Gas-phase intramolecular benzyl–benzyl interactions in protonated dibenzyl derivatives containing benzyl–oxygen, –sulfur and –nitrogen bonds

Abstract

Protonated molecules of a variety of benzyl diethers, diesters and ether-esters, produced by chemical ionization (CI), undergo a unique rearrangement yielding relatively abundant m/z 181 C₁₄H₁₃⁺ ions, both in the ion source and under collision-induced dissociation (CID) conditions. This highly general rearrangement involves an intramolecular C–C bond formation between the two benzyl groups, and the resulting C₁₄H₁₃⁺ ions have been shown by the analysis of their CID spectra to be an almost equimolar mixture of isomeric α-o-tolylbenzyl, α-p-tolylbenzyl and p-benzylbenzyl cation structures in all cases. This structural information suggests that this process may be viewed as gas-phase aromatic substitution of the non-charged benzoxy group by the benzyl cation originating from the protonated ether function involving a series of π- (and/or ion–neutral) and σ-complexes. The extent of this rearrangement process strongly depends on the nature of the benzyl bond heteroatoms. It is dramatically suppressed in the MH⁺ ions of benzyl disulfides and absent in diamines, diamides and amino-amides. The different behaviour of the O-derivatives vs. S- and N-analogues is explained in terms of the energies of the benzyl–XH⁺ bond heterolytic cleavages, which have been shown to increase in the order: O < S < N.