Phenols palladium catalysts

The kinetics of hydrogenation of phenol has already been studied in the liquid phase on Raney nickel (18). Cyclohexanone was proved to be the reaction intermediate, and the kinetics of single reactions were determined, however, by a somewhat simplified method. The description of the kinetics of the hydrogenation of phenol in gaseous phase on a supported palladium catalyst (62) was obtained by simultaneously solving a set of rate equations for the complicated reaction schemes containing six to seven constants. The same catalyst was used for a kinetic study also in the liquid phase (62a). 

Hartwig and Buchwald have developed a new methodology for arylation of amines or phenols with aryl halides and palladium catalysts.17 This reaction provides a very useful strategy for the preparation of various heterocyclic compounds such as phenazines, as shown in Scheme 9.4.18... 

These telomerization reactions of butadiene with nucleophiles are also catalyzed by nickel complexes. For example, amines (18-23), active methylene compounds (23, 24), alcohols (25, 26), and phenol (27) react with butadiene. However, the selectivity and catalytic activity of nickel catalysts are lower than those of palladium catalysts. In addition, a mixture of monomeric and dimeric telomers is usually formed with nickel catalysts ... 

The most characteristic reaction of butadiene catalyzed by palladium catalysts is the dimerization with incorporation of various nucleophiles [Eq. (11)]. The main product of this telomerization reaction is the 8-substituted 1,6-octadiene, 17. Also, 3-substituted 1,7-octadiene, 18, is formed as a minor product. So far, the following nucleophiles are known to react with butadiene to form corresponding telomers water, carboxylic acids, primary and secondary alcohols, phenols, ammonia, primary and secondary amines, enamines, active methylene compounds activated by two electron-attracting groups, and nitroalkanes. Some of these nucleophiles are known to react oxidatively with simple olefins in the presence of Pd2+ salts. Carbon monoxide and hydrosilanes also take part in the telomerization. The telomerization reactions are surveyed based on the classification by the nucleophiles. 

Although palladium catalysts have played the most prominent role in this area, other metals have also been found to catalyze allylic etherification reactions, often providing complementary stereochemical outcomes. A few ruthenium catalyst systems have been used for the O-allylation of phenols,143,144 including an enantioselective version utilizing [Cp Ru(MeCN)3]PF6 that provides promising ee s, albeit with diminished control of regioselectivity (Equation (25)).145... 

The proposed mechanism is given in Scheme 15. Initially the dissociation of water, maybe trapped by the molecular sieve, initiates the catalytic cycle. The substrate binds to the palladium followed by intramolecular deprotonation of the alcohol. The alkoxide then reacts by /i-hydride elimination and sets the carbonyl product free. Reductive elimination of HOAc from the hydride species followed by reoxidation of the intermediate with dioxygen reforms the catalytically active species. The structure of 13 could be confirmed by a solid-state structure [90]. A similar system was used in the cyclization reaction of suitable phenols to dihydrobenzofuranes [92]. The mechanism of the aerobic alcohol oxidation with palladium catalyst systems was also studied theoretically [93-96]. 

The role of the metal catalyst in the hydrogenation reaction and its stereoselectivity has been widely studied (9). Group Vtll metals are generally used, but different behaviours are observed. In the hydrogenation of disubstituted phenols palladium mainly gives cyclohexanone derivatives, rhodium and platinum are very selective for the c/5 isomer, whereas nickel is more selecive for the tra/ 5 isomer (9). 

The oxidative carbonylation of arenes to aromatic acids is a useful reaction which can be performed in the presence of Wacker-type palladium catalysts (equation 176). The stoichiometric reaction of Pd(OAc)2 with various aromatic compounds such as benzene, toluene or anisole at 100 °C in the presence of CO gives aromatic acids in low to fair yields.446 This reaction is thought to proceed via CO insertion between a palladium-carbon (arene) allyl chloride, but substantial amounts of phenol and coupling by-products are formed.447... 

The oxycarbonylation of phenol in the presence of a palladium catalyst, a tertiary amine and a manganese cocatalyst at room temperature and atmospheric pressure results in the formation of diphenyl carbonate in good yield (equation 184).453... 

Similarly, propynoic esters react with phenols in the presence of a palladium catalyst to afford coumarins in moderate to high yield (Equation 276) <1996JA6305, 2000JOC7516, 2002CL380, 2003BCJ1889, 2003JA4518>. 

A useful transformation that leads to chiral butenolides, in their own right useful chiral building blocks, is the reaction of the butenolide 8 with a nucleophile, such as a phenol, in the presence of a palladium catalyst (Scheme 22.21).150... 

Mahata, N., Vishwanathan, V., Kinetics of phenol hydrogenation over supported palladium catalysts, Catal. Today, 49,... 

Talukdar, A.K., Bhattacharyya, K.G., Hydrogenation of phenol over platinum and palladium catalysts, Appl. Catal. A General, 96, 229-239,1993... 

The 1,3-dioxolane ring is usually inert to catalytic hydrogenation unless forcing conditions are employed. Hydrogenolysis over rhodium or palladium catalysts in the presence of an acid catalyst gives mainly hydroxy ethers as shown in equation (8). Raney nickel cleaves the oxathiolane ring (Scheme 9) and converts 1,3-benzoxathioles into phenol derivatives. 

Narayanan, S. and Krishna, K. (1998). Hydrotalcite-supported palladium catalysts Part I preparation, characterization of hydrotalcites and palladium on uncalcined hydrotalcites for CO chemisorption and phenol hydrogenation. Appl. Catal. A 174, 221. 

Mahata, N. and Vishwanathan, V, Influence of palladium precursors on structural properties and phenol hydrogenation characteristics of supported palladium catalysts. J Catalysis 2000, 196 (2), 262-270. 

A number of patents have been applied for the synthesis of cyclohexanone by hydrogenation of phenol where palladium catalysts have been employed.134 The presence of basic components such as CaO, MgO, and NajCOj has often been claimed to be effective for this purpose. As an example, 1000 parts of phenol was hydrogenated over 1 part of Pd-C containing 0.01 part of Na2C03 and 5000 ppm of Na in a 16.5% solution of aqueous ethanol at 186°C and 0.48 MPa H2 for 2.5 h to give a mixture of 97.2% of cyclohexanone, less than 0.5% of phenol, and cyclohexanol.134a In another patent, the reaction of 1 mol of phenol and 2 mol of cyclohexanol to give 3 mol of cyclohexanone was carried out over a palladium catalyst at 100-200°C (eq. 11.26).135... 

The unreacted cyclohexane is distilled off and recycled. The ca. 1 1 mixture of cyclohexanol and cyclohexanone is then subjected to dehydrogenation over a palladium catalyst (the same catalyst as is used in phenol hydrogenation) to give pure cyclohexanone. 

The cyciization of o-alkynyl phenol derivatives under the influence of a base or copper or palladium catalysts is a simple and reliable route to benzo[3]furans (Equation 107) <1996H(43)101>, given that the substrates are easily... 

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