Asymmetric synthesis of bis(tertiary arsines): Highly stereoselective alkylations of diastereomers of a chiral phosphine-stabilized bis(arsenium triflate)

The addition of organolithium reagents to an equilibrating mixture of diastereomers of a phosphine-stabilized 1,2-ethanediylbis(phenylarsenium triflate) containing chiral arsenic stereocenters and an enantiomerically pure, atropisomeric tertiary phosphepine derived from lithiated (aR)-2,2' -dimethyl-1,1'-binaphthalene generates unequal mixtures of diastereomers and enantiomers of chelating 1,2-ethanediylbis(tertiary arsines), chiral at arsenic, with liberation of the (aR_P)-phosphepine. Thus, the addition of methyllithium in diethyl ether at -95 Celsius degree to a dichloromethane solution of the complex (R*_As,R*_As)-(+-)/(R*_As,S_As)-1,2[(R₃P)PhAsCH₂CH₂AsPh(PR₃)](OTf)₂, where R₃P is (aR_P)-[2-(methoxymethyl)phenyl]phosphepine, generates (R*_As,R*_As)-(()-1,2-ethanediylbis(methylphenylarsine) in 78% diastereoselectivity and 95% enantioselectivity in favor of the (RAs,RAs) enantiomer. Under similar conditions, the addition of n-butyllithium in hexanes to a solution of the bis(phosphepine-stabilized)-diarsenium triflate at-95 Celsius degree gives the corresponding (R*_As,R*_As)-(+-)-1,2-ethanediylbis[(n-butyl)phenylarsine) in 77% diastereoselectivity and 93% enantioselectivity in favor of the (R_As,R_As) enantiomer.