Of selenophenes

The photochemically induced [2 + 2] cycloaddition of selenophene with dimethylmaleic anhydride gives a 1 1 adduct (140), but attempts to form an oxetane by photoreaction with benzophenone failed (80JHC1151). 

The direct combination of selenium and acetylene provides the most convenient source of selenophene (76JHC1319). Lesser amounts of many other compounds are formed concurrently and include 2- and 3-alkylselenophenes, benzo[6]selenophene and isomeric selenoloselenophenes (76CS(10)159). The commercial availability of thiophene makes comparable reactions of little interest for the obtention of the parent heterocycle in the laboratory. However, the reaction of substituted acetylenes with morpholinyl disulfide is of some synthetic value. The process, which appears to entail the initial formation of thionitroxyl radicals, converts phenylacetylene into a 3 1 mixture of 2,4- and 2,5-diphenylthiophene, methyl propiolate into dimethyl thiophene-2,5-dicarboxylate, and ethyl phenylpropiolate into diethyl 3,4-diphenylthiophene-2,5-dicarboxylate (Scheme 83a) (77TL3413). Dimethyl thiophene-2,4-dicarboxylate is obtained from methyl propiolate by treatment with dimethyl sulfoxide and thionyl chloride (Scheme 83b) (66CB1558). The rhodium carbonyl catalyzed carbonylation of alkynes in alcohols provides 5-alkoxy-2(5//)-furanones (Scheme 83c) (81CL993). The inclusion of ethylene provides 5-ethyl-2(5//)-furanones instead (82NKK242). The nickel acetate catalyzed addition of r-butyl isocyanide to alkynes provides access to 2-aminopyrroles (Scheme 83d) (70S593). 

Thiyl radicals and their analogs in gas-phase syntheses and transformations of compounds of selenophene series 99ZOR11. 

The C-Se and C-Te bonds are formed by an internal homolytic substitution of vinyl or aryl radicals at selenium or tellurium with the preparation of selenophenes and tellurophenes, respectively. An example is shown below, where (TMSIsSiH was used in the cyclization of vinyl iodide 65 that affords... 

Thiophenes continue to play a major role in commercial applications as well as basic research. In addition to its aromatic properties that make it a useful replacement for benzene in small molecule syntheses, thiophene is a key element in superconductors, photochemical switches and polymers. The presence of sulfur-containing components (especially thiophene and benzothiophene) in crude petroleum requires development of new catalysts to promote their removal (hydrodesulfurization, HDS) at refineries. Interspersed with these commercial applications, basic research on thiophene has continued to study its role in electrocyclic reactions, newer routes for its formation and substitution and new derivatives of therapeutic potential. New reports of selenophenes and tellurophenes continue to be modest in number. 

This chapter follows the organization used in the past. A summary of the electronic properties leads into reports of electrocyclic chemistry. Recent reports of studies of HDS processes and catalysts are then summarized. Thiophene ring substitution reactions, ring-forming reactions, the formation of ring-annelated derivatives, and the use of thiophene molecules as intermediates are then reported. Applications of thiophene and its derivatives in polymers and in other small molecules of interest are highlighted. Finally, the few examples of selenophenes and tellurophenes reported in the past year are noted. 

Although the first selenophene derivative, 2,5-dimethylselenophene, has been known for almost 100 years, the real development of selenophene... 

Magnetic circular dichroism (MCD) has now become a tool with valuable applications to analytical and structural studies. The MCD spectrum of thiophene is only slightly perturbed by substituents, and this is also expected to be true of the quite similar MCD spectra of selenophene.18 These molecules can therefore be classified as hard chromophores. 

The photoelectron spectrum of selenophene vapor down to 1350 A has been studied. By analogy with the other heterocyclic derivatives, Rydberg-type transitions occur, leading to the first ionization potential of the molecule.23... 

Data from microwave spectra on the centrifugal effect of rotational transitions of selenophene and its deuterium-substituted derivatives have been determined experimentally and compared with the calculated theoretical values of the centrifugal stretching constants by means of the force constants determined from the solution of the inverse vibrational problem.26 The two sets of values show good agreement, indicating that the system of force constants obtained for selenophene correctly reflects the characteristic features of the force field of the molecule. 

The mass spectrum of selenophene has been recorded and the fragmentation pattern compared with those of the other congener heteroaromatic compounds.27 The analogous fragments observed for the four heterocycles are given in Scheme 1. 

One of the reasons for the slow development of selenophene chemistry was the lack of convenient procedures for obtaining selenophene, its homologs, and derivatives. However, Gronowitz et al.42 have reported a semilarge-scale synthesis of selenophene (10), which gives about 150-200 g ( 58% yield) of selenophene per day. The procedure is a pyrolytic reaction... 

A more general synthesis of selenophene derivatives, starting from diacetylenes and sodium hydrogen selenide, was investigated by Curtis.48 In the synthesis of new tetraphenylporphyrin molecules containing heteroatoms other than nitrogen, Ulman et al.49 used in principle the method of Curtis for the synthesis of the starting material (17). 

Electrophilic substitution in furan, thiophene, selenophene and pyrrole has, up to 1970, been comprehensively reviewed by Marino.66 Italian workers have determined the relative reactivities of selenophene and thiophene as well67 relative rates are given in Table I. Including furan, the order of reactivity is furan > selenophene > thiophene. 

Substrates 47 and 48 have also been used to study the relative reactivities of selenophene and thiophene and of selenophene and benzene.71 The higher reactivity of the selenophene ring was demonstrated by the fact that upon formylation 59% of compound 49a was formed and upon acylation 63% of derivative 49b. Acylation of 48 gave exclusively 2-acetyl-5-benzylselenophene. Structures of the products were determined by H NMR. 

Nucleophilic substitution reactions in the selenophene series have attracted some interest. Debromination of bromonitro compounds [(50, X = S, Se) and (53, X = S, Se)] with sodium thiophenoxide and sodium selenophen-oxide72 was studied. Selenophene compounds were four times more reactive than the thiophene derivatives. The position of attack, a or /), had very little influence on the rate ratio. The kinetics of the side-chain nucleophilic reactions of selenophene derivatives, shown in Scheme 4, has been reported.7 3... 

The greatest difference observed between the side-chain reactivities of selenophene and thiophene derivatives was in the reaction of aldehydes with aniline (cf. Eq. 13), a ratio of 4.2. 

Selenophene compounds react faster than the corresponding thiophene derivatives in both electrophilic and nucleophilic substitutions. This may be due to the capacity of selenophene to delocalize both positive and negative charges, since the selenium atom is larger and more polarizable than the sulfur atom and consequently selenophene can release its p-electrons and accept electrons into its free -orbitals more readily than thiophene. 

A useful method for obtaining indazoles in high yields involves treatment of ortho-azido phenyl ketones or aldehydes with hydrazine hydrate. When this reaction was applied to 3-azido-2-formylselenophene, selenolo[3,2-c] pyrazole (111) was obtained in low yield. The yield could not be improved when the amino derivative was prepared as an intermediate142 (Scheme 13). 2-Dialkylamino-5-formylselenophenes react with diazonium salts under deformylation conditions to give azo dyes (Eq. 39).143 Another nitrogen-containing derivative of selenophene is compound U2.144 ... 

At present, the chemisty of selenophenes and tellurophenes is a relatively scantily studied area. Nevertheless, a number of new valuable contributions dealing with their chemistry have emerged. Electrophilic cyclization of l-(l-alkynyl)-2-(methylseleno)arenes provides a route to a variety of 2,3-disubstituted benzo[fe]selenophenes, as illustrated by the preparation of the system 88. Other useful electrophiles for similar reactions are E or NBS <06JOC2307>. Similar chemistry has also been employed in preparation of 2,3-disubstituted benzo[f>]selenophenes on solid phase <06JCC163>. In addition, syntheses of 2,3-dihydroselenolo[2,3- >]pyridines have been achieved using radical chemistry <06OBC466>. 

Similarly, C—Se bonds are formed by an internal homolytic substitution of aryl radicals at selenium, with the preparation of selenophenes and benze-neselenophenes [75]. Scheme 7.7 illustrates the reaction of aryl iodides 61 with (TMS)3SiH, which afforded benzeneselenophenes in good yields. The presence of (TMS)3SiI after the reduction induced the final dehydration of the intermediate 3-hydroxyselenophenes, presumably through an intermediate silyl ether. 

Systems with two condensed selenophene rings were first mentioned by Briscoe et in search of an improved synthesis of selenophene from acetylene and selenium. They obtained at 400° a mixture, including benzene, selenophene, naphthalene, and a compound, b.p. 240°-250°, containing about 20% of selenium, considered to be seleno-phenoselenophene (11). 

Little has been reported on the oxidation of selenium functionalities by dioxiranes. One case is the oxidation of selenophenes by DMD " at subambient temperatures. Good yields of either the selenophene 1-oxide or 1,1-dioxide may be achieved, which depends on the amount of DMD used (Scheme 12) . These results are similar to those already presented for the sulfur congeners. ... 

A number of Meisenheimer adducts from the reaction of selenophene derivatives 141, which are analogs of series 121, with methoxide ion have been investigated and characterized.I<9- 81 Their formation is generally even easier than that of the related sulfur compounds. 

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