Transition from Concerted to Stepwise Cycloaddition Reactions of α,β-Unsaturated Carboxylic Esters

Article abstract of DOI:10.1021/jo00169a037

The stereospecificity of the <2 + 4> cycloaddition reaction of electrophilic α,β-unsaturated esters with cis- and trans-propenyl ethers and with trans-anethole, leading to 3,4-dihydro-2H-pyran derivatives, is studied.Reactions with trisubstituted electrophilic olefins are stereospecific and are considered to be concerted, except with methyl 4,4,4-trichloro-2-cyanocrotonate where accompanying cyclobutane formation indicates some stepwise behavior.With tetrasubstituted electrophilic olefins, the reaction products are not stereospecific, and simultaneous cyclobutane formation also indicates stepwise behavior.The reactivity of these olefins is determined by their electronegativity, but the transition from a concerted to a stepwise mechanism appears to be governed by steric hindrance.