Benzophenone photochemistry

Photopolymerization is traditionally initiated by direct photolysis of a precursor to provide free radicals via bond homolysis. Examples of such initiators include benzoin, and benzoin ethers, disulfides, and azoalkanes or dialkylperoxides. Hydrogen abstraction chemistry, typified by benzophenone photochemistry, is also recognized as extremely useful. However, a number of viable commercial photopolymer imaging systems are based upon ionic (especially cationic) polymerization. These systems will be discussed next. 

In the course of carrying out the benzophenone photochemistry described in the previous section, we noted that the use of CCI4 as solvent led to the formation of some chlorinated steroid products. Therefore we examined such halogenations further [36], and found that the simple free-radical chlorination of cholestanyl acetate with phenyliodine dichloride afforded the 9-chloro- and 14-chlorosteroids as the major products. Similar results were seen in bromination by bromotrichloromethane. We extended such selective radical halogenations to suppress reactions other than at C-9 and C-14 by appropriate substituent effects, but more significantly we used tethers to achieve essentially complete selectivity in the halogenations [36]. 

PROTON-TRANSFER REACTIONS IN BENZOPHENONE/ A,A-DIMETHYLANILINE PHOTOCHEMISTRY... 

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-... 

Proton-Transfer Reactions in Benzophenone/Af,A-Dimethylaniline Photochemistry (Peters). 27 51... 

Proton-Transfer Reactions in Benzophenone/ A, A -Dimethylaniline Photochemistry... 

In addition to the rather trivial differences mentioned above, laser irradiation can also lead to products as a result of reexcitaion of the carbenes. Thus, excitation of 30 in isooctane with a pulse of the 249-nm line from a KrF excimer laser results in the formation of 9,10-diphenylanthrancene (103), 9,10-diphenylphenanthrene (104), and fluorene, in addition to tetraphenylethylene (Scheme 9.31). Conventional lamp irradiation of 30 results in the formation of benzophenone azine as a major product. None of the products mentioned above are detected. Moreover, the yield of both 103 and fluorene increased markedly with increased laser power. While the details of the mechanism of this reaction are not certain yet, it is clear from the dependence on laser power that some of these products arise from carbene photochemistry. " ... 

Recent work on the photochemistry of ketimines has shown that they do not undergo reduction unless ketones are present. Thus chemical sensitization is entirely responsible for the photoreduction of benzophenone methylimine (22)105 while intramolecular chemical sensitization has been suggested as the mechanism for reduction of the acylketimine (23).119 In related work,... 

As an example, let us consider the stoichiometric oxidation of diphenylmethanol to benzophenone, one of the most commonly used photosensitizers in photochemistry (Figure 1.3). We will evaluate this reaction using the measures of product yield, product selectivity, E-factor, and atom economy. In this reaction, three equivalents of diphenylmethanol react with two equivalents of chromium trioxide and three equivalents of sulfuric acid, giving three equivalents of benzophenone. First, let us see how the reaction measures with respect to product yield and selectivity. Assume that this is an ideal chemical reaction which goes to completion, so one obtains 100% yield of the product, benzophenone. If no other (organic) by-product is obtained, the product selectivity is also 100%. This is all well and good, and indeed for many years this has been the way that chemical processes were evaluated, both in academia and in the (fine-) chemical industry. 

Addition of THF is necessary with benzophenone derivatives and enhances the efficiency of phenyl acetophenone derivatives as shown by the following reactions (the photochemistry of these compounds are very well known in homogeneous solutions) ... 

Another interesting example of sulfone photochemistry pertains to diyne 92, which was investigated as an early approach for the synthesis of the highly unstable nine-membered ene-diyne ring of neocarzinostatin Chrom A 1 by Wender et al. [56]. The benzophenone-sensitized desulfonation of 92 resulted in the formation of the desired, but highly unstable, compound 93 in 9-15% yield (Scheme 2.22). 

To study the photolysis of azo compounds, CIDNP was only recently introduced in the field of photochemistry. The CIDNP-effect consists of generating a geminate radical pair which still remembers the spin state of its precursor. So the multiplicity of the precursor can be determined from enhanced absorption or emission signals in azoalkane photolysis. The benzophenone sensitized photolysis of dia-zirine in deuteriochloroform leads to the triplet azo compound 24 which decomposes under elimination of a ground state nitrogen molecule and a triplet methylene 38>. This abstracts deuterium from deuteriochloroform to form the geminate radical pair 25. This can now recombine to give 26 or dissociate to afford the free radical products. 

In principle, the photoreactions of CT s are able to offer a great number of photoinitiator systems for radical polymerization. But, so far, this subject has only received little attention, and the current knowledge relative to the photochemistry of such complexes is poor. In addition to the amine complexes mentioned above, chinoline-bromine [124-127], chinoline-chlorine [128], 2-methylpyridine-chlorine [129], pyridine-bromine [130], IV-vinylpyrrolidone-bromine [131], acridone-bro-mine [132], acridone-chlorine [133], benzophenone-S02 [134], isoquinoline-S02 [135, 136], and 2-methylquinoline-S02 [136] combinations are used for radical polymerization of AN, alkyl methacrylates, acrylic and methacrylic acid, and for... 

Hammond, Wamser, Chang, and Baylor [54] studied the photochemistry of the BP/MK pair formally using flash photolysis techniques and showed that electron transfer occurs via triplet exciplexes that can be formed by excitation of either partner, though under normal circumstances MK absorbs the vast majority of the light. Benzophenone ketyl radical was observed as a transient, and a second species present was identified as either the cation radical of MK or its corresponding deprotonated radical. The coupling product of the latter two radicals (16) was observed as a product of steady-state irradiation, and it was formed with a quantum... 

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