Intramolecular benzyl–benzyl interactions in protonated benzyl diethers in the gas phase. Effects of internal hydrogen bonding

Abstract

Protonated molecules of a variety of benzyl diethers, 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 benzyloxy 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 fragmentation in alkane benzyl diethers PhCH₂O(CH₂)_nOCH₂Ph (n = 2–10,12) is strongly affected by the alkane chain length. Stereoisomeric benzyl diethers display an unusual steric effect: the trans-isomers give rise to more abundant C₁₄H₁₃⁺ ions than their cis-counterparts. The latter two effects are explained in terms of intramolecular hydrogen bonding between the two alkoxy groups. Bis(benzyloxy)benzenes and -naphthalenes exhibit very low abundance C₁₄H₁₃⁺ ions in contrast to the aliphatic analogues. This behavior is attributed to competing intramolecular benzylation involving the aromatic skeletons of these compounds.