1,4,2-Diazaphospholidine-3,5-diones and related compounds: a lecture on unpredictability in catalysis.

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.