Stereoselectivity in the rhodium(II) acetate catalysed cyclopropanations of 2-diazo-1-indanone with styrenes

The rhodium(II) acetate catalysed cyclopropanation reactions of 2-diazo-1-indanone 4 with various substituted styrenes 5 have been investigated. The cyclopropane diastereomer 6a bearing a trans relationship between the carbonyl and the aryl ring was in all cases the predominant isomer and the ratio of stereosiomers almost constant over a range of styrene substituents. Styrenes bearing electron-donating substituents gave slightly better stereoselectivity in favour of the trans isomer. These results are substantiated by a mechanistic proposal.