Palladium catalytic cycle

Further investigations including 109Ag NMR confirmed the in situ formation of alkynyl silver through jt complexation and proton abstraction.117 They also revealed that the best palladium catalyst, namely, palladium tetrakis(triphenylphosphine), played a key role, in that a phosphine liberated by decoordination ended up on the alkynyl silver, stabilizing it and rendering it more soluble.118 The resulting alkynyl silver then entered the palladium catalytic cycle at the transmetallation step (See section 10.6.2).105... 

The numerous palladium-catalyzed organic reactions have a relatively small number of elementary steps. Oxidative Addition, Reductive Elimination, ligand coordination, and addition to coordinated ligands (either intramolecular or intermolecu-lar) are the most important classes of transformations in most palladium catalytic cycles. The exact nature of the species within the coordination sphere of palladium and the order in which the steps take place are responsible for the variety of the organic products produced. Four representative and important palladium-catalyzed reactions are briefly discussed here to illustrate the range of organopalladium reactions. 

In terms of cost, the effectiveness of the catalytic cycle in the ring closure makes this process economical in palladium. The first three steps in the reaction sequence -- ring opening of an epoxide by a Grignard reagent, converison of an alcohol to an amine with inversion, and sulfonamide formation from the amine — are all standard synthetic processes. 

A catalytic amount of a reactive palladium(0)-complex 3 (i.e. PdLa in the catalytic cycle scheme shown below) is likely to be formed when the palladium(ll) acetate 2 oxidizes a small amount of the alkene ... 

The original Sonogashira reaction uses copper(l) iodide as a co-catalyst, which converts the alkyne in situ into a copper acetylide. In a subsequent transmeta-lation reaction, the copper is replaced by the palladium complex. The reaction mechanism, with respect to the catalytic cycle, largely corresponds to the Heck reaction.Besides the usual aryl and vinyl halides, i.e. bromides and iodides, trifluoromethanesulfonates (triflates) may be employed. The Sonogashira reaction is well-suited for the synthesis of unsymmetrical bis-2xy ethynes, e.g. 23, which can be prepared as outlined in the following scheme, in a one-pot reaction by applying the so-called sila-Sonogashira reaction ... 

As in case of other palladium-catalyzed reactions, the general mechanism of the Stille reaction is best described by a catalytic cycle—e.g. steps a) to c) ... 

The palladium-catalyzed hnear telomerization of 1,3-bntklienes provides a useful method for thepreparation of functionalized alkenes. A proposed catalytic cycle for the paliadinm-catalyzed... 

From intermediate C, the next step in the catalytic cycle involves a simple bond rotation to give D. This event is essential because it establishes the necessary syn relationship between a -hydrogen and the palladium atom. With a / -hydrogcn and the transition metal... 

Carbon-carbon bond formation reactions and the CH activation of methane are another example where NHC complexes have been used successfully in catalytic applications. Palladium-catalysed reactions include Heck-type reactions, especially the Mizoroki-Heck reaction itself [171-175], and various cross-coupling reactions [176-182]. They have also been found useful for related reactions like the Sonogashira coupling [183-185] or the Buchwald-Hartwig amination [186-189]. The reactions are similar concerning the first step of the catalytic cycle, the oxidative addition of aryl halides to palladium(O) species. This is facilitated by electron-donating substituents and therefore the development of highly active catalysts has focussed on NHC complexes. 

Scheme 78) [89]. Aryl chlorides with activating as well as deactivating substituents could also be coupled under the same conditions in high yields, ranging from 60% to 95%, within 30-60 min of microwave irradiation. The process does not require an inert atmosphere. The increased conversion observed with the addition of the ionic liquid reveals that it might have an additional function besides simply acting as a molecular irradiator . It cannot be excluded for instance that carbene palladium complexes are formed in situ and implicated in the catalytic cycle. 

The free HCl and Cl generated in the catalytic cycle produce environmentally harmful chlorinated by-products to the extent that more than 3 kg of HCl need to be added to the reactor per tonne of acetaldehyde produced to keep the catalytic cycle going. Modified catalysts such as ones based on palladium/ phosphomolybdovanadates have been suggested as a way of reducing byproduct formation to less than 1% of that of the conventional Wacker process. These catalysts have yet to make an impact on commercial acetic production, however. 

Various other transition metal complexes are also useful, including rhodium,195 palladium,196 and molybdenum197 compounds. The catalytic cycle can generally be represented as shown below.198... 

Many palladium-catalyzed reactions are initiated by the reaction of a palladium(O) complex with an acidic derivative.367 The catalytic cycle is considered to be induced by a hydridopalladium complex. When the acidic derivatives are strong acids (e.g., HBF4, HC1, CF3C02H, HOTs), the hydridopalladium formation may be regarded as the protonation of basic Pd° to afford complexes HPdL3 +368-374 or IIPdL2.S +,375 in which S = solvent (see Equation (1)) ... 

In summary, these results demonstrate that air-stable POPd, POPdl and POPd2 complexes can be directly employed to mediate the rate-limiting oxidative addition of unactivated aryl chlorides in the presence of bases, and that such processes can be incorporated into efficient catalytic cycles for a variety of cross-coupling reactions. Noteworthy are the efficiency for unactivated aryl chlorides simplicity of use, low cost, air- and moisture-stability, and ready accessibility of these complexes. Additional applications of these air-stable palladium complexes for catalysis are currently under investigation. 

Gomez-Sainero et al. (11) reported X-ray photoelectron spectroscopy results on their Pd/C catalysts prepared by an incipient wetness method. XPS showed that Pd° (metallic) and Pdn+ (electron-deficient) species are present on the catalyst surface and the properties depend on the reduction temperature and nature of the palladium precursor. With this understanding of the dual sites nature of Pd, it is believed that organic species S and A are chemisorbed on to Pdn+ (SI) and H2 is chemisorbed dissociatively on to Pd°(S2) in a noncompetitive manner. In the catalytic cycle, quasi-equilibrium ( ) was assumed for adsorption of reactants, SM and hydrogen in liquid phase and the product A (12). Applying Horiuti s concept of rate determining step (13,14), the surface reaction between the adsorbed SM on site SI and adsorbed hydrogen on S2 is the key step in the rate equation. 

Palladium-catalyzed carbon-carbon cross-coupling reactions are among the best studied reactions in recent decades since their discovery [102, 127-130], These processes involve molecular Pd complexes, and also palladium salts and ligand-free approaches, where palladium(O) species act as catalytically active species [131-135]. For example, the Heck reaction with aryl iodides or bromides is promoted by a plethora of Pd(II) and Pd(0) sources [128, 130], At least in the case of ligand-free palladium sources, the involvement of soluble Pd NPs as a reservoir for catalytically active species seems very plausible [136-138], Noteworthy, it is generally accepted that the true catalyst in the reactions catalyzed by Pd(0) NPs is probably molecular zerovalent species detached from the NP surface that enter the main catalytic cycle and subsequently agglomerate as N Ps or even as bulk metal. 

As described in the preceding sections, many domino reactions start with the formation of vinyl palladium species, these being formed by an oxidative addition of vinylic halides or triflates to Pd°. On the other hand, such an intermediate can also be obtained from the addition of a nucleophile to a divalent palladium-coordinated allene. Usually, some oxidant must be added to regenerate Pd11 from Pd° in order to achieve a catalytic cycle. Lu and coworkers [182] have used a protonolysis reaction of the formed carbon-palladium bond in the presence of excess halide ions to regenerate Pd2+ species. Thus, reaction of 6/1-386 and acrolein in the presence of Pd2+ and LiBr gave mainly 6/1-388. In some reactions 6/1-389 was formed as a side product (Scheme 6/1.98). 

Scheme 5.5. Catalytic cycle in the palladium-catalyzed telomerization of 1,3-butadiene...

A similar involvement of palladium hydride, palladium alkyl, and palladium acyl complexes as intermediates in the catalytic cycle of the Pd-catalyzed hydroxycarbonylation of alkenes was reported for the aqueous-phase analogs. The cationic hydride PdH(TPPTS)3]+ was formed via the reduction of the Pd11 complex with CO and H20 to [Pd(TPPTS)3] and subsequent protonation in the acidic medium. The reaction of the hydride complex with ethene produced two new compounds, [Pd(Et)(TPPTS)3]+ and Pd(Et)(solvent)(TPPTS)2]+. The sample containing the mixture of palladium alkyl complexes reacted readily with CO to afford trans-[Pd(C(Q)Et)(TPPTS)2]+.665... 

Only recently (2002) have the very first examples of the transition metal-catalyzed incorporation of acrylate monomers into linear polyethylene been demonstrated. In our opinion the most notable report is that of Drent and coworkers [48] who describe the use of a neutral palladium catalyst with a chelating P-O ligand to generate linear copolymers that included the incorporation of acrylate monomers (Drent s catalyst and proposed catalytic cycle are shown in Scheme 2). In these early results, there was only minor acrylate incorporation (limited to some 3-17 mol%) and the resulting polymers were of very low molecular weight (Mn 4000-15,000). 

---