Reaction Mechanism with Copper Catalysts

It is important to emphasize that the mobile, active sites are formed from adjacent copper atoms and surface oxide. Much of this work has been the subject of a review by the ICI group.  

Mixed oxide methanol synthesis catalysts containing copper are very selective and the selectivity of the conversion of carbon oxides to methanol exceeds 99%. This is simply explained because there is no sphtting of carbon-oxygen bonds when carbon oxides are adsorbed on the copper surface. 

Consequently, the methane and higher alcohols formed with zinc oxide-chromium oxide catalysts are no longer major by-products. Formaldehyde and methyl formate form only in small, equilibrium amoimts. 

Although dimethyl ether eould be produced with an acid catalyst by a simple dehydration reaction, the alumina component of the catalyst has low acidity and less than 200 ppm ether is found in cmde methanol. This is most probably due to the neutralizing effect of the basic zinc oxide. 

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The original Sonogashira reaction uses copper(l) iodide as a co-catalyst, which converts the alkyne in situ into a copper acetylide. In a subsequent transmeta-lation reaction, the copper is replaced by the palladium complex. The reaction mechanism, with respect to the catalytic cycle, largely corresponds to the Heck reaction.Besides the usual aryl and vinyl halides, i.e. bromides and iodides, trifluoromethanesulfonates (triflates) may be employed. The Sonogashira reaction is well-suited for the synthesis of unsymmetrical bis-2xy ethynes, e.g. 23, which can be prepared as outlined in the following scheme, in a one-pot reaction by applying the so-called sila-Sonogashira reaction ... 

Copper(II) triflate is quite inefficient in promoting cyclopropanation of allyl alcohol, and the use of f-butyl diazoacetate [164/(165+166) = 97/3%] brought no improvement over ethyl diazoacetate (67/6 %)162). If, however, copper(I) triflate was the catalyst, cyclopropanation with ethyl diazoacetate increased to 30% at the expense of O/H insertion (55%). As has already been discussed in Sect. 2.2.1, competitive coordination-type and carbenoid mechanisms may be involved in cyclopropanation with copper catalysts, and the ability of Cu(I) to coordinate efficiently with olefins may enhance this reaction in the intramolecular competition with O/H insertion. 

Asymmetric amplification, 551,700-712 in carbonyl-ene reaction, 551 in diethylzinc addition, 702 Asymmetric autoinduction, 713 Asymmetric aziridination, 317-322 mechanism of, 320 with copper catalysts, 317-321... 

Allylic oxidation (acyloxylation) can also be achieved with copper catalysts and stoichiometric amounts of peresters or an alkylhydroperoxide in a carboxylic acid as solvent [108], via a free radical mechanism (Fig. 4.40). The use of water-soluble ligands [109] or fluorous solvents [110] allows recycling of the copper catalyst. In view of the oxidants required, this reaction is economically viable only when valuable (chiral) products are obtained using asymmetric copper catalysts [111-113]. The scope of the reaction is rather limited however. 

From a historical perspective it is interesting to note that the Nozaki experiment was, in fact, a mechanistic probe to establish the intermediacy of a copper carbe-noid complex rather than an attempt to make enantiopure compounds for synthetic purposes. To achieve synthetically useful selectivities would require an extensive exploration of metals, ligands and reaction conditions along with a deeper understanding of the reaction mechanism. Modern methods for asymmetric cyclopropanation now encompass the use of countless metal complexes [2], but for the most part, the importance of diazoacetates as the carbenoid precursors still dominates the design of new catalytic systems. Highly effective catalysts developed in... 

The reaction mechanism is not rigorously known, but is likely to involve the following steps." " First the arenediazonium ion species 1 is reduced by a reaction with copper-(l) salt 2 to give an aryl radical species 4. In a second step the aryl radical abstracts a halogen atom from the CuXa compound 5, which is thus reduced to the copper-1 salt 2. Since the copper-(l) species is regenerated in the second step, it serves as a catalyst in the overall process. 

The proposed reaction mechanism involves intermolecular nucleophilic addition of the amido ligand to the olefin to produce a zwitterionic intermediate, followed by proton transfer to form a new copper amido complex. Reaction with additional amine (presnmably via coordination to Cn) yields the hydroamination prodnct and regenerates the original copper catalyst (Scheme 2.15). In addition to the NHC complexes 94 and 95, copper amido complexes with the chelating diphosphine l,2-bis-(di-tert-bntylphosphino)-ethane also catalyse the reaction [81, 82]. 

The Pschorr reaction is a method of synthesis of phenanthrenes from diazotized Z-2-aminostilbenes. A traditional procedure involves heating with a copper catalyst. Improved yields are often observed, however, if the diazonium ion is treated with iodide ion. Suggest a mechanism for the iodide-catalyzed reaction. 

Wacker (1) A general process for oxidizing aliphatic hydrocarbons to aldehydes or ketones by the use of oxygen, catalyzed by an aqueous solution of mixed palladium and copper chlorides. Ethylene is thus oxidized to acetaldehyde. If the reaction is conducted in acetic acid, the product is vinyl acetate. The process can be operated with the catalyst in solution, or with the catalyst deposited on a support such as activated caibon. There has been a considerable amount of fundamental research on the reaction mechanism, which is believed to proceed by alternate oxidation and reduction of the palladium ... 

Arylation of activated double bonds with diazonium salts in the presence of copper catalysts is known as the Meerwin reaction. The reaction is postulated to either proceed through an organocopper intermediate or through a chlorine atom transfer from chiral CuCl complex to the a-acyl radical intermediate. Brunner and Doyle carried out the addition of mesityldiazonium tetrafluoroborate with methyl acrylate using catalytic amounts of a Cu(I)-bisoxazoline ligand complex and were able to obtain 19.5% ee for the product (data not shown) [79]. Since the mechanism of the Meerwin reaction is unclear, it is difficult to rationalize the low ee s obtained and to plan for further modifications. 

In a context of industrial interest, the copper-catalyzed addition of acetic acid36 to 1 (hydroacetoxylation) in the absence of oxygen was shown to be non-regioselective, a 1 0.5 mixture of 1,2- and 1,4-addition products being obtained in a yield of 60% based on butadiene. The effect of various additives on the regiochemistry and the yield has been carefully studied. The butadiene conversion was mainly efficient with the CuBr-LiBr catalytic system (equation 12). The role of the catalyst in the reaction mechanism has been discussed but not fully understood. It has been shown that the dominant formation... 

The study of the molecular weight of the intermediate course is an effective method for the classification of polymerization as chain or stepwise reaction. In Figure 3, the molecular weight of the obtained polymer is plotted against the yield, for the oxidative polymerization of dimethylphenol with the copper catalyst and for the electro-oxidative polymerization. The molecular weight rises sharply in the last stage of the reaction for the copper-catalyzed polymerization. This behavior is explained by a stepwise growth mechanism. 

There are a large number of reports on copper(I)-catalyzed conjugate additions, yet there is only scant information available about their reaction mechanisms. Recently, the conjugate addition of organozinc compounds to enones was found by Kitamura, Noyori, et al. to be catalyzed by N-benzylbenzenesulfonamide and CuCN, and the mechanism was scrutinized (Fig. 10.1). The kinetic rate was found to be first order in the concentrations of the catalyst that exist in equilibrium with R2Zn and enone [77]. 

Various transition metals have been used in redox processes. For example, tandem sequences of cyclization have been initiated from malonate enolates by electron-transfer-induced oxidation with ferricenium ion Cp2pe+ (51) followed by cyclization and either radical or cationic termination (Scheme 41). ° Titanium, in the form of Cp2TiPh, has been used to initiate reductive radical cyclizations to give y- and 5-cyano esters in a 5- or 6-exo manner, respectively (Scheme 42). The Ti(III) reagent coordinates both to the C=0 and CN groups and cyclization proceeds irreversibly without formation of iminyl radical intermediates.The oxidation of benzylic and allylic alcohols in a two-phase system in the presence of r-butyl hydroperoxide, a copper catalyst, and a phase-transfer catalyst has been examined. The reactions were shown to proceed via a heterolytic mechanism however, the oxidations of related active methylene compounds (without the alcohol functionality) were determined to be free-radical processes. 

The reaction mechanism of and active sites for the reaction have been studied using various techniques combined with an isotopic tracer method. Infrared spectra were measured for detection of surface adsorbates on the Cu-ZSM5 zeolites ESR, XPS, phosphorescence, diffuse reflectance UV, and Cu-MASNMR have been used to reveal the states of the copper ions in the catalysts. CO adsorption and TPD experiments have been employed to measure quantitativdy the amounts of Cu ions, NO adsorbed, and O2 remaining on the surface. Based on these investigations, we can propose a reasonable reaction mechanism which includes Cu ions as active sites and NO" species as reaction intermediates. The reaction cycle is suggested to be as foDows elevated temp. + 2NO... 

Acyl cyanides1708 can be prepared by treatment of acyl halides with copper cyanide. The mechanism is not known and might be free-radical or nucleophilic substitution. The reaction has also been accomplished with thallium(I) cyanide,1709 with MejSiCN and an SnCl4 catalyst,1710 and with Bu3SnCN,1711 but these reagents are successful only when R = aryl or tertiary alkyl. KCN has also been used, along with ultrasound,1712 as has NaCN with phase transfer catalysts.1713 OS III, 119. 

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