The Gas Phase Payne Rearrangement. Part 2. The Effect of Methyl Substitution

Abstract

An ab initio study (at G2[MP2]) level shows that deprotonated 2-methyl-3,4-epoxybutan-2-ol (A) and 3-methyl-2,3-epoxybutan-1-ol (B) should interconvert via modest barriers (A → B [47 kJ mol⁻¹]; B → A [33 kJ mol⁻¹]) by three-centre Payne rearrangements. The competing four-centre rearrangements of A and B to deprotonated 2,2-dimethyl-3-hydroxyoxetane (C) have barriers of 117 and 121 kJ mol⁻¹, respectively. The collision-induced mass spectra of (A)–(C) are very similar and the corresponding spectra of a number of ²H- and ¹⁸O-labelled derivatives confirm that isomers A, B and C interconvert on collisional activation, i.e. the three-centre Payne and four-centre ‘oxetane’ cyclisations compete under the reaction conditions. The major cleavages noted in the spectra of A, B and C involve competitive losses of CH₂O and CO: these are both fragmentations of deprotonated 3-methyl-2,3-epoxybutan-2-ol (B).