PUB - Publikationen an der Universität Bielefeld

The successive loss of two benzyl radicals from the [M +Na](+) ions of the isomeric dihydroxybenzene dibenzyl ethers (2-4) and of the isomeric trihydroxybenzene tribenzyl ethers (5-7) under ESI/CID conditions has been studied by deuterium labelling, MS experiments and DFT calculations. The fragmentation of the [M + Na](+) ions of 2 and 4 consists exclusively of the consecutive losses of two C7H7* (benzyl) radicals. This process is largely suppressed in the corresponding [M + Na](+) ions of the meta isomer 3 and also in those of the 1,3,5-analog, phloroglucinol tribenzyl ether (6), suggesting the facile formation of sodiated ortho- and para-quinone ions, [C6H4O2 + Na](+), in the cases of 2 and 4, respectively. This finding is corroborated by a detailed investigation of the sodiated tribenzyl ethers of pyrogallol, [5 + Na](+), and 1,2,4-trihydroxybenzene, [7 + Na](+), and their isotopologs bearing differently labelled benzyl residues. Again, the successive loss of two C7H7* radicals is the only fragmentation channel for ions [7 + Na](+) and strongly predominates for ions [5 + Na](+), with the primary and secondary losses being highly regiospecific: ions [5 + Na](+) lose the first benzyl residue almost exclusively from the central position (0-2) and only then a lateral C7H7* radical (from 0-1 or 0-3). Surprisingly, the very minor primary loss of a lateral benzyl group is followed by that of the other lateral one, suggesting a two-step isomerization process initiated by a 1,4-H* shift. Ions [7 + Na](+) lose the first benzyl radical almost exclusively from 0-1, in very minor amounts from 0-2 but not at all from 0-4. The secondary loss of C7H7* subsequent to the major primary loss occurs from both 0-2 and 0-4 in similar amounts, reflecting the relative stabilities of the sodiated benzyloxy-substituted ortho- and para-benzoquinones. This and several other details of the energy profiles associated with the twofold benzyl loss from ions [5 + Na](+) and [7 + Na](+) were calculated by use of the DFT methodology and were found to agree very well with the regioselectivities observed. Finally, the pyrogallol-based ions [5 + Na](+) were found to eliminate benzaldehyde in minor but significant relative amounts, which takes place with high regioselectivity from 0-1 or 0-3. (C) 2014 Elsevier B.V. All rights reserved.