Intramolecular carbostannylation of allyl- and vinylstannanes via a radical chain process

Article abstract of DOI:10.1246/cl.2002.32

In the presence of Bu₃SnH and AIBN, 8-tributylstannyl-6-octen-1-ynes were efficiently cyclized to 2-allyl-1-(tributylstannylmethylene)cyclopentanes by intramolecular homolytic allylstannylation. 8-Tributylstannyl-1,6-octadienes as well underwen

Full text of DOI:10.1246/cl.2002.32

32
Chemistry Letters 2002
Intramolecular Carbostannylation of Allyl- and Vinylstannanes via a Radical Chain Process
Katsukiyo Miura, Naoki Fujisawa, Hiroshi Saito, Hisashi Nishikori, and Akira Hosomiꢀ
Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571
(Received October 18, 2001; CL-011021)
In the presence of Bu3SnH and AIBN, 8-tributylstannyl-6-
octen-1-ynes were efficiently cyclized to 2-allyl-1-(tributyl-
stannylmethylene)cyclopentanes by intramolecular homolytic
allylstannylation. 8-Tributylstannyl-1,6-octadienes as well un-
derwent the radical cyclization in high efficiency. This radical-
based method was applicable to the intramolecular vinylstanny-
lation of alkynes and alkenes.
Under the same reaction conditions using 10 mol% of
Bu3SnH (method A), allylstannanes 1b-f as well underwent the
intramolecular allylstannylation (Scheme 2). The result with 1c
indicates that the present cyclization is compatible with a hydroxy
group as an advantage of radical process. The cyclization of 1d
(R¼Me), bearing an internal triple bond, formed a mixture of two
regioisomers (E)-2d and 3a in a poor yield. Increasing the
amounts of AIBN and Bu3SnH (method B) improved the yield
without change in the isomeric ratio. In contrast with 1d, 1e
(R¼Ph) was converted into 3b exclusively. The complete regio-
control is probably due to the high radical-stabilizing ability of the
phenyl group. As shown in the case with 1f, the intramolecular
allylstannylation is applicable to a substrate with one more
methylene tether.
Intramolecular carbometalation provides a powerful tool for
the stereoselective construction of a wide range of carbocycles
and heterocycles.1;2 A notable advantage of this cyclization is that
the cyclized products can be easily utilized for further
transformation by reaction with various electrophiles. With some
exceptions,3;4 most of the known intramolecular carbometala-
tions proceed via a concerted path in which a reactive carbon-
metal bond participates. Previously, we have reported inter-
molecular homolytic allylstannylation of alkenes and alkynes
with allylstannanes bearing an electron-withdrawing group at the
ꢀ-position (eq 1).5;6 We herein describe an intramolecular
version of the allylstannylation in addition to intramolecular
homolytic vinylstannylation.7
We next investigated the radical-initiated allylstannylation of
Initially, 1,6-enyne 1a, an allylstannane bearing an alkynyl
group, was selected as a substrate. It could be easily preparedfrom
dimethyl malonate in three steps (62% total yield) as shown in
Scheme 1. The AIBN-initiated reaction of 1a in benzene at 80 ꢁC
formed the desired allylstannylation product 2a in only a poor
yield (eq 2). However, addition of Bu3SnH (10 mol%) to the
reaction mixture effectively promoted the cycloisomerization to
give (E)-2a in 82% isolated yield along with a small quantity of its
geometrical isomer.8 Thus, unlike the intermolecular homolytic
allylstannylation (eq 1), the present intramolecular version does
not require the substrate to have an electron-withdrawing group ꢀ
to the stannyl group.
Scheme 1.
Scheme 2.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
33
dienylstannanes 4. The results are summarized in Table 1. The
cyclization of 4a was not observed without Bu3SnH, while its
addition to the reaction system is quite effective in the formation
of 5a (entry 1). Introduction of the substituent R1 increased the
yield of 5 (entries 2–4). In contrast, substitution at the terminal sp2
carbon and homologation of the methylene tether resulted in no
cyclized products (entries 5 and 6).
Table 1. Cyclization of dienylstannanes 4a
Scheme 3.
Research from the Japan Society for the Promotion of Science
(JSPS).
Dedicated to Professor Teruaki Mukaiyama on the occasion
of his 75th birthday.
The propagation process of the present radical cyclization
consists of the reversible addition of ꢂSnBu3 and the subsequent
intramolecular homolytic substitution (eq 3).9 The addition of
Bu3SnH would promote the former step by increasing the
concentration of ꢂSnBu3. The latter radical cyclization, which is
much faster than intermolecular homolytic allylation, allows the
successful allylstannylation of unactivated allylstannanes. The E-
selectivity in the cyclization of 1 results from the fact that the C-C
bond formation takes place in the opposite side to the stannyl
group to avoid its steric hindrance.5
References and Notes
1
a) P. Knochel, in ‘‘Comprehensive Organic Synthesis,’’ ed. by B. M.
Trost and I. Fleming, Pergamon Press, Oxford (1991), Vol. 4, Chap. 4.4,
p 865. b) W. Oppolzer, Angew. Chem., Int. Ed. Engl., 28, 38(1989).
Recent papers: a) D. Cheng, S. Zhu, Z. Yu, and T. Cohen, J. Am. Chem.
Soc., 123, 30 (2001). b) D. Cheng, K. R. Knox, and T. Cohen, J. Am.
Chem. Soc., 122, 412 (2000). c) W. F. Bailey and M. J. Mealy, J. Am.
Chem. Soc., 122, 6787 (2000). d) O. Kitagawa, T. Suzuki, H. Fujiwara,
and T. Taguchi, Tetrahedron Lett., 40, 2549 (1999). e) E. Nakamura, G.
Sakata, and K. Kubota, Tetrahedron Lett., 39, 2157 (1998).
Lewis acid-catalyzed intramolecular carbosilylation: a) N. Asao, T.
Shimada, and Y. Yamamoto, J. Am. Chem. Soc., 121, 3797 (1999). b) K.
Imamura, E. Yoshikawa, V. Gevorgyan, and Y. Yamamoto, J. Am.
Chem. Soc., 120, 5339 (1998).
Intramolecular carbometalation of C-Co or C-Ge bond via a radical
chain mechanism: a) E. G. Samsel and J. K. Kochi, J. Am. Chem. Soc.,
108, 4790 (1986). b) D. P. Curran, Synthesis, 1988, 489 and references
therein. c) D. P. Curran and H. Liu, J. Org. Chem., 56, 3463 (1991).
K. Miura, H. Saito, D. Itoh, T. Matsuda, N. Fujisawa, D. Wang, and A.
Hosomi, J. Org. Chem., 66, 3348(2001).
2
3
4
We further attempted the intramolecular vinylstannylation of
alkynes and alkenes (Scheme 3). The reaction of 2-stannyl-1,6-
enyne 6a by method A did not afford the desired product 7a, but a
small amount of bisstannylated carbocycle 8 was obtained. This
product would be formed by hydrostannylation of 7a, which
should suppress the cycloisomerization of 6a to 7a. To retard the
side reaction, 1-substituted 1,6-enyne 6b was used as a substrate.
As a result, the desired dienylstannane 7b was obtained in a
moderate yield by method B. 2-Stannyl-1,6-diene 6c was
converted into the desired product 7c although the yield was
rather low. The cyclization of 1-phenyl-substituted 1,6-diene 6d
smoothly proceeded even under the conditions of method A.
In conclusion, we have demonstrated that the intramolecular
allylstannylation of alkynes and alkenes is effectively catalyzed
by Bu3SnH-AIBN. The present radical-based carbometalation
provides a new route to functionalized five- and six-membered
rings.
5
6
Related works: a) K. Miura, H. Saito, N. Fujisawa, and A. Hosomi, J.
Org. Chem., 65, 8119 (2000). b) K. Miura, H. Saito, T. Nakagawa, T.
Hondo, J. Tateiwa, M. Sonoda, and A. Hosomi, J. Org. Chem., 63, 5740
(1998).
7
Radical-initiated intramolecular allylsulfonylation of allylsulfones
bearing an alkynyl or alkenyl group has been reported. T. A. K. Smith
and G. H. Whitham, J. Chem. Soc., Chem. Commun., 1985, 897.
8Quite recently, Shin and RajanBabu have reported the Pd-catalyzed
intramolecular allylstannylation of alkynes, which proceeds by cis-
addition of allylstannanes unlike the present case. S. Shin and T. V.
RajanBabu, J. Am. Chem. Soc., 123, 8416 (2001).
9
Radical cyclizations of enynes and dienes with hydrostannanes involve
the same reversible process: a) K. Nozaki, K. Oshima, and K. Utimoto,
J. Am. Chem. Soc., 109, 2547(1987). b)G. Storkand R. Mook, Jr., J. Am.
´
Chem. Soc., 109, 2829 (1987). c) S. Hanessian and R. Leger, J. Am.
Chem. Soc., 114, 3115 (1992) and references cited therein. d) B.
This work was partly supported by CREST, Science and
Technology Corporation (JST) and Grants-in-Aid for Scientific
ꢀ
ꢀ
´
Alcaide, I. M. Rodrꢁguez-Campos, J. Rodrꢁguez-Lopez, and A.
ꢀ
Rodrꢁguez-Vicente, J. Org. Chem., 64, 5377 (1999).