Elimination and addition reactions. Part 41. Nucleophilic eliminative fission of cyclopropanes: the coiled spring effect of ring strain on nucleofugality and its evaluation

Abstract

Rates have been measured of sulphonyl-activated eliminative ring fissions of a series of six cyclopropanes in which the leaving group is stabilised by alkoxycarbonyl, cyano, or sulphonyl groups. The measurements allow assignment of ranks (nucleofugalities) to carbon leaving groups in systems in which the connection to the leaving group is strained by incorporation in a cyclopropane ring. The values obtained are compared with those obtained for a unstrained (acyclic) analogues. Rank enhancements of about 9 (log) units are obtained; these enhancements suggest that free energies of activation for leaving-group expulsion are reduced by about 53 kJ mol–1, or about 46% of the excess of enthalpy of the strained ring, notwithstanding the small degree of ring fission in the transition structure. The effect of phenyl substitution at the leaving group suggests that cleavage of the ring is very little advanced in the transition structure, although this is variable with the nature of the leaving-group stabilisation. This is the first direct determination of the effect of strain on nucleofugality.