Benzophenone radical anion

Commercially available anhydrous diethyl ether is distilled under nitrogen from a solution of the sodium benzophenone radical anion generated by treating a solution of 10 g of benzophenone and 1 L of ether with 10 g of sodium ribbon until a dark blue or purple color persists. 

Two classes of charged radicals derived from ketones have been well studied. Ketyls are radical anions formed by one-electron reduction of carbonyl compounds. The formation of the benzophenone radical anion by reduction with sodium metal is an example. This radical anion is deep blue in color and is veiy reactive toward both oxygen and protons. Many detailed studies on the structure and spectral properties of this and related radical anions have been carried out. A common chemical reaction of the ketyl radicals is coupling to form a diamagnetic dianion. This occurs reversibly for simple aromatic ketyls. The dimerization is promoted by protonation of one or both of the ketyls because the electrostatic repulsion is then removed. The coupling process leads to reductive dimerization of carbonyl compounds, a reaction that will be discussed in detail in Section 5.5.3 of Part B. 

Figure 3.56 Absorption spectra of the benzophenone radical anion in acetonitrile and water...

There is evidence [19] that the initial absorption at ca. 720 nm within 25 ps belongs to a kind of SSIP of benzophenone radical anion and dimethylaniline radical cation. This peak shifts during 200 — 800 ps to 690 nm. The blue shift corresponds to the formation of the thermodynamically more stable contact ion pair, identified according to work by Hogen-Esch and Smid [20]. Finally the ketyl radical is observed at 545 nm after ca. 2000 ps. 

Photoionization is also an important (although not the major) decay pathway for the excited diphenylketyl radical (161) in polar solvents such as acetonitrile [137] (Scheme 22). This was confirmed with the observation of benzophenone radical anion in laser flash experiments following 515-nm photolysis of ground-state 161. The anion radical is apparently produced as a result of electron ejection followed by trapping of the electron by ground-state benzophenone. 

Table 2 summarizes distillation conditions (pressure, temperature and contaminant absorbing materials for several important solvents). In brief, the best contaminant absorber for ether solvents is benzophenone radical anion, which is formed by the introduction of alkali metal chips into a solution of benzophenone in the ether. The metal is chosen, depending on the difference between its melting... 

The diffusion-layer imaging technique which was developed by McCreery is another method for studying intermediates in the diffusion layer [71-75]. A laser beam is directed in a parallel direction through the diffusion layer of the electrode and the light is then magnified and focused on a diode-array detector. With this method, spatial resolution of the diffusion layer of 1.25 pm is achieved, and concentration profiles in the diffusion layer are mapped. A detailed description of mass transport processes as well as the kinetics and spectra of intermediates can be obtained. Diffusion coefficients and extinction coefficients for, for example, the benzophenone radical anion were measured with this technique [74, 75]. 

The blue benzophenone radical anion (the first recognized radical anion [65]) is well-known as a color test for the absence of water and oxygen from ether solvents over Na. In the presence of water, it forms the colorless pinacol by dimerization of the ketyl radical Ph2C(0)H (i.e. the protonated form of the radical anion) [66]. In the presence of dioxygen it is rapidly oxidized back to benzophenone, and thus removes traces of O2 from N2 [67]. It has been very little used as a stoichiometric... 

One of the conclusions of this study is that the first ligand substitution proceeds with a much higher coulombic efficiency than the second one, simply because of the increasing bulk at the metal center [317], This trend was known from previous studies of carbonyl substitution by phosphanes in transition-metal carbonyl clusters catalyzed by benzophenone radical anion [318, 319]. In such a case, selective ETC catalysts may be designed as a function of the required exergonicity of the desired initiation step [317]. 

Many phosphane-substituted transition-metal clusters have been synthesized from late transition-metal carbonyl clusters and the appropriate phosphane using reductive ETC catalysis with reductive initiation [318-333]. Indeed such an initiation provides an exergonic cross electron-transfer propagation step. Most syntheses were carried out using a cathodic initiation or sodium benzophenone radical anion. The method was successful because it turned out that the first substitution of a carbonyl by a phosphane proceeds with high yield and coulombic efficiency in homoleptic metal carbonyl clusters and some others. 

Sensitized photooxidation of sulfur-containing amino acids in neutral aqueous solution occurs via electron transfer from the sulfur to the triplet state of the sensitizer (the mechanism can be adapted to other dye-sensitized photoinitiated polymerization). Electron transfer quenching in the system is followed mainly by diffusion of the sulfur-centered radical cation and benzophenone radical anion. Laser... 

Treatment of benzopinacol with Na(Hg) in a mixture of anhydrous ether and benzene gives a dark blue solution that fades. Benzhydrol can be isolated in a quantitative yield. It has been suggested that the disodium salt of benzopinacol disproportionates with C-C cleavage into benzhydrol and benzophenone the latter is reduced by Na(Hg) to benzopinacol, which then decomposes [119]. In that connection the cleavage of benzophenone pinacol to two benzophenone radical anions under the influence of electrogenerated bases [120] is germane. 

The ability of the thiolate to function as a nucleophile and a reducing agent is required for this transformation. Later Pez and coworkers [34] showed that careful addition of sodium benzophenone radical anion to perfluorodecalin affords... 

Protection of primary hydroxyl groups. The 5 -hydroxyl groups of nucleosides can be protected as the a-naphthyldiphenylmethyl ether. The group is selectively cleaved reductively in the presence of a p-methoxytrityl protecting group by sodium anthracenide in THE (HMPT). Sodium naphthalenide is less selective. The a-naphthyldiphenylmethyl group is relatively stable toward the benzophenone radical anion, which readily cleaves /3,/3,/3-trichloroethyl groups. 

Two classes of charged radicals derived from ketones have been well studied. The ketyk are radical anions formed by one-electron reduction of a carbonyl compound. The formation of the benzophenone radical anion by reduction with sodium metal is an example ... 

Radical anions generated from metal-organic compounds are another group of initiators. Based on polarographic investigations and Huckel calculations it was shown that the polymerization of crotonaldehyde with benzophenone radical anions proceeds via formation of a complex radical anion of crotonaldehyde [174]. This complex accepts a... 

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