Mass spectrometer pericyclic reactions

A major problem in interpreting pericyclic reactions of radical ions is the difficulty of distinguishing between concerted and nonconcerted paths. The same difficulty occurs in the case of thermal reactions, but the problem here is more acute since there is less scope for the application of subtle experimental techniques. Thus cyclohexene (6) undergoes a retro-Diels-Alder reaction to (7) in the mass spectrometer but the products formed do not enable us to distinguish between a two-step process via the intermediate radical ion (8) and a pericyclic process involving the cyclic transition state (9). 

The parallel between retro-Diels-Alder reactions in the mass spectrometer and the corresponding thermal reactions of neutral cyclohexenes does, however, suggest that both proceed by a common mechanism, i.e., a one-step pericyclic process. This conclusion has been strongly supported by Loudon et al in a very detailed comparison of the thermal and mass spec-... 

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