Mechanism of the Alkylation of Indoles with Nitrostyrenes Catalyzed by Chiral-at-Metal Complexes

Chiral-at-metal rhodium­(III) complexes
of the formula [Rh­(κ4C,N,N′,P-L)­A­(Solv)]­[SbF6]n (Solv = A = MeCN, n = 2 (1); Solv = H2O, A = MeCN, n =
2 (2); Solv = MeCN, A = Cl, n = 1 (3); Solv = H2O, A = Cl, n = 1
(4)) catalyze the Friedel–Crafts reaction of trans-β-nitrostyrene and N-methyl-2-methylindole.
Complex 4 reacts with trans-β-nitrostyrene,
affording [RhCl­(κ4C,N,N′,P-L)­(trans-β-nitrostyrene)]+ (5). In the presence of N-methyl-2-methylindole,
complex 5 reversibly gives a mixture of aci-nitro complexes 6 in high diastereoselectivity which,
in turn, evolve to the FC adducts through 1,3-prototropy mediated
by a water molecule. On the basis of experimental NMR studies and
DFT calculations, a detailed mechanistic pathway for the alkylation
reaction is proposed, including an explanation of the origin of the
measured enantiomeric ratio.