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 °C to a dichloromethane solution of the complex (R^*_AS, R^* _AS)-(±)(R^*_AS, S ^*_AS)-1,2-[(R₃P)PhAsCH₂CH ₂AsPh(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 (R_AS, R_AS) 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 (R_AS, R_AS) enantiomer.