Benzophenone, photoreactivity

Benzophenone photoreacts with 2-methylselenophene to yield the adduct (102)." The photoaddition of benzophenone to the tricyclooctanones (103) has been reported to yield oxetanes. ... 

The photochemically induced [2 + 2] cycloaddition of selenophene with dimethylmaleic anhydride gives a 1 1 adduct (140), but attempts to form an oxetane by photoreaction with benzophenone failed (80JHC1151). 

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

Conjugated dienes can undergo a variety of photoreactions, depending on whether excitation is direct or photosensitized. The benzophenone-sensitized excitation of 1,3-pentadiene, for example, results in stereochemical isomerization and dimerization ... 

We have already discussed one of the earliest photoreactions to be studied, that is, the (4w + 4w) photodimerization of anthracene. That the singlet state was involved in this reaction was conclusively shown in the period 1955-1957. The first reaction in which the triplet state of the molecule was shown to be involved was the photoreduction of benzophenone by Hammond and co-workersa) and Backstrom and co-workers<2) 1959-1961. This was the first in a series of many papers from Hammond s laboratory... 

In acidic solution the rearrangement of a-santonin to lumisantonin and then ultimately to photosantonic acid is not as efficient as rearrangement to the hydroxy ketone, isophotosantonic lactone (5), shown on page 308. Fisch and Richards(6) found that the photorearrangements of a-santonin could be sensitized with benzophenone or Michler s ketone. Moreover, if the irradiation of a-santonin (3660 A) is carried out in piperylene as solvent, the photoreaction is completely quenched. This suggests that the rearrangements proceed via triplet states. 

Of the following amine-reactive and photoreactive crosslinkers, the overwhelming majority use an aryl azide group as the photosensitive functional group. Only a few use alternative photoreactive chemistries, particularly perfluorinated aryl azide, benzophenone, or diazo compounds. For general background information on photoreactive crosslinkers, see Das and Fox (1979), Kiehm and Ji (1977), Vanin and Ji (1981), and Brunner (1993). 

Figure 18.23 Biotin-PEG3-benzophenone is a water-soluble photoreactive biotinylation reagent that can be used to add a biotin group to surfaces or molecules containing no easily derivatized functional groups.

Tao, T., Lamkin, M., and Schemer, C. (1984) Studies on the proximity relationships between thin filament proteins using benzophenone-4-maleimide as a site-specific photoreactive crosslinker. Biophys. J. 45,261. 

Fig. 4 Photoreaction of benzophenone with the n-alkanes. Plot of the second-order rate constants in CC14 at 25°C against the number of CH2 groups. (Data from Winnik and Maharaj, 1979)...

Stony Brook synthesized a photoreactive benzophenone containing inhibitor (39, Fig. 14) which efficiently labeled the active site of the enzyme. The photoinhibition was prevented by adding native Ras to the reaction mixture. That competition indicated that the labeling was specific at the active site. Peptide mapping of the labeled enzyme by HPLC, Edman sequencing and MALDI-MS allowed the identification of key amino acids in the substrate binding, as Asp-110 and Asp-112 in the a-subunit [126]. 

Several examples of aromatic hydrocarbon sensitized additions of NNP to the same arenes were demonstrated to occur if an acid is present this is in contrast to the failure of benzophenone to sensitize the photoreactions. Irradiation of anthracene in the presence of NNP and hydrochloric acid gives 308 in 70% yield and a small amount of 309 derived from the acid-catalysed elimination of piperidinium ion and addition of ethanol165 (equation 147). Anthracene possesses Es = 76.3 kcal mol 1,

[Pg.813]

An early investigation concerned the photoreactions of p-nitroanisole. Letsinger and Steller (1969) elegantly demonstrated not only that its photosubstitution by hydroxide ion and by pyridine could be sensitized (by benzophenone), but also that the product distribution of the photohydrolysis (/ -nitrophenol/p-methoxyphenol 1 4) was the same whether it was the result of direct irradiation or of a sensitized reaction. 

Although the photochemistry of aryl selenoesters and aryl telluroesters is not fully developed, there are some photoreactions of these compounds that resemble a photo-Fries process. Se-/ ara-tolyl selenobenzoate (212) gives, upon irradiation, selenocresol (214), benzaldehyde, and the benzophenone 213 (Scheme 56), which is clearly a photo-Fries product [157,158], Starting from Se-phenyl 2-chlorosele-... 

The photochemically induced reaction of selenophene and dimethylmaleic anhydride gives mainly the [2+2] cycloadduct (79), but attempts to form an oxetane by photoreaction with benzophenone failed (80JHC1151). Photoirradiated in the presence of benzophenone, benzo[6 ]selenophene and some of its derivatives form [2+2] cycloadducts with dichloroethylene. Photoreaction with dimethyl acetylenedicarboxylate gives addition products which lose selenium to yield naphthalene derivatives (77T2225). 

Porter has correlated solvent shifts on absorption spectra with the photoreactivity of substituted benzophenones.365 He has noted red shifts for the unreactive substituted ketones upon shifting from cyclohexane to 2-propanol, which effect he ascribes to absorption into an intramolecular charge-transfer band which would be stabilized by solvent polarity. He reasons further that the charge-transfer nature of... 

Several applications of photoreactive peptides require the presence of a radionuclide to allow specific and sensitive detection of the photo-cross-linked conjugates. In several cases, radioiodination of tyrosyl moieties and radiolabeled Bolton-Hunter reagents have been used. However, the presence of a radiolabel within the benzophenone photophore is desirable, particularly when the objective is to identify the site of photo-insertion of benzophenone. To this end some radiolabeled, benzophenone-based compounds have been developed and used in peptide synthesis, in particular tritiated Phe(4-Bz) (Scheme 24)J2161 [1-14C-carboxy]-4-benzoylbenzoic acid,1221 and 4-benzoyl-(2,3-3H2)-dihydrocinnamic acid.[154l In addition, 4-(4-hydroxybenzoyl)phenylalanine (Scheme 25) has been directly radioiodinated with Na125I and Chloramine-T)151 ... 

Since quenching experiments with piperylene and naphthalene showed no effect, it was suggested that the triplet state of the thiatriazole is not involved in the reactions.22 It was later shown that the photoreactions leading to nitrile, isothiocyanate, sulfur, and nitrogen actually take place from a singlet excited state of the thiatriazole.20 Thus the photoreduction of benzophenone in isopropyl alcohol is effectively quenched by addition of 5-phenylthiatriazole, but analysis revealed that all thiatriazole could be recovered from the photolysis mixture, indicating the lack of photoreactivity of the triplet state. 

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