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

Abstract

Chiral-at-metal rhodium(III) complexes of the formula [Rh(κ 4 C,N,N′,P-L)A(Solv)][SbF 6 ] n (Solv = A = MeCN, n = 2 (1); Solv = H 2 O, A = MeCN, n = 2 (2); Solv = MeCN, A = Cl, n = 1 (3); Solv = H 2 O, 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(κ 4 C,N,N′,P-L)(trans-βnitrostyrene)] + ( 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.

Stereogenic-at-Iron Catalysts with a Chiral Tripodal Pentadentate Ligand

“…Carmona recently introduced a chiral rhodium catalyst based on a tripodal tetradentate ligand with three different arms . Interestingly, the overall chirality of the rhodium complex exclusively relied on a stereogenic metal center induced by the asymmetric coordination of an achiral tetradentate tripodal ligand.…”

Catalytic Enantioselective Alkylation of Indoles with trans-4-Methylthio-β-Nitrostyrene

“…Signals at about 17 ppm, attributed to NO 2 H groups, are also observed. Complexes 5 should be the aci -nitro complexes formulated in Scheme , resulting from the formation of the C–C bond between the C3 carbon of the indole and the C β carbon of the coordinated nitroalkene in 3-O . Apart from the excess of indole (the C3 indole proton gives a singlet at about 6.2 ppm), trace B also shows the presence of two new organic compounds characterized by one singlet at around 14 ppm and a doublet centered at 5.45 ppm and a complex multiplet centered at about 5.2 ppm.…”

“…Further treatment at 223 K gave rise to the gradual disappearance of the indole together with the progressive formation of the FC adduct 7 at the expense of the free aci -nitro compound 6 (traces C and D ). The aci -nitro compound 6 rearranges to the FC adduct 7 , most probably, through the prototropic tautomerism shown in Scheme . , …”

“…Complexes 5 should be the aci-nitro complexes formulated in Scheme 2, resulting from the formation of the C− C bond between the C3 carbon of the indole and the C β carbon of the coordinated nitroalkene in 3-O. 18 Apart from the excess of indole (the C3 indole proton gives a singlet at about 6.2 ppm), trace B also shows the presence of two new organic compounds characterized by one singlet at around 14 ppm and a doublet centered at 5.45 ppm and a complex multiplet centered at about 5.2 ppm. The singlet and the doublet were attributed to the NO 2 H group and the H β proton, respectively, of the free acinitro ligand (6) displaced from the coordination sphere of the metal in 5 by a new molecule of MTNS (Scheme 2).…”

“…The aci-nitro compound 6 rearranges to the FC adduct 7, 19 most probably, through the prototropic tautomerism shown in Scheme 3. 13,18 The 31 P{ 1 H} NMR spectrum of the resting state of the catalyst consisted of a pair of double doublets in about 20/80 molar ratio (8-S, trace E). To determine the nature of these complexes, excess of racemic FC adduct 7 was added to a dichloromethane solution of the rhodium aqua-complex (S Rh ,R C )-[Cp*Rh{(R)-Prophos}(OH 2 )][SbF 6 ] 2 (1).…”