Evidence for 2-hexene-1,6-diyl diradicals accompanying the concerted Diels-Alder cycloaddition of acrylonitrile with nonpolar 1,3-dienes
作者:Yufei Li、Anne Buyle Padias、H. K. Hall
DOI:10.1021/jo00077a025
日期:1993.12
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 parallel 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.