Cleavage of blocked isocyanates within amino-type resins: Influence of metal catalysis on reaction pathways in model systems

In the present work, a detailed account on the deblocking reactions of various blocked isocyanates in aqueous reaction media in the presence of amido- and hydroxyl-nucleophiles is given. Pyrazole- and oxime-blocked phenylisocyanate were reacted with monomethylolurea (MMU), dimethylolurea (DMU) and urea (U) in the presence of the catalysts (dibutyltin dilaurate DBTL 1, zirconium(IV)dionate 2, zinc(II)acetate 3 and manganese (III)dionate 4). Product formation was studied by kinetic analysis of either the hydrolytic or the additive reaction pathways, revealing strong differences in the reactions pathways of the used catalysts. Both zirconium(IV)dionate 2 and DBTL 1 lead to the formation of N-phenyl-N′-carbamidurea and N,N′-diphenylcarbamidurea 15 as the preferred products after reaction with urea as nucleophile, whereas zinc(II)acetate and manganese(III)dionate indicated a significantly stronger promotion of hydrolytic reaction pathways. Zirconium(IV)dionate 2 catalyst proved advantageous over DBTL 1 displaying a lower deblocking temperature.