Copper-Based Catalytic Systems

Copper nanoclusters are active in Sonogashira reactions involving aryl iodides and some bromides when TBAA is used as base [90], 

Generally speaking, palladium-free catalysts for Sonogashira reactions must be regarded with extreme caution, since it is known that cross-couphng reactions can be performed when only ppb levels of palladium are present in some inorganic bases [91]. 

Reaction mechanisms, particularly in complex catalytic sequences, are most often obscured by the formation of transient species or intermediates in very low concentrations, which are often not observable by standard means of analysis, such as chromatography or nuclear magnetic resonance (NMR) and infrared (IR) spectroscopies. As a consequence, a mechanism should be considered as the best model involving all available data at a given hme, and thus may naturally be subject to further refinements. 

The selective and reversible alkynyl-transfer from copper to palladium is kineti-cally favored over an aryl-transfer, and thereby prevents formation of the biaryls products [96]. 

Oxidative addition of aryl haUdes to an electron-rich paUadium(O) species depends substantially on the nature of the C(sp )—X bond. Relatively weak bonds, as in aryl iodides or triflates, lead to a faster oxidative addition, but the activations of stronger bonds, as in aryl bromides-and especially chlorides-require very good donor ligands. This dependence of oxidative addition rates on the nature of the electrophile in multistep reactions may lead to different rate-deterriiiiittig steps. Apart from the nature of the aromatic substrate, the possible interaction of the terminal alkyne starting material and the internal alkyne product with the metal center of the catalyst can alter the kinetics of oxidative addition. The kinetics of addition of 

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Marko et al. developed an efficient copper-based catalytic system for the aerobic oxidation of a wide range of alcohols to the corresponding carbonyl products (Fig. 24) 88). Typically, 0.40 mol of the alcohol was reacted with pure dioxygen in 800 mL toluene between 80 and 90° C. The aerobic oxidation is catalyzed by 5mol% [copper(I) chloride(l,10-phenanthroline)], 5mol% di- cr -butylhydrazinedicarboxylate (DBADH2) and two equivalents of potassium carbonate. 

Lee and co-workers developed a copper-based catalytic system for the one-pot syntheses of benzothiazoles from 2-iodoanilines, aldehydes, and NaSH iiHaO (Scheme 2.144a). This one-pot reaction system represents a significant advantage in that it does not require the isolation of the intermediate prior to execution of C-S bond formation and in most instances does not require the preparation of the starting materials. As the authors stated, this was the first example of the employment of NaSH iiHaO as a sulfur source, not only in the synthesis of the benzothiazole, but also in transition-metal-catalyzed C-S bond formation. In addition, the... 

First, Suzuki and coworkers reported Cul-mediated synthesis of diaryl tellurides via C—Te cross-coupling of aryl iodides with benzene teUurolate ions [194]. Catalytic protocol has been subsequently developed using Cul/l,10-phenanthroline L80 for the coupling of aryl iodides with organotellurolates [195] (Scheme 20.58). Further improvement was achieved in the coupling of ditellurides with boronic acid derivatives using copper-based catalytic systems, Cul-bpy [166], CuCl-bpy [196], and CuSO -5H2O/l,10-phenanthroline [197]. 

Simply changing the solvent in the Pd-based catalytic system from water to a mixture of water and a perfluorocarboxylic acid (some water was necessary for the reaction, see Scheme 4) had no significant effect on product composition formic acid was still the principal product from methane. However, the addition of copper (I) or (II) chloride to the reaction mixture had a dramatic effect. Methanol and its ester now became the preferred products, with virtually no acetic and little formic acid being formed [42b] The activation parameters for the overall reaction determined under the condition when the rate was first-order in both methane and carbon monoxide were A=2xl04 s 1 Ea=15.3 kcalmol-1. Since methyl trifluoroacetate is both volatile and easily hydrolyzed back to the acid and methanol, it should be possible to design a system where the acid is recycled and methanol is the end-product. 

Interestingly most of the catalytic systems based on Tp M units contain a coinage metal such as copper or silver. This is also observed for NHC-based catalytic systems, with the addition of gold to those metals. This similarity can be explained in terms of a common feature of both Tp M and (NHC)M moieties with those metals they leave just one coordination site for the catalytic reaction to occur. A second coordination site may also accessible in some cases, for example with Cu(ll)-based systems where five-coordinate geometries are available. In the case of (NHC)M systems (M = Cu, Ag, Au), the linear complexes can accept an incoming ligand (reactant) through the transient formation of three-coordinate intermediates (Scheme 2). 

The analysis of the known and our own experimental data indicated that the properties required may be offered by a copper-containing cement-based catalytic system modified with alkali metals. In this catalyst, copper-containing active sites catalyze the oxidation of hydrogen chloride, whereas the activity of the catalyst in the dehydrochlorination reaction is determined by the acid—base surface properties, which are inherent to cements with different phase compositions. 

This CuBr2-bpy/TEMPO-based catalytic system can be considered as the first synthetic functional model of galactose oxidase, as both the achieved chemoselectivity, and the proposed reaction mechanism, resemble that of the biological copper enzyme. Nevertheless, this functional model is not able to compete with the natural enzyme in terms of catalytic efficiency. Indeed, the rate of turnover is only 0.006 s while a TOE of 800 s is reached by GOase for its native substrate. The objective of future research investigations is therefore to enhance the proficiency of the catalyst to obtain an economically interesting system for industrial applications. 

Since copper(l), silver(l), and, to a lesser extent, thallium(l) are common components of palladium- and platinum-based catalytic systems, it appears likely that bimetalhc species similar to those discussed in this chapter are involved in the corresponding catalytic cycles. Thus, while most of the studies on donor-acceptor bonds involving... 

In 2006, Dupont s group tilso reported a simple and efficient copper-free catalytic system based on some ptJladacycle cattilysts (Figure 1.31) [109]. The coupling of iodoarenes and activated bro-moarenes could be conducted with terminal alkynes at room temperature. The yields were generally good and TONs of up to 100 000 could be obtained with iodoarene substrates (Figure 1.31). 

Finally, C—N bond formation has been also accomplished through aerobic oxidative amination of arylboronic acids (Evans-Chan-Lam coupling) under poly-NHC-copper(II) catalysis. In particular, azoles and aromatic amines were successfully coupled with arylboronic acid using catalyst 73, with catalytic efficiencies comparable to those of other previously reported copper(II)-based catalytic systems for this reaction. 

It is only very recently that rhodium-based catalytic systems have been described in efficient oxidative olefination reactions. Inspired by the work of Satoh and Miura on rhodium/copper-catalyzed aerobic oxidative coupling of benzoic acids with internal allqmes or acrylates (Scheme 9.11), Glorius and co-workers described, in 2012, a rhodium-catalyzed directing group assisted olefination of 2-aryloxazolines under air. This method, which necessitated rhodium, silver and copper metal sources, afforded the desired olefin-oxazoline products in moderate-to-good yields (Scheme 9.12). 

Consorti CS, Flores FR, Rominger F, Dupont J. A simple and efficient copper-free catalytic system based on a palladacycle for the arylation of alkynes. Adv. Synth. Catal. 2006 348 133-141. 

The stoichiometric insertion of terminal alkenes into the Cu-B bond of the (NHC)Cu-B(cat) complex, and the isolation and full characterisation of the p-boryl-alkyl-copper (I) complex has been reported. The alkyl complex decomposes at higher temperatures by P-H elimination to vinylboronate ester [67]. These data provide experimental evidence for a mechanism involving insertion of alkenes into Cu-boryl bonds, and establish a versatile and inexpensive catalytic system of wide scope for the diboration of alkenes and alkynes based on copper. 

IH of alkynylamines has been performed with a variety of catalytic systems based on palladium [274-281], cobalt, rhodium, iridium, ruthenium, platinum, copper, silver, zinc, cadmium, mercury [279-281], nickel [279-282], gold [279-281, 283], and molybdenum [284] derivatives. 

Several catalytic systems based on copper can also achieve allylic oxidation. These reactions involve induced decomposition of peroxy esters (see Part A, Section 11.1.4). When chiral copper ligands are used, enantioselectivity can be achieved. Table 12.1 shows some results for the oxidation of cyclohexene under these conditions. 

Recently, great advancement has been made in the use of air and oxygen as the oxidant for the oxidation of alcohols in aqueous media. Both transition-metal catalysts and organocatalysts have been developed. Complexes of various transition-metals such as cobalt,31 copper [Cu(I) and Cu(II)],32 Fe(III),33 Co/Mn/Br-system,34 Ru(III and IV),35 and V0P04 2H20,36 have been used to catalyze aerobic oxidations of alcohols. Cu(I) complex-based catalytic aerobic oxidations provide a model of copper(I)-containing oxidase in nature.37 Palladium complexes such as water-soluble Pd-bathophenanthroline are selective catalysts for aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic... 

A wide range of catalytic materials have been investigated for the selective catalytic reduction of NOx. For stationary emissions, NH3-SCR using vanadium-tungsten oxides supported on titania is the most used method however, when there is a simultaneous emission of NO and NOz (in tail gas from nitric acid plants), copper-based zeolites or analogous systems have been proven to be preferable [31b], In fact, there are two main reactions for NH3-SCR ... 

The replacement of vanadia-based catalysts in the reduction of NOx with ammonia is of interest due to the toxicity of vanadium. Tentative investigations on the use of noble metals in the NO + NH3 reaction have been nicely reviewed by Bosch and Janssen [85], More recently, Seker et al. [86] did not completely succeed on Pt/Al203 with a significant formation of N20 according to the temperature and the water composition. Moreover, 25 ppm S02 has a detrimental effect on the selectivity with selectivity towards the oxidation of NH3 into NO enhanced above 300°C. Supported copper-based catalysts have shown to exhibit excellent activity for NOx abatement. Recently Suarez et al and Blanco et al. [87,88] reported high performances of Cu0/Ni0-Al203 monolithic catalysts with NO/NOz = 1 at low temperature. Different oxidic copper species have been previously identified in those catalytic systems with Cu2+, copper aluminate and CuO species [89], Subsequent additions of Ni2+ in octahedral sites of subsurface layers induce a redistribution of Cu2+ with a surface copper enrichment. Such redistribution... 

The use of metal-catalyzed aziridination methods with chiral ligands has also been reported. The copper-based system paired with ligand 56 provides the expected cinnamyl aziridine in good yield and excellent ee <06MI4568>. It is interesting to note that the /-butyl ester is obtained with 99% ee while the smaller methyl ester is obtained in only 88% ee. The binaphthyl ruthenium catalyst 57 has been found to aziridinate a number of olefins with moderate enantioselectivity <06TL1571>. Both p-nitrophenyl (Ns) and trimethylsilyloxy (SES) sulfonamides work well with this catalytic system. As is usually seen, the aziridination of aliphatic olefins proceeds in only 32% yield and 56% ee. 

Important advances in propargylic etherification have come from the use of copper-based systems that achieve efficient, catalytic O-progargylation of phenols (Scheme 8).245,246 While the mechanism of this transformation remains unclear, the products of these reactions have been readily converted into chromenes through subsequent Claisen rearrangement,... 

In the same year, Wang and Stack (211) reported the first truely catalytic system. They discovered that Complexes E and F shown in Fig. 28 can be oxidized by 1 equiv of tris(4-bromophenyl)aminiumhexachloroantimonate, (TPA)SbCl6, at -0.7 V Fc+/Fc to the corresponding (phenoxyl)copper(II) species, which in turn were shown to stoichiometrically convert 1 equiv of benzyl alcohol to benzaldehyde. In the presence of 20 equiv of (TPA)SbCl6 and base, Complex E displays catalytic behavior (-10 turnovers). 

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... 

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