Quantitative studies in carbene chemistry. Part I. Rate-constant ratios for addition of dichlorocarbene to α-methylstyrenes and related olefins

Abstract

A competitive method has been used to measure the relative reactivities at 0° of a series of α-methylstyrenes (ArCMeCH₂) and related olefins towards dichlorocarbene generated by the reaction of potassium t-butoxide with chloroform. Estimation of the unchanged olefins and the products, dichlorocyclopropanes, demonstrated the absence of side reactions. The results confirm the electrophilic character and singlet nature of the carbene. Data for the 3- and 4-substituted compounds correlate well (r= 0·99) with the Hammett–Brown σ⁺ constants yielding a ρ-value of –0·378 ± 0·015. It is suggested that in the transition state the olefinic π-electrons are asymmetrically partially delocalised into the vacant p-orbital of dichlorocarbene. The results for the 2-substituted α-methylstyrenes and other olefins demonstrate the importance of steric and conformational factors. The preparation of the dichlorocyclopropanes and a new modification of the Wittig reaction are described.