MEERWEIN-PONNDORF-VERLEY REDUCTION AND OPPENAUER OXIDATION CATALYZED BY LANTHANOID TRIISOPROPOXIDES

Lanthanoid triisopropoxides, [Ln(OPri)3]n, which were prepared from la nthanoid metals and 2-propanol, were very active catalysts for Meerwei n-Ponndorf-Verley reduction and Oppenauer oxidation. Their activities were higher than that of Al(OPri)3 in the former reaction, and various ketones were reduced to the corresponding alcohols in good yields und er mild conditions. The reduction of aldehydes was very fast even at l ow temperature, and gave alcohols in moderate yields. At high temperat ure, however, aldol-type side reactions occurred. The analogous side r eaction was observed in the Oppenauer oxidation of 1-phenylethanol wit h large excess of cyclohaxanone at high temperatures, and gave 2-(1-cy clohexenyl)cyclohexanone. The side reaction was repressed by lowering the reaction temperature and by diluting the ketone concentration with toluene. Thus, the Oppenauer oxidation at -10-degrees-C gave acetophe none in a 94% yield. In both reduction and oxidation reactions, the ca talytic activity of lanthanoid triisopropoxides increased in the order La<Nd<Gd is similar to Er is similar to Yb which corresponds with the sequence of metal ion size. This result suggests that the Lewis acidi ty of the lanthanoid catalysts is the dominant factor in the Meerwein- Ponndorf-Verley/Oppenauer reactions.