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.