Stille palladium removal

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. 

A butoxylcarbonylation reaction was conducted in a liquid-liquid biphasic system under process conditions, but the removal of the product was conducted in a liquid-solid biphasic system at a lower temperature (84). lodobenzene or 4-bromoacetophenone reacted with CO at a pressure of 1-8 atm in the presence of a palladium-benzothiazole complex catalyst in the ionic liquid [TBA]Br (m.p. = 110°C) in the presence of Et3N base. The catalyst/ionic liquid system was recycled by extractive removal of the butyl ester product with diethyl ether. The solid residue, containing the catalyst, [TBA]Br, and Et3N.HBr, remained effective in subsequent carbonylation tests. After each cycle, the yields were still close to the initial value. A slight decrease in yield was attributed to a loss of catalyst during handling. 

Alternatively, 138 can be converted to CBZ carbamate 142 using standard conditions (Scheme 4.30). This 5-bromoisoindoline was transformed to stannane 143 in one step with bistributyltin in the presence of a palladium catalyst. The stannane was treated with 136 under Stille conditions to afford coupled product 144. The resulting ester was hydrolyzed with sodium hydroxide, and the CBZ group was removed under hydrogenolysis conditions to deliver garenoxacin (4) (Hayashi et al., 2002). 

The Marques de Aranda had the government turn over its entire supply of platinum to Chabaneau for his difficult and puzzling researches. When Chabaneau removed the gold, mercury, lead, copper, iron, etc., he thought he had a single metal, platinum. As a matter of fact, however, he was still dealing with six metals, for rhodium, palladium,... 

Two grams of anhydrous palladium(II) chloride is suspended in 50 ml. of benzonitrile and the mixture warmed to 100°. After 20 minutes the greater part of the palladium ) chloride has dissolved to give a red solution. This solution is filtered while it is still warm and the filtrate is poured into 300 ml. of low-boiling petroleum ether. The product, light yellow in color, is removed by filtration and washed with additional quantities of petroleum ether. The yield is 4.0 g. or 93% of the theoretical yield. 

Antony 1 prepares pure iridium by precipitation from an acidulated solution of the chloride with metallic zinc. The precipitated metal is treated with nitric acid (density 1-35) containing nitrous acid, whereby osmium and palladium are removed. Repeated extraction with dilute aqua regia (3 parts HC1, 3 parts H20, and 1 part HN03) dissolves out the bulk of the platinum and any traces of osmium and palladium still remaining. 

Reduction with sodium/liquid anunonia was introduced 70 years ago for the removal of benzyl-type and tosyl protecting groups and still remains a weU-estabhshed protocol in peptide synthesis.P Benzyl esters are also cleaved with tcrt-butyldimethylsilane in the presence of palladium(II) acetate yielding the corresponding TBDMS esters which are readily hydrolyzed in the presence of ACOH/H2O (see Section 2.2.1.3).P ... 

Palladium black precipitates on the resin during Stille coupHng. After removal of the reaction solution and rinsing of the resin, a brief treatment with dilute KCN in DMSO clears the precipitate from the resin. 

At this stage of our study, we have compared the rate of deprotection of several phenyl acetic allyl esters under the above homogeneous or biphasic aqueous conditions. We found that in (CH3CN/H20) medium the dimethylallyl group is cleaved at a lower rate than the cinnamyl group in the presence of 2 to 2.5 mol % of palladium (0). Under the same conditions, the allyl moiety is instantaneously removed. By comparison, in a biphasic system (C3H CN/H20), the cinnamyl and the dimethylallyl groups remain intact in the presence of mol 5 % of Pd(0) water soluble catalyst, even after 3 days at room temperature whereas the allylic ester is still cleaved to give phenyl acetic acid in excellent yield. 

From these experiments the conclusion can be drawn that carbonaceous deposits were accumulated on the catalyst surface at the temperature of full conversion even in presence of oxygen. On this basis the obvious explanation for the maxima in CO2 and CO in low activity state is that, when the catalyst starts working, the m-xylene is combusted, but simultaneously a part of them interacts with the palladium surface which results in its deactivation, i. e. in the decrease of amount of CO2. Consequently, due to the retarded combustion, CO also appears in the gas phase. When the temperature is further increased the carbonaceous deposits resulting from the fragmentation of m-xylene is removed by oxygen and the CO2 conversion levels off at 100 %. hr high actiyity state no maxima are observed, still carbonaceous deposits are formed bonded with different strength to palladium surface. 

In one embodiment patented by Makrides et al., tantalum foils were electrolyti-cally etched in hydrofluoric acid, washed with acetone and, while still wet with acetone, placed in vacuum and dried by evacuation. Using an argon plasma at a pressure of about 1 mm Hg (133.3 Pa), palladium was deposited onto both sides of the membranes to recommended thicknesses between 10 and 100 nm. Membranes of niobium and vanadium were prepared in the same manner, except that, in addition, vanadium was degassed in vacuum at 1273 K (1000 °C) to remove oxygen. Unalloyed palladium as well as Pd-Ag, Pd-Au and Pd-B, were also patented as hydrogen dissociation catalysts and as protective layers for the highly reactive niobium, tantalum and vanadium. 

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