Copper(I) salt mediated 1,4-reduction of α,β-unsaturated ketones using hydrosilanes

Article abstract of DOI:10.1039/a706032g

Reduction of α,β-unsaturated ketones with HSiPhMe₂ in the presence of CuF(PPh₃)₃·2EtOH proceeds in a 1,4-selective manner to give the corresponding saturated ketones in excellent yields.

Full text of DOI:10.1039/a706032g

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Copper(i) salt mediated 1,4-reduction of a,b-unsaturated ketones using
hydrosilanes
Atsunori Mori,* Akinori Fujita, Yasushi Nishihara and Tamejiro Hiyama
Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Nagatsuta, Yokohama 226, Japan
Table 1 Copper(i) salt mediated reactions of a,b-unsaturated ketones with
hydrosilanes in the presence of CuF(PPh3)3·2EtOHa
Reduction of a,b-unsaturated ketones with HSiPhMe2 in the
presence of CuF(PPh3)3·2EtOH proceeds in a 1,4-selective
manner to give the corresponding saturated ketones in
excellent yields.
Substrate
Hydrosilane Product (% yield)
O
O
+
OH
Transfer of the organic groups of organosilanes to metallic
species, transmetalation, has been our recent concern since a
number of metal mediated synthetic reactions involve trans-
metalation as a key step.1 The organometallic species thus
formed may undergo further transformations, such as carbon–
carbon or carbon–heteroatom bond formation, via reaction with
several organic substrates.2 We have disclosed that organosili-
con compounds such as alkenyl-, aryl- and alkynyl-silanes
induce homo-coupling reactions to yield the corresponding
1,3-dienes, biaryls and 1,3-diynes, respectively, in the presence
of a copper(i) salt in a polar solvent.3 The findings suggest that
transmetalation of the organosilanes to copper occurs to give the
organocopper species, the novel transmetalation being remark-
able since an organic group on silicon is assumed to transfer
onto copper without using an additional activator.4,5 Our
interest has accordingly turned to the transmetalation of
hydrosilanes to copper to generate copper hydride species, with
which reduction of a variety of organic molecules has been
reported.6 Although the combination of a copper salt and
hydrosilane has rarely been studied when compared with
reagents made from copper salts and the hydrides of metals such
as aluminium and tin,7 the desired reaction, we considered,
would proceed with the proper choice of the counter ion and a
ligand of copper. After screening several CuI species, we found
that the CuF(PPh3)3–HSiPhMe2 system could effect 1,4-se-
lective reduction of a,b-unsaturated ketones under mild con-
ditions.
Ph
Ph
Ph
1a
1a
1a
1a
1a
1a
2a HSiPhMe2
2a HSiPhMe2
2a HSiPhMe2
2b HSiEt3
3a (>99)
3a (63)b
3a (69)
3a (95)
3a (49)
—
—
—
4a (trace)c
—
2c H2SiPh2
2d HSiPhCl2
4a (36)
—
O
O
1b
2a HSiPhMe2
3b (91)
O
O
H11C5
H11C5
1c
1d
2a HSiPhMe2
2a HSiPhMe2
3c (91)
O
O
Ph
Ph
Ph
O
Ph
3d (98)
O
The reaction was studied using 1-phenylbut-1-en-3-one 1a as
substrate and HSiPhMe 2a as the reducing agent in the presence
of a copper(i) salt. The molar ratio of the substrate, hydrosilane
and copper salt, as well as solvent, reaction temperature and
reaction time were examined. We found that the use of 2 equiv.
of 2a and 1 equiv. of CuF(PPh3)3·2EtOH8 in N,N-dimethylace-
tamide (DMA) were the optimum conditions to yield 1-phenyl-
butan-2-one 3a in a quantitative yield.† Results are summarized
in Table 1. The yield decreased to 63% when 1 equiv. of 2a was
used. Using a catalytic amount of CuF(PPh3)3·2EtOH, the
reaction similarly proceeded with a slight decrease in yield,
along with the formation of a small amount of unidentified
product as well as a trace amount of 1,2-reduction product 4a.
Concerning the hydrosilane, 2a and triethylsilane 2b exhibited
high 1,4-selectivities to give the corresponding ketone in
excellent yields, but diphenylsilane 2c considerably decreased
the selectivity to give 3a and 4a in 49 and 36% yields,
respectively, in contrast with the results on rhodium-catalysed
hydrosilylation of a,b-unsaturated ketones.9 No reaction oc-
curred with HSiPhCl2 2d.
d
3e (85)e
1e
2a HSiPhMe2
a Reactions were carried out using enone 1 (1.0 mmol), silane 2 (2.0 mmol)
and CuF(PPh3)3·2EtOH 3 (1.0 mmol) unless otherwise specified. The yields
are based on 1H NMR analyses using Cl2CNCHCl as an internal standard.
One equiv. of 2a was used. Accompanied by recovery of 1a (21%).
CuF(PPh3)3 (5 mol%) was used.
e Diastereoisomeric ratio was ca. 8:1.
b
c
d
4 equiv. of 2a was used.
which reduced enones 1f–h smoothly in a highly 1,4-selective
manner. In this sense, our systems appears to be highly
chemoselective. Indeed, the reaction of a 1:1 mixture of 1a and
1f with 2a in the presence of CuF(PPh3)3·2EtOH cleanly
O
O
O
Reactions using several other a,b-unsaturated ketones were
carried out using similar optimized condition, as shown in Table
1. The reactions of 1a–e proceeded in excellent yields with high
1,4-selectivity. However, sterically congested enones such as
1f–h were recovered unchanged.‡ The results also contrast with
those from the reactions catalysed by a rhodium complex,9
1g
1f
1h
produced 3a (99%), along with complete recovery of 1f (97%)
[eqn. (1)].
Chem. Commun., 1997
2159

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mixture was gradually raised to room temperature, and stirring was
continued for 6 h to furnish 4a in a quantitative yield after workup.
‡ The reaction of an a,b-unsaturated ester (methyl cinnamate) also resulted
in no reaction. On the other hand, an a,b-unsaturated aldehyde (cinnamalde-
hyde) was found to effect 1,2-reduction, predominantly.
O
O
2HSiPhMe2
1a
+
1f
+
(1)
CuF(PPh3)3•2H2O
Ph
3a (99%)
3f (0%)
1 J. P. Collman, L. S. Hegedus, J. R. Norton and R. G. Finke, Principles
and Applications of Organotransition Metal Chemistry, University
Science Books, Mill Valley, CA, 1987.
2 E. W. Colvin, Silicon Reagents in Organic Synthesis, Academic Press,
London, 1988.
3 K. Ikegashira, Y. Nishihara, K. Hirabayashi, A. Mori and T. Hiyama,
Chem. Commun., 1997, 1039; Y. Nishihara, K. Ikegashira, A. Mori and
T. Hiyama, Chem. Lett., in the press. See also: H. Ito, H.-o. Sensui,
K. Arimoto, K. Miura and A. Hosomi, Chem. Lett., 1997, 639; H. Ito,
K. Arimoto, H.-o. Sensui and A. Hosomi, Tetrahedron Lett., 1997, 38,
3977; S.-K. Kang, T.-H. Kim and S.-J. Pyun, J. Chem. Soc., Perkin
Trans. 1, 1997, 797.
4 The addition of a fluoride ion as an activator to form the pentacoordinate
organosilicate is necessary for the palladium-catalysed cross-coupling
of organosilanes: T. Hiyama and Y. Hatanaka, Pure Appl. Chem., 1994,
66, 1471 and references cited therein.
5 Another activation without fluoride ion is in situ generation of an
activating species from allylic carbonates or diene monoxides as a
substrate: H. Matsuhashi, S. Asai, K. Hirabayashi, Y. Hatanaka, A. Mori
and T. Hiyama, Bull. Chem. Soc. Jpn., 1997, 70, 1943.
6 W. S. Mahoney, D. M. Brestensky and J. M. Stryker, J. Am. Chem. Soc.,
1988, 110, 291 and references cited therein.
7 M. F. Semmelhack, R. D. Stauffer and A. Yamashita, J. Org. Chem.,
1977, 42, 3180; D. Masure, P. Coutrot and J. F. Normant, J. Organomet.
Chem., 1982, 226, C55; T. Tsuda, H. Satomi, T. Hayashi and
T. Saegusa, J. Org. Chem., 1987, 52, 439; B. H. Lipshutz, C. S. Ung and
S. Sengupta, Synlett, 1989, 64.
Although a mechanistic study of the transmetalation of
organosilane to copper has yet to be undertaken, the activation
of hydrosilanes by CuF(PPh3)3·2EtOH seems to be rather
different from that by Bu4NF or the related reagents. The
12
reaction of 1a and 2a in the presence of 5 mol% of Bu4NHF2
resulted in predominantly 1,2-reduced product 4a in 68% yield.
Thus, 1,4- and 1,2-selective reduction of a,b-unsaturated
ketones with hydrosilanes are accessible by a suitable choice of
copper(i) fluoride or Bu4NHF2 in a highly selective manner,
respectively [eqn. (2)].
Bu4NHF2
(5 mol%)
4a (68%)
DMA
1a
+
2a
(2)
CuF(PPh3)3•2EtOH
DMA
3a (>99%)
The authors thank Shin-Etsu Chemical Co. Ltd. for the
generous donation of organosilicon reagents. This work was
partially supported by a Grant-in-Aid for Scientific Research on
Priority Areas (No. 09239102) from the Ministry of Education,
Science, Sports and Culture, Japan.
8 D. J. Gulliver, W. Levason and M. Webster, Inorg. Chim. Acta, 1981,
52, 153.
9 I. Ojima and T. Kogure, Organometallics, 1982, 1, 1390; G. Z. Zheng
and T. H. Chan, Organometallics, 1995, 14, 70.
Footnotes and References
* E-mail: amori@res.titech.ac.jp
† Typical experimental procedure: CuF(PPh3)3·2EtOH (0.25 mmol) was
dissolved in 2 ml of DMA to give a colourless clear solution, to which was
added HSiPhMe2 2a (0.5 mmol) at 0 °C to give a red homogeneous solution.
The colour of the solution gradually changed to yellow after the addition of
1-phenylbut-1-en-3-one (1a, 0.25 mmol). The temperature of the reaction
10 A. Mori, A. Fujita, K. Ikegashira, Y. Nishihara and T. Hiyama, Synlett,
1997, 639. See also: M. Fujita and T. Hiyama, J. Am. Chem. Soc., 1984,
106, 4629.
Received in Cambridge, UK, 18th August 1997; 7/06032G
2160
Chem. Commun., 1997