Aryl Ring Migration Reaction in the Synthesis of 2,4-Diaryl-4H-3,1-benzothiazines

Abstract

A new rearrangement of 1-(diarylmethyl)-2-isothiocyanatobenzenes into 2,4-diaryl-4H-3,1-benzothiazine derivatives is described. Treatment of the starting compounds with aluminum trichloride under Friedel-Crafts conditions leads to migration of an aryl substituent by an electrophilic ipso-substitution mechanism, followed by cyclization to form a 3,1-benzothiazine ring. The expected intramolecular thiocarbamoylation reaction also takes place, and 5,11-dihydro-6H-dibenzo[b,e]azepine-5-thiones are isolated as byproducts. Factors influencing the mechanism and selectivity of the reaction are discussed. The corresponding 1-(arylmethyl)-2-isothiocyanatobenzene derivatives were also synthesized, and their behavior under Friedel-Crafts reaction conditions was investigated to confirm the crucial role of the stability of the intermediate benzhydryl/benzyl cations in determining the course of the reaction.