Selenium-containing heterocycles

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). 

In his approach toward selenium-containing heterocycles with potential biological activity, Abdel-Hafez reacted 2-amino-3-(4,5-dihydro-17/-imidazol-2-yl)-4,5,6,7-tetrahydro-l-benzoselenophene 297 with triethyl orthoformate and benzaldehyde to generate the tricyclic systems 296 and 298, respectively (Scheme 21) <2005RJ0396>. Similarly, reaction with carbon disulfide gave the corresponding thiourea 299. 

Electrochemical studies indicate that the redox behaviours of the different derivatives are dominated by the nature of the chalcogen at position 2, with the selenium containing cations more readily reduced (by more than 0.1 V) than the sulfur analogues.82 The difference has been ascribed to the extent of delocalisation of the positive charge away from the heterocycle, onto the fused aromatic ring. 

The heating of the 1,3-thiazetes 250 in xylene at 140°C leads to the formation of 252. Heating the same compound with P2Se5 produced the selenium-containing heterocycle 253. ... 

The synthesis and preliminary biological activity of substituted 7-alkylseleno-l,4-dihydro[l,6]naphthyridines have been reported <2000DOC218> along with their further synthetic ttansformations <2001RCB122>. There have also been reports of the preparation of selenium-containing fused heterocycles. The C=0 function in benzothiophen-3-ones, 3,4-dihydrothiopyrano[3,2-. ]benzothiophen-4(2//)-ones, and 3,4-dihydro-2//,5//-thiopyrano[2, 3 4,5]thiopyrano[3,2-/ ]ben-zothiophen-4-ones reacts with selenium dioxide and thionyl chloride to give fused 1,2,3-selena and thiadiazoles via their semicarbazones <1999IJB308>. 

The chalcogene heterocycles have been used as stable precursors for sulfur-and selenium-containing hetero-l,3-dienes in cycloaddition reactions 3//-1,2,4-Thiaselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-thia-3-aza-buta-l,3-dienes, and 3H-diselenazoles are a convenient source of 4,4-bis(trifluoromethyl)-l-selena-3-azabuta-l,3-dienes as well as bis(tnfluoro-methyl)-subsbtuted mtnle ylides [137]... 

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. 

Coordination compounds of selenium-containing heterocycles with an Se — M bond are represented by the complexes of 1,4-diselenane 346 348 [634] and seleno-phene 349 [8,643]. Among these complexes, while the Se — M coordination 350 was proposed [634] for the complexes on the basis of 1,2,3-benzoselenodiazole, only N—M binding 351 was found in these compounds [13,644] ... 

The structure of 881 was proved by x-ray diffraction [193]. The possibility of carrying out these syntheses may be explained as due to the sufficently high stability of sulfur-and selenium-containing azomethines within a heterocyclic series, in comparison with their aromatic analogues (compare the data of Sec. 3.5.2.1), which is related to the existence of 471 as the aminomethylene form b [15,194-197]. 

Selenium-containing six-membered ring heterocycles have proved to be useful catalysts in a variety of transformations. The Baylis-Hillman reaction involves the reaction of alkenes containing electron-withdrawing groups such as a,/3-unsaturated carbonyl compounds with aldehydes leading to carbon-carbon bond formation (Equation 79). The reaction is promoted by tertiary amines such as l,4-diazabicyclo[2.2.2]octane (DABCO), or tertiary phosphines and Lewis acids. Unfortunately, the Baylis-Hillman reaction is severely limited because it proceeds only very slowly <1998CC197>. Much research has been carried out in attempts to increase the rate of this reaction. 

J.S. Murray et al., Simultaneous a-hole and hydrogen bonding by sulfur- and selenium-containing heterocycles. Int. J. Quantum Chem. 108, 2770-2781 (2008)... 

The selenium-containing complex 72b has been used to prepare a variety of new heterocycles containing both selenium and germanium. For example, reaction with dichlorodiphenylgermanium and germanium tetrachloride gives 76 and 77, respectively (Scheme 8).56... 

In contrast to thionyl chloride, which is used frequently as a reagent in synthesis, the applications of seleninyl chloride (SeOCl2) are very limited. It can be used to synthesize selenium-bridged chromanones332 or in the synthesis of novel selenium-containing heterocycles.333 334... 

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). 

Compound 142 is a useful synthon for the synthesis of other selenium-containing heterocycles (48, 49). 

Five-membered heterocycles with two vicinal chalcogen atoms in the ring system can be used as stable precursors for sulfur as well as for selenium-containing hetero-1,3-dienes in cycloaddition reactions. Consequently, 3//-1,2,4-thiaselenazoles have been used for the in situ formation of 4,4-bis(trifluoromethyl)-l-thia-3-azabuta-1,3-dienes, which exist at room temperature only as 4,4-bis(trifluoromethyl)-2//-l,3-thiazetes. This strategy was applied to the synthesis of the first stable selenophosphorane from bis(trifluoromethyl)-substituted 3//-diselenazol and 2-methoxy-1,3,2-dioxaphospholan [78AG(E)774] (Scheme 83). 

A plethora of selenium-containing heterocycles are known to act as the donor component for donor acceptor complexes that have properties as electrical conductors. " Two examples that have provoked widespread interest are tetraselenafiilvalene (54) and tetraselenotetracene (55). A variety of selenium-containing polymers such as (7) are also of interest in this regard. 

The radical substitution at selenium is efficient when the leaving radical is stabilized (e.g. benzyl, tert-butyl). This reaction has been used for the preparation of selenium containing heterocycles [117]. In Eq. 50, the conversion of the Barton ester 156 to the tetrahydroselenophene derivative 157 is described [118, 119]. Mechanistic studies and ab initio calculations indicate that the substitution does not involve hypervalent selenium derivatives. 

A recent study of some parameters which influence the oxidation of 2- and 4-methylquinolines to aldehydes with selenium dioxide in dioxane has shown that the successful result of oxidation is dependent on the amount of the oxidant used, but not on its purity, and on neutralization of quinolinecarboxylic acid in the reaction product before isolation of an aldehyde [16]. When 2,2,5,5-tetramethyltetrahydrofuran-3-one (43) was oxidized with selenium dioxide in ethanol, the six-membered selenium-containing heterocycle 44 was formed along with the expected 3,4-furandione 45 [17] (Eq. 5). 

The best known inorganic selenurane, selenium tetrachloride (191), has recently been used for the construction of various selenium-containing heterocyclic structures. For example, the first synthesis of 3-benzoselenoepine (192) was based on the reaction of 2,2-o-bis(j5-lithiovinyl)benzene (193) with 191 [85] (Eq. 39). 

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). 

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