Methylsulfinyl (Dimsyl) Anion as Umpolung Catalyst for the Chemoselective Cross Benzoin Reaction of α-Diketones with Aldehydes

The hitherto unreported ability of the methylsulfinyl carbanion (dimsyl anion) to generate acyl anion equivalents is described. The dimsyl anion, in fact, efficiently catalyses chemoselective intermolecular cross- benzoin condensations of diaryl α-diketones (benzils) with various aromatic and aliphatic aldehydes to give the corresponding aryl-aryl and aryl-alkyl benzoin benzoates in an atom-economic fashion. The dimsyl anion acts as an environmentally friendly alternative to the toxic cyanide anion and it is obtained by in situ deprotonation of DMSO solvent with a catalytic amount of a strong base, being t- BuOK the optimal promoter. The assumption that the methylsulfinyl carbanion is the active catalyst in the title transformation is supported by electrospray ionization mass spectrometry (ESI-MS) experiments.