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

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.