Asymmetric Synthesis of Bis(tertiary arsines): Highly Stereoselective Alkylations of Diastereomers of a Chiral Phosphine-Stabilized Bis(arsenium triflate)

Abstract

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 degrees C to a dichloromethane solution of the complex (R*(As),R*(As))-(+/-)/(R*(As),S*(As))-1,2-[(R(3)P)PhAsCH(2)CH(2)AsPh(PR(3))](OTf)(2), where R(3)P 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 degrees 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.