Diastereoselective Friedel-Crafts alkylation of indoles with chiral (α-phenyl benzylic cations.: Asymmetric synthesis of anti-1,1,2-triarylalkanes

The reactions of chiral benzyl carbocations bearing a-phenyl substituents with N-sulfonylated indoles afford 1,1,2-triarylalkanes with ante selectivities. This outcome is a reversal of facial diastereoselectivity relative to Bach's (x-alkyl-bearing benzyl cations. The reactions are promoted by either a Bronsted acid (TFA) or Lewis acid (BF(3)center dot OEt(2)), offering differential diastereoselectivities and reactivities. The electronic properties of both reacting partners strongly influence the reaction rates and the product diastereoselectivities and appear to operate under kinetic control. This chemistry provides an efficient access to sterically congested tetrasubstituted ethanes.