Selenium and Sulfur Dichlorides

Selenium (and sulfur) dichlorides are useful reagents for the synthesis of organoselenium compounds,18 nitrogen selenium heterocycles19-21 and other selenium compounds. The key steps in these reactions usually involve a 

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A more versatile synthesis of 11.1 (and the selenium analogue) involves the cyclocondensation of trisilylated amidines with sulfur dichloride or SeCl2 generated in situ (Eq. 11.3). This route can be used to prepare the prototypal systems [HCNEEN]" (E = S, Se). It is also readily extended to the synthesis of multi-dichalcogenadiazolium cations such as 1,3- or 1,4-C6H4(CNEEN)2] (H-2, E = S, Se), °... 

The 1,5-isomers 13.3 (E = S) are colourless, air-stable solids. They are prepared by the cyclocondensation reaction of R2PN2(SiMe3)3 with sulfur dichloride or thionyl chloride. A similar cyclocondensation process, using a mixture of SeCU and Sc2Cl2 as a source of selenium, produces a mixture of the isomers 13.2 and 13.3 (E = Se, R = Ph). The structures of 13.3 (E = S, R = alkyl, aryl) are folded eight-membered rings with a cross-ring S S distance of ca. 2.50 This structural... 

Compound (1) reacted with selenium dioxide to afford the disubstituted selenium derivative (49). Nucleophilic attack at the pyrrolopyridine C-3 atom by diethyl selenite, generated from selenium dioxide and ethanol, afforded the product (49) in 44% yield (Equation 6). An analogous reaction was carried out using sulfur dichloride to give the thio derivative (50) (44%). Both reactions are shown in Equation (6) <92BCJ2992>. 

The reaction very likely proceeds through the disproportionation of 4-ethoxyphenyl tellurium monochloride to tellurium and the diaryl tellurium dichloride. Bis[trifluoromethyl] tellurium transferred its trifluoromethyl groups to iodine, sulfur, selenium, phosphorus, arsenic, and antimony on heating with the elements, sulfur dichloride, selenium tetrabromide, or the triiodides of phosphorus, arsenic, and antimony at 170 to 220° in sealed tubes3. 

In contrast to sulfur chemistry where dihalides are well established, the isolation of dihalides of selenium and tellurium has only been achieved for SeCl2 and SeBr2 (reactions 15.71 and 15.72). Selenium dichloride is a thermally unstable red oil SeBr2 is a red-brown solid. 

Synthesis and General Properties. Sulfur tetrachloride, a pale yellow crystalline solid, is prepared best by chlorination of sulfur dichloride at —78° (I). Selenium tetrachloride also pale yellow is available corn-... 

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