Whereas 1‐organyl‐phospholanes and 1‐organyl‐2,3‐dihydro‐1H‐phospholes catalyze isocyanate oligomerization, the reaction of isocyanates with 1‐organyl‐2,5‐dihydro‐1H‐phospholes results in the formation of 1,3‐dienes and a novel class of P‐heterocycles, 1,4,2‐diazaphospholidine‐3,5‐diones. Isothiocyanates and carbodiimides exhibit analogous behavior. The resultant species readily form P‐oxides, P‐sulfides (see picture), and quaternary onium salts.
