Benzophenone, sensitization

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 ... 

Lapin [33] suggested that photoinitiated cationic polymerization can proceed through reactions of free radicals, as shown below for benzophenone sensitized photoinitiation ... 

This behavior has been verified with bithiazole derivative 94 (Scheme 36). Neither acetophenone nor benzophenone sensitized the reaction, in agreement with an ICI mechanism via the excited singlet state (86TL6385 87JA938). 

The parent system oxonin (1) is generated by benzophenone sensitized irradiation with a Hanovia light source3 5 or direct irradiation6 of the monoepoxide of cyclooctatetraene (9-oxabicyclo[6.1.0]nona-2,4,6-triene, 6). 

The yield of trans product (18) is decreased by the presence of a radical scavenger such as 1,1-diphenylethylene and increased by dilution of the reactants with methylene chloride or butane, indicating this product to result from the triplet carbene. A heavy-atom effect on the carbene intermediate was observed by photolysis of a-methylmercuridiazoacetonitrile. With c/s-2-butene as the trapping agent either direct photolysis or triplet benzophenone-sensitized decomposition results in formation of cyclopropanes (19) and (20) in a 1 1 ratio ... 

The photostationary state composition for the benzophenone-sensitized isomerization of 2,4-hexadienes is given in Table 9.2. Table 9.3 gives the measured quantum yields for benzophenone-sensitized isomerization of 2,4-hexadienes along with the calculated quantum yields based on Eqs. (9.47)-(9.49) and the pss values given in Table 9.2. 

In addition to the photoxygenation/diimide (equation 6),16) silver salt (Eq. 22), 36) and triflate (Eq. 44)s6> routes, 9 has also been prepared by benzophenone-sensitized photodecomposition of the corresponding azo compound 59 and trapping of the resultant triplet diradical with oxygen (Eq. 45) 57). 

The decomposition of methanesulphonyl azide in isopropyl alcohol could be effected by selective irradiation of 2-acetonaphthone instead of benzophenone 21>. Since 2-acetonaphthone triplets are incapable of hydrogen abstraction from isopropyl alcohol 22>, initiation must occur via transfer of excitation energy to the azide. A marked difference was observed from benzophenone sensitization in that the reaction was extremely slow, gave a nitrogen yield of only 68%, and produced a yellow solution 21>. 

Fig. 8. Benzophenone-sensitized isomerization of E-pentadiene. UV.-absorption spectra and energy diagramm...

Though the triplet sensitized photolysis of isoprene (159) does, as noted above, produce a complex mixture of products, one of these adducts has been used in the context of complex molecule synthesis (equation 5)71. Cyclobutane 160, which was formed in ca 20% yield by the benzophenone sensitized photolysis of 159, could be easily transformed into fragrantolol, 161, an isomer of grandisol isolated from the roots of the Artemisia fragrans, by simple hydroboration/oxidation of the less hindered double bond. 

With m-nitroanisole in liquid ammonia the benzophenone-sensitized reaction yields inter alia 2-methoxy-4-nitroaniline as a product and no m-nitroaniline, which is formed in very high yield upon direct irradiation in liquid ammonia as well as in NH3/CH3OH. In the latter instance l/4> varies linearly with 1/[NH3], suggesting that the reaction is either singlet or triplet but not of a mixed type. 

As a separate issue, the ring-opening reaction of the spirooxazine and the spiropy-ran can be sensitized by triplet energy donors[73,113-115]. A typical absorption rise for a benzophenone-sensitized ring-opening reaction is compared to that of the unsensitized fast absorption rise for NOSI3 in Fig. 13 [73,113]. In this case, both solutions were optically matched at the excitation wavelength. 

In solution these esters undergo a variety of transformations which are dependent on the reaction conditions. In benzene, decomposition to carbon monoxide and carbonyl compounds is observed either upon direct irradiation94 or with benzophenone sensitization.33 In cyclohexane a complex product mixture is obtained.95 Addition of solvent to the carbonyl group is observed when the reaction is carried out in cyclohexene.54 At room temperature photoreduction takes place when the reaction is carried out in a secondary alcohol.96-97 However, in the case of the phenylglyoxylates quite a different reaction is observed when the reaction is carried out at elevated temperatures. The ester is reduced to the mandelate ester of the solvent alcohol, and the alcohol moiety of the ester is oxidized to the corresponding carbonyl compound. The pyruvates have not been studied at an elevated temperature. 

It has been shown that the benzophenone sensitized decomposition of benzoyl peroxide is due in part to formation of the benzophenone ketyl radical, which induces decomposition.98,99 Hydrocarbon sensitized peroxide decomposition is discussed in Section IV.A.4. The formation of benzonitrile from the benzophenone sensitized irradiation of benzalazine, which was originally attributed to hydrogen abstraction by benzophenone,100 actually results from a photooxidation.101... 

In a study of the benzophenone sensitized photoreduction of camphor-quinone (16), Monroe and Weiner108-110 found that the quantum yield of photoreduction decreased from 0.386 to 0.087 as [16] increased from 0.003 to 0.20M. 

A related phenomenon has been observed in the benzophenone sensitized isomerization of c/y-piperylene.150 The measured quantum yield of cis to trans isomerization increased from 0.55 to 0.90 as the concentration of piperylene increased from 0.08 to lOAf. This observation can be rationalized as arising from addition of the piperylene triplet to a ground state diene molecule to give a biradical intermediate which can either cyclize to the dimer151 or dissociate to give two molecules of the more thermodynamically stable trans-isomer. This mechanism predicts that the quantum yield for the isomerization of /runs-piperylene to cw-piperylene should decrease with increasing diene concentration, an experiment that has not yet been reported. 

Bicyclo[2.1.0]pentane (6) was also generated by benzophenone sensitized photolysis of2,3-diaz-abicyclo[2.2.1]hcpt-2-ene.15 The existence of the intermediate diradical in this reaction was substantiated by a trapping experiment.15... 

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