Chiral ligands for organometallic catalysis containing two dioxolane rings

Abstract

Treatment of D-mannitol 1,6-dibenzoate with a catalytic quantity of toluene-p-sulphonic acid in 2,2-dimethoxypropane leads to the formation of two cyclic acetals, 4R,8R,9R,10R-2,2′,6,6′-tetramethyl-4,8-bis(benzoyloxymethyl)-1,3,5,7-tetraoxadecalin and R,R′-4,4′-bi-(2,2-dimethyl-5-benzoyloxy-methyldioxolanyl) in comparable amounts. Successive reaction with sodium methoxide in methanol, methanesulphonyl chloride, and lithium or potassium diphenylphosphide gave the corresponding phosphines namely 4S, 8S,9S,10S-2,2′,6′6-tetramethyl-4-8-di[bis(diphenylphosphino)methyl]-1,3,5,7-tetraoxadecalin and S,S-4,4′-bi-[2,2-dimethyl-5-bis(diphenylphosphino) methyldioxolanyl]. Similarly, starting from L-iditol, the diphosphines 4R,8S,9S,10R-2,2′,6,′6-tetramethyl-4,8-di[bis-(diphenylphosphino)methyldioxolanyl] were obtained. It was demonstrated through ³¹P n.m.r. studies that both, cis- and trans-chelate complexes of rhodium were formed by these biphosphine ligands. Their potential in asymmetric hydrogenation was demonstrated, for low to moderate yields of amino acids were formed from a variety of enamide precursors.