Elimination and addition reactions. Part 42. Structural effects on reactivity and mechanism in eliminative fission of three-membered rings

Abstract

Three aspects of eliminative fission reactions of the type (i) have been studied. First, the effects of stabilisation of the carbon leaving group, Z, by nitro and by sulphonyl groups have been compared. The results show that stabilisation by nitro produces a 10⁴-fold increase in reactivity over sulphonyl, which is modest when the stabilities of the discrete carbanions are considered. The enhanced stabilisation also produces a change in mechanism from (E1cB)_R to E2. [graphic ommitted]

Secondly, the effect of gem-dimethyl substitution on reactivity has been compared for cyclopropanes with nitro- and with sulphonyl-stabilised leaving groups, and for oxiranes. When elimination involves rate-determining ring fission, the gem-dimethyl (Thorpe–Ingold) effect is substantial and clearly revealed in the activation parameters. When the processes of deprotonation and ring fission are concerted, the Thorpe–Ingold effect is small.

Thirdly, eliminative fission of a thiirane has been compared with that of oxiranes and with that of an unstrained analogue possessing an ethylthio leaving group. The thiirane is somewhat less reactive than the oxirane, but like the oxirane, undergoes fission via a concerted mechanism 10^5.9 times faster than the unstrained analogue. The unstrained analogue undergoes elimination of the ethylthio group via the stepwise (E1cB)_R mechanism, and ranking ( nucleofugality) for the EtS leaving group gives a rank value of 7.3. Detailed comparisons of the balance between nucleofugality and ring strain in such systems are vitiated by the different mechanisms followed by the strained and unstrained systems.