Scope and mechanism of the intermolecular addition of aromatic aldehydes to olefins catalyzed by Rh(I) olefin complexes

Article abstract of DOI:10.1021/ja066509x

Rhodium (I) bis-olefin complexes Cp*Rh(VTMS)₂ and Cp?Rh(VTMS)₂ (Cp* = C₅Me₅, Cp? = C₅-Me₄CF₃, VTMS = vinyl trimethylsilane) were found to catalyze the addition of aromatic aldehy

Full text of DOI:10.1021/ja066509x

Published on Web 01/31/2007
Scope and Mechanism of the Intermolecular Addition of
Aromatic Aldehydes to Olefins Catalyzed by Rh(I) Olefin
Complexes
Amy H. Roy, Christian P. Lenges, and Maurice Brookhart*
Contribution from the Department of Chemistry, UniVersity of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290
Received September 8, 2006; E-mail: mbrookhart@unc.edu
Abstract: Rhodium (I) bis-olefin complexes Cp*Rh(VTMS)2 and CpqRh(VTMS)2 (Cp* ) C5Me5, Cpq ) C5-
Me4CF3, VTMS ) vinyl trimethylsilane) were found to catalyze the addition of aromatic aldehydes to olefins
to form ketones. Use of the more electron-deficient catalyst CpqRh(VTMS)2 results in faster reaction rates,
better selectivity for linear ketone products from R-olefins, and broader reaction scope. NMR studies of the
hydroacylation of vinyltrimethylsilane showed that the starting Rh(I) bis-olefin complexes and the
corresponding Cp*/qRh(CH2CH2SiMe3)(CO)(Ar) complexes were catalyst resting states, with an equilibrium
established between them prior to turnover. Mechanistic studies suggested that CpqRh(VTMS)2 displayed
a faster turnover frequency (relative to Cp*Rh(VTMS)2) because of an increase in the rate of reductive
elimination, the turnover-limiting step, from the more electron-deficient metal center of CpqRh(VTMS)2.
Reaction of Cp*/qRh(CH2CH2SiMe3)(CO)(Ar) with PMe3 yields acyl complexes Cp*/qRh[C(O)CH2CH2SiMe3]-
(PMe3)(Ar); measured first-order rates of reductive elimination of ketone from these Rh(III) complexes
established that the Cpq ligand accelerates this process relative to the Cp* ligand.
Introduction
selectivity and eliminating a competing reaction, the transition
metal-mediated decarbonylation of aldehydes. Decarbonylation
During the last two decades, intensive research has been
directed toward the transition metal-mediated activation of C-H
bonds,1-4 including numerous investigations of the C-H bond
activation of aldehydes.4 Only recently have successful catalytic
processes been reported that combine C-H bond activation
reactions with C-C bond formation.5-10 One of the most
developed processes of this kind is the transition metal-catalyzed
hydroacylation of olefins. In contrast to established metal-
catalyzed X-H addition reactions like the hydrogenation,
hydrosilylation, or hydroboration of olefins, the catalytic addition
of an aldehyde C-H bond across an olefin to generate a ketone
allows the formation of a new carbon-carbon bond in one step
with maximum atom economy, while also introducing a versatile
functionality, the carbonyl group, to the substrate (eq 1).
generates the parent alkane and metal carbonyl complexes that
are typically inactive or inferior as catalysts for hydroacyla-
tion.11-17 To circumvent this problem, the groups of James and
Bosnich independently reported the intramolecular hydro-
acylation of 4-pentenals to generate cyclopentanones, with
chelation of the substrate believed to facilitate the reaction.18-20
However, this intramolecular reaction is specific for the forma-
tion of cyclopentanones, and attempts to extend the same
catalytic protocol to intermolecular reactions were not success-
ful.
Until recently, strategies utilized for catalytic intermolecular
hydroacylation of olefins typically exhibited low productivity
and were carried out at high temperatures and/or high pressures
to prevent decarbonylation.21-23 Alternatively, Jun and co-
workers reported the chelation-assisted, Rh-catalyzed intermo-
lecular hydroacylation of olefins in the presence of 2-aminopy-
Central to the challenge of developing a general and ap-
plicable hydroacylation protocol has been controlling regio-
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Addition of Aromatic Aldehydes to Olefins
A R T I C L E S
ridine as additive,7,24-28 inspired by Suggs’ report of the
rhodium-catalyzed hydroimination of olefins.29 The proposed
mechanism for the chelation-assisted hydroacylation is based
on the in situ condensation of the aldehyde with the amine
additive to generate a chelated imino-rhodium pyridine com-
plex. Subsequent olefin hydroimination and hydrolysis generate
the ketone product. This strategy allows olefin coupling to occur
while decarbonylation of the aldehyde is avoided. In addition,
Jun has extended this chelation-assisted chemistry to include
the hydroacylation of alkenes and alkynes with alcohols, as
aldehyde precursors, and amines, via imine formation and
subsequent hydroiminoacylation.30-36 Willis has also utilized a
chelation-assisted approach to achieve successful rhodium-
catalyzed intermolecular hydroacylation of alkenes or alkynes
with aldehydes and imines.37-39 Imines, â-thioacetal-substituted
esters, and methylsulfanyl-substituted aldehydes were shown
to react with acrylate esters, amides, and functionalized alkynes
in the presence of Rh(I) catalysts, such as Wilkinson’s catalyst
and [Rh(dppe)]ClO4.
We have previously reported the use of labile cobalt(I) bis-
olefin complexes as catalysts for the intermolecular anti-
Markovnikov addition of aldehydes to vinylsilanes (eq 2).40,41
Olefin dissociation from [C5Me5Co(C2H3SiMe3)2], 1, generated
a reactive 16-electron intermediate that could activate an
aldehyde C-H bond via oxidative addition. Olefin insertion and
reductive elimination from a Co(III) acyl alkyl/aryl intermediate
formed the ketone product and regenerated 1 after reaction with
olefin. The turnover frequency exhibited a first-order dependence
on aldehyde concentration and an inverse first-order dependence
on olefin concentration when aromatic aldehydes were em-
ployed. The use of alkyl aldehydes resulted in a more complex
dependency of the hydroacylation rates on substrate concentra-
tions, suggesting the involvement of a second resting state. This
second resting state was observed and identified as a cobalt-
(III) bis(alkyl) carbonyl complex, 2 (eq 2). The turnover-limiting
step for the process was established as the reductive elimination
from a cobalt(III) acyl alkyl intermediate formed upon CO
insertion, likely assisted by an η2-acyl interaction. A major
synthetic limitation of the cobalt system is that simple alkenes
such as ethylene, R-olefins, and cyclic olefins are not viable
substrates.
The thermolysis of transition metal alkyl or aryl carbonyl
complexes to generate ketones stoichiometrically has previously
been reported.42-45 Most applicable to the work described in
this manuscript, Maitlis and co-workers prepared rhodium
analogs of 2 via reaction of a rhodium dimethyl precursor with
aldehydes.46 These complexes generated acetophenone in the
presence of excess CO at 20-100 °C (eq 3). The Rh(III)-CO
intermediate 3 resembles the observed resting state in the cobalt-
catalyzed hydroacylation reactions. However, the barrier for
C-C bond formation from a second row metal is expected to
be higher than from a first row metal, thus more severe
conditions may be required for ketone formation from Rh
complexes.
We previously investigated the reactivity of labile rhodium-
(I) olefin complexes analogous to 1 as catalysts for transfer
hydrogenation and the addition of olefins to aromatic ketones.47-49
As expected, loss of vinyltrimethylsilane from [C5Me5Rh(C2H3-
SiMe3)2] (4) generated a reactive 16-electron intermediate that
could oxidatively add a variety of C-H bonds. In addition, the
bis(propene) analog of 4 has been described as a catalyst for
the activation and isomerization of alkyl aldehydes to generate
a mixture of isomeric aldehydes.50 A rhodium(III) bis(alkyl)
carbonyl species was observed as the catalyst resting state for
this isomerization process (eq 4), indicating that the C-H
activation of aldehydes and subsequent insertion reactions can
(21) Kondo, T.; Akazome, M.; Tsuji, Y.; Watanabe, Y. J. Org. Chem. 1990,
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(28) Loupy, A.; Chatti, S.; Delamare, S.; Lee, D. Y.; Chung, J. H.; Jun, C. H.
J. Chem. Soc. Perkin Trans. I 2002, 1280-1285.
(29) Suggs, J. W. J. Am. Chem. Soc. 1979, 101, 489.
(30) Chang, D. H.; Lee, D. Y.; Hong, B. S.; Choi, J. H.; Jun, C. H. J. Am.
Chem. Soc. 2004, 126, 424-425.
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F.; Zanobini, F. Organometallics 1989, 8, 337-345.
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metallics 1987, 6, 2453-2455.
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41 (12), 2146-2147.
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2003, 125, 6372-6373.
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G.; Bergman, R. G. J. Am. Chem. Soc. 1984, 106, 7451-7461.
(46) Sunley, G. J.; Fanizzi, F. P.; Saez, I. M.; Maitlis, P. M. J. Organomet.
Chem. 1987, 330 (3), C27.
(36) Lee, D. Y.; Moon, C. W.; Jun, C. H. J. Org. Chem. 2002, 67, 3945-3948.
(37) Willis, M. C.; McNally, S. J.; Beswick, P. J. Angew. Chem., Int. Ed. 2004,
43, 340-343.
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4385-4396.
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Lett. 2005, 7 (11), 2249-2251.
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125, 2038-2039.
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A R T I C L E S
Roy et al.
Analysis of the reaction mixture revealed the formation of
three different products whose ratios changed as the reaction
proceeded. First, the expected anti-Markovnikov addition
product ketone (7a,d) was observed, analogous to the cobalt-
catalyzed process. In addition, a second product based on
Markovnikov addition (8a,d) was observed in the early stages
of the catalytic reaction as indicated by 1H NMR spectroscopy
(quartet at 3.21 ppm, doublet at 1.38 ppm). As the reaction
proceeded, a rearrangement of this product occurred to generate
the SiMe3-enol of propiophenone (9a,d), as revealed by a
decrease in the ketone quartet resonance at 3.21 ppm at the same
rate as the formation of a new quartet resonance at 5.05 ppm,
assigned to the vinylic hydrogen. After 8 h of reaction, 18% of
the Markovnikov ketone was converted to the silyl enol ether
9a, and 92% conversion to 9a was observed after 13 h.
Figure 1. Structures of catalysts 4 and 5.
occur in this type of system. However, reductive elimination
of the hydroacylation product was not observed.
Here we report the use of 4 and a CF3-substituted derivative
of 4 (5, Figure 1) as catalysts for the intermolecular hydro-
acylation of a variety of olefins with aromatic aldehydes. The
results reported here combine the prior findings illustrated in
eq 3 by Maitlis with the C-H bond activation chemistry
observed for systems like 4. The scope and mechanism for this
catalytic process have been investigated, and a more detailed
analysis of the turnover-limiting reductive elimination of aryl
ketone is described.
For catalytic reactions on a larger scale, product analysis
showed complete conversion of aldehyde to a 2:1 mixture of
ketone 7a and enol 9a as the only products after extensive
reaction time in neat vinyltrimethylsilane. Intermediate 8a was
not detected at this stage.
Results and Discussion
A. Hydroacylation of Aromatic Aldehydes Using 4. The
catalytic hydroacylation of vinyltrimethylsilane with benzalde-
hyde was studied initially with catalyst 4. In an experiment
B. Variation of the Rh Catalyst and Impact on Hydro-
acylation Activity. Although catalyst 4 showed encouraging
activity for the hydroacylation of vinyltrimethylsilane, overall
catalytic activity and catalyst lifetime were moderate. While 4
efficiently catalyzed the reaction of electron-donating aldehydes,
it was less effective for the reactions of p-tolualdehyde and
benzaldehyde and was ineffective for reactions of electron-
deficient aldehydes. Previous hydroacylation results with cobalt
analogs indicated that reductive elimination of ketone could be
the rate-limiting step of the reaction.41 Anticipating that the rate
of reductive elimination would increase for a more electron-
deficient metal center, catalyst 4 was modified by the substitu-
tion of a CF3 group for a CH3 group on the Cp* ligand52 (Figure
1) in an effort to increase the activity and productivity of the
catalyst.
Catalyst 5, CpqRh(VTMS)2, was prepared by a similar
procedure to that of 4,47 with synthetic details presented in the
Experimental Section. Complexes 4 and 5 were compared under
identical conditions as catalysts for the reaction between
4-(dimethylamino)benzaldehyde and cyclopentene, with results
summarized in Table 1. After 1.5 h at 75 °C, nearly quantitative
conversion of aldehyde to ketone was observed for the reaction
with catalyst 5, whereas the reaction with catalyst 4 achieved
only 14% conversion of starting material. The substitution of
one methyl group in the Cp* ligand by one CF3 group resulted
in a significant increase in catalyst activity for this system.
1
followed by H NMR spectroscopy, 5 mol % of 4 was added
to a 1:1 mixture of vinyltrimethylsilane and benzaldehyde (6d)
in d8-toluene. Thermolysis at 110 °C resulted in the slow
formation of the hydroacylation product ketone 7d (eq 5) with
a turnover frequency of 2.1 TO/h during the initial stage of
catalysis. After 30% conversion, catalyst deactivation generated
a blue solution that contained [C5Me5Rh(CO)]2 as the major
rhodium species.51 The formation of this dimer, a product of
aldehyde decarbonylation, also explained the appearance of
benzene in the reaction mixture.
The use of the more electron-rich substrate p-(dimethylami-
no)benzaldehyde, 6a, resulted in successful hydroacylation and
improved catalyst lifetimes under identical conditions. At 110
°C, an initial turnover frequency of 5 TO/hr was observed to
generate ketone. Slow catalyst deactivation resulted in 70%
conversion of aldehyde after 20 h. Again, the stoichiometric
formation of the parent arene, N,N-dimethylaniline, supports
decarbonylation as the catalyst deactivation route. Accordingly,
the color of the reaction mixture changed during the reaction
from the initial orange color of 4 to brown during the catalysis,
and then to the intense blue color of the carbonyl-bridged dimer
[C5Me5Rh(CO)]2. During the initial phase of catalysis a new,
single Rh species was observed, which was assigned as the
catalyst resting state. This resting state is discussed in more
detail later.
(52) Gassman, P. G.; Mickelson, J. W.; Sowa, J. R., Jr. Inorg. Synth. 1997, 31,
232-239.
(51) Nutton, A.; Maitlis, P. M. J. Organomet. Chem. 1979, 166, C21.
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Addition of Aromatic Aldehydes to Olefins
A R T I C L E S
1
Table 1. Comparison of 4 and 5 as Catalysts for the
the Cp* resonance was observed in the H NMR spectrum of
10a. In addition, a series of complex resonances was observed
in the alkyl region along with a set of aromatic and -NMe2
resonances that indicate a rhodium-bound p-NMe2-C6H4 group.
Catalytic conversion of aldehyde was observed efficiently at
temperatures above 100 °C. However, the formation of inter-
mediate 10a was observed at 70 °C with a half-life of formation
of ca. 30 min, indicating that the steps to generate intermediate
10a have a lower barrier than the turnover-limiting step of the
reaction. This complex was assigned as Cp*Rh(CH2CH2SiMe3)-
(4-NMe2C6H4)(CO) (Scheme 1).54 However, the compound was
difficult to isolate cleanly for characterization due to the presence
of excess aldehyde and because reductive elimination of ketone
occurred from this intermediate over time.
Hydroacylation of Cyclopentene
entry
catalysta
time (min)
aldehyde:catalyst
% conversionb
1
2
3
4
5
4
4
4
5
5
90
180
360
90
20:1
20:1
20:1
20:1
20:1
14
24
50
98
>99
180
a 5 mol % relative to aldehyde. b Conversion monitored by GC.
C. Investigation of the Scope of the Hydroacylation of
Olefins Catalyzed by 5. Results for hydroacylation of several
olefins catalyzed by the CF3-substituted catalyst 5 are shown
in Table 2. Products were identified by gas chromatography
(GC) and NMR spectroscopy after isolation and purification.
Isomer mixtures were observed in some cases based on the
regiochemistry of the insertion step. The isolated yields reported
are the average of two runs.
Reactions were initially performed with a 10:1 ratio of olefin
to aldehyde to favor hydroacylation over decarbonylation, but
identical yields were obtained when only a 1:1 ratio of olefin
to aldehyde was employed (entry 2).53 The addition of both
electron-rich and electron-poor benzaldehydes occurred between
75 and 100 °C, but higher catalyst loadings were required and
lower yields were obtained for reactions of electron-poor
aldehydes. Higher temperatures were required for reactions of
unstrained olefins (entries 6 and 9), except when bulky olefins
were employed (entry 8). Aryl aldehydes performed well as
substrates for these hydroacylation reactions, but the use of alkyl
aldehydes resulted in aldehyde isomerization50 and no ketone
formation.
Because the rate of reductive elimination of ketone should
be slower with a more electron-poor aldehyde, we performed a
reaction between 4 and 4-trifluoromethylbenzaldehyde (1:1.1
ratio) in C6D6 at room temperature to synthesize a more stable
analog of the proposed Rh(III) intermediate. After one week,
almost complete conversion of 4 to the Cp*Rh(III) alkyl aryl
CO complex (10e, Scheme 1) was observed by 1H NMR
spectroscopy with very little ketone formation. After evaporation
of the volatile compounds and dissolution of the residue in C6D6,
1
the H NMR spectrum of 10e displayed singlet resonances at
1.35 and 0.04 ppm for the C5Me5 and SiMe3 groups. Highly
coupled multiplets characteristic of a -CH2CH2- unit (both
CH2 groups are diastereotopic methylene units) were present
at 1.86 and 1.77 ppm, (2H, Rh-CHH′), as well as at 1.09 and
1.01 ppm (2H, CHH′-Si). Aromatic resonances between 7.25
and 7.35 ppm (m, 4H) were also observed. The 13C NMR
spectrum of 10e also supports the assignment. Resonances for
-SiMe3 (-1.9 ppm, s) and C5Me5 (102.2 ppm, d; 8.9 ppm, s)
were easily identified. Resonances for the CH2CH2 fragment
were observed at 18.5 ppm (d) and 26.2 ppm (s). A large quartet
The reactions catalyzed by 5 were highly regioselective.
Reaction between 4-(dimethylamino)benzaldehyde and vinyl-
trimethylsilane (entry 6) produced only the linear isomer of the
ketone product, as opposed to the formation of both the linear
and branched products (2:1 ratio) when 4 was used as catalyst.
Both the linear and branched products were formed in the
reaction of 1-hexene with catalyst 5 (entry 9), but high selectivity
for the linear product was observed (22:1 linear:branched
product ratio), as compared to the 3:2 linear:branched ratio for
the same reaction catalyzed by 4.
assigned to the CF3 substituent was found at 126.1 ppm (JCF )
271 Hz), and the aromatic resonances were located at 124.0 (q,
JCF ) 2.0 Hz), 125.9. (q, JCF ) 32 Hz), 139.6 (s), and 162.8
ppm (d). The doublet resonance at 162.8 ppm suggested the
formation of an Rh-aryl bond and was confirmed by the
observation of a doublet resonance at 194.0 ppm, assigned to
the Rh-CO. No resonances for the branched isomer 10′e, Cpq-
Rh(CH(CH3)SiMe3)(C6H4CF3)(CO), could be detected (<ca.
5%, the detection limit). The formation of similar intermediates
was also observed for reactions with other aldehydes.
D. Identification of Catalyst Resting States. To identify
potential catalyst resting states for this process, the hydro-
acylation of vinyltrimethylsilane was performed under catalytic
conditions and monitored by 1H NMR spectroscopy. The
reaction of 5 mol % 4 with 4-(dimethylamino)benzaldehyde and
vinyltrimethylsilane at 110 °C resulted in the formation of a
new Cp*-containing species (1.54 ppm, s, 15H) while 4
decreased in intensity. This new complex (10a) persisted
throughout the initial stages of catalysis, but decreased at later
stages of the reaction while the amount of catalyst deactivation
products increased. A decrease in catalytic activity paralleled
the decrease in the concentration of 10a in the reaction mixture.
The presence of a -SiMe3 resonance in a 1:1 molar ratio with
The corresponding proposed Rh(III) resting state for hydro-
acylations catalyzed by 5 was synthesized via the reaction of 5
with 4-(dimethylamino)benzaldehyde at 50 °C (Scheme 1).
Again, species 11a easily underwent reductive elimination of
ketone, preventing clean isolation of the desired Rh(III)
1
intermediate. The H NMR spectrum of 11a generated in situ
revealed singlet resonances at 0.037 and 2.58 ppm for the SiMe3
and NMe2 functionalities. Four singlets appeared at 1.21, 1.28,
1.68, and 1.78 ppm for the inequivalent cyclopentadienyl methyl
groups, whereas 2 broad multiplets at 1.15 and 2.15 ppm were
assigned to the CH2 groups of the -CH2CH2SiMe3 unit bound
(54) Comparison of the signals of 10a with the analogous species, 10e, generated
from p-CF3C6H4CHO (see below) clearly establish 10a as Cp*(CH2CH2-
SiMe3)(C6H4NMe2)(CO). Traces of a second complex are evident (doublet
at 7.32 ppm, 2 overlapping doublets (1 corresponding to this second
complex, 1 to the silyl enol ether product) at 6.61 ppm, a singlet at 2.63
ppm for NMe2, and a singlet at 1.54 ppm for Cp*), which may be due to
the branched isomer 10′a, but this assignment is tentative.
(53) Although our standard conditions for comparison of substrates involves
the use of a 10:1 ratio of olefin:aldehyde, the single successful run
employing a 1:1 ratio of olefin:aldehyde (entry 2, Table 2) is likely to be
generally applicable to all olefins.
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A R T I C L E S
Roy et al.
Table 2. Hydroacylation of Olefins Catalyzed by 5
a Olefin:aldehyde ) 1:1. b Isolated yields are the average of 2 runs, unless otherwise noted. c GC yield.
Scheme 1
to rhodium. The aromatic protons exhibited doublet resonances
at 6.66 and 7.13 ppm.
substitution of an electron-withdrawing group on the Cp* ligand
shows a more pronounced effect on the CO stretching frequency,
as seen by the difference in frequency between 10d and 11d
(Figure 2).
The reaction between 4-(dimethylamino)benzaldehyde and
vinyltrimethylsilane (1:2 ratio) catalyzed by 5 (15 mol % relative
to aldehyde) was performed at 50 °C and monitored by 1H NMR
spectroscopy to identify the catalyst resting state(s) under
conditions where turnover is occurring, but at a sufficiently slow
rate that intermediates can be easily detected. Surprisingly, only
a trace amount of the Rh(III) species 11a is present in solution
after 6 h, even though hydroacylation product is slowly being
formed. The major Rh species present is the starting Rh(I) bis-
A series of resting state analogs for different aromatic
aldehydes was generated in situ and characterized by IR
spectroscopy. The νCO values for these resting state species are
shown in Figure 2. The para substituent of the Rh-bound phenyl
group appears not to strongly influence the CO stretch, as
suggested by the similar νCO values for both electron-donating
and electron-withdrawing substituents. However, the trend
shows a small increase in CO bond strength as the phenyl
substituent becomes more electron-deficient. As discussed
earlier, longer lifetimes and higher turnover numbers are
typically achieved with a more electron-rich substrate. The
9
2086 J. AM. CHEM. SOC. VOL. 129, NO. 7, 2007

Addition of Aromatic Aldehydes to Olefins
A R T I C L E S
In an NMR tube, complex 4 was dissolved in C6D6 and
combined with 20 equiv of 4-(dimethylamino)benzaldehyde-d1
and 20 equiv of vinyltrimethylsilane. The reaction mixture was
1
heated to 80 °C and followed by H NMR spectroscopy. As
hydroacylation was observed slowly under these conditions, a
1
1H aldehyde signal grew into the H NMR spectrum to about
20% of the normalized aldehyde resonance in the aryl region.
This observation suggests reversible aldehyde activation and
olefin insertion prior to slow, turnover-limiting reductive
elimination. Significantly, the -CH2- resonances of the gener-
ated ketone, H2NC6H4C(O)CH2CH2SiMe3, showed approxi-
mately equal integral areas and, as expected, a reduction in
integration due to deuterium incorporation. For example, after
3 days under these conditions only 60% intensity was observed
for each -CH2- group, which indicates a reversible H/D
exchange prior to ketone formation. The equal incorporation
of deuterium into both sites of the linear -CH2-CH2- chain
suggests that reversible 1,2 and 2,1 insertion occurs to scramble
D equally into the R and â sites prior to ketone formation (see
Scheme 2).
Figure 2. CO stretching frequencies for Rh(III) resting states 10 and 11.
Table 3. Dependence of Rh(I):Rh(III) Resting States on
Aldehyde:Olefin Ratioa
X
aldehyde:olefin
Rh(I):Rh(III)
5:11
CF3
1:2
5:1
17:1
2:1
4:10
CH3
1:2
5:1
4.5:1
0.8:1
a Typical reaction conditions: 140 mM aldehyde, 21 mM 4 or 5, 0.5
mL C6D6, 50 °C in a J Young tube. Reactions were monitored by 1H NMR
spectroscopy.
A similar reaction was performed with complex 5. In an NMR
tube, 20 equiv of 4-(dimethylamino)benzaldehyde-d1 and 20
equiv of vinyltrimethylsilane were added to a solution of 5 in
olefin complex 5. The ratio of the Rh(I):Rh(III) species was
studied as a function of the initial aldehyde:olefin ratio. This
data is summarized in Table 3 for both 5 and the less-
electrophilic complex 4. For each complex, the ratio of Rh-
(III):Rh(I) increases with an increase of the aldehyde:olefin ratio.
A comparison of the two systems under identical conditions
shows that the Rh(I):Rh(III) ratio is greater by a factor of 6-8
for the CF3-substituted complex 5. This outcome might be
expected because the stronger electron-withdrawing Cpq ligand
should favor Rh(I) over Rh(III) to a greater extent than the Cp*
ligand. Once product begins to appear, the ratios of the Rh(I):
Rh(III) species remain unchanged.
These experiments strongly suggest that the bis-olefin com-
plexes 4 and 5 are in equilibrium with the alkyl aryl carbonyl
Rh(III) species prior to product formation, as shown in eq 6.
The equilibration of these two rhodium species is further
examined and verified in the next section.
1
C6D6. The solution was heated at 50 °C and monitored by H
NMR spectroscopy. Again, a 1H aldehyde signal grew into the
1H NMR spectrum to about 30% of the normalized aldehyde
resonance, in agreement with reversible oxidative addition and
insertion observed for the reaction of 4. However, unlike the
reaction with 4, the -CH2- resonances of the generated ketone
did not integrate equally. Rather, the CH2 group R to the -SiMe3
group decreased in intensity more than the CH2 â to the -SiMe3
group. For example, as ketone formed after 4 days at 50 °C,
only 61% intensity was observed for the CH2 group R to
-SiMe3 whereas 91% intensity was observed for the CH2 group
â to -SiMe3. The lack of significant D incorporation into the
â site and the high selectivity for formation of the linear ketone
in the case of 5 (>99:1 linear:branched) indicates that, in
contrast to the catalytic reaction with 4 described above, little
2,1 insertion occurs prior to formation of linear ketone (see
Scheme 2).
Aldehyde exchange experiments were also performed to gain
insight about reversibility of the CO deinsertion step. Ten
equivalents of both 4-(dimethylamino)benzaldehyde (6a) and
p-tolualdehyde (6c) were added to 1 equiv of 4 in C6D6. The
reaction was performed at room temperature and monitored by
1H NMR spectroscopy. The formation of the corresponding
Cp*Rh(aryl)(CH2CH2SiMe3)(CO) complexes (10a,c) in a nearly
1:1 ratio was observed after 6 h, indicating no kinetic preference
for the formation of one product over the other (Scheme 3).
However, after 6 days, the tolualdehyde complex 10c was
favored over the corresponding (dimethylamino)benzaldehyde
complex (10a) by a ratio of 16:1, demonstrating the establish-
ment of an equilibrium between 4 and the Rh(III) intermediates
10a and 10c over time. In a second confirming experiment, a
1:15:1 ratio of 4:6a:6c was similarly monitored at 50 °C by 1H
NMR spectroscopy. Under these conditions, formation of the
Rh(III) (dimethylamino)benzaldehyde complex 10a was favored
at early reaction times due to the large excess of 4-(dimethyl-
amino)benzaldehyde in solution. However, after 4 days, a 1:1
ratio of the Cp*Rh(III) complexes was established, supporting
E. Investigation of Reversibility in the Hydroacylation
Mechanism. Resting states 10 and 11 are generated by a
sequence of reactions including C-H activation of the aldehyde,
olefin insertion into the Rh-H bond, and CO deinsertion from
the resulting rhodium acyl complex. Reactions between vinyl-
trimethylsilane and 4-(dimethylamino)benzaldehyde-d1 were
performed with both 4 and 5 to gain mechanistic information
about these three early steps of the catalytic cycle.
9
J. AM. CHEM. SOC. VOL. 129, NO. 7, 2007 2087

A R T I C L E S
Scheme 2
Roy et al.
Scheme 3
Scheme 4
there is a substantial resonance interaction of the Me2N group
with the carbonyl functionality in the transition state for
formation of complexes 10a,c from 4. In other words, para-
substituent effects on stabilities are similar for the aldehydes
6a,c and for the transition states for formation of 10a,c. A
transition state involving C-H addition to the Rh center (12,
Figure 3) is consistent with this view.
F. Reactivity of Resting State 10/11 with PMe3. We next
investigated the reactivity of intermediates of type 10/11 with
a trapping ligand to learn more about the reductive elimination
of ketone. Complex 10d was generated in situ by the reaction
of benzaldehyde with 4 (ratio 1:1.1) in toluene-d8. Excess
benzaldehyde was removed under vacuum and the isolated
complex 10d used without further purification. Ten equivalents
of PMe3 were added to a solution of 10d in C6D6 at room
temperature, and the reaction was followed by NMR spectros-
copy. Reversible CO insertion (see kinetic studies below)
generates an acyl aryl intermediate 13 that is rapidly trapped
by PMe3 to readily form the acyl-aryl complex 15d at room
temperature (eq 7).
the reversibility of the CO deinsertion step, along with the
oxidative addition and olefin insertion steps.
That 10c is favored over 10a in the equilibrium described in
Scheme 3 implies that K1 is greater than K2 in the equilibria
illustrated in Scheme 4.
The results in Scheme 4 are easily rationalized by considering
substituent effects on both the free aldehydes and rhodium aryl
species, 10a/10c. Aldehyde 6a is strongly stabilized through a
resonance interaction of the dimethylamino group with the
aldehyde functionality whereas no such stabilization is present
in tolualdehyde. In the rhodium aryl complex 10a, no equivalent
resonance stabilization is possible, thus the fact that K2 < K1
can be largely attributed to strong resonance stabilization of
6a. The observation that there is no kinetic preference for
formation of 10a vs 10c suggests that, as in the ground state,
Orange crystals of 15d were obtained from the reaction
mixture after cooling to -30 °C (Figure 4). The regiochemistry
Figure 3. Transition state structure for C-H bond activation step.
9
2088 J. AM. CHEM. SOC. VOL. 129, NO. 7, 2007

Addition of Aromatic Aldehydes to Olefins
A R T I C L E S
6H). The signals corresponding to the CH2 groups of 11c
disappeared and were replaced by highly coupled resonances
at 3.35 (1H), 2.50 (1H), and 1.20 (1H) ppm, with the fourth
resonance hidden underneath another peak. The appearance of
a new doublet at 1.05 ppm (d, J ) 9.6 Hz) indicates a rhodium-
bound PMe3 ligand.
Rate constants for CO insertion and subsequent trapping of
the resulting intermediate by PMe3 were measured by 1H NMR
spectroscopy at 25 °C. Complex 11c was generated in situ from
the reaction between 5 (0.035 M) and p-tolualdehyde (0.42 M)
in C6D6, with 1,3,5-trimethoxybenzene present as internal
standard. Once complete conversion of 5 to 11c was observed
by 1H NMR spectroscopy, 5 or 10 equiv of PMe3 (0.18 or 0.36
M) were added to the NMR tube, and the reaction to form 16c
was monitored by 1H NMR spectroscopy at 25 °C. Clean first-
order kinetics were observed with measured rate constants of
0.50 × 10-4 s-1 and 1.1 × 10-4 s-1 for reactions with 5 and
10 equiv PMe3, respectively. The observed first-order depen-
dence of rate on [PMe3] show that CO insertion is fast and
reversible, followed by rate-determining trapping by PMe3.
Identical reactions were performed with 10c, generated in situ
by reaction between 4 and p-tolualdehyde. Rate constants of
0.83 × 10-4 s-1 and 1.5 × 10-4 s-1 were measured for the
reaction to form 15c with 5 and 10 equiv of PMe3, respectively.
Figure 4. ORTEP diagram of complex 15d; ellipsoids are drawn with 50%
probability. Selected bond distances (Å) and angles (deg) are provided in
Table 4.
Table 4. Selected Bond Distances and Angles for 15d
bond lengths (Å)
bond angles (deg)
Rh-P
2.2534(3)
P-Rh-C2
86.26(4)
88.21
88.07(5)
126.62(10)
116.14(9)
117.23(11)
111.96(9)
126.84(10)
118.01(10)
Rh-C2
Rh-C11
O-C2
2.0183(13)
2.0636(12)
1.2203(16)
1.8710(14)
P-Rh-C11
C2-Rh-C11
Rh-C2-O
C4-Si
Rh-C2-C3
O-C2-C3
Si-C4-C3
Rh-C11-C12
Rh-C11-C16
of the proposed structure for 15d was confirmed by X-ray
crystallography. Insertion of CO into the rhodium-alkyl bond,
not the rhodium-aryl bond, is clearly observed in the crystal
structure. Selected bond distances and angles for 15d are
1
provided in Table 4. Key H NMR data for 15d include a
The formation of 15c and 16c must involve rapid and
reversible formation of acyl aryl complexes 13 and 14 followed
by rate-determining trapping with PMe3. In general, CO
insertions into Rh-C(sp3) bonds are slow compared to insertion
into Rh-C(sp2) bonds55 and thus it seems likely that carbonyl
complexes 10c, 11c are in rapid equilibrium with both the aryoyl
alkyl species 19, 20 and the acyl aryl species 13, 14. The latter
species are then selectively trapped by PMe3 to yield 15c, 16c
as shown in Scheme 5. Supporting the contention that 10c, 11c
are in equilibrium with 19, 20 are the prior experiments showing
that in the presence of VTMS 10c, 11c are in equilibrium with
bis-vinyl TMS complexes 4, 5 (via return through 19, 20) prior
to ketone formation.
doublet resonance at 1.03 ppm for the rhodium-bound PMe3
ligand, and four highly coupled multiplets at 1.18, 1.31, 2.69,
and 3.43 ppm corresponding to the two sets of diastereomeric
methylene protons of C(O)CH2CH2SiMe3. The multiplets at 2.69
and 3.43 ppm are shifted significantly downfield compared to
the Cp*Rh(CH2CH2SiMe3)(Ph)(CO) precursor 10d due to the
insertion of CO into the rhodium alkyl bond. The 13C NMR
spectrum displays a doublet for the rhodium-bound PMe3 ligand
at 14.85 ppm, and singlets at 13.47 and 51.75 ppm for the CH2
groups of the acyl ligand. The resonance at 51.75 ppm is again
shifted downfield compared to 10d due to the insertion of CO
into the rhodium alkyl bond.
The analogous intermediate with the more electron-deficient
catalyst 5 was also prepared. Complex 11c was generated in
situ by the reaction of 10 equiv of p-tolualdehyde with 5 at
room temperature in C6D6. Excess aldehyde was needed to
increase the rate of oxidative addition at room temperature, but
was then not able to be separated from 11c. Five equivalents
of PMe3 were added to the solution of 11c, and the formation
of PMe3-trapped 16c at room temperature was monitored by
1H and 31P NMR spectroscopy (eq 7). 31P NMR spectroscopy
showed the appearance of a doublet at 14.0 ppm (J ) 184.5
Hz), indicating a new rhodium-bound phosphine, as the peak
corresponding to free PMe3 decreased in intensity. The disap-
pearance of signals corresponding to the four cyclopentadienyl
Reactions between excess PMe3 and 15c or 16c at 50 °C were
performed to study the reductive elimination of ketone. Rate
constants for the reductive elimination of ketone from 15/16
1
and trapping with PMe3 to form 17/18 were measured by H
NMR spectroscopy at 50 °C by simply heating the reaction
mixtures that resulted from the kinetic studies of the CO
insertion step. Data is summarized in Table 5, column 6. Clean
first-order kinetics were observed with measured rate constants
of 2.0 × 10-5 and 2.6 × 10-5 s-1 for the reductive elimination
of ketone from 16c with 5 and 10 equiv PMe3, respectively.
The lack of dependence of the rate of reductive elimination on
[PMe3] suggests that C-C bond formation is the rate-determin-
ing step of the reductive elimination, followed by rapid loss of
ketone and formation of bis-PMe3 complex 18.
1
methyl groups was observed by H NMR spectroscopy, along
with the appearance of new cyclopentadienyl methyl signals at
1.92 (br d, 3H), 1.79 (br d, 3H), and 1.53 ppm (d, J ) 1.6 Hz,
(55) Luo, X.; Tang, D.; Li, M. THEOCHEM 2005, 730, 177-183.
9
J. AM. CHEM. SOC. VOL. 129, NO. 7, 2007 2089

A R T I C L E S
Scheme 5
Roy et al.
Scheme 6
elimination from the analogous complex with the Cpq ligand
(16a) could not be made because 16a underwent reductive
elimination of ketone at room temperature, and thus 16a could
not be cleanly isolated for subsequent studies. However, the
fact that 16a underwent reductive elimination at room
temperature, whereas 15a did not, indicates that 16a should have
a significantly lower barrier for reductive elimination than 15a.
This conclusion is supported by the observation that 16c
underwent reductive elimination at a measurable rate at 50 °C,
while 15c did not. Thus, the substitution of one CF3 group for
a CH3 group of the cyclopentadienyl ligand significantly
increases the rate of reductive elimination of ketone, as expected
based on a more electron-deficient rhodium center in the case
of the CF3-substituted system.
Table 5. Rate Constants for CO Insertion and Reductive
Elimination Steps
[PMe3]
(M)
k1
(×
104 s-1),
25
kRE
(
×
105 s-1),
50
compound
X
R
°
C
°C
15a
CH3
NMe2
0.18
0.36
0.18
0.36
0.18
0.36
0.18
0.36
N/A
N/A
N/A
N/A
0.83
1.5
0.33
0.33
16a
CF3
CH3
CF3
NMe2
CH3
fast at 25 °C
fast at 25 °C
very slow at 50 °C
very slow at 50 °C
2.0
10c/15c
11c/16c
CH3
0.50
1.1
2.6
Accurate rate constants for the reductive elimination of ketone
from 15c could not be measured due to negligible formation of
ketone from 15c at 50 °C. Instead, the rate of reductive
elimination of ketone from 15a, with a dimethylamino sub-
stituent on the aryl group, was measured. Rate constants of 0.33
× 10-5 s-1 were measured for reactions of 15a with 5 and 10
equiv of PMe3. A direct comparison to the rate of reductive
G. Summary of the Proposed Catalytic Cycle for Rh-
Catalyzed Hydroacylation of Olefins by Aryl Aldehydes. The
results described above are consistent with the mechanism
illustrated in Scheme 6 for hydroacylation of vinyltrimethylsi-
lane. Initial dissociation of olefin forms a 16-electron intermedi-
9
2090 J. AM. CHEM. SOC. VOL. 129, NO. 7, 2007

Addition of Aromatic Aldehydes to Olefins
A R T I C L E S
ate that rapidly binds aldehyde to form a reactive Rh aldehyde
olefin complex. Oxidative addition of the aldehyde C-H bond
is fast and reversible to form a Rh(III) acyl hydride intermediate
(step A). Olefin insertion can then occur with either regiochem-
istry to generate a linear (step B) or branched (step B′) alkyl-
acyl Rh intermediate. In the case of the Cp*-based system, steps
A and B,B′ must be reversible to account for loss of deuterium
from deuterated aldehyde and for the pattern of deuterium
labeling in the Rh(III) alkyl aryl carbonyl complex 10. For the
Cpq-based system, formation of the linear isomer 11 (via 20) is
fast and reversible, but the D labeling pattern suggests that the
barrier for formation of the branched isomer, 20′, is substantially
higher and that product formation occurs with a lower barrier
than formation of 11′, leading to high selectivity for the linear
ketone.
similar catalytic cycle applies to reactions of other olefins in
Table 2. However, the relative rates of interconversion of species
and the relative stabilities of the resting states will surely change
with the nature of the olefin. It is interesting to note that a similar
trend for regioselectivity in product formation is observed in
the case of 1-hexene. The Cpq-derived catalyst 5 yields highly
linear product (22:1, linear:branched), whereas the Cp*-based
catalyst 4 is unselective (3:2 linear:branched ketone).
Experimental Section
General Considerations. All manipulations were carried out using
standard Schlenk and glovebox techniques. Argon was purified by
passage through columns of BASF R3-11 (Chemalog) and 4 Å
molecular sieves. All solvents used for synthesis, except tetrahydrofuran,
were deoxygenated and dried via passage over a column of activated
alumina.56 Tetrahydrofuran was distilled from sodium benzophenone
ketyl prior to use. NMR spectra were recorded on Bruker DRX 400
and AMX 300 MHz instruments and are referenced to residual protio
solvent. 31P NMR chemical shifts are referenced to an external 85%
H3PO4 standard. GC analysis was performed on an Agilent 6850 Series
GC with a dimethylpolysiloxane column (Agilent HP-1). Elemental
analyses were performed by Atlantic Microlabs, Inc., of Norcross, GA.
[C5Me5Rh(C2H3SiMe3)2], (1,2,3,4)-Me4-5-CF3-cyclopentadiene (CpqH),
[CpqRhCl2]2, and 4-(dimethylamino)benzaldehyde-d1 were prepared
following reported procedures.47,53,57 All aldehydes and olefins listed
were purchased from Aldrich. Aldehydes were purified via recrystal-
lization or distillation before use, and olefins were dried and distilled
prior to use. Benzene-d6 and toluene-d8 were dried over potassium
benzophenone ketyl, vacuum transferred, and degassed. Acetone-d6 and
acetophenone-d8 were dried over CaH2, vacuum transferred and
degassed.
The deinsertion of CO in step C generates Rh(III) complexes
10/11, one of the dual resting states observed during the reaction.
Steps A, B, and C, leading to the formation of 10/11, must all
be reversible to explain the results of the aldehyde exchange
experiments. CO insertion then occurs reversibly in step D to
generate the rhodium acyl aryl intermediate that then reductively
eliminates ketone in the turnover-limiting step of the reaction.
The increased activity of catalyst 5, relative to 4, for the
hydroacylation of olefins is attributed to an increase in the rate
of reductive elimination for complexes with the more electron-
withdrawing Cpq ligand. While only very minor amounts of
branched Cp*Rh(CH2CH2SiMe3)(aryl)(CO) complex 10′ are
present, a significant fraction of the ketone product (ca 33%) is
branched, suggesting that reductive elimination from the
branched Cp*Rh(alkyl)(aryl)(CO) isomer 10′ (which is in
equilibrium with the linear isomer, 10) occurs considerably faster
than reductive elimination from the linear isomer.
Synthesis of CpqRh(H2CdCH-SiMe3)2 (5). Under an argon
atmosphere, 995 mg (1.37 mmol) of [CpqRhCl2]2 and 1.82 g (27.8
mmol) of Zn powder were weighed into an oven-dried Schlenk flask.
THF (25 mL) was added to the flask via syringe, followed by 4.0 mL
(27 mmol) of vinyltrimethylsilane. The red suspension was stirred
vigorously overnight. The resulting brown suspension was filtered
through celite to remove insoluble zinc species, and THF was
evaporated under vacuum. The product was extracted the pentane, and
the pentane was evaporated under vacuum to yield 1.10 g (81% yield)
of the clean, yellow/orange solid. This solid can also be recrystallized
Several additional points should be made concerning this
scheme: (1) Product ketones could form from the reductive
elimination from the alkyl aryoyl complexes.19,20 However, such
reductive elimination would require C(sp3)-C(sp2) coupling
which should be less favorable than C(sp2)-C(sp2) coupling
required for product formation from 13,14. Thus, we favor
product formation from 13,14. (2) The structures of the Rh(III)
acyl and aryoyl complexes 13,14 and 19,20 are depicted as 16-
electron species. However, they may well exist as 18-electron
η2 acyl complexes. We do not have experimental data to
distinguish these possibilities. (3) In addition to the fact that
the turnover frequency is greater for the more electron-deficient
Cpq-based system, the achievable turnover numbers are also
higher for the Cpq systems. This implies that the catalyst decay
rate relative to the turnover frequency must be greater for the
Cp* system. Catalyst decay occurs via decarbonylation and
formation of arene. Decarbonylation must involve reductive
elimination of aryl hydride from a Rh(I) center. However, the
mechanistic details of this decarbonylation process and the rate-
limiting step are not known, thus it is unclear why product
formation, relative to decarbonylation, is more favored in the
case of Cpq catalysts. (4) Despite the fact that between the
equilibrating resting states (4/5 with 10/11) the Rh(III) alkyl
aryl carbonyl species (10,11) are more populated for the
electron-deficient aldehydes, these aldehydes yield fewer total
turnovers. This must arise from some combination of more rapid
decarbonylation (catalyst death) and more sluggish reductive
elimination due to a stronger Rh-aryl bond. (5) It is likely a
1
from acetone at -78 °C. H NMR (CD2Cl2): δ 0.082 (s, 18H), 1.35
(br s, 1H), 1.38 (br s, 1H), 1.48-1.50 (br m, 1H), 1.51-1.53 (br m,
1H), 1.56-1.57 (3H), 1.71 (s, 3H), 1.74-1.75 (br, 3H), 2.03 (s, 3H),
2.54 (dd, J ) 11.2 Hz, 2.4 Hz, 2H). 13C NMR (acetone-d6): δ 2.17,
7.91 (br q, J ) 1.4 Hz), 8.61, 9.93 (br q, J ) 1.6 Hz), 10.95, 49.13 (d,
J ) 13.6 Hz), 49.56 (d, J ) 13.8 Hz), 90.49 (q of d, J ) 31.3 Hz, 6.1
Hz), 96.10, 98.20, 102.04, 104.25, 126.80 (q, J ) 269.1 Hz). Anal.
Calcd for C20H36F3Si2Rh: C, 48.76; H, 7.38. Found: C, 49.07; H, 7.63.
General Procedure for the Synthesis of Intermediates 10, 11. In
the drybox, rhodium bis-olefin complex 4 or 5 was weighed into a
flame-dried Kontes flask. The flask was removed from the box and
purged with argon on the Schlenk line. Anhydrous toluene (10 mL)
was added, along with 1.1 equiv of the desired aldehyde. The reaction
mixture was stirred at room temperature and was monitored periodically
by TLC and/or 1H NMR spectroscopy. Once the reaction was finished,
the volatiles were removed under vacuum and the residue dried under
vacuum overnight. Attempts to purify the products by recrystallization
(56) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers,
F. J. Organometallics 1996, 15, 1518-1520.
(57) Barletta, G. L.; Zou, Y.; Huskey, W. P.; Jordan, F. J. Am. Chem. Soc.
1997, 119, 2356-2362.
(58) Akhrem, I. S.; Churilova, I. M.; Orlinkov, A. V.; Afanas’eva, L. V.; Vitt,
S. V.; Petrovskii, P. V. Russ. Chem. Bull. 1998, 47 (5), 918-923.
(59) Fausett, B. W.; Liebeskind, L. S. J. Org. Chem. 2005, 70 (12), 4851-
4853.
9
J. AM. CHEM. SOC. VOL. 129, NO. 7, 2007 2091

A R T I C L E S
Roy et al.
1
or by chromatography failed, so the product was characterized by H
and 13C NMR and by IR spectroscopy with small amounts of residual
starting materials or ketone (from reductive elimination) present.
Cp*Rh(CH2CH2SiMe3)(CO)(4-NMe2-C6H4) (10a). Cp*Rh(VTMS)2
(84 mg, 0.19 mmol) and 4-(dimethylamino)benzaldehyde (31 mg, 0.21
mmol) were employed to prepare 10a. 1H NMR (C6D6): δ 0.064 (s,
SiMe3), 1.73 (dt, J ) 13.6, 4.4 Hz, 1H), 1.17-1.22 (m, 1H), 1.51 (s,
15H), 1.75-1.86 (m, 1H), 1.90-2.10 (m, 1H), 2.63 (s, 6H), 6.68 (d, J
) 8.8 Hz, 2H), 7.16 (d, 8.0 Hz, 2H).
CpqRh(CH2CH2SiMe3)(CO)(C6H5) (11d). CpqRh(VTMS)2 (81 mg,
0.17 mmol) and benzaldehyde (19 µL, 0.19 mmol) were employed to
prepare 11d. 1H NMR (C6D6): δ -0.001 (s, 9H), 1.04-1.17 (m, 2H),
1.14 (s, 3H), 1.22 (s, 3H), 1.60 (s, 3H), 1.74 (s, 3H), 2.05-2.11 (m,
2H), 7.01 (t, J ) 7.2 Hz, 1H), 7.06 (t, J ) 7.6 Hz, 2H), 7.23 (d, J )
7.2 Hz, 2H). 13C NMR (C6D6): δ -1.89 (s, SiMe3), 8.14 (s), 9.43 (s),
9.88 (s), 10.48 (s), 20.31 (d, J ) 23.2 Hz, Rh-Calkyl), 26.32 (s), 91.60
(q, J ) 33.9 Hz, C-CF3), 102.66 (s), 103.47 (s), 106.77 (s), 109.18 (s),
124.13 (s), 125.67 (q, J ) 270.6 Hz, CF3), 128.75 (s), 138.77 (s), 150.30
(d, J ) 34.6 Hz, Rh-Caryl), 192.25 (d, J ) 83.9 Hz, CO). IR: 2016
Cp*Rh(CH2CH2SiMe3)(CO)(4-OMe-C6H4) (10b). Cp*Rh(VTMS)2
(79 mg, 0.18 mmol) and 4-methoxybenzaldehyde (24 µL, 0.20 mmol)
were employed to prepare 10b. 1H NMR (C6D6): δ 0.054 (s, 9H), 1.05
(dt, J ) 14.0 Hz, 4.4 Hz, 1H), 1.17 (dt, J ) 13.6 Hz, 4.4 Hz, 1H), 1.48
(s, 15H), 1.76-1.84 (m, 1H), 1.92-1.99 (m, 1H), 3.43 (s, 3H), 6.83
(d, J ) 6.8 Hz, 2H), 7.17 (d, J ) 7.2 Hz, 2H). 13C NMR (C6D6): δ
-1.78 (s, SiMe3), 9.08 (s, C5Me5), 18.00 (d, J ) 24.1 Hz, Rh-Calkyl),
54.62 (s, OMe), 101.99 (s, C5Me5), 114.84 (s), 139.05 (s), 141.40 (d,
J ) 36.2 Hz, Rh-Caryl), 157.61 (s), 194.62 (d, J ) 81.7 Hz, CO). IR:
cm-1
.
Synthesis of Cp*Rh(C(O)CH2CH2SiMe3)(PMe3)(Ph) (15d). Into
a flame-dried Schlenk flask was weighed Cp*Rh(VTMS)2 (292.0 mg,
0.666 mmol). Anhydrous toluene (8 mL) and freshly distilled benzal-
dehyde (84 µL, 0.821 mmol) were added via syringe, and the mixture
was stirred under argon at room temperature for about a week. Volatiles
were evaporated under vacuum to leave brown oil. PMe3 (0.65 mL,
6.28 mmol) and 5 mL of toluene were added, and the mixture was
stirred at room temperature overnight. Volatiles were evaporated and
1994 cm-1
.
1
the resulting brown oil was dried under vacuum. H NMR (C6D6): δ
Cp*Rh(CH2CH2SiMe3)(CO)(4-CH3-C6H4) (10c). Cp*Rh(VTMS)2
(79 mg, 0.18 mmol) and 4-tolualdehyde (24.5 µL, 0.20 mmol) were
employed to prepare 10c. 1H NMR (C6D6): δ 0.057 (s, 9H), 1.06 (dt,
J ) 14.4 Hz, 4.0 Hz, 1H), 1.18 (dt, J ) 13.6 Hz, 4.4 Hz, 1H), 1.48 (s,
15H), 1.78-1.87 (m, 1H), 1.93-2.10 (m, 1H), 2.23 (s, 3H), 6.99 (d, J
) 7.6 Hz, 2H), 7.24 (d, J ) 6.4 Hz, 2H). 13C NMR (C6D6): δ -1.78
(s, SiMe3), 9.08 (s, C5Me5), 18.05 (d, J ) 24.2 Hz, Rh-Calkyl), 20.97
(s, tolyl CH3), 26.03 (s), 101.97 (s, C5Me5), 129.35 (s), 132.14 (s),
138.95 (s), 149.44 (d, J ) 35.6 Hz, Rh-Caryl), 194.61 (d, J ) 81.8 Hz,
0.071 (s, 9H), 1.03 (d, J ) 11.6 Hz, 9H), 1.18 (m, 1H), 1.31 (qd, J )
5.2, 12.0, 15.2 Hz, 1H), 1.61 (d, J ) 2.0 Hz, 15H), 2.69 (qd, J ) 4.8,
11.6, 16.8 Hz, 1H), 3.43 (qd, J ) 4.4, 11.6, 16.4 Hz, 1H), 6.98 (t, J )
6.8 Hz, 1H), 7.06 (t, J ) 7.2 Hz, 2H), 7.25 (d, J ) 8.0 Hz, 2H). 13C
NMR (C6D6): δ -1.69 (s, SiMe3), 10.37 (s, C5Me5), 13.47 (s, CH2),
14.85 (d, J ) 32.9 Hz, PMe3), 51.75 (s, CH2), 100.50 (t, J ) 9.2 Hz,
C5Me5), 121.76 (s), 126.71 (s), 140.84 (s), 156.5 (dd, JRh-C, JP-C )
20.3, 18.5 Hz), 199.30 (d, J ) 63.6 Hz, CO).
General Procedure for the Reaction of Aromatic Aldehydes with
Olefins. The reaction conditions and average yields for each reaction
are shown in Table 2. A typical procedure is given for the first entry
in Table 2.
CO). IR: 1995 cm-1
.
Cp*Rh(CH2CH2SiMe3)(CO)(C6H5) (10d). Cp*Rh(VTMS)2 (78 mg,
0.18 mmol) and benzaldehyde (20 µL, 0.20 mmol) were employed to
prepare 10d. 1H NMR (C6D6): δ 0.047 (s, 9H), 1.05 (dt, J ) 14.4 Hz,
4.0 Hz, 1H), 1.17 (dt, J ) 13.6 Hz, 4.8 Hz, 1H), 1.45 (s, 15H), 1.78-
1.86 (m, 1H), 1.92-2.10 (m, 1H), 7.05 (t, J ) 6.8 Hz, 1H), 7.13 (t, J
) 7.6 Hz, 2H), 7.33 (d, J ) 7.2 Hz, 2H). 13C NMR (C6D6): δ -1.78
(s, SiMe3), 9.04 (s, C5Me5), 18.12 (d, J ) 23.9 Hz, Rh-Calkyl), 26.09
(s), 102.00 (s, C5Me5), 123.49 (s), 128.36 (s), 139.25 (s), 154.35 (d, J
4-NMe2-C6H4-C(O)CH2CH2SiMe3.40 Into a thick-walled 4 mL
flask were weighed 123 mg (0.821 mmol) of 4-(dimethylamino)-
benzaldehyde and 20.2 mg (0.0410 mmol) of CpqRh(VTMS)2. Subse-
quently, 1.2 mL (8.2 mmol) of vinyltrimethylsilane and 1.2 mL of
toluene were added to the flask via syringe. The flask was sealed with
a Kontes Teflon screw cap and placed in a 100 °C bath. The solution
was stirred at 100 °C until the aldehyde was completely consumed, as
determined by GC. After evaporation of the solvent, the residue was
purified by chromatography on silica gel (100% hexanes to elute small
brown band, then hexanes:ethyl acetate ) 9:1 to elute colorless product)
to yield 164 mg (80% yield) of 4-NMe2-C6H4-C(O)CH2CH2SiMe3
) 35.5 Hz, Rh-Caryl), 194.53 (d, J ) 81.9 Hz, CO). IR: 1998 cm-1
.
Cp*Rh(CH2CH2SiMe3)(CO)(4-CF3-C6H4) (10e). Cp*Rh(VTMS)2
(102 mg, 0.23 mmol) and 4-trifluoromethylbenzaldehyde (34 µL, 0.25
1
mmol) were employed to prepare 10e. H NMR (C6D6): δ 0.040 (s,
9H), 1.01 (dt, J ) 14.0 Hz, 4.7 Hz, 1H), 1.09 (dt, J ) 14.0 Hz, 4.8 Hz,
1H), 1.35 (s, 15H), 1.77 (m, 1H), 1.86 (m, 1H), 7.31 (m, 4H). 13C
NMR (C6D6): δ -1.87 (s, SiMe3), 8.86 (s, C5Me5), 18.50 (d, J ) 23
Hz, Rh-Calkyl), 26.20 (s), 102.16 (s, C5Me5), 123.95 (q, J ) 2.0 Hz),
125.89 (q, J ) 32 Hz, Caryl-CF3), 126.13 (q, J ) 271 Hz, CF3), 139.56
(s), 162.8 (d, J ) 37 Hz, Rh-Caryl), 194.0 (d, J ) 81 Hz, CO). IR:
1
as a white solid. H NMR (CDCl3): δ 0.030 (s, 9H), 0.85-0.90 (m,
2H), 2.80-2.85 (m, 2H), 3.04 (s, 6H), 6.69 (d, J ) 9.2 Hz, 2H), 7.86
(d, J ) 9.2 Hz, 2H). 13C NMR (CD2Cl2): δ -1.66 (3C), 11.70, 32.57,
40.18 (2C), 110.96 (2C), 125.05, 130.36 (2C), 153.64, 199.33.
4-NMe2-C6H4-C(O)(C5H9). 4-NMe2-C6H4CHO (100 mg, 0.674
mmol), cyclopentene (0.60 mL, 6.79 mmol), CpqRh(VTMS)2 (16.6 mg,
0.0337 mmol), and 0.6 mL of toluene was used, and the solution was
stirred at 75 °C. The crude product was purified by chromatography
on silica gel (100% hexanes to elute small brown band, then hexanes:
ethyl acetate ) 9:1 to elute colorless product) to give 139 mg (95%
2000 cm-1
.
CpqRh(CH2CH2SiMe3)(CO)(4-NMe2-C6H4) (11a). CpqRh(VTMS)2
(8.3 mg, 0.017 mmol) and 4-(dimethylamino)benzaldehyde (24 mg,
1
0.16 mmol) were employed to prepare 11a in situ. H NMR (C6D6):
δ 0.037 (s, 9H), 1.15 (m, 2H), 1.21 (s, 3H), 1.28 (s, 3H), 1.68 (s, 3H),
1.78 (s, 3H), 2.15 (m, 2H), 2.58 (s, 6H), 6.66 (d, J ) 8.8 Hz, 2H),
7.13 (d, 2H; half of doublet overlaps with residual solvent peak, so no
J value calculated).
1
yield) of 4-NMe2-C6H4-C(O)C5H9 as a white solid. H NMR (CD2-
Cl2): δ 1.62-1.69 (m, 4H), 1.81-1.88 (m, 4H), 3.04 (s, 6H), 3.64
(quintet, J ) 7.6 Hz, 1H), 6.67 (d, J ) 7.2 Hz, 2H), 7.86 (d, J ) 6.8
Hz, 2H). 13C NMR (CD2Cl2): δ 26.71 (2C), 30.55 (2C), 40.17 (2C),
45.90, 110.95 (2C), 125.17, 130.69 (2C), 153.61, 200.78. Anal. Calcd
for C14H19NO: C, 77.36; H, 8.83; N, 6.45. Found: C, 77.12; H, 8.86;
N, 6.50.
4-MeO-C6H4-C(O)(C5H9).58 4-MeO-C6H4CHO (80 µL, 0.658
mmol), cyclopentene (0.60 mL, 6.79 mmol), CpqRh(VTMS)2 (22.7 mg,
0.0461 mmol), and 0.6 mL toluene were used, and the solution was
stirred at 75 °C. The crude product was purified by chromatography
on silica gel (100% hexanes to elute small brown band, then hexanes:
ethyl acetate ) 9:1 to elute colorless product) to give 124 mg (93%
CpqRh(CH2CH2SiMe3)(CO)(4-Me-C6H4) (11c). CpqRh(VTMS)2
(100 mg, 0.20 mmol) and 4-tolualdehyde (28 µL, 0.23 mmol) were
1
employed to prepare 11c. H NMR (C6D6): δ 0.055 (s, 9H), 1.12-
1.22 (m, 2H, CH2), 1.21 (s, 3H), 1.29 (s, 3H), 1.68 (s, 3H), 1.80 (s,
3H), 2.10-2.16 (m, 2H, CH2), 2.23 (s, 3H), 6.97 (d, J ) 7.6 Hz, 2H),
7.19 (d, J ) 8.0 Hz, 2H). 13C NMR (C6D6): δ -1.89 (s, SiMe3), 8.17
(s), 9.47 (s), 9.93 (s), 10.49 (s), 20.20 (d, J ) 23.3 Hz, Rh-Calkyl),
26.27 (s), 91.52 (q, J ) 34.5 Hz, C-CF3), 102.94 (s), 103.46 (s), 106.73
(s), 109.15 (s), 125.71 (q, J ) 270.5 Hz, CF3), 129.79 (s), 132.99 (s),
138.47 (s), 145.55 (d, J ) 34.7 Hz, Rh-Caryl), 192.36 (d, J ) 82.2 Hz,
CO).
9
2092 J. AM. CHEM. SOC. VOL. 129, NO. 7, 2007

Addition of Aromatic Aldehydes to Olefins
A R T I C L E S
yield) of 4-MeO-C6H4-C(O)(C5H9) as a white solid. 1H NMR (CD2-
Cl2): δ 1.63-1.72 (m, 4H), 1.83-1.91 (m, 4H), 3.67 (quintet, J ) 8.4
Hz, 1H), 3.86 (s, 3H), 6.95 (d, J ) 7.2 Hz, 2H), 7.95 (d, J ) 7.2 Hz,
2H). 13C NMR (CD2Cl2): δ 26.66 (2C), 30.38 (2C), 46.36, 55.82,
4-NMe2-C6H4-C(O)CH2CH2CH2CH2CH2CH3. 4-NMe2-C6H4-
CHO (124 mg, 0.832 mmol), 1-hexene (1.2 mL, 9.6 mmol), CpqRh-
(VTMS)2 (20.5 mg, 0.0416 mmol), and 1.2 mL toluene were used,
and the solution was stirred at 100 °C. The crude product was purified
by chromatography on silica gel (100% hexanes to elute small brown
band, then hexanes:ethyl acetate ) 9:1 to elute colorless product) to
give 173 mg (89% yield) of 4-NMe2-C6H4-C(O)CH2CH2CH2CH2-
113.94 (2C), 130.37, 130.89 (2C), 163.58, 201.36.
59
4-Me-C6H4-C(O)(C5H9).58,
4-Me-C6H4CHO (78 µL, 0.662
mmol), cyclopentene (0.60 mL, 6.79 mmol), CpqRh(VTMS)2 (22.8 mg,
0.0463 mmol), and 0.6 mL toluene were used, and the solution was
stirred at 75 °C. The crude product was purified by chromatography
on silica gel (100% hexanes to elute small brown band, then hexanes:
ethyl acetate ) 9:1 to elute colorless product) to give 101 mg (81%
1
CH2CH3 as a white solid. H NMR (CD2Cl2): δ 0.89 (m, 3H), 1.26-
1.40 (m, 6H), 1.67 (quintet, J ) 7.6 Hz, 2H), 2.86 (t, J ) 7.6 Hz, 2H),
3.04 (s, 6H), 6.66 (d, J ) 9.2 Hz, 2H), 7.84 (d, J ) 8.8 Hz, 2H). 13C
NMR (CD2Cl2): δ 14.25, 23.00, 25.31, 29.59, 32.17, 38.23, 40.21,
110.95 (2C), 125.50, 130.37 (2C), 153.71, 198.69. Anal. Calcd for
C15H23NO: C, 77.19; H, 9.95; N, 6.00. Found: C, 76.90; H, 9.98; N,
6.06.
1
yield) of 4-Me-C6H4-C(O)(C5H9) as a white solid. H NMR (CD2-
Cl2): δ 1.62-1.72 (m, 4H), 1.82-1.94 (m, 4H), 2.40 (s, 3H), 3.69
(quintet, J ) 7.2 Hz, 1H), 7.26 (d, J ) 8.0 Hz, 2H), 7.84 (d, J ) 8.0
Hz, 2H). 13C NMR (CD2Cl2): δ 21.65, 26.65 (2C), 30.31 (2C), 46.60,
128.80 (2C), 129.51 (2C), 134.78, 143.84, 204.84.
Representative Procedure for Measurement of Rate Constants
for Reaction of 10/11 with PMe3 to form 15/16. Complex 5 (9.8 mg,
0.020 mmol) was dissolved in 0.5 mL C6D6 in a J Young tube.
p-Tolualdehyde (30 µL, 0.25 mmol) and 1,3,5-trimethoxybenzene (100
µL of a 0.1 M stock solution in C6D6) were added, and the contents
were allowed to react at room temperature. The reaction was monitored
by 1H NMR spectroscopy. Upon full conversion to 11c, PMe3 (21 µL,
0.20 mmol) was added to the mixture, and the reaction was performed
4-NMe2-C6H4-C(O)(C7H11). 4-NMe2-C6H4CHO (124 mg, 0.828
mmol), norbornene (762 mg, 8.09 mmol), CpqRh(VTMS)2 (20.4 mg,
0.0414 mmol), and 1.2 mL toluene were used, and the solution was
stirred at 75 °C. The crude product was purified by chromatography
on silica gel (100% hexanes to elute small brown band, then hexanes:
ethyl acetate ) 9:1 to elute colorless product) to give 192 mg (95%
yield) of 4-NMe2-C6H4-C(O)(C7H11) as a white solid. 1H NMR (CD2-
Cl2): δ 1.10-1.13 (m, 1H), 1.29-1.31 (m, 1H), 1.36-1.47 (m, 3H),
1.55-1.60 (m, 2H), 1.92-1.99 (m, 1H), 2.29 (br, 1H), 2.43 (br, 1H),
3.04 (s, 6H), 3.14 (dd, J ) 5.6, 8.8 Hz, 1H), 6.67 (d, J ) 9.2 Hz, 2H),
7.85 (d, J ) 9.2 Hz, 2H). 13C NMR (CD2Cl2): δ 29.41, 30.04, 34.08,
36.49, 36.69, 40.16, 41.72, 48.96, 110.92, 124.77, 130.66, 153.51,
199.50. Anal. Calcd for C16H21NO: C, 78.95; H, 8.71; N, 5.76.
Found: C, 78.65; H, 8.71; N, 5.81.
4-NMe2-C6H4-C(O)CH2CH2C(CH3)3. 4-NMe2-C6H4CHO (123
mg, 0.822 mmol), 3,3-dimethyl-1-butene (0.94 mL, 7.29 mmol), Cpq-
Rh(VTMS)2 (20.2 mg, 0.0411 mmol), and 1.0 mL toluene were used,
and the solution was stirred at 75 °C. The crude product was purified
by chromatography on silica gel (100% hexanes to elute small brown
band, then hexanes:ethyl acetate ) 9:1 to elute colorless product) to
give 188 mg (98% yield) of 4-NMe2-C6H4-C(O)CH2CH2C(CH3)3 as
a white solid. 1H NMR (CD2Cl2): δ 0.95 (s, 9H), 1.55-1.60 (m, 2H),
2.79-2.84 (m, 2H), 3.04 (s, 6H), 6.67 (d, J ) 6.8 Hz, 2H), 7.84 (d, J
) 7.2 Hz, 2H). 13C NMR (CD2Cl2): δ 29.35 (3C), 30.46, 33.87, 39.10,
40.19 (2C), 110.94 (2C), 125.38, 130.37 (2C), 153.70, 199.11. Anal.
Calcd for C15H23NO: C, 77.19; H, 9.95; N, 6.00. Found: C, 76.75; H,
9.93; N, 6.02.
1
at 25 °C in the probe of a 500 MHz NMR spectrometer. H NMR
spectra were obtained at regular time intervals until 11c was fully
converted to 16c.
Representative Procedure for Measurement of Rate Constants
for Reductive Elimination of Ketone from 15/16. The J Young tube
containing the reaction mixture from the formation of 16c (above) was
heated to 50 °C in the probe of a 500 MHz NMR spectrometer, and
1H NMR spectra were obtained at regular time intervals over 3-5 half-
lives.
Acknowledgment. We gratefully acknowledge funding by
the National Institutes of Health (Grant No. GM 28938).
Supporting Information Available: CIF file containing X-ray
crystallographic data for complex 15d, and figures showing
kinetic plots for the conversion of 5 to 11c and for the reductive
elimination of ketone from 16c. This material is available free
of charge via the Internet at http://pubs.acs.org.
JA066509X
9
J. AM. CHEM. SOC. VOL. 129, NO. 7, 2007 2093