Imino−Rhodium(III) and −Ruthenium(II) Compounds with Stereogenic Metal Centers

Abstract

The synthesis and characterization of optically active imino complexes (RM or SM)-[(η5-C5Me5)RhCl(imine)][SbF6] (imine = Ln = N-(2-pyridylmethylene)-(R)-1-phenylethylamine (L1) (1a,a‘), N-(2-pyridylmethylene)-(R)-1-naphthylethylamine (L2) (2a,a‘), N-(2-pyridylmethylene)-(R)-1-cyclohexylethylamine (L3) (3a,a‘)) or [(η6-p-MeC6H4iPr)RuCl(imine)]A (A = SbF6, imine = L1 (4a,a‘), L2 (5a,a‘), L3 (6a,a‘), N-(2-pyridylmethylene)-(1R,2S,4R)-1-bornylamine) (L4) (7a,a‘); (A = BF4, imine = L1 (4b,b‘), L2 (5b,b‘), L3 (6b,b‘), L4 (7b,b‘)) is reported. The absolute crystal structures of the (RRh)-1a and (RRu)-7b epimers were determined by X-ray analysis. Both complexes possess a chiral metal center in a pseudo-octahedral environment, being bonded to an η5-C5Me5 group (1a) or to an (η6-p-MeC6H4iPr) ring (7b), a terminal chloride, and, in a chelating fashion, to the two nitrogen atoms of the imine ligand. At room temperature, in acetone or chloroform, the complexes are configurationally stable but in refluxing methanol epimerize at the metal center. Dichloromethane/acetone solutions of the solvate complexes [(ηn-ring)M(imine)S]2+ are active catalysts for the Diels−Alder reaction between methacrolein or acrolein and cyclopentadiene. The reaction occurs rapidly at room temperature and, in general, good exo/endo selectivities and moderate enantioselectivities are achieved.