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

Abstract

Observation of the [M + H] + ion in electrospray ionisation mass spectrometry (ESI-MS) for either product identification or further unimolecular reactivity studies is not always guaranteed. When a suitable donor is β to a good leaving group such as protonated alcohols, neighbouring group participation (NGP) is one process that can lead to facile in-source decomposition leading to threemembered ring product ions. In this study, lone pair rich heteroatoms are explored computationally as donors for the intramolecular displacement of either water or ammonia in protonated 2-heteroethanols and ethanamines, respectively. Density functional theory (DFT) calculations showed that for the halogens (Cl, Br and I) both the loss of water and ammonia are overall endothermic processes with modest barriers for the 2-haloethanol fragmentation by NGP. The chalcogens (S, Se and Te) displayed a significant shift with the loss of water becoming exothermic and the enthalpic barrier approximately zero in most cases leading to little [M + H] + ion observation. The loss of ammonia remained endothermic for both the chalcogens (S, Se and Te) and pnictogens (P, As and Sb), but the 2-pnictoethanol fragmentation showed mixed results as two pathways were accessible due to the increased s character of the pnictogen lone pair. Examination of the donor-acceptor interactions by natural bond orbital (NBO) theory reflected an increase in the heteroatom lone pair participation at the transition state, but that stabilisation due to the polarisable C X bond dominated and should not be discounted in processes believed to proceed by nonvertical neighbouring group participation.

Electronic and Steric Effects on the Reactivity of Seleniranium Ions with Alkenes in the Gas Phase

“…However, not only are the barriers very high to isomerization (Δ H ‡ > 200 kJ mol – 1 relative to the iranium ion), which is unsurprising given that a substantial amount of positive charge builds up on a primary carbon prior to hydride transfer, but there is no thermodynamic driving force toward selenonium ion formation for any of these species (Δ H > 25 kJ mol – 1 ). Potential isomerization was also considered during CID as around 70–80 kJ mol – 1 is required to displace ammonia, and the overall formation of the seleniranium ion is endothermic (Scheme A) . However, the transition state for the 1,2-hydride shift to the selenonium ion with the ammonia present is still more than double that of the formation of the seleniranium ion at 177–184 kJ mol – 1 (Scheme B), in agreement with previous DFT calculations for a similar seleniranium ion system .…”

“… a (A) Displacement of ammonia via q to give seleniranium ion c , (B) displacement of ammonia via r to give selenonium ion s (a syn elimination transition state 14r′ was also found, but was higher in relative enthalpy at +211 kJ mol –1 ), and (C) isomerization of seleniranium ion c to selenonium ion s via a 1,2-hydride shift in t . Enthalpies (Δ H ) are relative to the corresponding ammonium ion b precursor for each substituent (R = OMe, H, and CF 3 ) and are in kJ mol –1 at 298 K calculated at M06-2X-D3/def2-TZVP. …”

“…Potential isomerization was also considered during CID as around 70−80 kJ mol −1 is required to displace ammonia, and the overall formation of the seleniranium ion is endothermic (Scheme 6A). 45 However, the transition state for the 1,2-hydride shift to the selenonium ion with the ammonia present is still more than double that of the formation of the seleniranium ion at 177−184 kJ mol −1 (Scheme 6B), in agreement with previous DFT calculations for a similar seleniranium ion system. 25 The barriers and endothermicities of the unimolecular reactions all increase as the electron demand on selenium increases as the 1,2-hydride shift is assisted by the selenium atom lone pair, which becomes less nucleophilic as the substituent on the phenyl ring becomes more electron-withdrawing.…”