SYNTHESIS OF OPTICALLY-ACTIVE ALPHA-HYDROXY CARBONYL-COMPOUNDS BY THECATALYTIC, ENANTIOSELECTIVE OXIDATION OF SILYL ENOL ETHERS AND KETENEACETALS WITH (SALEN)MANGANESE(III) COMPLEXES

A ser of silyl enol ethers and ketene acetals 1a-h with alpha- and/or beta-phenyl as well as all;yl substituents of different steric bull; h as been submitted to the enantioselective catalytic oxidation by chira l (salen)Mn-III complexes 3. Highest conversions and best enantioselec tivities have been obtained with bleach rather than iodosobenzene as o xygen source for the active oxo-metal species. With regard to substrat e structure, ee values up to 89% have been achieved for enol ethers wi th short and unbranched alkyl substituents at the siloxy position. Whi le beta-phenyl groups are beneficial for enantiofacial control, phenyl substituents a to the siloxy functionality result in lower ee values, while the diphenyl-substituted derivative Id displays the lowest ster eoselectivity, The fact that beta- versus alpha-phenyl substituents ex hibit not only differences in the magnitude but also in the sense (opp osite absolute product configuration) of the stereoselectivity may be utilized as a valuable mechanistic probe to assess steric and electron ic effects in the substrate and the catalyst as a function of the type and pattern of substitution Our results display that steric interacti ons between the substrate and the ore-metal complex are mainly respons ible for the observed stereochemical preferences. Indeed, significantl y increased enantioselectivities are achieved even for the remote silo xy group when bulkier derivatives are:employed. In contrast, primarily electronic effects operate in the (salen)Mn-III catalyst 3 since elec tron-donating groups in the 5,5' positions of the salicylaldehyde liga nd afford higher ee values in this catalytic oxidation. The skewed sid e-on approach (trajectory b) of the substrate onto the oxo-metal catal yst is favored, the metallaoxetane mechanism adequately accounts for t he observed enantioselectivities. Herewith a synthetically-valuable me thod for the preparation of optically active alpha-hydroxy carbonyl pr oducts 2 has been made available through the catalytic, enantioselecti ve oxidation of the silyl enol ethers 1 by (salen)Mn-III complexes.