| dc.description | The structure and reactivity of the radical cations of methylenecyclopropane (1), 1,4-bis(methylene)cyclohexane (2), tricyclo (2.2.2.0$\sp{1,4}$) octane (3), dispiro (2.0.2.2) octane (4), and dispiro (2.1.2.1) octane (5) were studied by theoretical and experimental methods. Ab initio molecular orbital calculations show that several interconversions and rearrangements of these radical cations are energetically favourable. Structural limitations, however, will prevent most of these from actually taking place. Experimental studies included photochemical (PET) and electrochemical (EC) techniques under a number of distinctive conditions. Generation of 2$\sp{+\cdot}$ by PET in the presence of a nucleophile leads to nucleophilic addition to the radical cation. In the absence of a nucleophile several other pathways are followed; the mechanisms for these are complicated and involve radical ions, radicals, and ionic species. Electrochemical oxidation of 2 in the presence of a nucleophile leads to nucleophilic addition to the initially formed radical cation, often followed by a second oxidation and nucleophilic addition (ECE). In the absence of a nucleophile, multiple oxidation steps, as well as acid catalyzed reactions, lead to the formation of aromatic species. Deprotonation is commonly observed under all conditions but is favoured when no nucleophile is present. Photochemical reactions of 1 in the presence of an electron-acceptor (1,4-dicyanobenzene, 33) in acetonitrile lead to cycloadditions rather than products from electron transfer (ET). ET was predicted to occur based on the measured (CV) oxidation potential. It was shown that the measured oxidation potential of 1 represents the adiabatic ionization potential. For PET processes the value for the vertical ionization potential must be used. Generation of 1$\sp{+\cdot}$ (EC) without a nucleophile present results in the formation of one product: tert-butyl acetamide (92). A series of rearrangements leading to the tert-butyl cation is proposed. Addition of a nucleophile (methanol) to the mixture leads to the formation of 3-methoxy-2-(methoxymethyl)-1-propene (93). This product most likely arises from trapping of the initially formed ring-opened (trimethylenemethane) radical cation (1a$\sp{+\cdot})$ which undergoes a second oxidation and nucleophilic addition (ECE). Under these conditions, the tert-butyl cation is not expected to be an important intermediate. | en_US |
