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

The reaction of the rhodium aqua-complex (S Rh,R C)-[Cp*Rh{(R)-Prophos} (OH2)][SbF6]2 [Cp* = C5Me5, Prophos = propane-1,2-diyl-bis(diphenylphosphane)] (1) with trans-4-methylthio-β-nitrostyrene (MTNS) gives two linkage isomers (S Rh,R C)-[Cp*Rh{(R)-Prophos}(κ1 O-MTNS)]2+ (3-O) and (S Rh,R C)-[Cp*Rh{(R)-Prophos}(κ1 S-MTNS)]2+ (3-S) in which the nitrostyrene binds the metal through one of the oxygen atoms of the nitro group or through the sulfur atom, respectively. Both isomers are in equilibrium in dichloromethane solution, the equilibrium constant being affected by the temperature in such a way that when the temperature increases, the relative concentration of the oxygen-bonded isomer 3-O increases. The homologue aqua-complex of iridium, (S Ir,R C)-[Cp*Ir{(R)-Prophos}(OH2)][SbF6]2 (2), also reacts with MTNS; but only the sulfur-coordinated isomer (S Ir,R C)-[Cp*Ir{(R)-Prophos}(κ1 S-MTNS)]2+ (4-S) is detected in the solution by NMR spectroscopy. The crystal structures of 3-S and 4-S have been elucidated by X-ray diffractometric methods. Complexes 1 and 2 catalyze the Friedel-Crafts reaction of indole, N-methylindole, 2-methylindole, or N-methyl-2-methylindole with MTNS. Up to 93% ee has been achieved for N-methyl-2-methylindole. With this indole, the ee increases as conversion increases, ee at 263 K is lower than that obtained at 298 K, and the sign of the chirality of the major enantiomer changes at temperatures below 263 K. Detection and characterization of the catalytic intermediates metal-aci-nitro and the free aci-nitro compound as well as detection of the Friedel-Crafts (FC)-adduct complex involved in the catalysis allowed us to propose a plausible double cycle that accounts for the catalytic observations.