Abstract
The mechanism of alkyl radical loss from ionised alkenyl methyl ethers containing two γ-alkyl substituents, R1R2C=CHCH2OCH3+•, has been studied by investigating the collision-induced dissociation spectra of the resultant CnH2n–1O+ oxonium ions (n = 5–7). Comparison of these spectra with one another and those of reference ions generated by dissociative ionisation of secondary allylic alkenyl methyl ethers indicates that expulsion of a γ-alkyl group occurs without isomerisation of the heavy atom skeleton via an allylic rearrangement. This finding is consistent with the occurrence of two consecutive 1,2-H shifts in R1R2C=CHCH2OCH3+•, followed by γ-cleavage of the ionised enol ether, R1R2CHCH=CHOCH3+•, to give R1CH=CHCH=OCH3+ or R2CH=CHCH=OCH3+. Thus, CH3CH2CH2(CH3CH2)C=CHCH2OCH3+• loses C2H5• and C3H7• to yield CH3CH2CH2CH=CHCH=OCH3+ and CH3CH2CH=CHCH=OCH3+, respectively.
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