Benzil photolysis

Benzil. Photolysis of benzil vapor at 200° to give carbon monoxide and benzophenone was reported 121> in 1923. The gas phase reaction has not been investigated since then. 

The photosensitized dimerization of isoprene in the presence of benzil has been investigated. Mixtures of substituted cyclobutanes, cyclohexenes, and cyclooctadienes were formed and identified (53). The reaction is believed to proceed by formation of a reactive triplet intermediate. The energy for this triplet state presumably is obtained by interaction with the photoexcited benzil species. Under other conditions, photolysis results in the formation of a methylcydobutene (54,55). 

The addition of t-Bu2>Si to 1,4-diaza-l,3-butadienes competes with dimerization of the silylene only when the concentration of t-BinSi is low170. Subtle steric effects must also be responsible for the addition of /-BinSi to the W-cyclohexyl mono-imine of benzil, while only the silylene dimer undergoes addition under similar conditions in the presence of the IV-methyl mono-imine171. It may be that t-Bi Si and its dimer t-Bu2Si=SiBu-t2, both formed simultaneously upon photolysis of cyclo (t-Bu2Si)3, are in equilibrium, and the steric effect is upon the (2+4) cycloaddition of the disilene. 

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]) ... 

In fact, the addition of 1,4-dimethoxybenzene (DMB) and/or several similar compounds, at concentrations as low as 10 4 M, to a mixture of aryl-olefins and DCA almost completely inhibits the reactions. Concentration dependence and flash photolysis studies confirm that the primary electron-transfer process occurs between the singlet excited sensitizer and DMB (E01 = 1.34 V vs SCE) with the generation of the corresponding radical ion pair. As a consequence, quantum yields lower than 1, even at infinite substrate concentration, are measured [95]. In this regard, valuable confirmations came from the cyanoaromatic photoinduced electron-transfer oxygenation of alkynes [99], Farid and Mattes reported that the photooxygenation of diphenylacetylene DPA (E° = 1.85 V vs SCE) 25, leading to a mixture of benzil 26 and benzoic acid 27, was efficiently sensitized by DCA ( = 0.15), but poorly by TCA ( > < 0.001) [Eq. (12)] [99]. 

Further work on the mechanism of dione reactions with oxygen includes a detailed study 165> of reactions of biacetyl and benzil with emphasis on the latter. The earlier conclusion2 that photolysis of benzil does not produce benzoyl radicals... 

The effect of the micelle environment on the photolysis of adamantyl ketones is dramatic31). The major products in the photolysis of phenyladamantyl ketone (20) in benzene are shown in Scheme IX. However, in HDTC1 only (22) is formed in significant amounts. Minor products are (16) (7%) and two new products AdOH (25) (3 %) and benzil (18) (3 %). The appearance of large amounts of adamantane... 

The results of spin-trapping experiments 18) also support this view no radicals were trapped on photolysis of benzil in benzene solutions of compounds I and II (see Section II. A. 1) but in methanol solutions of II, CH OH radicals were scavenged, indicating that hydrr en abstraction by photoexcited benzil had occurred 18). Thus the behaviour of benzil in these experiments contrasted markedly with that of benzoin and benzoin methyl ether, described previously. 

Photoreduction of benzophenone by primary and secondary amines leads to the formation of benzpinacol and imines [145]. Quantum yields greater than unity for reduction of benzophenone indicated that the a-aminoalkyl radical could further reduce the ground state of benzophenone. Bhattacharyya and Das confirmed this in a laser-flash photolysis study of the benzophenone-triethylamine system, which showed that ketyl radical anion formation occurs by a fast and a slow process wherein the slow process corresponds to the reaction of a-aminoalkyl radical in the ground state of benzophenone [148]. Direct evidence for similar secondary reduction of benzil [149] and naphthalimides [150] by the a-aminoalkyl radical have also been reported. The secondary dark reaction of a-aminoalkyl radicals in photo-induced electron-transfer reactions with a variety of quinones, dyes, and metal complexes has been studied by Whitten and coworkers [151]. 

The study of the photodecomposition of a-diazo ketones continues to attract attention. The strong and weak triplet signals observed in the e.s.r. spectrum on photolysis of crystalline azi-benzil at 77 K have been assigned to the s-Z and the s-E stereo-... 

Triplet states of ring substituted -phenylpropiophenones have been characterized by laser flash photolysis and phosphorescence at -70°Spin relaxation of the benzil triplet has been studied... 

Photolysis of 4,5-diphenyl-l,2,3-selenadiazoIe in the presence of oxygen has been found to yield, besides diphenylacetylene in high yield, small amounts of tetraphenyl-l,4-diselenafulvene, benzil, and benzophenone. A series of 1,2,3-selenadiazoles substituted with arylsulfonyl moieties (139-141) have been prepared they could not be converted into the corresponding acetylenes by pyrolysis. Arylsulfonylacetylenes 142 and 143 could, however, be prepared by the photolysis of 1,2,3-selenadiazoles. ... 

FIGURE 3 - A typical set of experimental traces obtained using Diffuse Reflectance Laser Flash Photolysis for a microcrystalline sample of benzil (excitation at 354 nm and analysis at 520 nm). 

Microcrystalline benzophenone [38] and benzil [16] were two of the first systems studied by nanosecond diffuse reflectance flash photolysis. Both samples gave transient absorptions which were positively identified as triplet-triplet absorptions. In the case of benzophenone an absorption, centred at 540 nm, was obsejrved which has, within experimental error, identical kinetics to the phosphorescence decay, which is predominantly second order. In the case of benzil a transient absorption of 60% at 510 nm was observed after 354 nm excitation. The assignment as triplet-triplet absorption was made on the basis of the absorption and phosphorescence kinetics being virtually identical, namely a mixture of first and second order kinetics. Ikeda et al [39] have also studied microcrystalline benzophenone on the picosecond time scale. Another microcrystalline sample studied is 1,5-diphenyl-3-styryl-2-pyrazoline, in which the triplet-triplet transient absorption was identified within the microsecond time domain [15] (see figure 7(b)). However, as mentioned above (see section 4 and figure 5), the transient absorption due to the excited singlet state has been observed on a picosecond time domain [17]. 

On sensitized irradiation with carbonyl compounds such as benzophenone, it is expected that the triplet state of olehns will be produced. However, a radical chain mechanism is sometimes discussed when benzil or biacetyl is used as a sensitizer. Thus, some other experimental evidence such as direct observation of the triplet state by laser photolysis is needed to establish the triplet mechanism even on triplet sensitization. 

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)... 

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