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Paper | Special issue | Vol 37, No. 3, 1994, pp.1579-1602
Published online, 1st January, 1970
DOI: 10.3987/COM-93-S115
Use of 3,3'-Polymethylene-bridged Thiazolium Salts Plus Bases as Catalysts of the Benzoin Condensation and Its Mechanistic Implications: Proposal of a New Mechanism in Aprotic Conditions

Francisco López-Calahorra,* Josep Castells, Laura Domingo, Josep Martí, and Josep M. Bofill

*Departamento de Química Orgánica, Universitat de Barcelona, Martí i Franques, 1-11, E-08028 Barcelona, Spain

Abstract

3,3’-Polymethylene-bridged thiazolium and benzothiazolium salts are used as precatalysts for the benzoin condensation and it is found that catalytic activity depends strongly on the methylene bridge length. This result supports our previous postulation that, at least in strictly aprotic medium, bis(thiazolin-2-ylidene)s, and not thiazolin-2-ylidenes, are the catalytic species in the benzoin condensation catalyzed by thiazolium salts plus base. As a consequence, a new mechanism based in experimental data and AM1 calculations is proposed.