Pd compounds

In organic synthesis, two kinds of Pd compounds, namely Pd(II) salts and Pd(0) comple.xes, are used. Pd(II) compounds are used either as stoichiometric reagents or as catalysts and Pd(0) complexes as catalysts. Pd(Il) compounds such as PdCh and Pd(OAc)2 are commercially available and widely used as... 

The most useful reaction of Pd is a catalytic reaction, which can be carried out with only a small amount of expensive Pd compounds. The catalytic cycle for the Pd(0) catalyst, which is understood by the combination of the aforementioned reactions, is possible by reductive elimination to generate Pd(0), The Pd(0) thus generated undergoes oxidative addition and starts another catalytic cycle. A Pd(0) catalytic species is also regenerated by /3-elimination to form Pd—H which is followed by the insertion of the alkene to start the new catalytic cycle. These relationships can be expressed as shown. 

The transmetallation of various organometallic compounds (Hg, Tl, Sn, B, Si, etc.) with Pd(II) generates the reactive cr-aryl, alkenyl, and alkyl Pd compounds. These carbopalladation products can be used without isolation for further reactions. Pd(II) and Hg(II) salts have similar reactivity toward alkenes and aromatic compounds, but Hg(II) salts form stable mercuration products with alkenes and aromatic rings. The mercuration products are isolated and handled easily. On the other hand, the corresponding palladation products are too reactive to be isolated. The stable mercuration products can be used for various reactions based on facile transmetallation with Pd(II) salts to generate the very reactive palladation products 399 and 400 in rim[364,365]. 

Several Pd(0) complexes are effective catalysts of a variety of reactions, and these catalytic reactions are particularly useful because they are catalytic without adding other oxidants and proceed with catalytic amounts of expensive Pd compounds. These reactions are treated in this chapter. Among many substrates used for the catalytic reactions, organic halides and allylic esters are two of the most widely used, and they undergo facile oxidative additions to Pd(0) to form complexes which have o-Pd—C bonds. These intermediate complexes undergo several different transformations. Regeneration of Pd(0) species in the final step makes the reaction catalytic. These reactions of organic halides except allylic halides are treated in Section 1 and the reactions of various allylic compounds are surveyed in Section 2. Catalytic reactions of dienes, alkynes. and alkenes are treated in other sections. These reactions offer unique methods for carbon-carbon bond formation, which are impossible by other means. 

The 7V-methylbenzo[( e]quinoline 426 was prepared by trapping the insertion product of an internal alkyne with a tertiary dimethylamine. One methyl group is eliminated. The dimethylaminonaphthalene-Pd complex 427 is an active catalyst and other Pd compounds are inactive[290a]. 

A Ru—Si complex 169), a cobaloxime derivative 216a), a Ni complex 216), and the reported Pd compound (obtained in an impure state) 158) have also been made [Eqs. (20)-(23)]. 

The Pd lT) and Pt ll) complexes yilB dtc) HI, 46) are diamagnetic and have a square planar geometry 112,113) like the nickel analogues. In CH2CI2 they can be oxidised irreversibly on a rotating platinum electrode, the Pd compound at a higher potential than the Pt compound (1.21 and 0.92 V vs s.c.e., respectively) 150). 

A review12 with 26 references of the biocoordination chemistry of Pd, Pt, and Ru and the discovery of a new type of immunomodulating activity of the Pd compounds is presented. 

Solution of Pd compound in PhOH or reaction mixture were aged at 70 °C... 

The variety of products can be increased by an anion-capturing process of tbe intermediate Pd-compounds, and tbis has been intensively explored by Grigg and coworkers [18]. 

De Meijere and coworkers developed a nice example where three C-C-bonds are formed in one domino process [36]. Thus, reaction of 6/1-48 with the Hermann-Beller catalyst 6/1-15 led to 6/1-50 as the only product (Scheme 6/1.10). It can be assumed that the Pd-compound 6/1-49 is an intermediate. 

The reaction of alcohols with CO was catalyzed by Pd compounds, iodides and/or bromides, and amides (or thioamides). Thus, MeOH was carbonylated in the presence of Pd acetate, NiCl2, tV-methylpyrrolidone, Mel, and Lil to give HOAc. AcOH is prepared by the reaction of MeOH with CO in the presence of a catalyst system comprising a Pd compound, an ionic Br or I compound other than HBr or HI, a sulfone or sulfoxide, and, in some cases, a Ni compound and a phosphine oxide or a phosphinic acid.60 Palladium(II) salts catalyze the carbonylation of methyl iodide in methanol to methyl acetate in the presence of an excess of iodide, even without amine or phosphine co-ligands platinum(II) salts are less effective.61 A novel Pd11 complex (13) is a highly efficient catalyst for the carbonylation of organic alcohols and alkenes to carboxylic acids/esters.62... 

The distinction between Pd compounds and those of Ni and Pt is also evident in the NAO valence configurations of metal atoms in the three final HMMH species of Table 4.57. The values show that Ni and Pt have more nearly the promoted s1 d9 configuration for hybridized bonding, whereas Pd is more nearly the unhybridized... 

Finally, the uptake energy values only represent a contribution to the total free energy of coordination. In fact, an always unfavorable uptake entropy has to be accounted for. Although few experimental data are available, it is reasonable to assume that the -FAScontribution to the free energy of olefin coordination to group 4 metallocenes at room temperature is close to the 10 kcal/mol value observed at 300 K for Ni and Pd compounds [59], The few... 

As is almost always true when when the substrate in a Pd-catalyzed reaction is C(sp2)-X, the first step is oxidative addition of Pd(0) to the C-I bond to give an arylpalladium(II) intermediate. (Although the Pd compound that is added to the reaction mixture is Pd(II), it is reduced in situ to Pd(0) by the mechanism outlined in the text.) Coordination of CO and insertion into the C-Pd bond gives an acylpalladium(II) intermediate. Deprotonation of the alcohol is followed by nucleophilic attack on the carbonyl C. Expulsion of Pd(0) gives the product and completes the catalytic cycle. 

The mechanism begins with oxidative addition of Pd(II) to the C2-Brl bond to give an arylpalladium(II) compound. (Although the Pd compound that is added to the reaction mixture is Pd(IE), it is reduced in situ... 

Under the mild conditions of this distillation, the chance of decomposition of the product under the influence of traces of Pd-compounds is small. 

The brown colour may be caused by traces of palladium or Pd compounds. If the product has a reasonable thermal stability and the boiling point is not too high, purification may be carried out more quickly by distillation in vacuo. 

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