Selenium heterocycles

The ability of various selenium heterocycles to check the loss of orthophosphate caused by irradiation of ATP has been studied by Brucker and Bulka (92). They found that only 2-amino-4,5-dimethyiselenazole shows radioprotective properties, while other 2-aminoselenazoles, selenosemicarbazides, and acetone selenosemicar-bazones possess no such activity but are in addition very sensitive to radiation (93). 

Selenium heterocycles receive far less mention in the literature than do such homologs as oxazole, thiazole, or imidazole. In fact, preparative methods of selenium heterocycles are much more limited than for the other series, mainly because of manipulatory difficulties arising from the toxicity of selenium (hydrogen selenide is even more toxic) that can produce severe damage to the skin, lungs, kidneys, and eyes. Another source of difficulty is the reactivity of the heterocycle itself, which can easily undergo fission, depending on the reaction medium and the nature of the substituents. 

The similarity of the electronic structures of thiophene and tellurophene [72JCS (Pl)199] implies that their chemistry toward Fc3(CO)l2 should be the same. Thus, tellurophene produces [Fe3Fc2(CO)9], ferrole 83, and 114 [72JOM(42)C87,96JCS (D)1545]. The same trend is observed in the reactions of the derivatives of tellurium and selenium heterocycles [97JCS(D)1579, 98JCS(D)3947]. 

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. 

In common with the sulfur systems, the most important organophosphorus-selenium heterocycle is the four-membered ring R(Se)P(p-Se)2P(Se)R. In the solid state, the R groups adopt a trans arrangement with respect to the P2Sc2 ring (R = Ph ). Solid-state P and Se NMR spectroscopy has been... 

Phosphine Sulfides and Selenides, Sulfur and Selenium Heterocycles 980... 

An interesting reaction has been reported by Glass and coworkers344 consisting of the attack of a selenium heterocycle, Ebselen oxide 66, by a-toluenethiol (equation 69). In this reaction thiobenzaldehyde has been invoked as intermediate and could be trapped with cyclopentadiene. In the absence of a diene, dibenzyl disulfide is isolated instead of... 

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). 

Complex 15 illustrates in detail the possibility of simultaneous a-hole and hydrogen bonding, which has already been discussed in 15, there is one 0---S a-hole bond and three 0---H hydrogen bonds. Note the deviations of the hydrogen bonds from linearity. Another example in Table 6.2 is 14, in which the very short N---Si separation can be attributed to the presence of several H---F hydrogen bonds, in addition to the N—Si a-hole interaction [32]. We have encountered analogous situations elsewhere, for instance, with sulfur and selenium heterocycles [47]. 

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... 

Comparison OFpKR. Values for Nitrogen, Oxygen, Sulfur, and Selenium Heterocyclic Cations"... 

V. G. Pesin, Russ. Chem. Rev. (Engl. Transl.) 39, 923-943 (1970). Selenium heterocycles ... 

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>. 

A similar bisquinolone structure 115 was isolated in 72% yield when 112 was reacted with the iV-tosylenaminone 116 (Eq. 25). The selenide 115 was further used for the preparation of novel selenium heterocycles [581. 

As with selenoesters, selenoamides are excellent reagents for the synthesis of selenium-containing heterocycles. Anhydro-2,3,5-triphenyl-4-hydroxyselenazolium hydroxide (96), as the first mesoionic selenium heterocycle,selenophene derivatives (97) " and 1,3-selenazoles (98) have been synthesized as shown in Scheme 33 and equations (38) and (39). 

A. Five-Membered Selenium Heterocycles with One Nitrogen Atom.110... 

B. Six-Membered Selenium Heterocycles with Two Nitrogen Atoms.147... 

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... 

B. Five-Membered Selenium Heterocycles WITH Two Nitrogen Atoms... 

The synthesis of several novel hypervalent selenium heterocycles has been reported. Thus the l,6-bis-(2,4-dinitrophenyl) l,6-dihydro-3,4-dimethyl-l,2,3-selenadiazolo[5,l-c]selenadiazole-7-Se (167) was obtained through... 

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