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.

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(SRh,Rc)-(Methacrylonitrile)(η5-pentamethylcyclopentadienyl)(benphos)rhodium(III) hexafluoroantimonateC47H50F12RhNP2IrSb2Ee = 100%[α]D25 = +11.0 (c 0.5, CH2Cl2)Source of chirality: (R-Benphos)Absolute configuration: (SRh,Rc)

(SRh,Rc)-(Methacrylonitrile)(η5-pentamethylcyclopentadienyl)(cyphos)rhodium(III) hexafluoroantimonateC46H54F12RhNP2Sb2Ee = 100%[α]D25 = +5.8 (c 1.0, CH2Cl2)Source of chirality: (R-Cyphos)Absolute configuration: (SRh,Rc)

(SIr,Rc)-(Methacrylonitrile)(η5-pentamethylcyclopentadienyl)(benphos)iridium(III) hexafluoroantimonateC47H50F12IrNP2Sb2Ee = 100%[α]D25 = −10.2 (c 0.5, CH2Cl2)Source of chirality: (R-Benphos)Absolute configuration: (SIr,Rc)

(SIr,Rc)-(Methacrylonitrile)(η5-pentamethylcyclopentadienyl)(cyphos)iridium(III)hexafluoroantimonateC46H54F12IrNP2Sb2Ee = 100%[α]D25 = −19.1 (c 0.7, CH2Cl2)Source of chirality: (R-Cyphos)Absolute configuration: (SIr,Rc)