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

Ab initio calculations [at G2 level] indicate that an energised 2,3epoxypropoxide 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-tb 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 (H-2) and doubly labelled (H-2, O-18) 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 oxetane intermediate.