Benzene organic radical ions

New synthetic transformations are highly dependent on the dynamics of the contact ion pair, as well as reactivity of the individual radical ions. For example, the electron-transfer paradigm is most efficient with those organic donors yielding highly unstable cation radicals that undergo rapid unimolecular reactions. Thus, the hexamethyl(Dewar)benzene cation radical that is generated either via CT activation of the [D, A] complex with tropylium cation,74... 

The reaction of benzene with Cu(II) and Fe(III)-exchanged hectorites at elevated temperatures produced a variety of organic radical products, depending on the concentration of water in the reaction medium and the reaction time (90). The formation of free radicals was accompanied by a reduction in oxidation state of the metals, a process that had a zero-order dependence on the metal ion concentration. Under anhydrous conditions the free radicals appeared to populate sites in the interlayer region, the activation energies under these conditions being lower than in the hydrated samples. 

Electron spin resonance (ESR) spectroscopy is of application to organic species containing unpaired electrons radicals, radical ions and triplet states, and is much more sensitive than NMR it is an extremely powerful tool in the field of radical chemistry (see Chapter 10). Highly unstable radicals can be generated in situ or, if necessary, trapped into solid matrices at very low temperatures. Examples of the application of this techniques include study of the formation of radical cations of methoxylated benzenes by reaction with different strong oxidants in aqueous solution [45], and the study of the photodissociation of N-trityl-anilines [46],... 

EPR experiments have shown that the redox ability of WZ catalysts is sufficient to initiate a homolytic cleavage of C-H bonds in alkanes. Exposure of a WZ catalyst to n-pentane at 523 K led to the formation of W5+ species and organic radicals on the surface.27 The formation of organic radicals also occurred when WZ catalysts interacted with other hydrocarbons, including benzene.31 We therefore infer that one-electron transfer, although it is not regarded as a step in the catalytic cycle, can initiate catalysis by a process that leads to the formation of the carbenium ion chain carriers,27 as also occurs in acidic solutions.32 We emphasize that a strong redox reactivity is necessary but not sufficient for the catalytic activity of WZ. 

As a 3-step mechanism, the electron-transfer paradigm provides a pair of discrete intermediates [D, A] and D+, A for the prior organization and the activation, respectively, of the donor and the acceptor. The quantitative evaluation of these intermediates would allow the overall second-order reaction (k2) to be determined. Although the presence of [D, A] does not necessarily imply its transformation to D+, A-, a large number and variety of donor/ acceptor couples showing transient charge-transfer absorptions associated with [D, A] have now been identified. In each case, the product can be predicted from the expected behavior of the individual ion radicals D+ and A-. Consider for example, the labile 1 1 benzene complex with bromine that has been isolated at low temperatures and characterized crystallographically (Chart 9).256... 

As for solvents, liquid ammonia or dimethylsulfoxide are most often used. There are some cases when tert-butanol is used as a solvent. In principle, ion-radical reactions need aprotic solvents of expressed polarity. This facilitates the formation of such polar forms as ion-radicals are. Meanwhile, the polarity of the solvent assists ion-pair dissociation. This enhances reactivity of organic ions and sometimes enhances it to an unnecessary degree. Certainly, a decrease in the permissible limit of the solvent s polarity widens the possibilities for ion-radical synthesis. Interphase catalysis is a useful method to circumvent the solvent restriction. Thus, 18-crown-6-ether assists anion-radical formation in the reaction between benzoquinone and potassium triethylgermyl in benzene (Bravo-Zhivotovskii et al. 1980). In the presence of tri(dodecyl)methylammonium chloride, fluorenylpi-nacoline forms the anion-radical on the action of calcium hydroxide octahydrate in benzene. The cation of the onium salts stabilizes the anion-radical (Cazianis and Screttas 1983). Surprisingly, the fluorenylpinacoline anion-radicals are stable even in the presence of water. 

The fate cS the contact ion pair [RH A 1 is critical to electrm-transfer oxidation. Oxidative efficimey is the highest with those organic donors diat yield unstable radical cations, such as hexamethyl(Dewar benzene), which und goes spontaneous rearrangement (equation 7). > ... 

Organic fluorine compounds and methods for their preparation are the central topic of the next four procedures. Much of the synthetic versatility of methyl phenyl sulfone is embodied in FLUOROMETHYL PHENYL SULFONE and the fluoro Pummerer reaction of methyl phenyl sulfoxide with DAST is a key step in its preparation. The utility of this fluoromethyl sulfone in the preparation of fluoroalkenes Is demonstrated in a companion procedure for Z-[2-(FLUOROMETHYLENE) CYCLOHEXYL]BENZENE, a procedure with several prominent stereoselective features. Geminal difluoroalkenes are featured in the following procedure. (3,3 DIFLUOROALLYL)TRIMETHYLSILANE is prepared by a method in which the radical addition of dibromodifluoromethane to alkenes and the selective reduction of a-bromoalkylsilanes are key steps. A procedure for nucleophilic introduction of the trifluoromethyl group completes this set. The key reagent, (TRIFLUOROMETHYL)-TRIMETHYLSILANE is obtained by reductive coupling of TMS chloride and bromotrifluoromethane. Liberation of a CF3- equivalent with fluoride ion in the presence of cyclohexanone affords 1-TRIFLUOROMETHYL-1-CYCLOHEXANOL. 

It has been widely reported that organic molecules, and especially methyl-substituted benzenes coul easily reduce ions incorporated in calcined VAPO-5 at temperatures below 100 C [10]. Therefore, it is conceivable that the mechanism proceeds via the reduction of ions by toluene molecules followed by the formation of tBuO radicals on the V formed. Indeed, the chain complex B leads to the formation of radicals, with the following scheme ... 

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