Synthesis of novel (P,S) ligands based on chiral nonracemic episulfides. Use in asymmetric hydrogenation

A strategy for the synthesis of new chiral (P,S) ligands is described. It i s based on the opening of chiral nonracemic episulfides using phosphorus nu cleophiles. Chiral episulfides, CH2CH(R)S, 4, are derived either from the r eaction of thiourea with the corresponding epoxide (4a, R = CH3) or from th e stepwise conversion of chiral diols to the episulfide via the thiocarbona te (4b, R = cyclohexyl). The reaction of lithium salts of phosphines, R-2'P Li (R = Ph, cyclohexyl), with episulfides is regioselective and gives the r ing-opened products 5-8, PR2CH2CH(R')SLi. Upon treatment with electrophiles , R"Cl (R" = -CH2Ph, -CH2(C-5(CH3)(5)), -CH2(C14H9), -CH(C14H12)), they giv e novel chiral (P,S) ligands, PR'2CH2CH(R')SR", 9-17 in 31-93% yield. React ions of PCy2CH2CH(CH3)SCH2(C-6(CH3)(5)), 11, With LMCl2 (LM = (DME)Ni, (COD )Pd, and (NBD)Pt; DME = dimethoxyethane, COD = 1,5-cyclooctadiene, NBD = 2, 5-norbornadiene) yields the corresponding metal complexes, (11)MCl2, 18. Va riable-temperature H-1 NMR spectroscopy indicates that in these complexes s ulfur inversion occurs on the NMR time scale. Compound 18b (M = Pd) was cha racterized by single-crystal X-ray analysis. Reaction of PCy2CH2CH(CH3)SCH( C14H12), 13, with (COD)PdCl2 results in C-S bond cleavage and produces a di nuclear thiolato-bridged complex, dichlorobis{mu-[2-(dicyclohexylphosphino) -1-(methyl)ethanethiolato]-P,mu-S}dipalladium(II), 19. Complex 19 was chara cterized by single-crystal X-ray analysis. Reactions of ligands 9-17 with R h(COD)(2)+OTf- (COD = cyclooctadiene, OTf- = CF3SO3-) yields rhodium comple xes that have been tested in the asymmetric hydrogenation of alpha-enamide methyl esters, providing enantioselectivities of up to 51%. The rhodium com plex obtained with ligand 11, (11)Rh(COD)OTf, 22, was characterized by sing le-crystal X-ray analysis.