Palladium naming

Another important type of reactivity of palladium, namely oxidative addition to Pd(0), is the foundation for several methods of forming carbon-carbon bonds. Aryl126 and alkenyl127 halides react with alkenes in the presence of catalytic amounts of palladium to give net substitution of the halide by the alkenyl group. The reaction, known as the Heck reaction,128 is quite general and has been observed for simple alkenes, aryl-substituted alkenes, and substituted alkenes such as acrylate esters, vinyl ethers, and A-vinylamides.129... 

The conclusion that palladium particles in zeolites may carry a partial positive charge follows from the IR study of CO adsorption. This adsorbate can be considered to be a probe of the electronic state of palladium. Namely, the shift toward higher frequencies of the CO linear band (for Pd°-CO it appears at <2100 cm ) reflects a decrease in the back donation of electrons from Pd to CO. Along with such an interpretation, Figueras et al. (138) detected the presence of electron-deficient Pd species in Pd/ HY but not in Pd/Si02. More recently, Lokhov and Davydov (139) confirmed the presence of positively charged Pd species apart from Pd° in reduced (at 300°C) Pd/Y samples and ascribed a 2120- to 2140-cm"1 band to Pd+-CO complexes (Fig. 7). Similarly, Romannikov et al. (140) report that adsorption of CO on Pd/Y samples reduced at 300°C produces IR bands at >2100 cm 1 ascribed to Pd+-CO and Pdzeolite protons, because the IR band of the zeolite O-H group decreases when CO is released and increases when CO is added to the cluster (141, 142). 

Another aspect of evolution of Pd catalysts during a reaction is the change in the electronic properties of palladium. Namely, both in the reaction of ethylene dimerization (in the absence of H2 in the gas phase) and in the reaction of CO with H2, drastic changes in the activity and selectivity during initial stage of the reaction were correlated with the fact of formation of electron-deficient Pd species. 

The essential importance during stage of impregnation of SAPO material with precious metal is an origin of the latter. Usually tetraammine complexes of Pd or Pt are used for that purpose. Tetraammine complexes have definite composition and their water solution have practically neutral pH value. For the case of palladium loading we have checked the influence of another form of palladium, namely palladium dichloride, water solution of which possesses slightly acidic reaction. Catalytic data are presented in Table 4. 

Palladium [Named after the discovery of the asteroid, Pallas] (ICSD 64922 and PDF 46-1043) Pd or (Pd,Hg) Ms 106.42 (Native elements) Cubic a = 389.03 pm Al,cF4 (2 4) S.G. Fm3m Copper type Isotropic H 1 4.5-5 Habit granular. Color gray. Diaphaneity opaque. Luster metallic. L... 

A palladium-promoted and regioselective addition of thiophe-nol to allenes has been developed. For example, reaction of this thiol with 1,1-dimethylallene in the presence of 15 mol % of Pd(OAc)2 gave only the one adduct and there by avoided the production of regioisomers usually associated with this transformation (eq 80). The active species is thought to be a thiol adduct of palladium, namely [Pd(SPh)2]n... 

The third important type of reactivity of palladium, namely, oxidative addition to Pd(0), is the foundation for several methods of forming carbon-carbon bonds. Aryl and vinyl halides react with alkenes in the presence of catalytic amounts... 

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

This reagent, to which the name nioxime has been given, ia employed for the determination of palladium and may also be used for nickel it is soluble in water, and possesses advantages over dimethylglyoxime. The latter is used as a solution in alcohol and may therefore contaminate the palladium or nickel precipitate when added to an aqueous solution. 

Palladium was named after the asteroid Pallas, which was discovered at about the same time. Pallas was the Greek goddess of wisdom. 

Based on the above-mentioned stereochemistry of the allylation reactions, nucleophiles have been classified into Nu (overall retention group) and Nu (overall inversion group) by the following experiments with the cyclic exo- and ent/n-acetales 12 and 13[25], No Pd-catalyzed reaction takes place with the exo-allylic acetate 12, because attack of Pd(0) from the rear side to form Tr-allyl-palladium is sterically difficult. On the other hand, smooth 7r-allylpalladium complex formation should take place with the endo-sWyWc acetate 13. The Nu -type nucleophiles must attack the 7r-allylic ligand from the endo side 14, namely tram to the exo-oriented Pd, but this is difficult. On the other hand, the attack of the Nu -type nucleophiles is directed to the Pd. and subsequent reductive elimination affords the exo products 15. Thus the allylation reaction of 13 takes place with the Nu nucleophiles (PhZnCl, formate, indenide anion) and no reaction with Nu nucleophiles (malonate. secondary amines, LiP(S)Ph2, cyclopentadienide anion). 

In 1803, in the course of his study of platinum, Wollaston isolated and identified palladium from the mother liquor remaining after platinum had been precipitated as (NH4)2PtCl6 from its solution in aqua regia. He named it after the newly discovered asteroid, Pallas, itself named after the Greek goddess of wisdom (ytaXXd iov, palladion, of Pallas). 

Together with reactions named after Heck and Suzuki, the Stille reac-tion belongs to a class of modern, palladium-catalyzed carbon-carbon bond forming reactions. The palladium-catalyzed reaction of an organotin compound 2 with a carbon electrophile 1 is called Stille coupling. 

Hydrogenation of the vinyl ether (49) in ether solution in the presence of palladium-on-carbon catalyst afforded 6-deoxy-2,3-0-isopropyli-dene-/ D-arafemo-hexulofuranose (53) (17) as the only product. As with the vinyl ethers (39) and (43), reduction of the double bond occurred from the least hindered side of the molecule, namely opposite to the isopropylidene ring. 

Rhodium was discovered in 1803 by the eminent Norfolk scientist W.H. Wollaston he dissolved platinum metal concentrates in aqua regia and found that on removing platinum and palladium he was left with a red solution. From this he obtained the salt Na3RhCl6, which yielded the metal on reduction with hydrogen. The rose-red colour (Greek rhodon) of many rhodium salts gave the element its name. 

Palladium and platinum are the longest known and most studied of the six platinum metals [1-11], a reflection of their abundance and consequent availability. Platinum occurs naturally as the element, generally with small amounts of the other platinum metals. It was used as a silver substitute by Colombian Indians and first observed there by Ulloa (1736), who called it platina del Pinto ( little silver of the Pinto river ) but the first sample was actually brought to Europe in 1741 by Charles Wood, Assay Master of Jamaica. Palladium was isolated in 1803 by W.H. Wollaston, who was studying the aqua regia-soluble portion of platinum ores (he announced his discovery by an anonymous leaflet advertising its sale through a shop in Soho) and named it after the newly discovered asteroid Pallas [12],... 

As has been shown by the X-ray diffraction method the parent metals (i.e. Pd or Ni), the a-phase, and /3-phase all have the same type of crystal lattice, namely face centered cubic of the NaCl type. However, the /9-phase exhibits a significant expansion of the lattice in comparison with the metal itself. Extensive X-ray structural studies of the Pd-H system have been carried out by Owen and Williams (14), and on the Ni-H system by Janko (8), Majchrzak (15), and Janko and Pielaszek (16). The relevant details arc to be found in the references cited. It should be emphasized here, however, that at moderate temperatures palladium and nickel hydrides have lattices of the NaCl type with parameters respectively 3.6% and 6% larger than those of the parent metals. Within the limits of the solid solution the metal lattice expands also with increased hydrogen concentration, but the lattice parameter does not depart significantly from that of the pure metal (for palladium at least up to about 100°C). 

Moreover, in the case of hydride intervention, still a further factor, namely the kinetics of hydrogen diffusion into the metal, influences also the overall kinetics by removing a reactant from a reaction zone. In order to compare the velocity of reaction of hydrogen, catalyzed by palladium, with the velocity of the same reaction proceeding on the palladium hydride catalyst, it might be necessary to conduct the kinetic investigations under conditions when no hydride formation is possible and also when a specially prepared hydride is present in the system from the very beginning. 

Metallation and oxymetallation reactions have been observed with the salts of only a few metals, namely mercury(II) (66, 67), thallium(III) (66,67), lead(IV) (66, 67), palladium(II) (100), gold(III) (63), and platinum-(II) (29). These facts correlate well with what Chatt (1) has termed class b, and Pearson (130) has called "soft acid character. Soft acids are characterized by low charge, large size, and, often, d electrons in their outer shell. No class b metal is known, in fact, which contains fewer than five d... 

Helquist et al. [129] have reported molecular mechanics calculations to predict the suitability of a number of chiral-substituted phenanthrolines and their corresponding palladium-complexes for use in asymmetric nucleophilic substitutions of allylic acetates. Good correlation was obtained with experimental results, the highest levels of asymmetric induction being predicted and obtained with a readily available 2-(2-bornyl)-phenanthroline ligand (90 in Scheme 50). Kocovsky et al. [130] prepared a series of chiral bipyridines, also derived from monoterpene (namely pinocarvone or myrtenal). They synthesized and characterized corresponding Mo complexes, which were found to be moderately enantioselective in allylic substitution (up to 22%). 

Chitosan (Fig. 27) was deposited on sihca by precipitation. The palladium complex was shown to promote the enantioselective hydrogenation of ketones [80] with the results being highly dependent on the structure of the substrate. In the case of aromatic ketones, both yield and enantioselectiv-ity depend on the N/Pd molar ratio. Low palladium contents favored enan-tioselectivity but reduced the yield. Very high conversions were obtained with aliphatic ketones, although with modest enantioselectivities. More recently, the immobilized chitosan-Co complex was described as a catalyst for the enantioselective hydration of 1-octene [81]. Under optimal conditions, namely Co content 0.5 mmolg and 1-octene/Co molar ratio of 50, a 98% yield and 98% ee were obtained and the catalyst was reused five times without loss of activity or enantioselectivity. 

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