ortho-Effects in the acylation of substituted phenylureas with isocyanates and acetic anhydride

Abstract

The reactions of ortho-chloro-, -bromo-, -iodo-, and -trifluoromethyl-aniline with nitrourea in aqueous acetone yield both the corresponding phenylurea and the phenylbiuret, in marked contrast to other ortho- and parasubstituted anilines which yield only the urea. Biuret formation has been rationalised in terms of a mechanism involving electrophilic stabilisation of the conjugate base of the corresponding urea. ortho-Chloro-, -bromo-, and -iodo-substituents have also been shown to result in abnormal behaviour in the acylation of phenylureas with acetic anhydride in acetic acid. Here the effect has been attributed to electrophilic assistance in the departure of the leaving acetate group from the acetic anhydride. In contrast, no unusual ortho-effects were observed in the reactions of phenylureas with 3,4-dichlorophenyl isocyanate in acetonitrile, and the rate constants can be correlated accurately with the extended form of the Hammett equation.