Anionic Migration in Aromatic Systems Effected by Collisional Activation. Unusual Fragmentations of Deprotonated Anilides Containing Methoxyl and Ethoxyl Substituents

Abstract

Long-range cross-ring reactions occur when (M – H)⁻ ions of methoxy- and ethoxy-C₆H₄–⁻NCOR (R = H, CH₃, C₆H₅ and CH₃O) are subjected to collisional activation. These reactions are generally minor processes: a particular example is the cross-ring elimination p-C₂H₅O–C₆H₄–⁻NCOCH₃ → [CH₃⁻ (p-C₂H₅O–C₆H₄–NCO)] → p-(⁻O)–C₆H₄–NCO + C₂H₄ + CH₄. Major processes of these (M – H)⁻ ions involve (i) losses of radicals to form stabilised radical anions, e.g. (a) loss of a ring H^• or (b) CH₃^• (or C₂H₅^•) from the alkoxy group, and (ii) proximity effects when the two substituents are ortho, e.g. loss of CH₃OH from o-CH₃O–C₆H₄–⁻NCHO yields deprotonated benzoxazole. Another fragmentation of an arylmethoxyl anion involves loss of CH₂O. It is proposed that losses of CH₂O are initiated by anionic centres but the actual mechanisms in the cases studied depend upon the substitution pattern of the methoxyanilide: o⁻ and p-methoxyanilides may undergo ipso proton transfer/elimination reactions, whereas the m-analogues undergo proton transfer reactions to yield an o-CH₃O substituted aryl carbanion followed by proton transfer from CH₃O to the carbanion site with concomitant loss of CH₂O.