New chiral bis(oxazoline) ligands for asymmetric catalysis
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The design and synthesis of a new series of chiral bis(oxazoline) ligand has been described. The utility of their metal complexes has been demonstrated in the asymmetric Mukaiyama aldol reactions of silyl dienolate and silyl ketene acetals with pyruvate and glyoxylate esters. Aldol adducts were isolated in high enantioselectivities (up to 98% and >99% after a single recrystallization) and can be converted to versatile synthetic intermediates. An efficient method for the preparation of ortho-substituted phenyl glycinols using rhodium catalyzed asymmetric hydrogenation was also developed. The preparation of a new class of tetradentate bis(oxazoline) ligand was also described using chiral amino alcohols or from the deprotection of bidentate bis(oxazoline) ligands. Their corresponding metal complexes might serve as potential catalysts for a variety of asymmetric reactions.