Irradiation photoelimination reaction

The photoelimination reactions seem to present the cleanest progress the products mostly do not absorb at the irradiation wavelength. These reactions are the prime candidates for verification of the kinetic relations [12,40]. In many other cases, photoracemization, side reactions or, if the primary products absorb at the irradiation wavelength, secondary photolysis occur and obscure the kinetics of CD development. The isomerization of E-cyclooctene 42 also is a well-defined reaction. The a vs. time plots show a maximum [103], but a photostationary state will be reached at long times as the Z — E isomerization is also active under irradiation. 

Representative of recent applications of the reaction to the synthesis of heterocycles are the photodehydrochlorination of chlorobenzo[b]thiophen (347) to give the fused pyrimidone 348,287 the photoelimination of HI from iodobenzene derivatives 349 to give the benzazepines 350,288 and the synthesis of the medium ring aza-heterocycle 351 by irradiation of the chloro precursor 352.289 Included among the many other examples of... 

The photoelimination of nitrogen from diazo compounds provides a simple and versatile route for the generation of carbenes, and in certain instances, insertion reactions of carbenes can be employed in the synthesis of heterocycles. Carbenes are believed to be involved at least in part in the photochemically induced conversion of N,N-diethyldiazoacetamide (439) into the y-lactam 440 and the /Mactam 441,365 and a similar approach has been successfully employed in the synthesis of a carbapen-2-em366 and of 7-methylcephalosporin analogues.367 Carbene insertion of a different type has been observed on irradiation of the 6-anilino-5-diazouracils 442 to give the indolo[2,3-d]pyrimidines 443.368 Ring contractions in heterocycles... 

The irradiation of a-keto-pentanoic acid in water is reported to produce butyroin,10 a reaction similar to that of pyruvic acid in water. The low yield may have been due to competing Type II photoelimination. 

MisceUaneous Reactions.—Details of the formation of nitrous oxide by photofragmentation of methyl nitrite have been reported.Photorearrangement is observed, however, on irradiation of the nitrites (183) derived from 6-methyl- and 4,4,6-trimethyl-cholest-5-en-3-ol to give the cyclic hydroxamic acids (184). There is ample precedent for these transformations, the likely pathway for which is outlined in Scheme 12. An alkoxyl radical (185) is also thought to be involved in the photochemicaUy induced conversion of O-nitrobenzoin (186) into benzaldehyde (187) and 2-phenylbenzo[b]furan (188). Reductive photoelimination of vicinal dinitro-groups takes place by... 

The generation and reactions of arylnitrenes continue to attract attention. Evidence for a photoinitiated autocatalytic chain mechanism in the photodecomposition of phenyl azide has been reported, and the reaction of photochemically generated phenyl nitrene with oxygen has been reexamined. Irradiation of p-azidoaniline in aqueous solution yields triplet p-aminophenyl nitrene, which on reaction with water is converted into the highly reactive p-benzoquinone diimine. Both m- and p-nitrophenyl azides, on photoelimination of nitrogen at 77 K, afford the corresponding nitrenes, whereas o-nitrophenyl azide (93) is converted without the intermediacy of a nitrene into the benzofurazan (94). 4,4 -... 

Aryl and heteroaryl azides are known to undergo ring expansion to azepines on photoelimination of nitrogen in the presence of nucleophiles. Evidence for benzazirine and 1-azacycloheptatetraene intermediates has previously been described and both species continue to be invoked to explain transformations of this type. The presence and nature of substituents appears to influence the course of the reaction, with electron-withdrawing substituents promoting azepine formation. o-Substituted aryl azides (102) are converted on irradiation in methanol-tetrahydrofuran into the 3-substituted 2-methoxy-3H-azepines (103). A pathway via the benzazirine (104) is proposed in Scheme 4, although... 

Diazo-compounds can in turn be generated photochemically from sodium salts of toluenesulphonyl hydrazones. Irradiation of the cyclobutane tosylhydrazone (44) gave trans-tricyclo[5.1.0.0 ]-octane (45 R=Me) by the pathway shown in Scheme 4 44 the tricycle (45 R=H) can also be obtained by photoelimination of nitrogen from the diazatricyclo[ 5.2.1.0 4]decene (46). The unexpected conversion of the anthracenocycloheptatriene derivative (47) into triptycene with loss of one carbon atom was observed on irradiation in tetrahydrofuran 4 the mechanism of this unusual reaction remains obscure and merits further investigation, but neither 1-triptycenyl nor 2-triptycenyl carbene appears to be an intermediate in this transformation. 

Homolysis of the S-acyl bond is responsible for formation of phenylthio-cyclohexane and 3-phenylthiocyclohexene from. SiS -diphenyl dithiocarbonate by irradiation in cyclohexene.117 A cyclic mechanism, however, has been proposed to account for the photodecomposition of certain carbohydrate dithiobisthio-formates.118 On irradiation, 0-ethyl thioacetate is converted into cis- and / 5-2,3-diethoxybut-2-ene, together with small amounts of 2,3-diethoxybut-l-ene, 2,3-diethoxybuta-1,3-diene, and 1-ethoxyethane-l-thiol.119 121 Free sulphur was trapped by reaction with 1,2-dimethyIenecyclohexane. Details of the photoelimination of sulphur dioxide from D-glucofuranosyl phenyl sulphone acetates have been published.180 An efficient synthesis of another 1,8-bridged naphthalene... 

These groups are further subdivided into various reactions including photooxidation, hydroperoxide formation and conversion, Norrish Type 1 II and other carbonyl and carboxyl group reactions, photo-Fries rearrangement, photosubstitution, photoaddition, photoelimination, photodimerization, photocondensation, and others. This paper will not elucidate these mechanisms except to note issues regarding spectral irradiance. 

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