Asymmetric 1,3-dipolar cycloaddition reactions between enals and nitrones catalysed by half-sandwich rhodium or iridium diphosphane complexes

Abstract

The aqua complexes [(η⁵-C₅Me₅)M(PP*)(H₂O)][SbF₆]₂ (M = Rh, Ir; PP* = chiral diphosphane) have been tested as catalysts for the asymmetric 1,3-dipolar cycloaddition of nitrones to α,β-unsaturated aldehydes. Quantitative conversions with very high regioselectivity, perfect endo selectivity and excellent enantioselectivity (up to 99% ee) were achieved. The stereochemical outcome was analyzed on the basis of the stereoelectronic properties of the intermediate enal complexes of the formula [(η⁵-C₅Me₅)M(PP*)(enal)][SbF₆]₂.