Towards a sustainable synthesis of aromatic isocyanates : by the palladium diphosphane catalyzed reduction of nitrobenzene; a first step

Aromatic isocyanates are annually produced on the megaton scale from nitro arenes. A major problem with current synthetic strategies is that they utilize the extremely toxic phosgene to carbonylate a reduced nitroaromatic. More sustainable strategies have been known for decades, in particular the palladium catalyzed reductive carbonylation of nitro arenes with CO in methanol. The lack of a clear mechanistic understanding of this reaction, however, has thus far hampered the development of sufficiently active and selective catalysts; the generation of such understanding is therefore the prime aim of this thesis. Thus, the palladium catalyzed reductive carbonylation of nitrobenzene with CO in methanol was studied in detail, wherein palladium was supported by a variety of diphosphane ligands of different steric and electronic nature. The general mechanistic pictured that emerged form these studies, is that nitrobenzene reduction chemistry is catalytically coupled with methanol oxidation chemistry by a complex network of catalytic reactions that are centred around a palladium-imido complex (__P2Pd2+=NPh__). This catalytic coupling provided a unique opportunity to gather mechanistic insights for both reactions, but also makes the system needlessly complicated when only nitrobenzene carbonylation products are desired. Several other nucleophiles were considered and 2,2,2-trifluoroethanol was identified as promising alternative to methanol.UBL - phd migration 201