INVESTIGATION OF THE MECHANISM OF ALKYL RADICAL ELIMINATION FROM IONIZED 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 C4H 7O+ and C5H9O+ ions generated by loss of an alkyl radical from 11 isom ers of C5H9OCH3+. and 8 isomers of C6H11OCH3+. produced by ionisation of alkenyl methyl ethers derived from stable alkenols. The oxonium pro duct ions have acyclic structures (CH2=CHCH=O+CH3 for C4H7O+; CH2=CH(C H3)C=O+CH3, CH3CH=CHCH=O+CH3, or CH2=(CH3)CCH=O+CH3 in the case Of C5H 9O+). Elimination of a methyl radical does not always occur by simple alpha-cleavage. Expulsion of an alkyl substituent attached to a carbon atom at either end of the C=C double bond also takes place readily; t his process sometimes competes with or pre-empts alpha-cleavage, as is shown by H-2-labelling experiments. Plausible mechanisms for this sig ma'-cleavage are considered. A route involving a 1,2-H shift to the ra dical centre of a distonic ion, followed by gamma-cleavage of the resu ltant ionised enol ether, is shown to provide the most accurate unifyi ng description of this unusual fragmentation. The mechanistic signific ance of this interpretation of the sigma'-cleavage is discussed by ana lysing 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 ion ised alkenyl methyl ethers by sigma'-cleavage and the parallel process starting from ionised carboxylic acids or isomeric distonic ions deri ved from these CnH2n+1CO2H+. species. Both classes of fragmentation ar e best understood to occur via gamma-cleavage of a distonic ion of gen eral structure R1CH2CH.C+ (X)OR2 (R1 = alkyl; X = OH, R2 = H; or X = H , R2 = CH3), thus yielding (R1). and CH2=CHC+ (X)OR2.