Photochemical radical benzylic bromination with Br2: Computational modeling of the mechanism and microkinetic

Abstract

Halogenation of organic compounds is an important functionalization process for further chemical transformation and the benzylic hydrogen is reactive towards the photochemically generated Br atom. This work reports the free energy profile of the several steps of the reaction of the Br atom with toluene leading to benzyl bromide, (dibromomethyl)benzene, and (tribromomethyl)benzene using reliable theoretical B2GP-PLYP method. A detailed microkinetic analysis of the reaction rate and product distribution was also performed. Our analysis points out that polar solvent increases the reaction rate in relation to apolar one. Substitution on the aromatic ring cannot be explained by the electrophilic mechanism in an apolar medium. The reactivity of different benzylic substrates was evaluated, and we have found that the cyano electron-withdrawing group in the aromatic ring retards the reaction, while the ortho-methoxy group, and oxygen and nitrogen atoms bonded to the benzylic carbon, enhance the reaction rate.