Regioselective Aluminum Chloride Catalyzed Reactions of Unsaturated Electtrophiles with Thiazoles

Article abstract of DOI:10.1021/jo00141a008

Aluminum chloride promotes reactions of α,β-unsaturated esters and nitriles with 1,3-thiazoles to give 1:1 and 2:1 adducts, the nature and distribution of which depend on the substituent R at C₂ of the heterocycle and the nuleophile QN employed as a quencher of the reaction mixture.With 2-bromothiazole, ethyl propiolate, and dimethyl acetylenedicarboxylate give substituted (E)- and (Z)-N-vinylthiazolin-2-ones (QN, aqueous sodium bicarbonate) and 2-imino-N-vinylthiazolines (QN, benzylamine), while ethyl acrylate and acrylonitrile afford substituted N-propanoylthiazolin-2-ones (QN, aqueous sodium bicarbonate).With 1,3-thiazole and 2-ethylthiazole, ethyl propiolate gives (QN, aqueous sodium bicarbonate) N-formyl- and N-propanoylthiazolines, respectively, together with 2:1 open-chain adducts containing 2 mol of acetylene and 1 mol of thiazole.A plausible scheme accounting for the formation of the observed products involves a regioselective attack by C_β of the multiple bond of the ester or nitrile coordinated with AlCl3 on the nitrogen of the thiazole ring to give a zwitterion with allenic structure which by action of the QN undergoes C₂-R (R = Br) or C₂-S (R = H, C2H5) bond fission and forms the observed products.