Copper complex catalysis

Fortunately, in the presence of excess copper(II)nitrate, the elimination reaction is an order of magnitude slower than the desired Diels-Alder reaction with cyclopentadiene, so that upon addition of an excess of cyclopentadiene and copper(II)nitrate, 4.51 is converted smoothly into copper complex 4.53. Removal of the copper ions by treatment with an aqueous EDTA solution afforded in 71% yield crude Diels-Alder adduct 4.54. Catalysis of the Diels-Alder reaction by nickel(II)nitrate is also... 

Copper complexes of sparteine have also been used for the catalysis of asymmetric carbon-carbon bond formation. The copper-catalyzed reaction... 

In the very recent past, metal complex catalysis has been used with advantage for the stereo- and enantio selective syntheses based on the Henry and Michael reactions with SENAs (454-458). The characteristic features of these transformations can be exemplified by catalysis of the reactions of SENAs (327) with functionalized imides (328) by ligated trivalent scandium complexes or mono-and divalent copper complexes (454) (Scheme 3.192). Apparently, the catalyst initially forms a complex with imide (328), which reacts with nitronate (327) to give the key intermediate A. Evidently, diastereo- and enantioselectivity of the process are associated with preferable transformations of this intermediate. 

Activation of a C-H bond requires a metallocarbenoid of suitable reactivity and electrophilicity.105-115 Most of the early literature on metal-catalyzed carbenoid reactions used copper complexes as the catalysts.46,116 Several chiral complexes with Ce-symmetric ligands have been explored for selective C-H insertion in the last decade.117-127 However, only a few isolated cases have been reported of impressive asymmetric induction in copper-catalyzed C-H insertion reactions.118,124 The scope of carbenoid-induced C-H insertion expanded greatly with the introduction of dirhodium complexes as catalysts. Building on initial findings from achiral catalysts, four types of chiral rhodium(n) complexes have been developed for enantioselective catalysis in C-H activation reactions. They are rhodium(n) carboxylates, rhodium(n) carboxamidates, rhodium(n) phosphates, and < // < -metallated arylphosphine rhodium(n) complexes. 

The utilization of copper complexes (47) based on bisisoxazolines allows various silyl enol ethers to be added to aldehydes and ketones which possess an adjacent heteroatom e.g. pyruvate esters. An example is shown is Scheme 43[126]. C2-Symmetric Cu(II) complexes have also been used as chiral Lewis acids for the catalysis of enantioselective Michael additions of silylketene acetals to alkylidene malonates[127]. 

The nature of the counterion has had a profound impact on catalysis, as will be seen. Structurally, it was of considerable interest to delineate the factors that influence selectivity and to examine whether the counterion plays a role in the solid-state geometry of these catalysts. While the hexafluoroantimonate copper complexes of bis(oxazoline) 55c are completely dissociated in the solid state, analogous triflate complexes exhibit weak bonding to one counterion in the apical position (2.62 A from the metal), Fig. 23. Association of the triflates in the solid state was also noted for Complex 266d. The water molecules are distorted toward the phenyl substituents, similar to the SbF6 complex 265d. 

The addition of an enolsilane to an aldehyde, commonly referred to as the Mukaiyama aldol reaction, is readily promoted by Lewis acids and has been the subject of intense interest in the field of chiral Lewis acid catalysis. Copper-based Lewis acids have been applied to this process in an attempt to generate polyacetate and polypropionate synthons for natural product synthesis. Although the considerable Lewis acidity of many of these complexes is more than sufficient to activate a broad range of aldehydes, high selectivities have been observed predominantly with substrates capable of two-point coordination to the metal. Of these, benzy-loxyacetaldehyde and pyruvate esters have been most successful. 

Micelle-forming copper complexes were found to effectively discriminate between enantiomers in the hydrolysis of a-amino esters (257). Hydrolysis of (.V)-phenylalanine p-nitrophenyl ester is 14-fold faster than its enantiomer, Eq. 223. Leucine affords 10-fold faster hydrolysis. The authors note that the micellar nature of these systems is extremely important for both rate of hydrolysis and selectivity (258). For example, the /V-mcthyl-dcrivcd ligand 419b leads to inhibition of the hydrolysis process, relative to catalysis by Cu(II) ion alone. 

A hydroxoaqua copper complex containing N, N, N, A -tetramethyl-1,2-diamino-ethane (250) is an excellent catalyst for the hydrolysis of sarin, O-isopropyl methylphosphonofluoridate (251), and diethyl p-nitrophenyl phosphate (252 R = Et). The mechanism of the reaction probably involves bound hydroxide attacking the phosphoryl group with concomitant electrophilic catalysis by copper. 

In 2004, Kobayashi et al. introduced enecarbamates as nucleophiles to asymmetric catalysis [48], The addition of enecarbamates to imines in the presence of a chiral copper complex provides access to P-amino imines which can be hydrolyzed to the corresponding p-amino carbonyl compounds [49],... 

One of the most surprising findings is the observed catalyst-inhibitor conversion with copper complexes. Since a copper (III) chelate is not regarded as feasible, a possible explanation might be that copper(I) formed by reducing impurities is the active species at least as far as catalysis is concerned. 

An unprecedent skeletal rearrangement of O-propargyl oximes has been reported to go via copper complex catalysis, involving cleavage of five different covalent bonds (C=N, N-O, C-O, C-C, and C=C) and leading to reorganization into (3-lactams in good to excellent yields (Scheme 114), [244]. 

Oxidative coupling polymerization provides great utility for the synthesis of high-performance polymers. Oxidative polymerization is also observed in vivo as important biosynthetic processes that, when catalyzed by metalloenzymes, proceed smoothly under an air atmosphere at room temperature. For example, lignin, which composes 30% of wood tissue, is produced by the oxidative polymerization of coniferyl alcohol catalyzed by laccase, an enzyme containing a copper complex as a reactive center. Tyrosine is an a-amino acid and is oxidatively polymerized by tyrosinase (Cu enzyme) to melanin, the black pigment in animals. These reactions proceed efficiently at room temperature in the presence of 02 by means of catalysis by metalloenzymes. Oxidative polymerization is observed in vivo as an important biosynthetic process that proceeds efficiently by oxidases. 

The d-d absorption of the copper complex differs in each step of the catalysis because of the change in the coordination structure of the copper complex and in the oxidation state of copper. The change in the visible spectrum when phenol was added to the solution of the copper catalyst was observed by means of rapid-scanning spectroscopy [68], The absorbance at the d-d transition changes from that change the rate constants for each elementary step have been determined [69], From the comparison of the rate constants, the electron transfer process has been determined to be the rate-determining step in the catalytic cycle. 

B. Reaction Aspects of p,-Peroxo Dinuclear Copper Complexes Relevant to Tyrosinase Catalysis... 

The structures of type II copper sites have not as yet generated as much interest, but the hetero-dinuclear structures of type IIC copper sites and the unusual protein donor ligands found in type IIB copper sites are noteworthy. The synthetic model approach to gain insight into the structures and functions of these types of copper sites will be also described. The diverse functions of a variety of proteins containing type IIA copper sites inspired many chemists to mimic the functions with synthetic copper complexes, even though the spectroscopic properties of the complexes are not unusual. Results of these studies will be reviewed and different aspects of the reactions relating to enzymatic catalysis will be discussed. 

Some metal- (especially copper) complexes catalyse the dismutation of superoxide at rates that compare favourably with catalysis by superoxide dismutase. One could therefore argue that the presence of such complexes in vivo might be beneficial. There are, however, additional considerations (1) such metal complexes may also reduce hydrogen peroxide, which could result in the formation of hydroxyl radicals, and (2) it is extremely likely that the metal will be displaced from its ligands (even when those ligands are present in excess), and becomes bound to a biomolecule, thereby becoming less active as a superoxide dismutase mimic. As an example, copper binds well to DNA and catalyses the formation of hydroxyl radicals in the presence of hydrogen peroxide and ascorbate [30],... 

Spescha et al. [4] used the copper complex 6, which was obtained from a thioglucofuranose derivative, as catalyst for 1,4-additions of Grignard reagents to 3, and observed enantioselectivities of up to 60 % ee. The dihydrooxazolylthiophenolato copper complex 7 was employed by Pfaltz et al. 5] for the enantioselective catalysis of Michael additions to cyclic enones the best results were obtained with tetrahydrofuran as solvent and HMPA as additive. There was a pronounced dependence of the stereoselectivity on the ring size of the substrate 16-37 % ee for 2-cyclopente-none, 60-72 % ee for 3, and 83-87 % ee for 2-cycloheptenone. Alexakis et al. [6] used the heterocycle 8, which is readily accessible from... 

Scheme 2.7 Selective direct a-fluorination of carbonyl compounds. Copper salt catalysis supposedly acts via formation of the copper enolate complex [19, 20], The formation of the corresponding copper complex of monofluoromalonate, the precursor of difluorinated products, is energetically disfavored.

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