Collision-Induced Carbanion Rearrangements. The Elimination of Ethene From the 3,3-Dimethylheptan-4-one Enolate Ion

Abstract

The mechanisms of formation of the major product ions produced by collision activation of the enolate negative ion of 3,3-dimethylheptan-4-one have been studied by deuterium labelling. H₂ loss involves 1,2-elimination from the 6- and 7-positions. Methane elimination is complex involving three competitive rearrangement processes. Ethene loss produces the most abundant fragment ion and occurs by elimination of the C1-C2 ethyl group with concomitant proton transfer from the 1-position. In the case of the enolate ion from CD₃CH₂CMe₂COPr, the expected elimination of C₂H₂D₂ competes with loss of C₂H₄ from positions 6 and 7. Other decompositions are as follows: loss of C₃H₈ involves the methyl and ethyl substituents at positions 7, 1 and 2 respectively; loss of C₄H₈ occurs by two processes, ( i ) successive losses of two C₂H₄ units (from the 1, 2 then 6, 7 positions), and (ii) loss of CH₂=CMe₂ (from the 2 and 3 positions); and loss of C₅H₁₂ produces Et-C=C-0-. .