Mizoroki-Heck reaction electron-rich bulky phosphines

The mechanism of action of these base-free Mizoroki-Heck reactions is an intriguing question. These reactions are performed under very mild conditions, so any uncontrolled thermal decomposition pathways might be ruled out. Evidently, palladium(II) hydride intermediates sometimes regenerate palladium(0) spontaneously, either by reductive elimination or because of enhanced acidity. While there is no well-documented evidence of reductive elimination, Milstein and coworkers [11] presented a system with enhanced acidity that is rendered by an electron-rich bulky phosphine 14 in the coordination sphere. These authors described base-free Mizoroki-Heck reactions in the presence of ligand 14 (12 15, Scheme 2.3), which was shown to increase the acidity of palladium(II) hydride... 

The efficiency of bulky and electron-rich phosphines in Mizoroki-Heck reactions seems to be due to their ability to generate monophosphine-Pd(O) or -Pd(II) complexes in each step of the catalytic cycle (Scheme 1.55). Steric factors are probably more important than electronic factors. One sees from Fu s studies that the last step of the catalytic cycle in which the Pd(0) complex is regenerated in the presence of a base may be rate determining. The role of this last step has been underestimated for a long time. Provided this step is favoured (e.g. with P-r-Bus as ligand and Cy2NMe as base), the oxidative addition of aryl chlorides would appear to be rate determining. However, Mizoroki-Heck reactions performed from the same aryl chloride with the same Pd(0) catalyst and same base but... 

The Mizoroki-Heck reaction is a subtle and complex reaction which involves a great variety of intermediate palladium complexes. The four main steps proposed by Heck (oxidative addition, alkene insertion, )3-hydride elimination and reductive elimination) have been confirmed. However, they involved a considerable number of different Pd(0) and Pd(Il) intermediates whose structure and reactivity depend on the experimental conditions, namely the catalytic precursor (Pd(0) complexes, Pd(OAc)2, palladacycles), the Ugand (mono- or bis-phosphines, carbenes, bulky monophosphines), the additives (hahdes, acetates), the aryl derivatives (ArX, ArOTf), the alkenes (electron-rich versus electron-deficient ones), which may also be ligands for Pd(0) complexes, and at least the base, which can play a... 

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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