Dissociation reactions of low-energy pentenyl methyl ether radical cations C_5H_9OCH_3~(·+)

摘要

The dissociation chemistry of the low-energy C_5H_9OCH_3~(·+) 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 ~(13)C-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 C_6H_(12)O~(·+) species is considered. The closely similar reactions of C_2H_5CH = CHCH_2OCH_3~(·+), 3~(·+), CH_2 = CH-CH(C_2H_5)OCH_3~(·+), 4~(·+), and CH_2 = C(C_2H_5)CH_2OCH_3~(·+), 13~(·+), point to a common chemistry, which is rationalised in terms of facile 1,2-H and 1,2-C_2H_5 shifts via distonic ions. Each of the other isomers displays a distinct, though often related, chemistry. The eight allylic ionised ethers easily lose CH_3~· to produce C_5H_9O~+ oxonium ions, whose structure was established by CID experiments; ions 3~(·+)/4~(·+)/13~(·+) also readily expel C_2H_5~· to give C_4H_7O~+ ions of structure CH_2 = CH-C~+(H)OCH_3. Elimination of CH_3OH is also significant for 3~(·+)/4~(·+)/13~(·+) and for (CH_3)_2C = CHCH_2OCH_3~(·+), 8~(·+), and CH_3CH = C(CH_3)CH_2OCH_3~(·+), 11~(·+). Besides expelling CH_3~· and/or C_2H_5~· and CH_3OH, the three homoallylic isomers undergo dissociations which are (almost) absent for their allylic counterparts: thus, both CH_3CH = CH(CH_2)_2OCH_3~(·+), 2~(250L?), and CH_2 = CH-CH(CH_3)CH_2OCH_3~(·+), 10~(·+), lose H~· and H_2O, whereas CH_2 = C(CH_3)CH_2CH_2OCH_3~(·+), 7~(·+), is unique in predominantly losing CH_2O. For the losses of CH_2O and H_2O mechanisms are proposed in which ion-neutral complexes of the type [C_5H_(10)~(·+)/CH_2O] and [C_6H_(10)~(·+)/H_2O] are key intermediates. The behaviour of the non(homo)allylic isomer, CH_2 = CH(CH_2)_3OCH_3~(·+), 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.