Angewandte
Chemie
into ketones in good yields. This process demonstrates novel
utility of vinylsilanes in organic synthesis. Further research on
the oxidation of organosilicon compounds with molecular
oxygen is currently under way in our laboratory.
Received: October 10, 2002 [Z50337]
Scheme 4. Synthesis of carbonyl compounds from alkenylsilanes 2 as
2-oxoalkyl-equivalent radical acceptors. 2b,c,d: R3Si=Ph2MeSi, 2e:
R3Si=Me2PhSi. For details see Table 1.
[1] a) I. Fleming, Chemtracts: Org. Chem. 1996, 9, 1; b) K. Tamao, N.
Ishida, Y. Ito, M. Kumada, Org. Synth. Col. Vol. 1993, 8, 315.
[2] K. Kato, T. Mukaiyama, Chem. Lett. 1989, 2233.
[3] A. Inoue, J. Kondo, H. Shinokubo, K. Oshima, J. Am. Chem. Soc.
2001, 123, 11109.
[4] For the reaction of carbon-centered radicals with molecular
oxygen, see: a) C. Ollivier, P. Renaud in Radicals in Organic
Synthesis, Vol. 2 (Eds.: P. Renaud, M. P. Sibi), Wiley-VCH,
Weinheim, 2001, p. 93; b) N. Kihara, C. Ollivier, P. Renaud, Org.
Lett. 1999, 1, 1419, and references therein.
[5] For reviews on atom-transfer radical reactions, see: J. Byers in
Radicals in Organic Synthesis, Vol. 1 (Eds.: P. Renaud, M. P.
Sibi), Wiley-VCH, Weinheim, 2001, p. 72.
[6] For the use of phosphinic acid as a radical chain carrier, see:
a) D. H. R. Barton, D. O. Jang, J. C. Jaszberenyi, J. Org. Chem.
1993, 58, 6838; b) R. McCague, R. G. Pritchard, R. J. Stoodley,
D. S. Williamson, Chem. Commun. 1998, 2691; c) S. R. Graham,
J. A. Murphy, D. Coates, Tetrahedron Lett. 1999, 40, 2414; d) H.
Tokuyama, T. Yamashita, M. T. Reding, Y. Kaburagi, T. Fu-
kuyama, J. Am. Chem. Soc. 1999, 121, 3791; e) M. T. Reding, T.
Fukuyama, Org. Lett. 1999, 1, 973; f) C. G. Martin, J. A. Murphy,
C. R. Smith, Tetrahedron Lett. 2000, 41, 1833; g) D. O. Jang, S. H.
Song, Tetrahedron Lett. 2000, 41, 247; h) H. Yorimitsu, H.
Shinokubo, K. Oshima, Bull. Chem. Soc. Jpn. 2001, 74, 225; i) H.
Yorimitsu, H. Shinokubo, K. Oshima, Chem. Lett. 2000, 104.
[7] For reviews on Et3B as a radical initiator, see: a) C. Ollivier, P.
Renaud, Chem. Rev. 2001, 101, 3415; b) H. Yorimitsu, K.
Oshima in Radicals in Organic Synthesis, Vol. 1 (Eds.: P. Renaud,
M. P. Sibi), Wiley-VCH, Weinheim, 2001, p. 11.
One can hence regard vinylsilanes as a 2-oxoalkyl-equivalent
radical acceptor. Several characteristics of this process are
noteworthy: 1) The reaction of a-silylstyrene 2c also afforded
the desired ketones in good yields. 2) The reaction can
employ iodo ketones as a radical source (entry 7). However,
concurrent reduction of the iodo ketone via a water-unstable
boron enolate occurred,[12] and the use of excess iodo ketone
(3.0 equiv) was required. 3) The reaction allows efficient
introduction of a perfluoroalkyl group at the a position of
ketones (entries 8 and 10). 4) Direct oxidation of the iodide
can lower the yield of ketone (entry 6; the undesired
oxidation product is 2-hydroxypropionate).[4b] 5) Acylsilanes
can be prepared from 1,1-disilylethene 2e. The product 4 m
was converted into a,b-unsaturated acylsilane 4n during
purification over silica gel (entry 10). 6) In all cases, silanol
was obtained as a byproduct.
We propose the reaction pathway for the sequential
radical addition–oxidation reaction as illustrated in Scheme 5.
Addition of a radical 6 to alkenylsilane 2 provides an a-silyl
radical 7, which then reacts with oxygen to afford peroxy
R2I
[8] a-Iodosilane 3d is stable in water at room temperature. None of
the hydrolyzed alcohol could be detected.
EtI
R2
R2
[9] Caution: The addition of Et3B in methanol to an aqueous
mixture in air may be flammable. Accordingly, the solution of
Et3B was introduced under argon atmosphere, and then the
reaction flask was connected to a balloon filled with air. See
Supporting Information.
[10] For radical reactions in aqueous media, see: H. Yorimitsu, H.
Shinokubo, K. Oshima, Synlett 2002, 674.
[11] Unfortunately, the reaction with 1-substituted vinylsilanes was
unsuccessful. No addition reaction proceeded. The reaction with
2a in the one-pot procedure mainly provided a-silylalkyl iodides
3.
Et
Et3B
6
R3Si
R1
R3Si
R1
R3Si OOBEt2
O2 R3Si OO
R2
R2
R1
R1
9
2
7
8
R3Si
SiR3
R3Si OOH
R2
H2O
O
H
R1
O
R2
R1
OH
H
R1
10
R2
O
H2O
R2
+
R3SiOH
R1
4
[12] For the formation of boron enolates from a-iodo ketones, see:
a) K. Nozaki, K. Oshima, K. Utimoto, Tetrahedron Lett. 1988, 29,
1041; b) K. Nozaki, K. Oshima, K. Utimoto, Bull. Chem. Soc.
Jpn. 1991, 64, 403.
Scheme 5. Proposed reaction pathwayfor the sequential radical addi-
tion–oxidation reaction studied.
[13] We carried out DFT calculations on the migration of the silyl
group of a silylated hydroperoxide. See ref. [3].
radical 8. The reaction of radical 8 with Et3B furnishes
peroxyborane 9. Hydroperoxide 10, derived from the perox-
yborane by hydrolysis, is eventually converted into the
carbonyl product 4 through migration of the silyl group to
the internal oxygen atom.[13] The ethyl radical which results
from reaction 8!9 regenerates an alkyl radical from R2I.
In conclusion, we have achieved the synthesis of alde-
hydes and ketones from alkenylsilanes under radical con-
ditions with air as the oxidant. A tandem intermolecular
radical addition–oxidation sequence can convert vinylsilanes
Angew. Chem. Int. Ed. 2003, 42, No. 7
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