The degenerate Payne rearrangement of the 2,3-epoxypropoxide anion in the gas phase. A joint theoretical and experimental study

Abstract

Ab initio calculations [at G2 level] indicate that an energised 2,3-epoxypropoxide anion should undergo two competing cyclisation processes, i.e. (i) the degenerate Payne rearrangement (attack of O⁻ at the more substituted carbon of the ethylene oxide ring, to open that ring, and to form another ethylene oxide ring) (the computed barrier to the transition state is 45 kJ mol⁻¹), and (ii) attack of O⁻ at the less substituted carbon of the ethylene oxide ring. This cyclisation forms a more stable oxetane species, but the barrier (from the reactant to transition state) is 122 kJ mol⁻¹. Experimental results are in accord with this prediction. The major fragmentation of energised 2,3-epoxypropoxide is loss of CH₂O to yield a product anion identified as the acetaldehyde enolate anion (CH₂CHO). This cleavage can be used as a probe to investigate the relative extents of the two possible cyclisation processes. Comparison of the spectra of the 2,3-epoxypropoxide anion and the (M - H)⁻ ion from 3-hydroxyoxetane, together with studies of labelled (²H), and doubly labelled (²H, ¹⁸O) analogues, demonstrate (a) that 40% of CH₂O loss occurs by simple cleavage before any rearrangement of the 2,3-epoxypropoxide anion , (b) 25% of CH₂O loss follows Payne equilibration, and (iii) 35% of CH₂O loss occurs following equilibration of the Payne product and an oxetane intermediate.