Direct heterogeneous palladium catalyst

Ethanal is only used as an intermediate to acetic acid and its derivatives and in the near future production of ethanal will be replaced by other routes based on methanol and syn-gas to give acetic acid and acetic anhydride. Vinyl acetate can also be made via syn-gas routes, but the major producer in Europe employs a direct Wacker route with a heterogeneous palladium catalyst (former Hoechst ethene, acetic acid -from syn-gas-, and oxygen). 

Direct phenylation of pyridine was accomplished with a heterogeneous palladium catalyst in the presence of zinc and water, albeit in only moderate yield [80]. In contrast, 2,3,5,6-tetrafluoropyridine (119) can easily be directly arylated with aryl bromides or chlorides such as 52 as electrophiles, furnishing substituted products in high yields (Scheme 9.38) [72]. 

Sugar azides can be reduced directly to the corresponding acetamido-sugars with a mixture of thiolacetic acid and its potassium salt. A heterogenized homogeneous catalyst, interlamellar montmorillonite diphenylphosphine palladium(II), has been reported (two carbohydrate examples) to reduce azides selectively to amines without affecting benzyl ether or alkene moieties. ... 

When media other than water are used, related processes operate. Thus in acetic acid ethylene gives vinyl acetate, whereas vinyl ethers may be formed in alcohols. Both homogeneous and heterogeneous syntheses of vinyl acetate have been commercialized. The latter process (Hoechst) involves direct oxidation over a palladium-gold catalyst containing alkali acetate on a support ... 

A homogeneous catalytic solution to the alcohol inhibition problem (see the discussion under Uncatalyzed chain reactions of the oxidation of alcohol intermediates, above) does not appear to have been found. However, the presence of a heterogeneous oxidative dehydrogenation catalyst has been reported to be effective in the direct oxidation of alcohols to carbonyls and acids [109, 110]. The mechanism probably involves preliminaiy heterogeneous (oxidative) dehydrogenation of carbinols to carbonyls. If the carbonyl is an aldehyde, it is readily converted to the acid. Platinum, palladium, ruthenium, rhodium, and iridium catalysts, supported on carbon, are reported to be active and selective catalysts for the purpose [109]. Promoters such as cobalt and cadmium have been reported to be effective additives. 

Palladium nanoparticles and other heterogeneous catalysts are often invoked as catalysts in cross-coupling processes [194, 195). Direct evidence in support of an oxidative-addition-promoted leaching mechanism has been recently obtained in the Suzuki-Miyaura reactions with nanoparticle catalysts, suggesting that true surface catalysis remains largely unknown with these heterogeneous catalysts [196]. 

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