Copper -catalyzed dimerization

The first mechanism is. in fact, reminiscent of the well-known copper-catalyzed dimerization of acetylene viny(acetylene being the main by-product of this process. This side reaction can, however, be inhibited to some extent by the use of cobalt salts as additives [IS]. The cyanation of acetylene and of alkenyl halides is also promoted by Co and Ni cyanides and Pd catalysis. A reducing reagent, such as Zn or NaBll4, has been used in conjunction with cobalt cyanide complexes, and the formation of. succinonitrile has been reported to result from the basebase-catalyzed hydrocyanation of acrylonitrile. 

Exchanged copper ions in Y zeolite are located near supercages (47). As it has been proposed that copper carbenoid intermediates are involved in the copper-catalyzed dimerization of aryldiazomethanes (43), organocopper intermediates may be formed in narrow supercages. The excellent cis/trans selectivity is accounted for by the increased stability differences between the two intermediates (13c, 13t) leading to cis- and rrans-1,2-diarylethylenes, respectively, as shown in Fig. 5. Efficient catalysis of zeolite-encapsulated copper ions was proved in reactions of ethyl diazoacetate as well (48, 49). 

Formation of C—C links in the anthraquinone molecule, especially substitutions with aryl moieties, proceed via copper-catalyzed nucleophilic exchange of halogenated anthraquinone compounds. These methods include dimerization of 1 -aminoanthraquinone. 

The polymerization starting from a dimer or an oligomer is also an effective method for the classification of polymerization as chain or stepwise reaction. For the copper-catalyzed polymerization, the poly(phenyleneoxide) was also and rather rapidly obtained from the dimer of dimethylphenol(2,6-dimethylphenyl 3,5-dimethyl-4-hydroxy-... 

Copper-catalyzed oxidations of phenols by dioxygen have attracted considerable interest owing to their relevance to enzymic tyrosinases (which transform phenols into o-quinones equation 24) and laccases (which dimerize or polymerize diphenols),67 and owing to their importance for the synthesis of specialty polymers [poly(phenylene oxides)]599 and fine chemicals (p-benzoquinones, muconic acid). A wide variety of oxidative transformations of phenols can be accomplished in the presence of copper complexes, depending on the reaction conditions, the phenol substituents and the copper catalyst.56... 

A careful reexamination of the Pschorr synthesis has shown that, in the copper-catalyzed reaction, products are formed which are not found when the diazonium salt is decomposed thermally. Thus, in a typical example, the fluoroborate (92) of o-amino-A-methylbenzanilide forms both the spirocyclohexadienone (93) and the spirohexadiene dimer (94) as well as A-methylphenanthridone, and the proposed... 

Copper-catalyzed [2 + 2] photocycloadditions are related to the latter reactions. These transformations have been extensively studied, frequently in the context of application to organic synthesis [21], When irradiated in the presence of copper(I) triflate, norbomene 12 was effidently transformed into its dimer 13 (Scheme 5.3, reaction 4) [22]. Although complexes such as III are involved in the reaction mechanism [22, 23], it is unclear whether MLCT (metal to ligand charge transfer) or LMCT (ligand to metal charge transfer) excitation induces the transformation. 

Since then, experiments have been reported which indicate that (1) organocopper compounds will couple with aryl halides (2) arylcopper compounds can be oxidatively and thermally dimerized (3) arylcopper compounds are intermediates in the Ullmann reaction (4) organocopper compounds are intermediates in copper-catalyzed decarboxylations and (5) copper-promoted coupling reactions are not restricted to aromatic halides. The copper(I) oxide-promoted coupling reactions, however, have still to yield firm evidence of a copper intermediate. 

A radical species may also be generated by reduction of an electron-deficient compound and a classical entry to 4,4 -bipyridines is the reduction of a pyridine by sodium and subsequent rearomatization. Figure 8a illustrates the use of such a reduction in order to prepare the precursor of a sodium-ion molecular switch [30], A more general route, derived from the ancient copper-catalyzed Ullmann coupling, is the metal-induced dimerization of an aryl halide. The key step is a reductive elimination within the coordination sphere of the metal. A nickel(O) complex, in stoichiometric quantities, is usually selected for this purpose. Constable and Ward have used such a reaction to prepare a bis-terpyridine from an interesting synthon, which would have otherwise required a more specialized strategy with dedicated intermediates (Figure 8b) [31]. 

The copper-catalyzed reaction of silicon with chloromethane leads to a complex mixture of various monomeric and dimeric methylchlorosilanes termed crude silane. 

Among the methods at hand to synthesize cyclopropane derivatives, carbene addition to alkenes plays a prominent role 63). As a source of vinylcarbenes, cyclopropenes might be useful in this kind of approach. In 1963, Stechl was the first to observe a transition metal catalyzed cyclopropene-vinylcarbene rearrangement64). When treating 1,3,3-trimethylcyclopropene with copper salts, dimerization occurred to give 2,3,6,7-tetramethyl-octa-2,4,6-triene (9), the product from a formal recombination of the corresponding vinylcarbene (Eq. 8). 

The copper-catalyzed cyclopropanation of alkenes with diazoalkanes is a particularly important synthetic reaction (277). The reaction of styrene and ethyl diazoacetate catalyzed by bis[/V-(7 )- or (5)-a-phenyl-ethylsalicylaldiminato]Cu(II), reported in 1966, gives the cyclopropane adducts in less than 10% ee and was the first example of transition metal-catalyzed enantioselective reaction of prochiral compounds in homogeneous phase (Scheme 90) (272). Later systematic screening of the chiral Schiff base-Cu catalysts resulted in the innovative synthesis of a series of important cyclopropane derivatives such as chrysanthemic acid, which was produced in greater than 90% ee (Scheme 90) (273). The catalyst precursor has a dimeric Cu(II) structure, but the actual catalyst is in the Cu(I) oxidation state (274). (S)-2,2-Dimethylcyclopropanecar-boxylic acid thus formed is now used for commercial synthesis of ci-lastatin, an excellent inhibitor of dehydropeptidase-I that increases the in vivo stability of the caibapenem antibiotic imipenem (Sumitomo Chemical Co. and Merck Sharp Dohme Co.). Attempted enantioselective cyclopropanation using 1,1-diphenylethylene and ethyl diazoacetate has met with limited success (211b). A related Schiff base ligand achieved the best result, 66% optical yield, in the reaction of 1,1-diphenylethylene and ethyl diazoacetate (275). 

Another early example that followed the discovery of CuAAC, the copper-catalyzed reaction of nitrile oxides, is shown in Scheme 10.9. Similarly to azides, the uncatalyzed 1,3-dipolar cycloaddition of nitrile oxides and acetylenes has long been known, but its applications to the synthesis of the corresponding heterocycle (isoxazoles) are scarce. Yields of isoxazole products are often quite low, side reactions are common, and both regioisomers may be formed (although the selectivity of nitrile oxide cycloadditions is usually higher than in reactions of azides, favoring the 3,5-isomer) [127]. Furthermore, nitrile oxides are not very stable and readily dimerize. 

Gray, E.T., Taylor, R.W. and Margerum, D.W., Kinetics and mechanisms of the copper-catalyzed decomposition of hypochlorite and hypobromite. Properties of a dimeric copper(III) hydroxide intermediate, Inorg. Chem., 16, 3047, 1977. 

Copper-catalyzed decomposition of benzenesulfonyl azide in the presence of cyclohexene was the first reported evidence of a metal-catalyzed nitrene insertion reaction [25]. This seminal discovery was then followed by the pioneering work of Breslow and Gellman who introduced the use of iminoiodinanes as metal nitrene precursors as well as rhodium dimer complexes as catalysts [26,27]. They showed the formation of the corresponding benzosultam in 86% yield in the presence of rhodium (II) acetate dimer (Rh2(OAc)4) via an intramolecular metal nitrene C—H bond insertion reaction (Eq. (5.1)). 

Another proposed mechanism of the polymerization is a two-electron transfer mechanism, which involvs phenolate-bridged dinuclear copper(II) complex as starting species. The complex generated phenoxonium cations and phenolate anion through a double one-electron transfer from a phenolate to both copper centres (step v) and form the quinone-ketal intermediate via nucleophilic attack (step vi). This reaction pathway is supported by theoretical calculations of atomic charges of monomeric and dimeric species of 2,6-DMP where phenoxonium cations are proposed as key intermediates. Ab Initio calculations on 2,6-DMP and 4-(2,6-Dimethylphenoxy)-2,6-dimethylphenol provided evidence of the phenoxonium cation in the copper-catalyzed oxidative coupling reaction which proposed that the selective C-O coupling was achieved via the nucleophilic attack of a phenolate on the para-carbon of a phenoxonium cation (25). Based on the experimental evidence currently reported, both... 

Do H-D, Daugulis O (2011) A general method for copper-catalyzed arene cross-dimerization. J Am Chem Soc 34 13577-13586... 

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