Rearrangement reactions of deprotonated α-substituted ketones in the gas phase

Abstract

Ambident anions [CH2COC(X)Me2]–, on collisional activation in the gas phase, could, in principle, undergo cyclisations through (i) C– to form a Favorskii cyclopropanone ion complex, or (ii) O– to form an allene oxide ion complex. No Favorskii intermediates are detected, but allene oxide ion complexes are formed when X = OMe or SMe. These cyclisation 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 transfer produces the alkoxide [MeCOC(O–)Me2] which undergoes the acyloin rearrangement (to equilibrate the three methyl groups by 1,2-methyl anion migration) prior to decomposition.