Palladium triarylphosphine complexes

Arylation of olefins can also be achieved312 by treatment with an arylpalladium reagent that can be generated in situ by several313 methods (1) by treatment of an aryl bromide with a palladium-triarylphosphine complex (ArBr —> ArPdBr") 114 (2) by treatment of an aryl iodide315 with palladium acetate316 in the presence of a base such as tributylamine or... 

During arylations of carbon nucleophiles with aryl halides in the presence of palladium triarylphosphine complexes products containing the aryl group of the phosphine can result (Scheme8.16). These reactions proceed via reversible arylation of the Pd-bound phosphine, which occurs at temperatures above 50 °C, particularly readily in the presence of iodide [11] (see Section 8.2). Electron-deficient aryl groups usually migrate less readily than electron-rich groups [23, 25],... 

Carbon—carbon coupling reactions of aryl halides are commonly catalyzed by palladium triarylphosphine complexes and proceed well for aryl bromides and iodides while aryl chlorides are generally unreactive. More basic chelating trialkylphosphines, however, render palladium sufficiently electrophilic to undergo rapid oxidative addition with chlorobenzene ... 

R. F. Heck, Adv. Chem. Ser, 1982, 196, 213-230. Palladium-Triarylphosphine Complexes as Catalysts for Vinylic Halide Reactions. 

Palladium acetate triarylphosphine complexes catalyze the addition of vinylic groups from vinylic halides to olefinic compounds in the presence of amines. Conjugated dienes are major products from 0,/3-unsaturated acids, esters, or nitriles while unactivated olefinic compounds react best in the presence of secondary amines where allylic amines are major products. The reactions are usually regio- and stereospecific. The synthetic utility of the reaction is illustrated with a wide variety of examples. 

Nickel(O) complexes tend to undergo oxidative addition of aryl halides faster than palladium(O) complexes. There are some drawbacks to the use of the nickel complexes, described above, that can outweigh the lower cost of nickel. Nevertheless, the ruckel complexes containing triarylphosphines do undergo oxidative addition of aryl chlorides (Equation 19.39) and tosylates (Equation 19.40), and some mild conditions have been developed for nickel-catalyzed cross couplings of aryl chlorides and aryl tosylates with ligands such as triphenylphosphine or tricyclohexylphosphine. ... 

A coordinatively unsaturated 14-electron palladium(O) complex, usually coordinated with weak donor ligands (mostly tertiary phosphines), has meanwhile been proved to be the catalytically active species [13]. This active complex is always generated in situ, for example, from tetrakis(triphenylphosphine)palladium(0), which exists in equilibrium with tris(triphenylphosphine)palladium(0) and free triphe-nylphosphine in solution. The endergonic loss of a second phosphine ligand [14] leads to the catalytically active bis(triphenylphosphine)palladium(0). However, pal-ladium(II) complexes or salts such as bis(triphenylphosphine)palladium dichloride or palladium acetate, which are easily reduced (e.g., by triarylphosphines, see later discussion) in the reaction medium, are more commonly employed for convenience, as they are inherently stable toward air. The mechanistic situation is a bit more complicated with palladium acetate in that anionic acetoxypalladium species Pd(PPh3) (AcO ) (n = 2, 3) are formed in the presence of acetate ions [13], and these actually participate in the oxidative addition step as well as the following coupling reaction. 

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). 

From the results of the 1,3-diene addition reaction, the metal-catalyzed reaction of unactivated alkenes was examined, and it was found that the palladium complex effectively catalyzed the a rt-Markovnikov addition of triarylphosphines and bis(trifluoromethanesulfonyl)imide (Tf2NH).24... 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Reactions involving breaking of C-H bonds have been proposed. For palladium catalysed decomposition of triarylphosphines this is not the case [32], Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [33], Several rhodium complexes catalyse the exchange of aryl substituents at triarylphosphines [34] ... 

Methods (i) and (ii) require palladium(II) salts as reactants. Either palladium acetate, palladium chloride or lithium tetrachloropalladate(II) usually are used. These salts may also be used as catalysts in method (iii) but need to be reduced in situ to become active. The reduction usually occurs spontaneously in reactions carried out at 100 °C but may be slow or inefficient at lower temperatures. In these cases, zero valent complexes such as bis(dibenzylideneacetone)palladium(0) or tetrakis(triphenylphos-phine)palladium(O) may be used, or a reducing agent such as sodium borohydride, formic acid or hydrazine may be added to reaction mixtures containing palladium(II) salts to initiate the reactions. Triarylphosphines are usually added to the palladium catalysts in method (iii), but not in methods (i) or (ii). Normally, 2 equiv. of triphenylphosphine, or better, tri-o-tolylphosphine, are added per mol of the palladium compound. Larger amounts may be necessary in reactions where palladium metal tends to precipitate prematurely from the reaction mixtures. Large concentrations of phosphines are to be avoided, however, since they usually inhibit the reactions. 

Normally, the most practical vinyl substitutions are achieved by use of the oxidative additions of organic bromides, iodides, diazonium salts or triflates to palladium(0)-phosphine complexes in situ. The organic halide, diazonium salt or triflate, an alkene, a base to neutralize the acid formed and a catalytic amount of a palladium(II) salt, usually in conjunction with a triarylphosphine, are the usual reactants at about 25-100 C. This method is useful for reactions of aryl, heterocyclic and vinyl derviatives. Acid chlorides also react, usually yielding decarbonylated products, although there are a few exceptions. Likewise, arylsulfonyl chlorides lose sulfur dioxide and form arylated alkenes. Aryl chlorides have been reacted successfully in a few instances but only with the most reactive alkenes and usually under more vigorous conditions. Benzyl iodide, bromide and chloride will benzylate alkenes but other alkyl halides generally do not alkylate alkenes by this procedure. 

A number of metal / -diketonates have been used to catalyze the oxidation of sulfides to sulfoxides, important synthetic intermediates for the construction of various biologically active molecules . For example, an elegant study by Ishii and coworkers demonstrated that VO(acac)2 (35) selectively catalyzed the sulfoxidation of adamantane (41) by SO2/O2 to give 1-adamantane sulfonic acid (42) (equation 10) . Although a number of metal acac complexes were examined as catalysts for this reaction, all but the vanadium compound failed to promote the sulfoxidation. The catalytic oxidation of triarylphosphines using the palladium complex Pd(acac)2 (29) has also been investigated . 

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