Palladium metal reactions with

Palladium is attacked by concentrated nitric acid, particularly in the presence of nitrogen oxides. The reaction is slow in dilute nitric acid. Finely divided palladium metal reacts with warm nitric acid forming paUadium(ll) nitrate, Pd(NOs)2. Hydrochloric acid has no affect on the metal. Reaction with boiling sulfuric acid yields palladium sulfate, PdS04, and sulfur dioxide. 

Many such activated acyl derivatives have been developed, and the field has been reviewed [7-9]. The most commonly used irreversible acyl donors are various types of vinyl esters. During the acylation of the enzyme, vinyl alcohols are liberated, which rapidly tautomerize to non-nucleophilic carbonyl compounds (Scheme 4.5). The acyl-enzyme then reacts with the racemic nucleophile (e.g., an alcohol or amine). Many vinyl esters and isopropenyl acetate are commercially available, and others can be made from vinyl and isopropenyl acetate by Lewis acid- or palladium-catalyzed reactions with acids [10-12] or from transition metal-catalyzed additions to acetylenes [13-15]. If ethoxyacetylene is used in such reactions, R1 in the resulting acyl donor will be OEt (Scheme 4.5), and hence the end product from the acyl donor leaving group will be the innocuous ethyl acetate [16]. Other frequently used acylation agents that act as more or less irreversible acyl donors are the easily prepared 2,2,2-trifluoro- and 2,2,2-trichloro-ethyl esters [17-23]. Less frequently used are oxime esters and cyanomethyl ester [7]. S-ethyl thioesters such as the thiooctanoate has also been used, and here the ethanethiol formed is allowed to evaporate to displace the equilibrium [24, 25]. Some anhydrides can also serve as irreversible acyl donors. 

The mechanism of the Stille reaction involves an oxidative addition of the palladium(O) species into the carbon-halogen bond, a trans-metallation reaction with the organostannane and a final reductive elimination step releasing the product and thereby regenerating the palla-dium(O) catalyst. 

The most important class of allylic substitutions are palladium-catalyzed reactions with so-called soft nucleophiles such as stabilized carbanions or amines, and with few exceptions, the enantioselective transformations discussed in this chapter belong to this category. The mechanism of these reactions has been firmly established and a detailed picture of the catalytic cycle can be drawn [1, 2,3,4,5,6,13,14,15]. The course of allylic substitutions catalyzed by metals other than palladium is less clear and information about the intermediates involved is scarce. 

The metal-catalyzed transvinylation of vinylacetates or vinyl ethers with alcohols is initiated by an oxy-metallation reaction. With PdCl2(PhCN)2 as the catalyst and at low temperature, transvinylation takes place exclusively (Scheme 5a). At higher temperatures, acetal formation is observed after an induction period, in addition to precipitation of palladium (Scheme 5b). It is probable that the acetaliza-tion step is catalyzed exclusively by traces of Brpnsted acid (HCl), rather than via proto-de-metallation of a Pd(II) alkyl species [21, 22]. Consequently, a recent report on the protection of primary alcohols as tetrahydropyranyl- (THP-)ethers by addition to 2,3-dihydropyrane in the presence of PdCl2(MeCN)2 as catalyst... 

Palladium metal reacts with organic halides directly and organopalladium compounds are isolated by reaction with phosphine [44-48]. For example, organopalladium compounds are prepared by the reaction of organic halides with vaporized palladium or with metal slurry obtained by the reduction of palladium halide with an alkali metal as shown in eqs. (20.24)-(20.26). The last reaction shows higher yield. 

The equilibrium is more favorable to acetone at higher temperatures. At 325°C 97% conversion is theoretically possible. The kinetics of the reaction has been studied (23). A large number of catalysts have been investigated, including copper, silver, platinum, and palladium metals, as well as sulfides of transition metals of groups 4, 5, and 6 of the periodic table. These catalysts are made with inert supports and are used at 400—600°C (24). Lower temperature reactions (315—482°C) have been successhiUy conducted using 2inc oxide-zirconium oxide combinations (25), and combinations of copper-chromium oxide and of copper and silicon dioxide (26). 

Transition-Metal Catalyzed Cyclizations. o-Halogenated anilines and anilides can serve as indole precursors in a group of reactions which are typically cataly2ed by transition metals. Several catalysts have been developed which convert o-haloanilines or anilides to indoles by reaction with acetylenes. An early procedure involved coupling to a copper acetyUde with o-iodoaniline. A more versatile procedure involves palladium catalysis of the reaction of an o-bromo- or o-trifluoromethylsulfonyloxyanihde with a triaLkylstaimylalkyne. The reaction is conducted in two stages, first with a Pd(0) and then a Pd(II) catalyst (29). 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

In some cases, the catalyst is a solid substance on whose surface a reactant molecule can be held (adsorbed) in a position favorable for reaction until a molecule of another reactant reaches the same point on the solid. Metals such as iron, nickel, platinum and palladium seem to act in this way in reactions involving gases. There is evidence that in some cases of surface adsorption, bonds of reactant particles are weakened or actually broken, thus aiding reaction with another reactant particle. 

Another useful class of palladium-catalyzed cycloisomerizations is based on the general mechanistic pathway shown in Scheme 13. In this chemistry, a hydridopalladium acetate complex is regarded as the catalytically active species.27b-29 According to this pathway, coordination of a generic enyne such as 59 to the palladium metal center facilitates a hydropalladation reaction to give intermediate 60. With a pendant alkene, 60 can then participate in a ring-form-... 

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