Synthesis, Characterization, Properties, and Asymmetric Catalytic Diels−Alder Reactions of Chiral-at-Metal Imino−Iridium(III) Complexes

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Abstract

The synthesis and characterization of optically active imino complexes (RIr,RC)- and (SIr,RC)-[(η5-C5Me5)IrCl(imine)][SbF6] (imine = Ln = N-(2-pyridylmethylene)−(R)-1-phenylethylamine (L1; 1a,a‘), N-(2-pyridylmethylene)-(R)-1-naphthylethylamine (L2; 2a,a‘), N-(2-quinolylmethylene)−(R)-1-naphthylethylamine (L3; 3a,a‘), N-(6-methyl-2-pyridylmethylene)−(R)-1-naphthylethylamine (L4; 4a,a‘), N-(2-pyridylmethylene)−(R)-1-cyclohexylethylamine (L5; 5a,a‘), N-(2-pyridylmethylene)−(1R,2S,4R)-1-bornylamine) (L6; 6a,a‘)), (RIr,RC)- and (SIr,RC)-[(η5-C5Me5)IrCl(L2)][A] (A = Cl (7a,a‘), BF4 (8a,a‘), PF6 (9a,a‘), (1S)-camphor-10-sulfonate (R*SO3) (10a,a‘)) and (RIr,RC)- and (SIr,RC)-[(η5-C5Me5)(L6)(H2O)][SbF6]2 (11a,a‘)) are reported. The absolute crystal structures of (RIr,RC)-1a, (RIr,RC)-2a, and (RIr,RC)-3a were determined by X-ray analysis. All three complexes show the chiral metal center in a pseudo-octahedral environment, being bonded to an η5-C5Me5 ring, to a terminal chloride, and, in a chelate fashion, to the two nitrogen atoms of the imine ligands. For the chloride compounds 1−6, 1H NMR solution data reveal conformational differences between the crystal and solution structures. At room temperature, in acetone, complexes 1−11 are configurationally stable. At 62 °C, in methanol, the more labile chloro complex 5a epimerizes at Ir with a half-life of 68.0 min (activation parameters, ΔH⧧ = 97.7 ± 13.2 kJ mol-1 and ΔS⧧= −26.3 ± 6.2 J K-1mol-1; equilibrium constant, 5a‘/5a = 4.26 ± 1.65). Dichloromethane/acetone solutions of the solvate complexes [(η5-C5Me5)Ir(imine)S][SbF6]2, prepared in situ by treating complexes 1−6 with equimolar amounts of AgSbF6, are active catalysts for the Diels−Alder reaction between methacrolein and cyclopentadiene. The reaction occurs rapidly at room temperature with good exo:endo selectivity (84:16 to 95:5) and moderate enantioselectivities (up to 46%).

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