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 (R Rh and S Rh)-[(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‘)), (S Rh and R Rh)-[(η5-C5Me5)RhI(PN(Me))][SbF6] (4a,4a‘) and (R Ir and S Ir)-[(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 (S Rh and R Rh)-[(η5-C5Me5)Rh(PN)S][SbF6]2 (PN = PN(iPr) (8a,8a‘), PN(Me) (9a,9a‘), PN(Ind) (10a,10a‘); S = H2O, Me2CO) and (S Ir and R Ir)-[(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 (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 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)).

Half-sandwich iridium complexes—Synthesis and applications in catalysis

“…Under similar conditions, the reaction of [Cp*IrCl 2 ] 2 with chiral phosphinooxazolines also resulted in a mixture of two diastereomers (S Ir ,S C ) and (R Ir ,S C ); the dr varied with the substituents on the oxazoline ring and with the metathesis reagent used (77-79, Table 6, entries 2-4) [52]. However, single diastereomers of 77, 78 and 79 (>98% by 1 H NMR) were obtained by fractional crystallization from methanol.…”

The Stereospecific Ligand Exchange at a Pseudo‐Benzylic T‐4 Iridium Centre in Planar‐Chiral Cycloiridium (η6‐Arene)tricarbonylchromium Complexes

“…[10][11][12][13] If a restriction to the rearrangement of a ligand can be imposed, control of the facial approach of the metal by the incoming ligand can be achieved. [14][15][16][17][18] This may be realised with appended auxiliary ligands capable of reversibly binding the metal centre and constraining the ligand-exchange process to fewer stereochemical paths than in an unconAbstract: The stereospecificity of ligand exchange at the Ir III centre of a cycloiridium arenetricarbonylchromium complex has been established experimentally by various analytical methods as well as by X-ray diffraction structural analysis and computational investigations. Two new cases of phenyl and methyl iridiumA C H T U N G T R E N N U N G (III) complexes have been prepared by reaction of (S* p ,R* Ir )-chlorido{2-[(tricarbonyl)(h 6 -phenylenekC 1' )chromium(0)]pyridinekN}(pentamethylcyclopentadienyl)iridiumA C H T U N G T R E N N U N G (III) with PhMgBr and MeMgBr.…”

Catalytic Asymmetric Michael Additions of α-Cyanoacetates

“…The planar-chiral complex reported by Ito provided the best results in terms of enantioselectivity in the Michael addition of a-cyano esters 3-R 1 -Me. 3 The only example for an iridium system was reported by Carmona et al 13 Based on their findings that some iridium acetylacetonate complexes exist in the form of hydridoenolato complexes 14 enantioselective Michael additions were attempted. With the chiral P,N-ligand 13, enantiomeric excess values of up to 39% were obtained (Table 4).…”