Evidence for 2-Hexene-1,6-diyl Diradicals Accompanying the Concerted Diels-Alder Cycloaddition of Acrylonitrile with Nonpolar 1,3-Dienes

Article abstract of DOI:10.1021/jo00077a025

The spontaneous reactions of a series of alkyl 1,3-dienes with acrylonitrile (AN) were investigated.Reproducible spontaneous copolymerizations were shown to compete with the expected concerted <4+2> cycloadditions.For dienes which exist in s-cis/s-trans equilibrium, both copolymer and cycloadduct are formed.Kinetic measurements show that the alternating copolymerization and cycloaddition are two independent paralell second order reactions.With 1,3-cyclohexadiene and 1,2-dimethylenecyclohexane, for which s-gauche is in equilibrium with s-cis, copolymerization still competes with cycloaddition.The s-trans-locked verbenene forms only copolymer, while s-cis-locked cyclopentadiene and 1,2-dimethylenecyclopentane form only cycloadduct rapidly.Our explanation involves a 2-hexene-1,6-diradical, formed by combination between the terminal carbons of the s-gauche or s-trans diene and acrylonitrile.This does not cyclize but initiates copolymerization.Competitively s-cis conformer undergoes classical concerted <4+2> addition.