REARRANGEMENT REACTIONS OF DEPROTONATED ALPHA-SUBSTITUTED KETONES IN THE GAS-PHASE

Ambident anions [CH2COC(X)Me2]-, on collisional activation in the gas phase, could, in principle, undergo cyclisations through (i) C- to for m a Favorskii cyclopropanone ion complex, or (ii) O- to form an allene oxide ion complex. No Favorskii intermediates are detected, but allen e oxide ion complexes are formed when X = OMe or SMe. These cyclisatio n reactions are endothermic and would not be expected to occur should lower energy reaction channels be available. Such a scenario pertains when X = OH, i.e. no internal cyclisation occurs for ambident enolate [CH2COC(OH)Me2]- following collisional activation. Instead, proton tra nsfer produces the alkoxide [MeCOC(O-)Me2] which undergoes the acyloin rearrangement (to equilibrate the three methyl groups by 1,2-methyl a nion migration) prior to decomposition.