PUB - Publikationen an der Universität Bielefeld

1,3,5-Cycloheptatriene (1) and various 7-alkyl-1,3,5-cycloheptatrienes (3, 6, 9, 13, and 16-19) were subjected to gas-phase protonation under Cl(CH4) and Cl(iC(4)H(10)) conditions and the MIKE spectra of their [M + H](+) ions were measured Loss of CH, from the parent ion [1 + H](+) and almost exclusive loss of C2H4 from the methyl derivative [3 + H](+) indicate ring contraction of the dihydrotopylium ions to protonated toluene (toluenium ions) and protonated ethylbenzene (ethylbenzenium ions), respectively, prior to fragmentation. With increased exothermicity of protonation, ions [3 + H](+) also isomerize to xylenium ions. Similarly, higher protonated n-alkylcycloheptatrienes undergo skeletal isomerization to the corresponding "chain-elongated" (n + 1)-alkylbenzenium and to the corresponding n-alkyltoluennium ions. Starting with ethyldihydrotropylium ions, a competing isomerization channel is opened giving rise to expulsion of C2H4 from the constituents of the seven-membered ring, as evidenced bg deuterium labelling and an unusually high kinetic energy release, Isoalkyl analogues behave in a similar manner with increased hydrogen exchange between the alpha position of the side chain and the ring. Copyright (C) 1999 John Wiley & Sons, Ltd.