Meerwein-Ponndorf - Verley tables

Meerwein-Ponndorf-Verley-Oppenauer catalysts typically are aluminum alkox-ides or lanthanide alkoxides (see above). The application of catalysts based on metals such as ytterbium (see Table 20.7, entries 6 and 20) and zirconium [85, 86] has been reported. 

The racemization mechanism of sec-alcohols has been widely studied [40, 41]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), e.g., aluminum complexes in the Meerwein-Ponndorf-Verley/Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via a hydrogen transfer process through metal hydrides or metal dihydrides as intermediates (Scheme 5.21) [42]. 

Although bidentate Lewis acids still remain poorly studied, it is increasingly difficult to dismiss them as esoteric reagents of mere academic interest because truly efficient and useful synthetic applications have recently appeared. The authors reported a new catalytic Meerwein-Ponndorf-Verley reduction [60,61 ] system based on the bidentate Lewis acid chemistry [62]. Treatment of benzaldehyde with (2,7-dimethyl-l,8-biphe-nylenedioxy)bis(diisopropoxyaluminum) (57) at room temperature instantaneously produced the reduced benzyl alcohol almost quantitatively (entry 2, Table 1-10). Moreover, even with 5 mol% of the catalyst 57 the reduction proceeds quite smoothly at room temperature to furnish benzyl alcohol in 81 % yield after 1 h (entry 3, Table 1-10). This remarkable efficiency can be ascribed to the double electrophilic activation of carbonyls by the bidentate aluminum catalyst (Scheme 1-21). 

Enolizable 1,3-dicarbonyl compounds are not reduced by the Meerwein-Ponndorf-Verley reaction4,13. However, 1,2-cyclopentanedione56 and 1,2-cyclohexancdione 7 are converted to the corresponding 1,2-diols. The reaction of both cyclic diones predominantly leads to the corresponding cis-diols. The cis trans ratio depends on the ratio of substrate/aluminum isopro-poxide as shown in the following two tables. 

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