Preparation of Mono-1,3,2,4-Dithiadiazolylium Salts

This salt is most commonly prepared by oxidation of sulfur and S4N4 14 with AsF5 44 in liquid S02 in the presence of a catalytic quantity of bromine (82IC1679). However, this route is potentially hazardous in inexpert hands S4N4 is explosive and AsF5 is a highly toxic gas. The whole reaction is carried out under pressure ( 3-4 bar). 

Other preparations have also been reported [82IC1679 86IC4451 87JCS(D)1565], and more recently, directed syntheses from less haz- 

[SNS][AsF6] As a Building Block in Sulfur-Nitrogen Chemistry 

The reactions of 17a with nitriles have been examined extensively and have been shown (by nmr), in many cases, to proceed quantitatively in liquid S02. Yields are commonly in excess of 80%. A summary of all published reactions of this type is given in Table IX. In the majority of cases, reaction proceeds smoothly at room temperature over 1-24 h. 

Mechanistically, the cycloaddition process would appear to be highly asynchronous (91IC3342), and MO calculations indicate that a transition state involving [RC=N8. . . +SNS] interactions is involved, followed by cyclization. Although there are several other routes to 1,3,2,4-dithiadiazolylium (3+) salts (described below), cycloaddition of [SNS] [AsF6] with the parent nitrile would appear to constitute the only general quantitative route available. 

---