cis-Selective cyclopropanations using chiral 5,5-diaryl bis(oxazoline) catalysts

A series of 5,5-diaryl bis(oxazolines) have been prepared and used as ligands in the copper catalysed asymmetric cyclopropanation of styrene. Unusually, for bis(oxazolines), the diastereoselectivity of the process favoured the cis cyclopropanes with diastereomeric ratios of up to 65:35 being realised. The cis selectivity of the process was rationalised in terms of repulsion between the alkene substituent and the pro-R 5-aryl group of the bis(oxazoline).