Bimetallic Catalytic Systems

A two-component bimetallic catalytic system has been developed for the allylic etherification of aliphatic alcohols, where an Ir(i) catalyst acts on allylic carbonates to generate electrophiles, while the aliphatic alcohols are independently activated by Zn(n) coordination to function as nucleophiles (Equation (48)).194 A cationic iridium complex, [Ir(COD)2]BF4,195 and an Ru(n)-bipyridine complex196 have also been reported to effectively catalyze the O-allylation of aliphatic alcohols, although allyl acetate and MeOH, respectively, are employed in excess in these examples. 

Shi el al. reported one of the first examples of bimetallic catalytic systems that allowed the insertion of C02 into the rather unreactive fin-carbon bond [45], The concept behind this system was to exploit, in the same system, the ability of a transition metal to catalyze crosscoupling reactions and C02 activation. For instance, tributyl(allyll)tin does not react with C02 in solution even under high-pressure. To run the same reaction in the presence of zero-valent palladium species (Pd(PPh3)4 or Pd(PBu3)4) will quantitatively afford carboxylates 2 (90%) and 3 (10%) (Scheme 5.10), although the reactivity of the system is limited to allylstannanes. 

The most often-reported metallic systems used in the formation of carboxylic acids are most certainly those systems that involve a zero-valent nickel species as the active intermediate. Ochiai et al. reported on a bimetallic catalytic system which allowed the synthesis of various saturated carboxylic acid in good yields, under very mild conditions [53] (0.1 MPa C02, 4—8 h reaction time, temperatures ranging from room temperature to 323 K). The catalytic system was based on the use of organozinc reagents as carbon nucleophiles, which could be selectively carboxy-lated in the presence of Ni(acac)2 as the main catalyst. 

A formally related yet most likely Ullman type of arylation of nitrogen nucleophiles was reported most recently. A bimetallic catalytic system of Fe(acac)3—CuO was shown to be highly active in the simple arylation of a variety of N-heterocycles with aryl iodides and bromides in the presence of CsC03. Although a deeper mechanistic understanding awaits further investigations, the broad scope plus its operational simplicity make this procedure especially amenable to applications on an industrial scale (Scheme 7.6) [10]. 

The hydrolormylation of alkynes is usually accompanied by extensive hydrogenation, producing saturated aldehydes or alkenes. However, the use of bimetallic catalytic systems, such as Pd-Co, Pd-W or Pd-Fe. can achieve excellent yields in the hydroformylation of symmetric internal alkynes, affording conjugated un-salurated aldehydes (Eq. 14). ... 

The C=C bond in furfural is made much less reactive (compared with that in the above mentioned aldehydes) by the structure of the molecule. The reactions to he suppressed here are the hydrogenolyses leading to either 2 or 6. The routes 1 to 2 and 1 to 6 are the most important side-reactions which were observed in the hydrogenation of furfural over platinum catalysts. The reaction of 1 to 6 is a CO elimination and reaction 1 to 2 is most likely a common hydrogenolytic fission of a C-0 bond. To study the role of promoters we have chosen platinum as active metal in the bimetallic catalytic system. Platinum belongs to the most active metals for hydrogenation, while it shows a low activity for various hydrogenolytic fissions. 

Bersot, J.C., Jacquel, N., Saint-Loup, R. et al. (2011) Efficiency increase of poly(ethylene terephthalate-co-isosorbide terephthalate) synthesis using bimetallic catalytic systems. Macromolecular Chemistry and Physics, 212 (19), 2114-2120. 

The Cu(I)-catalyzed direct addition of terminal alkynes to imines generated in situ from aldehydes and amines affords synthetically useful propargylamines. This metal-catalyzed carbon-carbon bond formation is best performed in toluene but water is also a convenient solvent. While the achiral process is mediated by the bimetallic catalytic system RuCls/CuBr, CuOTf in conjunction with chiral bis(oxazolines) has been found optimal for enantioselective additions (eq 119). 

R. F. Heck, Pure Appl. Chem., 1981, 55, 2323-2332. Palladium-Catalyzed Syntheses of Conjugated Polyenes. E. Negishi, Pure Appl. Chem., 1981, 55, 2333-2356. Bimetallic Catalytic Systems Containing Ti, Zr, Ni, and Pd. Their Applications to Selective Organic Syntheses. 

The catalytic carbonylation of / -aminonitroarenes or, less efficiently, of p-dinitroarenes and aliphatic aldehydes, carried out at 180 °C and 70 atm of CO, in the presence of water and with RhClfPPhs) and PdCh as a bimetallic catalytic system, gives substituted phenanthrolines (49) in 45-55 % yield (cq. 24) [89] ... 

Various a-aryl vinyl azides and internal alkynes were transformed into highly substituted isoquinolines under the [Cp RhCl2]2-Cu(OAc)2 bimetallic catalytic system in DMF as the solvent (eq A) ... 

The results of the EXAFS studies on supported bimetallic catalysts have provided excellent confirmation of earlier conclusions (21-24) regarding the existence of bimetallic clusters in these catalysts. Moreover, major structural features of bimetallic clusters deduced from chemisorption and catalytic data (21-24), or anticipated from considerations of the miscibility or surface energies of the components (13-15), received additional support from the EXAFS data. From another point of view, it can also be said that the bimetallic catalyst systems provided a critical test of the EXAFS method for investigations of catalyst structure (17). The application of EXAFS in conjunction with studies employing ( mical probes and other types of physical probes was an important feature of the work (25). 

In addition to palladium catalysts, Co(OAc)2 shows a catalytic activity for the arylation of heterocycles, including thiazole, oxazole, imidazole, benzothiazole, benzoxazole, and benzimidazole.78 As shown in Scheme 6, the catalytic system Co(OAc)2/9/Cs2C03 gives G5 phenylated thiazole, while the bimetallic system Co(OAc)2/CuI/9/Cs2C03 furnishes the G2 phenylated thiazole. The rhodium-catalyzed reaction of heterocycles such as benzimidazoles, benzoxazole, dihydroquinazoline, and oxazoline provides the arylation product with the aid of [RhCl(coe)]2/PCy3 catalyst.79 The intermediacy of an isolable A-heterocyle carbene complex is proposed. 

Conventionally, organometallic chemistry and transition-metal catalysis are carried out under an inert gas atmosphere and the exclusion of moisture has been essential. In contrast, the catalytic actions of transition metals under ambient conditions of air and water have played a key role in various enzymatic reactions, which is in sharp contrast to most transition-metal-catalyzed reactions commonly used in the laboratory. Quasi-nature catalysis has now been developed using late transition metals in air and water, for instance copper-, palladium- and rhodium-catalyzed C-C bond formation, and ruthenium-catalyzed olefin isomerization, metathesis and C-H activation. Even a Grignard-type reaction could be realized in water using a bimetallic ruthenium-indium catalytic system [67]. 

Supported bimetallic catalysts have gained unquestionable importance in subjects such as refining, petrochemistry and fine chemistry since their earliest use in the 1950s [1, 2]. The catalytic behavior of such a system is influenced by the size of the metal particles and by the interactions among them and with the support and other catalyst components. The second metal may influence the first metal through electronic interactions or by modifying the architecture of the active site. Very often, the interactions between the two metals are complex and largely unknown, and consequently the preparation procedure critically influences the nature of the catalytic system obtained. 

Another example of promising research is the efficient electrochemical dicarbo-xylations of aryl-acetylenes with C02, using an uncomplicated bimetallic redox couple as the catalytic system. In this case, metallic nickel was used as the cathode and aluminum as the anode, to generate in situ carboxylation-active nickel species (Scheme 5.20) [61]. 

Table 1 Bimetallic DEN systems synthetic methods and catalytic reactions...

This section reports a series of examples of application of the cluster model approach to problems in chemisorption and catalysis. The first examples concern rather simple surface science systems such as the interaction of CO on metallic and bimetallic surfaces. The mechanism of H2 dissociation on bimetallic PdCu catalysts is discussed to illustrate the cluster model approach to a simple catalytic system. Next, we show how the cluster model can be used to gain insight into the understanding of promotion in catalysis using the activation of CO2 promoted by alkali metals as a key example. The oxidation of methanol to formaldehyde and the catalytic coupling of prop)me to benzene on copper surfaces constitute examples of more complex catalytic reactions. 

The synergistic effect often observed in bimetallic systems was further explored by Garland and coworkers. The hydroformylation of 3,3-dimethylbut-l-ene to form 4,4-dimethylpentanal in >95% selectivity at room temperature with [Rli4(CO)i2]-[Mn2(CO)io/HMn(CO)5] as catalyst coprecursors was investigated using in situ PT-IR spectroscopic techniques and kinetic studies revealing evidence of a bimetallic catalytic binuclear elimination reaction (CBER). 

The catalytic silylative carbonylation of aryl iodides using a bimetallic catalyst system, [PdCl2(PPh3)2-Co2(CO)8], has been reported by Hidai and coworkers. The carbonylation of... 

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