The remarkable electron impact mass spectrum of (2-benzyl-1,3-xylylene)-15-crown-4: expulsion of triethylene glycol by double hydrogen transfer

Abstract

During our investigations of the synthesis of magnesium-containing crown ethers, the mass spectral characterisation of a precursor, (2-benzyl-1,3-xylylene)-15-crown-4 (C₂₁H₂₆O₄), leads to a surprising result: its electron-impact mass spectrum was nearly identical with that of 1-methylanthracene, C₁₅H₁₂. Several deuteriated analogues and other model compounds were synthesised and investigated with respect to their ionisation-induced and collision-induced mass spectroscopic behaviour. A mechanism for the fragmentation of the ions of [2-benzyl-1,3-xylylene]-15-crown-4 to a [C₁₅H₁₂]˙⁺ species is proposed. It involves consecutive 1,5-H transfer, to generate an open-chain polyether group, cyclisation of the two aromatic moieties, to give an anthracene-type intermediate, and remote hydrogen transfer, to release the triethylene glycol neutral.