Palladium and Metal-containing Cocatalysts

In Chapter 2 we have discussed the use of PdCL in the presence of pyridine or isoquinoline for the synthesis of isocyanates. The same catalytic system has also been applied to the synthesis of carbamates [33-35], but the results are very poor. The system is able to catalyse the carbonylation even of dinitro compounds 

From the purpose of optimisation of the activity of the palladium catalyst, high cocatalyst/Pd ratios appear to give the best results, with turnovers of several thousands being obtained in some cases. However, problems with the highly corrosive nature of most of the active cocatalysts push towards the use of minor quantities of them. During the years, research has evolved towards the identification of more efficient cocatalysts, which may be used in small amounts. In the following we will try to evidence this evolution. 

Before starting the discussion on these palladium-based systems, we must note that many patents in this field also mention the use of other metals, especially rhodium and/or ruthenium, in place of palladium. However, under the same conditions, these last two metals appear to give always worst results with respect to palladium and they appear to have been included only for patenting purposes. Often their use is limited to one example and relevant data are not reported on Chemical Abstracts. Thus we will not discuss them here. A discussion of the catalytic systems specifically optimised with these metals is given in appropriate paragraphs. 

Some characteristics appear to be common to all of the systems discussed in this paragraph  

Possible explanations for these requirements are given in Chapter 6. 

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