Asymmetric 1,3-dipolar cycloaddition reactions between methacrylonitrile and nitrones catalysed by well-defined M(diphosphane) (M = Rh, Ir) complexes

The cationic half-sandwich aqua-complexes [(η5-C5Me5)M(PP∗)(H2O)][SbF6]2 [M=Rh, Ir; PP∗=(R)-Benphos, (R)-Cyphos, (2R,4R)-Norphos] catalyse the 1,3-dipolar cycloaddition reaction of nitrones with methacrylonitrile with perfect regioselectivity, low-to-perfect endo-selectivity and low-to-moderate enantioselectivity. The active species involved in the catalytic process, [(η5-C5Me5)M(PP∗)(methacrylonitrile)][SbF6]2, have been isolated and characterised as mixtures of the (S)- and (R)-at-metal epimers. NMR measurements of these mixtures indicated that the (RM)-isomers epimerise to the corresponding (SM) counterparts. The molecular structure of the rhodium complex (SRh,RC)-[(η5-C5Me5)Rh{(R)-Benphos}(methacrylonitrile)][SbF6]2 has been determined by X-ray diffraction methods. Diastereomerically pure (SRh,RC)-[(η5-C5Me5)Rh(PP∗)(methacrylonitrile)][SbF6]2 compounds catalyse stoichiometrically the above mentioned dipolar cycloaddition reaction with up to 90% enantiomeric excess, thus indicating the influence of the metal handedness on the catalytic stereochemical outcome. Catalysts can be recycled up to three times without a significant loss of either activity or selectivity.