Intramolecular Halogen Transfer via Halonium Ion Intermediates in the Gas Phase

Abstract

Abstract. The fragmentation of halogen-substituted protonated amines and quaternary ammonium ions (R . A potential energy surface scan using DFT calculation for CH 2 -N bond cleavage process of protonated 2-bromo-N,N-dimethylethanamine supports the formation of this intermediate. The bromonium ion intermediate-involved halogen transfer mechanism is supported by an examination of the ion/molecule reaction between isolated ethylenebromonium ion and triethylamine, which generates the N-bromo-N,N,N-triethylammonium cation. For other halogens, Cl and I also can be involved in similar intramolecular halogen transfer, but F cannot be involved. With the elongation of the carbon chain between the halogen (bromine as a representative example) and amine, the migration ability of halogen decreases. When the carbon chain contains two or three CH 2 units (n = 1, 2), formal bromine cation transfer can take place, and the transfer is easier when n = 1. When the carbon chain contains four or five CH 2 units (n = 3, 4), formal bromine cation transfer does not occur, probably because the fiveand six-membered cyclic bromonium ions are very stable and do not donate the bromine to the amine.

GC-MS Study of Mono- and Bishaloethylphosphonates Related to Schedule 2.B.04 of the Chemical Weapons Convention: The Discovery of a New Intramolecular Halogen Transfer

“…The final step of the mechanism consists of the extrusion of an ethylene molecule from I3 to give the observed 27 (M-43). This process is analogous to one reported for the fragmentation of halogen-substituted amines via intramolecular halogen transfer [34]. This rate-determining step is clearly favored for the more nucleophilic iodine (ΔG ‡ = 48.2 kcal mol ).…”

“…Similar to the results for compounds 1-12, the elimination of the methyl radical (M-15) that forms product-ions as 25 (Scheme 2, X = H) followed by the loss of an ethylene molecule leads to the base peaks with structure identical to 28. The EI mass spectra of all bisHAPs studied (13-24) lacked the odd-electron molecular ions (Tables 2 and 3, SF [25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]. The molecular masses of these compounds were confirmed by GC/ MS(CI) (SF 41-43).…”

Effects of the β‐heteroatom and leaving group on neighbouring group participation in the gas phase: A density functional theory study

“…Chai et al have utilised CID to generate haliranium ions (Cl, Br or I) from N , N ‐substituted 2‐halo‐ethanammonium ions 22 . [ 35 ] However, the mechanism discussed here for the formation of the three‐membered ring via NGP was not considered in their study, and instead a higher energy alternative conformer of the reactant 22′ was proposed that under CID conditions would lead to an ion–molecule complex 23 that could dissociate to give either C 2 H 4 X + 20c or the N ‐haloammonium ion 24 , where the latter was formed from intramolecular X + transfer to the amine (Pathways a and b, respectively, Scheme 2). Despite the overall enthalpy of the reaction favouring the intramolecular transfer over the haliranium ion formation, the energy required to reach the former was much higher meaning that NGP is a competitive pathway as we have proposed in our own study on haliranium ion reactivity (Pathway c, Scheme 2).…”

“…Possible pathways for the major products of fragmentation of 2‐bromoethanammonium ion 22 adapted from Chai et al [ 35 ] : formation of ion–molecule complex 23 leading to (A) N ‐bromoammonium ion 24 or (B) bromiranium ion 20c or alternatively (C) direct formation of the bromiranium ion 20c via neighbouring group participation (our proposal). *Represents an estimate of the transition state energy from a potential energy scan (see Figure 2 in Chai et al [ 35 ] for more detail). Energies are given in kJ mol −1 calculated at B3LYP/6‐311++G(2d,p)…”

Methyl group transfer upon gas phase decomposition of protonated methyl benzoate and similar compounds

“…On the other hand, such intramolecular transfers are very interesting from the gas phase ion chemistry point of view; therefore, there is a number of examples of detailed studies of such processes. The examples involve transfer of such moieties as amino, halogen, methoxy, benzyl, tosyl, trimethylsilyl, oxygen, alkyl, and hydroxyl, and, as expected, most examples involve transfer of methyl group …”