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 ¹⁸O 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(CH₂)_nO⁻ (n = 2–4). All ΔG values were calculated at 298 K. (1) PhO(CH₂)₂O⁻ effects an ipso Smiles rearrangement (ΔG_r = +35 kJ mol⁻¹; barrier to transition state ΔG_# = +40 kJ mol⁻¹) equilibrating the two oxygen atoms. The Smiles intermediate reverts to PhO(CH₂)₂O⁻ which then undergoes an S_Ni reaction to form PhO⁻ and ethylene oxide (ΔG_r = −24 kJ mol⁻¹; ΔG_# = +54 kJ mol⁻¹). (2) The formation of PhO⁻ from energized PhO(CH₂)₃O⁻ is more complex. Some 85% of the PhO⁻ formed originates via a Smiles intermediate (ΔG_r = +52 kJ mol⁻¹; ΔG_# = +61 kJ mol⁻¹). This species reconverts to PhO(CH₂)₃O⁻ which then fragments to PhO⁻ by two competing processes, namely, (a) an S_Ni process yielding PhO⁻ and trimethylene oxide (ΔG_r = −27 kJ mol⁻¹; ΔG_# = +69 kJ mol⁻¹), and (b) a dissociation process giving PhO⁻, ethylene and formaldehyde (ΔG_r = −65 kJ mol⁻¹; ΔG_# = +69 kJ mol⁻¹). The other fifteen percent of PhO⁻ is formed prior to formation of the Smiles intermediate, occurring directly by the S_Ni 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(CH₂)₄O⁻ fragments exclusively by an S_Ni process to form PhO⁻ and tetrahydrofuran (ΔG_r = −101 kJ mol⁻¹; ΔG_# = +53 kJ mol⁻¹). The Smiles ipso cyclization (ΔG_r = +64 kJ mol⁻¹; ΔG_# = +74 kJ mol⁻¹) is not detected in this system.