Hydrosilylation with biscarbene Rh(I) complexes: Experimental evidence for a silylene-based mechanism

Article abstract of DOI:10.1021/ja110017c

A detailed study investigating the mechanism of the hydrosilylation of 4-F-acetophenone by N-heterocyclic biscarbene rhodium(I) complexes was performed, delivering substantial experimental evidence for a recently proposed catalytic cycle and explaining the observed side-product formation. Labeling experiments, silylene trapping reactions, and specific catalytic reactions were employed to substantiate each step of the catalytic cycle and explain the differences observed for different types of chiral catalysts. It is further shown that hydrosilylation and dehydrocoupling reactions with dihydrosilanes are mechanistically related.

Full text of DOI:10.1021/ja110017c

ARTICLE
pubs.acs.org/JACS
Hydrosilylation with Biscarbene Rh(I) Complexes:
Experimental Evidence for a Silylene-Based Mechanism
Peter Gigler,†,‡ Bettina Bechlars,‡ Wolfgang A. Herrmann,*,‡ and Fritz E. K€uhn*,‡,§
†WACKER-Institut f€ur Siliciumchemie, ‡Chair of Inorganic Chemistry, §Molecular Catalysis, Catalysis Research Center,
Department of Chemistry, Technische Universit€at M€unchen, Lichtenbergstrasse 4, 85747 Garching b. M€unchen, Germany
S Supporting Information
b
ABSTRACT: A detailed study investigating the mechanism of the hydrosilylation of 4-F-
acetophenone by N-heterocyclic biscarbene rhodium(I) complexes was performed, delivering
substantial experimental evidence for a recently proposed catalytic cycle and explaining the
observed side-product formation. Labeling experiments, silylene trapping reactions, and
specific catalytic reactions were employed to substantiate each step of the catalytic cycle
and explain the differences observed for different types of chiral catalysts. It is further shown
that hydrosilylation and dehydrocoupling reactions with dihydrosilanes are mechanistically
related.
Scheme 1. Ojima Mechanism2a,2b
’ INTRODUCTION
Rhodium complexes are commonly used catalysts for the
hydrosilylation of ketones.1 Although the reaction is well estab-
lished, only a few mechanistic studies on rhodium-based hydro-
silylation reactions have been published.2 Based on their investi-
gations on Wilkinson's catalyst in the hydrosilylation of ketones,
Ojima et al. proposed a mechanism shown in Scheme 1, which
will be further addressed as the Ojima mechanism.2a,2b
In the first step of this mechanism, a Rh(III) species is formed
via oxidative addition of the silane R3SiH (IO). Then a ketone
coordinates to the metal center (IIO) before it inserts into the
Rh-Si bond to form a Rh(III) alkyl hydride species (IIIO).
These steps are followed by reductive elimination to yield the
product (R1R2CH-O-SiR3) and regenerate the catalyst LnRh
(IVO).
reductive elimination generates the product R1R2CH-O-
The isolation of [RhClH(SiEt3)(PPh3)2] provided experi-
SiR2H and recovers the catalyst (IVZ).
mental evidence for the existence and formation of the first
intermediate of this catalytic cycle.2a Experimental results ob-
Recently, Gade et al. reported on the enantioselective hydro-
tained by replacement of monohydrosilanes by dihydrosilanes,
however, do not support the Ojima mechanism. Considerable
rate enhancement was observed, and the product selectivities for
the hydrosilylation of R,β-unsaturated ketones differ significantly
silylation of ketones catalyzed by a Rh(I) complex bearing a che-
lating chiral NHC-oxazoline ligand.3 In addition to the afore-
mentioned rate enhancement for dihydrosilanes versus mono-
hydrosilanes they determined an inverse kinetic isotope effect,
which cannot be explained by either of the two mechanisms
described above. Therefore a third, silylene-based mechanism
was proposed (Scheme 3) for the hydrosilylation with dihydro-
silanes, supported by a theoretical study comparing the three
mechanisms.2d,2e According to the calculations of Gade et al.,2e
the Gade mechanism involves the lowest activation barrier for the
rate-determining step.
for mono- and dihydrosilanes (1,4- versus 1,2-addition).2a,2c
A
kinetic isotope effect of two (KIE = 2) was determined in case
of dihydrosilanes, which;along with the above-described
differences;led to the proposal of a different mechanism for
the hydrosilylation of ketones with dihydro- and trihydrosilanes
by Zheng and Chan,2c while the Ojima mechanism is still
accepted for the use of monohydrosilanes. The Zheng mechanism
is depicted in Scheme 2.
After the initial oxidative addition of the silane R2SiH2 to the
Rh(I) complex LnRh (IZ), a ketone is thought to coordinate to
the silicon atom (IIZ) and then to insert into the Si-H bond
forming a rhodium silyl hydride complex (IIIZ). In the last step,
Whereas the first step (IG) is consistent with both mechanisms
discussed above (IO and IZ), the second step involves the transfer
Received: November 8, 2010
Published: December 27, 2010
r
2010 American Chemical Society
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Scheme 2. Zheng Mechanism2c
Figure 1. (a) Nonchiral and (b) chiral rhodium biscarbene complexes.
Scheme 3. Gade Mechanism2d,2e
Figure 2. Chiral biscarbene complex E (a) and Gade's catalyst (b).
It was found that dehydrogenative coupling occurs when an
excess of dihydrosilane is added to A but not if monohydrosilanes
are employed instead. If stoichiometric amounts of styrene and
silane are reacted with A, hydrogenation prevails over hydro-
silylation, while hydrosilylation takes place under catalytic con-
ditions. In the past decade, Tilley et al. isolated various metal-
silylene species,6 but to the best of our knowledge no rhodium
silylene complex has yet been isolated.
Recently, we reported on a series of rhodium biscarbene com-
plexes D, depicted in Figure 1a.7 It was shown that the modifica-
tion of the biscarbene ligand with respect to the composition of
the heterocycle, the N-substituent, and the bridging group
significantly effects the catalytic activity and product selectivity
in the hydrosilylation of 4-F-acetophenone with diphenylsilane.
In all cases, the formation of relatively high amounts of silyleno-
lether was observed as side product.7 The analogous chiral
rhodium complexes E (Figure 1b), which were synthesized as
described in the Supporting Information, do not lead to signifi-
cant asymmetric induction in the hydrosilylation of 4-F-aceto-
phenone, not even at low temperatures. This is surprising,
because the very similar systems described by Gade et al. are
highly enantioselective (Figure 2).3
Scheme 4. Indirect Proof of a Rhodium Silylene Species by
Milstein et al.5
In order to understand these differences, we carried out a
detailed experimental study of the mechanism (vide infra). It
provides strong experimental evidence for the existence of an
intermediate silylene species and confirms the mechanism pro-
posed by Gade et al., extending it by revealing further informa-
tion on the side-product formation.
of the second hydrogen atom from the silyl ligand to the rhodium
center and thus the formation of a silylene rhodium dihydride
species (IIG). The ketone then coordinates to the electron-
deficient silicon atom (IIIG). After that a hydride is transferred
from the metal to the carbonyl carbon atom (IVG). The catalytic
cycle is closed by reductive elimination of the product (VG).
The existence of such a silylene species was discussed earlier
in connection with rhodium-catalyzed dehydrogenative coupl-
ing reactions.4 Goikhman and Milstein were the first to prove
the presence of a rhodium silylene species by reacting a dipho-
sphine rhodium triflate complex (A) with a stoichiometric amount
of diphenylsilane.5 Since the silylene species could not be directly
observed, it was trapped with t-BuMe2SiOH, while the corre-
sponding rhodium dihydride complex C was isolated (Scheme 4).
’ EXPERIMENTAL SECTION
General Remarks. All manipulations were carried out under a dry
argon atmosphere using standard Schlenk techniques. Solvents were
dried by standard methods8 and distilled under nitrogen. 1H, 2H, 13C,
19F, and 29Si NMR spectra were recorded on a JEOL JMX-GX 400 MHz
and a Bruker Avance III 400 MHz spectrometer at room temperature
and referenced to the residual solvent signal9 or to TMS (29Si). Spec-
tral assignments were achieved by combination of 13C DEPT, 1H-13C
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Scheme 5. Catalyst (1) and Catalytic Reaction
1
HMQC, and H-13C HMBC experiments. IR spectra were acquired
Scheme 6. Proposed Mechanism
using a Mettler-Toledo reactIR spectrometer equipped with a fluid cell.
GC/MS measurements were performed on a Varian CP-3800 gas
chromatograph equipped with a 1200 L Quadrupol MS/MS and a HP
6890 GC system with a HP 597 mass selective detector. Compound 1
was prepared following an earlier published method.7
NMR Scale Stoichiometric Hydrogenation Reaction. A NMR
tube was equipped with 8.5 mg (11.3 μmol) of 1, silane (11.3 μmol), and
1
0.5 mL of CD2Cl2, and the reaction was monitored by H NMR and
13C NMR spectroscopy. NMR data of cyclooctane: 1H NMR (CD2Cl2,
400 MHz) δ 1.53 (s, 16H); 13C NMR (CD2Cl2, 100 MHz) δ 27.2 (s).
NMR data of hydrogen: 1H NMR (CD2Cl2, 400 MHz) δ 4.59 (s).
NMR Scale Catalytic Hydrogenation Reaction. In a typical
experiment, a NMR tube was equipped with 6.8 mg (9.01 μmol) of 1,
olefin (0.45 mmol), silane (0.68 mmol), and 0.25 mL of CD2Cl2, and the
reaction was monitored by 1H NMR and 13C NMR spectroscopy.
NMR Scale Scrambling Reaction. In a typical experiment, a
NMR tube was equipped with 6.8 mg (9.01 μmol) of 1, evacuated in a
Schlenk flask, and refilled with D2. Then, silane (0.45 mmol) and 0.5 mL of
CD2Cl2 were added, and the reaction was followed by 1H NMR spectroscopy.
Diphenylsilane-HD. To a solution of 1.11 g (5.11 mmol) of
chlorodiphenylsilane in 10 mL of Et2O was added dropwise 6.13 mL of
LiAlD4 (6.13 mmol, 1 M in Et2O). The reaction mixture was stirred for
16 h at room temperature and then added to 10 mL of HCl (1 M aqueous
solution). The organic phase was extracted with 10 mL of saturated
NaCl solution, dried over Na2SO4, and filtered, before the solvent was
removed in vacuo. The crude product was purified by bulb-to-bulb
distillation (150 °C, 14 mbar) to afford 0.96 g (100%) of diphenylsilane-
’ RESULTS
For the mechanistic investigations, [Rh(COD)(bis-NHC)]-
[PF6] with R = Mes, X = CH, and n = 2 (1) was chosen due to its
comparatively high activity and selectivity.7 Compound 1 and the
catalytic model reaction are shown in Scheme 5.
1
HD as a colorless liquid. H NMR (C6D6, 400 MHz): δ 5.08 (s, 1H,
3
Si-H), 7.10-7.14 (m, 6H, Ar-H), 7.51 (d, JHH = 7.0 Hz, 4H, Ar-H).
13C NMR (C6D6, 100 MHz): δ 128.5 (s, Ar-C), 130.1 (s, Ar-C), 131.7
(s, Ar-C), 136.1 (s, Ar-C). 2H NMR (C6D6, 61 MHz): δ 5.09 (s, 1H, Si-
4-F-Acetophenone was selected as the substrate because it
offers the possibility to monitor the reaction progress by 19F
1
D). 29Si NMR (C6D6, 79 MHz): δ -33.6 (t, JSiD = 30.3 Hz). Anal.
NMR spectroscopy. In all H, 13C, 19F, and 29Si NMR and IR
1
Calcd for C12H11DSi (185.32): C, 77.77; H, 7.07; Si, 15.16. Found: C,
78.00; H, 6.61; Si, 15.86.
spectra, no direct evidence for a Rh(III) species was found;
within the time scales of these methods, only the original
complex and the evolving products are observable. The results of
various experiments, which indirectly prove the silylene-based
mechanism depicted in Scheme 6, are discussed in the following
paragraphs.
NMR Scale Catalytic Hydrosilylation Reactions. In a typical
experiment, a NMR tube was equipped with 6.8 mg (9.01 μmol) of 1,
55 μL (62.2 mg, 0.45 mmol) of 4-F-acetophenone, silane (0.68 mmol),
and 0.25 mL of CH2Cl2, and the reaction was monitored by 19F NMR
spectroscopy.
Steps I-V involving the intermediates 4-7 correspond to
the Gade mechanism. An alternative pathway that leads to the
formation of silylenolether 3, the byproduct observed in the
hydrosilylation reaction, is represented by the steps VI and VII. It
requires the ketone in the intermediate 6 to tautomerize to its
enol form under formation of silyl complex 8 and the release of
hydrogen (step VI, Scheme 6). By reductive elimination of 3,
the catalytically active species is regenerated in step VII. Each
elementary step (I-VII) of the mechanism was investigated by
specific experiments, which will be discussed in detail below.
Oxidative Addition (I). In the first step, the silane adds
oxidatively to the cationic Rh(I) center. As mentioned above,
no Rh(III) species nor any color change can be observed by
NMR, IR, or UV-vis spectroscopy. In order to understand how
NMR Scale Trapping Reaction. To 4.1 mg (5.40 μmol) of 1
dissolved in 0.5 mL of CD2Cl2, 50 μL (50.1 mg, 0.27 mmol) of Ph2SiH2
and 43 μL (36.1 mg, 0.27 mmol) of t-BuMe2SiOH were added, and the
reaction monitored by 1H NMR and 29Si NMR spectroscopy. NMR data
1
of t-BuSiMe2-O-SiHPh2: H NMR (CD2Cl2, 400 MHz) δ 0.08 (s,
6H, SiCH3), 0.90 (s, 9H, CCH3), 5.53 (s, 1H, SiH), 7.36 (m, 6H, Ar-H),
7.59 (m, 4H, Ar-H); 29Si NMR (CD2Cl2, 79 MHz) δ -22.5 (s,
Ph2HSiO), 15.0 (s, t-BuMe2SiO).
Determination of the KIE. To 3.0 mg (3.98 μmol) of 1 dissolved
in 1.5 mL of dichloroethane, 24.5 μL of 4-F-acetophenone was added
(27.7 mg, 0.20 mmol) and a 1:1 mixture of 41 μL of Ph2SiH2 (40.7 mg,
0.22 mmol) and 41 μL of Ph2SiD2 (41.1 mg, 0.22 mmol). The reaction
was then followed by an in situ IR measurement. Ph2SiH2: IR (cm-1
ν~SiH 2148. Ph2SiD2: IR (cm-1) ν~SiD 1559.
)
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Scheme 7. Reaction Pathways of 1 in Absence of Substrate
Scheme 8. Formation of 9, 10, and 11
the silane reacts with 1 in absence of the ketone, the reactions
a-e, shown in Scheme 7, were examined.
(m/z 108), 1,4-cod (m/z 108), 1,3-cod (m/z 108), and coe (m/z
110)).11
If stoichiometric amounts of diphenylsilane are reacted with 1
(reaction a, Scheme 7), a small amount of cyclooctane evolves
after several hours, giving rise to a singlet in the 1H and 13C NMR
spectrum at 1.53 and 27.2 ppm, respectively. A GC-mass spec-
trum confirms the formation of cyclooctane. Besides, hydrogen
In contrast to the conditions applied under a, complete
hydrogenation of the cyclooctadiene is not observed in the case
of c. This is attributed to the chelating effect of the excess 1,5-
cyclooctadiene. After hydrogenation of the first double bond,
regeneration of the chelate complex is favored over hydrogena-
tion of the second double bond. If cyclooctene is employed as
substrate, a small amount of cyclooctane is formed very slowly
(several days) (reaction d, Scheme 7).
In order to prove an intermediate existence of a dihydride
species, diphenylsilane is reacted with 2 mol % of 1 under a D2
atmosphere (reaction e, Scheme 7). For that case, formation of
Ph2SiD2 and H2 is expected, since the dihydride could be sub-
stituted by reductive elimination of hydrogen and subsequent
oxidative addition of deuterium (Scheme 9).
A complete HD scrambling at the diphenylsilane moiety can
be observed in the 1H and 2H NMR spectra, similar to the inves-
tigations of Comte et al. with their [(o-dppbe)Rh(COD)](OTf )
system.13 The appearance of Ph2SiHD can be attributed to a HD
exchange at 4 or the reaction of 5 with Ph2SiD2 (or vice versa)
(Scheme 9).
1
was detected by its characteristic signal at 4.59 ppm in the H
NMR spectrum. Simultaneously, a strong broadening of the
aromatic signals in the 1H and 13C NMR spectra was observed,
indicating an oligomeric diphenylsilane species, which is typical
for rhodium-catalyzed dehydrogenative coupling reactions.4,10
Hydrogenation of the ligand also occurs when 1 reacts with
hydrogen in dichloromethane (reaction b, Scheme 7). Addition
of equimolar amounts of 1,5-cyclooctadiene (1,5-cod) and diphe-
nylsilane to a solution of 2 mol % catalyst in dichloromethane
leads to the formation of cyclooctene (coe) and the 1,4- and 1,3-
isomers of cyclooctadiene (reaction c, Scheme 7), which can be
explained with an interplay of M-H insertion and β-H-elimina-
tion reactions (Scheme 8).
The products were identified by 1H and 13C NMR spectroscopy
(Table 1, entries 1-3) and GC-mass spectrometry (1,5-cod
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12
Table 1. Characteristic NMR Signals of coe, 1,4-cod, and 1,3-cod in CD2Cl2
entry
compound
selected 1H NMR shifts (multiplicity, integral), ppm
selected 13C NMR shifts, ppm
26.0, 26.7, 29.8
1
2
3
4
5
cyclooctene
1.49, 2.13
1,4-cyclooctadiene
1.41 (quin, 2H), 2.27 (quar, 4H), 2.79 (t, 2H)
1.50a, 2.17a
23.5, 25.3, 29.9
23.7, 28.6
25.9, 26.0, 26.3 (t, 1JHD = 19.1 Hz), 26.6, 29.7
23.4, 24.9 (t, 1JHD = 19.8 Hz), 25.2, 29.6
1,3-cyclooctadiene
[5-2H1]-cyclooctene
[6-2H1]-1,4-cyclooctadiene
covered by coe
covered by cod
a Covered by signal of coe; visible only in 2D spectra.
Scheme 9. HD-Scrambling of Ph2SiH2 under D2 Atmosphere
Scheme 10. Possible Intermolecular Reaction Pathway to a
Scheme 11. Hydrogenation of cod with Ph2SiHD
Rhodium Dihydride Species (Curtis-Epstein Mechanism)14
(13) and was confirmed by their characteristic masses of m/z 111
for 12 and 109 for 13. Selected 13C signals for 12 and 13 are
shown in Table 1 (entries 4 and 5). Both compounds show the
characteristic triplet pattern for the deuterium-bound carbon
atom with the corresponding 1JCD coupling constant of about
20 Hz. The product distribution corresponds to an intramolec-
ular hydrogenation transfer as depicted in Scheme 12.
All of the discussed experiments (reactions a-e, Scheme 7)
did not proceed in the absence of 1. These findings support the
formation of the intermediate rhodium dihydride complex 5
(Scheme 6), which is analogous to the one Goikhman and
Milstein reported.5 Hence, an oxidative addition as the first step
of the mechanistic cycle, as it is proposed in all other mechanisms
described so far, is evident, since it is necessary to explain the
hydrogenation and scrambling behavior, which only occurred
in presence of 1. The rhodium dihydride can subsequently be
formed by an intramolecular (as proposed) or an intermolecular
process, which will be discussed in the following section.
r-H Transfer (II). In the key step of the mechanistic cycle, it
was proposed2d,2e that a second silicon-bonded hydrogen atom is
transferred to the metal center affording the silylene rhodium
dihydride species 5. To prove the formation of 5 and exclude an
intermolecular reaction pathway14 (Scheme 10), it is necessary to
show that both hydrogen atoms involved in the reduction of 1,5-
COD (reaction c, Scheme 7) originate from one molecule of
diphenylsilane.
Therefore Ph2SiHD was synthesized and reacted with an equi-
molar amount of 1,5-cyclooctadiene using 2 mol % of 1, with the
expectation to give different product distributions for intra- and
intermolecular reaction pathways. Scheme 11 shows the five main
products of the reaction.
Besides the formation of coe (9), 1,4-cod (10), and 1,3-cod
(11), two deuterated products appear in the 13C NMR. Their forma-
tion can be attributed to [5-2H1]-coe (12) and [6-2H1]-1,4-cod
Coordinated 1,5-cyclooctadiene can insert either into the
Rh-H (F) or Rh-D (G) bond to form an intermediate alkyl
rhodium hydride/deuteride complex. Subsequently, the complex
can undergo reductive elimination to 12 or β-hydride elimination
either to 1,5-cod, 13, [5-2H1]-1,5-cyclooctadiene (14), or 10. In
the first step, insertion into the Rh-D bond should be thermo-
dynamically favored, due to the stronger C-D versus C-H
bond, and for the same reason β-hydride elimination should
favor the formation of the deuterated compounds 12 and 13. The
resulting rhodium dihydride complex can then give rise to the
formation of 9, 10, and 11, as well as reaction pathway G (cp
Scheme 8). Compound 14 cannot be identified in the 13C NMR
spectrum due to the overlapping signals of excess cod. 2H NMR
and GC-mass, however, provide evidence for the presence of
this compound. Hence, this experiment supports the assumption
of two subsequent intramolecular hydrogen transfers from the
silicon atom to the metal center by reacting 1 with diphenylsilane.
In the case of an intermolecular reaction pathway, an intermedi-
ate rhodium bisdeuterated species should occur and result in
DD-hydrogenation products (Scheme 13).
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Scheme 12. Possible Reaction Network for the Hydrogena-
tion of cod with Ph2SiHD
Figure 3. Different reactivity of secondary and tertiary silanes.
Scheme 14. Trapping Experiment with t-BuMe2SiOH and
the Possibly Involved Reaction Pathways
Scheme 13. Rhodium Dideuteride Complex by an Inter-
molecular Reaction Pathway
Based on these results, the reaction of a catalytic amount of 1
with diphenylsilane very likely leads to a highly reactive silylene
species 5. The formation of an oligomeric silane species in the
absence of substrate can be explained by the reaction of this
silylene species with another molecule of silane, as it has been
proposed earlier.4 The existence of such an accordant silylene
species in the presence of substrate was investigated and will be
discussed in the following section.
Monohydrosilanes, in contrast to dihydrosilanes, cannot un-
dergo the second hydrogen transfer (II, Scheme 6), and thus
pronounced differences are expected for the catalytic reaction.
Therefore the hydrosilylation of ketones with the dihydrosilanes
Ph2SiH2 and PhMeSiH2 and the monohydrosilane Ph2MeSiH in
the presence of catalytic amounts of 1 was investigated. In accor-
dance to other studies,2a-2d the use of monohydrosilane reduces
the reaction rate by several orders of magnitude (Ph2MeSIH,
TOF < 0.1 h-1) as compared with dihydrosilanes (1, TOF =
150 h-1) (Figure 3). The monohydrosilane, however, does not
produce any byproduct 3.
The rate enhancement using dihydrosilanes could be explained
with both the Zheng mechanism (step IIIZ in Scheme 2) and the
silylene-based Gade mechanism. To determine which of the
mechanisms is valid in our case, a typical silylene trapping experi-
ment was performed. t-BuMe2SiOH is known to react with a
silylene species by forming a disiloxane (see Scheme 4), which is
easy to characterize by 1H and 29Si NMR spectroscopy.5 Due to
its steric bulk, side reactions are minimized and coordination to
the metal center can be excluded. Therefore a catalytic amount of
complex 1 (2 mol %) was dissolved in dichloromethane, and
equimolar amounts of diphenylsilane and t-BuMe2SiOH were
added, while 1H and 29Si NMR spectroscopy were employed to
monitor the reaction progress. Upon contact of the reaction
partners, vigorous gas evolution was observed, and formation of
H2 was detected by 1H NMR spectroscopy. Simultaneously, the
signals of Ph2SiH2 (-33.5 ppm) and t-BuMe2SiOH (19.4 ppm)
decrease in the 29Si NMR spectrum and two new signals arise at
15.0 ppm and -22.5 ppm, which are assigned to the quantita-
tively formed disiloxane 15. The same reaction using Ph2SiD2
instead, leads to formation of HD and exclusive deuteration at
the diphenylsilane moiety, with its characteristic Si-D signal at
5.52 ppm in the 2H NMR spectrum (Scheme 14).
The reaction can occur via two possible reaction pathways
(Scheme 14). According to the Gade mechanism, the silanol
coordinates to the electron-deficient silylene Si atom of H, the
silyl complex I and H2 are formed, and 15 is generated by reduc-
tive elimination. Alternatively, the silanol may react with the silyl
rhodium(III) hydride complex J in a concerted mechanism. In
the latter case, no significant differences in reactivity using mono-
or dihydrosilanes would be expected. However, while the reac-
tion with Ph2SiH2 is completed within 1 h, no conversion of the
monohydrosilane takes place over a period of several days. Con-
sequently, intermolecular interactions can be ruled out, and a
second Si-bound hydrogen atom is required to explain the ob-
served reactivity. These considerations thus serve as an indirect
proof for the existence of a silylene rhodium(III) dihydride as key
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Table 2. Yields of Product (2) and Byproduct (3) for Various
Reaction Conditions Involving Electronically Different
Silanes
product
byproduct
silane
Ph2SiH2
solvent
yield (2) (%)
yield (3) (%)
CD2Cl2
THF
68
74
84
92
32
26
16
8
Ph2SiH2
PhMeSiH2
CD2Cl2
CD2Cl2
[(o-OMe)Ph]2SiH2
Figure 5. Hammet plot of the catalytic reaction using different para-
substitued diephenylsilanes 16.
acidity of the silylene silicon atom is responsible for the high
amounts of side-product formation. Accordingly, the monohy-
drosilane Ph2MeSiH, which cannot generate such a silylenespecies,
does not produce any byproduct.
The ketones' coordination at the silicon atom results in a long
distance interaction between the chiral information in [Rh(COD)-
(bis-NHC)](PF6) where bis-NHC = cyclohexyl-bridged imida-
zole-based biscarbene (Figure 2a) and the substrate. Conse-
quently, low asymmetric induction is observed. The ligands used
by Gade et al. (Figure 2b) are more flexible, and the chiral
information is located closer to the reaction site, which could
explain the better asymmetric induction reported.
Kinetic Isotopic Effect (KIE). All experiments discussed so far
give experimental evidence for the silylene-based mechanism, first
postulated by Gade and co-workers based on DFT calculations.
Hydride transfer from the metal to the substrate was assigned as
rate-determining step, explaining the observed inverse kinetic
isotopic effect for their system.
In order to investigate this aspect for our system, catalysis was
performed by the hydrosilylation reaction of 4-F-acetophenone
with a mixture of Ph2SiH2 and Ph2SiD2 (1:1) in dichloroethane,17
which was monitored by in situ IR spectroscopy. The change of
the relative intensity of the Si-H versus Si-D band over time is
represented in Figure 4.
The stronger decrease of the Si-D band results in an inverse
kinetic isotope effect of kH/kD = 0.57, which Gade et al. attrib-
uted to the thermodynamically stronger C-D vs C-H bond,
which is formed in step IV (Schemes 3 and 6).2d Additionally, the
absence of a Hammett correlation18 using different para-sub-
stituted silanes15 (Figure 5) indicates that neither of the first two
reaction steps is rate-determining. Otherwise the different elec-
tronic properties of 16 would influence the catalyst's activity in a
linear way.
Figure 4. Determination of the inverse KIE.
intermediate of the mechanistic cycle. Any attempts to stabilize 5
by using electron-donating di(2-methoxyphenyl)silane,15 steri-
cally hindered dimesitylsilane, or a NHC ligand with coordinat-
ing N-substituents were not successful, not even when carried
out at low temperatures.
Product and Side-Product Formation (IV-VII). As an effect
of the Lewis acidity of the silylene Si atom, the ketone coordi-
nates to the silicon atom instead of the rhodium center (III,
Scheme 6). After coordination to the silicon atom, two possible
reaction pathways arise, as discussed earlier. The driving force for
the formation of the undesired silylenolether 3 is the evolution of
hydrogen (VI, Scheme 6). This reaction pathway can be sup-
pressed by performing the reaction under H2 pressure as Comte
et al. showed for the analogous [(o-dppbe)Rh(COD)](OTf )
system.16 Another approach is to reduce the Lewis acidity of
the electron-deficient silicon atom, which would favor the keto
tautomer 6. For that purpose, THF as coordinating solvent and
various silanes were tested under catalytic conditions. The
resulting yields of 2 and 3 are summarized in Table 2.
Replacement of the nonccordinating solvent methylene chloride
by the coordinating tetrahydrofurane resulted in a slight increase
in product selectivity (68% f 74%). The same trend was observed
when Ph2SiH2 was replaced by the more electron-rich PhMe-
SiH2 (84%) or the ortho-methoxy-substituted diphenylsilane15
(92%). These observations confirm the assumption that the Lewis
’ CONCLUSION
In summary, experimental evidence for the formation of an inter-
mediate rhodiumsilylenespecies is given. This intermediate species
is generated by two subsequent hydrogen transfers from the silane
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Journal of the American Chemical Society
ARTICLE
to the metal, which is confirmed by several experiments, such as
the catalytic hydrogenation in the absence of substrate, the
different reactivity of mono- and dihydrosilanes, and trapping
with t-BuMe2SiOH. The experiments provide substantial evi-
dence that the ketone interacts with the catalyst by coordinating
to the electron-deficient silylene silicon atom. The resulting
transition state cannot be significantly affected by the distant
and rigid chiral ligand applied in this work, which explains the low
enantioselectivity.
The silylene species opens up two alternative reaction path-
ways. Besides the formation of product, the Lewis acidity of the
silicon atom competitively enables the formation of the enole
tautomer leading to the nondesired silylenolether as side prod-
uct. The inverse kinetic isotope effect observed by Gade et al. is
confirmed by our experiments. Based on this and the absence
of a Hammet correlation when using different para-substitued
diphenylsilanes, the product formation can be assigned as rate-
determining step.
The chiral biscarbene rhodium(I) complexes investigated here
are not well suited for the asymmetric hydrosilylation of ketones
with dihydrosilanes. The transfer of chiral information, however,
could be induced by locating the chirality closer to the generated
silylene silicon atom.
In addition to giving experimental evidence for a silylene-
based mechanism, our studies provide an explanation for the
formation of the side product and show the mechanistic con-
nection between Rh(I)-catalyzed hydrosilylation and dehydro-
coupling.
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’ ASSOCIATED CONTENT
(14) Curtis, M. D.; Epstein, P. S., . In Advances in Organometallic
Chemistry, Stone, F. G. A., West, R., Eds.; Academic Press: New York,
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S
Supporting Information. Experimental procedures for
b
the synthesis of E, as well as the Diamond plot of ECy and supplemen-
tary crystallographic data for ECy in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
(16) Comte, V.; Balan, C.; Gendre, P. L.; Moise, C. Chem. Commun.
2007, 713–715.
(17) Dichloroethane was used instead of dichloromethane, due to its
lower vapor pressure, to exclude changes in concentration.
(18) Hammett, L. P. J. Am. Chem. Soc. 1937, 59, 96–103.
’ AUTHOR INFORMATION
Corresponding Author
wolfgangherrmann@ch.tum.de; fritz.kuehn@ch.tum.de
’ ACKNOWLEDGMENT
We are grateful for the financial support of the Wacker Chemie
AG and thank Prof. Peter H€arter for helpful discussions.
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