Selective Metal-Free Hydrosilylation of CO2Catalyzed by Triphenylborane in Highly Polar, Aprotic Solvents

Article abstract of DOI:10.1002/chem.201601006

Triphenylborane (BPh₃) in highly polar, aprotic solvents catalyzes hydrosilylation of CO₂effectively under mild conditions to provide silyl formates with high chemoselectivity (>95 %) and without over-reduction. This system also promotes reductive hydrosilylation of tertiary amides as well as dehydrogenative coupling of silane with alcohols.

Full text of DOI:10.1002/chem.201601006

DOI: 10.1002/chem.201601006
Communication
&
Hydrosilylation
Selective Metal-Free Hydrosilylation of CO2 Catalyzed by
Triphenylborane in Highly Polar, Aprotic Solvents
Debabrata Mukherjee+, Daniel F. Sauer+, Alessandro Zanardi, and Jun Okuda*[a]
B(C6F5)3 as a versatile catalyst for addition of SiÀH moieties
Abstract: Triphenylborane (BPh3) in highly polar, aprotic
solvents catalyzes hydrosilylation of CO2 effectively under
mild conditions to provide silyl formates with high chemo-
selectivity (>95%) and without over-reduction. This
system also promotes reductive hydrosilylation of tertiary
amides as well as dehydrogenative coupling of silane with
alcohols.
across C=O,[8] C=N,[9] and C=C[10] bonds. Interestingly however,
effective CO2 hydrosilylation independently catalyzed by
B(C6F5)3 remains unknown.[1a,y] Piers and co-workers’ metal-free
tandem FLP/B(C6F5)3 system is uncontrollable for CO2 hydro-
silylation using Et3SiH with over-reduction leading to CH4 as
the end product along with (Et3Si)2O. Conceptually, the weaker
Lewis acid BPh3 should be less suited to bond activation. Oes-
treich and co-workers demonstrated that, unlike the fluorinat-
ed boranes, BPh3 is inefficient for catalytic hydrosilylation of
carbonyls, imines, and olefins in toluene.[11] Herein we report
that the apparently ‘weaker’ Lewis acid BPh3 catalyzes the hy-
drosilylation of CO2 to give silyl formates with exceptional se-
lectivity under mild conditions. Any catalytic transformation
solely mediated by BPh3 is unprecedented.
Hydrosilylation is an important catalytic process for reductive
transformation of CO2 that uses inexpensive hydrosilanes as
the readily available liquid reductants.[1] Additionally, the pri-
mary products of this reaction, silyl formates, are precursors for
formic acid, formamides, formates, and many other valuable
chemicals.[1a,o,x] However, selective one-step reduction of CO2 to
silyl formates [Eq. (1)] is challenging due to the easy over-re-
duction of carbonyl functions.
Initially, PhMeSiH2 was used in hydrosilylation of CO2 under
stoichiometric conditions in the presence of 10 mol% of BPh3.
Various organic solvents, ranging from aromatic to highly polar
and aprotic, were screened at 408C (Table 1).
Table 1. BPh3-catalyzed hydrosilylation of CO2 with PhMeSiH2.[a]
To date, several transition metal catalysts,[1i–l,n,r–t,v–x,z] either
alone or together with B(C6F5)3 as frustrated Lewis pairs (FLPs),
as well as main group metals such as zinc,[1o,p] have been re-
ported to catalyze this transformation selectively. However,
their practical applications are often limited, possibly owing to
factors like accessibility, air and moisture sensitivity, functional
group intolerance, and harsh reaction conditions (high pres-
sure or temperature).[1a] A few organocatalysts, such as 1,3,2-di-
azaphospholene[2] and N-heterocyclic carbenes,[3] have also
emerged recently as “metal-free” alternatives. Alkali metal fluo-
ride and acetate salts are also active catalysts for the same re-
action.[4] Some metal[1h,m,5] and nonmetal[6] catalysts promote
the N-formylation of amines by using CO2 and hydrosilanes,
a reaction in which silyl formates serve as intermediates.
Entry Solvent
Dielectric constant e0 t [h] Conversion [%]
1
2
3
4
5
6
7
8
9
10
[D6]benzene
[D8]toluene
[D8]THF
2.3
2.4
7.5
7
7
7
7
7
7
5
–
–
0
0
6
[D2]DCM
9.1
7
[D3]acetonitrile
benzonitrile
[D3]nitromethane
[D7]DMF
[D6]DMSO
propylene carbonate 64.0
37.5
26.0
35.8
38.0
47.0
>95
40
>95
–
–
2.5 >95
[a] BPh3 (0.021 mmol), PhMeSiH2 (0.210 mmol), 1,3,5-trimethoxybenzene
as internal standard (0.030 mmol), solvent (500 mL).
While investigating zinc hydrides and their reactivity towards
CO2,[1p,7] we discovered that triphenylborane (BPh3) is also ca-
pable of effecting this transformation. The groups of Piers and
Gevorgyan have described the more strongly Lewis acidic
No turnover occurred in nonpolar solvents such as
[D6]benzene or [D8]toluene (Table 1, entries 1 and 2). However,
the slightly more polar solvents [D8]THF and [D2]DCM (Table 1,
entries 3 and 4) led to trace amounts of PhMeSiH(O2CH). The
efficiency drastically improved with the more polar
[D3]acetonitrile (Table 1, entry 5), giving PhMeSiH(O2CH) quanti-
tatively within 7 h with remarkable selectivity and without
over-reduction (Figure 1). Notably, only one of the two silicon–
hydride bonds in PhMeSiH2 reacted selectively.[12] Whereas the
[a] Dr. D. Mukherjee,+ D. F. Sauer,+ Dr. A. Zanardi, Prof. Dr. J. Okuda
Institute of Inorganic Chemistry, RWTH Aachen University
Landoltweg 1, 52056 Aachen (Germany)
E-mail: jun.okuda@ac.rwth-aachen.de
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201601006.
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slower rate, presumably due to steric restraint. However,
(EtO)3SiH and polymethylhydrosiloxane (PMHS; Table 2, en-
tries 4 and 5) remained unreacted under the same conditions,
for reasons that are currently unknown.
Several batches of BPh3, freshly prepared as well as commer-
cially purchased, exhibited similar activity on repeated occa-
sions. Moreover, a blank run without the borane was not pro-
ductive. Decreased loadings of BPh3 prolonged the reaction
times. Isotopically labelled 13CO2 was incorporated and the for-
mation of PhMeSiH(O213CH) was confirmed by NMR spectrosco-
py. Adding another equivalent of PhMeSiH2 to PhMeSiH(O2CH)
did not show any change, even at elevated temperatures. Hy-
drosilylation of ethyl acetate in [D3]acetonitrile was also unsuc-
cessful. It is intriguing that this system transforms CO2 selec-
tively and reduces amides, but is inert to esters; such a behav-
ior may be compared to that of Beller’s catalyst system
Zn(OAc)2/(EtO)3SiH.[13] More detailed study on BPh3-mediated
selective amide reduction is currently ongoing.
Attempts to obtain direct evidence for the interaction of
BPh3 with CO2 or PhMeSiH2, individually or during catalysis,
proved unsuccessful, suggesting a weak and/or dynamic
nature. A recent quantum chemical calculation demonstrated
that AlCl3 as Lewis acid is self-sufficient to catalyze hydride
transfer to CO2.[14] BPh3-mediated hydride abstraction from the
b-agostic SiÀH moieties in [M{C(SiHMe2)3}2(thf)2] (M=Ca, Yb) is
already documented.[15] Indirect evidence for BPh3-mediated
silane activation was obtained through H/D exchange study.
Thus, a 1:1:1 mixture of BPh3, PhMeSiD2, and Et3SiH at 408C
provided an isotopically scrambled silane mixture within 6 h in
[D3]nitromethane or in [D3]acetonitrile, but not in [D6]benzene
or in the absence of BPh3. B(C6F5)3 is also known to promote
such H/D interchange between silanes in toluene.[16] Further-
more, catalysis was faster with PhMeSiH2 than with PhMeSiD2
Figure 1. 1H NMR spectra of a catalytic reaction mixture in [D3]acetonitrile.
Bottom: 30 min after adding CO2, RT; top: after complete consumption of
PhMeSiH2 (7 h, 408C).
activity was low in the less polar benzonitrile (Table 1, entry 6),
superior performance was found in [D3]nitromethane (entry 7).
In [D7]DMF (Table 1, entry 8), the amide group was readily re-
duced instead of CO2 to give (CH2D)N(CD3)2. The amide reduc-
tion was further confirmed as N,N-dimethylacetamide was also
converted into N,N-dimethylethylamine (NMe2Et) within 4 h at
ambient temperature in [D8]THF, [D2]DCM and [D3]acetonitrile.
[D6]DMSO (Table 1, entry 9) was also unsuitable, as an intract-
able product mixture was obtained even without CO2. A fur-
ther acceleration was recorded in propylene carbonate
(Table 1, entry 10), as the reaction proceeded to completion
within only 2.5 h. Therefore, the solvent polarity seems critical
in this context.
under identical conditions.
A
plot of conversion in
[D3]nitromethane against reaction time (Figure 2) leads to
a rough estimate of a primary kinetic isotope effect (kH/kD) of
1.28(4). This agrees well with results for the B(C6F5)3-catalyzed
hydrosilylation of PhC(O)CH3 using Ph3SiH/D [kH/kD =1.40(5)][16]
or for hydride abstraction from silanes by carbocations (kH/kD =
Even with primary silane PhSiH3 (Table 2, entry 1), PhSi-
H2(O2CH) was identified as the main product. A tertiary silane
Et3SiH (Table 2, entry 3) also provided Et3Si(O2CH) but at much
Table 2. BPh3-catalyzed hydrosilylation of CO2 in [D3]acetonitrile with dif-
ferent silanes.[a]
Entry
Silane
Product
t [h]
Conversion [%]
1
2
PhSiH3
PhMeSiH2
PhSiH2(O2CH)
PhMeSiH(O2CH)
7
7
7
36
7
7
86
>95
30
>95
0
3
Et3SiH
Et3Si(O2CH)
4
5
(EtO)3SiH
PMHS
–
–
0
[a] BPh3 (0.021 mmol), silane (0.210 mmol), 1,3,5-trimethoxybenzene as in-
ternal standard (0.030 mmol), [D3]acetonitrile (500 mL).
Figure 2. Comparison of % conversions of CO2 hydrosilylation with PhMeSiH2
and PhMeSiD2, in the presence of 10 mol% BPh3 in [D3]nitromethane.
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1.4–1.9).[17] Presumably, activation of the silicon–hydride bonds
determines the rate of the reaction.
perimentally verified, these possible interactions could be very
dynamic and hence should not be completely ruled out.
In Scheme 1, we preliminarily propose a dual activation
mechanism through a simplified schematic diagram. This
The effectiveness of BPh3 in highly polar, aprotic solvents is
further prominent in its catalytic activity towards dehydrogena-
tive silylation of alcohols. Although B(C6F5)3 is already known
to promote this transformation in toluene or dichlorome-
thane,[20] catalysis with much milder Lewis acidic BPh3 still de-
serves special attention. Most importantly, as in case of CO2 hy-
drosilylation, similar dependence on solvent polarity is envis-
aged (Table 3). A 1:1 mixture of ethanol and PhMeSiH2 reacted
Table 3. Dehydrogenative coupling of PhMeSiH2 with EtOH in the pres-
ence of 10 mol% BPh3.[a]
Scheme 1. Proposed dual activation mechanism for BPh3-catalyzed CO2 hy-
drosilylation in highly polar solvents.
Entry
Solvent
t [h]
Selectivity [%]
B
mechanism involves weak or dynamic coordination of BPh3 to
CO2, as well as to the SiÀH moiety, generating two partially po-
larized transient species, which ultimately lead to the products.
Polar solvents with high dielectric constants are known to sta-
bilize charged species, and thus positively influence such inci-
dent. Total inactivity in benzene against the fast catalysis in
acetonitrile, nitromethane, and propylene carbonate clearly im-
plies a strong solvent effect. Notably, the nickel catalyst [{(dip-
A
1
2
3
4
5
[D6]benzene
[D8]THF
[D3]acetonitrile
[D3]nitromethane
propylene carbonate
24
6
3
<1
<1
90
67
90
90
90
10
33
10
10
10
[a] BPh3 (0.021 mmol), PhMeSiH2 (0.210 mmol), EtOH (0.210 mmol), 1,3,5-
trimethoxybenzene as internal standard (0.030 mmol), solvent (500 mL).
pe)Ni(m-H)}2]
[dippe=1,2-bis(diisopropylphosphino)ethane]
also requires BEt3 as a co-catalyst, which is proposed to acti-
vate both CO2 and Et3SiÀH, albeit without experimental evi-
dence.[1j] A detailed mechanistic and computational study into
the true nature of BPh3-mediated activation is currently under-
way. Admittedly, the actual scenario could be more complicat-
ed than that shown in the simplified Scheme 1.
slowly in [D6]benzene (Table 3, entry 1), whereas the reaction
rate dramatically increased with solvent polarity (entries 2–5).
Rapid H2 evolution with strong effervescence was observed in
[D3]nitromethane and in propylene carbonate. The system was
again highly selective for PhMeSiH(OEt) (90%) over PhMe-
Si(OEt)2 (10%), except in [D8]THF. Detailed study on this aspect
of BPh3 along with a greater substrate scope will be published
elsewhere. Attempted N-formylation catalysis of amines with
CO2 and hydrosilanes in the presence of BPh3 resulted in dehy-
drogenative SiÀN bond formation under similar conditions and
will not be pursued further in the present context.
A FLP-type CO2 activation mechanism may also prevail. Polar
solvent molecules may act as Lewis bases to generate intermo-
lecular FLPs with BPh3 in situ and thereby activate CO2. The
nonfluorinated FLP tBu2PCH2BPh2 readily captures CO2 in tolu-
ene despite its substantially lower Lewis acidity.[18] However,
the 11B resonances of BPh3 at d=68 ppm in [D3]nitromethane
[Gutmann’s donor number, DN=2.7] and propylenecarbonate
(DN=15.1), which is also the case in [D6]benzene (DN=0.1), in-
dicates free borane in all of them, whereas the same resonance
at d=0 ppm in acetonitrile (DN=14.1) suggests solvent coor-
dination. Thus, although having similar donor ability in Gut-
mann’s scale based on SbCl5 coordination,[19] the significant dif-
ference between acetonitrile and propylene carbonate towards
Lewis acidic BPh3 is highly intriguing. Also notably, BPh3 alone
or in catalytic mixture, is poorly soluble in nitromethane and
propylene carbonate at ambient temperature, but goes into
solution at 408C. At this temperature, the 11B resonance still
appears at d=68 ppm and remains unchanged during cataly-
sis. Apparently, acetonitrile coordination to BPh3 inhibits cataly-
sis, as the observed rate difference compared to nitromethane
with similar dielectric but poorer donor ability. Apart from
BPh3, substrate activation from the polar solvents are also con-
sidered. However, the 29Si and 13C NMR data of PhMeSiH2 and
13C NMR of CO2 in [D6]benzene and [D3]nitromethane are com-
parable, and thus proved to be inconclusive. Although not ex-
In conclusion, the features of BPh3 highlighted herein dis-
pute the traditional belief that weaker nonfluorinated boranes
are catalytically ineffective. In addition, aside from conventional
strong catalyst–substrate interactions, weaker interactions
appear beneficial for achieving chemoselectivity under the ap-
propriate conditions. We are currently attempting to fine-tune
simple boranes for further reactivity enhancement and to
apply this concept to other important transformations.
Experimental Section
Typical CO2 hydrosilylation experiment with PhMeSiH2
A J. Young NMR tube was charged with BPh3 (5 mg, 0.021 mmol),
1,3,5-trimethoxybenzene (5 mg, 0.030 mmol) as an internal stan-
dard, deuterated solvent (500 mL), and PhMeSiH2 (25 mg,
0.210 mmol). Three freeze–pump–thaw cycles were applied before
the headspace (ca. 2.5 mL) was filled with CO2 (1 bar) to deliver ap-
proximately 0.21 mmol of CO2. After the temperature was in-
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Acknowledgements
We thank the Deutsche Forschungsgemeinschaft for financial
support through the International Research Training Group
“Selectivity in Chemo- and Biocatalysis” and the Alexander von
Humboldt Foundation for a fellowship to D.M.
Keywords: boranes · carbon dioxide · chemoselectivity ·
hydrosilylation · solvent effects
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Received: March 3, 2016
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Published online on April 26, 2016
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