An Asynchronous Concerted Mechanism and Its Origin in Lewis Acid-Mediated Carbonyl-Olefin [2+2] Cycloaddition

The concerted mechanism of thermal BBr3-mediated [2+2] carbonyl-olefin cycloaddition is intriguing considering the conflict against the Woodward-Hoffmann rule. In this work, we report a mechanistic study of the titled reaction using density functional theory calculations. The concerted [2+2] cycloaddition mechanism is operative even for the truncated model system of 2-methyl-2-butene and butanone. Intrinsic reaction coordinate analysis and potential surface mapping showed the asynchronous nature of the apparent concerted [2+2] cycloaddition, resulting from the proximity of carbocation-oxyanion in the transient zwitterionic structure. Solvent effects can change the reaction to a stepwise mechanism, highlighting the importance of zwitterion stability. Building upon this finding of the asynchronous concerted or stepwise [2+2] cycloaddition mechanism, two mechanistic classifications were elaborated based on substrates containing varied substituents and Lewis acids, with a focus on manipulating the stability of positive carbocation and negative oxyanion in the transient zwitterionic species.

Automated summary

The concerted mechanism of thermal BBr3-mediated [2+2] carbonyl-olefin cycloaddition is studied using density functional theory calculations. The reaction follows an asynchronous concerted [2+2] cycloaddition mechanism due to the proximity of carbocation-oxyanion in the transient zwitterionic structure. Solvent effects can change the reaction to a stepwise mechanism, emphasizing the importance of zwitterion stability. Two mechanistic classifications are developed based on substrates with varied substituents and Lewis acids, focusing on manipulating the stability of positive carbocation and negative oxyanion in the transient zwitterionic species.