General featuresCOPPER and its compounds are outstanding within the transition series for the variety and value of their applications as reagents or catalysts for organic reactions. The development of these processes has been largely empirical and their mechanisms are not well understood. This Review is confined to aspects of copper chemistry encountered in aromatic reactions, and is particularly concerned with recent observations in this area. Less attention is paid to long-established features which have been described in other reviews; these include accounts of the Ullmann method of biaryl synthesis,l the Sandmeyer r e a~t i o n ,~,~ and some other applications in nucleophilic aromatic substitution proce~ses.~ To systematise the diverse effects of copper described in the recent literature, they are grouped in accordance with the change undergone by the aromatic compound, i.e., as reduction, oxidation, substitution, or ring-enlargement processes. The following are examples : Reductions :2ArHal + 2Cu -+ Ar-Ar + 2Cu+ + 2Hal-2ArN,+ + 2Cu+ -+ AraAr + 2Cu2+ + 2N, ArHal + XH, --+ ArH + HHal + X CU' Oxidations:2Ar-MgX+ + 2Cu2+ --f Ar.Ar + 2Cu+ + 2MgX+ ArNHeNH, + 2Cu2+ -+ ArH + 2Cu+ + 2H+ + N, CU"ArH + 0, -+ ArOH etc. 2ArH + X u 3 + --f Ar-Ar + 2Cu2+ + 2H+ ArH + Br-+ 2Cu2+ -+ ArBr + 2Cu+ + H+ Replacements:

Bacon and Stewart 4953 912. Metal Ions and Complexes in Organic Reactions. Part I V.l Copper-promoted Preparations of Diaryl Ethers and Competing Hydrogen-transfer Processes By R. G. R. BACON and 0. J. STEWART Diaryl ethers have been prepared, e.g. in collidine or dimethylacetamide solution at -170", by the reaction: 2ArHal-t 2Ar'OH + Cu,Oin which a suspension of copper(1) oxide functions as reagent and catalyst; copper or copper(I1) oxide may also be used. The process has preparative value and may be superior to the conventional Ullmann condensation. With an alkali-metal phenoxide the copper oxide is not consumed and reaction may be faster than with the phenol, particularly in dimethylacetamide. Pronounced solvent effects were observed. Halide reactivity was in the order ArI > ArBr > ArCl. Substituent effects were studied.Copper-catalysed substitutive reduction, &Hal --+ ArH, was a common competing reaction] and was sometimes a major effect with an efficient hydrogen acceptor, e.g., o-bromonitrobenzene, or with an efficient hydrogen donor, e.g., o-nitrophenol. Sources of transferred hydrogen may be a substituent group, e.g., OH or NH,, or an aromatic or heterocyclic nucleus. Other side-reactions included decarboxylation, halogen exchange] coupling of the halide to a biaryl, and substitution of the halide by a nucleophile other than the phenol.ULLMANN and his co-workers discovered2 that small amounts of copper catalyse the formation of diary1 ethers in the reaction:

Potassium phthalimide (0.01 mol) + p-bromobenzonitrile (A) (0-01 mol) or p-bromonitrobenzene (B) (0.01 mol) + catalyst (0.01 mol) in solvent (100 nil), 24 h under nitrogen ArN(C0) ,C,H, Catalyst and solvent Yield (yo) (A) CuI, dimethylacetamide a 94 (A) Cu,O, dimethylacetamide 57 (A) C U ~+ , ~ dimethylacetamide a 55 (A) CuBr,, dimethylacetamide 12 (A) No catalyst, dimethylacetarnide a 8 (A) CuBr, dimethylacetamide a 82 (A) Cu, dimethylacetamide a 15 Temp. Catalyst and solvent ("C) CuI, dimethylacetamide 165 CuI, 2,4,6-collidine b 165 CuI, quinoline b 165 CuI, nitrobenzene 6 165 CuI, diethylene glycol dimethyl 162 Cur, dimethylformamide a 155 CuI, p-xylene 138 CuI, 2-picoline 0 128* For recent discussions of copper-assisted nucleophilic substitutions involving perfluoro-aryl and -alkyl halides see

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