Single electron transfer (SET) oxidation of [ 2.2lparacyclo-phane (1) with cerium ammonium nitrate (CAN) produces the corresponding radical cation (l"), which undergoes cleavage of the carbon-carbon bond of the ethano bridge to generate the double-benzylic radical cation 2+'. Trapping of this intermediate by oxygen and nucleophiles gives rise to aldehydes 3, while a second oxidation and subsequent nucleophilic trapping affords nitrates 4. This facile cleavage of the carbon-carbon bond is explained by alignment of the latter with the 7c systems in the rigid structure of [2.2]paracyclophane, which enables charge delocalization across both benzene rings. When the reaction is carried out in methanol, aromatic substitution takes also place as a minor reaction pathway, which is responsible for the formation of the polyfunctionalized cyclophane 5d.The mode of cleavage of radical cations of 1,2-diarylethanes has been a matter of some controversy [']. Early results obtained with cerium ammonium nitrate (CAN) oxidation of 1,2-diphenyIethane pointed to the favored scission of the central carbon-carbon bond to give a benzyl radical and a benzyl cation and the products derived therefromL21. Photosensitized electron transfer oxidation of related systems appears to be also in agreement with an initial carboncarbon bond However, more recent studies have provided support to the fact that the radical cations of 1 ,Zdiarylethanes undergo preferential loss of a benzylic proton to give side chain-functionalized primary product~ [~-"]. Further oxidation of the latter in a secondary process would justify the formation of fragmentation products.Actually, carbon-carbon bond breaking of the radical cations of 1,Zdiarylethanes is enhanced in the gas phase (EI mass spectrometry), while in solution the large negative heat of solvation for the proton favors deprotonation['l. Simple thermochemical calculations are in agreement with this rationalization [']. Nevertheless, stereoelectronic effects must be also taken into account to explain the fragmentation mode of this type of radical cations. Thus, if for geometrical constraints the carbon-carbon instead of the carbon-hydrogen bond is aligned with the 7c system, fragmentation of this bond occurs despite the unfavourable thermochemistry"].In this context, it has been hypothesized that the radical cations of 1 ,Zdiarylethanes have a sandwich structure, which enables charge delocalization between both benzene rings['0]. A rigid model for such sandwiched species would be the radical cation of [2.2]paracyclophane, in which alignment of the central bond of the ethano bridges with the .n systems should be expected to promote carbon-carbon bond cleavage. Strain relea~e['~,'~] would substantially contribute to this process from the thermochemical point of view.Such considerations have prompted us to investigate the Ce(IV)-induced SET oxidation of [2.2]paracyclophane in an effort to generate the double-benzylic radical cation 2+' under conditions which would facilitate its trapping by oxygen and a varie...
