Synthesis of phenyl isocyanate and derivates via oxidative carbonylation of aniline catalyzed by Pd(II)-based redox systems

One of the challenges in current day catalysis is to replace wasteful and dangerous industrial processes by more environmentally friendly and safer ones. An example of such a challenge is to replace the highly toxic and corrosive phosgene often used as carbonylating agent. An important example, from an industrial point of view, is represented by the synthesis of isocyanates, starting materials for the production of polyurethanes, actually performed by treating amines with phosgene. Among the numerous phosgene-free methods reported in the literature to produce isocyanates, carbamates and ureas, the reductive carbonylation of nitro compounds and the direct oxidative carbonylation of amines represents a valid alternative to the traditional methods. The Pd-catalyzed carbonylation of aniline to phenylisocyanate, phenylcarbamate or 1,3-diphenylurea has been deeply studied and appears particularly attractive from the standpoint of atom economy. In such a systems, the catalyst consists of a Pd(II) salt that, to form the desiderated products, is reduced to inactive Pd(0) species. The reoxidation of such species to Pd(II) active ones is a key step which permits the restarting of the catalytic cycle. As a matter of fact, the direct reoxidation of Pd(0) species by molecular oxygen is so slow that the precipitation of inactive palladium metal (palladium black) often competes with the reoxidation itself, making inefficient the catalysis. Several methods have been proposed and developed to circumvent this problem mainly based on two strategies which can work at the same time: 1) the stabilization of reoxidable Pd(0) complexes, avoiding the formation of Pd metal; 2) the increasing of the Pd(0) reoxidation rate. An interesting proposal widely adopted in literature is to increase the rate of Pd(0) reoxidation step by inserting an electron-transfer mediator (ETM) between the substrate-selective catalyst and O2 or H2O2 as terminal oxidant. This ETM would then carry the electrons from the metal to the oxidant along a low-energy pathway which would compete kinetically with side reactions of the reduced form of the metal (precipitation, decomposition). Actually, the ETM’s (redox cocatalysts) proposed in literature are mainly metal compounds of varying valences. Salts of Cu, Fe, Ag and heteropolyacids like H3PMo12O40 are the most frequently used cocatalysts, which are able to oxidize Pd(0) to Pd(II), allowing to restart the main reaction cycle. In the present work we synthesized directly phenyl isocyanate through the oxidative carbonylation of aniline catalyzed by a Pd(II)-based multistep redox system in which oxygen is used as terminal oxidant.