9.10- dicyanoanthracene as sensitizer

Irradiation of 2-phenylbenztriazole in aerated solvents leads remarkably to cleavage of the benzene ring and formation of 2-phenyl-l,2,3-triazole-4,5-dicaiboxylic acid (Amiet et al.). Ylidions, a new class of reactive intermediates, have been generated by photo-oxidation of phenacylsulphonium ylides using 9,10-dicyanoanthracene as sensitizer (Zhang and Schuster). 

The photooxygenation of 2,3,5,6-tetraphenyl-1,4-dioxin (19) in an acetonitrile-dichloromethane mixture, using 2,4,4,6-tetrabromocyclohexa-2,5-dienone or 9,10-dicyanoanthracene as sensitizer, yields Z-stilbenediol benzoate together with benzil <85TL4383,86MI609-01 >. 

Intramolecular photocyclization between the vinyl moieties of (189) occurs using 9,10-dicyanoanthracene as an electron transfer sensitizer. In general, the route leads to the moderately efficient synthesis of the macrocyclic ring system (190, 191). The stereoselectivity shown by the reaction is, in some instances, dependent on the solvent used. Thus with (189a, b, and c), the cis-isomers (190) were the major products with acetonitrile as solvent, but with benzene the frans-isomer (191) predominated. In the case of (189d, e, and f), the trans-isomers predominate with both solvents. 

As described above, upon photoexcitation, stilbene undergoes a two-way isomerization. However, upon 9,10-dicyanoanthracene (DCA) sensitization in acetonitrile, cis-stilbenes undergo essentially one-way isomerization via a radical cation chain mechanism. Either a unimolecular cis - - trans - or a bimolecular process involving addition of cis - to the neutral stilbene may lead to trans isomers [139-156]. 

The photooxidation of tertiary methylamines sensitized by electron acceptors such as 9,10-dicyanoanthracene in the presence of lithium perchlorate results in demethylation thus tropinone yields nortropinone257. Photoinduced cyanation of tertiary amines with oxygen, a sensitizer and trimethylsilyl cyanide results in a-cyano nitriles (equation 88)258. 

Although cyclic azoalkanes are well known as biradical precursors [159] they have been used as 1,2- and 1,3-radical cation precursors only recently [160-164]. Apart from the rearrangement products bicyclopentane 161 and cyclopentene 163, the PET-oxidation of bicyclic azoalkane 158 yields mostly unsaturated spirocyclic products [165]. Common sensitizers are triphenyl-pyrylium tetrafluoroborate and 9,10-dicyanoanthracene with biphenyl as a cosensitizer. The ethers 164 and 165 represent trapping products of the proposed 1,2-radical cation 162. Comparison of the PET chemistry of the azoalkane 158 and the corresponding bicyclopentane 161 additionally supports the notion that the non-rearranged diazenyl radical cation 159 is involved (Scheme 31). 

The 9,10-dicyanoanthracene sensitized irradiation of c/i-stilbene results in nearly quantitative isomerization (>98%) to the trans isomer with quantum yields greater than unity. Therefore, the isomerization was formulated as a free radical cation chain mechanism with two key features (1) rearrangement of the c/i-stilbene radical cation and (2) electron transfer from the unreacted cis-olefin to the rearranged (trans-) radical cation. 

Irradiation of oxadisilirane 82c in an acetonitrile-methylene chloride solvent mixture in the presence of 9,10-dicyanoanthracene (DCA) as photosensitizer with a tungsten-halogen lamp under an oxygen flow resulted in the formation of 83c in 69% yield. In the absence of the sensitizer, no reaction occurred and 82c was recovered quantitatively. A similar result was also obtained in the reaction of 82c with 302 in the presence of 10 mol% of tris(p-bromophenyl)aminium hexachloroantimonate [ /j-BrCr,H/ (N+SbClfU (BPHA) as the single ET reagent, which gave 83c in 58% yield (Table 17)97. 

The anti-Markownlkov orientation of addition in the presence of electron-acceptor sensitizers applies also to intramolecular reaction, and 5,5-dipheny pent-4-en-1-ol gives a tetrahydrofuran (2.SI) when irradiated in solution with 9,10-dicyanoanthracene, whereas its thermal reaction under proton-acid catalysis leads to 2,2-diphenyltetrahydropyran by Markownikov addition. Sometimes an added sensitizer is not required, if the alkene itself can act as a good electron-donor or electron-acceptor, and this is likely to be the reason why 1-lo-methoxyphenyl)propene adds photochemically to acetic acid (2.52), whereas l-phenylpropene does not. 

As in the dimerization of 1,3-cyclohexadienes (Sect. 2.4), the concentration dependence confirms the proposed mechanism in this case, too. Other acceptors, which favor ion pair separation, support the formation of the escape products 32 and 33. For example, with the pyrylium salt 22 as acceptor, the eage/escape ratio (31/32 + 33) decreases from 0.65 to 0.04 compared to the PET reaction using 9,10-dicyanoanthracene (28a) as electron transfer sensitizer [84]. This behaviour... 

A similar radical cyclization reaction of unsaturated amino acid derivatives has been recently reported by Steckhan et al. [36]. The PET-catalyzed cyclization reaction proceeds under mild conditions using 9,10-dicyanoanthracene (DCA) as sensitizer and biphenyl (BP) as co-sensitizer (Sch. 14). The diastereoselectivity of the cyclization step was found to be moderate to high depending on the substitution pattern of the starting material. In almost all cases examined, the raws-diastereoisomer was predominately formed. 

Significant degradation of compounds can also be obtained in the presence of electron-acceptor sensitizers. In a very recent example, triadimenol, a systemic pesticide widely applied in horticulture and viticulture that is very difficult to degrade by direct UV photolysis, could be significantly decomposed in the presence of electron acceptors such as 9,10-dicyanoanthracene or 2,4,6-triphenylpyrylium tetrafluoroborate. Decomposition was accelerated by the presence of oxygen [17]. 

For example, with the same singlet-excited sensitizer, i.e., 9,10-dicyanoanthracene DCA [Ered = —0.89 V vs SCE AEo 0 = 2.94 eV] [84] and two different electron donors, the quantum yield for the formation of separated radical ions increases from 0.03 to 0.3 as the exothermicity for back electron-transfer, in relation to the donor s oxidation potentials, increases from 2.1 to 2.8 eV. 

Redox photosensitization or co-sensitization by aromatic hydrocarbons has been utilized for enhancement of the efficiency of photoinduced electron transfer reactions. For example, the efficiency of the 9,10-dicyanoanthracene-sensitized photooxygenation of 1,2-diphenyloxirane in acetonitrile is enhanced appreciably by adding biphenyl as a co-sensitizer, giving 3,5-diphenyl-1,2,4-trioxolane in good yield [31-32]. This photoreaction does not take place in the absence of biphenyl. Schaap proposed that in this photoreaction the primary electron transfer reaction occurs from biphenyl (BP) to DCA to produce biphenyl radical cation BP and DCA . The secondary electron transfer from the oxirane to BP produces BP and the radical cation of the oxirane which is converted into the trioxolane (Scheme 5). 

Besides cyclobutane formation, alternative ring closures are sometimes observed. One example is the 9,10-dicyanoanthracene sensitized dimerization of 1,1-diphenylethylene (169). The six-membered ring is formed via the 1,4-radical cation, which results from the addition of the free radical cation to diphenylethylene as indicated in Scheme 56, while the 1,4-biradical generated by back electron transfer from the radical ion pair yields tetraphen-ylcyclobutane (170) (Mattes and Farid, 1983). 

There are many examples of such reactivity and some of these have been reviewed by Roth and coworkers, a research group that is extremely active in this area. An example that is typical of the processes encountered involves the cyclization of the diene geraniol (1). In this case the sensitizer is 9,10-dicyanoanthracene (DCA) and the reactions are carried out in methylene chloride. The authors state that a contact radical-ion parr is involved, i.e. the radical cation of the diene is in close proximity to the radical anion of the DCA. Reaction within this yields the cyclopentane derivatives 2 and 3 in the yields shown. The ring formation is the result of a five centre CC cyclization within the radical cation of 1. When a more powerful oxidant such as p-dicyanobenzene is used as the sensitizer in acetonitrile as solvent, separated radical-ion pairs are involved. This leads to intramolecular trapping and the formation of the bicyclic ethers 4 and 5 . The bicyclic ether incorporates an aryl group by reaction of the radical cation of the diene with the radical anion of the sensitizer (DCB). This type of reactivity is referred to later. Other naturally occurring compounds such as (/fj-f-bj-a-terpineol (6) and (R)-(- -)-limonene (7)... 

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