Selenium compounds heterocyclic

Selenazole is the selenium-containing compound in the series of heterocyclic 5-membered ring azoles with two different hetero atoms, of which the first two members are oxazole and thiazole. The numbering of the ring system is according to the scheme given (1). 

Aspects of the chemistry of compounds belonging to more than 20 systems covered by this chapter have been reported, but the chemistry of only a few of these systems has been studied in detail. Chief among these are the 1,2,3-selenadiazoles, which have generated wide interest owing to their ready conversion into selenium heterocycles, reactive intermediates, and nitrogen- and selenium-free products, and the 1,2,5-selenadiazoles and 1,2,3,5-diselenadiazole systems in which interest stems from their use in the synthesis of potentially useful conducting materials. Compounds belonging to several new systems have been synthesized. 

Heterocyclic Compounds of Quadricovalent Sulfur , D. H. Reid, in Organic Compounds of Sulphur, Selenium and Tellurium , ed. D. H. Reid, Royal Society of Chemistry, London, 1975, vol. 3, pp. 387-399. 

Metallacyclopentadienes undergo a range of synthetically versatile reactions which proceed with extrusion of the metal atom and attendant ligands. Thus, reactions with alkenes and alkynes afford cyclohexa-1,3-dienes and arenes (Scheme 6), and thiophenes, selena-cyclopentadienes, pyrroles and cyclopentadienones (indenones, fluorenones) can be obtained by treatment with sulfur, selenium, nitroso compounds and CO, respectively. The best studied substrates for such reactions are cobaltacyclopentadienes of the type (24a), which have been converted into a wide variety of arenes, cyclohexadienes and five-membered heterocycles, many of which would be very difficult to obtain by conventional organic procedures (74TL4549, 77JOM(139)169, 80JCS(P2)1344). 

These examples show the specificity of many reactions of organotellu-rium compounds that thereby prevent the simple extension of synthetic procedures developed for the preparation of sulfur and selenium heterocycles to their tellurium-containing analogs (see also 85MI6 86MI2). 

Interest in the chemistry and use of different selenium-containing heterocyclic compounds, including those having aromatic systems, as potential pharmaceuticals, as new materials, and as reagents and catalysts has expanded rapidly during last three decades and a lot of works in this field have been published. 

Treatment of 139 with selenium and tellurium did not lead to any reaction. However, upon addition of a small amount of a triethylphosphane, the corresponding five-membered heterocyclic compounds 155 and 156 were isolated in high yield (equation 40)142. 

Sulfur and selenium heterocycles have also been prepared by reaction of zirconium-benzyne complexes with the elemental chalcogens. In these reactions, chalcogens insert into both of the zirconium-carbon bonds (Scheme 7).53-55 Interestingly, neither para-bromo norpara-dimethylamino substituents in 65 interfere with their conversion to 66 (60-80% yields).54 Complexes 66 react further with mono- and bifunctionalized electrophiles (Scheme 7) to yield ort/io-dichalcogenated benzenic compounds 67-69. 

The electron impact mass spectra of a series of selenazoles have been compared with those of their thiazole analogs. The results indicate that, in general, the mass spectrometric fragmentation patterns of the selenium heterocycles can be predicted by an examination of the corresponding sulfur analog. One of the characteristics of the mass spectra of selenium compounds is that they are rich in ions due to the presence of the six relatively abundant stable selenium isotopes (81JHC1335). 

Three relatively stable compounds (210a-210c) of this class of hypervalent selenium heterocycles are known. The assignment of a bicyclic structure is based on physical and... 

Many of the chiral selenium electrophiles have also been employed in cyclization reactions. Various internal nucleophiles can be used and access to different heterocycles is possible. Not only oxygen nucleophiles can be used for the synthesis of heterocyclic compounds, but also nitrogen nucleophiles are widely employed and even carbon nucleophiles can be used for the synthesis of carbacycles with new stereogenic centers. Oxygen nucleophiles have been widely used and some selected examples of selenolactonizations of unsaturated acids 50 and 52 and seleno-etherifications of unsaturated alcohols 54 and 56 are shown in Scheme 10. 

These reactions have also been performed using enantiomerically pure selenium electrophiles to access heterocyclic compounds with stereogenic centers. The yields and selectivities obtained using some selected chiral electrophiles generated from the diselenides are given in Table 2. 

The ability of thienothiophenes and selenoloselenophenes to transmit substituent effects has been determined by H and l3C NMR, and also calculated by the CNDO/2 method [82CS(20)208 83CS(22)22]. As expected, resonance effects are better transmitted between the 2- and 5-positions in 8.185 than in 8.186, since only in the former is direct conjugation between these positions possible. Both resonance and inductive/field effects were greater in the sulfur- relative to the selenium-containing compounds, but in general, replacement of sulfur by selenium caused only minor perturbations of data, which follows from the fact that the main mode of transmission of substituent effects in five-membered heterocycles must be via the carbon chain (cf. Chapter 6, Section 9.C). 

The aromaticity of a heterocycle depends on how effectively the lone-pair of the heteroatom contributes to the aromatic sextet. The aromaticity of five-membered heterocyclic compounds may be estimated from their reactivity in the Diels-Alder reaction.94 Spectrophotometry shows that furan, thiophene, and selenophene resemble benzene in that with maleic anhydride 1 1 complexes are formed which are stable up to 150°C in the case of thiophene, decompose at 150°C with selenophene (whereby selenium is formed together with a diene which gives a further adduct with another molecule of maleic anhydride), and produce the usual adduct at 20°C with furan. Thus, only furan is a normal diene as regards the Diels-Alder reaction. 

R. C. Elderfield, "Heterocyclic Compounds, Vol. 2, Chapter 14. Wiley, New York, 1950. R. J. S. Beer, Organic Compounds of Sulphur, Selenium and Tellurium (Sr, Reporter,... 

A number of selenium heterocycles are prepared utilizing carbon-selenium double bonds as 271 dienophilic intermediates for [4+2] cycloadditions. However, Koketsu et al. reported a novel hetero Diels-Alder reaction wherein selenoazadienes 353 serve as 47t components. Accordingly, compound 353 reacts with dimethyl acetylenedicarboxylate to yield 4-selenazolone 354. The proposed mechanism involves the formation of hetero Diels-Alder adduct 355, which is converted into 4-selenazolone 354 following purification on silica gel. Protonation of cycloadduct 355 presumably affords the selenoamidine 356, which is converted into 357 by nucleophilic recyclization <03H(60)1211>. 

Owing to the high polarizability of the carbon-selenium double bond, selenoesters can easily be attacked by nucleophiles. This property makes a selenoester a useful reagent for the synthesis of hetero-cycles. Condensation of selenoesters with bifiinctional aromatic or aliphatic amines has yielded a variety of nitrogen-containing heterocyclic compounds (Scheme 19). 

In the book Organic Selenium Compounds Their Chemistry and Biology, edited by Klayman and Gunther, several chapters have been devoted to the review of selenium heterocycles including one chapter on... 

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