gg-disubstituted alkenyl methyl ethers



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Eur. J. Mass Spectrom. 2, 233 - 246 (1996)

The mechanism of alkyl radical elimination from ionised gg-disubstituted alkenyl methyl ethers

Richard D. Bowen and Paul Clifford
Chemistry and Chemical Technology, University of Bradford, Bradford, West Yorkshire BD7 1DP, UK.
Richard T. Gallagher
Department of Chemistry, University of Warwick, Coventry CV4 7AL, UK.

ABSTRACT:
The mechanism of alkyl radical loss from ionised alkenyl methyl ethers containing two g-alkyl substituents, R1R2C=CHCH2OCH3, has been studied by investigating the collision-induced dissociation spectra of the resultant CnH2n1O+ oxonium ions (n = 57). 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 g-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 g-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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