A-Heterocyclic Carbene-based Complexes

Their role in the Mizoroki-Heck reaction is different. An analysis of published data shows that, in the intermolecular Mizoroki-Heck reaction, the use of carbene ligands is 

Carbene ligands are used either as preformed palladium complexes or added as the respective salts. As soon as the complexes involve palladium(II), reductive preactivation is required, which is evidenced by rather long induction periods. Here lies one of many essential differences between carbene and phosphine systems While phosphines are known to lend assistance in the reduction of palladium(II) topalladium(O), carbenes are not capable of this. This accounts for the need for harsh conditions, under which the preactivation processes (partial or full dissociation of complex and reduction by other components of reacting system) occur. The addition of reducing agents sometimes results in activation under milder conditions [186] or shortening of the induction period [33], but this is not common practice. 

The only (more or less) clear trend is that usually the less stable precatalyst performs better in a given model reaction. Stability is affected by geometric parameters, such as size of the chelate (five- and six-membered rings being more stable than larger rings) and steric bulk. The absence of trends is indicative of the SRPCs, for which the difference between various forms is relatively unimportant as soon as the conditions under which a given precatalyst releases active palladium species are realized. 

Even hybrid carbene-phosphine precatalysts such as 136-139 showed weak and uneven dependence on the phosphine ligand (Table 2.4, entries 14-17) [204], The complexes with bulky electron-rich phosphines 138 and 139 were less reactive than those with conventional triarylphosphines 136 and 137. This can hardly be regarded as surprising, as the deligation of the carbene is likely to be less probable than the deligation of the phosphine at the applied temperatures activation of the precatalyst should inevitably occur by dissociation of the phosphine first. In this respect, the design of hybrid carbene-phosphine complexes for Mizoroki-Heck reactions is not promising. 

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