Dissociation Reactions of Low-Energy Pentenyl Methyl Ether Radical Cations C5H9OCH3•+

Abstract

The dissociation chemistry of the low-energy C5H9OCH3•+ ions generated from the 13 isomeric pentenyl methyl ethers derived from stable alkenols has been studied. This was done by examining their metastable ion characteristics, in conjunction with 2H and 13C-labelling as well as collision-induced dissociation and neutralisation–reionisation experiments. The influence of the position and substitution pattern of the double bond on the chemistry of these C6H12O•+ species is considered. The closely similar reactions of C2H5CH=CHCH2OCH3•+, 3•+, CH2=CH–CH(C2H5)OCH3•+, 4•+, and CH2=C(C2H5)CH2OCH3•+, 13•+, point to a common chemistry, which is rationalised in terms of facile 1,2-H and 1,2-C2H5 shifts via distonic ions. Each of the other isomers displays a distinct, though often related, chemistry. The eight allylic ionised ethers easily lose CH3• to produce C5H9O+ oxonium ions, whose structure was established by CID experiments; ions 3•+ / 4•+ / 13•+ also readily expel C2H5• to give C4H7O+ ions of structure CH2=CH–C+(H)OCH3. Elimination of CH3OH is also significant for 3•+ / 4•+ / 13•+ and for (CH3)2C=CHCH2OCH3•+, 8•+, and CH3CH=C(CH3)CH2OCH3•+, 11•+. Besides expelling CH3• and/or C2H5• and CH3OH, the three homoallylic isomers undergo dissociations which are (almost) absent for their allylic counterparts: thus, both CH3CH=CH(CH2)2OCH3•+, 2•+, and CH2=CH–CH(CH3)CH2OCH3•+, 10•+, lose H• and H2O, whereas CH2=C(CH3)CH2CH2OCH3•+, 7•+, is unique in predominantly losing CH2O. For the losses of CH2O and H2O mechanisms are proposed in which ion–neutral complexes of the type [C5H10•+ / CH2O] and [C6H10•+ / H2O] are key intermediates. The behaviour of the non-(homo)allylic isomer, CH2=CH(CH2)3OCH3•+, 1•+, is similar to that of 2•+ but the reactions occur in different proportions. A mechanism for the facile loss of an alkyl radical from 1+ is proposed in which 1,4-H shifts and distonic ions as well as communication with ionised cyclopentyl methyl ether, 14•+, play an important role.