The cationic half-sandwich aqua-complexes [(η-CMe)M(PP∗)(HO)][SbF] [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, [(η-CMe)M(PP∗)(methacrylonitrile)][SbF], have been isolated and characterised as mixtures of the (S)- and (R)-at-metal epimers. NMR measurements of these mixtures indicated that the (R)-isomers epimerise to the corresponding (S) counterparts. The molecular structure of the rhodium complex (S,R)-[(η-CMe)Rh{(R)-Benphos}(methacrylonitrile)][SbF] has been determined by X-ray diffraction methods. Diastereomerically pure (S,R)-[(η-CMe)Rh(PP∗)(methacrylonitrile)][SbF] 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.

We thank the Ministerio de Economía y Competitividad, Spain and European Social Fund (Grants CTQ2012-32095, CTQ2015-66079-P and CONSOLIDER INGENIO-2010 CSD2006-0015), European Union and Gobierno de Aragón and European Social Fund (Grupo Consolidado: Catalizadores Organometálicos Enantioselectivos), European Union for financial support. A.A. thanks the IUCH for a grant.

Peer Reviewed