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

Abstract

The synthesis and characterization of optically active phosphinooxazoline complexes (RRh and SRh)-[(η5-C5Me5)RhCl(PN)][A] (PN = (4S)-2-(2-diphenylphosphino)phenyl)-4-isopropyl-1,3-oxazoline (PN(iPr)), 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‘)), (SRh and RRh)-[(η5-C5Me5)RhI(PN(Me))][SbF6] (4a,4a‘) and (RIr and SIr)-[(η5-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 (SRh and RRh)-[(η5-C5Me5)Rh(PN)S][SbF6]2 (PN = PN(iPr) (8a,8a‘), PN(Me) (9a,9a‘), PN(Ind) (10a,10a‘); S = H2O, Me2CO) and (SIr and RIr)-[(η5-C5Me5)Ir(PN)S][A]2 (PN = PN(iPr), 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 (RRh)-1a, (SRh)-1a‘, (RRh)-2a, (SRh)-2a‘, (RRh)-2b, (RRh)-3a, (SRh)-4a, (RIr)-5b, (RIr)-6a, (SIr)-6a‘, and (RRh)-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 η5-C5Me5 ring, to the nitrogen and phosphorus atoms of the phosphinooxazoline ligand in a chelate fashion, and to a terminal chlorine (1a, 1a‘, 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 5S4 (unprimed complexes and 2a‘) and the 1S2 (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 1S2 and 5S4 conformations. This process was studied by 1H and 31P NMR spectroscopy. Dichloromethane solutions of the solvate complexes [(η5-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)).