Benzophenone photosensitized photolysis

In the presence of benzophenone, (8) was again the major product (>95°/0) and only trace amounts of the cyclohexane products were produced. These results suggest the intermediacy of a singlet 1,6-hexylene biradical in the direct photolysis and a longer lived triplet 1,6-diradical in the sensitized photolysis. In the triplet biradical more time is available for 1,6-hydrogen transfer to occur prior to spin inversion and hence more olefin (8) is produced. Similar results were reported for the direct and photosensitized photolysis of the 3,8-dimethyl derivative of (7). 

Comparison of these experimental results with the calculated charge densities (S0 and Si) at the 2 and 3 positions (Table 11.5) shows that this is the expected result. Except for those compounds discussed below, the failure to observe quenching with triplet quenchers and reaction in the presence of a photosensitizer indicated singlet reactions. Compound (89) was found to also undergo benzophenone-photosensitized substitution, indicating that the triplet state of this compound is also reactive. The reaction, however, was less clean than that observed in the direct photolysis. Similarly, 1,6-dinitro-naphthalene was found to undergo both direct and benzophenone-photosensitized substitution ... 

Cyclohexene. The experimental results are summarized in Table IV. The product ratios are similar for CH2 generated by CH2N2 photolysis and by CH2N2 pyrolysis at 265°C., but for CH2 generated by the benzophenone-photosensitized decomposition of CH2N2 the addition product norcarane is favored. 

The reduction potentials for S,S-dialkyl-S-(hydroxyphenyl) sulfonium salts do not appear to have been measured. These sulfonium salts do not undergo electron transfer photosensitization. Rather, photosensitization has been achieved using typical diaryl ketones such as benzophenone, Michler s ketone, and thioxanthone as photosensitizers. Strongly acidic solutions are produced during photosensitized photolysis, suggesting the formation of a protonic add as one of the products. 

A final comment on Table 4 concerns the reaction shown in entry 8. Because the di-TT-methane photorearrangement of benzonorbomadiene derivatives requires triplet energy sensitization, we could not use typical, passive amines such as (/ )-( + )-l-phenylethylamine as chiral auxiliaries. We therefore prepared an optically pure amine to which a sensitizing benzophenone moiety was tethered, namely, the 4-benzoylphenyl ester of l-valine [25]. Photolysis of the salt of this amine at wavelengths where only the benzophenone chromophore absorbs led to the photoproduct in 91% ee at 100% conversion, a gratifying vindication of the concept. Optically active photosensitizers have been used in solution with limited success [33], but this represents the first example of simultaneous triplet-triplet energy transfer and asymmetric induction in the crystalline state. 

There is much evidence that the photolysis of ketene, diazomethane, and diazirine produces methylene predominantly in the singlet state. It is, however, possible, by using mercury photosensitization or by photosensitization using benzophenone to produce triplet methylene. Since the triplet is the ground state for methylene, it is also possible to produce triplet methylene from singlet methylene by carrying out reactions in the presence of a large excess of inert gas. Recently, much evidence has accumulated to indicate that even in the normal photolysis of all three of the methylene precursors, some of the methylene is produced in the triplet state. ... 

Thus, the Pyrex-filtered irradiation of an acetonitrile solution of 2,3-diazabicy-clo[2.2.1]hept-2-ene 1 (R, R, R, R = H) in a Rayonet photoreactor in the presence of nitr-oxide and with benzophenone employed as a triplet photosensitizer gave bicyclo[2.1. Ojpentane (4) in 40% yield and the two bis-nitroxide adducts cis- and trans-5 in 21 and 39% yields, respectively, based on 60% conversion. Separately generated bicyclo[2.1.Ojpentane was shown to be unreactive towards the nitroxide under the conditions of the photolysis furthermore no reaction occurred in the absence of light. However, these bis-nitroxide photoadducts were also produced, albeit to a smaller extent, when the reaction was carried out in the absence of benzophenone, implying that the nitroxide itself may act as a sensitizer. This triplet cyclopenta-1,3-diyl diradical has also been generated in the gas phase. ... 

Benzophenone was used as photosensitizer in a study of the photo-decomposition of benzene solutions of polystyrene and poly(a-methyl styrene). Laser photolysis (A 265 nm) of poIy(a-methyl styrene) in chlorofonn and carbon tetrachloride proceeded by attack of solvent radicals in the polymer molecules resulting in main chain cleavage. " A series of papers have been devoted to the study of the photo-induced decomposition of poly(a-methyl styrene) in benzene solutions, using a,a -bisisobutyronitrile, benzophenone, and triphenyl-methyl cations as sensitizers. Spin-trapping techniques were used to detect transient radicals formed during benzophenone-sensitized degradation of poly-(a-methyl styrene). The rate constant for intramolecular reaction of the macroradical was estimated. 

Photosensitization of TEMPO and SG-1 based alkoxyamines with photosensitizers (e.g. benzophenone or 2-isopropylthioxanthone) also leads to the desired NO-C photolysis. However, side reactions were also observed. 

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