Towards a Phosgene-Free Synthesis of Aryl Isocyanates: Alcoholysis of N-phenylurea to N-phenyl-O-methyl Carbamate Promoted by Basic Metal Oxide Nanoparticles and Organocatalysts

Abstract

There is an urgent need to develop new processes for the synthesis of polyurethanes and polycarbonates different from the current technology based on the use of phosgene. In the case of polyurethanes, the reaction of phosgene with aromatic amines renders N-aryl isocyanates that are the co-monomers for these polymers. In the search for a phosgene-free process for the synthesis of N-aryl isocyanates, N-aryl carbamates could play a key role of synthetic precursors. N-Aryl carbamates can be prepared in turn by alcoholysis of N-aryl ureas. Herein, we compare the catalytic activity of three homogeneous catalysts and four heterogeneous metal oxides for the methanolysis of N-phenylurea as a model probe for other aromatic ureas. In the present work we have carried out this reaction in the presence of KOH, 1,8-diaminonaphthalene (a protonic sponge), a dimeric phosphazene (strong, neutral soluble base) and nanometric ZnO, MgO, CaO and a mixed Mg/Al metal oxide derived from hydrotalcite calcination. Besides the target N-phenyl O-methyl carbamate, N-phenyl isocyanate and undesired aniline were also formed. It was observed that using KOH and diaminonaphthalene, aniline was formed preferentially with respect to the desired carbamate. In contrast, dimeric phosphazene forms the target N-phenyl O-methyl carbamate with 90% selectivity at 99% N-phenylurea conversion. In the case of the metal oxides, the best-performing solid catalyst was CaO that exhibits similar activity (99% N-phenylurea conversion) and carbamate selectivity (89%) as phosphazene. CaO performs as a truly heterogeneous catalyst and could be reused for a second run with only a 4 and 2% decrease in conversion and selectivity, respectively.