Elimination and addition reactions. Part 43. Eliminative fission of cyclobutanes and the relationship between strain and reactivity in cyclobutanes and cyclopropanes

Abstract

Eliminative fission of cyclobutanes has been compared with that of analogous cyclopropanes; reactivity differences of 10^3.7—10^4.8 have been determined. In both sets of compounds, mechanisms have been shown to involve rate-determining ring fission. The possibilities that the large reactivity differences between the comparably strained systems are due either to peculiarity in the cyclobutane structures or to their abnormally slow deprotonation are rejected. Analysis of strain in the two systems suggests that as ring fission occurs, dispersal of the excess of enthalpy of the cyclobutanes is a less sensitive function of extension of a bond in the ring than for the cyclopropanes. The behaviour of these systems is compared with cleavage of cyclopropanes and cyclobutanes in other reactions; the results are remarkably similar. Calculations using the MINDO3 programme have been carried out for eliminative fissions of cyclopropylmethyl and cyclobutylmethyl carbanions. These reproduce remarkably closely the difference between the heats of formation of the species at the energy maxima for fission of each ring size; the maximum for fission of the cyclobutane is considerably displaced along the reaction co-ordinate towards product.