REACTIONS OF METASTABLE IONIZED GAMMA,GAMMA-DIALKYLALLYL METHYL ETHERS - RELATIVE RATES FOR ELIMINATION OF ALKYL RADICALS BY FORMAL GAMMA-CLEAVAGE

The reactions of seven metastable ionised allylic alkenyl methyl ether s of general structure, (RRC)-R-1-C-2 = CHCH2OCH3+. [R-2 = CH3, R-2 = C2H5, n-C3H7, n-C6H13, or iso-C3H7; R-1 = C2H5, R-2 = n-C3H7 or iso-C3 H7; R-1-n-C3H7, R-2=n-C4H9] with two different gamma-alkyl substituent s are reported and discussed. Two common reactions are observed: loss of a molecule of methanol or elimination of an alkyl radical. The seco nd fragmentation is interpretable by a mechanism in which two consecut ive 1,2-hydrogen shifts lead to an ionised enol ether, (RRCHCH)-R-1-C- 2 = CHOCH3+., which then undergoes gamma-cleavage to give either (RCH) -C-1 = CHCH = O+CH3, or (RCH)-C-2 = CHCH = O+CH3. Competition experime nts reveal that the relative rate of alkyl radical loss from these met astable radical-cations follows the order (CH3)(2)CH.> CH3CH2. greater than or equal to CH3CH2CH2CH2. >> CH3.. Metastable ionised ethers con taining a gamma-isopropyl group differ from their isomers with an n-pr opyl substituent in expelling propane at an appreciable rate. A unique process apparently involving loss of a neutral with a mass of 74 Da i s observed from metastable CH3CH2CH2CH2CH2CH2(CH3)C = CHCH2OCH3+., thi s fragmentation is attributed to consecutive elimination of two specie s, C3H6 followed by CH3OH, rather than direct expulsion of C4H10O. (C) 1997 Elsevier Science B.V.