Steric effects on the control of endo/exo-selectivity in the asymmetric cycloaddition reaction of 3,4-dimethyl-1-phenylarsole

The asymmetric cycloaddition reactions of 3,4-dimethyl-1-phenylarsole and (Z/E)-diphenyl-1-propenylphosphine/diphenyl-1-styrylphosphine promoted by a chiral organopalladium(II) complex derived from (S)[1-(dimethylamino)ethyl]naphthalene proceeded stereoselectively to generate different exo/endo-products. The reactions involving the (Z/E)-methyl substituted phosphines gave the individual optically pure exo- and endo-cycloadducts in very high stereoselectivity (33 : 1). However, when the methyl group was replaced by a Ph moiety both (Z/E)-phenyl substituted phosphines produced the same endo-cycloadduct in a stereoselectivity of 15 : 1. Every reaction produced five new chiral centers (four of them are sterically independent) in a single step and all three optically pure As P heterobidentate ligands were obtained in high yields. The mechanism involved in the conversion of exo- to endo-product was investigated via density functional theory calculations. Computational results were consistent with the experimentally observed endo/exo-selectivity.