Direct irradiation

The chemical pathways leading to acid generation for both direct irradiation and photosensitization (both electron transfer and triplet mechanisms) are complex and at present not fully characterized. Radicals, cations, and radical cations aH have been proposed as reactive intermediates, with the latter two species beHeved to be sources of the photogenerated acid (Fig. 20) (53). In the case of electron-transfer photosensitization, aromatic radical cations (generated from the photosensitizer) are beHeved to be a proton source as weU (54). 

Fig. 20. Proposed photochemical mechanisms for the generation of acid from sulfonium salt photolysis. Shown ate examples illustrating photon absorption by the onium salt (direct irradiation) as well as electron transfer sensitization, initiated by irradiation of an aromatic hydrocarbon.

A principal appHcation for photomedicine is the photodynamic treatment of cancer. Photochemical and clinical aspects of this topic have been reviewed (10,11). Direct irradiation of tumors coupled with adininistration of a sensitizer is used to effect necrosis of the malignancy. In this process, an excited state sensitizer interacts with dissolved in vivo to effect conversion of the oxygen from its triplet ground state to an excited singlet state, which is highly cytotoxic. In principle, excited sensitizers in either the singlet or the triplet state can effect this conversion of molecular oxygen (8). In... 

Ketones in which the double bond is located in the p,y position are likely candidates for a-cleavage because of the stability of the allyl radical that is formed. This is an important process on direct irradiation. Products then arise by recombination of the radicals or by recombination after decarbonylation. 

On direct irradiation of 1,3-pentadiene, Z-E isomerization is accompanied by cyclization to 1,3-dimethylcyclopropene and 3-methylcyclobutene ... 

Diphenylcyclohexadiene shows divergent photochemical behavior, depending on whether the reaction is induced by direct irradiation or by photosensitization. On direct irradiation, the electrocyclic conversion to 1,1-diphenylhexatriene is dominant, whereas a... 

Predict the structure, including all aspects of stereochemistry, for the product expected to result from direct irradiation of each compound. 

The reaction products are the same for both direct irradiation and acetophenone sensitization. When the reactant B is used in homochiral form, the product D is nearly racemic (6% e.e.). Relate the formation of the cyclobutanones to the more normal products of type E and E Why does the 5-aryl substituent favor formation of the cyclobutanones Give a complete mechanism for formation of D which is consistent with the stereochemical result. 

The direct irradiation of A gives predominantly B, but the photosensitized reaction gives more C. Explain. 

In the case of the direct irradiation, the singlet excited state is populated, hence the formation of the Dewar furan is energetically possible (Fig. 1). This result is in agreement both with the evidence for the formation of the Dewar furan in the direct irradiation and with the formation of isomeric furans. 

Mercury-sensitized irradiation of 3-methylfuran gave 2-butyne, 1,2-pentadiene, 1-methylcyclopropene, 1-butyne, and 1,3-butadiene (Scheme 6) (68JA2720 70JPC574). In the direct irradiation, 1-methylcyclopropene was obtained in lower yields while both 3-methylcyclopropene and 2-methylfuran appeared (Scheme 6) (70JPC574). 

By contrast, direct irradiation of 2,5-dimethylfuran gave 2,4-dimethylfuran as the only isolated product (68JA2720). 

When pyrrole is irradiated, only decomposition products were obtained. Theoretical data can fit this statement (Fig. 6). In fact, the direct irradiation populates the excited singlet state, which can be converted into the Dewar pyrrole or into the corresponding triplet state. Clearly, the intersystem crossing to the triplet state allows the system to reach the lowest energy state. The excited triplet state can give the biradical intermediate, and this intermediate can give either the decomposition... 

Direct irradiation of substituted 1,4-dihydro-1,4-epoxy naphthalenes gives small quantities of 3-benzoxepins 16 amongst other products.144 146,256 The primary product of this reaction is probably a 3-oxaquadricyclane derivative which can react to give the benzoxepin under thermal conditions. The formation of benzoylindene compounds as the main products from the photochemical rearrangement of l,4-diphenyl-l,4-dihydro-l,4-epoxynaphthalenes has been shown to proceed via the corresponding 3-benzoxepins.146 The irradiation of naphthalene 1,4-oxides in the presence of a photosensitizer did not lead to benzoxepin derivatives.144... 

Intramolecular [2 + 2] photocycloadditions of alkenes is an important method of formation of compounds containing four-membered rings.184 Direct irradiation of simple nonconjugated dienes leads to cyclobutanes.185 Strain makes the reaction unfavorable for 1,4-dienes but when the alkene units are separated by at least two carbon atoms cycloaddition becomes possible. 

Zimmerman and co-workers were also able to obtain some information regarding the multiplicities of the excited states responsible for the initial /9-cleavage through quenching and sensitization studies. It was found that both trans-to-cis and cis-to-trans isomerizations could be sensitized by chlorobenzene under conditions where the latter absorbed over 95% of the light. The same product ratio was obtained under these conditions as in the direct irradiation of the ketones. With 1,3-cyclohexadiene or 2,5-dimethyl-2,4-hexadiene as quenchers nearly 90% of the reaction of the trans isomer could be quenched. Again the ratio of the quenched reaction products was the same as in the unquenched reaction. The reaction of the cis isomer, on the other hand, could not be quenched by 1,3-cyclohexadiene or 2,5-dimethyl-2,4-... 

In addition,<1,48,48) it was noted that whereas the azulene effect on the sensitized reaction is sensitive to changes in solvent viscosity, the azulene effect on the direct photoreaction was independent of solvent viscosity, as would be predicted for Forster-type energy transfer. The inescapable conclusion is that cis-trans isomerization upon direct irradiation of stilbene takes place in the singlet manifold. 

The direct irradiation of cis- and frans-2-butenes in solution separately and in admixture gives stereospecific dimerization along with the more efficient cis-trans isomerization<78) ... 

The yields of cyclopropanes in this case are low in relation to the amount of acetophenone formed. However, similar cyclopropane product ratios are obtained when photolysis is carried out in the presence of Michler s ketone as sensitizer. Thus the carbene intermediate produced in the direct irradiation is thought to be a triplet, as suggested by the nonstereospecificity of its addition. Whether this intermediate arose from singlet diazoacetophenone (via singlet decomposition and intersystem crossing of the singlet carbene) or by decomposition of the triplet molecule was not determined. 

One of the first examples of photodimerization of an a,/3-unsaturated carbonyl was that of dibenzylideneacetone reported by Ciamician and Silber. Direct irradiation of this compound in ethanol or in isopropanol/benzene solution yields cyclobutane (66), although irradiation in the presence of uranyl chloride as sensitizer results in (67)(81) ... 

Nitrones in general undergo a four-electron photocyclization to afford the corresponding oxaziridines. The process is stereospecific,47 proceeds via the excited singlet state, and is in certain instances photochemically reversible.48 Theoretical studies support this proposed pathway49 The nitrones 52, on direct irradiation, afford the oxaziridines 53, which on further irradiation are converted into the isomeric amides 54.so In contrast, triplet... 

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