Palladium complexes precursor reactivity

Palladium complexes, immobilized through appropriate linkers on various supports, might be regarded as a palladium complex precursor. In our opinion, from the viewpoint of reactivity and catalytic performance, there is no significant difference between these and other common precatalysts. All of them enter into the Mizoroki-Heck catalytic cycle via a preactivation procedure. 

Moving to palladium complexes, related early studies on [PdX(Et4-dien)]+ also indicated the possible operation of a CB mechanism. Hydroxide ions reacted differently to other nucleophiles at these steri-cally hindered molecules in showing a distinct bimolecular (A2) dependence as well as a solvolytic pathway (85). This was unexpected because OH" is known to be a poor nucleophile in these systems, and it led to the proposal of the conjugate base contribution. The ions [PdX-(Et4Me-dien)]+, which have no acidic protons, did not show this behavior. The CBs reacted some 30 times faster than their precursors (a reactivity enhancement somewhat less than their gold(III) counterparts with the same ligands), again probably by an associative route. 

Scheme 1.49 Ionic mechanism for Mizoroki-Heck reactions catalysed by a Pd(0) coordinated to one or two C—C saturated or C=C unsaturated fi-heterocyclic monocarbenes (only one way for the coordination of the alkene is presented). The reactive species is PdP(Cb) for a bulky carbene and Prf(Cb)2 for a nonbulky carbene. The aryl-palladium complex formed in the oxidative addition is always ligated by two Cb ligands delivered by the Pd(0) or PdfII) precursor even if Pcf(Cb) is the reactive species.

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... 

The use of palladium(ll) complexes with chelating olefins such as COD or nbd, whether in catalysis or in other reactivity studies, where the diene ligand is accessory, or the complex is just the precursor for the in situ preparation of the actual catalyst by adding another ligand, is extremely frequent but is not dealt with in this section. For all these reasons, in spite of the importance of palladium(n) complexes in the chemistry of palladium, the size of this section is not proportional to its importance. Here, we will concentrate mostly on new isolated palladium complexes. 

Often catalyzed by a variety of palladium complexes or simply by a mixture of Pd(OAc)2 and PR3, these involve initial reduction of a Pd(II) precursor to Pd(0), normally stabilized by a single 2e ligand, L, typically a phosphine or an NHC. Subsequent oxidative addition of RX generates an R-Pd(II)(L)X intermediate. Basic, bulky phosphines, such as P(t-Bu)3 or X-phos (4.11) facilitate the OA by favoring the formation of this highly reactive zerovalent, 1 1 complex, PdL, in line with the idea that the microscopic reverse, RE from Pd(II), often takes place from a three-coordinate LPd(R)(X)... 

The reactivity of phosphasilenes with organic compounds, for example, [2 + 2]-cycloaddition reaction (8) has been intensively investigated. The reactivity of phosphasilenes with transition metal complexes, however, is comparatively unexplored (9,10). The described phosphasilene 1 can imdergo unprecedented EtZ isomerization of the Si=P bond upon coordination to tungsten (11). Another remarkable reactivity study describes the coordination of group 10 transition metals to phosphasilene 1 generating dinuclear platinum and palladium complexes with Si—P bond cleavage and a bissilylene nickel complex from a Ni(0) precursor (12). 

Because of their convenient preparation from palladium(II) salts and stable NHC-precursors (vide supra), paUadium(ll) complexes were first examined as potential catalysts for Heck-type reactions. Due to the high thermal stability, temperatures up to 150°C can be used to activate even less reactive substrates, like, e.g., aryl chlorides. Inunobilization of such catalysts has been shown recently (vide infra) ... 

Ligand size determines coordination numbers as well as reactivity. Thus phosphine complexes of Pd°, frequently used as precursors in palladium catalyzed reactions, may be 4-, 3-, or 2-coordinate, as in Pd(PMe3)4, Pd(PPr )3, and Pd(PPhBu 2)2, respectively. For nickel phosphine and phosphite complexes, the dissociation constant Kd for the equilibrium... 

Figure 5.24 Synthesis of a carbene precursor with an axially chiral backbone and reactivity of its palladium(ll) allyl complex.

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