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

Kuck D, Heitkamp S, Sproß J, Letzel MC, Ahmed I, Krohn K, Parker RG, Wang Y, Robbins VJ, Ames WM, Schettler PD, et al. (2015)
International Journal of Mass Spectrometry 377: 23-38.

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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, 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.
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Kuck D, Heitkamp S, Sproß J, et al. Consecutive losses of two benzyl radicals from the [M + Na](+) adduct ions of di- and tri(benzyloxy)benzenes under ESI/CID conditions. International Journal of Mass Spectrometry. 2015;377:23-38.
Kuck, D., Heitkamp, S., Sproß, J., Letzel, M. C., Ahmed, I., Krohn, K., Parker, R. G., et al. (2015). Consecutive losses of two benzyl radicals from the [M + Na](+) adduct ions of di- and tri(benzyloxy)benzenes under ESI/CID conditions. International Journal of Mass Spectrometry, 377, 23-38.
Kuck, D., Heitkamp, S., Sproß, J., Letzel, M. C., Ahmed, I., Krohn, K., Parker, R. G., Wang, Y., Robbins, V. J., Ames, W. M., et al. (2015). Consecutive losses of two benzyl radicals from the [M + Na](+) adduct ions of di- and tri(benzyloxy)benzenes under ESI/CID conditions. International Journal of Mass Spectrometry 377, 23-38.
Kuck, D., et al., 2015. Consecutive losses of two benzyl radicals from the [M + Na](+) adduct ions of di- and tri(benzyloxy)benzenes under ESI/CID conditions. International Journal of Mass Spectrometry, 377, p 23-38.
D. Kuck, et al., “Consecutive losses of two benzyl radicals from the [M + Na](+) adduct ions of di- and tri(benzyloxy)benzenes under ESI/CID conditions”, International Journal of Mass Spectrometry, vol. 377, 2015, pp. 23-38.
Kuck, D., Heitkamp, S., Sproß, J., Letzel, M.C., Ahmed, I., Krohn, K., Parker, R.G., Wang, Y., Robbins, V.J., Ames, W.M., Schettler, P.D., Hark, R.R.: Consecutive losses of two benzyl radicals from the [M + Na](+) adduct ions of di- and tri(benzyloxy)benzenes under ESI/CID conditions. International Journal of Mass Spectrometry. 377, 23-38 (2015).
Kuck, Dietmar, Heitkamp, Sandra, Sproß, Jens, Letzel, Matthias C., Ahmed, Ishtiaq, Krohn, Karsten, Parker, Robert G., Wang, Yulun, Robbins, Virginia J., Ames, William M., Schettler, Paul D., and Hark, Richard R. “Consecutive losses of two benzyl radicals from the [M + Na](+) adduct ions of di- and tri(benzyloxy)benzenes under ESI/CID conditions”. International Journal of Mass Spectrometry 377 (2015): 23-38.
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