Selenadiazoles, aromaticity

JMC1164), and when (148) was treated with sulfur monochloride, the aromatic system (150) was obtained (81JOC4065). The corresponding 1,2,5-selenadiazole was isolated when (148) was heated with selenium dioxide (52% yield). 

The structural indices of aromaticity, I, of oxadiazoles (145-148), thiadiazoles (150-153) and selenadiazoles (155, 156) are compared with that of the parent furan (144), thiophene (149) and selenophene (154) (Scheme 11). 1,2,3-Oxadiazole (145) is the least stable among them since all attempts to synthesize this compound were unsuccessful, most likely because of its easy isomerization to the acyclic isomer. At the same time its sulfur analogue (150) possesses good stability and has been synthesized. Its 2,4-diaza- (151), 3,4-diaza- (152) and 2,5-diaza-(153) isomers demonstrate even more the extent of n-electron delocalization. There exists a well-known tendency of decreasing aromaticity depending on the type of pyrrole-like heteroatom S > Se > O. However, there is no uniformity in the change in aromaticity in the horizontal rows, i. e., dependence on heteroatom disposition. 

The decreasing order of aromaticity based on DRE values (in kcal mol"1) is benzene (22.6) > thiophene (6.5) > selenophene > pyrrole (5.3) > tellurophene > furan (4.3). In benzoderivatives the effect of the heteroatom is similar to the azole series and the decreasing scale of aromaticity is benzo-2,l,3-selenadiazole > benzo-2,1,3-thiadiazole > benzo-2,l,3-oxadiazole [28],... 

Thermodynamically stable aromatic selenaheterocycles such as selenophene (1), 1,3-selenazole (3), 1,2,5-selenadiazole (5), and 1,2,4-selenadiazole undergo [l,4]cycloaddition followed by spontaneous deselenation, which is a convenient way for construction of nonselenium azaaromatic rings [3, 6, 42], 3,4-Diphenyl-1,2,5-selenadiazole reacted with DMAD to give methyl 2,3-diphenylpyrazine-5,6-dicarboxylate. The similar reaction of 1,2,4-selenadiazoles resulted in pyrimidine -5,6-dicarboxylate, while 2,1,3-benzoselenadiazoles reacted with DMAD to give the quinoxalines [6, 110]. (For deselenation see also Sect. 5.1). 

The treatment of 1,2,3-selenadiazoles (79) with a catalytic amount of tributyltin hydride and AIBN in the presence of an excess of olefin, gives 2,3-dihydroselen-ophenes (80) in good yield (Scheme 20). The reaction proceeds via tributyltin radical-promoted denitrogenation to give a vinyl radical, which then adds to the olefin followed by intramolecular cyclization. Under similar conditions 1,2,3-selenadiazoles and aliphatic or aromatic ketones afford alkynes as the sole products [114, 115]... 

Many 1,2,3-selenadiazoles have been prepared as intermediates for the ultimate preparation of other organic compounds or for biological activity evaluation [6], The most general, extensively applied method is based on the oxidation of semicar-bazones, mostly derived from aromatic and carbocyclic ketones, by selenium dioxide [6, 14, 188-191], Synthesis of 4-(2-naphthyl)-l,2,3-selenazole (117) from semicarbazone (116) is an example (Scheme 36). 

Only a few works on the synthesis of 1,3,4- and 1,2,5-diazoles have been reported, all in older literature [6,14], More attention has been paid to synthesis of benzo-2,l,3-selenadiazoles simply obtained by reaction of aromatic or heteroaromatic 1,2-diamines with selenium dioxide [196-199],... 

Chapter 7, by Jacek Mlochowski and Miroslaw Giurg deals with aromatic and related selenaheterocycles and their applications. The chapter presents firstly the chemical properties of selenaheterocyclic rings tautomerism, reactions on the carbon atom and on the heteroatom, and finally ring transformations. Next are presented the syntheses of selenaheterocyclic compounds like selenphanes, isoselenazoles, selenazoleds, selenadiazoles, and selenoporphyrines. Finally, an outline of the applications of selenaheterocycles is presented. 

Oxidation of fused 1,2,3-selenadiazole 223 with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) at room temperature gave naphtho[l,2-<7][l,2,3]selenadiazole 88 in 55% yield (Equation 26) <2000JHC1325>. Reaction of compound 224 with selenium powder in an equimolar ratio in an inert atmosphere gave aromatized product 225 (Equation 27) <1998JCM784>. 

The reaction of / -butoxycarbonyl hydrazones 263 with selenium oxychloride in CH2CI2 from — 20 °C to room temperature gave 2,3-dihydro-l,2,3-selenadiazoles 264 in 71-77% yields (Equation 33). Under these mild conditions, the aromatization process did not occur, and only 2,3-dihydro compounds were observed. The advantage of this reaction is the ease of workup. Pure compounds 264 were obtained simply by addition to the reaction mixture of an aqueous saturated solution of NaHCC>3 and subsequent evaporation of the organic solvent <2003JOC1947>. 

Benzoselenadiazoles were synthesized at room temperature in the solid state with ortho-aromatic diamines and selenium dioxide. Diamines and selenium dioxide were ground, respectively, and then were mixed in a ratio of 1 1 in a mortar at room temperature the process was monitored with X-ray diffraction (XRD) or IR. The results showed that the reactions were completed after 30 min of grinding and the desired products were obtained. The yields of the synthesized compounds are as follows 2,1,3-benzoselenadiazole 196 77% l,2,5-selenadiazolo-[3,4-A]pyridine 284 44% l,2,5-selenadiazolo[3,4-c]pyridine 285 23% 5-methyl-2,l,3-benzo-[3,4-c]selenadiazole 286 74% 1,2,5-selenadi-azole[3,4- /]pyrimidine-7-(6/7)-one 287 50% 5,7-dihydroxy-l,2,5-selenadiazolo-[3,4- 7]pyrimidine 288 19% and 2,l,3-naphtho-[2,3-c]-selenadiazole (289) 77% <2004MI1>. 

The syntheses from [4 + 1] atom fragments, in which the Group VIA heteroatom is introduced between two nitrogen atoms, are the most widely applicable and versatile methods available for construction of the 1,2,5-thiadiazole and 1,2,5-selenadiazole ring systems. These methods have been applied to the synthesis of monocyclic and polycyclic aromatic forms of these ring systems in addition to the direct formation of 1-oxides and 1,1-dioxides, 2-oxides, quaternary salts and reduced forms. The earliest use of the [4 + 1] synthesis dates back to 1889 when Hinsburg prepared 2,1,3-benzothiadiazole (1) and 2,1,3-benzoselenadiazole (2) from o-phenylenediamine and sodium bisulfite, or selenium dioxide, respectively. 

Selenadiazoles are useful intermediates for the preparation of alkenes because they can be easily decomposed with the loss of nitrogen and a selenium atom under free radical conditions. However, if 1,2,3-selenadiazoles such as 102 are treated with allyltributyl stannane/AIBN in the presence of an olefin or diene dihydroselenophenes such as 103 are formed provided the 1,2,3-selenadiazole has been derived from a cyclic ketone. Under similar conditions 1,2,3-selenadiazoles prepared from linear or aromatic ketones afford alkynes as the sole products <02JOC 1520>. 

Several NMR studies of the substituted 2,1,3-selenadiazoles have been reported. Thus Katritzky and Takeuchi ° have studied the effect of substituents on the coupling constants of the aromatic ring protons. The N-methyl chemical shift in quatemized 2,1,3-benzoselenadiazole (215) and its congeners have been studied. The shifts appear to be determined by resonance from the heteroatom, the order of donating ability being NMe > O > S Se. ... 

Straightforward approach to benzo[ r]selenophenes in which 4-(3-nitroaryl)-1,2,3-selenadiazoles undergo a base-promoted transformation to an intermediate eneselenoate followed by 5-exo-trig cyclization. The regiochemistry of the cyclization is dependent upon the conditions of the reaction. In the presence of an oxidant, the adduct is formed rapidly by the oxidative nucleophilic substitution of hydrogen (ONSH, SnAt ) followed by oxidative aromatization of the rapidly formed adduct. In the absence of an oxidant, the reaction proceeds via intermediate formation of the adduct, followed by nucleophilic aromatic substitution of the halogen (SnAt ). 

A similar base-promoted transformation of 4-(3-nitroaryl)-l,2,3-selenadiazoles 134 provides entries to various 2-aminobenzoselenophenes 150 and 152 (130L1744).The reaction presumes the formation of intermediate eneselenolates 148 followed by 5-ea o-trig cyclization. The regio-chemistry of the cyclization differs with the participation of an oxidant. In the presence of an oxidant, oxidative aromatization of the rapidly formed intermediates 149 takes place, leading to 2-aminobenzoselenophenes 150. In the absence of oxidant, the reactions progress through intermediates 151... 

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