Benzothiadiphosphole as phosphorus donating reagent for a new route to 2H-1,2,3-diazaphosphole derivatives

Conjugated azoalkenes 2 react at reflux toluene with fused benzothiadiphosphole 1 (or 4) to give the diazaphosphole 3 in 25–52% yields. With this route it is possible to isolate the so far unknown 3a which is difficult to obtain in other reported conditions. The P atom involved in this cycloaddition is the one between the two S atoms.