Competitive charge-remote and anion-induced fragmentations of the non-8-enoate anion. A charge-remote reaction which co-occurs with hydrogen scrambling

Abstract

The non-8-enoate anion undergoes losses of the elements of C 3 H 6 , C 4 H 8 and C 6 H 12 on collisional activation. The mechanisms of these processes have been elucidated by a combination of product ion and labelling ( 2 H and 13 C) studies, together with a neutralisation reionisation mass spectrometric study. These studies allow the following conclusions to be made. (i) The loss of C 3 H 6 involves cyclisation of the enolate anion of non-8-enoic acid to yield the cyclopentyl carboxylate anion and propene. (ii) The loss of 'C 4 H 8 ' is a charge-remote process (one which proceeds remote from the charged centre) which yields the pent-4enoate anion, butadiene and dihydrogen. This process co-occurs and competes with complex H scrambling. (iii) The major loss of 'C 6 H 12 ' occurs primarily by a charge-remote process yielding the acrylate anion, hexa-1,5-diene and dihydrogen, but in this case no H scrambling accompanies the process. (iv) It is argued that the major reason why the two charge-remote processes occur in preference to anion-induced losses of but-1-ene and hex-1-ene from the respective 4-and 2-anions is that although these anions are formed, they have alternative and lower energy fragmentation pathways than those involving the losses of but-1-ene and hex-1-ene; viz. the transient 4-anion undergoes facile proton transfer to yield a more stable anion, whereas the 2-(enolate) anion undergoes preferential cyclisation followed by elimination of propene [see (i) above].

The fragmentations of [M–H]− anions derived from underivatised peptides. The side‐chain loss of H2S from Cys. A joint experimental and theoretical study

“…It is often difficult to authenticate such processes experimentally, and recourse to theoretical considerations is necessary. A number of charge‐remote processes involving losses of odd‐ and of even‐electron neutrals from even‐electron anions have been authenticated,14–16 but, in the systems we have studied, they are the exception rather than the rule. We have not found any such processes for peptide anions in our previous studies,1 even though some have been proposed 12,13.…”

Selective Hydrogenation of Single Benzene Ring in Biphenyl Catalyzed by Skeletal Ni

“…This is a high energy process which can occur competitively with H scrambling along the carbon backbone 8. It appears that charge‐remote reactions may occur when charge‐directed reactions are energetically unfavourable in comparison 8 …”

Applications and mechanisms of charge-remote fragmentation

“…Another example of the competition between chargeremote and charge-driven processes is provided by a study on fragmentations of the non-8-enoate anion (Dua et al, 1997). Under collisional activation, the anion loses C 3 H 6 , C 4 H 8 , and C 6 H 12 .…”