Palladium center

Transmetallation—Transfer of substituent R from tin to the palladium center thus generating a palladium species 7 that contains both the fragments R and R that are to be coupled. 

Nevertheless it does not change the principle of the mechanism proposed by Scholten and Konvalinka, i.e. the ability to act catalytically of only the superficial palladium centers released from the vicinity of the interstitial hydrogen. Bearing in mind the dynamic character of the equilibrium in a palladium-hydrogen system as a whole is to regard such centers as being mobile in the surface layer of the hydride. 

Surprisingly, diethyl 2-vinyl-[l,3]-dioxolane-4,5-diacetate 2 is very reactive (Entry 1). Whatever the aryl bromide, complete conversions are observed after 3h and high isolated yields (> 73%) toward the expected compound are achieved. No product issued from the alternative syn-/i-H-elimination is detected. In that case, we suggest that a specific interaction between the ester group and the palladium center could occur leading to a stabilized 7 membered-ring intermediate 7 avoiding thus the formation of undesired product. 

The structure catalyst prepared using this ligand is shown in Fig. 11 and suggests that the high molecular weights can be attributed to the bulky substituent blocking at least one of the axial positions on the palladium center. 

To probe the reaction mechanism of the silane-mediated reaction, EtjSiD was substituted for PMHS in the cyclization of 1,6-enyne 34a.5 The mono-deuterated reductive cyclization product 34b was obtained as a single diastereomer. This result is consistent with entry of palladium into the catalytic cycle as the hydride derived from its reaction with acetic acid. Alkyne hydrometallation provides intermediate A-7, which upon cw-carbopalladation gives rise to cyclic intermediate B-6. Delivery of deuterium to the palladium center provides C-2, which upon reductive elimination provides the mono-deuterated product 34b, along with palladium(O) to close the catalytic cycle. The relative stereochemistry of 34b was not determined but was inferred on the basis of the aforementioned mechanism (Scheme 24). 

It was concluded that the high selectivity observed in the hydrogenation experiments using 26 b is explained by the relatively strong coordination of the alkyne to the palladium center, which only allows for the presence of small amounts of alkene complexes. Only the latter are responsible for the observed minor amounts of ( )-alkene, which was shown to be a secondary reaction product formed by a subsequent palladium-catalyzed, hydrogen-assisted isomerization reaction. Since no n-octane was detected in the reaction mixture, only a tiny... 

In order to keep the mild conditions, hydroxycarbonylation has been performed in biphasic media, maintaining the catalyst in the aqueous phase thanks to water-soluble mono- or diphosphine ligands. In the presence of the sodium salt of trisulfonated triphenylphosphine (TPPTS), palladium was shown to carbonylate efficiently acrylic ester [19], propene and light alkenes [20,21] in acidic media. For heavy alkenes the reduced activity due to the mass transfer problems between the aqueous and organic phases can be overcome by introducing an inverse phase transfer agent, and particularly dimeihyl-/-i-cyclodextrin [22,23]. Moreover, a dicationic palladium center coordinated by the bidentate diphosphine ligand 2,7-bis(sulfonato)xantphos (Fig. 2) catalyzes, in the presence of tolylsulfonic acid for stability reasons, the hydroxycarbonylation of ethylene, propene and styrene and provides a ca. 0.34 0.66 molar ratio for the two linear and branched acids [24],... 

Furukawa and co-workers (368,369) succeeded in applying the softer dicationic Pd-BINAP 260 as a catalyst for the 1,3-dipolar cycloaddition between 225 and 241a (Scheme 12.82). In most cases, mixtures of endo-243 and exo-243 were obtained, however, enantioselectives of up to 93% ee were observed for reactions of some derivatives of 225. A transition state structure has been proposed to account for the high selectivities obtained for some of the substrates (368). In the structure shown in Scheme 12.82, the two phosphorous atoms of the Tol-BINAP ligand and the two carbonyl oxygens of the crotonoyl oxazolidinone are arranged in a square-planar fashion around the palladium center. This leaves the ii-face of the alkene available for the cycloaddition reaction, while the re-face is shielded by one of the Tol-BINAP tolyl groups. 

Interestingly, the bis-urea pocket represents also a good host for anions and, indeed, upon the addition of ammonium chloride the pocket is occupied with the chloride anion. The binding pocket can change the reactivity ofthe metal center as it facilitates substitution processes at the palladium center. Upon the introduction of... 

The selective t 2 coordination of the diene to the palladium center is suggested.141... 

The first detailed study of the individual steps of the cationic pathway of the intramolecular Heck reaction was recently described by Brown (Scheme 8G.21) [46], Oxidative addition of aryl iodide 21.1 to [l,l -bis(diphenylphosphino)ferrocene](cyclooctatetraene)palladium generated 21,2. Complex 21.2 was stable at room temperature and was characterized by X-ray crystallography no interaction between the palladium center and the tethered alkene was observed in this intermediate. Treatment of 21.2 with AgOTf at -78°C removed iodide from the palladium coordination sphere, which facilitated a rapid alkene coordination and subsequent... 

The structure of 70 was established by X-ray diffraction analysis (Figure 22). The palladium center is only coordinated by the phosphorus atom of 32 (the sulfur atom remains pendant) and the dvds coligand (r 2 r 2 fashion), resulting in a slightly distorted trigonal planar geometry around the metal. The rigid thioxanthene linker maintains the boron atom remote from the coordination sphere of the metal. 

It is assumed that the mechanism of the palladium-catalyzed cross-coupling reactions of iodonium salts involves the initial oxidative addition step, followed by ligand coupling at the iodine and then at the palladium centers analogously to the mechanism shown in Scheme 31 [63,66]. 

In a continuous set-up using Go-5, a considerable decrease in activity was also observed in comparison with the monomeric models. This effect was partly explained by deactivation of the catalyst due to the reaction of the catalytic units with the membrane or to some dendritic effect (due to the proximity of the unit centers leading to double or multiple phosphine complexation with the same palladium center), since the formation of a palladium black precipitate was observed on the membrane. Tests with the Go dendrimer model ligand (without palladium) showed a retention degree of 85%. This fact could also partly explain the decrease in activity due to the washout of catalyst during the reaction. However, a first-generation Gi-5 catalyst, with a higher retention, showed almost the same deactivation behavior. Thus, catalyst decomposition is most probably the main reason for the observed deactivation. 

In batch processes, the monodentate catalysts showed lower activity compared to their bidentate analogs. The activity per palladium center was constant upon increasing the dendrimer generation of the dendritic Pd(allyl) complexes, indicating that all active sites act as independent catalysts. In addition, the selectivity between the E- and Z-products was similar to that induced by analogous mononuclear palladium complexes. Although a considerable amount of the branched product was observed, the authors did not put forward an explanation for its formation. 

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