Organocopper Compounds as Intermediates in Organic Syntheses

Two reviews have detailed the reactions of aryl halides with copper and copper compounds these are (1) the Ullmann biaryl synthesis, by Fanta 113) and (2) copper-promoted reactions, by Bacon and Hill (9). The mechanism of the Ullmann biaryl synthesis was discussed the possible routes are shown in Eq. (110). No evidence had been obtained 

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. 

An organocopper intermediate was detected by Lewin and Cohen in the reaction of / -iodotoluene with copper in a good complexing solvent (184). Analysis of protonated aliquots from a reaction performed in quinoline indicated an accumulation of / -tolylcopper to a maximum of 43% after 95 hours, at which point the iodide was consumed, and then a slow decrease to by dimerization. Other experiments also indicate the formation of an arylcopper compound in Ullmann reactions (127,141, 210). The isolation of deuterated products, presumably from the decomposition of an intermediate organocopper species in deuterated benzene and cyclohexane, suggested decomposition to free radicals (127). Decompositions of certain o-haloarylcopper intermediates by a benzyne mechanism cannot be totally excluded. The formation of a dichlorobenzene and by-products such as dibenzofuran and triphenylene from only the ortho isomer of the chloroiodobenzenes in Ullmann coupling reactions (210) 

The mass spectrum of copper(II) benzoate 239) at an electron energy of 50 V and 220°C indicates that a binuclear copper(I) benzoate is formed initially and then decomposes the parent ion (CeH5C02)2Cu2, the 

An inert atmosphere is necessary for decarboxylation via organocopper intermediates to suppress oxidative decarboxylation to esters and salicylates (150), a well-known industrial route to phenols. 

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