NHC-catalyzed chemo- and regioselective hydrosilylation of carbonyl derivatives

Article abstract of DOI:10.1055/s-0031-1290208

The hydrosilylation of carbonyl derivatives has been explored by the activation of diphenylsilane in the presence of a catalytic amount of an N-heterocyclic carbene (NHC). Presumably, a hypervalent silicon intermediate featuring strong Lewis acid character allows dual activation of both the carbonyl moiety and the hydride at the silicon center. Reduction under mild conditions could be accomplished using this organocatalytic process. Some interesting selectivities have been encountered. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1290208

LETTER
433
NHC-Catalyzed Chemo- and Regioselective Hydrosilylation of
Carbonyl Derivatives
Regioselective
Hy
i
drosilyl
w
ation of Carbonyl
u
Derivatives Zhao,a Dennis P. Curran,b Max Malacria,a Louis Fensterbank,*a Jean-Philippe Goddard,*a Emmanuel Lacôte*a,1
a
UPMC Univ Paris 06, Institut Parisien de Chimie Moléculaire (UMR CNRS 7201), C. 229, 4 place Jussieu, 75005 Paris, France
Fax +33(1)44277360; E-mail: louis.fensterbank@upmc.fr; E-mail: jean-philippe.goddard@upmc.fr; E-mail: emmanuel.lacote@icsn.cn-
rs-gif.fr
Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA
b
Received 4 November 2011
ed at the end of the reaction in order to isolate the alcohols.
Sodium hydroxide (2 M solution) could also be used, but
TBAF was selected for our study.
Abstract: The hydrosilylation of carbonyl derivatives has been ex-
plored by the activation of diphenylsilane in the presence of a cata-
lytic amount of an N-heterocyclic carbene (NHC). Presumably, a
hypervalent silicon intermediate featuring strong Lewis acid char-
acter allows dual activation of both the carbonyl moiety and the hy-
dride at the silicon center. Reduction under mild conditions could be
accomplished using this organocatalytic process. Some interesting
selectivities have been encountered.
Thiazolylidene (from A) and ICy (from B) did not allow
the formation of 2a (Table 1, entries 1 and 2). Under the
same conditions, IPr (from C) gave rise to the desired
product in 15% yield (Table 1, entry 3). SIMes (from D)
was more active (60%; Table 1, entry 4). The best candi-
date was the triazolylidene derived from E, which yielded
93% 2a (Table 1, entry 5).
Key words: hydrosilylation, N-heterocyclic carbenes, organocatal-
ysis, reduction, green chemistry
Because t-BuOK also activates the silane and leads to re-
duction in the absence of carbene (Table 1, entry 6),10 it
was necessary to make sure that the catalytic activity de-
rived from carbene activation. For this purpose, a base-
free sample11 of F was tested and it was found that per-
forming the reaction under these conditions also led to a
productive outcome (Table 1, entry 7). This result shows
that NHCs are indeed able to catalyze the hydrosilylation
of ketones. We believe there is an incentive to use NHCs
rather than t-BuOK because they are not as strong Brønsted
bases and chiral versions can be made.
Hydrosilylation of carbonyl derivatives is an essential
functional group transformation in synthetic organic
chemistry.2 However, to date, the reaction is usually
achieved using noble metal catalysts (Rh, Pt, Ru),3 or
greener and cheaper metal complexes involving copper4
or iron.5 Nevertheless, the cost of the catalysts and/or
leaching are still challenges to be solved for efficient hy-
drosilylation. Hence, the development of effective metal-
free organocatalyzed processes for the hydrosilylation of
carbonyl derivatives is important and has attracted grow-
ing interest.6
Nonetheless, in order to sidestep this issue, we switched to
non-nucleophilic NaH for the deprotonation of E. This
modification resulted in lower yield (77%; Table 1, entry
8), however, it was improved when N,N-dimethylform-
amide (DMF) was used as solvent (94%; Table 1, entry 9).
Interestingly, the amount of silane could be decreased to
0.6 equivalents without loss of activity (90%; Table 1, en-
try 10), which suggests that the two hydrides on the silane
are available for the hydrosilylation process. As expected,
no reaction took place in the absence of NHC or base
(Table 1, entries 11 and 12).
Since we have shown that the Si–H bond can be activated
by N-heterocyclic carbenes (NHCs) and harnessed for the
hydrosilylation of olefins,7 we decided to also explore the
NHC-catalyzed reduction of carbonyls. Such a process
has been described recently with poly-NHC particles, but,
to the best of our knowledge, no molecular equivalent has
been reported.8 The formation of a more Lewis acidic hy-
pervalent silicon intermediate9 could promote dual activa-
tion of the carbonyl as well as the hydride. Herein, we
report our results.
First, we optimized the hydrosilylation/desilylation se-
quence with acetophenone (1a) as a model system to give
phenylethanol (2a). A range of azolium salts were select-
ed as precursors of NHCs (Figure 1). The corresponding
carbenes were generated in situ by deprotonation of azoli-
um salts A–E with either t-BuOK or NaH. Diphenylsilane
was found to be the best hydride source for this transfor-
mation. Tetrabutylammonium fluoride (TBAF) was add-
Bn
N
Cy
Cy
i-Pr
Cl
i-Pr
N
N
N
N
Cl
BF4
S
A
B
C
••
i-Pr
N
i-Pr
Mes
Mes
Cl
N
N
N
N
N
Ph
Cl
N
SYNLETT 2012, 23, 433–437
x
x.
x
x.
2
0
1
2
D
E
F
Advanced online publication: 26.01.2012
DOI: 10.1055/s-0031-1290208; Art ID: D62711ST
© Georg Thieme Verlag Stuttgart · New York
Figure 1

434
Q. Zhao et al.
LETTER
With the optimized conditions in hand (Table 1, entry 10), (Table 2, entry 14). This method has been successfully ex-
we investigated the scope of the reaction. Acetophenone tended to the reduction of N-Boc-protected imine 3, lead-
derivatives were selected to screen the impact of phenyl ing to the formation of carbamate 4 in 75% yield (Table 2,
ring substituents. Substrates 1b–e, bearing electron-with- entry 15). Such a transformation is generally not possible
drawing or electron-donating groups, gave good to excel- under transition-metal catalysis due to the reactivity of the
lent yields of the corresponding alcohols 2b–e (Table 2, carbamoyl group.14
entries 2–5). High chemoselectivity was observed since
We then turned our attention to unsaturated carbonyl com-
nitrile (1c)12 or ester moieties (1d)13,6a,10b were not re-
pounds. To our delight, b-ionone 5 was regioselectively
duced. 2-Naphthyl methyl ketone 1f quantitatively deliv-
reduced to the 1,2-adduct 6 in 74% yield. No 1,4- or 1,6-
ered 2f (Table 2, entry 6). Propiophenone 1g was
adduct was observed (Scheme 1). Aldehydes reacted with
efficiently reduced in five hours (Table 2, entry 7). Re-
duction of cyclic aromatic ketones 1h and 1i required
stoichiometric amounts of diphenylsilane to reach com-
NHCs to form the corresponding Breslow intermediates.15
This process is fast and reversible but can be favored
under protic conditions.15e Under our conditions, (E)-p-
pletion (Table 2, entries 8 and 9). Epoxychalcone 1j was
methoxycinnamaldehyde (7) was chemoselectively re-
chemoselectively reduced to the epoxyalcohol 2j in 92%
duced to form the allylic alcohol 8 exclusively in 85%
yield, leaving the reactive oxirane untouched (Table 2, en-
yield.
try 10).
1. E (10 mol%)
Table 1 Reduction of 1a
O
OH
NaH (10 mol%)
H2SiPh2 (0.8 equiv)
DMF, r.t., 6 h
OH
O
1. catalyst (10 mol%), base (10 mol%)
H2SiPh2 (1.1 equiv), solvent, r.t., 2–6 h
2. TBAF (1 equiv)
r.t., 0.5 h
5
6, 74%
2. NaOH (2 M) or TBAF (1 equiv), 0.5 h
1a
2a
1. E (10 mol%)
NaH (10 mol%)
H2SiPh2 (0.6 equiv)
DMF, r.t., 3 h
Entry
1
Catalyst
Base
Solvent
THF
THF
THF
THF
THF
THF
THF
THF
DMF
DMF
DMF
DMF
Yield (%)a
OH
O
A
B
C
D
E
–
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
–
0
H
2. TBAF (1 equiv)
r.t., 0.5 h
2
0
MeO
MeO
7
8, 85%
3
15
Scheme 1
4
60
To our delight, alcohols 10a and 10b were obtained in 75
and 65% yields, respectively, from esters 9a and 9b
(Scheme 2). The reactions proceeded smoothly with sto-
ichiometric amounts of silane, although longer reaction
times were needed compared to the reaction with ketones.
5
93
6
95
7
F
E
E
E
–
54
8
NaH
77
PhCO2Et
PhCH2OH
9
NaH
94
1. E (10 mol%)
NaH (10 mol%)
10a, 75% (conv. 80%)
9a
10c
11
12
NaH
92 (90)b
0
H2SiPh2 (1.1 equiv)
DMF, r.t., 15 h
2. TBAF (1 equiv), r.t., 0.5 h
NaH
CO2Et
Ph
Ph
OH
10b, 65% (conv. 70%)
9b
–
–
0
a Calculated from NMR spectra using Ph3CH as internal standard.
b Isolated yield given in parentheses.
Scheme 2
c H2SiPh2 (0.6 equiv) was used.
The reduction of ethyl benzoylacetate (11a) was investi-
gated, but no reaction was observed even when 1.1 equiv-
alent of diphenylsilane was used (Scheme 3).
Acetophenone was added to this reaction mixture in order
to assess whether the catalytic activity was maintained in
the presence of 11a; under these conditions, no reaction
occurred, indicating that inactivation of the catalyst had
occured. We surmised that the acidic protons might proto-
nate the NHC. When cyclopropyl derivative 11b was sub-
mitted to the reduction conditions, 1,3-diol 12 was
obtained in 25% yield and further improved to 82% with
two equivalents of diphenylsilane. The selective reduction
The selectivity against the reduction of alkene was high-
lighted by the efficient formation of alcohol 2k, in which
the olefin remained untouched, in 81% yield (Table 2, en-
try 11). Cyclohexyl methyl ketone was reduced sluggish-
ly, and 2l was isolated in 83% yield after one day with a
stoichiometric amount of silane (Table 2, entry 12). The
presence of an amino group did not hamper the process
since 2m was obtained in 85% yield (Table 2, entry 13).
The reduction of norcamphor 1n proceeded with complete
diastereoselectivity in favor of the endo-alcohol 2n
Synlett 2012, 23, 433–437
© Thieme Stuttgart · New York

LETTER
Regioselective Hydrosilylation of Carbonyl Derivatives
435
of the ketone group was not observed. This contrasts with leading to the corresponding alkoxysilane with a hydride
the selective reduction of ketone 1d in a shorter reaction suitably positioned for intramolecular transfer onto the es-
time with the same amount of silane (Table 1, entry 4). ter group.16
We postulated a fast reduction of the ketone group in 11b,
Table 2 Scope of the Reduction Process
X
XH
1. E (10 mol%), NaH (10 mol%)
H2SiPh2 (0.6 equiv), DMF, r.t., 2–6 h
R1
R2
R1
R2
2. TBAF(1 equiv), r.t., 0.5 h
X = O or NBoc
1a–n, 3
2a–n, 4
Entry
Substrate
Product
Yield (%)a
1
2
3
4
1a; R = H
1b; R = OMe
1c; R = CN
1d; R = CO2Me
2a
2b
2c
2d
90
99
83
99
OH
O
R
R
O
OH
5
6
7
1e
1f
2e
2f
87
99
87
F
F
O
OH
O
O
OH
1g
2g
OH
8b
9b
1h (n = 0)
1i (n = 1)
2h
2i
90
89
n
n
O
OH
10
11
1j
2j
92c
81
Ph
Ph
Ph
O
Ph
O
1k
2k
O
O
OH
OH
12b
13
1l
2l
83
85
1m
2m
BnN
O
BnN
OH
14
1n
3
2n
4
68
75
O
OH
NHBoc
NBoc
15
Ph
Ph
a Isolated yield.
b H2SiPh2 (1.1 equiv) was used in 24 h.
c Ratio syn/anti = 2.5:1 based on 1H NMR analysis of the crude reaction mixture.
© Thieme Stuttgart · New York
Synlett 2012, 23, 433–437

436
Q. Zhao et al.
LETTER
Weinheim, 2000. (e) Marko, I.; Sterin, S.; Buisine, O.;
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In conclusion, we have demonstrated the ability of NHCs
to catalyze the metal-free hydrosilylation of aldehydes,
ketones, esters and ketimines.17 This organocatalyzed re-
action exhibits high selectivity. All of the hydrides on the
silicon atom can be transferred. We are currently investi-
gating the mechanism for this transformation and the abil-
ity of chiral NHCs to promote an enantioselective variant
of the reaction.
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O
optimized condtions
CO2Et
CO2Et
N. R.
N. R.
Ph
Ph
11a
11a
O
O
O
optimized condtions
+
Ph
1a
1. E (10 mol%)
NaH (10 mol%)
H2SiPh2 (n equiv)
DMF, r.t., 15 h
OH
OH
CO2Et
Ph
Ph
2. TBAF (1 equiv), r.t., 0.5 h
11b
12
n = 0.6, 25% (conv. 30%)
n = 2, 82% (conv. 100%)
Scheme 3
(5) For iron catalysts, see: (a) Nishiyama, H.; Furuta, A. Chem.
Commun. 2007, 760. (b) Shaikh, N. S.; Junge, K.; Beller, M.
Org. Lett. 2007, 9, 5429. (c) Shaikh, N. S.; Enthaler, S.;
Junge, K.; Beller, M. Angew. Chem. Int. Ed. 2008, 47, 2497.
(d) Sui-Seng, C.; Freutel, F.; Lough, A. J.; Morris, R. H.
Angew. Chem. Int. Ed. 2008, 47, 940. (e) Gutsulyak, D. V.;
Kuzmina, L. G.; Howard, J. A. K.; Vyboishchikov, S. F.;
Nikonov, G. I. J. Am. Chem. Soc. 2008, 130, 3732.
(f) Tondreau, A. M.; Lobkovsky, E.; Chirik, P. J. Org. Lett.
2008, 10, 2789. (g) Yang, J.; Tilley, T. D. Angew. Chem. Int.
Ed. 2010, 47, 940.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
This work was supported by grants from UPMC, CNRS, IUF (L.F.
and M.M.), the US National Science Foundation and ANR (ANR
08-CEXC-011-01 Borane). Technical assistance was generously
offered through FR 2769.
(6) For organocatalytic hydrosilylation of carbonyl functions,
see: (a) Parks, D. J.; Piers, W. E. J. Am. Chem. Soc. 1996,
118, 9440. (b) Parks, D. J.; Blackwell, J. M.; Piers, W. E.
J. Org. Chem. 2000, 65, 3090. (c) Asao, N.; Ohishi, T.;
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(d) Iwasaki, F.; Onomura, O.; Mishima, K.; Maki, T.;
Matsumura, Y. Tetrahedron Lett. 1999, 40, 7507.
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Synlett 2012, 23, 433–437
© Thieme Stuttgart · New York

LETTER
Y.; Taton, D. Macromolecules 2010, 43, 8853. (g) For the
Regioselective Hydrosilylation of Carbonyl Derivatives
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(17) The triazolium chloride salt (24.4 mg, 0.1equiv) was added
to a suspension of NaH (60% in mineral oil, 4 mg, 0.1 equiv)
in anhydrous DMF (1 mL) at r.t. After stirring for 30 min,
H2SiPh2 (111 mg, 0.6 equiv) and the substrate (1 mmol)
dissolved in anhydrous DMF (1 mL) were added to the
reaction mixture. When no more substrate was seen by TLC
analysis, TBAF (1.0 M in THF, 1 mL, 1 equiv) was added
to the solution. Stirring was continued for 30 min and
quenching was achieved with H2O (10 mL). The mixture
was extracted with EtOAc (3 × 10 mL) and the combined
organic layers were washed with brine (10 mL), dried with
anhydrous Na2SO4, filtered, and the solution was concen-
trated in vacuo. The crude product was purified by flash
chromatography.
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Analytical data for some typical examples: Compound 2c:
1H NMR (400 MHz, CDCl3): d = 7.61 (d, J = 8.0 Hz, 2 H),
7.47 (q, J = 8.0 Hz, 2 H), 4.93 (q, J = 6.0 Hz, 1 H), 2.31 (br
s, 3 H), 1.47 (d, J = 6.8 Hz, 3 H). 13C NMR (100 MHz,
CDCl3): d = 151.2, 132.3, 126.1, 118.9, 111.0, 70.0, 25.4.
Compound 2d: 1H NMR (400 MHz, CDCl3): d = 7.97 (d,
J = 8.0 Hz, 2 H), 7.40 (q, J = 8.0 Hz, 2 H), 4.92 (q,
J = 6.4 Hz, 1 H), 3.88 (s, 3 H), 2.49 (br s, 1 H), 1.47 (d,
J = 6.4 Hz, 3 H). 13C NMR (100 MHz, CDCl3): d = 167.1,
151.1, 129.8, 129.1, 125.3, 69.9, 52.1, 25.3. Compound 2h:
1H NMR (400 MHz, CDCl3): d = 7.42 (d, J = 5.6 Hz, 1 H),
7.29–7.22 (m, 3 H), 5.24 (t, J = 6.0 Hz, 1 H), 3.06 (m, 1 H),
(11) (a) Kuhn, N.; Kartz, T.; Bläser, D.; Boese, R. Chem. Ber.
1995, 128, 245. (b) For the preparation of base-free NHC
samples, see: Read de Alaniz, J.; Rovis, T. J. Am. Chem.
Soc. 2005, 127, 6284.
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(a) Khalimon, A. Y.; Simionescu, R.; Kuzmina, L. G.;
Howard, J. A. K.; Nikonov, G. I. Angew. Chem. Int. Ed.
2008, 47, 7701. (b) Peterson, E.; Khalimon, A. Y.;
Simionescu, R.; Kuzmina, L. G.; Howard, J. A. K.; Nikonov,
G. I. J. Am. Chem. Soc. 2009, 131, 908. (c) Gutsulyak, D.
V.; Nikonov, G. I. Angew. Chem. Int. Ed. 2010, 49, 7553.
(d) Watanabe, T.; Hashimoto, H.; Tobita, H. J. Am. Chem.
Soc. 2007, 128, 2176. (e) Ochiai, M.; Hashimoto, H.;
Tobita, H. Angew. Chem. Int. Ed. 2007, 46, 8192.
(13) For representative hydrosilylation of esters, see: (a) Mao,
Z.; Gregg, B. T.; Cutler, A. R. J. Am. Chem. Soc. 1995, 117,
10139. (b) Ojima, I.; Kogure, T.; Kumagai, M. J. Org.
Chem. 1977, 42, 1671. (c) Igarashi, M.; Mizuno, R.;
Fuchikami, T. Tetrahedron Lett. 2001, 42, 2149. (d) Berc,
S. C.; Kreutzer, K. A.; Buchwald, S. L. J. Am. Chem. Soc.
1991, 113, 5093. (e) Berc, S. C.; Buchwald, S. L. J. Org.
Chem. 1992, 57, 3751.
2.82 (m, 1 H), 2.53–2.44 (m, 1 H), 1.99–1.90 (m, 2 H). 13
C
NMR (100 MHz, CDCl3): d = 145.0, 143.3, 128.3, 126.7,
124.9, 124.2, 76.4, 35.9, 29.8. Compound 2j (syn/anti,
2.5:1): 1H NMR (400 MHz, CDCl3): d = 7.50–7.27 (m, 20 H,
syn and anti), 5.00 (t, J = 2.0 Hz, 1 H, anti), 4.73 (t,
J = 4.2 Hz, 1 H, syn), 4.17 (d, J = 2.0 Hz, 1 H, anti), 4.04 (d,
J = 2.0 Hz, 1 H, syn), 3.34 (dd, J = 4.2, 2.0 Hz, 1 H, syn),
3.32 (dd, J = 2.8, 2.0 Hz, 1 H, anti), 3.01 (d, J = 4.2 Hz, 1 H,
syn), 2.85 (d, J = 2.4 Hz, 1 H, anti). 13C NMR (100 MHz,
CDCl3): d = 140.2 (syn), 139.4 (anti), 136.6 (anti), 136.4
(syn), 128.8 (syn), 128.7 (anti), 128.6 (syn and anti), 128.5
(syn), 128.4 (anti), 128.3 (syn and anti), 126.7 (anti), 126.3
(syn), 125.8 (syn and anti), 73.5 (syn), 71.3 (anti), 65.9 (syn),
65.1 (anti), 57.0(syn), 55.2 (anti). Compound 2l: 1H NMR
(400 MHz, CDCl3): d = 3.54 (m, 1 H), 1.84–1.67 (m, 5 H),
1.30–0.91 (m, 10 H). 13C NMR (100 MHz, CDCl3): d = 72.4,
45.1, 28.7, 28.4, 26.5, 26.2, 26.1, 20.4. Compound 4: 1H
NMR (400 MHz, CDCl3): d = 7.35–7.24 (m, 5 H), 4.90 (br
s, 1 H), 4.31 (d, J = 4.8 Hz, 2 H), 1.47 (s, 9 H). 13C NMR
(100 MHz, CDCl3): d = 155.9, 139.0, 128.6, 127.5, 127.3,
79.5, 44.7, 28.4. Compound 6: 1H NMR (400 MHz, CDCl3):
d = 6.04 (d, J = 16.0 Hz, 1 H), 5.48 (dd, J = 16.0, 6.8 Hz,
1 H), 4.35 (sext., J = 6.4 Hz, 1 H), 1.97 (t, J = 6.4 Hz, 2 H),
1.66 (s, 3 H), 1.62–1.51 (m, 3 H), 1.44 (m, 2 H), 1.32 (d,
J = 6.0 Hz, 3 H), 0.98 (s, 6 H). 13C NMR (100 MHz, CDCl3):
d = 137.6, 136.6, 128.8, 127.5, 69.5, 39.4, 33.9, 32.7, 28.7,
28.6, 23.6, 21.3, 19.2. Compound 12: 1H NMR (400 MHz,
CDCl3): d = 7.40–7.26 (m, 5 H), 4.1 (s, 1 H), 3.74 (d,
J = 11.6 Hz, 2 H), 3.19 (d, J = 11.6 Hz, 1 H), 2.92 (br s,
1 H), 0.73–0.62 (m, 3 H), 0.48–0.44 (m, 1 H). 13C NMR
(100 MHz, CDCl3): d = 142.1, 128.2, 127.5, 126.2, 79.7,
68.4, 27.5, 9.8, 8.0.
(14) Hanada, S.; Yuasa, A.; Kuroiwa, H.; Motoyama, Y.;
Nagashima, H. Eur. J. Org. Chem. 2010, 1021.
(15) For reviews of N-heterocyclic carbenes as organocatalysts,
see: (a) Enders, D.; Balensiefer, T. Acc. Chem. Res. 2004,
37, 53. (b) Marion, N.; Díez-González, S.; Nolan, S. P.
Angew. Chem. Int. Ed. 2007, 46, 2988. (c) Enders, D.;
Niemeier, O.; Henseler, A. Chem. Rev. 2007, 107, 5606.
(d) Nair, V.; Vellalath, V.; Babu, B. P. Chem. Soc. Rev.
2008, 37, 2691. (e) DiRocco, D. A.; Rovis, T. J. Am. Chem.
Soc. 2011, 133, 10402.
(16) For an intramolecular hydrosilylation of b-silyloxy ketones,
see: O’Neil, G. W.; Miller, M. M.; Carter, K. P. Org. Lett.
2010, 12, 5350.
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Synlett 2012, 23, 433–437

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