Functional Group Interaction in the Fragmentation of Protonated 2,7-Octanedione

Article abstract of DOI:10.1002/oms.1210240304

The fragmentation of 2,7-octanedione, induced by chemical ionization with methane as a reagent gas (CI (CH4)), is shown to be extensively governed by the interaction of the two carbonyl groups.Tandem mass spectrometry reveals that a sequential loss of H2O and C2H4O from the ⁺ ion competes with sequential loss of H2O and C6H10, and that both processes occur via the same ⁺ intermediate.This intermediate is likely to be formed via intramolecular gas-phase aldol condensation and subsequent dehydration.The resulting C(1) protonated 1-acetyl-2-methylcyclopentene structure readily accounts for the observed further decomposition to CH3C<*>O⁺ and 1-methylcyclopentene (C6H10) or, alternatively, to ⁺ (e.g. 1-methylcyclopentenylium) ions and acetaldehyde (C2H4O).Support for this mechanistic rationale is derived from deuterium isotope labelling and low-energy collision-induced dissociation (CID) of the ⁺ ion.The common intermediate shows a CID behaviour indistinguishable by these techniques from that of reference ions, which are produced by gas-phase protonation of the authentic cyclic aldol or by gas-phase addition of an acetyl cation to 1-methylcyclopentene in a CI (CH3COOCH3) experiment.