Alkyl selenazoles

Alkyl and aryl selenazoles are weakly basic, and their quaternary salts are easily hydrolvzed in aqueous solution. 

The condensation of a-haloketones with monosubstituted alkyl or aryl-selenoureas (25) leads to 2-alkylamino- (26. 27) or 2-arylaminOselana-zoles (28) while disubstituted selenoureas give 2-(dialkylamino) selenazoles (26. 27) (Table X-3a). 

From the point of view of reactivity, there is little difference betvv een 2-amino-selenazoles and aryl- Or alkyl-2-aminoselenazoles, except that the A"-arvl derivatives are generallv less basic and that their salts are more easily hydrolyzed. 

All attempts to prepare selenazole derivatives by the Gatterman (for-mylation) or Friedel-Crafts (alkylation) methods failed (19, 26). indicating that the electrophilic reactivity of the 5-position is less than that of benzene or toluene. 

The 2-benzamido 4-aryl(alkyl)selenazoles (96) form the corresponding 5-nitro derivatives under mild conditions using the nitrate-sulfuric acid method (Scheme 31). The nitro compounds are well-defined, ciy s-talline compounds. They may be most favorably obtained by dissolving the 2-benzamidoselenazoles in acetone and adding concentrated nitric... 

Alkyl selenazoles have been sulfonated. Thus 2,4-dimethylselenazole with fuming sulfuric acid (5% SO3) at 100°C gives 2.4-dimethy -5-sulfonylselenazole [m.p. 238 C (decomp.) (26)], which is relatively unstable. decomposing to give metallic selenium. 

Seemingly the presence of an aromatic substituent in the 4-position of the selenazole ring is necessary for obtaining well-crystallized products compounds with an alkyl group in the 4-position are amorphous and quite difficult to obtain as crystals. 

Quaternarj salts are obtained by alkylation of selenazole bases, the heterocyclic nitrogen atom playing the role of nucleophile with regard to the electrophilic carbon of the alkylating, agent. 

Selenazole, 2-acetamido-nitration, 6, 341 Selenazole, 2-alkyl-nitration, 6, 340 Selenazole, 2-amino-bromination, 6, 341 sulfonation, 6, 341 synthesis, 6, 342... 

The difficulties encountered in the synthesis of 2-alkyl- and 2-aryl-substituted selenazoles lie principally in the preparation of the corresponding selenoamides. In this respect, a method is worthy of note in which the use of selenoamides is dispensed with. For this, a nitrile, a hydrogen selenide, and an a-halogenoketone are reacted together in the presence of a condensation catalyst. Phosphorus oxychloride, alone or mixed with zinc chloride or phosphorus trichloride, is specially suitable. The yields of the corresponding 2-alkylseIenazoles are up to a maximum of 25%,... 

Table I gives details of the alkyl- and aryl-substituted selenazoles...

The 2-amino-selenazoles are crystalline compounds, and sometimes unstable, for example, the parent compound on heating in water undergoes complete decomposition. A few of these selenazoles which are substituted in the amino group are oily liquids. The basic character is more pronounced than in the alkyl- and aryl-selenazoles. The hydrochloride salts are, therefore, not so easily hydrolyzed in aqueous solution. 2-Amino- and 2-methylamino-4-methylselenazole have been considered to exist partly in the tautomeric selenazol-2-one imine form from comparison of their ultraviolet spectrum with that of 2-diethylamino-4-methylselenazole. ... 

Because of the basic nitrogen atom, alkyl-selenazoles form quaternary salts. 2,4-Dimethyl-3-ethylselenazolium iodide (mp 157-158 0) was prepared by Brooker et al in 87% yield as colorless crystals by heating of 2,4-dimethylselenazole in excess ethyl iodide for 2 days. By reaction with the corresponding quaternary salts, the following cyanine dyes (8) were prepared" l 3-diethyl-4-methylselenazolo-2 -... 

Alkyl A-2-selenazolines, preparation. 259 Amidinium ion, ionic system of, 68 2-Amino-4-aryl-5-acetic acid selenazoles. 

The isoselenazole ring in unsubstituted 3-hydroxy-l-benzo-l,2-selenazole (26) and in its 7-azaanalog (27) exists in nonaromatic amide form such as in 28 or 29 (Scheme 4). The amide proton is easily exchanged, via potassium salt, by alkyl, acyl, or sulfonyl groups. The additional evidence for amide structure is based on spectral data. For example, the amide band nc=0 = 1646 cm 1 in the IR spectrum and the broad singlet at 9.34 ppm in the 11 NMR spectrum were observed for 29 [49-51],... 

Electrophiles react with 2-substituted 1,3-selenazoles at the 5-position. Nitration of 2-alkyl-l,3-selenazoles and 2-acetylamino-l,2-selenazole affords 5-nitroderiva-... 

The reactions of selenocarbonyl compounds with electrophiles are also well-established procedures. Alkylations or acylations of the selenium atom of selenoamides409 or selenoureas410 are known. Selenonium salts are formed initially they can then be converted into diselenides, selenazoles, or cyclic selenides depending on their structure. Reactions of selenocarbonyls with bromine and iodine have also been widely exploited. Selenocarbonates, sele-nothiocarbonates,411-415 and selenoureas416-418 can be employed, the reaction of 209 with 1 equiv. bromine led to the hypervalent 10-Se-3 complex 210, whereas an excess of bromine gave rise to a cleavage of the carbon-selenium double bond and formation of product 211 (Scheme 64). 

Alkylthio-selenazoles are alkylated on nitrogen using Meerwein salts to produce the corresponding 2-alkylthio-3-methyl-l,3-selenazolium salts. The alkylthio group in these salts can be replaced with selenium using NaHSe to afford 3-methyl-l,3-selenazole-2-selenone 70 (Scheme 5) <2003NJC1622>. 

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