The gas phase Smiles rearrangement of anions PhO(CH2)nO− (n = 2–4). A joint theoretical and experimental approach

Abstract

A combination of experimental data [using (18)O labelling fragmentation data together with metastable ion studies in a reverse sector mass spectrometer (from a previous study)] and ab initio reaction coordinate studies at the CCSD(T)/6-31++G(d,p)//B3LYP/6-31++G(d,p) level of theory, have provided the following data concerning the formation of PhO(-) in the gas-phase from energized systems PhO(CH(2))(n)O(-) (n = 2-4). All DeltaG values were calculated at 298 K. (1) PhO(CH(2))(2)O(-) effects an ipso Smiles rearrangement (DeltaG(r) = +35 kJ mol(-1); barrier to transition state DeltaG(#) = +40 kJ mol(-1)) equilibrating the two oxygen atoms. The Smiles intermediate reverts to PhO(CH(2))(2)O(-) which then undergoes an S(N)i reaction to form PhO(-) and ethylene oxide (DeltaG(r) = -24 kJ mol(-1); DeltaG(#) = +54 kJ mol(-1)). (2) The formation of PhO(-) from energized PhO(CH(2))(3)O(-) is more complex. Some 85% of the PhO(-) formed originates via a Smiles intermediate (DeltaG(r) = +52 kJ mol(-1); DeltaG(#) = +61 kJ mol(-1)). This species reconverts to PhO(CH(2))(3)O(-) which then fragments to PhO(-) by two competing processes, namely, (a) an S(N)i process yielding PhO(-) and trimethylene oxide (DeltaG(r) = -27 kJ mol(-1); DeltaG(#) = +69 kJ mol(-1)), and (b) a dissociation process giving PhO(-), ethylene and formaldehyde (DeltaG(r) = -65 kJ mol(-1); DeltaG(#) = +69 kJ mol(-1)). The other fifteen percent of PhO(-) is formed prior to formation of the Smiles intermediate, occurring directly by the S(N)i and dissociation processes outlined above. The operation of two fragmentation pathways is supported by the presence of a composite metastable ion peak. (3) Energized PhO(CH(2))(4)O(-) fragments exclusively by an S(N)i process to form PhO(-) and tetrahydrofuran (DeltaG(r) = -101 kJ mol(-1); DeltaG(#) = +53 kJ mol(-1)). The Smiles ipso cyclization (DeltaG(r) = +64 kJ mol(-1); DeltaG(#) = +74 kJ mol(-1)) is not detected in this system.

Evidences for the Key Role of Hydrogen Bonds in Nucleophilic Aromatic Substitution Reactions

“…Starting with the plain phenol (11), we found that the spiro was a TS with a relative energy of 28.8 kcal mol À1 , a similar magnitude was reported in the literature. [29] This high energy barrier is consistent with the fact that the reaction experimentally fails. [27] Similar results were obtained when considering the 2-chloro and 2methoxyphenols (12) and (13): the energies of the corresponding spiro transition states were 26.6 and 29.2 kcal mol À1 , respectively ( Table 2), and the reactions fail.…”

Gas‐phase Smiles rearrangement reactions of deprotonated N‐phenylbenzamides studied by electrospray ionization tandem mass spectrometry

“…In contrast to studies on the solution phase, gas‐phase Smiles rearrangement reactions can occur upon collisional activation, even without the activation of the aromatic ring by electron‐withdrawing groups. For example, the product ion PhO – obtained in the fragmentation of PhO(CH 2 ) 2 O − was generated exclusively via the Smiles rearrangement, which was confirmed by heavy atom ( 13 C and 18 O) labeling study and validated by theoretical calculation . In the fragmentations of deprotonated 2‐(4,6‐dimethoxypyrimidin‐2‐ylsulfanyl)‐ N ‐phenylbenzamide and sulfonylurea herbicides, gas‐phase Smiles rearrangements have also been proposed …”

“…For example, the product ion PhOobtained in the fragmentation of PhO(CH 2 ) 2 O À was generated exclusively via the Smiles rearrangement, which was confirmed by heavy atom ( 13 C and 18 O) labeling study and validated by theoretical calculation. [9,10] In the fragmentations of deprotonated 2-(4,6dimethoxypyrimidin-2-ylsulfanyl)-N-phenylbenzamide and sulfonylurea herbicides, gas-phase Smiles rearrangements have also been proposed. [11,12] Amide compounds are pervasive in nature and have been extensively studied by mass spectrometry.…”

Benzyl anion transfer in the fragmentation of N-(phenylsulfonyl)-benzeneacetamides: a gas-phase intramolecular SNAr reaction

“…[38][39][40] Several other common fragmentations were also observed, such as the ion at m/z 256 that results from the loss of H 2 O, and the benzyl anion at m/z 91 that results from the direct decomposition of A-1. Such cases are not unusual in the gas-phase anionic fragmentations.…”