Reactions of metastable ionized γ, γ-dialkylallyl methyl ethers: relative rates for elimination of alkyl radicals by formal γ-cleavage

The reactions of seven metastable ionised allylic alkenyl methyl ethers of general structure, R1R2C= CHCH2OCH3+ [R1=CH3, R2=C2H5, n-C3H7, n-C6H13, or iso-C3H7; R1=C2H5, R2=n-C3H7 or iso-C3H7; R1 = n-C3H7, R2 = n-C4H9] with two different γ-alkyl substituents are reported and discussed. Two common reactions are observed: loss of a molecule of methanol or elimination of an alkyl radical. The second fragmentation is interpretable by a mechanism in which two consecutive 1,2-hydrogen shifts lead to an ionised enol ether, R1R2CHCH  CHOCH3+, which then undergoes γ-cleavage to give either R1CH  CHCH  O+CH3 or R2CH  CHCH  O+CH3. Competition experiments reveal that the relative rate of alkyl radical loss from these metastable radical-cations follows the order (CH3)2CH· > CH3CH2· > CH3CH2CH2· ≥ CH3CH2CH2CH2·⪢ CH3· Metastable ionised ethers containing a γ-isopropyl group differ from their isomers with an n-propyl substituent in expelling propane at an appreciable rate. A unique process apparently involving loss of a neutral with a mass of 74 Da is observed from metastable CH3CH2CH2CH2CH2CH2(CH3)C  CHCH2OCH3+, this fragmentation is attributed to consecutive elimination of two species, C3H6 followed by CH3OH, rather than direct expulsion of C4H10O.