BiBr3-catalyzed benzylation of alcohols. Stereochemistry and mechanistic investigations

We have investigated the benzylation of optically active aliphatic alcohols (octan-2-ol and butan-2-ol) as well as cis and trans 2-methylcyclohexanol or (S)-(-)-menthol, catalyzed by bismuth (III) bromide in the presence of(- ) or (+/-)-1-phenylethanol (PeOH). Under mild conditions, aliphatic alcohol s provided an equimolar mixture of diastereomeric ethers while alicyclic al cohols gave rise to ethers with retention of configuration. To explain thes e results, we assumed that BiBr3, acting as Lewis acid, would lead to the f ormation of a 6-fold coordinated bismuth intermediate involving two molecul es of 1-phenylethanol and one molecule of aliphatic alcohol. We have propos ed a mechanism to explain these BiBr3-promoted etherification reactions. (C ) 2001 Elsevier Science Ltd. All rights reserved.