The collision induced loss of carbon monoxide from deprotonated benzyl benzoate in the gas phase. An anionic 1,2-Wittig type rearrangement

Abstract

The ion PhCO2––CHPh, upon collision activation, undergoes competitive losses of CO and CO2 of which the former process produces the base peak of the spectrum. Product ion and substituent effect (Hammett) studies indicate that PhCO2––CHPh cyclises to a deprotonated hydroxydiphenyloxirane which ring opens to PhCOCH(O–)Ph. This anion then undergoes an anionic 1,2-Wittig type rearrangement {through [PhCO– (PhCHO)]} to form Ph2CHO– and CO. The mechanism of the 1,2-rearrangement has been probed by an ab initio study [at MP4(SDTQ)/6-31++G(d,p) level] of the model system HCOCH2O– → MeO– + CO. The analogous system RCO2––CHPh (R = alkyl) similarly loses CO, and the migratory aptitudes of the alkyl R groups in this reaction are But > Me > Et ≈Pri). This trend correlates with the order of anion basicities (i.e. the order of ΔGoacid values of RH), supporting the operation of an anion migration process. The loss of CO2 from PhCO2––CHPh yields Ph2CH– as the anionic product: several mechanistic scenarios are possible, one of which involves an initial ipso nucleophilic substitution.