Irradiation/photolysis

The overall reaction process consists of three contributions direct oxidation by UV irradiation (photolysis), direct oxidation by ozone (ozonation), and oxidation by free radicals ... 

Useful synthetic methodologies are based on the cyclization, rearrangement, or fragmentation of the alkoxy radicals generated in the reaction of alcohols with [bis(acyloxy)iodo]arenes in the presence of iodine under photochemical conditions or in the absence of irradiation. Photolysis of PhI(OAc)2 with cyclic alcohols in the presence of iodine leads to the generation of the respective alkoxy radical, which can undergo various... 

Mechanism for Chemical Surface Modification. Since PTFE films having no significant absorption at 193 nm were not excited upon the irradiation with an ArF laser, hydrazine is only photodissociated by the irradiation. Photolysis of hydrazine upon the ArF irradiation has been investigated in detail by Hawkins (31) and Lindberg (32). The irradiation of hydrazine in a gas phase produces reactive species such as N2H3, NH2, and H radicals and NH nitrene in high efficiency. By assuming that these species attack the PTFE surface, we studied a preliminary reaction path for these reactive species and perfluorocarbons, that is, which species can react with perfluoroalkyl molecules. 

Hz( = c/s) Hertz (=cycles per second) UV-irr., irradiation, photolysis by UV light... 

In sharp contrast with OSl, compound OS2 does not cleave dinitrogen. Instead, it evolves N2 gas, and, whilst the compound is somewhat thermally unstable and slowly releases nitrogen even without irradiation, photolysis causes violent evolution of N2. The difference between the reactivity of OSl and OS2 can be explained by differences in the energies of their MLCT states the MLCT (Os N2) is considerably higher in energy than Os° N2 MLCT, and is not... 

A reverse mechanism may occur upon irradiation (photolysis or radiolysis) of a. semiconducting oxide (hematite, magnetite, Ti02, ZnO, WO3, etc.) [79]. Electron-hole pairs are created in the colloid and sent into the conduction band. Fast recombination can be prevented by adsorbed species such as anions or neutral molecules acting as traps for electrons or holes. Polyvinyl alcohol strongly adsorbed on Ti02 traps the holes, and the lifetime of the electron in the colloid ranges from minutes to hours. They can react with species in solution and, for example, reduce tetranitromethane [80] ... 

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.

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

Electrical Discharge, Irradiation, and Photolysis. Early reports of the decomposition of SiH in an electrical discharge indicated that the main products were hydrogen, soHd sHicon subhydrides of composition SiH 2 i smaH quantities of higher silanes (37). However, more recent... 

Pyridazin-3(2H)-ones rearrange to l-amino-3-pyrrolin-2-ones (29) and (30) upon irradiation in neutral methanol (Scheme 10), while photolysis of 5-amino-4-chloro-2-phenylpyridazin-3(2H)-one gives the intermediate (31) which cyclizes readily to the bis-pyridazinopyrazine derivative (32 Scheme 11). 

Oxidopyridazinium betaines isomerize photochemically into pyrimidin-4(3H)-ones (33). Irradiation of 3-oxidopyridazinium betaine or 1-oxidophthalazinium betaine in water affords similarly the corresponding pyridazin-3(2H)-one (35) and phthalazin-l(2H)-one derivative (37). However, photolysis in acetonitrile affords stable diaziridines (34) and (36) which can be converted in the presence of water to the final products (35) and (37) (Scheme 12) (79JCS(P1)1199). 

The light-induced rearrangement of 2-phenyl- to 3-phenyl-thiophene may occur by a similar mechanism an equilibrium between the bicyclic intermediate (26) and the cyclopro-penylthioaldehyde (27) has been suggested (Scheme 2). The formation of IV-substituted pyrroles on irradiation of either furans or thiophenes in the presence of a primary amine supports this suggestion (Scheme 3). Irradiation of 2-phenylselenophene yields, in addition to 3-phenylselenophene, the enyne PhC=C—CH=CH2 and selenium. Photolysis of 2-phenyltellurophene furnishes solely the enyne and tellurium (76JOM(108)183). 

Photolysis of 2,3-diphenyl-A -azirine (442) generates benzonitrile ylide (443). Irradiation in the presence of ethyl cyanoformate resulted in a mixture of the oxazoline (444) and the imidazole (445) by 1,3-dlpolar cycloaddition to the carbonyl and nitrile group, respectively (72HCA919). 

Photolysis of spiro[fluorene-9,3 -indazole] (384) to the tribenzopentalene (385) has been rationalized in terms of the initial formation of triplet diradical (386) (76JOC2120). The spiroindazole (387) behaves differently and on irradiation in THF is converted into the dimer (388) and the stable iV-ylide (389) (76CB2596). 

The photolysis of 4-substituted 2,3-dimethyl-3-isoxazolin-5-ones has been studied. Irradiation in methanol or ethanol with a 100 W high-pressure mercury lamp through a Pyrex filter of a 4-phenylthio compound produced a semithioacetal (Scheme 5). In contrast, an H, Cl or OPh moiety gave no reaction. The use of alkylthio substitution gave similar products. Cyclic compounds yielded cyclic products (Scheme 5), and the photolysis of (29) in benzene... 

The photolysis of 3-( p-cyanophenyl)-2-isoxazoline in benzene produced a tricyclic product along with six other materials (Scheme 46) (B-79MI41616). Irradiation of the bicyclic 2-isoxazoline (155) produced benzonitrile, /3-cyanonaphthalene and polymer via a proposed biradical intermediate (156) (Scheme 47) (B-79MI41615). 

Whereas the cycloaddition of arylazirines with simple alkenes produces A -pyrrolines, a rearranged isomer can be formed when the alkene and the azirine moieties are suitably arranged in the same molecule. This type of intramolecular photocycloaddition was first detected using 2-vinyl-substituted azirines (75JA4682). Irradiation of azirine (54) in benzene afforded a 2,3-disubstituted pyrrole (55), while thermolysis gave a 2,5-disubstituted pyrrole (56). Photolysis of azirine (57) proceeded similarly and gave 1,2-diphenylimidazole (58) as the exclusive photoproduct. This stands in marked contrast to the thermal reaction of (57) which afforded 1,3-diphenylpyrazole (59) as the only product. 

Nitrile ylides derived from the photolysis of 1-azirines have also been found to undergo a novel intramolecular 1,1-cycloaddition reaction (75JA3862). Irradiation of (65) gave a 1 1 mixture of azabicyclohexenes (67) and (68). On further irradiation (67) was quantitatively isomerized to (68). Photolysis of (65) in the presence of excess dimethyl acetylenedicar-boxylate resulted in the 1,3-dipolar trapping of the normal nitrile ylide. Under these conditions, the formation of azabicyclohexenes (67) and (68) was entirely suppressed. The photoreaction of the closely related methyl-substituted azirine (65b) gave azabicyclohexene (68b) as the primary photoproduct. The formation of the thermodynamically less favored endo isomer, i.e. (68b), corresponds to a complete inversion of stereochemistry about the TT-system in the cycloaddition process. 

Studies on the UV irradiation of the 1-phthalimidoaziridine system have been carried out (71JCS(C)988). When the photolysis was performed in the presence of an alkene, an exchange reaction was observed. Evidence was presented which showed that the reaction involved concerted fragmentation of the phthalimidoaziridine (114) to an alkene and... 

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