Transition Metal Compounds and Organic Ligands as Catalysts

Transition Metal Compounds and Organic Ligands as Catalysts 

Cobalt(II) iodide and dicobalt octacarbonyl are not particularly effective as catalysts in the carbonylation of nitrobenzene [35, 36]. With Co2(CO)g at 190 °C and 200 atm, PhNCO was obtained in 25 % yield. Nitroso, azoxy and azo derivatives have also been used as substrates. Better yields have been achieved by adding to anhydrous cobalt(II) chloride, nitrogen donor ligands such as pyridine, 2,2 -bipyridine (Bipy), pyrimidine, quinoline and isoquinoline [37], In the presence of Bipy, at 180 °C and 120 atm, dry C0CI2 gave PhNCO in 68 % 

Analogously to cobalt, the addition of ligands such as pyridine and quinoline to PdCU and RhCh notably increases the activity of the metal salts. Thus PhNCO and TDI were obtained in 70-80 % yields at 200-210 C and 120 atm, by adding pyridine to PdCh, in chloro- or o-dichlorobenzene as solvents [38, 39]. The catalyst was recovered as the pyridine complex, PdPyiCh, and recycled. The use of RhCls under similar conditions was also claimed [38]. Between 4 and 8 moles of pyridine per mol of catalyst were used lower and higher quantities gave lower yields of isocyanate. The use of substituted pyridines [40], of quinoline and substituted quinolines, and of preformed complexes with these ligands [40, 

has also been reported. In the last case, the activity and selectivity were increased when excess ligand was added to the preformed palladium(II) complex [42]. Electron-donating alkyl substituents not in the a-position of the pyridine ring increase the degree of conversion of PhNOa and the yield of PhNCO [42, 

Conversely, when an electron-withdrawing substituent was introduced in the P-position, nitrobenzene was not carbonylated. Bulky substituents in the a-position of the pyridine ring have a negative effect as they should reduce, like electron-withdrawing substituents, the coordinating ability of these ligands. 

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