yield as a 2+1 mixture of diastereoisomers.‡ The silyl radical
addition is probably rather slow for this cyclisation. The
hydrosilylation was even worse with silyl ether 8 (? 13, 14%
yield, dr = 1+1). We assume that the reduction of the hindered
tertiary radical is not efficient with the silylated cyclohexadiene
8. Surprisingly, no selectivity was observed for the reduction of
the tertiary radical formed after the cyclisation of the silyl
radical derived from 8 (1,2-induction).
In conclusion, we present a new method for the intra-
molecular radical hydrosilylation. Furthermore, we have dis-
covered the first example of a radical 5-endo-dig cyclisation
reaction.
We thank the Swiss Science National Foundation
(2100-055280.98/1) for funding our work. The work described
herein is part of the planned dissertation of S.A.
Finally, we studied the intramolecular cyclisation using
silylated propargyl alcohol 14, which was prepared in 50% yield
starting from propargyl alcohol. We were very pleased to
observe that the intramolecular 5-endo-dig process could be
accomplished (Scheme 4). Intramolecular hydrosilylation fol-
lowed by ionic ring-opening provided alcohol 15 in 55% yield
as a diastereoisomerically pure compound with (Z)-configura-
tion. To the best of our knowledge this is the first report of a
radical 5-endo-dig process. To exclude the possibility that
product 15 was formed via an intermolecular process, we
performed the intermolecular hydrosilylation of TBDMS-
protected alcohol 16 using reagent 1 (AIBN (0.3 equiv.), hexane
(0.25 M), 80–85 °C). For an intermolecular radical addition to
an alkyl substituted alkyne low selectivity has to be expected.§
Indeed, the hydrosilylation of alkyne 16 provided vinyl silane
17 as a 1+1 mixture of diastereoisomers (38%). This clearly
proves that the vinyl silane 15 must have been formed via an
intramolecular radical reaction.
Notes and references
‡ The relative configuration of the major isomer was not assigned.
§ Alkyl substituted (non-conjugated) vinyl radicals are sp2-hybridized and
they invert with a very low barrier. The ratio of products formed depend on
the equilibrium constant of the two interconverting vinyl radicals and on the
rate constant of the reduction. In general low selectivities are obtained for
the reduction of these s-type vinyl radicals.13
1 K. Tamao, T. Tanaka, T. Nakajima, R. Sumiya, H. Arai and Y. Ito,
Tetrahedron Lett., 1986, 27, 3377; K. Tamao, T. Nakajima, R. Sumiya,
H. Arai, N. Higuchi and Y. Ito, J. Am. Chem. Soc., 1986, 108, 6090; K.
Tamao, T. Yamauchi and Y. Ito, Chem. Lett., 1987, 171; S. H. Bergens,
P. Noheda, J. Whelan and B. Bosnich, J. Am. Chem. Soc., 1992, 114,
2121; S. H. Bergens, P. Noheda, J. Whelan and B. Bosnich, J. Am.
Chem. Soc., 1992, 114, 2128; X. Wang and B. Bosnich, Organome-
tallics, 1994, 13, 4131.
2 Y. Cai and B. P. Roberts, J. Chem. Soc., Perkin Trans. 1, 1998, 467.
3 D. L. J. Clive and M. Cantin, J. Chem. Soc., Chem. Commun., 1995, 319;
D. L. J. Clive, W. Yang, A. C. MacDonald, Z. Wang and M. Cantin, J.
Org. Chem., 2001, 66, 1966 and references cited therein.
4 G. R. Jones and Y. Landais, Tetrahedron, 1996, 52, 7599.
5 S. Amrein, A. Timmermann and A. Studer, Org. Lett., 2001, 3, 2357.
6 Substituted cyclohexadienes in radical chain reactions: G. Binmore, J.
C. Walton and L. Cardellini, J. Chem. Soc., Chem. Commun., 1995, 27;
P. A. Baguley, L. V. Jackson and J. C. Walton, J. Chem. Soc., Perkin
Trans. 1, 2002, 304 and references cited therein.
7 A. Studer and S. Amrein, Angew. Chem., Int. Ed., 2000, 39, 3080.
8 K. Tamao, A. Kawachi and Y. Ito, J. Am. Chem. Soc., 1992, 114,
3989.
9 J. Ohshita, A. Iwata, H. Tang, Y. Yamamoto, C. Matui and A. Kunai,
Chem. Lett., 2001, 740.
10 T. J. Barton and A. Revis, J. Am. Chem. Soc., 1984, 106, 3802; C.
Chatgilialoglu, H. Woynar, K. U. Ingold and A. G. Davies, J. Chem.
Soc., Perkin Trans. 2, 1983, 555; J. P. Sarasa, J. Igual and J. M. Poblet,
J. Chem. Soc., Perkin Trans 2, 1986, 861.
11 Review on radical 5-endo-trig reactions: H. Ishibashi, T. Sato and M.
Ikeda, Synthesis, 2002, 695.
12 G. D. Mendenhall, Tetrahedron Lett., 1983, 24, 451.
13 B. Giese, Angew. Chem., Int. Ed. Engl., 1989, 28, 969.
Scheme 4 First radical 5-endo-dig reaction.
CHEM. COMMUN., 2002, 1592–1593
1593