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 Bu^t > Me > Et ≈Prⁱ). This trend correlates with the order of anion basicities (i.e. the order of ΔG^o_acid 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.