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

The synthesis and characterization of optically active phosphinooxazoline complexes (R-Rh and S-Rh)-[(eta(5)-C5Me5)RhCl(PN)][A] (PN = (4S)-2-(2-diphenylphosphino)phenyl)-4-isopropyl-1,3-oxazoline (PN(Pr-i)), A = SbF6 (1a,1a'), A = BF4 (1b,1b'); PN = (4S)-2-(2-diphenylphosphino)phenyl)-4-methyl-1,3-oxazoline (PN(Me)), A = SbF6 (2a,2a'), A = BF4 (2b,2b'); PN = (3aS,8aR)-2-(2-diphenylphosphino)phenyl)-3a,8a-dihydroindane[1,2-d]oxazole] (PN(Ind)), A = SbF6 (3a,3a')), (S-Rh and R-Rh)-[(eta(5)-C5Me5)RhI(PN(Me))][SbF6] (4a,4a') and (R-Ir and S-Ir)[(eta(5)-C5Me5)IrCl(PN)] [A] (PN = PN(iPr), A SbF6 (5a,5a'), A = BF4 (5b,5b'); PN = PN(Me), A = SbF6 (6a,6a'), A = BF4 (6b,6b'); PN = PN(Ind), A = SbF6 (7a,7a')), and the solvate complexes (S-Rh and R-Rh)-[(eta(5)-C5Me5)Rh(PN)S][SbF6](2) (PN = PN(Pr-i) (8a,8a'), PN(Me) (9a,9a'), PN(Ind) (10a,10a'); S = H2O, Me2CO) and (S-Ir and R-Ir)-[(eta(5)-C5Me5)Ir(PN)S][A](2) (PN = PN(Pr-i), A = SbF6 (11a'), A = BF4 (11b'); PN = PN(Me), A = SbF6 (12a'), A = BF4 (12b'); PN = PN(Ind), A = SbF6 (13a,13a')) are reported. The crystal structures of the (R-Rh)-1a, (S-Rh)-1a', (R-Rh)-2a, (S-Rh)-2a', (R-Rh)-2b, (R-Rh)-3a, (S-Rh)-4a, (R-Ir)-5b, (R-Ir)-6a, (S-Ir)-6a', and (R-Rh)-9a' epimers were determined by X-ray diffractometric methods. All the complexes show the chiral metal center in a pseudo-octahedral environment, being bonded to an eta(5)-C5Me5 ring, to the nitrogen and phosphorus atoms of the phosphinooxazoline ligand in a chelate fashion, and to a terminal chlorine (1a, la', 2a, 2a', 2b, 3a, 5b, 6a, 6a') or iodine (4a), or to the oxygen of an acetone molecule (9a'). Two conformations of the M-P-C-C-C-N metallacycle have been found in the crystals: the S-5(4) (unprimed complexes and 2a') and the S-1(2) (primed complexes and 2a) screw-boat conformations. In solution, complexes 2, 4, 6, 8a', 9, 10, 12, and 13 exist as a mixture of conformers, most probably arising from the interconversion of the S-1(2) and S-5(4) conformations. This process was studied by H-1 and P-31 NMR spectroscopy. Dichloromethane solutions of the solvate complexes [(eta(5)-C5Me5)M(PN)S][SbF6](2) are active catalysts for the Diels-Alder reaction between methacrolein and cyclopentadiene. The reaction occurs rapidly at room temperature with good exo:endo ratio (from 81:19 to 95:5) and moderate enantioselectivity (up to 67% (Rh compounds), 65% (Ir compounds)).