A new variation of the Meenvein-Ponndorf-Verley reduction based on mechanistic considerations is presented. Under optimized conditions 1-(4-dimethylaminophenyl)ethanol was used as the reducing alcohol (2-4 equiv.), Zr(O-tBu)4 as the catalyst (0.2 equiv.), and toluene or cyclohexane as the solvent. Aldehydes and ketones (if not extremely sterically hindered) were reduced to the corresponding alcohols at room temperature mostly within 2-4 h in essentially quantitative yield. a,P-Unsaturated carbonyl compounds cleanly react in a 1,2-mode to afford the corresponding allylic alcohols.In spite of the cheap hydrogen equivalent (e.g. 2-propanol) the classical protocol of the Meenvein-Ponndorf-Verley reduction (MPV reduction, has been largely replaced by reducing agents such as borohydrides or aluminium hydridesL3]. One of the reasons for the decrease in application may be the relatively drastic reaction conditions (2a) of Nern~t [~]. Logarithmation and insertion of the constants result in equation (2b). For example, a difference in redox potential of 0.1 V corresponds to an equilibrium constant of K = 2390.

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Twelve chiral ligands 718 were tested in the new kinetically controlled variation of zirconium alkoxide catalysed Meerwein-Ponndorf-Verley reduction of 2'-chloroacetophenone (2) using rac-4 as the reducing alcohol (Scheme 3). The tetraaryl-1,3-dioxolanes 11 (ca. 30% enantiomeric excess, ee) and the triphenylethanediol 14 (62% ee, 99% yield) gave the best results. A relatively low dependence on the amount of the catalyst (aluminum alkoxide gave no asymmetric induction) and the ligand excess was observed compared with a very high dependence on incubation time of Zr(O'Bu), with the chiral ligand 14 and also on the reaction temperature.

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