Dissociation of carbanions from acyl iridium compounds: An experimental and computational investigation

Article abstract of DOI:10.1021/ja045859l

Instead of reductive elimination of aldehyde, or decarbonylation to give a trifluoroalkyl hydride, heating Cp*(PMe₃)Ir(H)[C(O)CF ₃] (1) leads to the quantitative formation of Cp*(PMe ₃)Ir(CO) (2) and CF₃H. Kinetic experiments, isotope labeling studies, solvent effect studies, and solvent-inclusive DFT calculations support a mechanism that involves initial dissociation of trifluoromethyl anion to give the transient ion-pair intermediate [Cp*(PMe₃)Ir(H) (CO)]⁺[CF₃]^-. Further evidence for the ability of CF₃^- to act as a leaving group came from the investigation of the analogous methyl and chloride derivatives Cp*(PMe₃)Ir(Me)[C(O)CF₃] and Cp*(PMe ₃)Ir(Cl)[C(O)CF₃], Both of these compounds undergo a similar loss of trifluoromethyl anion, generating an iridium carbonyl cation and CF₃D in CD₃OD. Three additional acyl hydrides, Cp*(PMe₃)Ir-(H)[C(O)R_F] (where R_F = CF₂CF₃, CF₂CF₂CF₃, or CF₂(CF₂)₆CF₃) undergo R _F-H elimination to give 2 at a faster rate than CF₃H elimination from 1. Stereochemical studies using a chiral acyl hydride with a stereocenter at the β-position reveal that ionization of the carbanion occurs to form a tight ion-pair with high retention of configuration and enantiomeric purity upon proton transfer from iridium.

Full text of DOI:10.1021/ja045859l

Published on Web 12/07/2004
Dissociation of Carbanions from Acyl Iridium Compounds:
An Experimental and Computational Investigation
Joseph G. Cordaro and Robert G. Bergman*
Contribution from the Department of Chemistry, UniVersity of California,
Berkeley, California 94720
Received July 9, 2004; E-mail: bergman@cchem.berkeley.edu
Abstract: Instead of reductive elimination of aldehyde, or decarbonylation to give a trifluoroalkyl hydride,
heating Cp*(PMe3)Ir(H)[C(O)CF3] (1) leads to the quantitative formation of Cp*(PMe3)Ir(CO) (2) and CF3H.
Kinetic experiments, isotope labeling studies, solvent effect studies, and solvent-inclusive DFT calculations
support a mechanism that involves initial dissociation of trifluoromethyl anion to give the transient ion-pair
intermediate [Cp*(PMe3)Ir(H)(CO)]+[CF3]-. Further evidence for the ability of CF3- to act as a leaving group
came from the investigation of the analogous methyl and chloride derivatives Cp*(PMe3)Ir(Me)[C(O)CF3]
and Cp*(PMe3)Ir(Cl)[C(O)CF3]. Both of these compounds undergo a similar loss of trifluoromethyl anion,
generating an iridium carbonyl cation and CF3D in CD3OD. Three additional acyl hydrides, Cp*(PMe3)Ir-
(H)[C(O)RF] (where RF ) CF2CF3, CF2CF2CF3, or CF2(CF2)6CF3) undergo RF-H elimination to give 2 at a
faster rate than CF3H elimination from 1. Stereochemical studies using a chiral acyl hydride with a
stereocenter at the â-position reveal that ionization of the carbanion occurs to form a tight ion-pair with
high retention of configuration and enantiomeric purity upon proton transfer from iridium.
Introduction
alkyl complexes. Aldehydes have also been produced from alkyl
carbonyl complexes upon treatment with a hydride source.
The general interest in studying metal acyl complexes comes
from their ubiquitous presence in stoichiometric and catalytic
reactions involving carbon monoxide. For example, in the oxo
reaction, CO and H2 are added to olefins to produce aldehydes
using catalytic cobalt or rhodium complexes. An important
intermediate in this reaction is the metal acyl complex, which
is cleaved either by a metal hydride or by H2.1 Many stoichio-
metric reactions, which introduce a carbonyl group into organic
substrates, employ metal carbonyls and proceed via acyl
intermediates. Isolable metal acyl complexes can be accessed
by a wide range of methods, including oxidative addition of
aldehydes or acid halides, insertion of CO into metal alkyl
bonds, and nucleophilic addition to metal carbonyls.2 Much of
the reactivity of metal acyl complexes can be categorized into
two broad areas: decarbonylation reactions and reductive
elimination reactions.
Aldehyde decarbonylation using transition metal complexes
requires a reactive species which oxidatively inserts into the
C-H bond of an aldehyde. The newly formed metal acyl
hydride species can undergo decarbonylation if a vacant
coordination site is made available adjacent to the acyl group.
Various techniques have been employed to produce an adjacent
coordination site (e.g., ligand dissociation via photolysis or
thermolysis, hydride abstraction).
Evidence for the intermediacy of an acyl complex has been
obtained in many of these reactions. The rate of aldehyde
reductive elimination from acyl hydrides is often very rapid and
has been correlated to the metal-carbon bond strength.3
Because of the greater stability of metal perfluoroacyl and
perfluoroalkyl complexes as compared to their all-hydrogen
analogues, these complexes were among the earliest known
organometallic species.4 The stronger M-CF3 bond as compared
to M-CH3 is partially responsible for the enhanced stability of
the former toward thermal decomposition.5-7 Routes for syn-
thesizing metal perfluoroacyl complexes generally involve
oxidative addition of a perfluoroacyl halide to a late transition
metal carbonyl.8,9 Rapid loss of CO followed by RF migration
(RF ) perfluoroalkyl) to generate the (CO)M-RF species can
occur, often precluding the isolation of the metal acyl species.
Isolation of the acyl MC(O)RF species can be achieved when a
reactive open coordination site is not available at the metal
center. Examples of CO inserting into a M-RF bond to give a
perfluoroacyl complex are scarce.10 Sulfur dioxide insertion into
M-RF bonds has been observed;11 however, due to the strong
M-RF bond, perfluoroalkyl complexes are generally regarded
as stable toward insertion reactions.12
(3) See ref 1, Chapter 5.
(4) Treichel, P. M.; Stone, F. G. A. AdV. Organomet. Chem. 1964, 1, 143-
Reductive elimination of aldehydes and ketones from metal
complexes can occur upon thermolysis of acyl hydride or acyl
220.
(5) Morrison, J. A. AdV. Inorg. Chem. 1983, 27, 293-316.
(6) Brothers, P. J.; Roper, W. R. Chem. ReV. 1988, 88, 1293-1326.
(7) Morrison, J. A. AdV. Organomet. Chem. 1993, 35, 211-239.
(8) King, R. B.; Bisnette, M. B. J. Organomet. Chem. 1964, 2, 15-37.
(9) King, R. B. Acc. Chem. Res. 1970, 3, 417-427.
(10) Jablonski, C. R. Inorg. Chem. 1981, 20, 3940-3947.
(11) Von Werner, K.; Blank, H. J. Organomet. Chem. 1980, 195, C25-C28.
(1) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and
Applications of Organotransition Metal Chemistry; University Science
Books: Mill Valley, CA, 1987; Chapter 12.
(2) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals;
John Wiley & Sons: New York, 1994; Chapter 8.
9
16912
J. AM. CHEM. SOC. 2004, 126, 16912-16929
10.1021/ja045859l CCC: $27.50 © 2004 American Chemical Society

Dissociation of Carbanions from Acyl Iridium Compounds
A R T I C L E S
Scheme 2. Two Methods for the Synthesis of Iridium Acyl
Scheme 1. Thermolysis of 1 Gives CF3H, Not Decarbonylation
Hydrides from 4
Perfluoroalkylmetal species are not as easy to synthesize using
alkylation reactions of the type commonly used with their all-
hydrogen cousins. The instability of perfluoroalkyllithium and
-magnesium reagents toward fluoride elimination limits their
use as perfluoroalkylating reagents.13 Methods employing Cd-
(CF3)2 and Hg(CF3)2 to transfer trifluoromethyl to M-X species
have been developed, but these routes are less popular due to
safety and environmental concerns.14-17 Oxidative addition of
RF-I to a metal can give a M(I)RF complex, but this method
requires the initial formation of an unsaturated metal center.18-20
More recently, Ruppert’s reagent CF3Si(CH3)3 has been suc-
cessfully used to transfer CF3 to both early and late transition
metal halides.21-23
When we recently found that our new method for synthesizing
iridium acyl hydrides24 could be applied to the preparations of
fluoroacyl complexes such as Cp*(PMe3)Ir(H)[C(O)CF3] (1,
Cp* ) η5-C5Me5), we were intrigued by the possibility of
investigating such systems. Upon heating a C6D6 solution of 1
at 105 °C, we discovered that neither CO loss nor aldehyde
reductive elimination occurred (Scheme 1). To our surprise, the
products of this thermolysis reaction were CF3H and the iridium-
(I) carbonyl Cp*(PMe3)Ir(CO) (2).25 The product of decarbo-
nylation, Cp*(PMe3)Ir(CF3)H (3), was never observed. In this
paper, we report our work on the mechanism of this unprec-
edented formal R-fluoroalkyl elimination reaction using both
experimental and computational methods.
in good yield. However, compound 5 was made by the reaction
of [Cp*(PMe3)IrHLi]x with pentafluoropropanoic anhydride
because the acid chloride was not readily available (method 226).
This method gives significantly lower yields due to the
requirement for laborious purification. Two additional perfluo-
roacyl hydride complexes (6 and 7) were synthesized from the
commercially available acid chlorides and 4 as shown in method
1. All solids were stored at -38 °C and showed no decomposi-
tion at this temperature. NMR (1H, 19F, 13C, and 31P) and IR
spectroscopy and elemental analyses were used to characterize
new compounds. (Crystallographic characterization of a related
acyl hydride has been reported.24) All compounds exhibited
1
Experimental Results and Discussion
broad doublets, due to P-H coupling, upfield in the H NMR
spectrum near -16 ppm characteristic of an iridium hydride.
In the 31P NMR spectrum, a signal near -38 ppm was observed
for the PMe3 ligand. Coupling of the phosphorus signal to the
metal hydride was usually observed (JH-P ≈ 30-40 Hz). Solid
or solution IR spectroscopy of the acyl hydrides showed a broad
Ir-H stretch in the region 2110-2160 cm-1 and a strong
Synthesis of Acyl Complexes. The synthesis of 1 and the
other acyl hydrides was accomplished using methods previously
reported from this group (Scheme 2). Method 124 was preferred;
treatment of Cp*(PMe3)Ir(H)2 (4) with an acid chloride and base
in benzene or toluene gave the desired acyl hydride complex
carbonyl stretch near 1620 cm-1
.
(12) Klabunde, K. J.; Campostrini, R. J. Fluorine Chem. 1989, 42, 93-104.
(13) For a recent review on the addition of a trifluoromethyl group to various
organic compounds, see: Langlois, B. R.; Billard, T. Synthesis 2003, 185-
194.
(14) Lagow, R. J.; Eujen, R.; Gerchman, L. L.; Morrison, J. A. J. Am. Chem.
Soc. 1978, 100, 1722-1726.
Five non-fluorine containing acyl hydrides were synthesized
following method 1 and were used in this study. The phenyl-
substituted acyl hydride 8 and methyl ester acyl hydride 9 were
reported in the preliminary communication.25 The cyanoacyl
hydride 10 was synthesized in good yield and showed a
characteristic CtN stretch in the IR spectrum at 2225 cm-1 in
addition to absorptions for the Ir-H and acyl CdO at 2095
and 1589 cm-1, respectively. Methyl substituents at the â-posi-
tion were used to prevent H2O elimination, which has been
observed to give a vinyl complex.24 The 2,2-dimethylpropionyl
(11) and para-nitrobenzoyl (12) hydrides were originally
synthesized by Paisner et al.24
(15) Krause, L. J.; Morrison, J. A. J. Am. Chem. Soc. 1981, 103, 2995-3001.
(16) Dukat, W.; Naumann, D. J. Chem. Soc., Dalton Trans. 1989, 739-744.
(17) Eujen, R.; Hoge, B. J. Organomet. Chem. 1995, 503, C51-C54.
(18) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411-428.
(19) Hughes, R. P.; Smith, J. M.; Liable-Sands, L. M.; Concolino, T. E.; Lam,
K. C.; Incarvito, C.; Rheingold, A. L. J. Chem. Soc., Dalton Trans. 2000,
873-879.
(20) Hughes, R. P.; Maddock, S. M.; Guzei, I. A.; Liable-Sands, L. M.;
Rheingold, A. L. J. Am. Chem. Soc. 2001, 123, 3279-3288.
(21) Huang, D. J.; Koren, P. R.; Folting, K.; Davidson, E. R.; Caulton, K. G. J.
Am. Chem. Soc. 2000, 122, 8916-8931.
(22) Vicente, J.; Gil-Rubio, J.; Bautista, D. Inorg. Chem. 2001, 40, 2636-2637.
(23) Taw, F. L.; Scott, B. L.; Kiplinger, J. L. J. Am. Chem. Soc. 2003, 125,
14712-14713.
(24) Paisner, S. N.; Burger, P.; Bergman, R. G. Organometallics 2000, 19, 2073-
2083.
(26) Peterson, T. H.; Golden, J. T.; Bergman, R. G. Organometallics 1999, 18,
2005-2020.
(25) Cordaro, J. G.; Bergman, R. G. J. Am. Chem. Soc. 2004, 126, 3432-3433.
9
J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004 16913

A R T I C L E S
Cordaro and Bergman
Scheme 3. Possible Mechanisms for the Loss of CF3H from 1
Scheme 4. Possible Acid/Base-Catalyzed Mechanisms for the
Loss of CF3H from 1
The first five mechanisms are intramolecular reactions
(Scheme 3). Path A is a concerted, irreversible elimination of
CF3H from 1 via TS-1 to generate the observed products. This
proposed concerted R-fluoroalkyl elimination is unprecedented
in organometallic chemistry. Path B, also unprecedented,27
requires that the CF3 fragment, functioning as a leaving group,
reversibly dissociates to generate an intermediate metal cation/
Two nonhydride iridium acyl compounds were synthesized
from 1. The methyl-substituted acyl complex 13 was synthesized
by vacuum transfer of anhydrous THF into a Schlenk tube
containing 1 and solid tert-butyllithium at -196 °C. The solution
was warmed to -15 °C to ensure complete deprotonation of
the metal hydride. Recooling the orange solution to -196 °C
followed by vacuum transfer of iodomethane to this solution
generated the alkylated product Cp*(PMe3)Ir[C(O)CF3](CH3)
(13) in 72% isolated yield (eq 1). The chloride-substituted acyl
-
carbanion pair 15. CF3 irreversibly deprotonates the iridium
cation to generate the observed products. Alternatively, the
reaction can be envisioned to proceed through a radical
mechanism (Path C); reversible homolytic cleavage of the
C(O)-CF3 bond generates a metal acyl/fluoroalkyl radical pair
16. The CF3 radical then abstracts a hydrogen atom from iridium
to generate the observed products. Path D illustrates a more
common mechanism for the reactivity of metal acyl complexes:
reversible migration of CF3 to iridium, which occurs simulta-
neously with Cp* ring slippage to provide an open coordination
site, produces intermediate 17. This species then undergoes
reductive elimination of CF3H, concurrent with recoordination
of the Cp* ligand, to generate the observed products. However,
it seems likely that intermediate 17 would also dissociate CO
to generate the iridium(III) species Cp*(PMe3)IrH(CF3) (3),
which we do not observe. In the fifth possible mechanism,
reversible PMe3 dissociation from 1 generates 18, a reactive
16-electron intermediate species with an open coordination site
at iridium (path E). Decarbonylation to generate the iridium-
(III) carbonyl (19) would be facile. Reductive elimination of
CF3H, possibly induced by PMe3 association, yields the
observed products. CO dissociation could also occur upon
binding PMe3 to furnish the trifluoromethyl iridium(III) hydride
3.
complex 14 was synthesized in approximately 83% yield by
treating dihydride 4 and proton sponge (1,8-bis(dimethylamino)-
naphthalene) with a 10-fold excess of trifluoroacetyl chloride
(eq 2). Presumably, under these reaction conditions, 1 is initially
formed but then chlorinated by excess acid chloride. Alterna-
tively, 14 was synthesized by treating a solution of 1 in CD2-
Cl2 with CCl4. Both 13 and 14 exhibited strong absorptions in
the IR spectrum for the carbonyl group at 1619 and 1627 cm-1
respectively.
,
The remaining two proposed mechanisms involve a Lewis
acid or base catalyst (Scheme 4). In path F, a Lewis acid (LA)
reversibly coordinates to the metal acyl oxygen to give adduct
20, which has resonance contributions from forms 20a and 20b.
Elimination of CF3-LA generates a stable iridium carbonyl
Mechanisms for Elimination of CF3H from 1. Heating a
C6D6 solution of 1 in a sealed NMR tube gave the iridium
carbonyl 2 and CF3H in quantitative yield. We considered seven
mechanisms for this unusual transformation (Schemes 3 and
4).
(27) A fluoroalkyl carbanion/metal cation pair has been reported in W and Mo
chemistry. See: Hughes, R. P.; Maddock, S. M.; Guzei, I. A.; Liable-Sands,
L. M.; Rheingold, A. L. J. Am. Chem. Soc. 2001, 123, 3279-3288.
9
16914 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004

Dissociation of Carbanions from Acyl Iridium Compounds
A R T I C L E S
Table 1. Temperature and Solvent Dependence of kobs for the Elimination of CF3 Anion from 1
solvent
temp (
°C)
kobs (s-1) (
×
104)
solvent
temp (
°
C)
kobs (s-1) ( 104)
×
C6D6
94.5
104.9
120.2
136
136
142.2
80
94.3
104.9
104.9
0.0375 ((0.0025)
0.089 ((0.004)
0.396 ((0.008)
1.41 ((0.05)
1.39 ((0.01)
2.29 ((0.11)
3.2 ((0.1b)
CD3CN
DMF-d7
94.3
94.3
70
79.9
85.5
90.8
90.8
90.8
94.3
99.3
7.18 ((0.4)
4.1 ((0.6)
C6D6
C6D6
C6D6
C6D6
C6D6
CD3OD
CD3OD
CD3OD
CD3ODc
DMSO-d6
DMSO-d6
DMSO-d6
DMSO-d6
DMSO-d6
DMSO-d6
DMSO-d6
DMSO-d6
0.91 ((0.02b)
2.52 ((0.06)
4.1 ((0.6)
a
7.02 ((0.44)
7.12 ((0.05)
6.2 ((0.4)
a
d
1.7 ((0.1b)
2.2 ((0b)
10.6 ((1.7)
7.0 ((0b)
16.1 ((0.1b)
a Reaction done with deuterium labeled 1-d1. b Error calculated for fit. c Done with added 13. d Done with 1 equiv of MgCl2.
cation 21 and the CF3-LA adduct. Proton transfer from iridium
cation 21 to the CF3-LA adduct liberates LA and forms the
observed products. The final possible mechanism that we
propose is an elimination reaction catalyzed by a Brønsted base
(path G). Deprotonation of the iridium metal hydride with base
(either reversibly or irreversibly depending on the strength of
the base) generates the reactive iridate species 22. The anionic
-
intermediate 22 eliminates CF3 to give the iridium carbonyl
2. Proton transfer from the conjugate acid to CF3- yields CF3H
and the free base.
Rate Studies on the Elimination of CF3H from 1. The rate
of disappearance of 1 and the formation of 2 followed clean
first-order kinetics for all temperatures and solvents monitored.
The proposed first-order rate law for the conversion of 1 to 2 is
shown in eq 3. The rate was found to be independent of initial
concentrations of 1 and was not affected by the addition of 5
equiv of PMe3; therefore, reversible PMe3 dissociation before
the rate-limiting step to give an open coordination site at iridium
as in path E is ruled out (Scheme 3). This observation is
Figure 1. Eyring plots for the elimination of CF3H from 1 in (a) C6D6 (R2
) 0.9991) and (b) DMSO-d6 (R2 ) 0.9986).
consistent with other work from this group showing that PMe3
dissociation from Cp*(PMe3)Ir complexes is very slow.28 While
Lewis acid coordination to the oxygen of an acyl group has
been shown to increase the rate of alkyl-carbonyl migratory
insertion reactions,29 we found no effect of such reagents on
the rate of reaction. For example, the addition of 1 equiv of
MgCl2 had no effect on the rate of CF3H loss from 1 when
measured in DMSO-d6 (studies investigating the effect of H+
on the rate of anion dissociation from 13 are discussed below).
Treatment of 1 with the stronger Lewis acid triphenylcarbenium
tetrakis(pentafluorophenyl)borate resulted in hydride abstraction
and decarbonylation to give the iridium trifluoromethyl carbonyl
cation (23) (eq 4). No elimination products were observed.
influenced by the medium, the rate of the reaction was measured
using 1H NMR spectroscopy in various solvents. We found that
in polar aprotic solvents such as CD3CN and DMSO-d6 the rate
of the reaction at 94 °C was accelerated approximately 200-
fold compared to the rate in C6D6. Other solvents exhibited
varying degrees of rate enhancement (Table 1).
The activation parameters for the reaction in C6D6 and
DMSO-d6 were found to be: C6D6, ∆Hq ) 25.7 kcal/mol and
∆Sq ) -14.0 kcal/mol; DMSO-d6, ∆Hq ) 24.3 kcal/mol and
∆Sq ) -6.5 kcal/mol. (See Figure 1 for Eyring plots.) The
∆∆Gq between the two solvents is consistent with the observed
rate difference.
The significant solvent effect on the rate of reaction was the
first indication that the reaction might proceed through a polar
transition state(s) and/or charged intermediate(s) (paths B, F,
and G). Table 2 shows the correlations between rates in different
solvents and various solvent parameters. While some trends can
be observed, the best categorization of solvents screened appears
to simply be nonpolar (benzene), dipolar protic (methanol), and
aprotic dipolar solvents (CH3CN, DFM, and DMSO), the latter
group of solvents showing the largest effect on rate enhance-
ment.
Solvent Effects. Solvent can play a critical role in the rate
and product distribution of many organic and organometallic
reactions. To investigate whether CF3H loss from 1 was
(28) Buchanan, J. M.; Stryker, J. M.; Bergman, R. G. J. Am. Chem. Soc. 1986,
108, 1537-1550.
(29) See ref 1, p 372.
9
J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004 16915

A R T I C L E S
Cordaro and Bergman
Table 2. Correlation between Solvent Effect on Rate of Reaction
Table 3. Rate of Elimination for Acyl Compounds Other than 1
and Solvent Parameters
compound
solvent
temp (
°
C)
kobs (s-1) ( 104)
×
Relative Rates of Reaction Measured at 94 °C:
CF3CF2 5
C6D6
C6D6
C6D6
C6D6
C6D6
DMSO-d6
tol-d8
CD3OD
CD3OD
DMF-d7
104.9
104.9
104.9
135.8
135.8
135.8
104.9
104.9
104.9
104.9
1.80 ((0.02)
14.9 ((0.6)
26 ((1)
0.147 ((0.003)
0.043 ((0.002)
4.9 ((0.1)
14.1 ((0.4)
30.1 ((0.4)
0.138 ((0.001)
0.036 ((0.001)
C6D6
1
< CD3OD
< DMF-d7
< CD3CN
< DMSO-d6
CF3CF2CF2 6
CF3(CF2)6CF2 7
dimethylbenzyl 8
ester 9
ester 9
cyano 10
methyl 13
chloride 14
chloride 14
45
110
191
283
ꢀ: Dielectric Constants of Solventsa
C6H6
2.28
< CH3OH
32.7
< DMF
36.7
< CH3CN
37.5
< DMSO
46.68
µ: Dipole Moments of Solvents (debye)a
C6H6
0
< CH3OH
2.87
< CH3CN
3.44
< DMF
3.86
< DMSO
3.90
b
ET(30): Electronic Transition Energies of Solvents (kcal mol-1
)
C6H6
34.3
< DMF
< DMSO
< CH3CN
< CH3OH
The similar BDEs of DMSO-d6 (BDE ) 94 kcal/mol),36 CD3-
CN (BDE ) 93 kcal/mol),37 and THF-d8 (BDE ) 92-93)38,39
indicate that a mixture of CF3H and CF3D would be formed in
all three solvents if a radical mechanism was operative, not just
in CD3CN or DMSO-d6. The acidities of the solvents, however,
do coincide with protonation of CF3-. The pKa of CF3H has
been estimated to be approximately 29 in DMSO40 or 27 in
43.2
45.1
45.6
55.4
a Lowry, T. H.; Richardson, K. S. Mechanism and Theory in Organic
Chemistry; Harper Collins: New York, 1987. b Reichardt, C. Chem. ReV.
1994, 94, 2319-2358.
Isotope Effect and Isotope Tracer Studies. Rate studies
using deuteride 1-d showed no kinetic isotope effect (KIE) in
either C6D6 or DMSO-d6 (kH/kD ) 1.01 in C6D6 and 0.994 in
DMSO-d6). This result necessitates that deprotonation of 1 is
not rate-determining, as might be expected if path G wereop-
erative. Additionally, the negligible KIE suggests that reductive
elimination of CF3H from either of the proposed intermediates
17 or 19 is not rate-determining. The observation of an isotope
effect is common in rate-determining reductive elimination
reactions of late transition metal complexes.28,30-33 The lack of
a KIE is consistent with a mechanism involving CF3 dissociation
from 1 via an ionic or radical mechanism (paths B or C).
Further support for a mechanism involving initial dissociation
water at 0 °C.41 CF3 is probably not basic enough to
-
deprotonate DMF or THF even at elevated temperatures.
However, deprotonation of CH3CN (pKa ) 26.5 in CH3CN),42
CH3OH (pKa ) 17 in CH3OH), or DMSO (pKa ) 35.1 in
DMSO)43 by CF3 at elevated temperatures would be pos-
-
sible.
Elimination of R-H from Other Acyl Hydrides. Three
additional perfluoroalkyl hydride complexes (5-7) were found
to undergo elimination of RF-H upon heating. Significantly
faster rate constants for elimination were measured for the
complexes with longer perfluoroalkyl chains (Table 3). Whereas
in C6D6 at 105 °C the half-life for CF3H elimination from 1
was 21.6 h, the half-life for HCF2CF3 elimination from 5 was
1.1 h. The half-lives for elimination of HCF2CF2CF3 from 6
and HCF2(CF2)6CF3 from 7 under the same conditions were
only 7.8 and 4.5 min, respectively. Due to the volatile nature
of the elimination product from 1, the rate of reaction could
not be accurately monitored by 1H or 19F NMR spectroscopies
using the resonances for CF3H. In contrast, measuring the rate
of formation of HCF2CF2CF3 from 6 and HCF2(CF2)6CF3 from
7 by 1H NMR gave rate constants equivalent to the rate constants
determined by monitoring the formation of 2 or the disappear-
ance of starting materials. The yields of the longer perfluoro-
alkane products were >90% as determined by NMR integration.
No fluoroalkenes, the product of fluoride elimination, were
observed for the reactions in C6D6, but in polar solvents,
elimination products were observed. These results provide
further support for the intermediacy of perfluorocarbanions,
which are stabilized inductively and live long enough in polar
solvent to undergo some fluoride elimination.
-
of CF3 came from isotope tracer studies. When the reaction
was carried out in CD3OD, the product of the elimination
reaction was not CF3H, but CF3D. Control experiments showed
that (1) heating 1 in CD3OD at temperatures lower than those
required to form 2 did not incorporate D into the hydride
position of 1 and (2) heating 2 and CF3H in CD3OD did not
incorporate D into trifluoromethane. Using CH3OD, only 2 and
CF3D were formed, confirming that the H(D) arose from the
OH(OD) group of the methanol solvent. This result conclusively
ruled out the possibility of a radical mechanism.34 In CH3OD,
•
one would expect CF3H to form only if CF3 were generated,
because the trifluoromethyl radical would abstract a hydrogen
from the weaker C-H bond (BDE ) 96.1 ( 0.2 kcal/mol) in
preference to the more strongly bound hydroxyl-deuterium atom
(>104.6 ( 0.7 kcal/mol).35 We conclude that deuterium
incorporation into the trifluoromethane product occurs via an
ionic mechanism (path B).
Thermolysis of 1 in DMSO-d6, CD3CN, THF-d8, and DMF-
d7 provided more evidence for the formation of CF3D via an
ionic mechanism. In CD3CN or DMSO-d6, both CF3H and CF3D
were generated in approximately 1:1.9 and 1:0.9 ratios. In THF-
d8 or DMF-d7, only 2 and CF3H were generated; no CF3D was
observed. The pKa values and bond dissociation energies (BDE)
of these solvents are consistent with the observed deuterium
incorporation into the trifluoromethane product via protonolysis.
Two nonfluoroacyl groups were shown to undergo carbanion
dissociation, albeit at higher temperatures. Compound 8 under-
(36) Bordwell, F. G.; Liu, W. Z. J. Phys. Org. Chem. 1998, 11, 397-406.
(37) Bordwell, F. G.; Zhang, X. J. Org. Chem. 1990, 55, 6078-6079.
(38) Muedas, C. A.; Ferguson, R. R.; Brown, S. H.; Crabtree, R. H. J. Am.
Chem. Soc. 1991, 113, 2233-2242.
(39) Laarhoven, L. J. J.; Mulder, P. J. Phys. Chem. B 1997, 101, 73-77.
(40) Andrieux, C. P.; Gelis, L.; Medebielle, M.; Pinson, J.; Saveant, J. M. J.
Am. Chem. Soc. 1990, 112, 3509-3520.
(30) Periana, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1986, 108, 7332-7346.
(31) Churchill, D. G.; Janak, K. E.; Wittenberg, J. S.; Parkin, G. J. Am. Chem.
Soc. 2003, 125, 1403-1420.
(41) Symons, E. A.; Clermont, M. J. J. Am. Chem. Soc. 1981, 103, 3127-
3130.
(32) Janak, K. E.; Parkin, G. J. Am. Chem. Soc. 2003, 125, 6889-6891.
(33) Jones, W. D. Acc. Chem. Res. 2003, 36, 140-146.
(42) Coetzee, J. F.; Padmanabhan, G. R. J. Phys. Chem. 1962, 66, 1708-1713.
(43) Matthews, W. S.; Bares, J. E.; Bartmess, J. E.; Bordwell, F. G.; Cornforth,
F. J.; Drucker, G. E.; Margolin, Z.; McCallum, R. J.; McCollum, G. J.;
Vanier, N. R. J. Am. Chem. Soc. 1975, 97, 7006-7014.
(34) For related experiments, see ref 27 and references therein.
(35) Blanksby, S. J.; Ellison, G. B. Acc. Chem. Res. 2003, 36, 255-263.
9
16916 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004

Dissociation of Carbanions from Acyl Iridium Compounds
A R T I C L E S
Scheme 6. Reaction of Dihydride with Fluorenonyl Chlorides
Scheme 5. Decomposition of 9 May Arise from Ketene Formation
went elimination slowly as compared to 1 (t1/2 ) 13.3 h at 136°C
in C6D6), to give 2 and 2-phenylpropane (>95% yield, eq 5).
in the production of Cp*(PMe3)IrCl2 (26). We hypothesized that
the â-hydrogen at the 9-position of the fluorene was undergoing
deprotonation during the reaction to generate a ketene. To pre-
vent this undesired transformation, 9-methyl-9-fluorenonyl chlo-
ride (27) was synthesized and reacted with dihydride 4 and pro-
ton sponge in C6D6. After 36 h at 25 °C, an equal amount of
9-methylfluorene and iridium carbonyl 2 in addition to unreacted
starting materials with trace amounts of Cp*(PMe3)IrHCl (28)
were observed by NMR spectroscopy. None of the desired iri-
dium acyl hydride (29) was detected. Replacement of the 9-hy-
drogen with a methyl group successfully blocked the decom-
position pathways, but the rapid elimination of 9-methylfluorene
prevented the isolation of 29. These experiments suggest that
the stability of the anion and pKa are not the only important
factors that control which acyl groups will undergo facile
elimination.
Stabilization of the transition state for anion dissociation via
an inductive effect was demonstrated for the thermolysis of
compounds 1 and 5-9. With increasingly longer perfluoroalkyl
chains, the rate of anion elimination increased. For the nonper-
fluoroacyl compounds, stabilization of the transition state was
predicted to occur via both resonance and inductive effects; the
π-system of the phenyl substituent in 8 and the ester substituent
in 9 was expected to facilitate anion dissociation by delocal-
ization of the negative charge. Surprisingly, acyl complexes 8
and 9, which should lead to phenyl- and carbonyl-stabilized
anions, respectively, were very slow to undergo anion dissocia-
tion. In contrast, isolation of the fluorenyl-substituted acyl
complex 29 was not possible due to rapid elimination which
furnished 2 and 9-methyfluorene (see Scheme 6).
The elimination reaction of 8 in CD3OD generated 2-deuterio-
2-phenylpropane. The slow rate of elimination from the acyl
hydride 8 is consistent with the inability of the dimethylbenzyl
anion to efficiently stabilize a negative charge. Compound 9
decomposed to 2 in low yield and unidentifiable organic
products at a considerably longer reaction time as compared to
1 (reaction time for 9: t1/2 ) 24 min at 135 °C in DMSO-d6
and t1/2 ) 45 h at 135 °C C6D6). The longer half-life measured
for the elimination reaction from 9 was surprising. Furthermore,
decomposition of the organic products, and eventually the
iridium carbonyl 2, at longer reaction times prevented further
investigation. One possible explanation for the decomposition
products is that, although 9 dissociates to the intermediate ion-
pair, the resulting carbanion eliminates methoxide to form a
ketene product (Scheme 5). At the high temperature required
for dissociation, the resulting ketene product is expected to
undergo decomposition.
In contrast to 1, the 2,2-dimethylpropionyl (11) and para-
nitrobenzoyl-(12) iridium compounds were unreactive upon
heating in C6D6 or DMSO-d6 below 135 °C; above this
temperature, complete decomposition occurred (eq 6). The
prohibitively high activation energy required to generate tert-
butyl carbanion or phenyl anion explains the lack of reactivity
of these complexes.
A reason for the slow rates of elimination from 8 and 9, but
rapid in situ elimination of 9-methylfluorene from 29, may be
the orientation of the π-system of the acyl substituents. Figure
2 illustrates possible low-energy conformations of 8 and 9. In
these conformations, the phenyl group in 8 and ester group in
9 lie in a “staggered” orientation that minimizes steric repulsion
between these groups and the two methyl groups on the adjacent
carbon. If this conformation is maintained as the C-carbonyl
bond at the â-position cleaves, the sp3 orbital in the breaking
C-C bond is nearly orthogonal to the π-system of the aryl and
ester substituents. This orientation will minimize orbital overlap
and stabilization of the developing negative charge as the
transition state is approached. In contrast, the arene π-system
in 29 is locked into a conformation in which there is good
overlap between the developing orbital in the cleaving bond
and the fluorenyl π-system. This conformation would lead to
Fluorenide anion has found utility in our group as a stable
organic carbanion.44 Substituted fluorenyl acyl iridium com-
plexes appeared to be good candidates for investigating the
effects of pKa changes on the rate of anion dissociation. A wide
range of pKa values of substituted fluorenes are known; fluorene
itself has a pKa of 22.6 in DMSO.43 Unfortunately, all attempts
to synthesize the fluorenyl analogue (24) of the iridium acyl
hydride under a variety of conditions were unsuccessful (Scheme
6). Treatment of iridium dihydride 4 and proton sponge with
9-fluorenonyl chloride (25) in C6D6, C6D12, or THF-d8 resulted
(44) Fulton, J. R.; Sklenak, S.; Bouwkamp, M. W.; Bergman, R. G. J. Am. Chem.
Soc. 2002, 124, 4722-4737.
9
J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004 16917

A R T I C L E S
Cordaro and Bergman
Scheme 7. Deprotonation of 1 with tert-Butyllithium Gives 2 and
Difluoroketene 30
examining a â-chiral enantioenriched acyl hydride complex.47
Upon synthesis of an iridium acyl hydride from an R-chiral acid
chloride, a chiral center should be generated at the iridium center
giving four enantiomers, two of which are diastereomers
(Scheme 8). Elimination from this compound would give the
achiral iridium carbonyl 2, and an organic product with a tertiary
carbon stereocenter. The enantiomeric excess (ee) of the organic
product formed upon elimination would provide evidence about
its mechanism. If elimination occurred through a concerted
mechanism (path A), then the organic product would remain
enantioenriched. If dissociation of R- or R• occurred to give a
tight ion- or radical-pair, then R-H would remain enantioen-
riched, possibly with some loss of ee. However, if elimination
gave solvent-separated ions or radicals, then the R-H product
would be racemic. Migration of R to iridium followed by
reductive elimination of R-H would likely occur with high
overall retention of configuration. We decided to use enan-
tioenriched Mosher’s acid chloride (3,3,3-trifluoro-2-methoxy-
2-phenylpropionyl chloride) to investigate the stereochemistry
of the R-H elimination.
Reaction of 4 with Mosher’s Acid Chloride Reagent. The
reaction of Cp*(PMe3)IrH2 (4) and proton sponge with enan-
tioenriched (R)- or (S)-Mosher’s acid chloride (99% ee) (1:1:1
ratio) in C6D6 or CD3CN did not give any observable acyl
hydride 31a or 31b. After approximately 9 days at 25 °C, or
12 h at 75 °C in C6D6, the only iridium-containing materials
identified by NMR spectroscopy were dihydride 4, hydrido-
chloride 28, and carbonyl 2 in a 1:1:3 ratio with a trace amount
of dichloride 26 (Scheme 9). In CD3CN, complete consumption
of the acid chloride occurred after approximately 20 h at 25
°C, or 2 h at 75 °C.48 The iridium-containing products in CD3-
CN identified by 1H and 31P NMR spectroscopy were a mixture
of 4, 28, hydrido carbonyl cation 21,49 and an unidentified
iridium hydride product in a ratio of 1.0:0.81:2.9:0.41. A small
amount of dichloride 26 was detected in the 31P NMR spectrum.
No intermediate species were observed over the course of the
reaction in either solvent.
Figure 2. The dimethylmethylene linker inhibits rotation of the phenyl
and ester substituents, so that in the lowest-energy conformations, the
illustrated orbitals in 8 and 9 are orthogonal to one another. However, in
compound 29, the fluorenyl π-system overlaps well with the sp3 bond being
broken in 29 because of the structural rigidity of the system, and in 10
because of the cylindrical shape of the cyano substituent. (Eight carbon
atoms of the fluorene substituent 29 have been omitted for clarity.)
rapid anion dissociation from 29 because of the better resonance
stabilization of the negative charge.
To test this mechanistic hypothesis, we synthesized the cyano-
substituted acyl hydride 10. Because of the cylindrical symmetry
of the π-system of the cyano group, its overlap with the develop-
ing orbital in the cleaving C-C bond should not be conforma-
tion-dependent. Therefore, our hypothesis predicts that the cyano
substituent will stabilize the carbanion generated in the transition
state no matter what its lowest-energy transition state conforma-
tion is.
Upon thermolysis of 10 in toluene-d8 at 105 °C, 2 and
2-cyanopropane were generated in quantitative yield (eq 7). The
rate of elimination occurred with t1/2 ) 8.2 min, hundreds of
times faster than the rate of elimination from either 8 or 9 (Table
3).45 This result supports our hypothesis that π-bond overlap is
critical in stabilizing the transition state for anion dissociation.
Reaction of Cp*(PMe3)Ir(H)[C(O)CF3] with Base. To test
the possibility of a base-catalyzed mechanism as illustrated in
path G, Scheme 4, we treated 1 with both weak and strong bases.
Thermolysis of 1 in C6D6 at 75 °C with 10 equiv of aniline or
n-butylamine showed no catalytic reactivity for the elimination
of CF3H. The deprotonation of 1 with tert-butyllithium,
however, was shown to occur at low temperature to give iridate
22-Li (vide supra). Upon warming to 25 °C, iridate 22-Li
The reaction in C6D6 generated two major organic products.
These products were identified by NMR spectroscopy, GC/MS,
and chiral GC as 3,3,3-trifluoro-2-methoxy-2-phenylpropional-
dehyde (32), the aldehyde derivative of Mosher’s acid, and 2,2,2-
trifluoro-1-methoxyethylbenzene (33), the product of decarbo-
nylation/reduction of Mosher’s acid chloride. The ratio of the
aldehyde to alkane in C6D6 was approximately 1:2.5 (similar
-
eliminated CF3 to give 2 and eliminated fluoride to give
difluoroketene adduct 30 (Scheme 7).46 No fluoride elimination
was ever observed in the thermolysis of 1. These results indicate
that path G is operative in the presence of a strong base, where
the reaction is fast even at low temperatures. It is unlikely,
however, that the thermal reaction to generate 2 and CF3H
proceeds through intermediate 22.
(47) For some early examples on the mechanism and stereochemistry of aldehyde
and acid chloride decarbonylation, see: (a) Walborsky, H. M.; Allen, L.
E. J. Am. Chem. Soc. 1971, 93, 5465-5468. (b) Stille, J. K.; Fries, R. W.
J. Am. Chem. Soc. 1974, 96, 1514-1518. (c) Lau, K. S. Y.; Becker, Y.;
Huang, F.; Baenziger, N.; Stille, J. K. J. Am. Chem. Soc. 1977, 99, 5664-
5672.
(48) Monitoring the reaction after 70 h indicated no further changes in product
ratios.
(49) The solubility of the ammonium chloride salt in acetonitrile accounts for
the presence of the protonate iridium carbonyl 21.
Stereochemical Studies of R-H Elimination. Next, we
initiated a study of the stereochemistry of R-H elimination by
(45) A significant solvent effect on the rate of elimination is not expected upon
substituting toluene for benzene.
(46) Cordaro, J. G.; van Halbeek, H.; Bergman, R. G. Angew. Chem., Int. Ed.
2004, 43, 6366-6369.
9
16918 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004

Dissociation of Carbanions from Acyl Iridium Compounds
A R T I C L E S
Scheme 8. Generation of Two Diastereomers upon Reaction of 4 with an R-Chiral Acid Chloride
Scheme 9. Reaction of Dihydride 4 and Proton Sponge with Mosher’s Acid Chloride
of the volatile materials by 1H and 19F NMR spectroscopy and
chiral GC showed that the major product was alkane 33,
generated in >98% ee along with minor amounts of alkene 34.
This sequence of reactions, similar to method 2 (Scheme 2) for
synthesizing acyl hydrides, confirms that acyl hydride 31a/31b
is a likely intermediate that quickly eliminates alkane 33 with
high ee in the reactions depicted in both Scheme 9 and eq 8.
(S)-Aldehyde 32 was independently synthesized in >95% ee
and 98% yield (by chiral GC) from (S)-Mosher’s acid chloride
and tributyltin hydride in C6D6 at 25 °C (Scheme 10).51 The
reaction of alkyl tin hydrides with acid chlorides was originally
proposed to occur through a radical chain mechanism,52 but later
work showed that this reduction did not involve a radical chain
process.51
To ensure that the radical (Ph)(MeO)(CF3)C•, if formed,
would be configurationally unstable and gave stereorandomized
products, alkane 33 was generated via a radical mechanism using
the method pioneered by Barton and co-workers (Scheme 10).53
Metathesis of the thallium N-hydroxypyridine-2-thione salt54
with (R)-Mosher’s acid chloride furnished the pyridine thioether
derivative 35 after radical-decarboxylative rearrangement. Thio-
ether 35 was shown to be racemic (ca. 6.5% ee by chiral
HPLC).55 Photolysis of 35 in C6D6 in the presence of 9,10-
to the ratio of hydridochloride 28 to carbonyl 2). Trace amounts
of an alkene, identified as 1,1-difluoro-2-methoxy-2-phen-
ylethene (34), were also observed in the 19F NMR spectrum
(Scheme 9). The ee values of the aldehyde 32 and alkane 33
determined by chiral GC were >98% and >96%, respectively.50
In CD3CN, three major products were identified as aldehyde
32, alkane 33, and alkene 34 present in approximately a 1:1:1
ratio. Two unidentified fluorine-containing products (ca. 40%
by integration) were detected. Using chiral GC, the ee was
measured to be approximately 98% for the aldehyde and >96%
for the alkane.
In the absence of proton sponge, the reaction of dihydride 4
with Mosher’s acid chloride in C6D6 or CD3CN gave the iridium
hydrido carbonyl cation 21 rather than the carbonyl 2. Two
unidentified iridium-containing products were also observed.
More of the iridium dichloride 26 was observed than when base
was added, suggesting that free HCl was responsible for
chlorination. Aldehyde 32, alkane 33, and alkene 34 were also
formed in ratios similar to those observed in the reactions with
added base. The ee’s of the aldehyde and alkane as determined
by chiral GC were high in both solvents (>97%).
In an alternative route to 31a/31b, dihydride 4 was depro-
tonated with tert-butyllithium to generate [Cp*(PMe3)IrHLi]x
and was subsequently treated with (S,S)-Mosher’s anhydride in
C6D6 at 25 °C. The only organometallic product identified by
NMR spectroscopy was the iridium carbonyl 2 (eq 8). Analysis
(50) In THF-d8, the reaction was slightly faster than in C6D6 and gave rise to
more alkene 34.
(51) Lusztyk, J.; Lusztyk, E.; Maillard, B.; Ingold, K. U. J. Am. Chem. Soc.
1984, 106, 2923-2931.
(52) Kuivila, H. G.; Walsh, E. J. J. Am. Chem. Soc. 1966, 88, 571-576.
(53) Barton, D. H. R.; Crich, D.; Motherwell, W. B. Tetrahedron 1985, 41,
3901-3924.
(54) Aveline, B. M.; Kochevar, I. E.; Redmond, R. W. J. Am. Chem. Soc. 1995,
117, 9699-9708.
(55) Isolated crystals of thioether 35 showed no optical rotation, and analysis
by chiral HPLC revealed that 35 consisted of a 1:1 ratio of enantiomers.
Chiral HPLC of the crude reaction mixture containing 35 showed that the
product was nearly racemic (9% ee).
9
J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004 16919

A R T I C L E S
Cordaro and Bergman
Scheme 10. Independent Synthesis of Aldehyde 32 and Alkane
33 from Mosher’s Acid Chloride
Scheme 11. Possible Mechanisms for the Formation of Hydrido
Iridium Carbonyl Cation from 14 in CH3OH
-
hydride ligand and metal cation for CF3 loss from 1 has not
been investigated. Therefore, to examine the role that might be
played by the hydride ligand in the reactivity of compounds
such as 1, substituted trifluoroacyl iridium complexes with CH3
(13) and Cl (14) in place of H were synthesized (vide supra).
It was found that heating a CD3OD solution of the acyl methyl
compound 13 (eq 9) or acyl chloride compound 14 (eq 10) at
105 °C gave CF3D and an iridium cation product in quantitative
yield. In aprotic solvents (e.g., C6D6 or DMF-d7), complete
decomposition was observed. Formation of 1,1,1-trifluoroethane
or trifluoromethyl chloride was not seen. The methyl acyl
compound 13 underwent dissociation to give Cp*(PMe3)Ir(CO)-
(CH3)+ (37) and CF3-, and the latter reacted with CD3OD to
give CF3D and methoxide-d3 as the counterion. Upon thermoly-
sis of 14, the chloro carbonyl cation was not observed. NMR
dihydroanthracene (DHA) gave a mixture of fluorine-containing
products, including alkane 33. Alternatively, thermolysis of 35
with 12 equiv of tributyltin hydride at 75 °C in C6D6 gave 33
in >95% yield by NMR. Analysis of the volatile materials by
chiral GC confirmed that alkane 33 was racemic. These
experiments show that the formation of (Ph)(MeO)(CF3)C• from
an enantioenriched precursor leads to racemized products and
rule out the intervention of this radical in the elimination from
31a/31b.
In the reaction of Mosher’s acid chloride with Cp*(PMe3)-
IrH2 and proton sponge, the rate-limiting step is most likely
initial nucleophilic attack at the carbonyl carbon of the acid
chloride by the iridium metal center of the dihydride 4.24 This
step is slow due to the steric bulk of the acid chloride; however,
it appears to be faster in acetonitrile than in benzene (oxidative
addition of RC(O)Cl to Rh(CO)L3Cl was reported to be much
faster in acetonitrile than in benzene or toluene56). Rapid
deprotonation of the resulting iridium cation 36 by proton sponge
generates the acyl hydride product 31. This complex, however,
is unstable and undergoes elimination to generate alkane 33 and
carbonyl complex 2. The fast rate of elimination from 31a/31b
is likely due to inductive stabilization of the carbanion and
possibly hyperconjugation from the trifluoromethyl substituent.
The high enantioselectivity of alkane formation indicates a tight
ion-pair intermediate which transfers a proton to the carbanion
to give the observed products. In polar solvents, fluoride
elimination to give alkene 34 occurs from the carbanion at a
rate which is competitive with proton transfer. The ion-pair must
react to give alkane 33 much more rapidly than the ions separate,
because the ee of 33 is high in both solvents. Aldehyde 32 and
hydridochloride 28 likely form by aldehyde reductive elimina-
tion from the acyl iridium dihydride cation 36 (Scheme 9).
and IR analysis of the material isolated from this thermolysis
reaction in protiated methanol revealed that the product was
the hydrido iridium carbonyl cation 21. This compound was
independently synthesized by the method described by Angelici
et al.57 The chloro iridium carbonyl cation intermediate is most
likely formed in situ. Two mechanisms for the transformation
of the chloro cation in the presence of methoxide to 21 are
proposed (Scheme 11). Both mechanisms involve displacement
of chloride, hydride elimination, and liberation of formaldehyde.
In CD3OD, the rate of disappearance of 13 to give the methyl
cation followed first-order kinetics with a rate approximately 4
times faster than the rate for the conversion of 1 to 2 (Table 3).
The disappearance of the chloro acyl complex 14 was found to
be 51 times slower than the disappearance of 1 under identical
conditions. CF3- dissociation is, not surprisingly, faster for the
more electron-donating methyl compound 13 as compared to
the chloro complex 14. The methyl ligand is expected to stabilize
-
Elimination of CF3 from 13 and 14. The reactivity of
the trifluoroacyl group has provided strong support for the loss
of CF3H from 1 via an ion-pair mechanism (path B). Additional
experiments have focused on the leaving group ability of the
carbanion (e.g., CF3, alkyl, or aryl). The importance of the
(56) Stille, J. K.; Fries, R. W. J. Am. Chem. Soc. 1974, 96, 1514-1518.
(57) Wang, D. M.; Angelici, R. J. Inorg. Chem. 1996, 35, 1321-1331.
9
16920 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004

Dissociation of Carbanions from Acyl Iridium Compounds
A R T I C L E S
Scheme 12. CF3 Migration to Iridium Would Result in CO
Dissociation
Table 4. Rate Constants for the Disappearance of
Cp*(PMe3)Ir(Me)[C(O)CF3] (13) in DMF-d7 at 98 °C with Added
Acid (HA) and Cp*(PMe3)Ir(Me)(CO)+ (37)a
entry
13
37
HA
[HA]/[37]
kobs (s-1) ( 104)
×
1
2
0.015b
0.015b
0.015b
0.015b
0.014
0.017
0.015
0.013
0.043
0.028
0.014
0.014
0.015
0.018
0
0
NA
NA
NA
NA
1.9
NA
0.33
0.53
1.1
2.0
3.8
0.52
3.7
NA
19c
27c
30c
32c
15
0.000
0.000
0.000
0.036
0.0
0.059b,d
0.11b,d
0.23b,d
0.069d
0.060e
0.046e
0.043e
0.14e
3
4
5
6
23f
28
7
0.140
0.086
0.130
0.095
0.050
0.087
0.054
0
8
29
9
29
10
11
12
13
14
0.18e
34
0.19e
40
0.045g
0.20g
21
23
excess
17 ((3)h
the resulting metal cation and possibly lower the activation
energy. Conversely, the more electron-withdrawing chloro
ligand destabilizes the iridium cation upon CF3 dissociation,
a Concentrations given in mol/L. b Concentration is approximate. c Initial
rate data only. d Used 9-phenyl fluorine as HA. e Done with NMe3HCl.f Rate
of disappearance was equal to rate of product growth. g Done with 94%
NMe3DCl. h Average of five runs using a ratio of NMe3HCl/NMe3DCl
between 1.3 and 0.0.
-
resulting in a slower rate of reaction.
The results of hydride substitution with methyl- or chloro-
ligands strongly disfavor both of the migration mechanisms
(paths D or E in Scheme 2). Had migration of CF3 to iridium
occurred to give intermediate 38 (R ) Me) or 39 (R ) Cl),
then rapid CO loss would be expected to give Cp*(PMe3)Ir-
(CF3)(R) (40, R ) Me, or 41, R ) Cl). Reductive elimination
of CF3-R from these intermediates or protonolysis from CD3-
OD solvent to give CF3D and the observed products would be
unlikely (Scheme 12).
is due to the known decomposition of CF3- to fluoride ion and
the reactive carbene CF2.
A stronger acid, therefore, was selected to react with CF3
-
and reduce the amount of decomposition. When 3.5 equiv of
Me3NHCl was added, clean first-order kinetics were seen for
the entire reaction profile even in the absence of added 37 (entry
6, Table 4). The rate of starting material disappearance was equal
to the growth of product (-d[13]/dt ) d[37]dt), conditions not
achieved when using 9-phenylfluorene as HA. Having found
an acid that appeared to prevent CF3- decomposition, the ratio
of [HA]/[37] was varied to look for product inhibition (entries
7-13, Table 4). Initial concentrations of starting acyl complex
13 were varied between 0.013 and 0.043 M. Concentrations of
product 37 were varied between 0 and 0.14 M, and the
concentrations of HA were varied between 0.045 and 0.20 M.
Two runs using Me3NDCl as HA showed a slight, but probably
insignificant, decrease in kobs (entries 12 and 13). A modest
increase in kobs with the highest ratios of [HA]/[37] (entries 10
and 11) as compared to kobs for the lowest ratios of [HA]/[37]
(entries 7, 8, and 12) was recorded. Overall, variations in the
measured kobs values were too minor to conclude that product
inhibition was observed when Me3NHCl was used as the acid.
These results suggest that, under these conditions, fast proto-
nation by HA ensured that k2[HA] . k-1[37]. The HA term
dominates the denominator of eq 14, reducing the rate law to
the simple first-order expression in eq 15.
Kinetic Studies on the Methyl Acyl Iridium Complex 13.
Without the hydride attached to the metal center, as in the methyl
acyl complex 13, ionization of the CF3 group can be studied
-
directly. Proposed elementary steps for the dissociation of CF3
from methyl acyl complex 13 are illustrated in eqs 11 and 12.
The first step, dissociation of CF3-, is potentially reversible.
-
The second step, protonation of CF3 with an external acid
(HA), is considered irreversible due to the high pKa of CF3H
(pKa ≈ 29 in DMSO),40 as long as HA is a substantially stronger
acid. Using the steady-state approximation for the concentration
of CF3-, a rate expression for d[13]/dt was derived (eqs 13 and
14).
It is possible that addition of Me3NHCl resulted in protonation
of iridium or the acyl oxygen prior to CF3- dissociation, chang-
ing the mechanism to a Lewis acid-catalyzed reaction (similar
to path F, Scheme 4). However, the measured kobs with no added
HA (entry 1) is similar to kobs with added NMe3HCl, suggesting
that H+ is not acting as a catalyst. Furthermore, in experiments
using mixtures of Me3NH/DCl (in excess relative to 13), the
ratio of CF3H to CF3D was approximately equal to the initial
ratio of H+/D+ (entry 14, Table 4). No anomalous deuterium
incorporation was observed, confirming that a preequilibrium,
in which a proton coordinates to 13, is not occurring.
This rate expression predicts that, in the presence of excess
HA and 37, the rate should exhibit pseudo first-order kinetics,
with a dependence of kobs on both the concentration of 37 and
HA. In principle, the value of kobs should depend on the
concentration of HA at low [HA] and high [37]; that is, inhibi-
tion by 37 and saturation in HA should both be observed. In
practice, we found that when the rate of protonation was too
slow, for example with high pKa acids such as fluorene and
9-phenylfluorene, we observed complicated product mixtures
and irreproducible rates (entries 1-4, Table 4). We believe this
Reactivity of Cp*(PMe3)IrH(CF2CF2CF3) (42) with CO.
To test the possibility of fast and reversible CO dissociation
9
J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004 16921

A R T I C L E S
Cordaro and Bergman
Figure 3. Computed Gibbs free energy of CF3H elimination (B3LYP/LACVP**) values are in bold. Enthalpy of reaction (B3LYP/LACVP**++//B3LYP/
LACVP**) values are in parentheses.
Table 5. Calculated Energies (kcal/mol) Relative to 1: Gas-Phase
and Solution-Phase Calculations
from intermediate 17 to give 3 versus RF-H reductive elimina-
tion to give 2 (eq 16), we synthesized the perfluoroalkyl hydride
complex Cp*(PMe3)IrH(CF2CF2CF3) (42)58 and treated it with
CO. (Attempts to make 3 were not successful.) Heating a DMF-
Gibb’s
free
CH3OH CH3OH C6D6
single- SCF single- SCF
C6D6
single- CH3OH
Esolv
Esolv
C6H6
com-
SCF
pound energya energyb pointc energya pointc energya pointc (kcal/mol) (kcal/mol)
1
0
0
0
0
0
0
0
-10.9
-4.2
-3.7
2 +
-5.4 -19.6 -11.7 -11.0 -13.8 -6.3 -13.1 -9.0
CF3H
3 + CO 9.1 -6.6
17
TS-1
TS-2
TS-3
5.0
7.6
9.8
9.7
6.7 10.4
9.4 8.0
6.4 -9.0
7.5 -9.1
-2.6
-4.2
8.1
49.5
54.4
47.0
3.7
44.9 40.8 35.6 21.3 44.3 32.5 -30.4 -12.5
49.0 52.4 56.7 53.2 55.9 52.3 -10.1
-4.2
-4.2
41.8 47.2 49.8 50.4 46.9 47.2 -7.7
d7 solution of 42 under an atmosphere of CO at 75 °C gave no
iridium carbonyl 2 after 20 min (eq 17). In comparison, acyl
hydride 6 eliminated HCF2CF2CF3 to give 2 in <20 min at the
same temperature. In C6D6, similar results were observed. While
this result eliminates the possibility of fast and reversible CO
dissociation from intermediate 17, it does not preclude this spe-
cies as an intermediate if CO dissociation is slow. Computational
studies were therefore used to further elucidate the mechanism
of CF3H elimination from 1 and investigate the nature of a ring-
slipped intermediate like 17.
a Geometry optimization done at the DFT B3LYP/LACVP** level.
b Gibb’s free energy calculated at 298.15 K and 1 atm. c Single-point
calculations repeated using LACVP**++ basis set.
transition states for two of the mechanisms depicted in Scheme
3 were calculated using B3LYP density functional theory (DFT).
Calculations for the loss of CF3H from 1 focused on (1) the
concerted elimination illustrated in path A and (2) migration of
CF3 to iridium followed by reductive elimination illustrated in
path D, because experimentally these two mechanisms were the
most difficult to distinguish.
Thermodynamically, the conversion of 1 to CF3H and iridium
carbonyl 2 was calculated to be more exergonic (∆G(298.15
K) ) -19.6 kcal/mol) than was the conversion of 1 to CO and
iridium trifluoromethyl hydride 3 (∆G(298.15 K) ) -6.6 kcal/
mol) (Figure 3). Entropy and zero-point-energy (ZPE) factors
contribute approximately -9 kcal/mol to ∆G for the loss of
either CO or CF3H. The energies of all calculated species
relative to 1 are given in Table 5.
Computational Results and Discussion
Loss of CF3H from 1: Gas-Phase Calculations. The geome-
tries, energies, vibrational frequencies, and natural bond orders
(NBO) of reactants, products, and possible intermediates and
9
16922 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004

Dissociation of Carbanions from Acyl Iridium Compounds
A R T I C L E S
Table 6. Selected Bond Length (Å) Changes for CF3H Loss via Two Calculated Mechanisms
bond distance (Å)
structure
Ir−H
Ir−(CO)
Ir
−(PMe3)
C
−O
C(O)
−CF3
Ir−C(4)
Ir
−C(5)
Ir
−C(12)
Ir
−C(10)
Ir
−C(8)
H
−(CF3)
Ir−(CF3)
1
1.596
1.578
NA
1.591
1.623
1.736
1.588
2.010
1.890
1.834
1.870
1.947
1.854
NA
2.286
2.343
2.274
2.248
2.334
2.319
2.294
1.227
1.155
1.173
1.176
1.154
1.183
NA
1.568
2.884
NA
2.033
NA
2.316
2.301
2.356
3.409
3.419
3.303
2.324
2.318
2.276
2.286
3.083
2.987
3.037
2.261
2.386
2.471
2.340
2.968
3.064
2.779
2.366
2.357
2.440
2.394
2.322
2.378
2.246
2.376
2.338
2.285
2.370
2.358
2.250
2.291
2.328
3.122
2.249
NA
3.005
2.557
1.401
NA
NA
NA
NA
2.723
2.066
2.367
2.042
TS-1
2
TS-2
17
TS-3
3
3.661
NA
A transition state (TS-1) located for the direct conversion of
1 to 2 was found to be 44.9 kcal/mol above 1. It is clear that
∆Gq for this process is prohibitively high for a normal, solution-
phase thermal organometallic reaction. However, upon further
analysis, it becomes evident that the energy of TS-1 is not
unreasonable for this gas-phase calculation. The calculated
geometric differences between 1 and TS-1 are minimal except
for the elongation of the acyl C-CF3 bond length (see Table 6
for selected bond lengths). Calculations predicted the C-CF3
bond length of the acyl group to increase from 1.568 to 2.884
Å, while there was negligible change (0.018 Å) in the Ir-H
bond length upon going from 1 to TS-1. Most important to the
energy of TS-1 is the change in charge distribution. Results of
NBO analysis showed that the calculated negative charge on
the CF3 group in TS-1 increased from -0.09 in 1 to -0.769.
The large negative charge on the CF3 fragment suggests that
TS-1 more closely resembles a metal cation/carbanion pair, and
the calculated high activation energy is, therefore, not unreason-
able for a gas-phase heterolytic bond cleavage process.
An alternative mechanism, involving CF3 migration to iridium
followed by CF3H reductive elimination, was also calculated
(path D, Scheme 3). The transition state (TS-2) connecting 1
and intermediate 17 was found to be 49.0 kcal/mol in energy
above 1 (Figure 3). Simultaneous with CF3 migration to iridium
in TS-2 was a pronounced η5- to η2-ring slip of the Cp* ligand
(Table 6). The unusual η2- rather than η3-coordination of the
Cp* ligand was evident both geometrically and electronically.
In 17, the geometry at iridium was best described as trigonal
bipyramidal with the hydride and CO ligands axial and the
PMe3, CF3, and η2-Cp* ligands equatorial. A comparison
between 1 and 17 revealed that, whereas in 1 the charges on all
five tertiary carbon atoms of the Cp* ligand were negative
(-0.08 to -0.11 for each or -0.48 on all five carbons), in 17
the two carbon atoms closest to iridium had a combined charge
of -0.42 while the three displaced carbon atoms had a combined
charge of + 0.06. Only two of the Cp* carbon atoms are bound
to iridium in 17. The energy of intermediate 17 was found to
be only 3.7 kcal/mol higher than 1. This result suggests that a
large amount of reorganization energy is required in the
transition state to displace the Cp* ligand. Reductive elimination
of CF3H from 17, simultaneous with recoordination of the Cp*
ring (η2- to η5-Cp*), was calculated to occur via TS-3 with ∆Gq-
(298.15 K) ) 41.8 kcal/mol relative to 1. Presumably, an equal
amount of reorganization energy is required to recoordinate the
Cp* ligand in TS-3 as was needed in TS-2. However, the loss
of CF3H in TS-3 contributes to the lowering of the barrier for
reductive elimination because of entropic effects.
We also considered the possibility of CO dissociation from
intermediate 17 to give 3. Unfortunately, attempts to locate a
transition state connecting 17 and 3 were unsuccessful. Sys-
tematically increasing the Ir-(CO) bond distance in 17 and
performing transition state searches either resulted in immediate
recoordination of CO to iridium or loss of CO to give 3. We
believe, therefore, that if intermediate 17 were formed in the
reaction, CO loss would be rapid to give 3 with a barrier much
lower than that required to reach TS-3. Because neither
intermediate 17 nor product 3 were observed experimentally,
and treatment of 42 with CO did not induce RF-H reductive
elimination (eq 17, vide supra), we conclude the mechanism
illustrated in path D is not kinetically feasible.
Loss of CF3H from 1: Solution-Phase Calculations. The
calculations for the loss of CF3H from 1 were repeated to
compute solvation energies and structures (Table 5 and Figure
4). In solvation calculations, structures are first optimized in
the gas phase, and then the results are subjected to solvation
effects, which include reoptimization of the geometries. This
method utilizes the Poisson-Boltzmann continuum approxima-
tion, which represents the solvent as a layer of charges at the
molecular surface using the dielectric constant and probe radius
for the selected solvent.59,60 Recent work has appeared in the
literature employing this technique.61-63 The solvation energy
(Esolv) of a structure is defined as the (optimized gas-phase
energy) - (optimized solution-phase energy). For all organo-
metallic structures (except for TS-1), Esolv was calculated to be
approximately -4 kcal/mol in benzene and -8 to -11 kcal/
mol in methanol. TS-1 was calculated to have an Esolv value of
-12.5 kcal/mol in benzene and -30.4 kcal/mol in methanol,
significantly larger than the other calculated solvation energies.
The geometries of solvated species were not significantly
perturbed compared to the gas-phase optimized structures except
for TS-1 (see Table S-3 in the Supporting Information for
selected bond lengths). In TS-1, the acyl C-CF3 bond distance
was predicted to increase from 2.884 Å in the gas-phase
calculation to 3.105 Å in the benzene-solvent calculation, and
to 3.484 Å in the methanol-solvent calculation. The Ir-H
calculated distance in TS-1 remained at 1.580 Å in the gas-
phase and benzene-solvent calculations, but increased slightly
to 1.589 Å in the methanol-solvent calculation. NBO analyses
of TS-1 revealed that a -0.88 and -0.94 charge accumulated
(59) Tannor, D. J.; Marten, B.; Murphy, R.; Friesner, R. A.; Sitkoff, D.; Nicholls,
A.; Ringnalda, M.; Goddard, W. A.; Honig, B. J. Am. Chem. Soc. 1994,
116, 11875-11882.
(60) Marten, B.; Kim, K.; Cortis, C.; Friesner, R. A.; Murphy, R. B.; Ringnalda,
M. N.; Sitkoff, D.; Honig, B. J. Phys. Chem. 1996, 100, 11775-11788.
(61) Oxgaard, J.; Muller, R. P.; Goddard, W. A.; Periana, R. A. J. Am. Chem.
Soc. 2004, 126, 352-363.
(62) Xu, X.; Kua, J.; Periana, R. A.; Goddard, W. A. Organometallics 2003,
22, 2057-2068.
(58) Hughes, R. P.; Kovacik, I.; Lindner, D. C.; Smith, J. M.; Willemsen, S.;
Zhang, D. H.; Guzei, I. A.; Rheingold, A. L. Organometallics 2001, 20,
3190-3197.
(63) Baik, M. H.; Friesner, R. A.; Lippard, S. J. J. Am. Chem. Soc. 2003, 125,
14082-14092.
9
J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004 16923

A R T I C L E S
Cordaro and Bergman
Figure 4. Energy diagram for CF3H elimination from 1 calculated in the gas phase, benzene, and methanol. Geometries were optimized using B3LYP/
LACVP**. Single-point calculations were performed at the B3LYP/LACVP**++ level of theory. ZPE and Gibbs free energy corrections are not included.
Table 7. Calculated Isotope Effect for the Elimination of CF3H/D
from 1
on the CF3 fragment for the benzene and methanol-solvent
calculations. No significant geometrical changes occurred for
TS-2 or TS-3 in the solution-phase calculations.
∆
G
°
of D
∆
Gq of ground
and H for given
compound
state and TS
for H and D
(kcal/mol)
The effects of introducing solvent into the molecular environ-
ment on the structures and energies of the calculated species
were most pronounced for TS-1 and were the result of compen-
sating for the change in charge distribution. The large Esolv and
the elongation of the acyl C-CF3 bond length calculated upon
going from the gas-phase to benzene and finally methanol sol-
vent for TS-1 are consistent with experimental observations
which demonstrated that the reaction described in Scheme 1
occurs through an ion-pair mechanism. While we cannot
definitely state that the structure labeled TS-1 is truly a transition
state and not an intermediate when solvent-inclusive calculations
are performed, experimentally we know that this “ion pair” has
a long enough lifetime for the trifluoromethyl anion to be
trapped by proton donation from solvent. In addition, M-H
bond-breaking must be very weakly developed in this species
to account for the negligible isotope effect (vide infra).
compound
(kcal/mol)
kH/kD
1a,b
1-Da,b
1a,c
-1.64
-1.55
-1.64
-1.55
-1.56
-1.26
1-Da,c
17
17-D
TS-1
TS-1-D
TS-2
TS-2-D
TS-3
TS-3-D
46.02
46.12
50.89
50.87
38.17
38.55
1.17
0.97
1.90
a Two rotomers for 1 were found. It was determined that one conforma-
tion led directly to TS-1 while the other conformation led to TS-2. b Rotomer
leading to TS-1. c Rotomer leading to TS-2.
illustrated in Figure 3. Calculations predicted a KIE of 1.17 for
the concerted but highly asynchronous mechanism, 0.97 for
migration of CF3 to Ir, and 1.90 for loss of CF3H(D).
Calculated Isotope Effect for CF3H/D Elimination from
1. Substitution of deuterium for hydrogen at the iridium center
was done to determine whether a KIE would be predicted for
the loss of CF3H from 1 (experimentally, no KIE was measured
for the loss of CF3H from 1). The Gibbs free energies of all
calculated structures decreased upon D for H substitution. For
example, the calculated ∆∆G°(1 - 1-D) ) -1.64 kcal/mol.
The calculated ∆∆G° caused by substituting D and H for all
other compounds is given in Table 7. Using the difference in
activation energy (∆∆Gq) between the deuterated and protiated
compounds, the KIE was calculated for both mechanisms
The negligible KIE of 0.97 for migration of CF3 to iridium
is expected because the metal hydride bond is not being broken.
A KIE of 1.90 was calculated for the reductive elimination of
CF3H from intermediate 17. However, because this step occurs
after the rate-determining TS-2, the KIE would not be observed
experimentally. The small calculated KIE for the concerted
elimination reaction is consistent with the experiment results
and re-enforces the notion that CF3- dissociation occurs before
the Ir-H bond is broken.
9
16924 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004

Dissociation of Carbanions from Acyl Iridium Compounds
A R T I C L E S
chemical shifts were referenced to the residual protiated solvent peak.
13C NMR spectra are proton decoupled and were recorded at 125 or at
100 MHz; chemical shifts were referenced to the solvent. 19F NMR
spectra were recorded at 376 MHz, and chemical shifts are reported
relative to external CFCl3. 31P NMR spectra were recorded at 162 MHz;
chemical shifts were referenced to an external standard of trimethyl
phosphate. Pyrex NMR tubes equipped with resealable Teflon stopcocks
were used in some procedures and are referred to as J. Young tubes.
Gases and some volatile liquids were condensed into reaction vessels
at -196 °C employing known-volume bulbs and a Baratron pressure
gauge. Infrared (IR) spectra were recorded using either a Mattson
Instrument Galaxy 300 Fourier transformer spectrometer (for solution
IR) or a Thermal Nicolet Avatar 370 FT-IR spectrometer equipped with
a Smart Performer ATR (for solid IR) and are reported in wavenumbers
(cm-1). Elemental analyses were performed at the UC-Berkeley
Microanalytical facility with a Perkin-Elmer 2400 Series II CHNO/S
Analyzer. Gas chromatograph-mass spectroscopy (GC-MS) data were
obtained using an Agilent Technologies Instrument 6890N GC (column
#HP-5MS, 30.0 m × 250 µm × 0.25 µm calibrated) and 5973N MS.
Mass spectroscopy on compound 7 was collected using a Quattro triple
quadrupole instrument (Micromass) equipped with an electrospray ion
source. The source was operated in positive mode, capillary voltage
-4 kV, cone voltage -52 V. The sample flow rate was 10 µL/min.
Chiral gas chromatography was carried out using a HP 5890 Series
II instrument equipped with a Chiraldex G-TA column. The column
was pressurized at 20 psi. The oven temperature profile was as
follows: initial temperature 70 °C for 25 min, ramp 5 °C/min to 120
°C, hold at 120 °C for 10 min, then cool 15 °C/min back to 70 °C. The
alkene 34 was detected at 11.5 min. The alkane 33 was detected at
12.8 and 19.9 min depending on the enantiomer. The (R)-aldehyde 32
was detected at 35.3 min, and the (S)-aldehyde 32 was detected at 36.4
min. Chiral HPLC analyses were performed using a HP 1100 system
with a HP 1100 Diode Array Detector (monitoring at 210, 220, 230,
254, and 280 nm) using a Daicel CHIRACEL OD column (4.6 × 250
mm) with a flow rate of 1 mL/min. Optical rotations were measured
on a Perkin-Elmer model 241 polarimeter equipped with a sodium lamp.
Synthesis. The following compounds were made according to
literature procedures: Cp*(PMe3)Ir(H)2 (4),64 Cp*(PMe3)Ir(D)2,26 Cp*-
(PMe3)Ir(CO) (2),65 Cp*(PMe3)Ir(H)[C(O)-tert-C4H9] (11) and Cp*-
(PMe3)Ir(H)[C(O)-p-NO2C6H4] (12),24 [Cp*(PMe3)Ir(H)(CO)]O3SCF3
(21),57 [Cp*(PMe3)Ir(Me)(CO)]O3SCF3 (37-OTf),66 Cp*(PMe3)IrH(CF2-
CF2CF3) (42),58 9H-fluorene-9-carbonyl chloride (24),67 9-methyl-9H-
fluorene-9-carboxylic acid,68 2-methyl-2-phenyl-propionyl chloride,69
cyano-dimethylacetyl chloride,70 [Ph3C][B(C6F5)4],71 and thallium(I)
N-hydroxypyridine-2-thione.54 All new iridium acyl hydrides, except
5, were synthesized on the basis of the method developed by Paisner
et al.24
Conclusions
The conversion of 1 into CF3H and the iridium carbonyl
product 2 defines a new reaction of metal acyl complexes. The
following experimental observations provide strong evidence
for a mechanism that invokes a metal cation/carbanion ion-pair
intermediate. (1) The rate of reaction was greatly accelerated
in polar aprotic solvents. (2) No KIE was observed for the
elimination of CF3H from 1, suggesting that the Ir-H bond
undergoes little bond-breaking in the rate-limiting step. (3)
Isotope tracer studies revealed that, in deuterated solvents,
deuterium was incorporated into the trifluoromethane product
by protonolysis of CF3-. (4) Other acyl complexes were
synthesized and found to also undergo anion dissociation. The
rate at which R-H elimination occurred appears to be related
to the electron-withdrawing ability of the dissociated anion and
steric bulk at the acyl group. For example, complexes with
longer perfluoroacyl substituents eliminated RF-H at faster rates
than 1. (5) Additionally, whether the molecular orbitals of the
acyl substituent are configurationally aligned with the acyl group
in the transition state, so as to stabilize the developing carbanion,
strongly influences the rate of anion dissociation. (6) Using an
R-chiral acid chloride, it was shown that R-H elimination
occurred with a high degree of retention of configuration. (7)
â-Elimination products arising from fluoride loss were observed.
In support of the above experimental results, computational
studies showed that inclusion of solvation effects was essential
to predicting the barrier heights for CF3H elimination from 1.
In gas-phase calculations, the energy of CF3- dissociation was
found to be prohibitively high for the thermal reaction, which
occurred experimentally with t1/2 < 10 min at 105 °C in
methanol. However, by introducing solvent into the molecular
environment, the structure of TS-1 changed to one which more
closely resembles an intermediate ion pair with a drastic decrease
in energy relative to alternative transition states. An alternative
mechanism involving migration of CF3 to iridium was calcu-
lated. This mechanism was discarded because of the high
reaction barriers and a low energy path that led to the incorrect
products.
Experimental Section
General. All experiments were performed using standard Schlenk
and vacuum line techniques or in a Vacuum Atmosphere glovebox
under an inert atmosphere of N2 unless otherwise noted. Glassware
was dried at 160 °C prior to use. All reagents were used as received
from commercial suppliers unless otherwise noted. THF (Fisher) was
distilled from sodium/benzophenone ketyl under nitrogen and stored
over 3 Å molecular sieves in the glovebox. Diethyl ether, benzene,
toluene, hexane, pentane, and dichloromethane (Fisher) were passed
through a column of activated alumina (type A2, size 12 × 32, Purify
Co.) under nitrogen pressure, sparged with N2, and stored over 3 Å
molecular sieves in the glovebox prior to use. Deuterated solvents were
purchased from Cambridge Isotope Laboratories, degassed and distilled
from the proper drying agent, and stored over 3 Å sieves. tert-
Butyllithium (Aldrich) was recrystallized multiple times from pentane
at -38 °C (until white) and stored at -38 °C. Methyl iodide was stored
under vacuum over copper wire. Silica gel used for chromatography
was dried under vacuum at 300 °C for 2 days and stored in the
glovebox.
Cp*(PMe3)Ir(H)[C(O)CF3] (1). A resealable 250-mL Schlenk flask
equipped with a stir bar and Teflon stopper was charged with Cp*-
(PMe3)Ir(H)2 (401 mg, 1.00 mmol), 1,8-bis(dimethylamino)-naphthalene
(257 mg, 1.20 mmol), and toluene (30 mL). The flask was connected
to a vacuum manifold, and the solution was subjected to three freeze-
pump-thaw cycles. At -196 °C, trifluoroacetyl chloride (1.20 mmol)
was condensed into the flask. The solution was then thawed by warming
to room temperature to give a yellow solution containing copious
amounts of white precipitate. After the mixture was stirred for 18 h,
(64) Heinekey, D. M.; Hinkle, A. S.; Close, J. D. J. Am. Chem. Soc. 1996, 118,
5353-5361.
(65) Del Paggio, A. A.; Muetterties, E. L.; Heinekey, D. M.; Day, V. W.; Day,
C. S. Organometallics 1986, 5, 575-581.
(66) Alaimo, P. J.; Arndtsen, B. A.; Bergman, R. G. Organometallics 2000, 19,
2130-2143.
(67) Robarge, M. J.; Husbands, S. M.; Kieltyka, A.; Brodbeck, R.; Thurkauf,
A.; Newman, A. H. J. Med. Chem. 2001, 44, 3175-3186.
(68) Bavin, P. M. G. Anal. Chem. 1960, 32, 554-556.
(69) Campaigne, E.; Maulding, D. R. J. Org. Chem. 1963, 28, 1391.
(70) Biechler, S. S.; Taft, R. W. J. Am. Chem. Soc. 1957, 79, 4927-4935.
(71) Ihara, E.; Young, V. G.; Jordan, R. F. J. Am. Chem. Soc. 1998, 120, 8277-
8278.
All NMR spectra were obtained using Bruker AV-400 or DRX-500
MHz spectrometers. Unless otherwise indicated, NMR spectra were
recorded at 22 °C. Chemical shifts are reported in parts per million
(ppm). 1H NMR spectra were recorded at 400 or 500 MHz, and
9
J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004 16925

A R T I C L E S
Cordaro and Bergman
the volatile materials were removed under reduced pressure. The
residual solid was taken up in diethyl ether and filtered through a small
pad of silica gel on a medium-pore glass frit eluting with diethyl ether
(ca. 35 mL). The yellow filtrate was evaporated to dryness in vacuo to
give a yellow residue which was extracted with hexane until the extracts
were colorless (ca. 12 mL). The combined hexane extracts were
concentrated in vacuo to ca. 5 mL and then allowed to stand at -38
°C. After 1 d, yellow needlelike crystals formed. The mother liquor
was removed via pipet, and the crystals were washed with cold pentane
and dried under vacuum giving analytically pure material. Yield: 319
mg (64%). Additional crops could be isolated by combining the mother
liquor and pentane washes, concentrating the solution in vacuo, and
allowing it to stand at -38 °C for crystallization. Combined yields
typically ranged from 70% to 90%. The monodeuterated compound
1-d was synthesized from Cp*(PMe3)Ir(D)2 following the same
procedure. 1H NMR (C6D6): δ 1.76 (d, 4JP-H ) 1.2 Hz, 15H, CpCH3),
1.12 (d, JP-H ) 10.5 Hz, 9H, PCH3), -16.45 (d, JP-H ) 33.5 Hz,
1H, IrH). 13C{1H} NMR (CD2Cl2): δ 211.0 (m, carbonyl), 121.0 (q,
1JF-C ) 303 Hz, CF3), 95.0 d, (2JC-P ) 1.3 Hz, CqMe5), 19.3 (d, 1JC-P
) 39.0 Hz, PCH3), 10.2 (s, Cp*CH3). 31P{1H} NMR (C6D6): -40.4.
19F NMR (C6D6): -79.7. IR (CD2Cl2): 2113 (mb, Ir-H) and 1613 (s,
CO), 1287, 1233, 1172, 1120, and 879. Anal. Calcd for C15H25F3OPIr:
C, 35.92; H, 5.03. Found: C, 36.06; H, 5.15.
ether. The yellow filtrate was evaporated to dryness under reduced
pressure to give a yellow residue which was extracted with hexane
until the extracts were colorless. The hexane extracts were filtered
through a glass fiber filter, and the volatile materials were evaporated
under reduced pressure. The resulting oil, contaminated with starting
material, was dissolved in a minimum amount of pentane and added
to a column of silica gel (1.5” packed in a Pasteur pipet). First, the
column was eluted with pentane (ca. 20 mL) to collect a faint yellow
fraction containing unreacted proton sponge. Second, the column was
eluted with diethyl ether (10 mL) to collect a yellow fraction containing
product. The volatile materials from the yellow fraction were removed
under reduced pressure. The resulting oil was dissolved in pentane and
allowed to stand at -38 °C. After 2 days, small crystals grew. The
mother liquor was removed by pipet, the crystals were washed with
cold pentane, and the excess solvent was removed under reduced
pressure to give 35 mg of analytically pure material. The mother liquor
and pentane washes were combined, concentrated, and allowed to stand
at -38 °C to obtain additional crops. Combined yield: 100 mg (32%).
2
2
4
1H NMR (C6D6): δ 1.73 (d, JP-H ) 1.2 Hz, 15H, CpCH3), 1.10 (d,
2
2JP-H ) 10.4 Hz, 9H, PCH3), -16.45 (d, JP-H ) 33.6 Hz, 1H, IrH).
3
19F NMR (C6D6): δ -79.9 (t, JF-F ) 11.3 Hz, 3F, CF3), -109.6 (d,
2JF-F ) 290 Hz, 1F, -CF2-), -111.4 (d, 2JF-F ) 297 Hz, 1F, -CF2-
3
), -124.7 (q, JF-F ) 5.6 Hz, 2F, -CF2-). 31P{1H} NMR (C6D6):
-40.3. IR (Solid ATR): 2112 (br, Ir-H), 1608 (s, CdO). Anal. Calcd
for C17H25F7OPIr: C, 33.94; H, 4.19. Found: C, 34.18; H, 4.26.
Cp*(PMe3)Ir(H)[C(O)CF2CF3] (5). Because the acid chloride was
not commercially available, this compound was synthesized using the
method developed by Peterson et al. from the iridate [Cp*(PMe3)IrHLi]x
and the acid anhydride.26 In the glovebox, a 20-mL scintillation vial
equipped with a Teflon stir bar was charged with Cp*(PMe3)Ir(H)2
(214 mg, 0.53 mmol), tert-butyllithium (56 mg, 0.74 mmol), and
benzene (8 mL) and stirred for 10 min. Pentafluoropropionic anhydride
(229 mg, 0.74 mmol) dissolved in 2 mL of benzene was added dropwise
to the stirred solution, giving a dark brown suspension. After 2 h, the
suspension was filtered through a pad of silica gel on a medium-pore
fitted glass frit eluting with diethyl ether (ca. 50 mL). The resulting
yellow-orange filtrate was evaporated to dryness, giving a brown oil.
The oil was extracted with benzene/hexane (1:1) until colorless, and
the extract was filtered through a glass fiber filter. The resulting yellow
solution was evaporated to dryness under reduced pressure to give an
oil. The oil was dissolved in a minimum amount of pentane and added
to a column of silica gel (1.5” packed in a Pasteur pipet). The column
was eluted with pentane collecting approximately 6 mL of a colorless
fraction. Next, the column was eluted with pentane/diethyl ether (1:1)
to collect 10 mL of a yellow fraction containing product. The volatile
materials from the second fraction were removed under reduced pressure
to give a yellow oil. The oil was dissolved in a minimum amount of
pentane (ca. 2 mL) and allowed to stand at -38 °C for crystallization.
After 2 days, small clusters of yellow microcrystals grew. The mother
liquor was removed via pipet, and the crystals were washed with cold
pentane (2 mL) and dried under reduced pressure. The mother liquor
and pentane washes were combined, reduced to approximately 2 mL,
and allowed to stand at -38 °C for two addition crops. Combined
Cp*(PMe3)Ir(H)[C(O)(CF2)7CF3] (7). The reaction was set up as
described for the synthesis of 6 using Cp*(PMe3)Ir(H)2 (122 mg, 0.30
mmol), 1,8-bis(dimethylamino)-naphthalene (77.0 mg, 0.36 mmol), and
perfluorononanoyl chloride (174 mg, 0.36 mmol). After being stirred
for 18 h at room temperature, the solution was filtered though a small
pad of silica gel on a medium-pore glass frit eluting with diethyl ether.
The resulting solution was evaporated in vacuo to afford a yellow oil.
The oil was dissolved in pentane (1 mL) and added to a column of
silica gel (2” packed in a Pasteur pipet). Unreacted starting material
was eluted from the column with pentane (ca. 35 mL, flash chroma-
tography) to give a colorless fraction. The column was then flushed
with diethyl ether to collect a yellow fraction containing 7. This fraction
was evaporated under reduced pressure to afford a yellow oil (yield:
127 mg, 50%) of >95% purity by NMR spectroscopy. This oil could
be crystallized from a minimum amount of pentane at -38 °C to
provide analytically pure material. 1H NMR (C6D6): δ 1.74 (dd, 4JP-H
4
2
) 1.8 Hz, JH-H 0.6, 15H, Cp*CH3), 1.11 (d, JP-H ) 10.5 Hz, 9H,
PCH3), -16.46 (d, JP-H ) 33.1 Hz, 1H IrH). 31P{1H} NMR (C6D6):
2
-40.4. 19F NMR (C6D6): δ - 80.90 (t, J ) 9.7 Hz), -110.0 (four
lines, each split into a triplet, J ) 13.5 Hz), -120.2 (s), -120.4 (s),
-121.6 (m) -122.6 (s), -126.1 (s). IR (CH2Cl2): 3055, 2985, 2911,
2118 (mb, Ir-H), 1612 (s, CO), 1422, 1242, 715. Anal. Calcd for
C22H25F17OPIr: C, 31.03; H, 2.96. Found: C, 31.33; H, 2.85.
Cp*(PMe3)Ir(H)[C(O)C(CH3)2(C6H5) ] (8). The reaction was set
up as described for the synthesis of 6 using Cp*(PMe3)Ir(H)2 (122 mg,
0.30 mmol), 1,8-bis(dimethylamino)naphthalene (77.0 mg, 0.36 mmol),
and 2-methyl-2-phenyl-propionyl chloride (72 mg, 0.45 mmol). After
the solution was stirred for 18 h, it was filtered through a medium-
pore glass frit, and the volatile materials were removed under reduced
pressure. The resulting orange solid was extracted with pentane (3 ×
4 mL), and the pentane extracts were filtered through a glass fiber filter.
The yellow filtrate was concentrated in vacuo to ca. 5 mL and allowed
to stand at -38 °C overnight to afford pale yellow crystals. After the
mother liquor was removed via pipet, the crystals were recrystallized
from pentane at -38 °C to obtain analytically pure material. Yield:
1
4
yield: 92 mg (32%). H NMR (C6D6): δ 1.74 (d, JP-H ) 1.2 Hz,
15H, CpCH3), 1.09 (d, 2JP-H ) 10.8 Hz, 9H, PCH3), -16.41 (d, 2JP-H
) 32.0 Hz, 1H, IrH). 19F NMR (C6D6): δ -78.9 (s, 3F, CF3), -113.2
2
2
(d, JF-F ) 294 Hz, 1F, CF2), -115.0 (d, JF-F ) 294 Hz, 1F, CF2).
31P{1H} NMR (C6D6): -40.3. IR (Solid ATR): 2128 (br, Ir-H), 1602
(s, CdO), 1211 (s), 1156 (s), 955 (s), 807 (s), 723 (s). FAB HRMS:
m/z calcd for C16H24F5OPIr 551.112589 [M]+, found 551.111437.
Cp*(PMe3)Ir(H)[C(O)CF2CF2CF3] (6). In the glovebox, a 20-mL
scintillation vial equipped with a Teflon stir bar was charged with Cp*-
(PMe3)Ir(H)2 (211 mg, 0.52 mmol), 1,8-bis(dimethylamino)-naphthalene
(139 mg, 0.63 mmol), and benzene (10 mL). To the stirred solution,
heptafluorobutyryl chloride (146 mg, 0.63 mmol) was added, producing
a yellow solution with copious amounts of white precipitate. After being
stirred for 18 h at room temperature, the solution was filtered though
a small pad of silica gel on a medium-pore glass frit eluting with diethyl
1
3
4
75 mg (47%). H NMR (C6D6): δ 7.52 (dt, JH-H ) 8.5 Hz, JH-H
)
1.5 Hz, 2H, aryl), 7.19 (tt, 3JH-H ) 8.5 Hz, 4JH-H ) 5.5 Hz, 2H, aryl),
3
4
7.07 (tt, JH-H ) 7.3 Hz, JH-H ) 1.2 Hz, 1H, aryl), 1.83 (s, 3H,
benzylCH3), 1.78 (dd, 4JP-H ) 1.8 Hz, 4JH-H ) 0.6 Hz 15H, CpCH3),
1.76 (s, 3H, benzylCH3), 0.88 (d, 2JP-H ) 10.4 Hz, 9H, PCH3), -16.79
(d, 2JP-H ) 35.4 Hz, 1H, IrH). 13C{1H} NMR (C6D6): δ 228.5, 146.1,
9
16926 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004

Dissociation of Carbanions from Acyl Iridium Compounds
A R T I C L E S
128.5, 127.8, 125.4, 94.1, 53.9, 29.8, 29.3, 18.8 (d, JC-P ) 37.9 Hz,
PCH3), 10.7. 31P{1H} (C6D6): δ -40.6. IR (pentane): 2111 (mb, Ir-
H), 1588 (s, CO), 1281, 955, 855, and 899. Anal. Calcd for C23H36-
OPIr: C, 50.07; H, 6.58 Found: C, 50.18; H, 6.54.
tane): 1619 (s, CdO), 1284, 1233, 1173, 1125, 956, 877, 723, 678.
Anal. Calcd for C15H24ClF3OPIr: C, 37.22; H, 5.28. Found: C, 36.97;
H, 5.49.
Cp*(PMe3)Ir(Cl)[C(O)CF3] (14). The reaction was set up as
described for the synthesis of 1 using Cp*(PMe3)Ir(H)2 (201 mg, 0.50
mmol), 1,8-bis(dimethylamino)naphthalene (129 mg, 0.50 mmol),
hexane (15 mL), and excess trifluoroacetyl chloride (6.50 mmol). After
the suspension was stirred for 18 h at room temperature, the volatile
materials were removed under reduced pressure. The resulting yellow-
orange solid was dissolved in CH2Cl2 and filtered through a pad of
silica gel on a medium-pore glass frit. The yellow filtrate was
concentrated in vacuo to approximately 3 mL, layered with pentane,
and allowed to stand at -38 °C. After 24 h, reddish-yellow blocklike
crystals grew. The mother liquor was removed via pipet, and the excess
solvent was removed under reduced pressure to give analytically pure
material. Yield: 158 mg (59% yield). Additional crops could be isolated
by concentrating the mother liquor in vacuo, layering with pentane,
and cooling to -38 °C. Combined yields were typically about 85%.
Cp*(PMe3)Ir(H)[C(O)C(CH3)2(CO2CH3)] (9). The reaction was
set up as described for the synthesis of 6 using Cp*(PMe3)Ir(H)2 (122
mg, 0.30 mmol), 1,8-bis(dimethylamino)naphthalene (77.0 mg, 0.36
mmol), 2-chlorocarbonyl-2-methylpropionic acid methyl ester (59.0 mg,
0.36 mmol), and toluene (10 mL). After 2 days, the reaction was worked
up as described for the synthesis of 8 to afford white microcrystals.
Yield: 78 mg (49%). An additional crop could be isolated from the
mother liquor after concentrating it in vacuo and allowing it to stand
1
at -38 °C. Crop 2 yield: 28 mg (18%). H NMR (C6D6): δ 1.98 (s,
3H, OCH3), 1.80 (m, 18H, propylCH3 and CpCH3), 1.70 (s, 3H,
propylCH3), 1.29 (d, 2JP-H ) 10.5 Hz, 9H, PCH3), -17.12 (d, 2JP-H
)
32.4 Hz, 1H, IrH). 13C{1H} NMR (C6D6): δ 219.9, 169.5, 98.1, 94.5,
1
25.6, 25.0, 22.4, 19.6 (d, JC-P ) 38.0 Hz, PCH3), 11.6. 31P{1H}
(C6D6): δ -42.0. IR (pentane): 2151 (br, Ir-H), 1738 and 1607 (two
CdO), 1290, 1148, 956, 879, and 729. Anal. Calcd for C19H34O3PIr:
C, 42.76; H, 6.24 Found: C, 42.95; H, 6.07.
4
1H NMR (CD2Cl2): δ 1.68 (d, JP-H ) 2.0 Hz, 15H, Cp*CH3), 1.53
2
(d, JP-H ) 11.5 Hz, 9H, PCH3). 13C {1H} NMR (CD2Cl2): δ 216.3
2
2
3
(dq, JP-C ) 16 Hz, JF-C ) 31, carbonylC), 122.0 (dq JP-C ) 4 Hz,
Cp*(PMe3)Ir(H)[C(O)C(CH3)2(CN)] (10). The reaction was set up
as described for the synthesis of 6 using Cp*(PMe3)Ir(H)2 (142 mg,
0.35 mmol), 1,8-bis(dimethylamino)naphthalene (77.0 mg, 0.36 mmol),
cyano-dimethyl-acetyl chloride (59.0 mg, 0.45 mmol), and toluene (10
mL). After 4 h, the reaction was worked up as described for the
synthesis of 8 to afford yellow cubic microcrystals. Yield: 78 mg
(49%). An additional crop could be isolated from the mother liquor
after concentrating it in vacuo and allowing it to stand at -38 °C. Crop
2JF-C ) 305 Hz, acylCF3), 95.4 (d, JC-P ) 2.6 Hz, CqMe5), 13.9 (d,
2
1JC-P ) 39.5 Hz, P-CH3), 8.8 (s, Cp*CH3). 31P{1H} NMR (CD2Cl2):
-27.9. 19F NMR (CD2Cl2): -75.5. IR (CH2Cl2): 2980, 2917, 1630 (s,
CdO), 1285, 1232, 1177, 1129, 958, 874. Anal. Calcd for C15H24ClF3-
OPIr: C, 33.61; H, 4.51. Found: C, 33.76; H, 4.39.
[Cp*(PMe3)Ir(CO)(CF3)][B(C6F5)4] (23). In a 20-mL scintillation
vial equipped with a stir bar, 1 (99 mg, 0.197 mmol) and [Ph3C]-
[B(C6F5)4] (182 mg, 0.197 mmol) were added followed by toluene (10
mL). After the yellow suspension was stirred for 18 h, pentane (5 mL)
was added and the suspension was allowed to settle. The mother liquor
was removed by pipet, and the yellow solid was washed with pentane
(3 × 5 mL). The solid was dissolved in CH2Cl2, layered with pentane,
and put at -38 °C for crystallization. After 1 day, brown crystals grew.
The mother liquor was removed by pipet, the crystals were washed
with cold CH2Cl2 (2 mL), and the excess solvent was removed under
reduced pressure. Yield: 49 mg, 21.1%. The mother liquor and CH2-
Cl2 washes were combined, concentrated, and put at -38 °C for a
second crop of faint yellow crystals. Yield: 76 mg, 32.7%. A third
crop was obtained upon further concentration of the mother liquor
(yield: 18 mg, 7.9%). All three crops were equally pure by NMR
1
2 yield: 28 mg (18%). H NMR (C6D6): δ 1.80 (s, 15H, CpCH3),
2
1.32 (s, 3H, CH3), 1.30 (s, 3H, CH3), 1.19 (d, JP-H ) 10.4 Hz, 9H,
PCH3), -16.25 (d, 2JP-H ) 33.6 Hz, 1H, IrH). 13C{1H} NMR (C6D6):
1
δ 216.2 (m, carbonyl), 127.4, 94.6, 25.2, 24.7, 19.4 (d, JC-P ) 37.1
Hz, PCH3), 10.5 (one signal, presumably for the cyano carbon, was
not located). 31P{1H} (C6D6): δ -40.6. IR (Solid ATR): 2974, 2909,
2225 (sm, C-N), 2095 (br, Ir-H), 1589 (s, CdO), 960, 942, 857,
776, and 733. Anal. Calcd for C18H31NOPIr: C, 43.18; H, 6.24; N,
2.80. Found: C, 43.05; H, 6.36; N, 3.14.
Cp*(PMe3)Ir(CH3)[C(O)CF3] (13). A 250-mL thick-walled, re-
sealable flask equipped with a stir bar was charged with Cp*(PMe3)-
Ir(H)[C(O)CF3] (1) (155 mg, 0.308 mmol) and tert-butyllithium (25.7
mg, 0.401 mmol). The flask was attached to a vacuum manifold and
cooled to -196 °C. THF (20 mL) was vacuum transferred into the
flask. The frozen reaction mixture was warmed to -15 °C over 45
min and stirred at this temperature for 10 min. The resulting orange
solution was cooled to -196 °C, and MeI (0.43 mmol) was condensed
into the flask. The reaction mixture was allowed to warm to -78 °C
and stirred. After 20 min, the mixture was warmed to 25 °C to produce
a pale yellow solution. After the mixture was stirred at 25 °C for 30
min, the volatile materials were removed in vacuo. The crude material
was extracted with diethyl ether (2 mL) and filtered through a pad of
silica gel on a medium-pore glass frit eluting with diethyl ether (ca. 8
mL). The resulting red-orange solution was evaporated in vacuo to
dryness. The residual solid was dissolved in 2 mL of diethyl ether,
layered with pentane, and allowed to stand at -38 °C. After 18 h,
analytically pure, red microcrystals were collected and washed with
pentane, and the excess solvent was removed under reduced pressure.
Yield: 114 mg (72%). An additional crop was isolated from the mother
liquor by concentrating the solution in vacuo and cooling it to -38 °C
for crystallization. Crop 2 yield: 21 mg (12%). 1H NMR (CD2Cl2): δ
1
analysis; however, crop 2 was used for elemental analysis. H NMR
(CD2Cl2): δ 2.10 (d, 4JP-H ) 2.0 Hz, 15H, Cp*CH3), 1.83 (d, 2JP-H
)
11.6 Hz, 9H, PCH3). 13C{1H} NMR (CD2Cl2): δ 163.22 (s, carbonyl),
105.6 (s, CqMe5), 16.7 (d, 1JC-P ) 41.3 Hz, P-CH3), 9.3 (s, Cp*CH3)
(resonances for the trifluoromethyl-bound iridium and borate counterion
were not resolved in the 13C NMR). 19F NMR (CD2Cl2): -5.7 (s, 3F,
Ir(CF3)), -133.0 (s, 8F, CarylF), -163.5 (s, 4F, CarylF), -167.4 (s, 8F,
C
arylF). 31P{1H} NMR (CD2Cl2): -37.8 (q, 3JP-F ) 5.4 Hz). IR (CH2-
Cl2): 2065 (s, CdO), 1643, 1513, 1465, 1084, 980. Anal. Calcd for
C39H24BF23OPIr: C, 39.71; H, 2.05. Found: C, 39.90; H, 1.85.
3,3,3-Trifluoro-2-methoxy-2-phenylpropionaldehyde (32). Treat-
ment of (S)-Mosher’s acid chloride (15.6 mg, 0.062 mmol) with n-Bu3-
SnH (24 mg, 0.74 mmol) in C6D6 (0.5 mL) afforded aldehyde 32 in
98% yield based on NMR and GC analysis after 3 days at 25 °C. The
volatile materials were vacuum transferred away from tin compounds
to give a solution of 32 in C6D6. Analysis of the product by chiral GC
showed that the aldehyde was generated in >98% ee. 1H NMR
4
(C6D6): δ 9.27 (q, JH-F ) 2.6 Hz, 1H, C(O)H), 7.35 (m, 2H, aryl),
6.00 (m, 3H, aryl), 3.05 (s, 3H, OCH3). 19F NMR (C6D6): δ -71.1 (s,
3F, CF3).
4
2
1.73 (d, JP-H ) 1.5 Hz, 15H, Cp-CH3), 1.42 (d, JP-H ) 10.5 Hz,
9H, P-CH3), 0.36 (d, 3JP-H ) 6.0 Hz, 3H, IrCH3). 13C{1H} NMR (CD2-
Cl2): δ 225.0 (m, carbonylCq), 121.7 (dq 3JP-C ) 2.1 Hz, 2JF-C ) 302
(()-2,2,2-Trifluoro-1-methoxyethylbenzene (33). This compound
was originally synthesized enatiomerically enriched by Mosher et al.
in a study to assign the absolute stereochemistry.72 The procedure
reported here was developed to synthesize racemic 33. In a J-Young
NMR tube, 35 (12.5 mg, 0.036 mmol) and n-Bu3SnH (130 mg, 0.45
2
1
Hz, acylCF3), 95.8 (d, JC-P ) 2.8 Hz, CqMe5), 15.0 (d, JC-P ) 38.9
Hz, PCH3), 9.2 (s, Cp*CH3), -24.7 (d, JC-P ) 6.3 Hz, IrCH3). 31P-
2
{1H} NMR (CD2Cl2): -35.3. 19F NMR (CD2Cl2): -76.5. IR (pen-
9
J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004 16927

A R T I C L E S
Cordaro and Bergman
mmol) were dissolved in C6D6 (0.6 mL). The solution was heated at
75 °C for 3 days to give 33 in 98% yield by NMR with no other
fluorine-containing products. The volatile materials were vacuum
transferred to give a solution of 33 in C6D6 contaminated with alkyl
tin products. The product was racemic as determined by chiral GC. 1H
NMR (C6D6): δ 7.25 (m, 2H, aryl), 7.06 (m, 3H, aryl), 4.07 (q, 3JH-F
) 6.8 Hz, 1H, CH), 2.91 (s, 3H, OCH3). 19F NMR (C6D6): δ -76.3
material and product concentrations were monitored versus the internal
standard and fit to a single-exponential decay or growth function. For
1, the data obtained by monitoring the decay of the Cp* or PMe3
resonances had less scatter in the exponential fit than the fits for the
data obtained by monitoring the decay of the iridium hydride resonance.
(Presumably, this error is due to a decreasing signal-to-noise ratio for
the smaller, upfield hydride resonance over the course of the reaction.)
The reported rate constants are, therefore, those measured using the
Cp* and PMe3 signals. Usually, the rate constants calculated for the
first-order decay and first-order growth processes were in very good
agreement, which is indicative of the absence of side reactions.
Exceptions occurred in a few reactions carried out in CD3OD in which
the product Cp* ligand suffered from H/D exchange with the solvent;
in those cases, the product growth could not be monitored via Cp*
appearance. The starting Cp* resonance of 1 in CD3CN overlapped
with the residual solvent peak; therefore, the rate constant was
determined by monitoring the PMe3 methyl resonance. Rate constants
reported for the thermal reaction of 14 in C6D6 and DMSO-d6 were
obtained by monitoring the disappearance of starting material because
no identifiable product was generated over the course of the reaction.
Rate constants measured from trapping experiments with 13 and added
acid were obtained by monitoring the disappearance of starting material.
3
(d, JF-H ) 4 Hz, 3F, CF3).
2-(2,2,2-Trifluoro-1-methoxy-1-phenyl-ethylsulfanyl)-pyridine (35).
In a 20-mL scintillation vial equipped with a stir bar, thallium(I)
N-hydroxypyridine-2-thione (318 mg, 0.96 mmol)54 and (R)-Mosher’s
acid chloride (243 mg, 0.96 mmol) were mixed as a suspension in 18
mL of diethyl ether. The suspension was stirred for 14 h, and then
filtered through a medium-pore fitted glass frit eluting with diethyl
ether (20 mL). The resulting yellow solution was concentrated in vacuo
under reduced pressure to give an oil. Diethyl ether (5 mL) was added
to the oil and then removed under reduced pressure. After this process
was repeated three times, the oil began to solidify. The crude material
was dissolved in a minimum amount of diethyl ether and put at -38
°C. After 2 days, large colorless cubic-like crystals formed. The mother
liquor was removed by pipet, and the crystals were recrystallized from
diethyl ether at -38 °C. Yield: 103 mg (36%). An additional crop
was grown from the combined mother liquors after concentrating the
solution under reduced pressure and allowing it to stand at -38 °C.
Optical rotation for crystals [R]23D ) 0.00 (c ) 1.2, in CHCl3). Chiral
HPLC of isolated crystals detected 35 as a 1:1 ratio of enantiomers at
5.61 and 6.41 min (hexanes:isoproponal 95:5). The mother liquor was
also analyzed by chiral HPLC. The two enantiomers were detected with
ca. 9% ee. 1H NMR (CD3CN): δ 8.45 (ddd, J ) 5, 2, 1 Hz, 1H), 7.70
(m, 2H), 7.62 (dt, J ) 8, 2 Hz, 1H), 7.50 (d, J ) 8 Hz, 1H), 7.42 (m,
J ) 5, 2 Hz, 3H), 7.22 (ddd, J ) 8, 5, 1 Hz, 1H), 3.73 (s, 3H, OCH3).
19F NMR (CD3CN): δ -74.7 (s, 3F, CF3). 13C{1H} NMR (CD3CN):
δ 154.6, 150.8, 138.1, 134.9, 130.8, 129.5, 129.4, 129.3, 128.4, 126.2,
123.9, 121.6, 94.4 (q, JC-F ) 29.3 Hz), 54.2. IR (Solid ATR): 2978.2
(w), 2943.8 (w), 1573.5 (m), 1561.5 (m), 1448.6 (m), 1418.2 (m),
1268.7 (m), 1177.6 (s), 1168.6 (s), 1149.5 (s), 1093.2 (m), 1079.1 (m),
972.9 (m), 878.9 (s), 772.7 (m), 762.8 (s), 720.5 (s), 700.2 (m), 600.0
(m). Anal. Calcd for C14H12F3O1NS: C, 56.18; H, 4.04; N, 4.68.
Found: C, 56.15; H, 4.08; N, 4.94. EI/MS: [M+] ) 299. FAB/MS:
[M + 1] ) 300.
Computational Details
Method. Quantum mechanical calculations were performed at the
University of California, Berkeley Molecular Graphics Facility using
a 98 cpu-cluster with 2.8 GHz Xeon processors. DFT calculations were
carried out by implementing the B3LYP exchange-correlation function-
al74-76 with the LACVP** basis set using Jaguar 5.0 or 5.5.77 This
method replaces the innermost 60 electrons of iridium with a nonlocal
effect core potential (ECP).78 All other atoms are described by the
6-31G** basis set (valence double-ú plus polarization) developed by
Pople and co-workers.79 Geometries were fully optimized in the gas
phase using the LACVP** basis set followed by single-point calcula-
tions using the LACVP**++ basis set, which places polarization
functions and diffuse orbitals on all atoms except iridium. NBO analyses
were calculated during the single-point calculations.80 Vibrational
frequencies were calculated to ensure that local minima lacked any
negative frequencies and to confirm that transition states had only one
imaginary (negative) frequency. Occasionally, “soft” vibrations (<10
cm-1) were found during frequency calculations that were not included
in the zero-point energy. Visual inspection of these vibrations using
the Molden software program81 revealed that they were small “wags”
or “twists”, usually in a methyl group, and, hence, determined to be
artifacts of negligible significance to the overall energy, structure, or
location along the reaction coordinate surface. Quick pseudospectral
calculations developed by Jaguar were used for most calculations.
However, TS-1 and 3 did not converge using the standard method.
For these calculations, fully analytical numerical methods were required,
and the accuracy of the SCF grids was set to ultrafine (nops)1 and
iacc)1 added to input). The magnitude of the pseudospectral error was
estimated to be <0.5 kcal/mol by comparing the energies of 1 and 2
calculated at both the standard method and the fully analytical method.
Vibrational frequency analyses were not performed on solvent-
inclusive calculations. Ground states and transition states were con-
firmed by vibrational frequency analyses for gas-phase calculations.
These optimized structures were then used in solvation calculations.
Elimination of CF3 from 14. Upon heating 14 (7 mg, 0.013 mmol)
in CD3OD (0.5 mL) at 105 °C for 6 days, the expected carbonyl chloride
cation [Cp*(PMe3)Ir(Cl)(CO)]+ was not observed via NMR spectro-
scopy. Instead, [Cp*(PMe3)Ir(CO)(D)]+Cl was identified by NMR and
IR spectroscopy. The identity of this carbonyl cation was confirmed
1
by repeating the reaction in CH3OH and comparing the H and 31P
NMR spectra of the product in DMSO-d6, and the IR spectrum in CH2-
Cl2, to those of an authentic sample of [Cp*(PMe3)Ir(CO)(H)]+[O3-
SCF3], which was synthesized by the method of Angelici and
co-workers.57
Kinetics. Rate constants were measured by NMR spectroscopy.
Samples were prepared in NMR tubes, which were then sealed under
vacuum. For first-order reactions, concentrations of approximately 0.02
M were used for starting material and the internal standard 1,3,5-
trimethoxybenzene.73 For trapping experiments with 13 and added 37-
OTF (OTf ) O3S(CF3) as the anion) and acid, concentrations were
determined by the weight of added reagents and checked by NMR
integration. NMR tubes were either heated in a constant temperature
oil bath or in the probe of the NMR spectrometer. Ethylene glycol
(100%) in a sealed NMR tube was used to calibrate the temperature of
the probe. Reactions were monitored to at least 95% completion using
one-pulse 1H NMR spectroscopy at specified time intervals. The starting
(74) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200-1211.
(75) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(76) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785-789.
(77) Jaguar 5.0; Schro¨dinger LLC: Portland, OR, 2002.
(78) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310.
(79) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980,
72, 650-654.
(72) Peters, H. M.; Feigl, D. M.; Mosher, H. S. J. Org. Chem. 1968, 33, 4245-
(80) NBO, 5.0; Theoretical Chemistry Institute, University of Wisconsin,
Madison: Madison, WI, 2001.
4250.
(73) Higher sample concentrations did not affect the observed rates of
reactions.
(81) Schaftenaar, G.; Noordik, J. H. J. Comput.-Aided Mol. Des. 2000, 14, 123-
134.
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16928 J. AM. CHEM. SOC. VOL. 126, NO. 51, 2004

Dissociation of Carbanions from Acyl Iridium Compounds
A R T I C L E S
The Gibb’s free energies could not be assessed due to this limitation.
Therefore, energies for solvation calculations are discussed only with
respect to enthalpy at 0 K relative to 1.
To calculate kinetic isotope effects, frequency calculations were
repeated in which hydrogen was replaced with deuterium. All calcula-
tions to determine the KIE of CF3H/D elimination from 1 were done
using Jaguar 5.5 and the fully analytical method with an ultrafine SCF
grid to prevent the occurrence of systematic errors.
provided by the National Science Foundation (Grant No. CHE-
0094349). We are also grateful for an equipment grant from
the NSF (Grant No. CHE-0233882) for the computational
facilities.
Supporting Information Available: Chart for compound
numbering. Atom labeling scheme of 1. Molecular representa-
tions of calculated species of all structures described in Figure
3. Cartesian coordinates, bond lengths, and results of vibrational
frequency analyses of all compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank Dr. Forrest Michael, Dr. Jen-
nifer Krumper, and Ms. Cathleen M. Yung for helpful discus-
sions. We also thank Mr. Ian C. Stuart and Ms. Rebecca M.
Wilson for help with acquiring HPLC data. Funding was
JA045859L
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