Synthesis, Separation, and Stereochemical Studies of Chiral-at-Metal Rhodium(III) Complexes. Crystal Structure of (SRh,RC)-[(η5-C5Me5)RhCl{Ph2PCH(Me)CH2PPh2}]BF4

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.

Conformational preferences driven by the C-methyl substituent in chelated o-diphenylphosphino-alpha-methyl-N,N-dimethylbenzylamine rhodium complexes

The stereochemistry of the chelate rings of a number of rhodium aminophosphine complexes is studied by NMR spectroscopy. The similarity in the variable-temperature behavior for the different compounds is consistent with them having in common highly preferred chelate ring conformations. The six-membered metallacycle of coordinated (R)-PN (PN = o-diphenylphosphino-alpha-methyl-N,N-dimethylbenzylamine) adopts a delta conformation in the solid state. NMR experiments indicate that this conformation is strongly favored in solution as well. The preferred sense of helicity is imposed by the absolute configuration of the stereogenic carbon atom on the ligand, which exerts an important steric control. The complex [Rh(TFB)((C(6)H(4)CHMeNMe(2))(2)P(C(6)H(4)CHMeNHMe(2)))](BF(4))(2).H(2)O.Me(2)CO crystallizes in the monoclinic space group P2(1) with a = 12.0548(11) A, b = 16.139(2) A, c = 12.1804(10) A, beta = 100.742(9) degrees, Z = 4.