The benzoin condensation catalysis by bis(azolin-2-ylidene)s and bis(azolidin-2-ylidene)s and its interpretation within the context of nucleophilic carbene chemistry

The preparation of bis(thiazolin-2-ylidene)s by passing a methanol solution of the corresponding thiazolium salt through an ion exchange column (basic form) is reported and the use of these “dimers” as benzoin condensation catalysts is studied. The “dimers” show better catalytic activity than the corresponding thiazolium salt plus base. A general discussion of the benzoin condensation catalysis within the framework of nucleophilic carbene chemistry is carried out and as a result of it the important role played by the “dimers” is emphasized. Mechanistic suggestions related with this fact are put forward.