Palladium hydroxide complex

Upon pressurization of a solution of the palladium hydroxide complex (tBupcp)pd-OH (3) in CgDg with 7atm of H2 at room temperature, quantitative conversion to the Pd-H complex 1 was observed over 60 h (Scheme 11.8) [37, 38]. Unusual kinetics were observed for this reaction as the reaction rate was first order in [H2] but only half-order in [3] [38]. Well-behaved kinetic behavior was also observed only when the reaction was carried out in the presence of an excess of water. Notably, water was found to inhibit the reaction rate and, with water being a product of the reaction, it was important to keep the concentration of water constant through the use of an excess of water. It was initially thought that the inhibition by water was due to an equilibrium effect, essentially pushing... 

Scheme 2.23 Reaction of a palladium hydroxide complex with an imidazolium salt, leading to reductive elimination of a 2-phenylimidazolium species.

The postulated steps that constitute the Suzuki coupling process are shown in Scheme 25. After oxidative addition of the organic halide to the palladium(o) catalyst, it is presumed that a metathetical displacement of the halide substituent in the palladium(ii) complex A by ethoxide ion (or hydroxide ion) takes place to give an alkoxo-palladium(ff) complex B. The latter complex then reacts with the alkenylborane, generating the diorganopalladium complex C. Finally, reductive elimination of C furnishes the cross-coupling product (D) and regenerates the palladium(o) catalyst. 

Kostic et al. recently reported the use of various palladium(II) aqua complexes as catalysts for the hydration of nitriles.456 crossrefil. 34 Reactivity of coordination These complexes, some of which are shown in Figure 36, also catalyze hydrolytic cleavage of peptides, decomposition of urea to carbon dioxide and ammonia, and alcoholysis of urea to ammonia and various carbamate esters.420-424, 427,429,456,457 Qggj-jy palladium(II) aqua complexes are versatile catalysts for hydrolytic reactions. Their catalytic properties arise from the presence of labile water or other solvent ligands which can be displaced by a substrate. In many cases the coordinated substrate becomes activated toward nucleophilic additions of water/hydroxide or alcohols. New palladium(II) complexes cis-[Pd(dtod)Cl2] and c - Pd(dtod)(sol)2]2+ contain the bidentate ligand 3,6-dithiaoctane-l,8-diol (dtod) and unidentate ligands, chloride anions, or the solvent (sol) molecules. The latter complex is an efficient catalyst for the hydration and methanolysis of nitriles, reactions shown in Equation (3) 435... 

The physical properties of many macrocyclic polyethers and their salt complexes have been already described. - Dibenzo-18-crown-6 polyether is useful for the preparation of sharpmelting salt complexes. Dicyclohexyl-18-crown-6 polyether has the convenient property of solubilizing sodium and potassium salts in aprotic solvents, as exemplified by the formation of a toluene solution of the potassium hydroxide complex (Note 13). Crystals of potassium permanganate, potassium Lbutoxide, and potassium palladium(II) tetrachloride (PdClj + KCl) can be made to dissolve in liquid aromatic hydrocarbons merely by adding dicyclohexyl-18-crown-6 polyether. The solubilizing power of the saturated macrocyclic polyethers permits ionic reactions to occur in aprotic media. It is expected that this [)ropcrty will find practical use in catalysis, enhancement of... 

The mechanism is very similar to that of the Stille coupling. Oxidative addition of the vinylic or aromatic halide to the palladium(O) complex generates a palladium(II) intermediate. This then undergoes a transmetallation with the alkenyl boronate, from which the product is expelled by reductive elimination, regenerating the palladium(O) catalyst. The important difference is the transmetallation step, which explains the need for an additional base, usually sodium or potassium ethoxide or hydroxide, in the Suzuki coupling. The base accelerates the transmetallation step leading to the borate directly presumably via a more nucleophilic ate complex,... 

The concept can be adapted to the introduction of only one (chiral) carboxylic acid wingtip group, even without the introduction of a second one [94]. The adaptation comprises the drop-wise addition of a mixture of ammonia, sodium hydroxide and the a-amino acid in water to a solution of glyoxal and formaldehyde in water at 50°C. Yields are moderate (but excellent compared with the 40-50% achieved by the parent protocol [97]). In the event, Strassner and coworkers [94] used the chiral carboxylic acid functionalised imidazole for the synthesis of the corresponding ester functionalised bis-carbene ligand and their palladium(ll) complexes. 

Most of the mechanistic work on this reaction has been devoted to determining the role of the base. Its most obvious function would be to complex the Lewis-acidic boron reagent, rendering it nucleophihc and thus activating it toward transmetallation. However, Miyaura, Suzuki, and coworkers noted that an electron-rich tetracoordinate boronate complex was less reactive than a bivalent boronic ester. From this, they surmised that the role of the base was not to activate the boron toward transmetallation, but rather to transform the palladium halide intermediate to the hydroxide or alkoxide species, which would then be more reactive toward boron. However, in a mass spectrometry study of a reaction between a pyridyl halide substrate and an aryl boroiuc acid, Aliprantis and Canary saw no evidence of palladium hydroxide or alkoxide intermediates, despite observing signals in the mass spectra assignable to every other palladium intermediate of the proposed catalytic cycle. ... 

The kinetics and mechanism of methane combustion have been the subject of many investigations, e.g.. Refs. 43-47, because of the importance of natural gas as a potential fuel for catalytic combustors. Under conditions expected in catalytic combustors, i.e., excess oxygen, a first order in methane is generally observed [48], whercas a variety of orders has been observed for other hydrocarbons [13]. The actual mechanism appears to be quite complex and depends on the fuel used. For instance, inhibiting effects are observed for the products carbon dioxide and water in methane combustion over supported palladium catalysts [49,50]. The inhibition of methane adsorption and the formation of a surface palladium hydroxide were proposed to explain the observation. 

While potassium hydroxide is beneficial in catalysts containing palladium loads near 5% with low palladium loads it appears that potassium hydroxide is not always necessary. The semihydrogenation of dehydrolinalool (19) gives 100% of linalool (20) over 0.5% PCI/AI2O3 in alcoholic solvents as shown in Eqn. 16.25. Perhaps with lower palladium-loaded catalysts reactant diffusion is not a factor in the reaction, so the presumed enhanced adsorption of the potassium hydroxide complex is not as important. 

Preparation of the jcy/o-configurated deoxyimino sugars 805 and 807 from 802 or 806 illustrates the value of tartaric acid in enantiospecific syntheses of valuable target molecules. Ozonolysis of 802 followed by reduction with sodium borohydride in methanol provides 803. Subsequent borane-dimethylsulfide—THF complex reduction, OTBS deprotection with 60% aqueous acetic acid, and purification with Amberite IRA400(OH) resin provides, after acidification, 804 in 75% yield. Catalytic debenzylation in the presence of palladium hydroxide occurs quantitatively to afford (27, 3/ ,4R)-2-(2-hydroxyethyl)-3,4-dihydroxypyrrolidine hydrochloride (805) in an overall yield of 53% (Scheme 176). 

Several interesting variations on the reaction have recently been introduced. While the reaction with water is often carried out using a sulfonated, and hence water-soluble, tri-arylphosphine, the use of a palladium(O) complex of l,3,5-triaza-7-phosphaadamantane (77, Scheme 25) was recently reported. Monflier and co-workers reported that in addition to running the reaction under a CO2 atmosphere, the reaction of butadiene with water is facilitated by the use of cationic or neutral (nonionic) surfactants. The role of cationic surfactants such as dodecylammonium hydroxide is conceived to be threefold ... 

Scheme 11.10). Kinetic studies found that the hydrogenolysis reaction was first order in both [22] and [Hjj. A mechanism similar to that proposed for the hydrogenolysis of the hydroxide 3 was then invoked for the hydrogenolysis of the pincer palladium alkoxide complexes. 

The negatively charged base reacts with the arylpalladium(II) halide to give the arylpalladium hydroxide or alkoxide complex, which is able to form the dimeric palladium-boron complex XXIII what is crucial for the transmetallation process [2-6]. It is apparent that the metal cation (from the base) accelerates the formation of the latter, as clearly showed by Zhang and coworkers [15]. They have developed the SM coupling procedure for sterically bulky arylboronic acids when the clear influence of the anion basicity and the cation effect were discovered. The cationic radius is presumably an important parameter which influences the formation of dimeric... 

The three basic steps in the palladium-catalysed Suzuki-Miyaura reaction involve oxidative addition, transmetalation, and reductive elimination. A systematic study of the transmetalation step has found that the major process involves the reaction of a palladium hydroxo complex with boronic acid, path B in Scheme 3, rather than the reaction of a palladium halide complex with trihydroxyborate, path A. A kinetic study using electrochemical techniques of Suzuki—Miyaura reactions in DMF has also emphasized the important function of hydroxide ions. These ions favour reaction by forming the reactive palladium hydroxo complex and also by promoting reductive elimination. However, their role is a compromise as they disfavour reaction by forming of unreactive anionic trihydroxyborate. A method for coupling arylboronic acids with aryl sulfonates or halides has been developed using a nickel-naphthyl complex as a pre-catalyst. It works at room temperature in toluene solvent in the presence of water and potassium carbonate. ... 

Anion displacement of halides with azolyl anions is one common route to azolyl complexes. One example of such a synthesis is shown in Equation 4.21. In other cases, azolyl complexes have been prepared by proton transfer between the free azole and a metal alkox-ide or hydroxide. An example involving the synthesis of palladium-azolyl complexes is shown in Equation 4.22. In some rare cases, reactions of pyrrole and d early metal alkyls also lead to the formation of a metal-nitrogen bond via o-bond metathesis, as shown in Equation 4.23. Finally, several late-transition-metal-azolyl complexes possessing accompanying hydride Hgands have been prepared by N-H activation of pyrrole and other azoles. 

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