Catalytic Meerwein-Ponndorf-Verley (MPV) and Oppenauer (OPP) reactions: Remarkable acceleration of the hydride transfer by powerful bidentate aluminum alkoxides

A highly efficient catalytic procedure for Meerwein-Ponndorf-Verley (MPV) reduction has been devised by employing (2,7-dimethyl-1,8-biphenylenedioxy)bis(dialkoxyaluminum) (3) as a catalyst. For example, reduction of 4-phenylcyclohexanone in CH₂Cl₂ with i-PrOH (1 equiv) as a hydride source in the presence of a catalytic amount of the in situ generated 3a (1 mol%) smoothly proceeded at room temperature within 1 hour to furnish 4-phenylcyclohexanol in 82% isolated yield. In sharp contrast, the conventional MPV reduction with stoichiometric Al(i-PrO)₃ resulted in the recovery of the starting ketone under similar conditions. Our approach also enabled the reverse reaction of MPV reduction, i.e., Oppenauer (OPP) oxidation in a truly catalytic manner. Furthermore, these findings prompted us to develop a simultaneous intramolecular reduction/oxidation of hydroxy carbonyl substrates via the MPV reaction process in the presence of bidentate aluminum catalyst 1, where highly chemoselective hydride transfer from a secondary alcohol moiety to the remote carbonyl group took place. Finally, we demonstrate that the modern MPV system is applicable to high-speed, catalytic Tishchenko reactions. For instance, the dimerization product of cyclohexanecarbaldehyde was obtained in almost quantitative yield using only 0.2 mol% of catalyst 3a. Based on this result, highly stereoselective intramolecular Tishchenko reduction of a β-hydroxy ketone was also accomplished successfully.