Unusual Collision-Induced Fragmentations of Deprotonated Methoxybenzyl Formates in the Gas Phase. 1,2-Wittig Rearrangements,Cross-Ring Nucleophilic Substitution and Ortho Effects

Abstract

Deprotonated benzyl formate undergoes loss of carbon monoxide via the rearrangement sequence Ph-⁻CHOCHO →PhCH(O⁻)CHO → [(PhCHO) HCO⁻]→PhCH₂O⁻ + CO. The last step involves a 1,2-Wittig anionic rearrangement involving hydride transfer from a formyl anion. When a methoxyl group is placed on the benzene ring, the Wittig rearrangement competes with a cross-ring S_N2 process, i.e. [(o-, m- or p-MeOC₆H₄CHO) HCO⁻]→o-, m- or p-OHCC₆H₄O– + MeCHO and/or (CH₄ + CO). The ortho-isomer also undergoes several ortho-effects including loss of MeOH to produce [PhCOCHO – H]⁻.