Palladium associative exchange

When the reaction is carried out with a racemic mixture of complexes, the product is a racemic mixture of the isotactic polymers. It was of interest to see what would happen if, after formation of a chiral block with one enantiomer of the bisoxazoline ligand, an equivalent of the other enantiomer was added. It was found that an excess of ligand changes the tacticity completely and the second block was syndiotactic In these diimine palladium complexes exchange of ligand is relatively fast and it can often be observed on the NMRtime scale as a broadening in the H NMR spectra. The process may well be associative. 

Wilkinson had observed an associative exchange of CjH, rings analogous to that of (77-allyl)(7r-cyclopentadienyl)palladium and other related complexes discussed below. 

Platinum catalysts were prepared by ion-exchange of activated charcoal. A powdered support was used for batch experiments (CECA SOS) and a granular form (Norit Rox 0.8) was employed in the continuous reactor. Oxidised sites on the surface of the support were created by treatment with aqueous sodium hypochlorite (3%) and ion-exchange of the associated protons with Pt(NH3)42+ ions was performed as described previously [13,14]. The palladium catalyst mentioned in section 3.1 was prepared by impregnation, as described in [8]. Bimetallic PtBi/C catalysts were prepared by two methods (1) bismuth was deposited onto a platinum catalyst, previously prepared by the exchange method outlined above, using the surface redox reaction ... 

Thus, evidence has accumulated in support of hydrogen exchange in benzene by a mechanism involving associatively chemisorbed benzene, and without the necessity to postulate the participation of chemisorbed C Hs. One attractive test of these ideas which, so far as we know, has not been made, would be to repeat, for example, the reaction of para-xylene with deuterium using as catalyst a palladium thimble. This system would allow the exchange reaction to proceed either in the presence of molecular deuterium (both reactants on same side of the thimble) or in the presence of atomic deuterium only (xylene and molecular deuterium on opposite sides of the thimble, so that the hydrocarbon reacts only with chemisorbed atomic deuterium that arrives at the surface after diffusion through the metal). 

Apparently, in the reaction of olefins with hydrogen on catalysts such as palladium and platinum, both the dissociative and the associative mechanisms operate for isomerization and exchange. However, the dissociative mechanism accompanies those factors which tend to slow the addition or accelerate the removal of hydrogen from either substrate or intermediate. These factors may be any of the independent variables, such as the pressure of hydrogen, the structure of the substrate, or the catalyst (5). 

Matsubara I, Takeda Yand Ishida K (2000) Improved recovery of trace amounts of gold (III), palladium (II) and platinum (IV) from large amounts of associated base metals using anion-exchange resins. Eresenius J Anal Chem 366 213 — 217. 

Associative Ligand Exchange at Square-Planar Palladium(II)... 

The palladium-catalyzed Suzuki-Miyaura reaction is a cross-coupling reaction between aryl halides and horon derivatives (Scheme 19.40) [54]. Snznki received the Nobel Prize in 2010 [54h]. Reactions are performed under mild conditions and many functional gronps are tolerated. Water is not a problem and may be even beneficial (vide infra). The first reported reactions hy Suzuki in 1979 insisted about the presence of a base, even if no proton is exchanged [54a, b]. Various bases have been used EtO, MeO", OH", CO ", and F" associated with countercations such as Na, K% Cs+, LP, nBu N+, Ag+, or TP [54, 55]. The role of the base has led to many mechanistic interpretations involving among them anionic arylborates as reagents [54g, 56]. 

Three DNA metallization chemistries have been used to convert DNA into a conducting wire [5,8-10]. These chemistries fall into two reaction categories (1) ion-exchange of a metal (silver) ion for the positive sodium counter ion associated with the phosphate groups of the DNA backbone and (2) formation of a covalent bond between a metal ion (palladium or platinum) and amine groups of the DNA bases. In both cases, the attached ions are reduced to form a metal that can act as a catalytic site for the deposition of an alternate metal on the surface of the DNA [11],... 

The most active catalysts for many tritiolyses are palladium metal (freshly prepared from PdO) or palladium supported on charcoal, barium sulfate or aluminum oxide, but palladium catalysts are sometimes associated with side reactions such as O- and /V-debenzylations, isomerizations and labeling of certain sites by catalytic exchange (see Chapter 3, Section 3.3). For example, in the case of [ H]papaverine (10 and the [ H]pargyline precursor... 

BimetalHc catalysts Pt-Pd/H-beta were prepared by incipient wetness or by ion exchange [258]. The platinum dispersion was improved by the presence of palladium, but the overall dispersion did not exeed 0.59 it was not established whether the two metals were associated or occurred as separate phases. 

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