New Mono‐Quarternized Bis‐Cinchona Alkaloid Ligands for Asymmetric Dihydroxylation of Olefins in Aqueous Medium: Unprecedented High Enantioselectivity and Recyclability

Advanced Synthesis & Catalysis
2006.0

Abstract

New mono-quaternized allyl bromide salts of bis-Cinchona alkaloid ligands, [(QD)(2)PHAL-Allyl]Br and [(QN)2PHAL-Allyl]Br, have been synthesized which can be converted into their highly water-soluble multihydroxylated derivatives under asymmetric dihydroxylation (AD) conditions and, thus, easily recovered by a simple extraction method after reaction and reused. These mono-quaternized ligands exhibited superior catalytic efficiency to their neutral counterparts such as (DHQD)(2)PHAL and (DHQ)(2)PHAL for the AD reactions of mono- and disubstituted styrenes under Upjohn conditions. Merely 0.1 mol% of osmium was enough to complete the reactions of mono- and disubstituted styrenes and, moreover, these ligands showed the highest enantioselectivities (e.g., for styrene, 97% ee with [(QD)(2)PHAL-Allyl]Br) among those ever achieved under Upjohn conditions.

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