Reactions Involving Palladium

SECTION 6.4. SYNTHETIC APPLICATIONS OF OTHER TRANSITION METALS 

The second type of palladium intermediate is a rr-allyl complex of Pd(II). The TT-allyl complexes can be obtained from Pd(II) salts and allylic acetates, allyl ethers, and other allylic compounds having potential leaving groups.The same rr-allyl complexes can be prepared from alkenes by reaction with palladium chloride or palladium trifluoroacetate. For unsymmetrical alkenes with more than one reactive allylic position, the latter method gives rise to mixtures of rr-allyl complexes. 

The overall reaction is formulated as an electrophilic bond formation, followed by loss of a proton. The proton loss probably proceeds via an unstable species in which hydrogen is bound to palladium. Dimerization or addition of other ligands to 

These 7r-allyl complexes are electrophilic in character and undergo reaction with a variety of nucleophiles. 

After the nucleophilic addition occurs, the resulting palladium intermediate usually breaks down by elimination of Pd(0) and H.  

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W. S. Schneider, "Symposium of Homogeneous Catalytic Reactions involving Palladium. Division of Petroleum Chemistry, Inc. and Division of Inorganic Chemistry, Organometallic Subdivision, American Chemical Society, Minneapolis, Minnesota, 1969. 

Each step of the synthesis usually needs optimisation of reaction conditions (time, temperature, solvents, concentrations). Different techniques of reaction activation can also be used. Microwave heating has been shown to give faster, cleaner and more selective reactions [22,23] than conventional heating. Ultrasound, although promising [24], has not known the same development. Finally, catalysed reactions involving palladium complexes have been developed in car-bone- 11 chemistry [25 ] over the last few years. They have not been widely studied in fluorine-18 chemistry. 

Aniline that is orf/io-substituted with a hexa-2,5-dienyl side-chain undergoes catalytic, palladium-assisted cyclization to 2-propylquinoline (Scheme 34) this is only one of a series of ring-closure reactions involving palladium-promoted nucleophilic attack on an alkene. A tetrahydroquinoline (55) is produced by the action of trifluoroacetic acid on the hydroxylamine (54). This method has also been used to prepare l,4-benzoxazines. ... 

Solution-phase synthetic procedures leading to (NHC)2Pd complexes revolve around ligand displacement reactions involving palladium centers bearing weakly coordinating ligands such as phosphines or olefins (see Scheme 1). For instance, a 14-electron species such as Pd(P(o-tolyl)3)2 reacts with free NHC... 

In reactions catalyzed by metal complexes, irreversible formation of bulk metal is frequently a major catalyst deactivation roure, e.g., in nearly any reaction involving palladium complexes. Stopping the aggregation at the stage of small particles in combination with a re-oxidation step can enhance catalyst activity. 

The Heck-Mizoroki reaction involves palladium-catalyzed coupling of an alkene with an alkenyl or aryl halide, leading to a substituted alkene. The alkene product is generally trans due to a 1,2-elimination step in the mechanism. 

T3.3 Carhonylative Reactions Involving Palladium(ll) Salts 345 ----------- Carbonyl derivative + Pd(0)... 

Carbonylative Reactions Involving Palladium(ll) Salts 3S3 Pdl2, Kl ... 

The current intensified interests in the preparation of PEMs have prompted us to synthesize aromatic monomers such as trifluorovinyl ethers functionalized by acid groups. In particular, we reported the first preparation of 4-[(a,, -trifluorovinyl)oxy] benzene phosphonic acid (Fig. 2.21) [99]. [(a,, -Trifluorovinyl)oxy]benzene dialkyl phosphonate was prepared by several phosphonation methods, such as Michaehs-Arbuzov, Michaelis-Becker, or palladium-catalyzed arylation in the presence of various reactants. The reaction involving palladium triphenylphosphine as the catalyst led to the best yield [99]. 

The arylation of carbon-hydrogen bonds in polycyclic aromatic hydrocarbons, using aryl boron compoimds or aryl silanes, may be achieved with a palladium acetate/o-chloranil catalyst. The Suzuki-Miyaura reaction involves palladium-catalysed coupling of an arylboronic acid with an aryl hahde in the presence of base. After oxidative addition of palladium to the hahde, reaction with base may form intermediates such as (105). Transmetalation with the boronic acid followed by reductive elimination yields... 

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