Dirhodium(II) tetrakis(perfluorobutyrate)-catalyzed 1,4-hydrosilylation of α,β-unsaturated carbonyl compounds

Article abstract of DOI:10.1248/cpb.54.1622

The use of dirhodium(II) catalysts in the 1,4-hydrosilylation of α,β-unsaturated ketones and aldehydes was explored. Dirhodium(II) tetrakis(perfluorobutyrate), Rh₂(pfb)₄, proved to be the catalyst of choice for this process, providin

Full text of DOI:10.1248/cpb.54.1622

1622
Communications to the Editor
Chem. Pharm. Bull. 54(11) 1622—1623 (2006)
Vol. 54, No. 11
Dirhodium(II)
Tetrakis(perfluorobutyrate)-Catalyzed
1,4-Hydrosilylation of , -Unsaturated
Carbonyl Compounds
other solvents such as benzene, toluene, and THF were also
suitable, the reaction times in these solvents were extended
(entries 3—5). Using dichloromethane as the solvent, we
next evaluated the performance of other dirhodium(II) com-
plexes, Rh2(OAc)4 (1b), Rh2(oct)4 (1c), Rh2(tpa)4 (1d), and
Rh2(cap)4 (1e) (entries 6—9). Although these dirhodium(II)
complexes could also be used for this process, 0.1 mol% of
catalyst was required for completion of the reaction.
Rh2(pfb)4, which features strongly electron-withdrawing car-
boxylate ligands, proved to be the catalyst of choice in terms
of catalyst activity. As expected from the robust nature and
high reactivity of Rh2(pfb)4, the hydrosilylation of 2a with 3a
proceeded smoothly with very low catalyst loadings (0.0002
mol%) without compromising product yield (entry 10). In
addition to triethylsilane, a variety of monohydrosilanes such
a b
Masahiro ANADA, Masahiko TANAKA, Kanami SUZUKI,
Hisanori NAMBU, and Shunichi HASHIMOTO*
Faculty of Pharmaceutical Sciences, Hokkaido University; Kita 12,
Nishi 6, Kita-ku, Sapporo 060–0812, Japan.
Received August 28, 2006; accepted September 6, 2006; published
online September 19, 2006
The use of dirhodium(II) catalysts in the 1,4-hydrosilylation
of a,b-unsaturated ketones and aldehydes was explored.
Dirhodium(II) tetrakis(perfluorobutyrate), Rh2(pfb)4, proved to as diethylmethylsilane (3b), dimethylphenylsilane (3c), and
be the catalyst of choice for this process, providing the corre-
sponding silyl enol ethers in high yields.
triethoxysilane (3d) could be used in the Rh2(pfb)4-catalyzed
1,4-hydrosilylation of 2a (entries 11—13). On the other
hand, the reaction of 2a with diphenylsilane (3e) led to the
exclusive formation of the 1,2-hydrosilylation product 5, as
observed in the (Ph3P)3RhCl-catalyzed hydrosilylation with
Key words dirhodium(II) complex; 1,4-hydrosilylation; silyl enol ether;
a,b-enone
Silyl enol ethers are useful and versatile synthetic interme- dihydrosilanes (Chart 2).3)
diates available to the organic chemist.1,2) The catalytic 1,4-
The applicability of this catalytic system to a range of
hydrosilylation of a,b-unsaturated carbonyl compounds is a,b-unsaturated carbonyl compounds was then investigated
generally recognized as a powerful method for the direct and
regioselective synthesis of silyl enol ethers. Consequently, a
Table 1. 1,4-Hydrosilylation of 2-Cyclohexen-1-one (2a) Catalyzed by
number of transition metal complexes based on rhodium,3—5)
Dirhodium(II) Complexesa)
platinum,6) and copper7) as well as Lewis acids such as
B(C6F5)38) have been developed for that purpose.
Aside from their effectiveness in diazo decomposition,9,10)
dirhodium(II) complexes are also recognized as Lewis acid
catalysts11,12) as well as catalysts for hydrosilylation of termi-
nal alkynes,13) terminal alkenes,14) and enamides,15) silyl-
Product
formylation of terminal alkynes,16) and silane alcoholysis.17)
However, to the best of our knowledge, no examples of
the dirhodium(II) complex-catalyzed 1,4-hydrosilylation of
a,b-unsaturated carbonyl compounds have been reported.
As part of a program to explore dirhodium(II) complex-
catalyzed transformations,18) we wish to report herein that
dirhodium(II) tetrakis(perfluorobutyrate), Rh2(pfb)4 (1a), is
an exceptionally effective catalyst for the 1,4-hydrosilylation
of a,b-unsaturated ketones and aldehydes.
At the outset of this work, the 1,4-hydrosilylation of 2-cy-
clohexen-1-one (2a) with triethylsilane (3a) in the presence
of 0.01 mol% of Rh2(pfb)4 (1a) was examined. The reaction
in dichloromethane or dichloroethane proceeded smoothly at
40 °C, giving the corresponding silyl enol ether 4a in 96%
and 95% yields, respectively (Table 1, entries 1, 2).19) No
signs of the 1,2-hydrosilylation product were detected in the
crude reaction mixture by NMR spectroscopy. Although
Time
(h)
Entry
Rh(II) complex
Silane
Solvent
Yieldb)
(%)
1
2
3
4
5
6
7
8
9
Rh2(pfb)4 (1a)
Rh2(pfb)4 (1a)
Rh2(pfb)4 (1a)
Rh2(pfb)4 (1a)
Rh2(pfb)4 (1a)
Et3SiH (3a)
Et3SiH (3a)
Et3SiH (3a)
Et3SiH (3a)
Et3SiH (3a)
CH2Cl2
(CH2Cl)2
Benzene
Toluene
THF
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
1
1
2
2
2
1
1
8
3
4a 96
4a 95
4a 93
4a 94
4a 92
4a 95
4a 95
4a 88
4a 88
4a 92
4b 96
Rh2(OAc)4 (1b)c) Et3SiH (3a)
Rh2(oct)4 (1c)c)
Et3SiH (3a)
Rh2(tpa)4 (1d)c) Et3SiH (3a)
Rh2(cap)4 (1e)c) Et3SiH (3a)
Rh2(pfb)4 (1a)d) Et3SiH (3a)
10
11
12
0.5
Rh2(pfb)4 (1a)
Rh2(pfb)4 (1a)
Rh2(pfb)4 (1a)
Et2MeSiH
(3b)
Me2PhSiH
(3c)
(EtO)3SiH
(3d)
12
13
CH2Cl2
CH2Cl2
1
6
4c 94
4d 90
a) Unless otherwise noted, reactions were carried out as follows: 3 (1.2 eq) was
added to a solution of 2a (960 mg, 10 mmol) and Rh(II) complex (0.01 mol%) in the
indicated solvent (3 ml) and the mixture was stirred at 40 °C. b) Isolated yield.
c) 0.1 mol% of catalyst was used. d) 0.0002 mol% of catalyst was used.
Chart 1
Chart 2
∗ To whom correspondence should be addressed. e-mail: hsmt@pharm.hokudai.ac.jp
© 2006 Pharmaceutical Society of Japan

November 2006
1623
Table 2. 1,4-Hydrosilylation of a,b-Unsaturated Carbonyl Compounds 4l, m in high yields with no evidence of 1,2-hydrosilylation
Catalyzed by Dirhodium(II) Carboxylatesa)
(entries 9, 11).
In summary, we have reported the first example of the 1,4-
Rh(II) Time
Product
Entry
1
Substrate
hydrosilylation of a,b-unsaturated ketones and aldehydes
catalyzed by dirhodium(II) complexes and have shown that
Rh2(pfb)4 is an exceptionally effective catalyst for this
process. The effective use of Rh2(OAc)4 as a backup catalyst
for Rh2(pfb)4 was also demonstrated, in cases where b-
phenyl substituted substrates are used. The dirhodium(II)
carboxylate catalysts are air-stable, and easily handled. Fur-
ther studies of the scope of the reaction as well as mechanis-
tic studies are currently in progress.
(mol%)
(h)
Yieldb) (%)
1a
0.3
(0.05)
2
3
1a
(0.01)
0.5
0.5
1a
(0.01)
Acknowledgements This research was supported, in part, by a Grant-in-
Aid for Scientific Research on Priority Areas “Advanced Molecular Trans-
formations of Carbon Resources” from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. We thank S. Oka, M. Kiuchi, A.
Maeda, H. Matsumoto and T. Hirose of the Center for Instrumental Analysis
at Hokkaido University for mass measurements.
4
5
1a
(0.01)
0.5
0.3
1a
(0.01)
References and Notes
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1
1
1a
(0.01)
8
9
1a
(0.01)
2
3
1
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1b
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a) All reactions were performed on a 5 mmol scale with 1.2 eq of 3a in CH2Cl2
under reflux. b) Isolated yield. c) Determined by 1H-NMR. d) Combined yield
of 1,4-adduct and 1,2-adduct.
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and the results are summarized in Table 2. The use of cyclic
a,b-enones, including 2-cyclopenten-1-one (2b), 2-cyclo-
hepten-1-one (2c), and 4,4-dimethyl-2-cyclohexen-1-one
(2d), afforded the corresponding silyl enol ethers 4e—g in
high yields (entries 1—3).20) (E)-2-Ethylidenecyclohexanone
(2e) was converted to silyl enol ether 4g in high yield (entry
4). The 1,4-hydrosilylation also proceeded smoothly with
mesityl oxide (2f), 3-heptene-2-one (2g), and crotonaldehyde
(2h), although the products were a mixture of stereoisomers
(entries 5—7). However, in the cases of benzalacetone (2i)
and (E)-cinnamaldehyde (2j), containing a phenyl group at
the b-position, reactions with 3a gave predominantly silyl
enol ethers 4l, m, along with very small amounts of 1,2-hy-
drosilylation products 6a, b (entries 8, 10). Gratifyingly, this
problem could be overcome by changing the catalyst from
Rh2(pfb)4 to Rh2(OAc)4. The reactions in the presence of
0.1 mol% of Rh2(OAc)4 afforded exclusively silyl enol ethers
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19) Typical procedure (Table 1, entry 1): Et3SiH (3a) (1.9 ml, 12 mmol)
was added to a solution of 2-cyclohexen-1-one (2a) (960 mg, 10 mmol)
and Rh2(pfb)4 (1a) (1.1 mg, 0.001 mmol, 0.01 mol%) in CH2Cl2 (3 ml)
and the mixture was refluxed for 1 h. Evaporation in vacuo gave the
crude product (2.2 g), which was purified by short-path column chro-
matography (5 g of WAKO-gel C-200, 94.3 : 4.7 : 1 hexane/ether/Et3N)
followed by Kugelrohr distillation (5 mmHg, 100 °C) to provide 4a
(2.05 g, 96%) as a colorless oil.
20) The present method was found to be somewhat sensitive to the struc-
ture of the substrate; 3-methyl-2-cyclohexen-1-one and 2-methyl-2-cy-
clohexen-1-one were recovered unchanged.