THE DEGENERATE PAYNE REARRANGEMENT OF THE 2,3-EPOXYPROPOXIDE ANION INTHE GAS-PHASE - A JOINT THEORETICAL AND EXPERIMENTAL-STUDY

Ab initio calculations [at G2 level] indicate that an energised 2,3epo xypropoxide 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 tran sition state is 45 KJ mol(-1)), and (ii) attack of O- at the less subs tituted carbon of the ethylene oxide ring. This cyclisation forms a mo re stable oxetane species, but the barrier (from the reactant to trans ition state) is 122 kJ mol(-1). Experimental results are in accord wit h this prediction. The major fragmentation of energised 2,3-epoxypropo xide is loss of CH2O to yield a product anion identified as the acetal dehyde 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 a nd the (M -H)(-) ion from 3-hydroxyoxetane, together with studies of l abelled (H-2) and doubly labelled (H-2, O-18) analogues, demonstrate ( a) that 40% of CH2O loss occurs by simple cleavage before any rearrang ement of the 2,3-epoxypropoxide anion, (b) 25% of CH2O loss follows Pa yne equilibration, and (iii) 35% of CH2O loss occurs following equilib ration of oxetane intermediate.