Addition of Arylchlorocarbenes to α,β-Unsaturated Esters. Absolute Rates, Substituent Effects, and Variable Reactivities

Article abstract of DOI:10.1021/ja00229a031

Addition of arylchlorocarbenes to α,β-unsaturated esters occurs with remarkable facility, even though these carbenes are characterized as electrophilic.Reactions with diethyl maleate, which exhibits isomerization to diethyl fumarate and produces cyclopropanes formally derived from both olefin geometries, are most simply interpreted as concerted additions with isomerization of diethyl maleate resulting from a chemical species generated during decomposition of the reactant diazirine.Absolute rate constants for quenching of a series of para-substituted arylchlorocarbenes with electron-rich and electron-poor alkenes have been obtained.Consistent with concerted addition, diethyl fumarate is more reactive toward cyclopropanation by arylchlorocarbenes than diethyl maleate by factors of more than 20.Absolute rate constants for other alkenes, including n-butyl vinyl ether, 1-hexene, and 1-pentene together with the α,β-unsaturated esters ethyl acrylate, ethyl crotonate, and diethyl ethylidenemalonate are also reported.Relative reactivities for comparatively electron-rich alkenes are less than those for some electron-poor α,β-unsaturated esters.Increasing solvent polarity has only a moderate influence on the reaction rates of phenylchlorocarbene additions to electron-rich and electron-poor alkenes, and activation parameters do not distinguish between these electronically disparate olefins.Both classifications of alkenes undergo cycloadditions that are dominated by entropy.Hammett correlations have been obtained for reactions of para-susbstituted arylchlorocarbenes with each alkene substrate, and the resultant positive ρ-values are dependent on the nature of the olefin substituent.Frontier orbital analysis establishes the necesssity of including the nucleophilic term in estimating the stabilization energy of the transition state when electron-poor olefins add to reactive carbenes.