Palladium-Phosphine-Alkylammonium Halide for the

PALLADIUM-PHOSPHINE-ALKYLAMMONIUM HALIDE FOR THE SYNTHESIS OF UREAS 

Concerning the effect of the substituents, electron-withdrawing groups on the nitroarene or electron-releasing groups on the aniline increased the reaction rate with respect to the use of nitrobenzene and aniline, whereas the reverse occurred 

Although the use of an atmospheric CO pressure is interesting, the limits of the aforementioned system lie in the low catalytic ratio, long reaction time and only moderate selectivity. In recent years, Lee et al. have reinvestigated this catalytic system and much improved it [141-149]. The main results obtained by these authors are the following  

0 The addition of BujN in the previously described system appears only to increase the solubility of the palladium complex. If conditions are chosen such that the complex is completely soluble in the reaction medium (the use of excess aromatic amine is effective in this respect), the addition of the tertiary amine is useless [141, 142]. 

The process associated with the second equation is favoured by the use of an high aniline concentration. The occurrence of a process that can be described as an aniline oxidative carbonylation using nitrobenzene as an oxidant closely parallels many reactions described in paragraph 3.2.1. and the Pd(OAc)2/DPPP/AcOH-catalysed synthesis of carbamates [125] described in paragraph 3.2.2. The mechanistic implications of these competing stoichiometries are discussed in detail in Chapter 6. 

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