Palladium occurrence

Microwave promoted, palladium-catalyzed, DPPP-controlled arylation of butyl vinyl ether with 4-tert-butylphenyl triflate afforded the branched arylation product and the corresponding methyl ketone, indicating the occurrence of selective internal a-arylation. Addition of water to the reaction mixture and microwave-heating for 2.8 min at 55 W smoothly produced the hydrolyzed product, 4-tert-butylacetophe-none, with an isolated yield of 77% (Eq. 11.2) [17]. 

The mechanism of the evolution of hydrogen on palladium in alkaline medium and associated internal damage phenomena were discussed in a recent paper by Bockris and Minevski [141]. The aim of the paper was to get support for their belief concerning the occurrence of nuclear reaction inside the metal. However, the authors arrived at the conclusion that the results obtained could be explained by an atomic-molecular (nonnuclear) model. 

In 1809 Wollaston demonstrated the presence of grains of native palladium and native platinum in a Brazilian alluvial gold ore presented to him by the Portuguese ambassador, H. E. Chev. de Souza Coutinho. Wollaston was led to this discovery by the observation that some of the grains, although they looked like platinum, dissolved faster in aqua regia (44, 122, 123). In 1825 Alexander von Humboldt also reported the occurrence of native palladium in Brazil (109). 

Although t/3-allyl complexes of platinum(II) are not rare, their occurrence is not as frequent as for -alkene complexes. This situation is reversed for palladium(II) where r 3-allyl complexes are very common, and much of modern organopalladium chemistry is becoming dominated by the reactivity of j)3-allyl complexes. 

Although complexes with these ligands are common in palladium(II) chemistry, their occurrence is more scarce in platinum(II) compounds. Nevertheless these complexes can be prepared, examples being platinum(II) complexes of the optically active quadridentate Schiff base of salicylaldehyde and (R)-l, 2-diamines.1212 An alternative synthesis involves formation of the Schiff base by reaction of a complexed amino ligand on platinum(II) with amide acetates (equation 372).1213... 

The occurrence of a palladium(O) intermediate prompted us to examine the same reaction with a supported palladium catalyst with the aim of preventing the loss of palladium and separating more easily the catalyst from the reaction products. 

The ability of the boron atom of 59 to engage in a donor-acceptor interaction was illustrated with DMAP and DABCO (DABCO = diazabi-cyclo-[2.2.2]-octane) that readily formed the corresponding Lewis adducts. Interestingly, a similar behavior was retained after coordination of the phosphorus atom to palladium. The formation of the Lewis base adducts 66a and 66b of complex 65 (Scheme 38) was supported by solid-state 31P and nB CP/MAS-NMR spectroscopy (<5 1 B = 5-6 ppm), although the occurrence of decomposition and/or dissociation processes impeded spectroscopic characterization in solution and recrystallization to obtain X-ray quality crystals. Compounds 66a and 66b substantiate the ability of ambiphilic compounds to engage concomitantly into the coordination of donor and acceptor moieties. Such a dual behavior opens interesting perspectives for the preparation of metallo-polymers and multimetallic complexes. 

Occurrence and History.—Palladium occurs native along with platinum in Brazil,1 St. Domingo, and the Urals. It is also found alloyed with gold as the mineral porpesite, the name being derived from Porpez in Brazil. Of tliis alloy the following may be taken as a typical analysis 2 ... 

Occurrence and History of Palladium—Preparation—Physical Varieties Physioal Properties—Permeability to Hydrogen--Occlusion of Canos Occlusion of Hydrogen—Chemical Properties--Catalytic Activity Crystalline Palladium—Colloidal Palladium—Spongy Palladium —Palladium Black —Uses—Atomic Weight—Alloys. 

The complexes of nickel and palladium, with a coordination number four mentioned above, are interesting because here a plane configuration occurs, which is inexplicable on the basis of the electrostatic theory the latter leads to a tetrahedral model. The diamagnetism also points to the formation of electron pairs in the bonding. The possibility of the occurrence of cis and trans isomers, as in the octahedral complexes, is in agreement with a plane structure, but incompatible with a tetrahedral arrangement. 

The scientific world was stunned in March of 1989 when two electrochemists, Stanley Pons and Martin Fleischmann, reported that they had obtained evidence for the occurrence of nuclear fusion at room temperatures. During the electrolysis of heavy water (deuterium oxide), it appeared that the fusion of deuterons was made possible by the presence of palladium electrodes used in the reaction. If such an observation could have been confirmed by other scientists, it would have been truly revolutionary. It would have meant that energy could be obtained from fusion reactions at moderate temperatures. 

Palladium hydride complexes are of considerable interest in the catalytic chemistry of palladium because of their postulated occurrence as intermediates in a number of reactions such as hydrogenations, isomerizations, and oxidation reactions. In contrast to Pt(II), which forms stable hydrides, most Pd(II) hydrides are unstable. Although there are earlier reports of unstable Pd(II) hydride complexes being formed, the first stable hydride was prepared by Brooks and Clocking by the following reaction (20, 21) ... 

In addition to ablation products such as nickel, copper, and iron now commonly present in exhaust, there is the potential occurrence in exhaust particulate of metals in use or considered for use as catalyst materials. Their detection would depend upon both increased use and/or increased analytical sensitivity. Efforts to quantify by routine methods emissions of platinum and palladium, now in widespread use in the oxidation catalyst, have failed to substantiate the presence of these metals in exhaust. An emission factor of 3.1 X 10" g/mile for platinum has been estimated (53). Other metals which have been considered for use in automotive catalysts include ruthenium and vanadium. 

The experimental results that both branched and linear allylic ethers are obtained in the palladium-catalyzed decarboxylation of branched allylic carbonate indicate occurrence of direct oxidative addition involving the C-0 bond cleavage followed by the nucleophilic attack of the alkoxide liberated on either the substituted or non-substituted terminus of the allylic ligand (Scheme 4) [1]. 

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