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

The mechanism of alkyl radical loss from ionised alkenyl methyl ethers containing two gamma-alkyl substituents, R(1)R(2)C=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 gamma-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 R(1)R(2)C=CHCH2OCH3+., followed by gamma-cleavage of the ionised enol ether, R(1)R(2)CHCH=CHOCH3+., to give R(1)CH=CHCH=OCH3+ or R(2)CH=CHCH=OCH3+. Thus, CH3CH2CH2(CH3CH2)C=CHCH2OCH3+. loses C2H5. and C3H7. to yield CH3CH2CH2CH=CHCH=OCH3+ and CH3CH2CH=CBCH=OCH3+, respectively