Simple cracking and hydrogen rearrangement. Cleavage for oxetanes under electron impact. Substituent effects and energetics

Mass spectral fragmentation of oxetanes substituted with aryl and other groups (9 to 16) gives simple patterns of product ions associated with carbonyl and olefin cycloelimination fragments. For oxetanes substituted with aryl groups in the 2 position, ions resulting from cleavage plus hydrogen rearrangement (ArRC--OH+) are also prominent. The direction of two-bond cleavage without rearrangement does not correlate well with electron affinities of fragment ions alone. There is general agreement, however, between regioselectivity and overall stability of ions and neutrals, assessed from heats of formation of product pairs. The regiochemistry of fragmentation for 2-aryloxetanes is also correctly predicted by assessment of the relative stabilities of ring-opened valence isomers of the oxetane molecular ions. The competition between cleavage with and without hydrogen migration for 2-aryloxetanes is dependent on ionizing energy, with the former relatively more important at low voltages. A sizable β-secondary deuterium isotope effect on simple cleavage for 16 is also observed (k/sub H//k/sub D/ = 1.08 to 1.02, per deuterium, at 10 to 70 eV). A similarity in regioselectivity for mass spectral and thermal cracking for 2-aryloxetanes is noted