Photochemistry of Thiophens

Photochemistry of Thiophens.—Wynberg and co-workers have continued their investigations on the photochemistry of thiophens, and an excellent essay by Wynberg on the photochemistry and other aspects of thiophen chemistry has recently been published. The photoisomerizations of thiophens are also discussed in a detailed review article on the photoisomerization of five-membered heterocyclics. Irradiation of 2-methyl-, 2-benzyl-, 2-neopentyl-, and 2-t-butyl-thiophens leads to their irreversible transformation to the corresponding 3-substituted derivatives in 8— 27% yields. 2,5-Di-t-butylthiophen rearranges irreversibly to the 2,4-isomer, which is remarkably stable under the reaction conditions. Irradiation of 2,2 -bithienyl leads to 2,3 -bithienyl and a small amount of benzo[6]thio-phen. 2,3 -Bithienyl affords 3,3 -bithienyl and benzo[6]thiophen as major products. 5,5 -Dideuterio-2,2 -bithienyl gives 5,5 -dideuterio-2,3 -bithienyl 

Lablache-Combier and M.-A. Remy, Bull. Soc. chim. France, 1971, 679. 

In an investigation on the benzophenone-sensitized photoaddition of citraconic anhydride and 2,3-dimethylmaleic anhydride to several thiophen and benzo[6]thiophen derivatives, it was found that, when 2,5-dimethyl-thiophen was used as substrate, only small amounts of the expected adduct (103) were formed, the main product being (104). Using only benzo-phenone in the photoaddition, the yield of (104) was 62%. The reaction 

irradiation of 3,4-diformylthiophen in CCI4 yields the lactone (109), whereas 2,3-diformylthiophen gives a mixture of (109a) and (109b). Upon irradiation, many 2-acylthiophens undergo 2 + 2 cycloaddition to simple olefins at the 2,3-position of the ring in the same way as carbocyclic 

Wassennan and W. Strehlow, Tetrahedron Letters, 1970, 795. isia Q pauhnier, J. Bourguignon, J. Morel, and P. Pastour, Compt. rend., 1970, 270, C, 494. 

Singlet oxygen reacted stereospecifically with 2-vinyl-thiophens in a 1,4-cycloaddition to give thermally stable 1,4-endoperoxides (147). The previously discovered [2 + 2] photoaddition of benzophenone to 2,5-dimethylthiophen to give oxetans has been extended to other carbonyl derivatives. Mechanisms explaining the formation of pyrroles, obtained by u.v. irradiation of a number of thiophens in the presence of propylamine, have been discussed. Irradiation of (148) led to the Dewar thiophen (149). The structures of dimerization products and Diels-Alder adducts with furans were elucidated.  

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The photochemistry of heterocyclic compounds was the object of a book published in 1976 and edited by Buchardt (76MI1). Here, the photochemistry of thiophene covers eight pages written by Lablache-Combier (76MI123). Some reviews have appeared since on the photochemical behavior of pyrrole and furan derivatives (89G419 96H(43)1305 96H(43) 1529) but apparently none on thiophene photochemistry. 

When standing in solution, a slow conversion of the thiophene S-oxide takes place and after some time a noticeable amount of the corresponding thiophenes is formed, among other products. The conversion is accelerated upon exposure to light. In order to understand the conversion processes under these conditions, the authors undertook a study on the photochemistry of thiophene S-oxide. 

The second chapter is concerned with the photochemical and photophysical behavior of thiophene derivatives and is written by Maurizio D Auria of the University of the Basilicata, Italy. The last readily available review of this topic is apparently the eight pages on the photochemistry of thiophene written by Lablache-Combier, which appeared in a book on the photochemistry of heterocyclic compounds published in 1976. In recent years, photochemical and photophysical properties of thiophenes have become of great importance in a variety of technical and commercial applications thus, the present chapter should fill an important place in the literature. 

Kiesewetter, R. and Margaretha, P. (1989) Photochemistry of thiophen-2(5H)-ones. Helvetica Chimica Acta, 72, 83—92. 

Thiophene absorbs very strongly below 2400 A, and the transition is probably of the n, n ) type. The photochemistry of thiophene is very complex but thorough investigation of the effects of wavelength of irradiation (2139 and 2288 A), temperature (25-305 °C), pressure, light intensities and inert and scavenger gases such as O2 has shown that at least three intermediates lead to the observed products , viz. 

The photochemistry of organosulfur compounds in general has been the subject both of annual reports [1,2] and of timely scattered reviews [3-5], Light-induced reactions of specific chromophores, such as sulfoxides and sulfones [6], thiocarbonyl compounds [7], and thiols [8] have been discussed in more detail, as has the photochemistry of thiophenes and certain other specific ring systems [9,10],... 

Excited potential energy curves of higher multiplicities may also be involved in the UV photochemistry of thiophene. In the future, the detailed examination of the UV photodissociation dynamics of thiophene will require a higher-level theoretical study. 

Kellogg, R.M., and H. Wynberg The Photochemistry of Thiophenes. V. Investigation of Phenylthiophene Photorearrangements by Deuterium Labeling Techniques. J. Amer. Chem. Soc. 89, 3495 (1967). 

Kellogg, R.M., Photochemistry of thiophenes. X. Model for the photochemically induced valence bond isomerizations of thiophenes, Tetrahedron Lett., 1429, 1972. 

To study the photochemistry of the dithiins 51b (Scheme 13) in solution, the compounds were irradiated with visible light at —60 to —75 °C to afford the thiiranes 118 in excellent yields which were characterized by H and NMR spectroscopy. Upon warming or further exposure to light, the thiiranes 118 afforded the thiophenes 119 (Scheme 29) <2000MI159, 1996JA4719>. 

Valence isomer formation is a feature also of the photochemistry of naphthalenes (3.33) and anthracenes for naphthalenes, as for benzenes, the extent ol steric crowding helps to determine which type of valence isomer predominates, since there is more severe interaction in the bicyclohexadiene products than in the benzvalene products. Amongst five-membered heteroaromalic compounds there are many known ring photoisomerizations that involve conversion of a 2-substituled to a 3-substituted system (e.g. 3.34). In some cases non-aromatic products can be isolated, such as bicyclo[2.1.0]pentene analogues from thiophenes 13.35). or acylcycfopropenes from furans (3.36) related species may be... 

This, the eleventh volume of Advances in Heterocyclic Chemistry, includes surveys of the chemistry of the following groups of heterocyclic compounds benzo[6]thiophenes (B. Iddon and ft. M. Scrow-ston), naphthyridines (W. W. Paudler and T. J. Kress), and quinu-clidines (L, N. Yakhontov). In addition, R. A. Jones covers the application of physical methods to pyrrole chemistry and a very topical subject, the photochemistry of heterocycles, is reviewed by S. T. Reid. 

The photochemistry of trisilanes 16 and 19 has been investigated in some detail (Schemes 1 and 2)21. Upon irradiation of compound 16 only the Si—SiMe2Ph bond is broken and the initially formed silyl radical 17 undergoes a rearrangement to the more stable silacyclobutenyl radical 18 whose EPR spectrum has been recorded (Scheme l)21a. Irradiation of trisilane 19 with a medium pressure mercury lamp resulted in the formation of hexamethyldisilane, 2-(trimethylsilyl)thiophene and 20, with 20 dominating (Scheme 2)21b. In the presence of carbon tetrachloride, a significant yield (19.2%) of the... 

Effenberger and Wonner63 have described the preparation and photochemistry of an anthrylvinyl thiophen fulgide (51). 

The photochemistry of substituted thiophenes has recently opened up a new and stimulating field of reaserch in heterocyclic compounds, due to the efforts of Wynberg et al.. The photochemical rearrangement of 2-phenylthiophene into 3-phenylthiophene... 

A wide variety of photoreactions have been reported in sulphur-containing compounds, with particular attention being paid to the photochemistry of thiones. Electrocyclic ring closure to the eyelobutabenzo[b]thiophenes (131) is the principal reaction observed on irradiation of the mono-substituted 1-benzo-thiepines (132). Photorearrangement of the 2-methyl-1,5-benzo-thiazepines (133) takes a different course and affords the isomeric... 

Photochemistry of Monocyclic Thiophens. - The HCCS radical has been identified, by theoretical calculations, as a transient species during the flash photolysis of thiophen. Various physical measurements, such as differential scanning calorimetry, thermogravimetry, and mass-spectral studies, have been... 

The format of this Chapter follows that used previously in this series with the exception that the current literature concerning the photochemistry of furan, thiophen, and pyrrole is now considered by reaction type in the appropriate section. 

The photochemistry of monocyclic a,p-unsaturated thiolactones has only been investigated recently. One reason for this is that the first syntheses of 2(5//)-thiophenones from thiophenes were only developed in the late 1960s and early 1970s and that the first report on a 5,6-dihydrothiin-2-one was published even later [60]. 

Topics reviewed during the year include the photochemistry of indoles, sulfoxides, pyrazoles and isothiazoles, (S-hetero)cyclic unsaturated carbonyl compounds, photoinduced single electron transfer (SET) reactions of amines and of azo compounds, SET reactions of organosilanes and organostannanes with Qo and ketones, photochromic polypeptides and di(hetero)arylethenes, processes in chromophore sequences on a-helical polypeptides,aryl-aryl coupling in furans, thiophenes and pyrroles," [3+2]cycloaddition of aromatic nitriles (and esters) with alkenes, and reactions of benzylsilane derivatives. ... 

Thiophene-1-oxides Thiophene-1,1 -dioxides. 2.1 Reaction with nucleophiles. 2.2 Cycloaddition reactions. 2.3 Other reactions Thiophene S,N-ylides and S,C-ylides. 3.1 Cycloaddition reactions of S,N-ylides and S,C-ylides. 3.2 Cyclopropanation reactions of S,C-ylides. 3.3 Thermal transformations of thiophene S,N- and S,C-ylides Oxothiophenes Tautomeric with Hydroxythiophenes. 4.1 Tautomerism. 4.2 Alkylation and acylation. 4.3 Condensation reactions at the methylene group. 4.4 Halogenation. 4.5 Oxidative coupling. 4.6 Cycloaddition. 4.7 2,3-Diones 4 Sulfoxides and sulfones. 4.9 Photochemistry. 4.10 Macrocyclic polyethers... 

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