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 C 4 H 7 O + and C 5 H 9 O + ions generated by loss of an alkyl radical from 11 isomers of C 5 H 9 OCH 3 +• and 8 isomers of C 6 H 11 OCH 3 +• produced by ionisation of alkenyl methyl ethers derived from stable alkenols. The oxonium product ions have acyclic structures (CH=CHCH=O + CH 3 for C 4 H 7 O + ; CH 2 =CH(CH 3 )C=O + CH 3 , CH 3 CH=CHCH=O + CH 3 , or CH 2 =(CH 3 )CCH=O + CH 3 in the case of C 5 H 9 O + ). 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 2 H-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 C n H 2n+1 CO 2 H +• species. Both classes of fragmentation are best understood to occur via γ-cleavage of a distonic ion of general structure R 1 CH 2 CH•C + (X)OR 2 (R 1  = alkyl; X = OH, R 2  = H; or X = H, R 2  = CH 3 ), thus yielding (R ′ )• and CH 2  = CHC + (X)OR 2 .