Investigation of the mechanism of alkyl radical elimination from ionised pentenyl methyl and hexenyl methyl ethers by analysis of the collision-induced dissociation mass spectra of C4H7O+ and C5H9O+ ions

Collision-induced dissociation (CID) mass spectra are reported for C4H7O+ and C5H9O+ ions generated by loss of an alkyl radical from 11 isomers of C5H9OCH3+• and 8 isomers of C6H11OCH3+• produced by ionisation of alkenyl methyl ethers derived from stable alkenols. The oxonium product ions have acyclic structures (CH=CHCH=O+CH3 for C4H7O+; CH2=CH(CH3)C=O+CH3, CH3CH=CHCH=O+CH3, or CH2=(CH3)CCH=O+CH3 in the case of C5H9O+). Elimination of a methyl radical does not always occur by simple α-cleavage. Expulsion of an alkyl substituent attached to a carbon atom at either end of the C=C double bond also takes place readily; this process sometimes competes with or pre-empts α-cleavage, as is shown by 2H-labelling experiments. Plausible mechanisms for this σ′-cleavage are considered. A route involving a 1,2-H shift to the radical centre of a distonic ion, followed by γ-cleavage of the resultant ionised enol ether, is shown to provide the most accurate unifying description of this unusual fragmentation. The mechanistic significance of this interpretation of the σ′-cleavage is discussed by analysing the reverse reaction (addition of an alkyl radical to a methyl cationated enal) in frontier molecular orbital terms. A comparison is made between the mechanisms by which an alkyl radical is lost from ionised alkenyl methyl ethers by σ′-cleavage and the parallel process starting from ionised carboxylic acids or isomeric distonic ions derived from these CnH2n+1CO2H+• species. Both classes of fragmentation are best understood to occur via γ-cleavage of a distonic ion of general structure R1CH2CH•C+(X)OR2 (R1 = alkyl; X = OH, R2 = H; or X = H, R2 = CH3), thus yielding (R′)• and CH2 = CHC+(X)OR2.