Palladium and Chelating Nitrogen Ligands

Palladium-phenanthroline catalytic systems, which have been developed in the last decade, appear to be the most promising catalytic systems for possible industrial applications, due to their high activity, versatility and lack of corrosion problems. These systems can afford several products in excellent yields, depending on the reagents used together with the nitro compound. Their use for the synthesis of isocyanates and heterocyclic compound is discussed in Chapters 2 and 5 respectively. In this chapter, only the synthesis of carbamates and ureas (including acylureas) will be discussed. 

Ligand (mmol) Acid (mmol) Reaction rate (mol PhN02/mol Pd h) Selectivity  

The question of the identity of the best acids when a phenanthroline ligand is used is not completely set. Sulphonic acids were said to be better than TMBH, but, when we tested the use of / -TsOH in the synthesis of isocyanates, only negative results were obtained (however, experimental conditions had been optimised on TMBH) [136]. Moreover, workers in other companies also apper to prefer the use of TMBH [137]. More recently, addition of / -TsOH to a [Pd(Phen)2][OTf]2 catalyst was found to lead to complete decomposition to palladium black even in the presence of an alcohol by another group [131]. The difference with respect to the data reported by Drent was attributed to the presence of the acetate ion in the system used by this author. The formed acetic acid may have a stabilising effect on the catalyst. It is possible that the identity of the best acid is a function of the experimental conditions, although the published data is still insufficient to address this question in more detail. The 

Compared to the attention paid to the synthesis of carbamates, little has been reported on the use of these catalytic systems for the synthesis of ureas. In one patent, the use of Pd(OAc)2 in conjunction with DPPP and AcOH (conditions which are not the ideal ones for the synthesis of carbamates, vide supra) for the 

A comparison of a catalytic system related to the ones here discussed with other systems for the synthesis of ureas is also reported in the next paragraph. 

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