Radical cations in solution

The majority of reactions in solution involve neutral molecules, radicals, and even-electron ions. Such processes were discussed in Chapters 4 and 5. There are, however, cases where radical ions play a role in solution chemistry. We have deferred discussing them until now because they can be understood most easily in terms of the gas-phase reactions examined in the earlier parts of this chapter. Here we will consider such reactions involving radical cations. 

In order to prepare a radical cation in solution, an electron must in some way be abstracted from a normal neutral molecule. There are three main ways in which this can be achieved. In the first, the electron is transferred to a powerful electron acceptor such as a metal in a high valence state (e.g., Mn , Co ) or a high-energy quinone (e.g., DDQ, dichlorodicyanoquinone). In the second, the electron is transferred to the positive electrode (anode) in 

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Asmus et al. unambiguously identified a variety of [R2S.. SR2] radical cations in solution and measured their optical absorption spectra using pulse radiolysis techniques [133]. They proposed that the spectrum of [H2S. .SH2] arises from the transition in the three-electron S.. S... 

Radiation techniques, application to the study of organic radicals, 12, 223 Radical addition reactions, gas-phase, directive effects in, 16, 51 Radicals, cation in solution, formation, properties and reactions of, 13, 155 Radicals, organic application of radiation techniques, 12,223 Radicals, organic cation, in solution kinetics and mechanisms of reaction of, 20, 55 Radicals, organic free, identification by electron spin resonance, 1,284 Radicals, short-lived organic, electron spin resonance studies of, 5, 53 Rates and mechanisms of solvolytic reactions, medium effects on, 14, 1 Reaction kinetics, polarography and, 5, 1... 

Radicals, cation in solution, formation, properties and reactions of, 13, 155... 

The N2H4 energy and coupling constant hypersurfaces and their application to approximate the structure of trialkyIsilyl-stibstituted hydrazine radical cations in solution. 

Summarizing, this example provides several take-home lessons complete sets of hypersurface calculations for main-frame models of compounds can be quite helpful in close correlation to experimental data. Obviously, both the radical cation ground state structure and the angular dependence of the coupling constants are correctly predicted. In return, by introducing experimental data into the established correlations, the structure of radical cations in solution may be cautiously approximated. Altogether, this example teaches another lesson on how drastic those structural changes may be, which accompany even one-electron redox reactions. 

Research on carbon-centered radical cations in solution accelerated dramatically with the development of time-resolved optical absorption and emission techniques. The research group of Th. Forster in Germany pioneered photochemical methods of production of radical cations and anions, as well as exciplexes." While the Forster group focused on structure and lifetimes, the later work of D. R. Arnold in Canada, and of H. D. Roth in the United States," reported the reactivity of photochemically generated radical cations from a mechanistic perspective. These studies of radical ion chemistry evolved into the field we now know as electron donor-acceptor interactions, arich area of science in which carbon-centered radical cations are stiU actively smdied. 

For several tautomeric systems ketones/enols, imines/enamin and others) a distinct reversal of the stability order is observed when going from the neutral compounds to the radical cations, the first use of which in a new preparative a-Umpolung reaction has been documented for keto/enol systems. The present review provides a critical evaluation of the chemistry of enol radical cations in solution with a special emphasis on the Umpolung reaction and the intermediates thereof. Other enol type of radical cations are discussed with respect to their potential to provide a-carbonyl radical and a-carbonyl cation intermediates. Hence, this article does not constitute a comprehensive summary on all enol type of radical cation reactions. All potentials in this review are referenced versus SCE, unless noted otherwise. Potentials measured against the ferrocene/ferrocenium couple were converted to SCE by adding 0.334 V. 

Despite the importance of keto/enol tautomers [57] only a small amount of work has been devoted to the study of enol radical cations in condensed phase. This is directly related to the fact that simple enols as the thermodynamically less stable tautomers [58] are usually not isolable, sinre the kinetic barrier for ketonization is rather low [59,60]. Much more is known about the chemistry of enol and keto radical cations in the gas-phase [61]. For details the reader is referred to recent comprehensive reviews [62]. The only available data on the thermodynamics of enol/keto radical cations in solution stem from a recent study [63]. Using stable dimesityl substituted enols the relative stabilities were determined by a thermochemical cycle approach. 

Fig 1. Thermochemical cycle used for the evaluation of the relative stabilities for enol and keto radical cations in solution [63]... 

All together, route 2 (Fig. 2) seems to be much more promising for offering access to enol radical cations in solution. As a consequence of the inversion of the stability order in keto/enol systems upon one-electron oxidation, the... 

Whereas little is known about ketone and enol radical cations in solution, the related one-electron oxidation of phenols has been extensively studied [72]. Nowadays, anodic oxidation of phenols constitutes a valuable synthetic access to phenoxenium ions [73] which are important intermediates for carbon-carbon bond formation processes [74-76] and to various natural products [77]. In light of the biological relevance of phenol oxidation [78,79] redox potentials of phenols [72,80] and phenolates [80-85] as well as pK values of phenol radical cations [80,86,87] are documented in various solvents. Some of the data will be quoted later in comparison with enol systems. 

Fortunately, the low enol content in simple ketone systems does not necessarily impose an obstacle to generating the corresponding enol radical cations in solution. As outlined in Sect. 2 the selective oxidation of the enol tautomer even in the presence of a vast excess of the ketone opens up an indirect, but quantitative access to enol radical cation intermediates for all systems, if an appropriate oxidant has been chosen. The first, albeit indirect evidence for this selective oxidation step stems from kinetic studies by Henry [109] and Littler [110-112] and will be discussed in more detail in Sect. 3.3. Direct evidence for a specific oxidation of enols was provided by Orliac-Le Moing and Simonet [108]. Using voltammetry at a rotating disc electrode they were able to establish a linear correlation between the anodic current and the enol content for various a-cyano ketones 11. In electrolysis experiments the corresponding 1,4-diketones 13 were obtained in high current yield (ca. 90%). 

A novel pathway to enol radical cations in solution through protonation of ) -hydroxy vinyl radicals 39 has recently been postulated by Gilbert [126]. Using ESR techniques it was demonstrated that l -hydroxyl vinyl radicals rearrange to a-carbonyl radicals 41 when the pH was lowered, and enol radical cations were proposed as intermediates. 

In a combined experimental and theoretical study, Borovkov et alP examined the properties of w-alkane radical cations in solution. Here, we shall just focus on one single aspect of their work, i.e., the conformers of one of the alkanes, w-nonane, C9H2o-In its simplest form of the highest symmetry, its structure can be described as a zigzag chain formed by the 9 carbon atoms. To each of the carbon atoms, 2 hydrogen atoms... 

More recently, Dinnocenzo and colleagues showed that a 2,3-dimethyl derivative (30, R = CH3) and several 1-phenyl- and l,l-diphenyl-2-alkyl-substituted cyclopropanes are captured with complete inversion of configuration [154, 155]. The observed stereochemistry requires an intermediate radical cation, 30, with the unperturbed stereochemistry of the parent molecule. This result unambiguously rules out a ring-opened cyclopropane radical cation in solution. 

An interesting example is the anodic dimerization of 2-phenylnorbornene [Eq. (40)] [90]. The experimental results indicate that the dimer radical cation in this case oxidizes the substrate to the corresponding radical cation in solution via Eq. (41), so fast that followup reactions leading to the degradation of the dimer are avoided. 

III. GENERATION OF ORGANOSILICON RADICAL CATIONS IN SOLUTION AS PREDICTED BY FIRST PES IONIZATION ENERGIES AND... 

To study organosilicon radical cations in solution, a general procedure has been developed, indeed a recipe, for their generation based on photoelectron spectroscopy16 ... 

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