Electrophilic cleavage of cyclopropylmethystannanes: an experimental comparison of sigma-sigma and sigma-pi conjugation.
[reaction: see text] Cyclopropylmethyltrimethylstannanes undergo electrophilic cyclopropane cleavage in chloroform with simple inorganic electrophiles (H(+), SO(2), I(2)) in a homologous reaction to the S(E)' cleavage of allylic stannanes. The sigma-sigma conjugation between the carbon-tin bond and cyclopropane orbitals observed spectroscopically in the parent cyclopropylmethyltrimethylstannane is responsible for a rate enhancement of ca. 10(2) toward iodinolysis, relative to comparable alkyl stannanes. This acceleration is considerably less, however, than the ca. 10(9)-fold rate enhancement provided by the corresponding sigma-pi conjugation in allylic stannanes. Methanol-tin coordination appears to reduce the activating influence of the metal, promoting methyl cleavage over cyclopropane fission with acid and iodine. Decreased sigma-sigma conjugation can also explain the decreased reactivity of cyclopropyltriphenylstannane compared with its trimethyltin counterpart. Cyclopropylmethylstannanes do not undergo the synthetically useful addition of aldehydes under conditions that facilitate the corresponding reaction of allylic stannanes.
Lucke
Andrew J
AJ
School of Science, Griffith University, Nathan 4111, Brisbane, Australia.
Young
David J
DJ