Rhodium and Palladium as Catalysts

In 1967, Hardy and Bennet reported the conversion of aromatic mononitro compounds to isocyanates by carbon monoxide, catalysed by palladium or rhodium on alumina or carbon, with dry Feds as cocatalyst (eq. 7) [24,25]  

Reaction conditions were 200 °C or less and at least 200 atm of carbon monoxide. Benzene, chlorobenzene, cyclohexane, or 1,1,2-trichloro-1,2,2-trifluoroethane were used as solvents. The noble metal could be supported, but even unsupported or in the form of salt, oxide, or complex. Among the Lewis acid cocatalysts used successfully were FeCU, FeCU, FeBrs, AICI3, AlBrs, SnCl4, CuCL, and anhydrous HCl, but the highest yields were obtained with supported metals and with FeCL as cocatalyst. The presence of FeCL allowed for the use of milder experimental conditions with respect to the monometallic system, also affording higher conversions and better selectivities [24-26]. For example, nitrobenzene (24.6 g), Rh/C (5 % on Rh, 5.0 g), anhydrous ferric chloride (0.4 g), under 500 atm of carbon monoxide and at 190 °C for 5.5 h in benzene (100 ml), gave 100 % conversion with formation of PhNCO (35 % after vacuum fractional distillation). Diphenylurea, PhNHC(0)NHPh, and 1,3,5- 

Metallic palladium on zeolite gives PhNCO fi om nitrobenzene with 35 % selectivity in dichlorobenzene at 240 °C and 200 atm [29], It has also been reported that zeolite Y exchanged with palladium (II), alone or in the presence of pyridine, gives PhNCO from nitrobenzene at 220-240 °C and 250 atm [30]. At 

Using RI1/AI2O3 in o-ChC, in the presence of pyridine, at 240 °C and under 200 atm CO, PhNCO is obtained in 70 % yield from PhN02, with 100 % conversion [34]. Among the by-products, only azobenzene (0.25 %) was identified. Based on IR studies, a reaction mechanism was proposed, which is discussed in Chapter 6. 

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Transition-metal-catalyzed carbonylation reactions have shown impressive progress during past few decades especially, the use of ruthenium, rhodium, and palladium as catalysts is widespread. More recently, iron and copper catalysts have also been attracting the attention of synthetic chemists because of their low cost and environmentally benign properties. 

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