Cation radicals, organic, in solution, and

Cation radicals, in solution, formation, properties and reactions of, 13,155 Cation radicals, organic, in solution, and mechanisms of reactions of, 20, 55 Cations, vinyl, 9,135... 

Importantly, the purple color is completely restored upon recooling the solution. Thus, the thermal electron-transfer equilibrium depicted in equation (35) is completely reversible over multiple cooling/warming cycles. On the other hand, the isolation of the pure cation-radical salt in quantitative yield is readily achieved by in vacuo removal of the gaseous nitric oxide and precipitation of the MA+ BF4 salt with diethyl ether. This methodology has been employed for the isolation of a variety of organic cation radicals from aromatic, olefinic and heteroatom-centered donors.174 However, competitive donor/acceptor complexation complicates the isolation process in some cases.175... 

Scheme 3.21 (Zwier et al. 2001). The initial neutral compound shown in Scheme 3.21 is very sensitive to air as a solid and is stable only for a few hours. The fluoroborate salt of the l,6-diazabicyclo[4.4.4] tetradecane cation-radical can be isolated as a dark red solid. The solid is indefinitely stable and in the absence of a base is stable for months in organic or even aqueous solutions(Alder and Sessions 1979). Three-electron N—N bound cation-radicals are known in a wide range of structures. Most examples are, however, unstable outside the glassy or solid state (for a review, see Alder 1983). The tricyclic cation-radical portrayed in Scheme 3.21 is structurally protected from the (N.. N)+ bond being cleaved. This protection provides it a long solution lifetime (Nelsen et al. 1980).

Electrons are transferred singly to any species in solution and not in pairs. Organic electrochemical reactions therefore involve radical intermediates. Electron transfer between the electrode and a n-system, leads to the formation of a radical-ion. Arenes, for example are oxidised to a radical-cation and reduced to a radical-anion and in both of these intermediates the free electron is delocalised along the 7t system. Under some conditions, where the intermediate has sufficient lifetime, these electron transfer steps are reversible and a standard electrode potential for the process can be measured. The final products from an electrochemical reaction result from a cascade of chemical and electron transfer steps. 

C-H transformation of alkanes by SET is still a developing area of preparative organic chemistry. Generation of cr-radical cations from alkanes in solution requires strong oxidants, and is achieved by photochemical and electrochemical oxidation. Under these conditions even unstrained strained alkanes may be functionalized readily. The C-H substitution is selective if the hydrocarbon forms a radical cation with a definite structure and/or deprotonation from a certain C-H position of the radical cation dominates. Overoxidations are the most typical side reactions that lead to disubstituted alkanes. This can usually be avoided by running the reactions at low alkane conversions. 

To start with gas-phase data, ionization potentials (IP) and the derived heats of formation of radical cations are available for a large number of organic species (Franklin et al., 1969 Gutmann and Lyons, 1967 Turner, 1966), whereas electron affinities (EA) are far more scarce (for a recent review, see Janousek and Brauman, 1979). For both types of data one has to estimate heats of solvation for participating species in order to obtain E° in solution, and this is known to be an uncertain procedure (Mortimer, 1962). An alternative is to use the rather good correlations that are available between gas phase and solution data for estimating unknown solution values (see below). 

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