Enantioselective Catalysts for the Diels–Alder Reaction between Methacrolein and Cyclopentadiene Based on the Chiral Fragment (η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)M(chiral diphosphane) (M = Rh, Ir)
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The aqua complexes [(η5-C5Me5)M(PP*)(H2O)][SbF6]2 (M = Rh, Ir; PP* = chiral diphosphane) (1–10) are prepared and characterized. These complexes efficiently catalyze the Diels–Alder reaction between methacrolein and HCp with enantioselectivities of up to 96% e.e. The norphos complexes 9 and 10, which are obtained as diastereomeric mixtures of the two epimers at the metal, afford the highest e.e. values. The intermediate complexes [(η5-C5Me5)M(PP*)(methacrolein)][SbF6]2 (11–20) are also isolated and characterized, including the molecular structure determination of the [(η5-C5Me5)M(benphos)(methacrolein)][SbF6]2 (M = Rh (13), Ir (14)) derivatives. An NMR study of the reactivity of the metallic intermediates under catalytic conditions explains the high enantioselectivities achieved with the norphos systems.
