Reduction of conjugate double bonds with trichlorosilane

Article abstract of DOI:10.1139/v99-238

A variety of α,β-unsaturated esters and cyclic ketones underwent smooth reduction of the carbon-carbon double bond with a combination of inexpensive and readily available trichlorosilane and CoCl₂. The reactions are performed under very mild conditions and products are obtained in high yields.

Full text of DOI:10.1139/v99-238

1396
Reduction of conjugate double bonds with
trichlorosilane
Moni Chauhan and Philip Boudjouk
Abstract: A variety of α ,β -unsaturated esters and cyclic ketones underwent smooth reduction of the carbon–carbon
double bond with a combination of inexpensive and readily available trichlorosilane and CoCl2. The reactions are
performed under very mild conditions and products are obtained in high yields.
Key words: reduction, carbonyl compounds, trichlorosilane, ketones, chemoselectivity.
Résumé : Une grande variété d’esters et de cétones cycliques α ,β -insaturés subissent une réduction facile de la double
liaison carbone–carbone sous l’influence combinée du CoCl2 et du trichlorométhane facilement accessible et peu
dispendieux. Les réactions se produisent dans des conditions très douces et les produits sont obtenus avec des
rendements élevés.
Mots clés : réduction, composés carbonyles, trichlorosilane, cétones, chimiosélectivité.
[Traduit par la Rédaction] Chauhan and Boudjouk 1398
Introduction
dimethyl-2-imidazolidinone) or DMPU (1,3-dimethyl-
3,4,5,6-tetrahydro-2(1 H)-pyrimidinone).
Among the various reducible substrates of great relevance
are those containing both a carbonyl group and a C=C bond
such as α ,β-unsaturated esters and ketones. Indeed, the ca-
pability of reducing only one group, leaving the other one
unaffected, is a challenging task in organic synthesis, partic-
ularly for the preparation of pharmaceutical and agrochemi-
cal products. Chemoselective hydrogenation of double bonds
in α ,β-unsaturated carbonyl compounds has attracted much
interest for more than two decades. A very convenient and
fundamentally important route to these reactions is by metal-
catalyzed hydrogenation (1). Practical motivations arise
from the fact that such reactions are convenient in large
scale synthesis since there is no need to employ a high hy-
drogen pressure or to use hazardous reducing agents. Trans-
fer of organic groups of organosilanes to metallic species,
transmetalation, and subsequent homocoupling reactions un-
der mild conditions are well known (2). Recently, there have
been reports of generating copper hydride species via
transmetalation of a hydrogen atom on silicon to copper (3).
Although this combination of a Cu(I) salt and dimethyl-
phenylsilane is a useful reagent for conjugate reduction of
α ,β-unsaturated compounds, the hydrosilane is a fairly ex-
pensive starting material. Consequently this report intro-
duces the use of the cheapest and most readily available
silane, trichlorosilane, as a hydride source catalyzed by
CoCl2 in the presence of small amounts of DMI (1,3-
In our effort to find a new catalyst for hydrosilylation of
acrylonitriles (4) and acrylates, we found that CoCl2 and
TMEDA (N,N,N ′,N ′-tetramethylethylenediamine) catalyzed
the reduction of the double bond in acrylates and
hydrosilylation (eq. [1]).
CoCl 2 , TMEDA
[1]
CH2=CHCO2Me + HSiCl3
→
CD3 CN, 70°C, 8h
Cl3SiCH2CH2CO2Me + CH3CH2CO2Me
In contrast, in the presence of DMI or DMPU reduction of
the carbon–carbon double bond was the exclusive reaction
path resulting in efficient, high yield syntheses of saturated
ketones and esters under mild conditions.
Experimental
General procedure
All reactions and manipulations were routinely performed
under an argon atmosphere by using standard Schlenk tech-
niques. Commercially available hexahydrated CoCl2 was
oven dried. All acrylates and ketones are commercial prod-
ucts and were distilled in an inert atmosphere prior to use.
Trichlorosilane was purified by distillation over magnesium
metal. TMEDA, DMI, or DMPU were used as received
without further purification. Deuterated acetonitrile for the
Received April 23, 1999. Published on the NRC Research
Press website on October 20, 2000.
M. Chauhan and P. Boudjouk.1 Center for Main Group
Chemistry, Department of Chemistry, North Dakota State
University, Fargo ND 58105–5516, U.S.A.
1
reaction was dried over molecular sieves. H and 13C NMR
spectra were recorded on a JEOL GSX270 and JEOL
GSX400 (operating at 100.53 MHz for 13C) spectrometers
respectively. All chemical shifts are reported in ppm and
1Author to whom correspondence may be addressed.
Telephone: (701) 231-8601. Fax: (701) 231-7947.
e-mail: boudjouk@plains.nodak.edu
1
were measured relative to residual H or 13C resonances in a
deuterated solvent (CD3CN, δ 1.94).
Can. J. Chem 78: 1396–1398 (2000)
© 2000 NRC Canada

Chauhan and Boudjouk
1397
Table 1.
CoCl2
(mmol)
DMI or DMPU
(mmol)
Entry Alkenes (mmol) / Silane (mmol)
Rxn. Conditions
Product, Yield
1
2
3
4
CH2=CHCO2Me(8) / HSiCl3(16)
CH2=CHCO2Et(8) / HSiCl3(16)
PhCH=CHCO2Et(8) / HSiCl3(16)
2-Cyclohexen-1-one(8) / HSiCl3(16)
1.4
1.4
1.4
4.0
4.0
4.0
CD3CN (2 mL) 70°C, 5 h
CD3CN (2 mL) 70°C, 5 h
CD3CN (2 mL) 70°C, 5 h
Neat or CD3CN (2 mL),
R.T., 5 h
CH3CH2CO2Me, 90%
CH3CH2CO2Et, 70%
PhCH2CH2CO2Et, 50%
1-cyclohexanone, 88%
1.4 or 0.4 4.0 or 2.0
5
6
7
8
9
2-Cyclopenten-1-one(8) / HSiCl3(16)
1-Acetyl-1-cyclohexene(8) / HSiCl3(16)
1.4
1.4
1.4
1.4
4.0
4.0
4.0
4.0
4.0
Neat or CD3CN (2 mL),
R.T., 4 h
CD3CN (2 mL) 70°C, 2 h
1-cyclopentanone, 92%
1-acetyl cyclohexane,
75%
3,5-Dimethyl-2-cyclohexen-1-one(8) /
HSiCl3(16)
CH2=CHCO2Et(8) / Me2PhSiH(16)
CD3CN (2 mL) 70°C, 12 h 3,5-dimethyl-1-
cyclohexanone, 50%
CD3CN (2 mL) 80°C, 10 h CH3CH2CO2Et, 85%,
Me2PhSiOSiMe2Ph.
2-Cyclopenten-1-one(8) / HSiMe2Ph(16) 1.4
Neat, 80°C, 10 h
1-cyclopentanone 65%,
Me2PhSiOSiMe2Ph.
meta-position might be responsible for its low reactivity.
Unfortunately, the silicon species formed in the present ca-
talysis were difficult to identify.
Catalysis experiments
A typical procedure is given as follows: to a mixture of
CoCl2 (0.180 g, 1.4 mmol) and DMI (0.5 mL, 4 mmol) in
CD3CN (2 mL), methylacrylate (0.72 mL, 8 mmol) and then
trichlorosilane (1.76 mL, 16 mmol) was added at room tem-
perature. The resulting mixture was refluxed at 70°C and
To get an insight into the mechanism of these reactions,
we treated dimethylphenylsilane with ethylacrylate and with
2-cyclopenten-1-one (entry 8 and 9). Notably, both reactions
were homogeneous to give the corresponding saturated prod-
ucts. The silicon compounds formed in these reactions were
isolated by dissolving the crude product in pentane and were
identified by NMR as dimethylphenyldisiloxane. It is rele-
vant to note that Mori et al. (3b and 3c) have recently re-
ported the formation of this disiloxane from the reaction of
CuCl and PhMe2SiH in DMF at room temperature for 1 h.
The authors suggested that CuH and chlorodimethyl-
phenylsilane first formed are converted to disiloxane via
hydrolytic condensation. By analogy, we assume a similar
pathway for the reactions in entry 8 and 9.
1
was monitored by H and 13C NMR. After 5 h the color of
the reaction solution changed from blue to green, which in-
dicated the complete consumption of methylacrylate. The
solid formed in the reaction was filtered through a short-
column chromatography (Florisil, CD3CN:pentane = 1:2) to
furnish pure reduction product.
Results and discussion
The catalysis was found to be quite general for the selec-
tive reduction of C=C bonds of α,β-unsaturated esters and
ketones. Our results are summarized in Table 1 and show
that all reactions exhibit excellent chemoselectivity, good
catalytic activity, and essentially only one product: the corre-
sponding saturated ester or ketone. The optimum reaction
conditions from entry 1 were applied to ethylacrylate (entry
2) and ethylcinnamate (entry 3). This last reaction proceeds
much slower and gives a lower yield of the desired product
(50%) in comparison to the reactions in entry 1 (90%) and 2
(70%). We attribute this to a combination of steric and elec-
tronic effects arising from the phenyl ring.
Conclusions
Trichlorosilane effects conjugate reduction with the aid of
CoCl2. This reduction method constitutes a simple and cheap
alternative to literature procedures for the selective conju-
gate reduction of α ,β-unsaturated carbonyl compounds. Fur-
ther characterization of the reactive intermediates and
elucidation of the mechanism is under active investigation.
We applied our reduction system to the α ,β-unsaturated
cyclic ketones. As is evident from Table 1, 2-cyclohexen-1-
one (entry 4) and 2-cyclopenten-1-one (entry 5) underwent
reduction reactions at room temperature which resulted in
excellent yields (88–92%). It was observed that a reduction
in the amounts of catalyst did not affect the rate or the yields
of the reaction. In fact, the amount of CoCl2 in this reaction
can be reduced to 5 mmol% and DMI or DMPU can be used
as low as 25 mmol%. On the other hand, 1-acetyl-1-
cyclohexene and 3,5-dimethyl-2-cyclohexen-1-one were not
reactive at room temperature. 1-acetyl cyclohexane was ob-
tained in 75% yield after heating at 70°C for 2 h (entry 6)
and 3,5-dimethyl-2-cyclohexane was obtained only in 50%
yield after 12 h. Steric influence from methyl groups at the
Acknowledgements
The financial Support of the National Science Foundation
through Grant no. 9452892 is gratefully acknowledged.
Fruitful discussions with Bhanu P. S. Chauhan are also
gratefully acknowledged.
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