Methoxymethyl group migration in his (methoxymethyl)arene radical cations: a new outlook on the rearrangement of dialkylarene radical cations

Harting N, Thielking G, Grützmacher H-F. Methoxymethyl group migration in his (methoxymethyl)arene radical cations: a new outlook on the rearrangement of dialkylarene radical cations. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY. 1997;167-168:335-352.The molecular ions of certain bis(methoxymethyl)benzenes and bis(methoxymethyl)naphthalenes decompose by the formation of a dimethoxymethyl cation a and a benzyl radical or naphthylmethyl radical by functional group interaction between two remote methoxymethyl side chains. The mechanism of this rearrangement was investigated using MIKE spectrometry and derivatives of bis(methoxymethyl)arenes specifically labelled by deuterium. The results show that formation of ion a occurs from the molecular ions. It is suppressed by elimination of methanol by ortho and peri effects and is rather weak if the side chains are located at different rings of the aromatic system. The labelling studies reveal that the formation of ion a occurs from intact methoxy groups and moreover from one intact [CH-OCH3] moiety of a side chain. This excludes a repeated and reversible rearrangement of the toluene-cycloheptatriene radical cation type. The MIKE spectra of derivatives deuterated display H/D isotope effects due to a 'hidden hydrogen rearrangement' preceding the formation of ion a and the loss of a complete methoxymethyl side chain from the molecular ions. A mechanism is presented for the formation of the dimethoxymethyl cation a. The crucial steps of this mechanism are multiple 1,2-shifts of a [CH-OCH3] group around the aromatic ring which an initiated by a 1,2-H shift from a [CHS-OCH3] side chain to the aromatic ring. This initial 1,2-H shift is identical to that of the rearrangement of toluene and cycloheptatriene radical cations but obviously requires much less activation energy. (C) 1997 Elsevier Science B.V