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
UNSPECIFIED. (1993) 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. CANADIAN JOURNAL OF CHEMISTRY-REVUE CANADIENNE DE CHIMIE, 71 (7). pp. 1073-1085. ISSN 0008-4042Full text not available from this repository.
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 (CH2=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 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; this process sometimes competes with or pre-empts alpha-cleavage, as is shown by H-2-labelling experiments. Plausible mechanisms for this sigma'-cleavage are considered. A route involving a 1,2-H shift to the radical centre of a distonic ion, followed by gamma-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 sigma'-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 sigma'-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 gamma-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 (R1). and CH2=CHC+ (X)OR2.
|Item Type:||Journal Article|
|Subjects:||Q Science > QD Chemistry|
|Journal or Publication Title:||CANADIAN JOURNAL OF CHEMISTRY-REVUE CANADIENNE DE CHIMIE|
|Publisher:||NATL RESEARCH COUNCIL CANADA|
|Number of Pages:||13|
|Page Range:||pp. 1073-1085|
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