Theoretical study of protonated xylenes: ethene elimination and H,C-scramling reactions

Quantum chemical calculations have been carried out to investigate various unimolecular rearrangements that can take place in protonated gas-phase xylenes. Hydrogen and methyl group ring migrations were investigated. The barriers for hydrogen migrations are lower than the barriers for methyl group migrations. Mechanisms for ring expansion to seven-membered rings, and for contraction to five-membered rings were studied. Both of these mechanisms can eventually lead to ethene elimination. The most favourable ring expansion step goes through a 1,3-hydrogen shift from a methyl group onto the arenium ring, forming a protonated methylcycloheptatriene. Interconversions between various ring-expanded forms have been investigated. Re-contraction can lead to an ethylbenzenium ion that could subsequently split off ethene. Alternatively, the xylenium ion can contract to a five-membered ring. The immediate product is a bicyclic ion (bicyclo[3.1.0]hexane skeleton) that can rearrange further to give an ethylbenzenium ion, or the five-ring system can split off ethene, and be converted into a cyclopentadienyl ion that can isomerize into a benzenium ion. Stable structures and transition states are calculated both at the B3LYP/cc-pVTZ//B3LYP/6-311 G(d,p) and at the MP2/cc-pVTZ//MP2/6-31 G(d) levels. The energies needed for ring expansion or ring contraction are not very different, and the calculations suggest that both reaction paths are possible, but the energy needed for actually splitting off an ethene molecule is lower along the expansion path. Copyright (C) 2004 John Wiley Sons, Ltd.