Benzilic radical intermediate

Ketones are oxidatively cleaved by Cr(VI) or Mn(VII) reagents. The reaction is sometimes of utility in the synthesis of difunctional molecules by ring cleavage. The mechanism for both reagents is believed to involve an enol intermediate.206 A study involving both kinetic data and quantitative product studies has permitted a fairly complete description of the Cr(VI) oxidation of benzyl phenyl ketone.207 The products include both oxidative-cleavage products and benzil, 7, which results from oxidation a to the carbonyl. In addition, the dimeric product 8, which is suggestive of radical intermediates, is formed under some conditions. 

The reactivity of stable germaphosphenes has been previously investigated and reviewed.3,4,7 The chemical behavior of Mes2Ge = PAr 155 toward orthoquinones (tetrachloro-o-benzoquinone, 3,5-di-terr-butyl-o-benzoqui-none, and 1,2-naphthoquinone) and ce-diketones (benzil and biacetyl) has been examined. The reaction with o-quinones involved probably radical intermediates, and in each case only the less encumbered isomer of the corresponding [2 + 4] cycloadduct 156 was formed. Benzil led in a very similar manner to a l,4,2,3-dioxagermaphosphin-5-ene 157, whereas, as was observed with acetone, an ene-reaction occurred with biacetyl to afford 158138 (Scheme 29). 

By ESR studies, the benzilic acid rearrangement in solution has been proven to proceed via a radical intermediate [14], For the benzilic acid reanangement in the solid state, a radical intermediate was also detected. For example, a freshly prepared mixture of finely powdered 35e and KOH showed a strong ESR signal (g=2.0049), and this signal declined as the reaction proceeded. 

However, benzil [PhC(O)C(O)Ph] cannot enolize and is dioxygenated by Oz - to give two benzoate ions. Scheme 7-11 outlines a proposed mechanism that is initiated by nucleophilic attack. An alternative pathway has been proposed i in which the initial step is electron transfer from 02 - to the carbonyl, followed by coupling of the benzil radical with dioxygen to give the cyclic dioxetanelike intermediate. However, the outer-sphere reduction potential for electron transfer from O2 to a carbonyl carbon is insufficient (-0.60 V versus NHE, Chapter 2). 

A similar mechanism can be written for the formation of phenyl acetone in an irradiated aqueous solution of biacetyl and phenylacetic acid.55 Abstraction of a benzilic hydrogen from phenylacetic acid by a photoexcited biacetyl would produce 3 and a benzylic radical which can add to a second biacetyl molecule. Cleavage of this intermediate would yield a jS-keto acid which, on... 

The analysis of similar processes with benzophenone (1) and benzil (7) requires a higher time resolution of the experimental setup. Using ns-laser flash photolysis, we observed the formation of radical ion intermediates, depending on solvent polarity, added salts and competing H-abstraction [36]. Summarizing all these experiments, one can draw the following conclusions (cf. Figs. 3—5, see also Ref. [33]) ... 

Kinetic parameters can be obtained from time resolved measurements. The time profile of the transients (formation and decay) of the photoreaction of benzil (7) with triethylamine (6) in acetonitrile is shown in Fig. 5, together with the influence of added lithium perchlorate on the lifetime of the radical anion intermediates. 

Laurent was the first to react a carbonyl compound, benzil, with an alkali metal. These attempts were more eventful than successful instead of the first radical anion he observed spontaneous ignition [34], Some fifty years later, Beckmann and Paul were more successful, when they reacted aromatic ketones (benzo-phenone) and diketones (benzil) with sodium [35]. They noted the sensitivity of the solutions and of the colored solid products to air and moisture and worked in a hydrogen atmosphere. Although they demonstrated admirable laboratory skills, (cf. Fig. 3), they did not understand the nature of the colored intermediates. 

The interception of the radical anion by add or the formation of 28 should have the same effect. In fact, the back electron-transfer reaction from (TCA ) to (DPA ) is suppressed, or at least reduced, maximizing the cage escape efliriency of the radical cation and its further reaction with molecular oxygen. In these reactions benzil 26 is probably formed via the dioxete intermediate, also suggested by de Mayo and co-workers [100], whereas the mechanism leading to benzoic add is still uncertain. Other examples of photoinduced oxygenations, preceded by nucleophilic addition, have been also reported. 

Both N-N and N-C bond fission occurs on irradiation of the hydrazone derivatives (191). The photodegradation of the phenylhydrazone and the hydrazone of benzil have also been described. a-Ketoiminyl radicals are formed on irradiation of oximino ketones at low temperature. A study of the photochemical decomposition of sulfamic esters and their use as initiators of cross-linking of a melamine resin have been described. The bispyridinyl radical (192) is formed by one electron reduction of the corresponding pyridinium salts. The irradiation of this biradical at 77 K results in C-N bond fission with the formation of benzene-1,3-diyl. The predominant products from the irradiation (X,> 340 nm) of (193) in methanol were identified as A -hydroxy-2-pyridone and (194) from the fission of the C-O bond. Other products were 2-pyridone, (195) and (196) that arise from O-N bond fission. The reaction is to some extent substituent dependent and a detailed analysis of the reaction systems has identified an intramolecular exciplex as the key intermediate in the C-O bond heterolysis. 

The inference that semidione radicals are intermediates in diketone photolysis has been widely accepted for many years since it provides a satisfactory rationale for the results observed in many reactions. Recently, direct evidence for the presence of these radicals has been obtained from flash photolysis 13> of benzil in alcoholic solvents and esr studie of irradiated solutions of biacetyl 12,171,183 camphorquinone 9,i83)> and other diones 183>. The related radical anions, generated by chemical means, have been studied extensively 5,3i,i3 ) and reports of the radical cations have also appeared m.137)... 

Photo-epoxidation of alkenes in the presence of benzoins and oxygen has been shown to proceed via the benzoylperoxy radical (2), which is effectively trapped by alkene and subsequently yields predominantly trans-epoxides The same intermediate radical (and, as a result, similar reactivities) has been observed during photo-epoxidation using benzoylformic acid (PhCOCOOH), but the reactivities of the alkenes were different from those obtained using peroxy-acids. A correction has been published to some previous studies on the efficiency of benzil-sensitized photo-epoxidation of trinorbornene. The new results indicate a lower yield of < 2 moles of epoxide per mole of diketone that is consumed and thus suggest that a chain mechanism is not involved for such reactions. 

Oxalic and malonic acids, as well as a-hydroxy acids, easily react with cerium(IV) salts (Sheldon and Kochi, 1968). Simple alkanoic acids are much more resistant to attack by cerium(IV) salts. However, silver(I) salts catalyze the thermal decarboxylation of alkanoic acids by ammonium hexanitratocerate(IV) (Nagori et al., 1981). Cerium(IV) carboxylates can be decomposed by either a thermal or a photochemical reaction (Sheldon and Kochi, 1968). Alkyl radicals are released by the decarboxylation reaction, which yields alkanes, alkenes, esters and carbon dioxide. The oxidation of substituted benzilic acids by cerium(IV) salts affords the corresponding benzilic acids in quantitative yield (scheme 19) (Hanna and Sarac, 1977). Trahanovsky and coworkers reported that phenylacetic acid is decarboxylated by reaction with ammonium hexanitratocerate(IV) in aqueous acetonitrile containing nitric acid (Trahanovsky et al., 1974). The reaction products are benzyl alcohol, benzaldehyde, benzyl nitrate and carbon dioxide. The reaction is also applicable to substituted phenylacetic acids. The decarboxylation is a one-electron process and radicals are formed as intermediates. The rate-determining step is the decomposition of the phenylacetic acid/cerium(IV) complex into a benzyl radical and carbon dioxide. 

All the results are consistent with intermediate formation of aroyl or acyl and solvent-derived radicals, as illustrated below for the reaction with p-xylene. It was not possible to determine if these are formed via an initial H-atom abstraction by trione from solvent or via an a-cleavage of 13 followed by Id-abstraction. The low quantum yields precluded a variety of experiments, such as, for example, examination of deuterium isotope effects on quantum yields. A report that 13 in benzene solution was converted to benzil in Cairo sunlight could not be confirmed in Haifa sunlight. ... 

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