Carbon-hydrogen bonds palladium© acetate

Regioselective functionalization of unreactive carbon-hydrogen bonds, in particular, arylation of pyridines by using aryl iodide, silver acetate, and catalytic palladium acetate 06SL3382. 

Fuchita, Y, Hiraki, K., Kamogawa, Y. et al. (1989) Activation of aromatic carbon-hydrogen bonds by palladium(II) acetate-dialkyl sulfide systems. Formation and characterization of novel diphenyltripalladium(Il) complexes. Bull. Chem. Soc. Jpn., 62, 1081-5. 

R, and ligands on the palladium, and the dominance of meta over para substitution is an electronic effect of the substituent on the cleaving carbon—hydrogen bond. In a variation of the Heck reaction, it has been shown that arylsulfinic acids may be substituted by alkenes in the presence of a palladium acetate catalyst, with copper... 

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... 

Since sulfur is the most effective of all catalyst poisons, the hydrogenation of sulfur containing heterocycles is not easily accomplished unless there are no unshared electron pairs on the sulfur atom or the catalyst used is not affected by the poison. The hydrogenation of the cyelie sulfone, 58, takes palace over an excess of palladium in acetic acid at room temperature and atmospheric pressure (Eqn. 17.57). Thiophene, itself, can be hydrogenated to tetrahydrothiophene over rhenium heptasulfide at 250°C and 300 atmospheres of hydrogen or over a large excess of palladium in methanolic sulfuric acid at room temperature and 3-4 atmospheres. No hydrogenolysis of the carbon-sulfiir bond was observed in these reactions. 

Perhaps the best known alkyne semi-hydrogenation catalyst is that developed by Lindlar which comprises calcium carbonate-supported palladium, modified by addition of lead acetate and, often, quinoline to improve selectivity [51]. Selective hydrogenation of 1-bromo-ll-hexadecyne (Eq. 8) has been shown to occur in high yield and without hydrogenolysis of the carbon-bromine bond, over Lin-dlar s catalyst treated with aromatic amine oxides such as pyridine A-oxide... 

Reaction of organic halides with alkenes catalyzed by palladium compounds (Heck type reaction) is known to be a useful method for carbon-carbon bond formation at unsubstituted vinylic positions. The first reports of the application of MW methodology to this type of reaction were published by Hallberg et al. in 1996 [109] and by Diaz-Ortiz et al. in 1997 [llOj both used in triethylamine solutions. Later, Ville-min et al. studied the possibility of Heck coupling of iodoarenes with methyl acrylate in aqueous solution under pressurized conditions [111]. The reactions were conducted in a Teflon autoclave under the action of MW irradiation in the presence of palladium acetate, different phosphine ligands, and tetrabutylammonium hydrogen sulfate (TBAHS) as PTC catalyst, to afford the desired coupling products in 40 to 90% yield. 

Palladium-carbon/acetic acid Hydrogenation of carbon-carbon double bonds with reduction of formyl to methyl groups... 

Examples of the formylation of aryl halides with synthesis gas catalyzed by palladium complexes are summarized in Equation 19.90. These reactions relied upon the development of ligands with particular steric and electronic properties. The dia-damantyl-n-butyl phosphine shown in the equation, in combination with palladium acetate, leads to the formation of aromatic aldehydes in high yields from electron-rich and electron-poor aryl bromides. Reactions of nitroarenes and 2-bromopyridine provided the aldehydes in low yield, but other examples occurred in satisfactor) yield with only 0.1-0.75 mol % catalyst. The identity of the base is important in this process, and TMEDA was the most effective base. The mechanism of this process was not proposed in the initial work, but is likely to occur by oxidative addition of the aryl halide, insertion of the carbon monoxide into the palladium-aryl bond, and a combination of hydrogenolysis of the acyl intermediate and elimination of hydrogen halide to regenerate palladium(O). The base would then be involved in the hydrogenol5 sis and consumption of hydrogen halide. 

It is very well documented that the carbon-carbon triple bonds (e.g., alkynes) on catalytic hydrogenation gives the completely reduced product, viz. alkanes. Alkynes can also be reduced partially to give z-alkenes by palladium-calcium carbonate catalyst which has been deactivated (partially poisoned) by the addition of lead acetate (Lindlar catalyst) or Pd-BaSO deactivated by quinoline. The lead treatment poisoned the palladium catalyst, rendering it less active and the reaction is more selective. Some examples are given (Scheme 98). 

An alternative mechanism involves the use of metal hydrides (Scheme 11.76). Palladium(O) complexes can oxidatively add to acetic acid to generate such a hydride. Rather than an oxidative cyclization occurring, a sequence of insertion into the palladium-hydrogen bond, followed by insertion into the palladium-carbon bond forms the new bicyclic complex 11.230. P-Hydride elimination then releases the product 11.227 and regenerates the acetoxypalladium hydride. ... 

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