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 )-[(η 5 -C 5 Me 5 )RhCl(PN)][A] (PN = (4 S )-2-(2-diphenylphosphino)phenyl)-4-isopropyl-1,3-oxazoline (PN( i Pr)), A = SbF 6 ( 1a, 1a ‘), A = BF 4 ( 1b, 1b ‘); PN = (4 S )-2-(2-diphenylphosphino)phenyl)-4-methyl-1,3-oxazoline (PN(Me)), A = SbF 6 ( 2a, 2a ‘), A = BF 4 ( 2b, 2b ‘); PN = (3a S,8a R )-2-(2-diphenylphosphino)phenyl)-3a,8a-dihydroindane[1,2-d]oxazole] (PN(Ind)), A = SbF 6 ( 3a, 3a ‘)), ( S Rh and R Rh )-[(η 5 -C 5 Me 5 )RhI(PN(Me))][SbF 6 ] ( 4a, 4a ‘) and ( R Ir and S Ir )-[(η 5 -C 5 Me 5 )IrCl(PN)][A] (PN = PN( i Pr), A = SbF 6 ( 5a, 5a ‘), A = BF 4 ( 5b, 5b ‘); PN = PN(Me), A = SbF 6 ( 6a, 6a ‘), A = BF 4 ( 6b, 6b ‘); PN = PN(Ind), A = SbF 6 ( 7a, 7a ‘)), and the solvate complexes ( S Rh and R Rh )-[(η 5 -C 5 Me 5 )Rh(PN)S][SbF 6 ] 2 (PN = PN( i Pr) ( 8a, 8a ‘), PN(Me) ( 9a, 9a ‘), PN(Ind) ( 10a, 10a ‘); S = H 2 O, Me 2 CO) and ( S Ir and R Ir )-[(η 5 -C 5 Me 5 )Ir(PN)S][A] 2 (PN = PN( i Pr), A = SbF 6 ( 11a ‘), A = BF 4 ( 11b ‘); PN = PN(Me), A = SbF 6 ( 12a ‘), A = BF 4 ( 12b ‘); PN = PN(Ind), A = SbF 6 ( 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 η 5 -C 5 Me 5 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 5 S 4 (unprimed complexes and 2a ‘) and the 1 S 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 1 S 2 and 5 S 4 conformations. This process was studied by 1 H and 31 P NMR spectroscopy. Dichloromethane solutions of the solvate complexes [(η 5 -C 5 Me 5 )M(PN)S][SbF 6 ] 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)).