Mechanism of β-hydrogen elimination from square planar iridium(I) alkoxide complexes with labile dative ligands

Article abstract of DOI:10.1021/ja010417k

Mechanistic studies were conducted on β-hydrogen elimination from complexes of the general formula [Ir(CO)(PPh₃)₂(OR)], which are square planar alkoxo complexes with labile ligands. The dependence of rate, isotope effect, and alkoxide racemization on phosphine concentration revealed unusually detailed information on the reaction pathway. The alkoxo complexes were remarkably stable, including those with a variety of electronically and sterically distinct groups at the β-carbon. These complexes were much more stable than the corresponding alkyl complexes. Thermolysis of these complexes in the presence of PPh₃ yielded the iridium hydride [Ir(CO)(PPh₃)₃H] and the corresponding aldehyde or ketone with rate constants that were affected little by the groups at the β-carbon. The reactions were first order in iridium complexes. At low [PPh₃], the reaction rate was nearly zero order in PPh₃, but reactions at high [PPh₃] revealed an inverse dependence of reaction rate on PPh₃. The rate constants were similar in toluene, THF, and chlorobenzene. The y-intercept of a 1/k_obs VS [PPh₃] plot displayed a primary isotope effect, indicating that the y-intercept did not simply correspond to phosphine dissociation. These data and a dependence of alkoxide racemization on [PPh₃] showed that the elementary β-hydrogen elimination step was reversible. A mechanism involving reversible β-hydrogen elimination followed by associative displacement of the coordinated ketone or aldehyde by PPh₃ was consistent with all of our data. This mechanism stands in contrast with the pathways proposed recently for alkoxide β-hydrogen elimination involving direct elimination, protic catalysts, or binuclear mechanisms and shows that alkoxide elimination can follow pathways similar to those for β-hydrogen elimination from alkyl complexes.

Full text of DOI:10.1021/ja010417k

7220
J. Am. Chem. Soc. 2001, 123, 7220-7227
Mechanism of â-Hydrogen Elimination from Square Planar Iridium(I)
Alkoxide Complexes with Labile Dative Ligands
Jing Zhao, Heather Hesslink, and John F. Hartwig*
Contribution from the Department of Chemistry, Yale UniVersity, P.O. Box 208107,
New HaVen, Connecticut 06520-8017
ReceiVed February 16, 2001
Abstract: Mechanistic studies were conducted on â-hydrogen elimination from complexes of the general formula
[Ir(CO)(PPh3)2(OR)], which are square planar alkoxo complexes with labile ligands. The dependence of rate,
isotope effect, and alkoxide racemization on phosphine concentration revealed unusually detailed information
on the reaction pathway. The alkoxo complexes were remarkably stable, including those with a variety of
electronically and sterically distinct groups at the â-carbon. These complexes were much more stable than the
corresponding alkyl complexes. Thermolysis of these complexes in the presence of PPh3 yielded the iridium
hydride [Ir(CO)(PPh3)3H] and the corresponding aldehyde or ketone with rate constants that were affected
little by the groups at the â-carbon. The reactions were first order in iridium complexes. At low [PPh3], the
reaction rate was nearly zero order in PPh3, but reactions at high [PPh3] revealed an inverse dependence of
reaction rate on PPh3. The rate constants were similar in toluene, THF, and chlorobenzene. The y-intercept of
a 1/kobs vs [PPh3] plot displayed a primary isotope effect, indicating that the y-intercept did not simply correspond
to phosphine dissociation. These data and a dependence of alkoxide racemization on [PPh3] showed that the
elementary â-hydrogen elimination step was reversible. A mechanism involving reversible â-hydrogen
elimination followed by associative displacement of the coordinated ketone or aldehyde by PPh3 was consistent
with all of our data. This mechanism stands in contrast with the pathways proposed recently for alkoxide
â-hydrogen elimination involving direct elimination, protic catalysts, or binuclear mechanisms and shows that
alkoxide elimination can follow pathways similar to those for â-hydrogen elimination from alkyl complexes.
Introduction
The microscopic reverse of this reaction is olefin insertion into
a metal hydride, which occurs in olefin hydrogenation.13
â-Hydrogen elimination from a metal alkyl must be suppressed
to observe cross-coupling14 of main group alkyl reagents with
vinyl and aryl electrophiles.
â-Hydrogen elimination from alkoxo and amido complexes
and the microscopic reverse of this reaction are similarly
involved in catalytic processes. These processes include transfer
hydrogenations using alcohol reducing agents15-17 and cyclo-
isomerizations involving aldehydes.18 The microsopic reverse
of this reaction is involved in aldehyde, ketone, and imine
hydrogenation.16,19,20 â-Hydrogen elimination from alkoxo
complexes must be avoided if aromatic etherification is to
encompass alcohol substrates that bear a methylene or methine
â-Hydrogen elimination and migratory insertion1,2 are com-
mon elementary reactions in both homogeneous and heteroge-
neous catalysis. Extensive mechanistic studies have been
conducted on â-hydrogen elimination from transition metal alkyl
complexes.1 These mechanistic studies have revealed a common
reaction pathway that requires an open coordination site and a
transition state involving a syn coplanar arrangement of the R-
and â-carbons, the â-hydrogen, and the metal center. This
transition state generates an olefin hydride complex, which often
generates the final product after olefin dissociation.
â-Hydrogen elimination from a metal alkyl and the micro-
scopic reverse of this reaction occurs in a variety of catalytic
processes including dehydrogenation of alkanes,3-6 Mizoroki-
Heck olefination,7 olefin isomerization,8 cycloisomerizations,9
and hydroformylation of internal olefins to n-aldehydes.10-12
(10) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. In
Principles and Applications of Organotransition Metal Chemistry, 2nd ed.;
University Science Books: Mill Valley, 1987; p 622.
(11) van der Veen, L. A.; Kamer, P. C. J.; van Leeuwen, P. Angew.
Chem. Int. Ed. Engl. 1999, 38, 336-338.
(12) Breit, B.; Winde, R.; Harms, K. J. Chem. Soc. 1997, 2681-2682.
(13) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. In
Principles and Applications of Organotransition Metal Chemistry, 2nd ed.;
University Science Books: Mill Valley, 1987; p 523.
(14) Hegedus, L. S. Transition Metals in the Synthesis of Complex
Organic Molecules; University Science Books: Mill Valey, CA, 1994.
(15) Uematsu, N.; Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J.
Am. Chem. Soc. 1996, 118, 4916-4917.
(16) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97-102.
(17) Mestroni, G.; Zassinovich, G.; Camus, A.; Martinelli, F. J. Organo-
met. Chem. 1980, 198, 87-96.
(18) Montgomery, J. Acc. Chem. Res. 2000, 33, 467-473.
(19) Palmer, M. J.; Wills, M. Tetrahedron: Asymmetry 1999, 2045-
2061.
(1) Cross, R. J. In The Chemistry of the Metal-Carbon Bond; Hartley,
F. R., Patai, S., Eds.; John Wiley: New York, 1985; Vol. 2, pp 559-624.
(2) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. In
Principles and Applications of Organotransition Metal Chemistry, 2nd ed.;
University Science Books: Mill Valley, 1987; pp 383-388.
(3) Crabtree, R. H.; Mellea, M. F.; Mihelcic, J. M.; Quirk, J. M. J. Am.
Chem. Soc. 1982, 104, 107-113.
(4) Felkin, H.; Fillebeen-Khan, T.; Gault, Y.; Holmes-Smith, R.;
Zakrzewski, J. Tetrahedron Lett. 1984, 25, 1279-1282.
(5) Liu, F.; Pak, E. B.; Singh, B.; Jensen, C. M.; Goldman, A. S. J. Am.
Chem. Soc. 1999, 121, 4086-4087.
(6) Liu, F. C.; Goldman, A. S. Chem. Commun. 1999, 655-656.
(7) Crisp, G. T. Chem. Soc. ReV. 1998, 27, 427-436.
(8) For a recent example see: Wakamatsu, H.; Nishida, M.; Adachi, N.;
Mori, M. J. Org. Chem. 2000, 65, 3966-70 and references therein.
(9) Trost, B. M.; Krische, M. J. Synlett 1998, 1-16.
(20) James, B. R. Chem. Ind. 1995, 62, 167-180.
10.1021/ja010417k CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/06/2001

â-H Elimination from Square Planar Iridium Alkoxide Complexes
J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7221
hydrogen R to oxygen.21,22 Despite being common steps of
catalytic processes, direct observations of â-hydrogen elimina-
tion from alkoxo and amido complexes are much less common
than those from alkyl complexes,23-29 and â-hydrogen elimina-
tion from alkoxo complexes is not well defined mechanistically.
Scheme 1
Evidence for â-hydrogen elimination from alkoxides is
indirect in most cases. Yet, a few mechanistic studies have been
conducted on isolated alkoxo complexes that cleanly generate
aldehyde or ketone and metal hydride product, and in each case
the mechanism has been proposed or shown to be distinct from
the conventional mechanism for â-hydrogen elimination from
alkyls. In one case, alkoxide â-hydrogen elimination from a
16-electron square planar complex [(DPPE)Pt(OMe)2] oc-
curred.28 This reaction was faster than â-hydrogen elimination
from classic platinum dialkyls with the same ligand. It was
proposed that an 18-electron aldehyde hydride complex is
formed as an intermediate. However, chelation of the phosphine
prevented detailed data on possible ligand dissociation paths.
Partial dissociation of the chelating phosphine is generally
undetectable by kinetic studies.
More detailed mechanistic data have been obtained on
â-hydrogen elimination from saturated Ir(III) alkoxo complexes,
and these studies revealed unusual mechanisms.23,26,27 In two
cases the reaction occurred in the presence of added alcohol,23,27
and the order of the reaction in added alcohol was positive and
noninteger. Thus, the authors could not pinpoint a transition
state that contains the number of methanol groups in the
experimental rate equation, but proposed a structure with
hydrogen bonding involving a variable number of alcohols. The
other example of â-hydrogen elimination from an Ir(III) alkoxo
complex involved a dinuclear transition state formed from
catalytic amounts of an iridium triflate.26
drogen elimination that are similar to those of the 18-electron
Ir(III) complexes or to those of classic square planar alkyl
complexes, we studied alkoxo complexes based on the Vaska
fragment. Atwood had reported previously some of the alkoxo
complexes we used for the study. He had shown that they were
isolable and would generate iridium hydride upon thermolysis.31
Moreover, we had recently provided some mechanistic data on
â-hydrogen elimination from related amido complexes,24 al-
lowing comparisons between the mechanism for â-hydrogen
elimination from alkoxo and amido complexes.
We report our results from this study, which provided an
unusually detailed reaction path for the â-hydrogen elimination.
This pathway involves a reversible, elementary â-hydrogen
elimination step. An unexpected influence of phosphine con-
centration on reaction rate and racemization of the alkoxide
complex uncovered the reversibility of this step. The isotope
effect on the y-intercept of a plot correlating 1/kobs with
phosphine concentration also revealed this reversibility.
Results
One study of rhenium alkoxo complexes probed â-hydrogen
elimination indirectly.30 In this case, the data supported a
mechanism that is similar to the typical one for â-hydrogen
elimination from alkyl complexes. [Cp*Rh(NO)(PPh3)OR]
catalyzed the racemization of alcohols, and this racemization
was proposed to occur by â-hydrogen elimination of the rhenium
alkoxide. The rate of racemization depended inversely on
phosphine concentration. This information implied that â-hy-
drogen elimination was preceded by phosphine dissociation and
that the phosphine dissociation allowed formation of an aldehyde
hydride complex. This species provided the diastereomeric
alkoxo complex by reinsertion.
1. Synthesis and Thermolysis of Vaska-Type Alkoxo
Complexes. The synthesis and thermolysis of Vaska-type
alkoxide complexes are summarized in Scheme 1. The isopro-
poxide was prepared previously by Atwood,31,32 who first
showed that it underwent â-hydrogen elimination. In our hands,
sodium alkoxides reacted with Vaska’s complex within an hour
in THF solvent to form stable, monomeric, terminal alkoxo or
benzyloxo complexes bearing â-hydrogens in 46-84% yield.
Enantiopure (R)-6a was prepared by a one-pot procedure
involving reaction of the alcohol and Vaska’s complex in the
presence of NaH in THF solvent.
Thermolysis of these complexes occurred smoothly in the
presence of added PPh3 in aromatic solvents between 95 and
108 °C to form the ketone or aldehyde and [Ir(CO)(PPh3)3H].33
No reaction of the product hydride with the starting alkoxide31
occurred in dry solvents. The organic products were identified
by 1H NMR and mass spectroscopy, as well as by GC or HPLC
retention times.
Complexes containing a square planar geometry with labile
dative ligands are common structures in catalytic processes. In
contrast to data on the â-hydrogen elimination from alkoxo
complexes with the geometries of the above examples, data on
â-hydrogen elimination from alkoxo complexes with a square
planar geometry and labile dative ligands are scarce. To test
whether these complexes would show mechanisms for â-hy-
Experiments involving thermolysis of the alkoxide in the
presence of a ketone that is different from the product ketone
showed that the overall â-hydrogen elimination process was
(21) Hartwig, J. F. Angew. Chem., Int. Ed. Engl. 1998, 37, 2046-2067.
(22) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc. Chem.
Res. 1998, 31, 805-818.
2
1
irreversible. H, H, and 31P NMR spectroscopy showed that
isopropoxide 2 and phenethoxide 6a did not incorporate
deuterated or protiated acetone, respectively, during the â-
elimination at 80 and 110 °C. At the concentrations employed,
(23) Blum, O.; Milstein, D. J. Am. Chem. Soc. 1995, 117, 4582-4594.
(24) Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 7010-7011.
(25) Hartwig, J. F.; Richards, S.; Baranano, D.; Paul, F. J. Am. Chem.
Soc. 1996, 118, 3626-3633.
(26) Ritter, J. C. M.; Bergman, R. G. J. Am. Chem. Soc. 1998, 120,
6826-6827.
2
we would be able to detect by H and 31P NMR spectroscopy
(27) Blum, O.; Milstein, D. J. Organomet. Chem. 2000, 593-594, 479-
484.
(31) Atwood, J. D.; Bernard, K. A.; Rees, W. M. Organometallics 1986,
5, 390-391.
(28) Bryndza, H. E.; Calabrese, J. C.; Marsi, M.; Roe, D. C.; Tam, W.;
Bercaw, J. E. J. Am. Chem. Soc. 1986, 108, 4805-4813.
(29) Fryzuk, M. D.; Piers, W. E. Organometallics 1990, 9, 986-98.
(30) Saura-Llamas, I.; Gladysz, J. A. J. Am. Chem. Soc. 1992, 114, 2136-
2144.
(32) Rees, W. M.; Churchill, M. R.; Fettinger, J. C.; Atwood, J. D.
Organometallics 1985, 4, 2179-2185.
(33) Ahmad, N.; Robinson, S. D.; Uttley, M. F. J. Chem. Soc., Dalton
Trans. 1972, 843-847.

7222 J. Am. Chem. Soc., Vol. 123, No. 30, 2001
Zhao et al.
Table 1. Structural Effects on the Rate Constants at 95 °C for
â-Hydrogen Elimination from Ir(I) Alkoxo Complexesa
a [Ir] ) [trans-Ir(CO)(PPh3)2].
Figure 2. Plot of 1/kobs vs [PPh3] for protiated and deuterated versions
of 2 and 6a. Reactions were conducted at 110 °C.
Table 2. Dependence of Alkoxide Racemization on [PPh3]
starting
ee (%)
[PPh3]
(M)
conversion
(%)
ee (%) after
conversion
84
84
>95
>95
>95
0.3
0.3
0.02
0.02
0.02
40
75
20
40
80
84
81
64
18
17
observed previously by Gladysz for ketone complexes of
rhenium34,35 and argues against the first explanation. However,
the ketone complex that is formed first could be η1-bound and
would then be more stable when the ketone is more electron
rich. Concerning the difference in ground-state energies control-
ling the reaction rates, the differences in thermodynamic stability
of these phenethoxide complexes should be small.36 Yet the
differences in transition state energies that account for these
rate differences are also small. Thus, this explanation cannot
be ruled out without firm thermodynamic data. The third
explanation is most conventional, however, and would be
consistent with hydridic character for the electron-rich iridium
product.
2. Mechanistic Studies. The influence of added phosphine
on the overall reaction rate and on the degree of racemization
of the enantiopure phenethoxide complex (R)-6a was used to
reveal the precise reaction pathway. The rate constants for
reaction of 2 and 6a were measured in C7D8 with added [PPh3-
d15] ranging from 0.02 to 0.30 M. The reactions were clearly
first order in the iridium alkoxide.
Figure 1. Hammett plot for â-hydrogen elimination from 6a-d. Cpd,
kobs × 104: 6a, 3.5; 6b, 7.3; 6c, 2.0; 6d, 1.2.
1
10% of the exchanged product and by H NMR 5% or less of
the exchanged product.
The series of alkoxo complexes revealed a surprising lack of
dependence of reaction rate on the steric and electronic
properties at the â-hydrogen of the alkoxide. The kobs values
are provided in Table 1. These data show less than a factor of
3 difference in rate for benzyloxo vs alkoxo and for methoxo
vs hindered alkoxo complexes. Although compound 6a reacted
more slowly than 5, suggesting that a substituent on the â-carbon
retards the reaction, compound 2 reacted with nearly the same
rate constant as compound 1, suggesting that this effect is subtle
if present at all. One trend that was consistent, however, was
the observation of slightly faster rates when increased steric
hindrance was present about the γ-carbon. For example,
compound 3 reacted faster than compound 2, and compound
4b reacted faster than 4a. Qualitative studies on solvent effects
showed similar rates in toluene, THF, and chlorobenzene.
The effect of electron density at the â-position was revealed
by a Hammett analysis of â-phenethoxide complexes 6a-d.
The dependence of 1/kobs on added [PPh3-d15] is shown in
Figure 2. Within a conventional range of small amounts of added
[PPh3-d15], the reaction appeared to be zero order in free ligand.
However, rate constants measured with a large increase in the
concentration of added PPh3-d15 showed a small but measurable
decrease in kobs upon increasing [PPh3-d15].
1
The reaction rates measured by H NMR spectroscopy under
In a mechanistically revealing set of experiments, the degree
of racemization of (R)-6a was shown to depend on the
concentration of added PPh3 (Table 2). For example, (R)-6a
underwent little or no detectable racemization during reactions
run at high concentrations of added PPh3-d15, but (R)-6a showed
substantial racemization during reactions run at low [PPh3-d15].
An increase in racemized material was observed with increased
conditions of a large excess of added PPh3 (vide infra)
demonstrated that electron-withdrawing substituents decreased
the reaction rate, and a Hammett plot (Figure 1) showed a
F-value of -0.94. This effect can be rationalized in several
ways: by greater stability of the resulting electron-rich ketone
complex and concomitant transition state stabilization for the
C-H bond cleavage, by reduced stability of the electron-rich
starting benzyloxo complex, or by migration of the hydrogen
with partial hydridic character. A π-bound complex of an
electron-poor ketone, which might be the product formed
initially from the elimination, should be more, rather than less,
stable than that of an electron-rich ketone. This effect has been
(34) Mendez, N. Q.; Seyler, J. W.; Arif, A. M.; Gladysz, J. A. J. Am.
Chem. Soc. 1993, 115, 2323-2334.
(35) Klein, D. P.; Dalton, D. M.; Mendez, N. Q.; Arif, A. M.; Gladysz,
J. A. J. Organomet. Chem. 1991, 412, C7-C10.
(36) Holland, P. L.; Andersen, R. A.; Bergman, R. G.; Huang, J. K.;
Nolan, S. P. J. Am. Chem. Soc. 1997, 119, 12800-12814.

â-H Elimination from Square Planar Iridium Alkoxide Complexes
J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7223
conversion. In these experiments, racemization was evaluated
by first hydrolyzing the alkoxo complex and then analyzing the
extruded alcohol by chiral stationary phase HPLC and chiral
GC.
A comparison of the stability of the alkoxo complexes with
the related amido complexes24 shows that these classes of
complexes undergo â-hydrogen elimination at similar rates. This
similar rate for â-hydrogen elimination of square planar alkoxo
and amido complexes that possess labile dative ligands has
several consequences for catalytic processes. The slower rate
for C-O bond-forming reductive elimination vs C-N reductive
elimination, along with this similar rate for â-hydrogen elimina-
tion, explains the difficulty in developing catalytic systems that
couple alcohols containing hydrogens R to oxygen with un-
activated aryl halides.41-44 Second, the similar rates for â-
hydrogen elimination suggest that for the opposite process,
insertion, imines should be as reactive as ketones toward
transition metal hydrides. Although rapid rates for ketone and
imine hydrogenations have now been observed,45,46 some rapid
imine hydrogenations are proposed to occur by a pathway that
bypasses imine insertion.47,48 The faster rate for â-hydrogen
elimination from alkyls implies that olefin insertion will be
faster, and this assertion is consistent with the greater diversity
of structures for olefin hydrogenation catalysts than imine or
ketone hydrogenation catalysts.
The deuterium isotope effect on the thermolysis of 2 and 6a
was also evaluated as a function of phosphine concentration.
This measurement was conducted as a function of [PPh3-d15]
because the kinetic importance of the C-H bond cleavage step
depends on whether dissociation of PPh3-d15 and â-hydrogen
elimination are reversible. The reversibility of these steps may
depend on [PPh3-d15]. The consequences of different reagent
concentrations on observed kinetic isotope effects are often
overlooked.
Figure 2 shows the dependence of the rate constant on
[PPh3-d15] for reaction of 2 and 2-d1 and for reaction of 6a and
6a-d1. These data reveal a significant isotope effect on the
y-intercept and slope of a plot of 1/kobs vs [PPh3-d15]. The isotope
effect on the y-intercept was 2.3 ( 0.2 for 2 and 2.6 ( 0.3 for
6a. The isotope effect on the slope was 2.3 ( 0.5 for 2 and 3.0
( 1.0 for 6a.
2. Mechanistic Studies. Experimental data and previous
thermodynamic studies rule out mechanisms involving CO
dissociation for the â-hydrogen elimination. The most convinc-
ing argument against a pathway involving CO dissociation is
based on previous measurements of Ir-CO bond energies for
closely related compounds. These measured bond energies are
between 72 and 84.4 kcal/mol, which are remarkably high for
a dative ligand.49 These values are 40-50 kcal/mol too high to
allow CO to be a part of the reaction pathway for the observed
â-hydrogen elimination.49 In addition, thermolysis of the alkoxo
complexes in the presence of CO led to the formation of
carboalkoxy complexes, as described previously.32 This result
prevented our obtaining a reaction order in CO that would probe
for CO dissociation. However, this result also suggests that
carboalkoxy complexes would be formed, at least in small
amounts, if CO dissociation did occur.
Instead, pathways involving alkoxide dissociation, direct
â-hydrogen elimination, or initial phosphine dissociation are
more likely. Six such pathways are shown in Scheme 2. All of
the pathways shown would be first order in iridium alkoxide,
but would differ in their dependence of reaction rate and degree
of racemization on solvent and added [PPh3].50
Reaction by pathway A, involving alkoxide dissociation,
would be zero order in added [PPh3] and highly dependent on
solvent polarity. Reaction by pathway B, involving direct
â-hydrogen elimination, would be zero order in added phos-
phine, but relatively independent of solvent polarity. The
stereochemistry of 6a during reactions by either pathway would
Discussion
1. Comparative Stability of Alkoxo, Amido, and Alkyl
Complexes. It was commonly believed that low-valent, late
transition metal alkoxo complexes would have weak bonds due
to an unfavorable hard-soft match between ligand and metal.37
Thermodynamic studies showed that this prediction of homolytic
bond strengths was unjustified and that metal alkoxo bond
dissociation energies are often higher than those of metal
alkyls.38 However, some reaction pathways that are high in
energy for metal alkyl complexes are low in energy for metal
alkoxides. For example, the basic alkoxide nonbonded electrons
can facilitate direct reaction at the alkoxo ligand or dissociation
of ancillary ligands due to π-stabilization of the resulting
unsaturated intermediate.39,40 Moreover, the greater stability of
alkoxide anions can induce bond heterolysis to generate reactive
ion pairs with a metal cation and an alkoxo anion.
It is these properties that allow the unusual mechanisms for
â-hydrogen elimination from Ir(III) alkoxo complexes noted in
the Introduction to be more energetically accessible than they
are for the analogous alkyl complexes. Yet, Bryndza’s study of
[(DPPE)Pt(OMe)(Et)] showed that the alkyl group could be
more reactive than the alkoxide toward â-hydrogen elimination
within the same complex. Perhaps no late metal system shows
as great a difference in stability of alkoxo vs alkyl complexes
as the Vaska-type system reported here. Schwartz’s group
generated the analogous alkyl complexes at low temperature,
and these complexes underwent â-hydrogen elimination near 0
°C. All of the alkoxo complexes prepared by Atwood and by
us require hours to decompose at 80-110 °C. As will be
discussed below, the mechanism for the â-hydrogen elimination
is similar to that for late metal alkyl complexes. Yet, â-hydrogen
elimination from the Vaska-type iridium complexes is much
slower for the alkoxides than it is for the alkyls. Thus,
â-hydrogen elimination of alkoxo and alkyl complexes of the
same metal fragment may occur by a similar mechanism with
(41) Mann, G.; Incarvito, C.; Rheingold, A. L.; Hartwig, J. F. J. Am.
Chem. Soc. 1999, 121, 3224-3225.
(42) Shelby, Q.; Kataoka, N.; Mann, G.; Hartwig, J. F. J. Am. Chem.
Soc. 2000, 122, 10718-10719.
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122, 12907-12908.
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P.; Buchwald, S. L. J. Am. Chem. Soc. 1999, 121, 4369-4378.
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1990, 29, 558-559.
the relative rate kM-R . kM-OR
.
(47) Haack, K.-J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R.
Angew. Chem., Int. Ed. Engl. 1997, 36, 285-288.
(48) Yamakawa, M.; Ito, H.; Noyori, R. J. Am. Chem. Soc. 2000, 122,
1466-1478.
(49) Rosini, G. P.; Liu, F.; Krogh-Jespersen, K.; Goldman, A. S.; Li,
C.; Nolan, S. P. J. Am. Chem. Soc. 1998, 120, 9256-9266.
(50) Experiments were conducted with PPh3-d15, but for simplicity we
state the effect of added PPh3.
(37) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163-1188.
(38) Bryndza, H. E.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 1444-
1456.
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Organometallics 1989, 8, 379-385.

7224 J. Am. Chem. Soc., Vol. 123, No. 30, 2001
Zhao et al.
Scheme 2
â-hydrogen elimination is irreversible. Under these conditions,
the phosphine is, therefore, involved only in a dissociative
preequilibrium before the irreversible step of the reaction and
the process is inhibited by added [PPh3].
d[6a]/dt ) kobs[Ir-OR]
(1)
1/kobs ) -1/k1 - k-1[PPh3]/(k1k2) for path D
1/kobs ) -1/K1k2 - k-2[PPh3]/(K1k2k3) for path E
1/kobs ) -k-2/(K1k2k3) - [PPh3]/(K1k2) for path F
In addition to the ability to account for the alkoxide
racemization, paths E and F, but not path D, account for the
isotope effect on the y-intercept of the plot in Figure 2. For
mechanisms E and F, the value of the y-intercept would contain
the rate constant for â-hydrogen elimination and would show a
primary isotope effect. In contrast, the y-intercept of a plot of
1/kobs vs [PPh3] for mechanism D, involving irreversible
â-hydrogen elimination, corresponds to the rate constant for
ligand dissociation. Thus, the y-intercept would not display a
significant isotope effect. The y-intercept of Figure 1 shows an
isotope effect of 2.3 ( 0.2 for reaction of 2 and 2.6 ( 0.3 for
reaction of 6a. These data provide further evidence against
pathway D and support pathway E or F. A comparison of the
magnitudes of the isotope effect for the y-intercept (2.3 ( 0.2
for 2 and 2.6 ( 0.3 for 6a) and slope (2.3 ( 0.5 for 2 and 3.0
( 1.0 for 6a) provides an approximate isotope effect for the
insertion reaction. Assuming k3, the replacement of coordinated
ketone by phosphine, has no isotope effect, the ratio of the
isotope effect for the y-intercept to the isotope effect for the
be independent of added PPh3. Reaction by pathway C,
involving irreversible phosphine dissociation, would exhibit no
dependence of reaction rate or degree of racemization on [PPh3].
Our data are not consistent with this set of predictions for paths
Α-C.
Pathway D, involving reversible phosphine dissociation and
irreversible â-hydrogen elimination, would display an inverse
first order dependence on [PPh3], at least at high [PPh3] (eq 1).
Although this dependence was observed, path D does not
account for the observed racemization of (R)-6a.
Pathways E and F involve reversible phosphine dissociation
and reversible â-hydrogen elimination. Path E involves dis-
sociation of the final free ketone product in the last step, while
path F involves associative substitution of phosphine for ketone
in the final step. These two pathways show similar dependencies
of kobs on added [PPh3] to that of path D. However, they do
allow 6a to undergo racemization because â-hydrogen elimina-
tion is reversible.
slope provides the isotope effect for the insertion of ketone k-2
.
This ratio is nearly 1. A similarly small isotope effect has been
observed for some alkene insertions studied previously.51,52
It remains to distinguish between pathways E and F. Al-
though the reversibility of the â-hydrogen elimination in both
pathways allows for racemization of 6a, these mechanisms are
clearly distinguished by the dependence of the degree of
racemization on the concentrations of added PPh3. As shown
in Scheme 3, the concentration of PPh3 can influence the amount
of racemization by altering the relative rates for reinsertion and
for ketone displacement. In pathway E, increased concentrations
of PPh3 will not affect the rate for reinsertion or dissociation of
ketone because neither step involves reaction with this species.
Thus, pathway E does not account for an influence of [PPh3]
on the rate of racemization of starting material.
However, pathway F, which involves associative displacement
of ketone by PPh3, predicts a dependence of the degree of
racemization on [PPh3]. The relative rates for the k3 and k-2
steps depend on [PPh3] because k3 is associative. At high [PPh3],
the associative displacement of ketone is faster than reinsertion,
the â-hydrogen elimination becomes irreversible, and essentially
no racemization of (R)-6a is observed. At low [PPh3], reinsertion
of the ketone competes with its associative displacement, the
â-hydrogen elimination is reversible, and racemization is
observed. Thus, the observed dependence of the amount of
racemization of 6a on [PPh3], along with the apparent irrevers-
ible formation of free ketone, is inconsistent with pathway E
and is consistent with pathway F.
The rate equations corresponding to these reaction paths are
provided in eq 1. The dependence of paths E and F on [PPh3]
is similar, but for different reasons. At low [PPh3], path E can
be nearly zero order in added [PPh3] because the â-hydrogen
elimination step is much faster than recoordination of PPh3.
However, at high [PPh3] the reaction can be strongly dependent
on [PPh3] because recoordination of phosphine is faster than
â-hydrogen elimination and phosphine dissociation becomes
reversible. Pathway F is also initiated by phosphine dissociation,
but at low [PPh3] reversible phosphine dissociation, reversible
â-hydrogen elimination, and associative displacement of ketone
by phosphine all occur. Because the phosphine is involved in
both dissociative and associative steps before or during the
irreversible step, a nearly-zero order dependence of kobs on
[PPh3] is observed when [PPh3] is low. At high [PPh3], the
ketone is displaced more rapidly than it reinserts, and the
(51) Doherty, N. M.; Bercaw, J. E. J. Am. Chem. Soc. 1985, 107, 2670-
2682.
(52) Landis, C. R.; Hilfenhaus, P.; Feldgus, S. J. Am. Chem. Soc. 1999,
121, 8741-8754 and references therein.

â-H Elimination from Square Planar Iridium Alkoxide Complexes
J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7225
Scheme 3
Scheme 4
Scheme 5
However, if path E were modified to include reversible
dissociation of ketone and irreversible coordination of phosphine
to the resulting 14-electron intermediate, then a dependence of
racemization on [PPh3] would be observed. If this pathway were
followed, though, incorporation of free added ketone into the
pool of alkoxide complex should have been observed. For
example, reactions of isopropoxide 2 conducted in the presence
of deuterated acetone and of phenethoxide 6a in the presence
of protiated acetone should show formation of 2-d6 from 2 and
2 from 6a. In neither case were detectable amounts of the
exchange product observed. We estimated that the detection limit
for ketone incorporation in these experiments would be 10%
for reaction with deuterated acetone and 5% or less for formation
of 2 from 6a. Considering the large amount of racemization in
the absence of PPh3, the products from incorporation of free
ketone must be larger than 5-10% of the alkoxide in solution
if path E operates. Thus, pathway F is the only mechanism in
Scheme 2 that is consistent with all of our data.
In addition to our experimental data, well-established mech-
anisms for ligand substitution favor pathway F over E. Square
planar d8 complexes typically react by associative mechanisms,
such as that in the last step of path F. Dissociative ligand
substitution reactions of square planar compounds, such as the
acetone complex in path E, are rare.53
As has been emphasized in other systems previously,34,35,54
the ketone complexes that are generated by â-hydrogen elimina-
tion from the Vaska-type alkoxide complexes are chiral. If
racemization is to be observed, reinsertion must be preceded
by rotation about the Ir-O bond if the ketone is bound η1 or
migration of the metal from one ketone face to the other if the
ketone is bound η2. Thus, rotation about the Ir-O bond of an
η1-ketone complex,34 or conversion of an η2- to η1-ketone
complex and rotation about the Ir-O bond, must be similar to
or greater than the rate for reinsertion.
reaction forms an η2-ketone complex after â-hydrogen elimina-
tion, then, by the Hammond postulate, one might have expected
the stability of the η2-ketone to affect the overall reaction rate,
and reactions that form less hindered ketones to occur faster.
Moreover, aldehydes bind more tightly than ketones, suggesting
that â-hydrogen eliminations that form aldehydes would be
faster than those forming ketones. The direct formation of an
η1-aldehyde or ketone complex without an intermediate η2-
complex could explain the small steric effect we observe on
reaction rate because the bulk of the coordinated carbonyl
compound would be distal from the metal (Scheme 4).
By this logic, â-hydrogen elimination from alkyl complexes
should show a greater steric effect than we observe here because
the olefin in the olefin hydride intermediate must be bound η2.
Such trends for â-hydrogen elimination from isolated alkyl
complexes have not been studied thoroughly, but we located
two examples that probe the steric effect. Both examples showed
a similar small dependence of the rate for â-hydrogen elimina-
tion on the steric properties of the alkyl group.55,56 The examples
at the top of Scheme 5 show that â-hydrogen elimination to
form ethylene, propene, butene, and isobutylene all occur at
similar rates. The examples at the bottom show that an
3. Mechanistic Implications of the Relative Stability of
Alkoxo Complexes. Only subtle differences in the rate constants
for â-hydrogen elimination were observed as a function of large
steric differences in the environment of the â-hydrogen. If the
(53) Lanza, S.; Minniti, D.; Moore, P.; Sachinidis, J.; Romeo, R.; Tobe,
M. Inorg. Chem. 1984, 23, 4428-4433.
(54) Klein, D. P.; Gladysz, J. A. J. Am. Chem. Soc. 1992, 114, 8710-
8711.

7226 J. Am. Chem. Soc., Vol. 123, No. 30, 2001
Zhao et al.
shifts were referenced to 85% H3PO4. Elemental analyses were
performed by Robertson Microlit Laboratories, Inc., Madison, NJ 07940
or Atlantic Microlab, Inc., Norcross, GA 30071. IR spectra were
recorded on a MIDAC Fourier Transform spectrometer. GC analyses
were conducted on a Hewlett-Packard 5890 instrument connected to a
3395 integrator. Enantiomeric excess was determined on a HPLC
Chiralcel OD-H column (2% i-PrOH in hexane, 1 mL/min) or a GC
Chiraldex B-PM column.
General Procedure for the Preparation of Sodium Alkoxides.
Into a Schlenk flask was placed 300 mg of NaH (12.5 mmol). This
solid was suspended in pentane. The flask was placed under nitrogen
with stirring and was cooled with ice. The appropriate alcohol (15.0
mmol) was added into the flask using a 1 mL syringe. The solution
started bubbling. The reaction mixture was allowed to warm to room
temperature over the course of 3 h. After this time, the pentane was
evaporated under vacuum. The resulting white powder was collected
on a medium fritted funnel, washed with pentane, and was used without
further purification.
General Procedure for the Preparation of Iridium Alkoxides
from Sodium Alkoxides. Into a 20 mL vial was weighed 150 mg of
Vaska’s complex (0.192 mmol). This material was suspended in 10
mL of THF, and 1.2 equiv (0.230 mmol) of sodium alkoxide was added.
The reaction was stirred at room temperature for 1 h. After this time,
the THF was evaporated under reduced pressure. The resulting yellow
film was dissolved in toluene, and the yellow suspension was filtered
through Celite. The resulting solution was concentrated under vacuum,
and crystallized by layering the concentrated toluene solution with
pentane or ether and cooling at -35 °C.
General Procedure for the Preparation of Iridium Alkoxides
from Sodium Hydride and Alcohols. Into a 20 mL vial was weighed
150 mg of Vaska’s complex (0.192 mmol). This material was suspended
in 10 mL of THF, and 1.2 equiv (0.230 mmol) of sodium hydride was
added. Then 1.5 equiv (0.288 mmol) of alcohol was added. The reaction
was stirred at 0 °C for 1 h. After this time, the THF was evaporated
under reduced pressure. The resulting yellow film was dissolved in
toluene, and the yellow suspension was filtered through Celite. The
resulting solution was concentrated under vacuum, and crystallized by
layering the concentrated toluene solution with pentane or ether and
cooling at -35 °C.
intramolecular selectivity to form one of two olefins, one more
substituted than the other, was again small. Perhaps the steric
properties of the alkyl or alkoxide complexes are not dramati-
cally different from those of the olefin or ketone complexes,
the transition state is early, or the substituents favor the cyclic
structure of the transition state, as in the well-known gem-dialkyl
effect.57-59
Thus, the modest steric effects on the rates for â-hydrogen
elimination from the alkoxo complexes have precedent in the
â-hydrogen elimination from alkyl complexes. It is, therefore,
unnecessary to invoke direct formation of an η1-aldehyde or
ketone complex to explain the small steric effect.
Conclusions
A combination of phosphine dependence on stereochemical
integrity, isotope effects on the y-intercept of a Lineweaver-
Burk plot, and phosphine dependence on kobs have clearly
revealed a mechanism for â-hydrogen elimination that stands
in contrast with the solvent-assisted ligand dissociation,23,27
direct elimination,28 and bimolecular hydride abstraction26
deduced from previous studies. Instead, it is more akin to the
classic â-hydrogen elimination from alkyl complexes.1
Further studies will be necessary to determine whether the
more common pathway for â-hydrogen elimination of alkoxides
will be the one outlined here and revealed by Gladysz in a
different rhenium system or the pathways observed previously
with saturated Ir(III) complexes. In either case, our studies show
that an alkoxo complex need not be coordinatively saturated
with tightly bound ligands to be stable. The alkoxo complexes
studied here are far more stable than the analogous alkyls
bearing â-hydrogens,60 despite the open coordination site in the
complex and its labile monodentate phosphines.
Experimental Section
General Methods. Unless otherwise noted, all reactions, recrystal-
lizations, and routine manipulations were performed at ambient
temperature in a nitrogen-filled glovebox, or by using standard Schlenk
techniques. Kinetic experiments were carried out in screw-capped NMR
tubes in deuterated solvents and were monitored by 1H NMR
spectroscopy. n-Pentane (tech grade) was distilled under nitrogen from
purple sodium/benzophenone ketyl made soluble by addition of
tetraglyme to the still. Benzene, toluene, tetrahydrofuran (THF), and
diethyl ether were distilled from sodium/benzophenone ketyl under
nitrogen. Deuterated solvents for use in NMR experiments were dried
as their protiated analogues, but were vacuum transferred from the
drying agent. Vaska’s complex was prepared according to published
methods. Triphenylphosphine was sublimed prior to use in kinetic
experiments. All other chemicals were used as received from com-
mercial suppliers.
trans-[Ir(PPh3)2](CO)(OMe) (1). Following the general procedure
involving alkoxide reagents, Vaska’s complex (120 mg, 0.154 mmol)
was combined with NaOMe (60.0 mg, 1.11 mmol) in THF. Workup
and crystallization from toluene/pentane at -35 °C gave yellow crystals
1
of 1 (55 mg, 0.071 mmol, 46%). H NMR (C6D6): δ 3.54 (s, 3H),
7.01-7.07 (m, 18H), 8.02 (m, 12H). 31P{1H} NMR (C6D6): δ 22.61.
13C{1H} NMR (C6D6): δ 60.58 (t, J ) 3.8 Hz), 128.35 (t, J ) 4.9
Hz), 130.17 (s), 133.67 (t, J ) 25.7 Hz), 135.18 (t, J ) 6.4 Hz), 175.36
(t, J ) 10.7 Hz). IR (C6D6, cm-1): 1944 (s, CO).
trans-[Ir(PPh3)2](CO)(OCHMe2) (2). Following the general pro-
cedure involving alkoxide reagents, Vaska’s complex (146 mg, 0.185
mmol) was combined with NaOCHMe2 (24.0 mg, 0.293 mmol) in THF.
Workup and crystallization from toluene/pentane at -35 °C gave yellow
crystals of 2 (121 mg, 0.150 mmol, 81%). 1H NMR (C6D6): δ 0.75 (d,
J ) 6.0 Hz, 6H), 4.04 (sept, J ) 5.6 Hz, 1H), 7.01-7.10 (m, 18H),
8.03 (m, 12H). 13C{1H} NMR (C6D6): δ 29.23 (s), 73.64 (t, J ) 5.9
Hz), 128.17 (t, J ) 4.7 Hz), 130.14 (s), 133.78 (t, J ) 25.2 Hz), 135.31
(t, J ) 6.0 Hz), 176.06 (t, J ) 11.6 Hz). 31P{1H} NMR (C6D6): δ
24.19. IR(C6D6, cm-1): 1943 (s, CO).
1H and 13C {1H} NMR spectra were obtained on either a GE QE
300 MHz or a Bruker 400 MHz Fourier Transform spectrometer.
31P{1H} and 2H NMR spectra were obtained on a GE Ω 300 or GE Ω
1
2
500 Fourier Transform spectrometer. H, 13C, and H NMR chemical
shifts are reported in parts per million downfield from tetramethylsilane
and were referenced to residual protiated (1H) or deuterated solvent
(13C) or natural abundance deuterated solvent (2H). 31P NMR chemical
trans-[Ir(PPh3)2](CO)(OCDMe2) (2-d1). Following the general
procedure involving alkoxide reagents, Vaska’s complex (153 mg, 0.196
mmol) was combined with NaOCDMe2 (24.3 mg, 0.289 mmol) in THF.
Workup and crystallization from toluene/pentane at -35 °C gave yellow
crystals of 2-d1 (127 mg, 0.158 mmol, 81%). 1H NMR (C6D6): δ 0.74
(s, 6H), 7.01-7.11 (m, 18H), 8.03 (m, 12H). 31P{1H} NMR (C6D6): δ
(55) Komiya, S.; Yamamoto, A.; Yamamoto, T. Chem. Lett. 1978, 1273-
1276.
(56) McDermott, J. X.; White, J. F.; Whitesides, G. M. J. Am. Chem.
Soc. 1976, 98, 6521-6528.
2
24.20. H NMR (C6H6): δ 3.97.
(57) Beesley, R. M.; Ingold, C. K.; Thorpe, J. F. J. Chem. Soc. 1915,
107, 1080-1106.
trans-[Ir(PPh3)2](CO)(OCHMeC(CH3)3) (3). Following the general
procedure involving alkoxide reagents, Vaska’s complex (153 mg, 0.196
mmol) was combined with NaOCHMeC(CH3)3 (34.6 mg, 0.279 mmol)
in THF. Workup and crystallization from toluene/pentane at -35 °C
gave yellow crystals of 3 (132 mg, 0.156 mmol, 79%). 1H NMR
(C6D6): δ 0.60 (d, J ) 6.0 Hz, 3H), 0.70 (s, 9H), 3.60 (q, J ) 5.7 Hz,
(58) Capon, B.; McManus, S. P. In Neighboring Group Participation;
Plenum: New York, 1976; pp 58-75.
(59) Lightstone, F. C.; Bruice, T. C. J. Am. Chem. Soc. 1996, 118, 2595-
2605.
(60) Cannon, J. B.; Schwartz, J. J. Am. Chem. Soc. 1974, 96, 2276-
2278.

â-H Elimination from Square Planar Iridium Alkoxide Complexes
J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7227
1H), 7.01-7.11 (m, 18H), 8.00 (m, 12H). 31P{1H} NMR (C6D6): δ
23.36. 13C{1H} NMR (C6D6): δ 24.69 (s), 27.02 (s), 38.36 (s), 84.05
(t, J ) 4.5 Hz), 128.23 (t, J ) 4.9 Hz), 130.12 (s), 133.87 (t, J ) 25.6
Hz), 135.25 (t, J ) 5.9 Hz), 175.78 (t, J ) 12.1 Hz). IR (C6D6, cm-1):
1936 (s, CO). Anal. Calcd for C43H43P2O2Ir: C, 61.04; H, 5.08.
Found: C, 60.79; H, 5.24.
trans-[Ir(PPh3)2](CO)(OCH2CH(CH3)2) (4a). Following the general
procedure involving alkoxide reagents, Vaska’s complex (151 mg, 0.194
mmol) was combined with NaOCH2CH(CH3)2 (28.0 mg, 0.292 mmol)
in THF. Workup and crystallization from toluene/pentane at -35 °C
gave yellow crystals of 4a (125 mg, 0.153 mmol, 79%). 1H NMR
(C6D6): δ 0.53 (d, J ) 7.2 Hz, 6H), 1.20 (nonet, J ) 6.4 Hz, 1H),
3.30 (d, J ) 6.0 Hz, 2H), 7.00-7.12 (m, 18H), 8.00 (m, 12H). 31P{1H}
NMR (C6D6): δ 23.83. 13C{1H} NMR (C6D6): δ 19.54 (s), 34.03 (s),
80.45 (t, J ) 3.3 Hz), 128.32 (t, J ) 5.6 Hz), 130.17 (s), 133.72 (t, J
) 25.7 Hz), 135.23 (t, J ) 7.0 Hz), 175.73 (t, J ) 10.4 Hz). IR (C6D6,
cm-1): 1945 (s, CO). Anal. Calcd for C41H39P2O2Ir: C, 60.23; H, 4.77.
Found: C, 60.51; H, 4.78.
ether mixtures at -35 °C gave yellow crystals of 6b (102 mg, 0.114
1
mmol, 60%). H NMR (C6D6): δ 0.89(d, J ) 6.0 Hz, 3H), 3.38 (s,
3H), 4.96 (q, J ) 6.0 Hz, 1H), 6.66 (apparent d, J ) 8.8 Hz, 2H), 6.71
(apparent d, J ) 8.8 Hz, 2H), 7.0 (m, 18H), 7.19 (d, J ) 8.3 Hz, 2H),
8.00 (m, 12H). 31P{1H} NMR (C6D6): δ 25.07. 13C{1H} NMR
(C6D6): δ 30.73 (s), 54.72 (s), 80.83 (t, J ) 6.2 Hz), 112.91 (s), 126.94
(s), 128.27 (t, J ) 4.6 Hz), 130.21 (s), 133.56 (t, J ) 26.8 Hz), 135.32
(t, J ) 6.3 Hz), 145.53 (s), 157.88 (s), 175.70 (t, J ) 11.6 Hz). IR
(C6D6, cm-1): 1944 (s, CO). Anal. Calcd for C46H41P2O3Ir: C, 61.66;
H, 4.61. Found: C, 61.62; H, 4.85.
trans-[Ir(PPh3)2](CO)(OCHMe(p-C6H4Cl)) (6c). Following the
general procedure involving alkoxide reagents, Vaska’s complex (242
mg, 0.310 mmol) was combined with NaOCHMe(p-C6H4Cl) (74.9 mg,
0.419 mmol) in THF. Workup and crystallization from toluene/pentane
mixtures at -35 °C gave yellow crystals of 6c (180 mg, 0.200 mmol,
1
64%). H NMR (C6D6): δ 0.87 (d, J ) 6.4 Hz, 3H), 4.85 (q, J ) 6.0
Hz, 1H), 6.46 (d, J ) 8.8 Hz, 2H), 7.0 (m, 20H), 7.94 (m, 12H). 31P{1H}
NMR (C6D6): δ 25.07. 13C{1H} NMR (C6D6): δ 30.30 (s), 80.45 (t,
J ) 6.2 Hz), 127.44 (s), 127.35 (s), 128.29 (t, J ) 4.5 Hz), 130.31 (s),
130.49 (s), 133.35 (t, J ) 25.9 Hz), 135.23 (t, J ) 6.0 Hz), 156.86 (s),
175.42 (t, J ) 11.1 Hz). IR (C6D6, cm-1): 1944 (s, CO). Anal. Calcd
for C45H38P2O2ClIr: C, 60.00; H, 4.22. Found: C, 60.25; H, 4.13.
trans-[Ir(PPh3)2](CO)(OCHMe(p-C6H4CF3)) (6d). Following the
general procedure involving reaction of alcohol (92.0 µL, 0.60 mmol)
with Vaska’s complex (150 mg, 0.192 mmol) in the presence of NaH
(11 mg, 0.46 mmol), 93 mg (0.100 mmol, 52%) of product was obtained
after crystallization from toluene/pentane at -35 °C. 1H NMR (C6D6):
δ 0.91 (d, J ) 6.1 Hz, 3H), 4.89 (q, J ) 6.0 Hz, 1H), 6.57 (d, J ) 8.3
Hz, 2H), 7.0 (m, 18H), 7.19 (d, J ) 8.3 Hz, 2H), 7.93 (m, 12H). 31P{1H}
NMR (C6D6): δ 25.64. 13C{1H} NMR (CD2Cl2): δ 29.67 (s), 80.11
(t, J ) 6.1 Hz), 123.91 (q, J ) 4.0 Hz), 125.68 (q, J ) 271.6 Hz),
125.79 (s), 126.71 (q, J ) 31.3 Hz), 128.41 (t, J ) 5.3 Hz), 130.61
(s), 132.98 (t, J ) 25.6 Hz), 135.14 (t, J ) 5.9 Hz), 157.24 (s), 175.34
(t, J ) 11.1 Hz). 19F NMR (CD2Cl2): δ -60.44. IR (C6D6, cm-1):
1942 (s, CO). Anal. Calcd for C46H38P2O2F3Ir: C, 59.16; H, 4.10.
Found: C, 58.99; H, 4.03.
trans-[Ir(PPh3)2](CO)(OCH2(cyclohexyl)) (4b). Following the gen-
eral procedure involving alkoxide reagents, Vaska’s complex (100 mg,
0.128 mmol) was combined with NaOCH2(cyclohexyl) (26.2 mg, 0.192
mmol) in THF. Workup and crystallization from toluene/pentane at
1
-35 °C gave yellow crystals of 4b (68 mg, 0.079 mmol, 62%). H
NMR (C6D6): δ 0.57 (q, J ) 11.2 Hz, 2H), 0.86 (m, 1H), 1.00 (m,
3H), 1.26 (m, 2H), 1.57 (d, J ) 9.6 Hz, 3H), 3.38 (d, J ) 5.2 Hz, 2H),
7.02-7.12 (m, 18H), 8.00 (m, 12H). 31P{1H} NMR (C6D6): δ 23.82.
13C{1H} NMR (C6D6): δ 26.90 (s), 27.39 (s), 30.19 (s), 44.24 (s), 79.51
(t, J ) 4.5 Hz), 128.25 (t, J ) 4.8 Hz), 130.14 (s), 133.74 (t, J ) 26.0
Hz), 135.25 (t, J ) 6.3 Hz), 175.74 (t, J ) 10.8 Hz). IR (C6D6, cm -1):
1944 (s, CO). Anal. Calcd for C44H43P2O2Ir: C, 61.62; H, 5.01.
Found: C, 61.70; H, 5.04.
trans-[Ir(PPh3)2](CO)(OCH2Ph) (5). Following the general proce-
dure involving alkoxide reagents, Vaska’s complex (176 mg, 0.226
mmol) was combined with NaOCH2Ph (48.4 mg, 0.372 mmol) in THF.
Workup and crystallization from toluene/pentane at -35 °C gave yellow
crystals of 5 (162 mg, 0.190 mmol, 84%). 1H NMR (C6D6): δ 4.64 (s,
2H), 6.76-6.79 (m, 2H), 6.98-7.15 (m, 21H), 7.97-8.00 (m, 12H).
31P{1H} NMR (C6D6): δ 24.0. 13C{1H} NMR (C6D6): δ 74.98 (t, 3.0
Hz), 125.23 (s), 126.46 (s), 127.4 (s), 128.37 (s), 130.23 (s), 133.50 (t,
25.5 Hz), 135.15 (t, 6.0 Hz), 148.28 (s), 175.43 (t, 11.8 Hz). IR (C6D6,
cm-1): 1947 (s, CO). Anal. Calcd for C44H37P2O2Ir: C, 62.06; H, 4.35.
Found: C, 62.20; H, 4.45.
Kinetic Measurements of â-Hydrogen Elimination from Iridium
Alkoxides in C7D8 and Data Processing. A stock solution consisting
of 0.0050 M iridium alkoxide was prepared in toluene-d8 and stored in
a -35 °C freezer. A typical reaction mixture was prepared by mixing
0.65 mL of the stock solution with 3.6 mg (0.013 mmol, 0.020 M) to
54 mg (0.19 mmol, 0.30 M) of PPh3-d15. The â-hydrogen elimination
was monitored by proton NMR spectroscopy at 95 or 108 °C. The
spectra were measured using 5-mm screw-capped NMR tubes, which
were inserted into a pre-shimmed and pre-warmed NMR spectrometer
probe. An automated acquisition program was started after 4-5 min
of temperature equilibration. The reactions were monitored for at least
three half-lives. Raw data from the spectra were processed using the
KaleidaGraph program on a Macintosh personal computer.
Evaluation of Alkoxide Racemization. A 5-mm screw-capped
NMR tube was charged with a solution containing 5.6 mg (0.0065
mmol, 0.0050 M) of complex (R)-6a and 102 mg of PPh3 (0.389 mmol,
0.300 M) or 6.8 mg of PPh3 (0.026 mmol, 0.020 M) in 1.3 mL of
toluene. The sample tube was capped with a septum. The tube was
placed into a 100 °C oil bath. The reaction was monitored by 31P NMR
spectrometry. Three or four aliquots of roughly 0.3 mL volume were
removed at different conversions of alkoxide complex. Each sample
was hydrolyzed by addition of excess water, and the enantiomeric
excess of the free alcohol was then determined by chiral capillary GC
or chiral stationary phase HPLC. Identical enantiomeric excess was
obtained by chiral capillary GC without the hydrolysis step.
trans-[Ir(PPh3)2](CO)(OCHMePh) (6a). Following the general
procedure involving alkoxide reagents, Vaska’s complex (201 mg, 0.258
mmol) was combined with NaOCHMePh (59.0 mg, 0.410 mmol) in
THF. Workup and crystallization from toluene/pentane at -35 °C gave
1
yellow crystals of 6a (170 mg, 0.196 mmol, 76%). H NMR (C6D6):
δ 0.89(d, J ) 6.0 Hz, 3H), 4.96 (q, J ) 5.9 Hz, 1H), 6.77 (m, 2H),
7.00-7.13 (m, 21H), 8.00 (m, 12H). 31P{1H} NMR (C6D6): δ 25.26.
13C{1H} NMR (C6D6): δ 30.54 (s), 81.28 (t, J ) 6.2 Hz), 125.10 (s),
126.03 (s), 127.37 (s), 128.27 (t, J ) 4.2 Hz), 130.22 (s), 133.51 (t, J
) 25.6 Hz), 135.30 (t, J ) 6.2 Hz), 153.22 (s), 175.63 (t, J ) 11.6
Hz). IR (C6D6, cm-1): 1945 (s, CO). Anal. Calcd for C45H39P2O2Ir:
C, 62.44; H, 4.51. Found: C, 62.26; H, 4.44. Complex (R)-6a was
prepared using the general procedure involving reaction of commercial
(R)-phenethyl alcohol (57.0 µL, 0.473 mmol) with Vaska’s complex
(200 mg, 0.256 mmol) in the presence of NaH (9.5 mg, 0.40 mmol) to
give 126 mg (0.145 mmol, 57%) of enantiopure product.
trans-[Ir(PPh3)2](CO)(OCDMePh) (6a-d1). Following the general
procedure involving alkoxide reagents, Vaska’s complex (151 mg, 0.194
mmol) was combined with NaOCDMePh (42 mg, 0.290 mmol) in THF.
Workup and crystallization from toluene/pentane at -35 °C gave yellow
1
Acknowledgment. We thank the PRF for partial support of
this work, along with the DOE. We are also grateful to Johnson-
Matthey for a gift of IrCl3‚3H2O. We thank Prof. Jack Faller
for helpful comments on the stereochemical requirements for
alkoxide racemization.
crystals of 6a-d1 (100 mg, 0.115 mmol, 59%). H NMR (C6D6): δ
0.88 (s, 3H), 6.77 (m, 2H), 7.00-7.07 (m, 21H), 8.00 (m, 12H). 31P{1H}
2
NMR (C6D6): δ 25.28. H NMR (C6H6): δ 4.91.
trans-[Ir(PPh3)2](CO)(OCHMe(p-C6H4OMe)) (6b). Following the
general procedure involving alkoxide reagents, Vaska’s complex (148
mg, 0.190 mmol) was combined with NaOCHMe(p-C6H4OMe) (51.5
mg, 0.296 mmol) in THF. Workup and crystallization from toluene/
JA010417K