Interconversion of gaseous bicyclo[3.2.1]oct-2-en-4-yl cations and protonated 7-alkylcycloheptatrienes: [5+2] cycloreversion in competition with fragmentation by way of alkylbenzenium ions

Several bicyclo[3.2.1]oct-2-en-3-yl cations, as isomers of protonated 7-methyl- and 7-ethylcycloheptatriene and of the protonated C8H10 and C9H12 alkylbenzenes, respectively, have been studied by deuterium labeling and mass-analyzed ion kinetic energy (MIKE) and collision-induced dissociation/MIKE spectrometry. Labeling reveals that the bicyclic framework undergoes fast and apparently complete hydrogen equilibration prior to fragmentation, involving a series of skeletal and hydrogen rearrangements (1,2-C and 1,2-H shifts). Fragmentation of the bicyclic ions C8H11+ and C9H13+ is manyfold: It occurs in part by way of the isomeric alkylbenzenium ions, e.g. CH3CH2C6H6+ and CH3C6H5CH3+, and C2H5C6H5CH3+ and CH3CH2CH2C6H6+, respectively, with the corresponding 7-alkyldihydrotropylium ions as intermediates. Another fraction of the bicyclic ions does not fragment by way of alkylbenzenium ions but apparently by [5 + 2] cycloreversion of bicyclo[3.2.1]octenyl framework itself. This process is indicated by ethene expulsion associated with an unusually large kinetic energy release (T* approximate to 300 meV). The characteristic high-KER ethene loss was also found for protonated 7-ethylcycloheptatriene but not for protonated 7-methylcycloheptatriene, suggesting a delicate balance of the activation energies and confirming, in turn, that bicyclo[3.2.1]oct-2-en-yl cations are intermediates during the fragmentation of higher alkyldihydrotropylium ions. (C) 2001 Elsevier Science B.V.