Consecutive losses of two benzyl radicals from the [M + Na]+ adduct ions of di- and tri(benzyloxy)benzenes under ESI/CID conditions

Abstract

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, MSn 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 (O-2) and only then a lateral C7H7 radical (from O-1 or O-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 O-1, in very minor amounts from O-2 but not at all from O-4. The secondary loss of C7H7 subsequent to the major primary loss occurs from both O-2 and O-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 O-1 or O-3.