4.5- Disubstituted 1,2,3-selenadiazoles

Disubstituted 1,2,3-selenadiazoles react with [(i/ -Cp)Mo(CO)2]2 to form a series of ring-opened and finally deselenated products (92JA5467). 

Table 20 Selected 13C NMR spectra of 4-substituted- and 4,5-disubstituted-1,2,3-selenadiazoles ...

Using the appropriate ketone semicarbazones, several 4-substituted and 4,5-disubstituted 1,2,3-selenadiazoles were prepared by this method. Unsubstituted and 5-substituted rings were obtained by the reaction of the corresponding aldehydes. Ketones possessing two different a-methylene or methyl groups give a mixture of the two isomeric 1,2,3-selenadiazoles 107 and 108 [Eq. (28)]. 

Very little work has been reported on the 1,3,4-selenadiazole system. Ring-opening of 1,3,4-oxadiazolium salts (132) with hydrogen selenide gives 1,2-disubstituted hydrazines (133) which are cyclized with perchloric or fluoroboric acid to give 1,3,4-selenadiazolium salts (134) (Scheme 9)... 

Selenazole and the 1,2,4- and 1,2,5-selenadiazoles serve as heterodienes in the Diels-Alder reaction. [l,4]Cycloaddition of the active dienophiles to these selenaheterocyclic compounds, followed by deselenation, is a convenient means of synthesis of a nonselenium azaaromatic ring. Cycloaddition of 2,4-disubstituted 1,3-selenazole (131) with DMAD forms a bicyclo intermediate (132) that undergo extrusion of elemental selenium liberating the pyridine product (133) (Scheme 42) [110]. 

Disubstituted 1,2,4-selenadiazoles reacted with DMAD with extrusion of selenium to give pyrimidine-5,6-dicarboxylates, while 1,2,5-selenadiazoles produce pyrazines. In a similar way, quinoxaline can be obtained from benzo-2,1,3-selenadiazoles [6, 110]. (For deselenation see also Sect. 3.4). 

Nuclear magnetic resonance (NMR) spectra of 1,2,3-selenadiazoles have been described previously <1996CHEC-11(4)743, 1981ZNB1017>. The characteristic 111 chemical shifts of H-4 and H-5 lie in the range S 8.2-8.4 and S 8.8-9.4ppm, respectively. The newly reported data also show similar chemical shifts (Table 19). 1J H-Se for 4-substituted-l,2,3-selenadiazoles and V and 2J C-Se coupling constants for 4,5-disubstituted-l,2,3-selenadiazoles were also reported (Tables 19 and 20). 

Unlike the 1,2,3-thiadiazole ring systems that are thermally stable, mono and disubstituted-l,2,3-selenadiazoles easily decompose on heating to give the acetylenes 111 in high yields (Eq. (30)]. 

Substituted 1,2,3-selenadiazoles were found to react with a strong base to form 2,( -disubstituted-l,4-diselenafulvenes 123. The stereochemistry of compounds 123 was established by NMR. It is suggested that the cis-trans isomerization of the 1,4-diselenafulvenes occurs via the diselenolium ion (124) [Eq. (33)]. 

Disubstituted-l,3,4-selenadiazoles (174) were obtained by Stolle and Gutmann in 1904, through the reaction of iV,iV -diacylhydrazines (173) with phosphorus pentaselenide. [Eq. (45)]. 

While unsubstituted or monosubstituted 1,2,5-selenadiazoles are reportedly quite stable to aqueous or alcoholic hydroxide solutions, the disubstituted derivatives (201) react quite readily even at — 70° with Grignard reagents or alkyl lithiums. The reaction is believed to proceed through a selenoether intermediate (202) to give 1,2-dicarbonyl compounds (203) ° [Eq.(49)]. 

Substituted 1,2,3-selenadiazoles react with strong base to form /3.w-disubstituted-l,4-diselenafulvenes (117). The cis isomer (117) is formed initially and is then slowly isomerized to the trans form. The rate of isomerization is enhanced by the addition of a trace of an acid, via the diselenolylium ion (Scheme 52) (73JOC338). The kinetics and mechanism of the base-catalyzed decomposition of 4-aryl-l,2,3-selenadiazoles have been studied. It has been shown that arylethynylselenolates (118) and arylselenoketenes (119) are intermediates in the diselenafulvene formation (74JOC3906). 

Thiadiazoles and 1,2,5-Selenadiazoles. - Synthesis. 4-Substi-tuted 3-aroyl-1,2,5-thiadiazoles (340 Ar=Ph,4-Me,4-ClCgHj, thienyl, R=Ph,4-Me-,4-ClCgHjj,Me,CF2) result from ArCOCH COR and 2,4-Disubstituted 1,2,5-thiadiazol-3(2H)-ones (341 alkyl,... 

Examples of well-established 1,3,4-thiadiazole syntheses have been extended to the production and study of their selenium analogues." 1,4-Disubstituted selenosemicarbazides (191 R = Aik, Ar, or Ac) are readily accessible in good yield by the addition of isoselenocyanate esters (including benzoyl isoselenocyanate) to acylhydrazines." Their cyclization in acidic media affords the stable crystalline 1,3,4-selenadiazoles (192 R = Aik, Ar, or Ac) " the members of the aroylamino-series are stable to alkalis, but are hydrolysable by acids to the parent amines (192 R = H) these are convertible into conventional derivatives, as expected." Bis-selenoureas of type (194) are similarly accessible, but usually cyclize spontaneously to the substituted 2,5-dianilino-1,3,4-selenadiazoles (195). The action of acetic anhydride on (194) yields diacetyl derivatives of (195)... 

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