Catalytic Action of Azolium Salts. VI. Preparation of Benzoins and Acyloins by Condensation of Aldehydes Catalyzed by Azolium Salts

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Benzoins 4 (2-hydroxyethanones substituted with aryl groups at the 1- and 2-positions) were prepared by self-condensation of aromatic aldehydes 3 using catalytic amounts of azolium salts 1 and 2 in excellent yields.1, 3-Dimethylbenzimidazolium iodide (2) was an effective catalyst for the preparation of acyloins 6 (2-hydroxyethanones substituted with alkyl groups at the 1- and 2-positions) by self-condensation of aliphatic aldehydes 5. On the other hand, an attempt at the condensation of hexanal (5d) catalyzed by 1, 3-dimethylimidazolium iodide (1) failed to yield the acyloin 6d, and instead the aldol-type condensed product 8d was obtained.