AlIII-Calix[4]arene Catalysts for Asymmetric Meerwein-Ponndorf-Verley Reduction

Chiral Al-III-calixarene complexes were investigated as catalysts for the asymmetric Meerwein-Ponndorf-Verley (MPV) reduction reaction when using chiral and achiral secondary alcohols as reductants. The most enantioselective catalyst consisted of a new axially chiral vaulted-hemispherical calix[4]arene phosphite ligand, which attained an enantioselective excess of 9996. This ligand consists of two lower-rim hydroxyl groups, with the remaining two lower-rim oxygens directly connected to the phosphorus of the phosphite, which is derived from a chiral diol. The results emphasize the importance of the rigid calix[4]arene lower-rim substituents and point to a possible role of a lower-rim chiral pocket and Lewis-basic phosphorus lone pairs in enhancing asymmetric hydride transfer.