Reductive Asymmetric Cross Pinacol Coupling Utilizing Planar Chiral Transition Metal Complexes

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Enantiomerically pure β-diols, diamines, and amino alcohols have found widespread use as chiral ligands or auxiliaries as asymmetric reactions, and are also important in key synthetic intermediates for biologically active natural products. While optically pure b-diols have been conveniently prepared by catalytic aymetric dihydroxylation of the olefins with high optical purity, the generally accepted method for the preparation of chiral diamines involvesan optical resolution of racemic compounds with certain chiral carboxylic acids. A reductive coupling of carbonyl or imine compounds, homo-pinacol coupling, is the most direct method for the preparation of b-diols or diamine. Although this reaction can be achieved in high yield by lanthanoid metals or low valent transition metals, highly diastereoselective formation of b-diols or diamines is problematic under conventional reductive coupling. We have succeeded that samarium idode-mediated cross-coupling of N-tosyl ferrocenylideneamine with planar chiral ferrocenecarboxaldehydes or benzaldehyde chromium complexes gave diastereoselectively the corresponding anti-β-amino alcohols by coupling with planar chiral arylaldehydes. Dynamic kinetic resolution of a configurationally equilibrated reactive species generated from achiral N-tosyl ferrocenilideneamine and benzylideneamine by reduction with samarium iodide was observed in the cross-coupling with planar chiral arylaldehydes giving both antipodes of β-amino alcohols depending on thee planar chirality. The obtained anti-β-amino with the ferrocene ring was utilized as a ligand for catalytic asymmetric reduction of acetophenone.