Photocycloaddition, thiophene

The benzo[Z)] fused systems participate in a number of [2 + 2] cycloaddition reactions (81JOC3939, 81TL521). The photocycloaddition products of benzo[Z)]thiophenes and DMAD are dependent on the irradiation wavelength (Scheme 56). 

Sensitized photocycloaddition of thiophene and 2,5-dimethylthiophene to acetylenedicar-boxylic ester leads to the phthalic esters (303) (73CB674). In the mechanism suggested, the triplet state of the thiophene adds to the alkyne to form a biradical, which cyclizes to the... 

Thiophene on irradiation in presence of dihalogenomaleimides leads to low yields of the 2-substituted products (Scheme 89) (78TL125). When benzo[6]thiophene is irradiated in the presence of dibromomaleic anhydride, both photosubstitution and photocycloaddition are observed (Scheme 90) (77BCJ1797). The ratio of photosubstitution relative to cycloaddition increases with the temperature and polarity of the solvent. A common triplet exciplex has been suggested for both processes. 

In addition to furan, other heterocycles have been examined." Thiophene undergoes efficient photocycloaddition with benzaldehyde to afford a single exo photoproduct (183) in 60% yield. As reported by Jones and coworkers,the photolysis of IV-methylpyrrole in the presence of aldehydes or ketones yields the corresponding 3-hydroxyalkyl derivative (184), even when the reaction mixture is free from any trace of acid. In order to use the pyrrole nucleus for stereoselective alkaloid synthesis (cf. caesalpinine, 185) in the fashion developed with the furan nucleus, pyrrole substituents that stabilize the presumed intermediate bicyclic oxetane must be discovered. 

A further report of the photocycloaddition reactions of benzothiophenes has been made. The two photoproducts (199) and (200) from the benzophenone-sensitized cycloaddition of bromoethene to the 2,3-dichloro-derivative (201a) have been demonstrated to have the stereochemistry shown.The photoaddition of cis-dichloroethene to (201a) is stereoselective and affords predominantly the adduct (202). When 2,3-dibromobenzothiophene (201b) is used with bromoethane, a different reaction path is followed, yeilding (203). Linear products are also obtained with dichloroethene and (201b). [2 + 2] Photocycloaddition of furan or thiophene to the iso-oxazolines (204) affords the adducts... 

A number of examples of photocycloaddition to sulphur-containing heterocycles have been reported. 2,3-Dichlorobenzo(b]thiophene (226) undergoes [ 2 + 2] benzophenone-sensitized photoaddition to vinyl bromide to give the stereoisomeric adducts (227). A potentially more useful cycloaddition has been described for 4-methoxy-l-thiocoumarin (228) which with isobutene, for example, affords the adduct (229). The structures assigned to adducts of... 

Bolivar, R. A., Rivas, C. Quencher effect of thiophene and its monomethyl derivatives on photoreduction and photocycloaddition reactions of ketones. J. Photochem. 1982,19, 95-99. 

The sensitized photocycloaddition of maleic anhydride derivatives to thiophenes to give the exo adducts (76) has been known for... 

Benzo[b]thiophene 1,1-dioxide 38 undergoes [2+2] photocycloaddition with vinyl acetate [120] and l-(diethylamino)propyne [121] to give the corresponding adducts in excellent yields (Scheme 22). The reaction appears synthetically important, and its scope and limitation deserve examination in detail. 

Several novel photocycloaddition processes together with further examples of known additions have been reported in sulphur-containing systems. [ 2 + 2]Dimerization is the principal photoreaction observed in 2- and 3-arylbenzo[Z ]thiophens, and a mixture of all four [,2 +, 2] photodimers was... 

Cycloadditions involving side-chain n-systems in addition to the thiophene double bond. 9.3 Photocycloadditions. 9.4 Cycloadditions of 3,4-dimethylenethiophene Reactions of Strained Thiophenes... 

The highly twisted Ti-conjugated macrocycle (184) with two allq nyl moieties underwent intramolecular [2 + 2] photocycloaddition to give the thiophene-fused bisdehydro[12]annulene (185) in 61% yield. Photolysis of the bis(dithienyl)ethynes (186) in the presence of iodine resulted in sequential electrophilic and photochemical cyclizations to yield the tetrathienonaphthalenes (183) in one pot reactions. The compounds (187) showed a significant potential as a cruciform scaffold for nanostructured Ji-electron materials. The alkenyl-substituted bis(dithienyl)ethynes (188) underwent p-benzoquinone photosensitized double 5-exo-cyclization to give the diarylated dithienofulvalenes (189). The atylpyridinylethynes (190) and (191) in aqueous HCl solution underwent photo-dehydro-Diels-Alder reaction to afford... 

Photochemistiy of Thiophens.—A model for the photochemically induced valence-bond isomerizations of thiophens has been proposed. Irradiation of the thiophenium ion (126), obtained by protonation of thiophens with fluorosulphonic acid at low temperature, led to no reactimethylated derivatives (127) smoothly gave migration to (128). Both thiophen and 2,5-dimethylthiophen give photocycloaddition... 

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