Influence of Electron Transfer Mediators in the Pd(II)-Catalyzed Oxidative Carbonylation of Aniline.

Currently, the most promising alternative to the use of the phosgenation reaction, for large-scale production of isocyanates, ureas, and carbamates, appears to be the Pd-catalyzed oxidative carbonylation of arylamines. During the reaction, the Pd(II) catalytic species are reduced to Pd(0) and the addition of sacrificial oxidizing agents is usually necessary to restart the catalytic cycle. Among these oxidizing agents, molecular oxygen is undoubtedly the more appealing, from an economical and green point of view, but it is not so efficient, whereas several metal salts (named cocatalysts) can be used, able to form redox couples with Pd(0) or to act as electron transfer mediators with oxygen itself. Testing several Pd(II) complexes, metal cocatalysts, and promoters, we have found that the [PdCl₂(dppf)]/FeCl₃/LiBr = 1/1200/200 (mol/mol) system efficiently catalyzes the carbonylation of aniline to form 1,3-diphenylurea selectively (100%) with a TOF of ca. 1177 h^-1. On the other hand, the addition of oxygen to such a system strongly increases the aniline conversion (0.3 MPa of O₂ increases the TOF at ca. 3930 h^-1), but it moves the selectivity towards the phenyl isocyanate (65%, mol/mol).