Homobimetallic rhodium NHC complexes as versatile catalysts for hydrosilylation of a multitude of substrates in the presence of ambient air

Article abstract of DOI:10.1021/om400452q

Two recently reported air- and water-stable di-Rh complexes based on 1,3-bis(3′-butylbenzimidazol-2′-ylidene)benzene were utilized as catalysts for hydrosilylation. Among the substrates investigated were aldehydes, ketones, α,β-unsaturated carbonyls, acyl chlorides, nitriles, alkenes, nitro groups, isocyanates, and tertiary amides. Additionally, carbon dioxide underwent hydrosilylation to produce dimethylphenylsilylformate. The catalysts compared well to other previously reported hydrosilylation catalysts, and the Rh-Cl catalyst was found to be faster and more selective than the Rh-I complex in each case.

Full text of DOI:10.1021/om400452q

Article
pubs.acs.org/Organometallics
Homobimetallic Rhodium NHC Complexes as Versatile Catalysts for
Hydrosilylation of a Multitude of Substrates in the Presence of
Ambient Air
Aron J. Huckaba, T. Keith Hollis,* and Sean W. Reilly
aDepartment of Chemistry and Biochemistry, The University of Mississippi, University, Mississippi 38677, United States.
S
* Supporting Information
ABSTRACT: Two recently reported air- and water-stable di-Rh
complexes based on 1,3-bis(3′-butylbenzimidazol-2′-ylidene)-
benzene were utilized as catalysts for hydrosilylation. Among the
substrates investigated were aldehydes, ketones, α,β-unsaturated
carbonyls, acyl chlorides, nitriles, alkenes, nitro groups, isocyanates,
and tertiary amides. Additionally, carbon dioxide underwent
hydrosilylation to produce dimethylphenylsilylformate. The cata-
lysts compared well to other previously reported hydrosilylation
catalysts, and the Rh−Cl catalyst was found to be faster and more
selective than the Rh−I complex in each case.
carbenes, carbene analogues, and their metal complexes have
allowed for a deeper understanding of the nature of these
ligands.20 Furthermore, reports of metal NHC complexes have
increased dramatically and have described both asymmetric and
selective catalyses,20s,21 descriptions of new structural mo-
tifs,20h,22 transmetalation,23 pharmacological applications,20f,24
and enhanced metal reactivity,25 to name a few.
In 2005, one of us reported the synthesis of a homobimetallic
Rh complex synthesized from the ligand prescursor 1,3-bis(3′-
butyl-1′-imidazolium)benzene diiodide26 that was a catalyst for
the hydrosilylation of alkynes.27 However, those original di-Rh
complexes were not found to hydrosilylate ketones, aldehydes,
or alkenes, even at high temperature and extended reaction
time. Earlier this year, we reported the synthesis of analogous
homobimetallic Rh complexes 1 or 2 synthesized from
metalation of the ligand precursor 1,3-bis(3′-butyl-1′-
benzimidazolium)benzene diiodide with Ag2O and trans-
metalation to Rh (Figure 1).28 These complexes were observed
to equilibrate between rac and meso isomers and were highly
active catalysts for the hydrosilylation of phenylacetylene.
This report’s focus is the hydrosilylation of unsaturated
bonds. Herein we report the use of di-Rh complexes 1 and 2 as
INTRODUCTION
■
Carbon−carbon unsaturated bonds were first reported as
substrates for hydrosilylation, catalyzed by acetyl peroxide, in
1947.1 Since that time, the transition-metal-catalyzed hydro-
silylation of C−C unsaturated bonds has become an important
application of homogeneous catalysis.2 This reaction is used to
synthesize industrially important functionalized organosilanes,
silicone polymers (such as silicone oils and resins), and
functional siloxanes.3 Another application is the cross-linking of
multifunctional silicone polymers, which are the basis for the
manufacturing of silicone elastomers and release coatings. All
these products (and more) contribute to the success of the
silicone industry, which has been forecasted to approach $20B
in annual sales by 2017.4 Therefore, the development of
efficient, selective catalysts for a range of hydrosilylation
reactions will expand the synthetic reach of functionalized
organosilanes and lower the production costs for a multibillion-
dollar industry.
Hydrosilylation is both operationally simpler and offers a
milder method of reduction than hydrogenation or transfer
hydrogenation.5 In addition, hydrosilylation is atom econom-
ical6 and can be used to prepare either a Si−C6 or Si−
heteroatom7 bond. The resulting organosilanes are synthetically
useful intermediates.7,8 Reports of organic CO hydro-
silylation have not been as numerous as olefin or alkyne
hydrosilylation. Even so, a multitude of transition-metal
catalysts have been reported, including Au,9 Cu,10 Fe,11 Ir,12
Mo,13 Pt,14 Rh,12d,15 and Ru16 as well as Ti, Zr, Yb, Cr, W, Mo,
Mn, Re, Os, Co, and Zn.7,17
The first N-heterocyclic carbene (NHC) transition-metal
complexes were reported in the late 1960s,18 but the ligands
were largely regarded as a novelty until the discovery and
elegant syntheses of stable carbenes by Bertrand and Arduengo
20 years later.19 Advances in the understanding of free
Figure 1. Di-Rh catalysts used in hydrosilylation studies.
Received: May 22, 2013
Published: October 24, 2013
© 2013 American Chemical Society
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catalysts in the hydrosilylation of aldehydes, ketones, α,β-
unsaturated carbonyls, nitriles, acid chlorides, carbon dioxide,
carbodiimides, nitro groups, isocyanates, and tertiary amides
without the exclusion of air or moisture.
increasing temperature. Because the catalyst was active at room
temperature, it was decided to evaluate the substrates at room
temperature. If the substrate’s reaction rate was slow, it was re-
evaluated at 100 °C. Reduction of the nitro group in Tables 1
and 2 was not observed, although some other nitro groups’
reactivity in hydrosilylation was evaluated (Table 8, entries 4
and 5, vide infra), their reduction took extended reaction time
at 100 °C if there was reaction.
Hydrosilylation of Alkenes. As an analogous di-Rh
catalyst was not reported to catalyze alkene hydrosilylation,
this class of substrate was studied first.27 Alkenes have been
perhaps the most studied substrate in hydrosilylation, dating
back to the first published reports over 50 years ago.1,29
Regioselectivity is important, as unsymmetrical alkenes yield
either α (A) or β (B) silanes upon hydrosilylation, as illustrated
in Scheme 2. The vast majority of catalysts developed for this
RESULTS AND DISCUSSION
■
Optimization. Once complex 1 was found to catalyze the
hydrosilylation of 4-nitrobenzaldehyde in preliminary experi-
ments (Scheme 1), the catalyst loading was optimized to
Scheme 1. Catalytic Hydrosilylation of 4-Nitrobenzaldehyde
Using Complex 1 as Catalyst
Scheme 2. Catalytic Hydrosilylation of Alkenes
a
Table 1. Effect of Catalyst Loading on Reaction Time
entry
cat. amt (mol %)
time (min)
conversn (%)
1
2
3
4
2
2.5
5
92
>99
91
reaction yield only the β isomer, B, through the widely accepted
Chalk−Harrod or modified Chalk−Harrod mechanisms.30
Transition-metal complexes utilized as catalysts in this reaction
have been based on Ni,31 Pd,32 Pt,29,30,33 Co,34 Rh,35 Ir,12a Fe,36
Ru,16a and others.17g,37 In industrial applications, Pt and Rh are
most often used.3 A di-Rh catalyst previously reported by one
of us was not reported to hydrosilylate alkenes, even at high
temperature and extended time.27
0.5
0.019
0.001
25
211
21
a
General conditions: HSiMe2Ph (0.077 mL, 0.50 mmol), C6D6 (0.5
mL), and complex 1 were combined and incubated at 100 °C for 3
min. After addition of the substrate (0.46 mmol), the reaction was
monitored and product conversions were determined by H NMR
spectroscopy. Reactions were performed at 100 °C in all cases.
1
The results of alkene hydrosilylation with Rh complexes 1
and 2 are described in Table 3. Where possible, product
resonances were referenced to literature reports.35b,38 In the
first experiment, 1-octene underwent hydrosilylation at room
temperature to yield mainly the β isomer, B, in excellent yield
(entry 1). A substrate containing a nitrile functionality, 4-
pentenenitrile, selectively underwent hydrosilylation at room
temperature at the double bond only. Nitriles were also shown
to undergo hydrosilylation (Table 8, entries 1−3, vide infra) at
extended reaction time and high temperature. Catalysis by the
Rh−Cl complex 1 yielded mainly the β isomer, B, while
catalysis with the Rh−I complex 2 afforded no product (entry
2). This result mirrored previously reported catalytic results
from an analogous di-Rh−I complex.27 Hydrosilylation of an
electron-deficient styrene also yielded mainly the β isomer, B,
at room temperature (entry 3). Propenyl ethyl ether did not
undergo hydrosilylation at room temperature, and even at 100
°C the reaction was slow (entry 4). Even so, selectivity for the β
isomer was observed. While allylamine did not react at room
temperature, only 10 min at 100 °C was needed to convert
most of the substrate to product with selectivity for the β
isomer, B (entry 5). The sterically hindered α-pinene did not
undergo hydrosilylation, even at 100 °C and extended time
(entry 6). Isomerization from β-pinene to its α isomer was the
only observed product upon hydrosilylation, likely assisted by a
Rh−H catalytic intermediate (entry 7).39
balance conversion and time (Table 1). Catalyst loadings were
varied from 0.001 to 2 mol %. Using 2 mol % of catalyst
resulted in 92% product conversion in 2.5 min (entry 1), while
decreasing the catalyst loading to 0.5 mol % led to the time of
reaction doubling to 5 min (entry 2). Further decreasing the
catalyst loading by an order of 10 to 0.019 mol % increased the
reaction time to 25 min (entry 3), while decreasing the catalyst
loading further to 0.001 mol % greatly increased the reaction
time (entry 4). A control experiment where the Rh catalyst was
not added yielded no reaction. From these results, 0.5 mol %
catalyst loading was chosen for further trials.
Using 0.5 mol % catalyst loading, the reaction temperature
was optimized for the hydrosilylation of 4-nitrobenzaldehyde
with complex 1 (Table 2). The reaction temperature was varied
from room temperature to 100 °C. The observed reaction rate
was modest at room temperature but increased quickly with
a
Table 2. Effect of Temperature on Reaction Time
entry
T (°C)
100
time (min)
conversn (%)
1
2
3
4
5
5
20
>99
82
80
60
90
98
40
600
1140
63
room temp
trace
Hydrosilylation of Aldehydes. After the scope of
hydrosilylation was expanded to alkenes, the investigations
were extended to aldehydes (Table 4). Where possible, product
peaks were compared to those reported in the literatur-
e.10c,15d,40 With the exception of p-nitrobenzaldehyde (entry 1),
which yielded only trace amounts of product at room
a
General conditions: HSiMe2Ph (0.077 mL, 0.50 mmol), C6D6 (0.5
mL), and complex 1 (0.5 mol %) were combined and incubated at 100
°C for 3 min. After addition of the substrate (0.46 mmol), the reaction
was monitored and product conversions were determined by 1H NMR
spectroscopy.
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a
a
Table 3. Catalytic Hydrosilylation of Alkenes
Table 4. Hydrosilylation of Aldehydes
a
General conditions: HSiMe2Ph (0.077 mL, 0.50 mmol), C6D6 (0.5
mL), and complex 1 (0.5 mol %) were combined and incubated at 100
°C for 3 min. After addition of the substrate (0.46 mmol), the reaction
was monitored and product conversions were determined by 1H NMR
spectroscopy. Product ratios are given with respect to each other only.
b
Reactions were performed at room temperature. Reaction was
c
d
performed at 100 °C. Complex 2 used as catalyst. Olefin
isomerization (46%) to form α-pinene observed as product and
confirmed by GC.
a
General conditions: HSiMe2Ph (0.077 mL, 0.50 mmol), C6D6 (0.5
mL), and complex 1 (0.5 mol %) were combined and incubated at 100
°C for 3 min. After addition of the substrate (0.46 mmol), the reaction
was monitored and product conversions were determined by 1H NMR
spectroscopy. Reactions were performed at room temperature.
temperature, electron-deficient systems reacted more quickly
than did electron-rich systems. When unpurified p-nitro-
benzaldehyde was used, a much faster rate was observed,
however, and investigation into the difference in rates is
currently ongoing. m-Nitrobenzaldehyde fastest to react (entry
2 vs entries 1 and 8). Benzaldehyde was converted more
quickly than were electron-rich systems (entry 3 vs entries 5
and 6). The less aromatic substrate furfural was converted more
quickly than electron-rich aryl aldehydes (entry 7 vs entries 5
and 6). Naphthaldehyde was converted more quickly than
benzaldehyde by Rh−Cl complex 1, while Rh−I complex 2 did
not catalyze the reaction at room temperature (entry 4 vs entry
3). Isophthaldehyde was also converted more quickly than
benzaldehyde (entry 8 vs entry 3). Unpurified butyraldehyde
reacted much more quickly than the benzaldehydes, while
purified substrate reacted more sluggishly (entry 9). Addition-
ally, β-hydride abstraction to form a silyl enol ether
product(but-1-en-1-yloxy)dimethyl(phenyl)silanewas ob-
served in the hydrosilylation of butyraldehyde. The results
compiled in Table 4 are consistent with the general mechanism
for hydrosilylation of carbonyls by tertiary silanes proposed by
Ojima41 and further expounded upon by Prock and Giering.42
b
c
Complex 2 used as catalyst. Doubly reacted product was also
d
observed (24%). Silyl enol ether was also observed (4%).
Hydrosilylation of Ketones. The investigation was
extended to ketone hydrosilylation (Scheme 3). In the 1970s,
Scheme 3. Catalytic Hydrosilylation of Ketones
Ojima and co-workers15d,41 and Corriu and Moreau43 reported
Rh phosphine complexes as carbonyl hydrosilylation catalysts.
Later Nile35b and Lappert35c used Rh NHC catalysts, while
Ritleng and co-workers used a Ni NHC catalyst.44 Even though
ketones are generally regarded as less reactive than aldehydes, a
recent report describes that the reactions of ketones are faster
than those of aldehydes when an air-sensitive Fe amide catalyst
is used.11j In this report, primary silanes worked well, but
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secondary and tertiary silanes did not, presumably due to
sterics.
Scheme 4. Catalytic Hydrosilylation of α,β-Unsaturated
Carbonyls
An evaluation of Rh complexes 1 and 2 for ketone
hydrosilylation is given in Table 5. Where possible, product
a
Table 5. Hydrosilylation of Ketones
α,β-unsaturated carbonyls that exhibited selectivity based upon
the silane.15c In their report, secondary silanes provided almost
exclusively the 1,2-addition product, while tertiary silanes
afforded almost exclusively the 1,4-product. Zheng and Chan
have proposed a reasonable mechanism for the observed
product distributions, in which the α,β-unsaturated carbonyl
coordinates to the Si via the carbonyl oxygen and side on (η2)
to Rh with the olefin after oxidative addition of the silane.46
The results of hydrosilylation of α,β-unsaturated aldehydes
and ketones with Rh complexes 1 and 2 are highlighted in
Table 6. Where possible, product peaks were compared to
a
Table 6. Hydrosilylation of α,β-Unsaturated Carbonyls
a
General conditions: HSiMe2Ph (0.077 mL, 0.50 mmol), C6D6 (0.5
mL), and complex 1 (0.5 mol %) were combined and incubated at 100
°C for 3 min. After addition of the substrate (0.46 mmol), the reaction
was monitored and product conversions were determined by 1H NMR
b
spectroscopy. Reactions were performed at 100 °C. Reaction was
c
performed at room temperature. Complex 2 was used as catalyst.
a
General conditions: HSiMe2Ph (0.077 mL, 0.50 mmol), C6D6 (0.5
peaks were compared to those reported in the literature.45
Conjugated aryl ketones did not undergo hydrosilylation at
room temperature and exhibited slow reaction rates at elevated
temperature (entries 1−4). Aliphatic ketones also had slow
reaction rates at elevated temperature (entries 5−8).
Acetophenone underwent reaction much more quickly than
benzophenone, likely due to sterics (entry 4 vs entry 3). In the
hydrosilylation of acetone, 2-butanone, and 2-octanone, the
data indicated that the reaction rate slowed with increasing
steric bulk (entry 6 vs entries 7 and 8). While the Rh−Cl
complex 1 catalyzed both sterically unhindered and sterically
demanding ketones at 100 °C, the Rh−I complex 2 yielded
only trace amounts of product in the hydrosilylation of acetone
(entry 7), which was similar to previous results of an analogous
di-Rh−I complex.27 The results compiled in Table 5 are
consistent with the mechanism for carbonyl hydrosilylation
proposed by Ojima.41
mL), and complex 1 (0.5 mol %) were combined and incubated at 100
°C for 3 min. After addition of the substrate (0.46 mmol), the reaction
was monitored and product conversions were determined by 1H NMR
spectroscopy. Product ratios are given with respect to each other only.
b
Reactions were performed at room temperature. Observed E:Z ratio
c
d
of product C: 13:5. Observed E:Z ratio of product A: 1:4. Complex
2 used as catalyst.
those reported in the literature.45b,47 Both aliphatic α,β-
unsaturated aldehydes and ketones were hydrosilylated, as
were α,β-unsaturated ketones in conjugation with aromatic
systems. The reaction of 2-cyclopentenone quickly yielded the
1,4-product exclusively at room temperature (entry 1). While
hydrosilylation of benzylideneacetone (entry 2) was not
particularly selective for C (5:3), the E:Z ratio of C indicated
much better selectivity (13:5). Increasing the steric bulk in
aliphatic systems (entries 3−5) slowed the rate of reaction,
especially at the α carbon (entry 3 vs 4). Using 1 as catalyst
afforded faster product conversion and better selectivity for the
1,4-addition product (entry 5). The data for catalysis by Rh
complex 1 compiled in Table 4 are consistent with the
proposed monohydrosilane mechanism of Zheng and Chan.
Use of Rh complex 2, however, yielded a 70:30 ratio of C to D.
Hydrosilylation of α,β-Unsaturated Carbonyls. To
assess the regioselectivity of Rh complexes 1 and 2, the
hydrosilylation of α,β-unsaturated aldehydes and ketones were
undertaken (Scheme 4). The addition of silane and hydride can
occur in either a 1,4- (C) or 1,2-manner (D). In 1982, Ojima
and Kogure reported highly regioselective hydrosilylations of
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a
This is not an expected result following Zheng and Chan’s
proposed mechanism. Studies into the nature of this selectivity
difference are currently under investigation.
Table 7. Hydrosilylation of Acyl Chlorides
Hydrosilylation of Acyl Chlorides. To expand the scope
of hydrosilylation to lesser-investigated carbonyl functional
groups, acyl chlorides were tested (Scheme 5). Only scattered
Scheme 5. Catalytic Hydrosilylation of Acyl Chlorides
reports of the reduction of acyl chlorides by hydrosilylation can
be found in the literature. In 1990, Oka and co-workers were
the first to report the hydrosilylation of acyl chlorides, albeit by
a γ-ray-initiated free radical mechanism.48 Last year, the
catalytic reduction of acyl chlorides to aldehydes with silanes
as the reductant was independently reported by Nikonov49 and
Tsuji.50 These reports, however, are analogous to those
describing the Rosenmund reduction, which is a catalytic
reduction of acyl chlorides to aldehydes, often using hydrogen
gas over a poisoned Pd-on-carbon catalyst.51
Four products were observed in the reaction illustrated in
Scheme 5. Chloro silyl ether product E is formed from
hydrosilylation of the acyl chloride substrate. Silyl enol ether
product F forms presumably from β-hydride elimination to give
HCl after carbonyl insertion into the the M−Si bond. Silyl
chloride elimination may form the aldehyde product G. Silyl
ether product H can form by two reasonable mechanims:
through hydrosilylation of aldehyde product G or Rh−H-
assisted reduction of chloro silyl ether product E. To the best of
our knowledge, the formation of halo silyl ether product C
from the hydrosilylation of acyl chlorides has not been
previously reported.
a
General conditions: HSiMe2Ph (0.077 mL, 0.50 mmol), C6D6 (0.5
mL), and complex 1 (0.5 mol %) were combined and incubated at 100
°C for 3 min. After addition of the substrate (0.46 mmol), the reaction
was monitored and product conversions were determined by 1H NMR
spectroscopy. Product ratios are given with respect to each other only.
b
Reactions were performed at 100 °C. Reaction was performed at
room temperature. Two equivalents of silane was used. Complex 2
used as catalyst. ClMe2SiH used instead of PhMe2SiH.
c
d
e
the more reactive silane ClMe2SiH at 100 °C, the reaction
afforded the silyl ether product H in >99% yield in less than 5
min (entry 4). The aldehyde G and silyl ether H were observed
without formation of chloro silyl ether E when aroyl chlorides
were used, likely due to the preference for resonance in forming
the aldehyde product.52 The chlorosilane byproduct resulting
from elimination of ClSiR3 to form the aldehyde product was
observed by NMR spectrsocopy (entry 4; see the Supporting
Information).
The results of acyl chloride hydrosilylation with Rh
complexes 1 or 2 are highlighted in Table 7. Upon
hydrosilylation of butyryl chloride (entry 1) with Rh−Cl
1
complex 1, the major product gave a H NMR resonance at δ
6.52. In a 2D heteronuclear multiple-quantum correlation
(HMQC) experiment, this resonance was correlated to a 13C
resonance at δ 84.4, which corresponds to the chlorosilyl ether
carbon. In 2002, Denmark and co-workers reported the
synthesis of trichlorosilylcyclohexylmethyl chloride by addition
of SiCl4 to cyclohexanecarboxaldehyde.52 The chloro silyl ether
product’s 1H NMR spectrum contained a doublet at δ 5.78 that
was reported to correspond to the chloro silyl ether proton, and
the 13C NMR spectrum contained a signal at δ 92.95 that was
reported to correspond to the chloro silyl ether carbon.
Complex 2 gave faster conversion, but the selectivity for chloro
silyl ether E was reduced. In preliminary trials using unpurified
butyryl chloride, the observed reaction rate was much faster and
more selective for the product E in comparison to the case
when purified substrate was used. Investigations into this rate
increase when unpurified acyl chloride was used are ongoing.
The hydrosilylation of cyclohexane carbonyl chloride (Table
7, entry 2) afforded a mixture of the four possible products,
with chloro silyl ether E being the major product. The
hydrosilylation of benzoyl or naphthoyl chloride did not result
in chloro silyl ether product formation (entries 3 and 4). Much
less aldehyde product was observed from the reaction of
naphthoyl chloride, likely from increased reactivity of
naphthaldehyde toward hydrosilylation as compared to
benzaldehyde (entry 4 vs entry 3). Upon hydrosilylation with
Hydrosilylation of Nitrogen-Containing Substrates.
Hydrosilylation of a number of different nitrogen-containing
substrates was investigated, as illustrated in Table 8. Where
possible, each product resonance was referenced to literature
values.13d,53 The first class of substrates we chose to include was
nitriles, the products of which can be silylimines53f or
silazanes.13d,53d Conversion to the corresponding disilazane
was observed in each case (entries 1−3). Benzonitrile was more
reactive than diphenylacetonitrile, which was more reactive
than propionitrile. These results mirrored the reactivity profile
observed for carbonyl substrates (Table 3, entry 2; Table 5,
entries 3 and 6). Hydrosilylation of substrates containing nitro
(−NO2) functional groups was also investigated.53a The
hydrosilylation of nitrobenzene proceeded slowly at 100 °C
to yield aniline (Table 8, entry 4); however, 2-nitropropane did
not undergo hydrosilylation (Table 8, entry 5). The reaction of
phenyl isocyanate proceeded at a rate comparable to that of
other benzylic substrates to yield a formamide product (Table
8, entry 6 vs entry 1; Table 3, entry 2). Dicyclohexylcarbodii-
mide (DCC) underwent hydrosilylation at a rate analogous to
that of aromatic substrates to yield a formamidine product
(Table 8, entry 7 vs entries 5 and 6; Table 3, entry 2).53b In an
experiment aimed at hydrosilylating DCC with no Rh catalyst,
no product formation was observed. A tertiary amide, N,N-
dimethylformamide (DMF), also underwent hydrosilylation
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a
quickly at 100 °C to form trimethylamine (Table 8, entry 8).
Reduction of the carbonyl group to form the amine is the
normal reactivity observed for hydrosilylation of tertiary
amides.15e,54 Attempts to hydrosilylate a variety of secondary
amides, including aryl amides, alkyl amides, and carbamates,
yielded no conversion (Table 8, entries 9−13).
Table 8. Hydrosilylation of Nitrogen-Containing Substrates
Hydrosilylation of CO2. The final substrate evaluated for
hydrosilylation was CO2. Many reports have described CO2
hydrogenation with H2, but those often use high pressures of
hydrogen gas and high temperatures to obtain conversion to
reduced products.55 Hydrosilylation offers a mild method for
CO2 reduction,56 and this methodology has been reported to
generate primarily formic acid,57 methane,58 and methanol.59
Other reports have cited CO2 hydrosilylation with a variety of
transition-metal-based catalysts.60 Preliminary carbon dioxide
hydrosilylation experiments catalyzed by 1 utilizing dry ice as
the source of CO2 provided justification for further study, as
product conversion was observed. On the basis of those
preliminary results, the hydrosilylation of isotopically labeled
13CO2 with HSiMe2Ph was undertaken, as illustrated in Scheme
6.
While hydrosilylation of carbon dioxide (1 atm) did not
proceed at room temperature, it did occur at 100 °C with both
Rh complexes 1 and 2, as illustrated in Table 9. Three products
a
Table 9. Hydrosilylation of 13CO2
b
c
entry
cat.
t (h)
products I:J:K
TN
1
2
1
2
19
19
6:1:1
1:0:0
4
1
a
General conditions: HSiMe2Ph (0.077 mL, 0.50 mmol), C6D6 (0.5
mL), and complex 1 (0.5 mol %) were combined and incubated at 100
°C for 3 min. After addition of 1 atm of 13CO2 gas, the reaction was
1
monitored and product conversions were determined by H NMR
spectroscopy. Product ratios are given with respect to each other only.
b
Reactions were performed at 100 °C. See Scheme 6 for products.
c
TN = turnover number, determined by total conversion.
were observed, including silylformate I, silyl methyl ether J, and
silyl ether acetal K. Rh complex 1 was found to be a more active
catalyst than was complex 2 (entry 1 vs 2). Both 13C{1H} and
13C spectra of the product mixture from catalysis by complex 1
after 19 h contained a doublet at δ 204.2 (JC−Rh = 49.0 Hz).
In a report by Komiya and co-workers, η1(C)-carbon dioxide
ligated to a Fe(II) metal center was activated using a Sn
electrophile to generate a metal carboxyl complex.61
A
13C
NMR spectrum of the Fe carboxyl complex contained a
resonance at δ 239.5 that was assigned to a carbon with
significant carboxylate (as opposed to dioxycarbene) character.
A report in 1991 by Gibson and co-workers described Fe(II)
carboxylates to resonate in 13C NMR spectroscopy at δ ∼215.62
Rh η1(C)-carboxylato complexes have been reported before,
but their 13C NMR chemical shifts have not.63 In 1996, Milstein
and co-workers reported the reaction of CO2 with a T-shaped
Rh(I) complex,64 in which a 13C NMR spectrum of the product
contained a doublet at δ 156.0 (JC−Rh = 18.4 Hz) that was
assigned to a Rh η2(C,O)-CO2 complex. Therfore, the
a
General conditions: HSiMe2Ph (0.077 mL, 0.50 mmol), C6D6 (0.5
mL), and complex 1 (0.5 mol %) were combined and incubated at 100
°C for 3 min. After addition of the substrate (0.46 mmol), the reaction
was monitored and product conversions were determined by 1H NMR
b
spectroscopy. Reactions were performed at 100 °C. Reaction was
performed at room temperature. 3 equiv of HSiMe2Ph and 2 mol %
of Rh catalyst were used. 2 equiv of HSiMe2Ph was used. 2 mol % of
catalyst was used.
c
d
e
Scheme 6. Hydrosilylation of CO2
6253
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Article
(hexamethyldisilazanyl)styrene),53e propionitrile ((CH3)2CHCH2N-
(SiMe2Ph)2),53f CO2 (Ph2MeSiOCHO57 and SiOMe).59b
General Considerations. Liquid aldehydes and acyl chlorides
were distilled and were stored under Ar prior to use. Solid aldehydes
were recrystallized prior to use. All other substrates were purchased
and were used as received. Rh catalysts 1 and 2 were prepared
according to literature procedures.28,66
resonance observed at δ 204.2 was assigned to a Rh η1(C)-
silylcarboxylate complex, as shown in Figure 2. Because the
reaction likely proceeds through the Ojima mechanism for
hydrosilylation, this complex represents the product of 13CO2
insertion into the M−Si bond.41
General Hydrosilylation Procedure. Catalyst (0.5 mol %),
HSiMe2Ph (0.077 mL, 0.50 mmol), and C6D6 (0.5 mL) were
combined and incubated at 100 °C for 3 min. After addition of the
substrate (0.46 mmol), the reaction was monitored and product
conversions were determined by 1H NMR spectroscopy. Product
ratios were determined relative to each other.
Figure 2. Proposed Rh η1(C)-silylcarboxylate formed during hydro-
silylation of CO2.
ASSOCIATED CONTENT
* Supporting Information
Figures giving selected key spectroscopic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
CONCLUSIONS
■
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
■
The previously reported di-Rh complexes 1 and 2 were found
to be effective catalysts in the hydrosilylation of a wide variety
of substrates, including aldehydes, ketones, α,β-unsaturated
carbonyls, acyl chlorides, nitriles, alkenes, nitro groups,
isocyanates, and tertiary amides. Furthermore, in the hydro-
silylation of some acid chlorides, chloro silyl ethers were
observed as major products of catalysis for the first time. Each
reaction was performed in the presence of both air and
moisture, which is a major advantage over air-sensitive
transition-metal catalyst systems.
ACKNOWLEDGMENTS
We gratefully acknowledge the National Science Foundation
(NSF Grant Nos. CHE0809732, EPS0903787).
■
REFERENCES
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Data for the hydrosilylation products from the following substrates
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dx.doi.org/10.1021/om400452q | Organometallics 2013, 32, 6248−6256