Synthesis, characterization, properties, and asymmetric catalytic Diels-Alder reactions of chiral-at-metal imino-iridium(III) complexes

The synthesis and characterization of optically active imino complexes (R-Ir,R-C)- and (SIrRC) [(eta(5)-C5Me5)IrCl(imine)] [SbF6] (imine = L-n = N-(2-pyridylmethylene)-(R)-1-phenylethylamine (L-1; 1a,a'), N-(2-pyridylmethylene)-(R)-1-naphthylethylamine (L-2; 2a,a'), N-(2-quinolylmethylene)-(R)-1-naphthylethylamine (L-3; 3a,a'), N-(6-methyl-2-pyridylmethylene)-(R)-1-naphthylethylamine (L-4; 4a,a'), N-(2-pyridylmethylene)-(R)-1-cyclohexylethylamine (Ll(5); 5a,a'), N-(2-pyridylmethylene)-(1R, 2S, 4R)-1-bornylamine) (L-6; 6a,a')), (R-Ir,R-C)- and (S-Ir,R-C)[(eta(5)-C5Me5)IrCl(L-2)][A] (A = Cl (7a,a'), BF4, (8a,a'), PF6 (9a,a'), (1S)-camphor-10-sulfonate (R*SO3) (10a,a')) and (R-Ir,R-C)- and (S-Ir,R-C)-[eta(5)-C5Me5)(L-6)(H2O)][SbF6](2) (11a,a')) are reported. The absolute crystal structures of (RIrRC)-1a, (R-Ir,R-C)-2a, and (R-Ir,R-C)-3a were determined by X-ray analysis. All three complexes show the chiral metal center in a pseudo-octahedral environment, being bonded to an eta(5)-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, H-1 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 degrees C, in methanol, the more labile chloro complex 5a epimerizes at Ir with a half-life of 68.0 min (activation parameters, Delta H double dagger = 97.7 +/- 13.2 kJ mol(-1) and Delta S double dagger= -26.3 +/- 6.2 J K-1 mol(-1); equilibrium constant, 5a'/5a = 4.26 +/- 1.65). Dichloromethane/acetone solutions of the solvate complexes [(eta(5)-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%).