Chemistry Letters 2002
909
The regioselectivity of formation of the acetate products, 2
and 3, depends on the nature of the 4-substituent of the
cyclohexenyl precursor 1. Both methyl and phenyl derivatives
(1a and 1c) give nearly equal amounts of the regioisomers as
expected for cyclohexyne intermediate, but the tert-butyl
substrate 1b gives three times more regio-migrated product 3b
than the non-migrated substitution product 2b. This unbalanced
formation of the regioisomers is rather unexpected for 4-
substituted cyclohexyne. Cyclohexyne 6b can be generated also
from the regioisomeric iodonium salt 1b0, and so 1b0 was
subjected to the acetate reaction (Scheme 1). The reaction of 1b0
gave 2b and 3b in the ratio of 20/80, which is almost identical with
the ratio of 25/75–28/72 obtained from 1b. This is compatible
with the common cyclohexyne intermediate 6b in the reactions of
1b and 1b0. The regio-convergent product ratio observed
indicates that it essentially reflects the intrinsic regioselectivity
of the reaction of the cyclohexyne intermediate with acetate ion; a
3–4 fold preference of 3b over 2b. Some ambiguities arise
because of the possibility of any direct substitution reactions of 1b
and 1b0, which give selectively 2b and 3b, respectively,13;18 but
the effects thereof on the observed ratio 2b : 3b are small. The
cyclohexyne intermediate giving the acetate products is also
confirmed by deuterium incorporation at the vinylic position of
the acetate products, 2 and 3, in case the reaction is performed in
the presence of methanol-d.
access to the precursors and simple manipulations needed to
generate cyclohexynes open routes to revealing chemistry of this
unstable class of compounds.
We acknowledge financial support from the Japan Society for
the Promotion of Science and the Mitsubishi Chemical Corpora-
tion Fund.
References and Notes
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The addition of methanol (1% v/v) as a proton source did not
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´
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In summary, cyclohexenyliodonium salts can be used as
effective cyclohexyne precursors under mild conditions. The easy