Selenophenes preparation

Polyselenophene (Fig. 16c) has been prepared. However, due to the difficulty in obtaining the monomer, the polymer has not been extensively investigated. Polymers of selenophene prepared electrochemically under appropriate conditions yield films with maximum conductivities of 10"- S cm [330,331]. Samples of p-doped selenophene produced chemically have conductivities on the same order of magnitude [332]. Applying 3-10 V between two electrodes in an electrolyte of 0.1 to 1 M lithium tetrafluoroborate or lithium perchlorate dissolved in benzonitrile or propylene carbonate gives polyselenophene films, as does the combination of tetrabutylammonium tetrafluoroborate in benzonitrile. However, other salts such as lithium hexafluoroarsenate, lithium hexafluorophosphate, tetrabutylammonium perchlorate, or silver perchlorate in combination with solvents such as acetonitrile or nitrobenzene were reported to produce either powders or no products at all [330,331,333]. 

Contrary to benzo[b]thiophen, benzo[b]selenophen is acylated at position 2. 2-Bromobenzo[b]selenophen, prepared from the 2-lithium derivative and bromine, is acylated at position 3, and from this derivative the 3-isomers could be synthesized. A large number of 2- and 3-aroylbenzo[b]seleno-phens were synthesized, either by Friedel-Crafts reaction of benzo-[bjselenophencarboxylic acid chlorides and benzenes or from benzol bjselenophens and benzoic acid chlorides. Also the reaction between Grignard reagents and nitriles was used for the synthesis of aroylbenzo[b]selenophens. 4,S-Diaminobenzo[b]selenophen has been synthesized via nitration of 5-aminobenzo[b]selenophen or via the reaction of the diazonium salt derived from the S-amino-derivative with p-sulphanilic acid. From the diamino-derivative, several fused benzo[b]selenophens, e.g. (525), (526), and (527), were prepared. Reaction of the phenyl-... 

In the thiophene and selenophene series, a-halogens are preferentially removed by reduction with zinc and acetic acid, as is illustrated by the preparation of 3-bromothiophene... 

Treatment with NCS in carbon tetrachloride converted the parent into the 2,3-dichloro derivative. The 2,3-dibromo compound was made similarly with NBS (74BSF2239), or with bromine in chloroform in the presence of sodium acetate (72CHE13). Monobromination is possible, but generally mixtures form with 2- and 3-bromo products in ratios of the order of 1 3 (72CHE13). It was possible to prepare 3-bromobenzo[h]sele-nophene by reaction of the 2,3-dibromo derivative with butyl lithium followed by hydrolysis. Four moles of bromine gave the 2,3,6-tribromo derivative from benzo[h]selenophene (74BSF2239). 

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... 

In a semimicro synthesis selenophene is prepared from bis(trimethylsilyl)-1,3-butadiyn and NaHSe generated in situ from Se and NaBH4 in aqueous dimethylformamide.52 Other ring cyclization reactions have been performed... 

Phosphorus pentaselenide prepared from red, amorphous selenium effects cyclization of 1,4-diketones (22) to substituted selenophenes (23) in moderate... 

Lalezari and co-workers prepared selenophene derivatives from cycloalka-... 

It is well-known that 2- and 3-selenienyllithium derivatives are valuable synthetic intermediates for the preparation of various 2- and 3-substituted selenophenes. In this way, 2- and 3-chloroselenophenes have been obtained by reaction with hexachloroethane at — 70°C.76 Another example has already been shown in connection with inverted reactivity (Section VI,B), and many applications will be demonstrated in subsequent sections. 

The formyl group in orthoformylselenophenecarboxylic acids on reduction to an hydroxymethyl group undergoes ring closure to lactones.100 To obtain 2,4-disubstituted selenophenes, 4-bromo-2-formylselenophene was prepared by the swamping catalyst method.101 Selenopheno[2,3-b]thiophene derivatives may be prepared from compound 84.102 Another route to substituted... 

This gives tautomeric mixtures119 when the tert-butyl group is removed. The methyl ether has been used to obtain 3-hydroxy-2-carbonyl derivatives in the selenophene series.120 The unsubstituted 2-hydroxyselenophene system has been prepared by hydrogen peroxide oxidation of 2-selenophene-boronic acid.121 However, in the 5-methyl-substituted system deboronation became such an important side reaction that 5-methyl-2-hydroxyselenophene had to be prepared by acid-catalyzed dealkylation of 5-methyl-2-fert-butoxy-selenophene. Both 2-hydroxy- and 5-methyl-2-hydroxyselenophene exist mainly as 3-selenolene-2-ones (93) and for the 5-methyl derivative it was possible to isolate the / ,y-unsaturated form (92) and follow the tautomeric isomerization. The activation parameters thus obtained were compared with those for the corresponding furan and thiophene systems. 

Cyano-substituted selenophenes are valuable precursors of formyl derivatives.89 They are also used as starting materials for the preparation of carboxylic acids and their derivatives.128 By treating the cyano compounds with hydrogen selenide, selenoloamides (101) were obtained. As shown in... 

A way to introduce the primary amino group directly onto the selenophene ring is via the azido compound, obtained by nucleophilic substitution of the bromo derivative with sodium azide. Useful transformations of the azido group are shown in Scheme 12.117 The amino aldehyde (109) is a suitable starting material for the preparation of selenolo[3,2-b]pyridine (110) by the Friedlander reaction.138 Not only can the azido be reduced to an amino... 

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 ... 

Because of their interest in physiologically active selenophenes, Magde-sieva and co-workers147 I4S have prepared some a-amino alcohols (115). Ketones (114) were nitrosated and the oximes thus obtained were reduced with lithium aluminium hydride. 

The discovery of high conductivity in some organic compounds has initiated a whole era of research. The first and most used acceptor molecule is 7,7,8,8-tetracyanoquinomethane. A selenophene analog has been prepared starting from 2,5-dibromoselenophene (118) as shown in Eq. (41).151 Data for the solution electrochemistry and ESR spectroscopy of 119 and other acceptors containing dicyanomethylene units have been determined.152 In an attempt to correlate oxidation potentials with LUMO energies no obvious... 

A direct synthesis of the thiophene nucleus has been achieved by allowing air-stable cobaltacyclopentadiene complexes (66) to react with sulfur the organometallic complexes are prepared in variable yields in a stepwise fashion from f75-cyclopentadienyIbis(triphenylphosphine)cobalt (65) (Scheme 82) 147,148 Reactions of the complexes 66 with selenium and nitrosobenzene give rise to selenophenes and pyrroles, respectively. 

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>. 

The 5,10,15,20-tetraaryl-2-aza-21-carba-22-selenaporphyrin possessing an inverted pyrrole ring has been prepared by the [3+1] condensation of 2,5-Ws(phenylhydroxymethyl)-selenophene and 5,10-ditolyltripyrrin . 

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. 

Halogen can be removed by catalytic hydrogenation and so it is possible to use halogen as a blocking group in pyrrole chemistry. In the thiophene and selenophene series, a-halogens are preferentially removed by reduction with zinc and acetic acid, as illustrated by the preparation of 3-bromothiophene (390) from 2,3,5-tribromothiophene (389) (81SC25). 

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