Mechanistic Study on the Formation of Compounds from Thioureas

Abstract

Formation of 2-(N-arylamino)benzothiazole takes place, when N,N 0 -diphenylthioureas are treated with polymer-supported tribromide or with iodine-alumina as catalyst under solvent free conditions. However, when N-substituted-N 0 -benzoylthioureas are treated with polymer-supported tribromide or with iodine-alumina as catalyst either under various conditions or under solvent free conditions, decomposition takes place to give the respective benzamides and thiobenzamides. Mechanistic study of the formation of these compounds is studied using DFT calculations. It is found that electron donating group at the para-position of the aryl group of benzoylthiourea favors the formation of benzamide whereas the presence of electron withdrawing group at para-position of the aryl group of benzoylthiourea, formation of thiobenzamide takes place. When the catalyst is changed to diacetoxyiodobenzene (DIB) under similar reaction conditions, benzoxazole amides are formed; expected benzothiazoles or the decomposition products are not obtained. Mechanistic study of the reaction using DFT calculation again shows that the reaction followed through carbodiimide intermediate undergoes the formation of C-O bond in benzoxazole moiety, instead of the expected C-S bond formation of benzothiazole moiety via a sequential acylation and deacylation process.