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 (aRP)-phosphepine. Thus, the addition of methyllithium in diethyl ether at −95 °C 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 (aRP)-[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 °C gives the corresponding (R*As,R*As)-(±)-1,2-ethanediylbis[(n-butyl)phenylarsine) in 77% diastereoselectivity and 93% enantioselectivity in favor of the (RAs,RAs) enantiomer.