Selenadiazole

Selenadiazole-4-carboxylic acid ethyl ester photolysis, 6, 351... 

Selenadiazole-5-carboxylic acid, 4-methyl-ethyl ester... 

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). 

The oxidation of the semicarbazone (114) with selenium dioxide in glacial acetic acid afforded the 1,2,3-selenadiazole (115) <96PS7>. This method has been used to prepare a number of 1,2,3-selenadiazoles <96PJC1143,96LA239> and <96PS155>. 

A novel synthesis of 1,2,3-selenadiazoles 338 starts with the Michael addition of 2-nitropropane to a,P-unsaturated ketones 336 under basic conditions <06JHC149>. The resulting adducts are treated with semicarbazide hydrochloride to give semicarbazones 337, which are converted to 1,2,3-selenadiazoles 338 by reaction with selenium dioxide in THF (the choice of the solvent appears to be important in this case). 

The synthesis of 1,2,3-selenadiazole derivatives has been reported. The reaction of aroyl chlorides such as 102 with potassium isoselenocyanate and ethyl diazoacetate yielded 5-(aroylimino)-2,5-dihydro-l, 2,3-selenadiazole-4-carboxylate esters such as 104. A reaction mechanism via the initial formation of the corresponding aroyl isoselenocyanate 103 followed by a 1,3-dipolar cycloaddition of the diazo compound with the C=Se bond is proposed <00HCA539>. 

In order to confirm the existence of the strained cyclophyne 77, Chan also generated 77 by an emphatic route, using the 1,2,3-selenadiazole 83 as a starting material. Thus, pyrolysis of 83 at 220 °C in the presence of tetraphenylcyclopen-tadienone (tetracyclone) gave 84, after extrusion of CO. The existence of cyclophyne 77 as an intermediate seemed evident (Scheme 18). 

The 1,2,3-thiadiazole literature was extensively reviewed in the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) <84CHEC-I(6)447>. It covered the literature up to 1981 and cited many excellent previous references to 1,2,3-thiadiazoles. A further review on the chemistry of 1,2,3-selenadiazoles and 1,2,3-thiadiazoles, while comparing and contrasting these heterocycles, focused more on the former <90HACl >. Another short review of 1,2,3-thiadiazoles addressed their rearrangement products <92PHC(4)128>. This review (second edition) covers the 1,2,3-thiadiazole literature up to the early part of 1994. It includes some references to CHEC-I when no reference to a particular subject is available between 1982 and 1994. 

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. 

A comprehensive table of C NMR data for 37 1,2,3-selenadiazoles has been published earlier <81ZN(B)1017>. Additional data are available for the following groups of 1,2,3-selenadiazoles 4,5-cycloalkeno-1,2,3-selenadiazoles with double or triple bonds in the carbocyclic ring <88LA221,... 

Table 5 Carbon-13, nitrogen-15, and selenium-77 NMR spectral data for the 1,2,3-selenadiazoles...

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