Coupling reactions palladium® chloride - metal salts

In early studies, it was observed that when the NHG was already attached to the metal center, reaction times were shortened since the time for the deprotonation of the salt and coordination to the metal center was no longer required. The use of well-defined systems also allows for a better understanding of the actual amount of stabilized palladium available in the system. Herrmann reported on two similar Pd(0) complexes bearing two carbenes, 37 and 38. The latter was used in 2002 as the first example of coupling of aryl chlorides (activated and unactivated) with arylboronic acids at room temperature, in high yields, and reaction times between 2 and 24 h in the presence of GsF as base. 

Semmelhack demonstrated that the addition of alkali metal chloride salts can sometimes markedly increase the yields of coupling products in heterogeneous Pd-catalyzed reactions, especially when the olefin component contains an amide function [15]. It has been claimed that palladium-grafted mesoporous material (MCM-41), designated Pd-TMSll, is one of the most active heterogeneous catalysts for the Heck reaction and enables C-C formation with activated and non-activated aryl substrates [16a,b]. Nanoscale particles of palladium clusters prepared by the ultrasonic reduction of Pd(OAc)2 and NR4X in THF or methanol, were also active for C-C couplings [17]. 

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