Thermal electrocyclic ring closure of alkenylhexa-1,3,5-trienes and the question of anchimeric π-bond participation

Abstract

The anomalously fast electrocyclic ring closure of 3-vinylhexa-1,3,5-triene to yield 2-vinylcyclohexa-1,3-diene had previously been interpreted as involving anchimeric π-bond participation of the 3-vinyl group in the electrocyclization transition state. In order to determine whether this observation and interpretation was indeed characteristic of alkenyl side-chains, 3-allylhexa-1,3,5-triene and 3-propylhexa-1,3,5-trienes were synthesized and their electrocyclization rates and activation parameters compared both to each other and to the corresponding 3-ethyl and 3-vinyl trienes. The results conclusively show that the accelerative effect operative in 3-vinylhexa-1,3,5-triene ring closure does not extend to non-conjugated alkenyl side chains.