METAL IONS AND COMPLEXES IN ORGANIC REACTIONS .7. COPPER-PROMOTED HYDROGEN TRANSFER FROM AROMATIC DONORS TO HALIDES

Copper-promoted hydrogen transfer from various aromatic donors to aryl bromides, e.g., in 2,4,6-collidine or dimethylacetamide at ∼170°, resulted in reductive substitution, ArBr → ArH (up to ∼80%), sometimes in competition with nucleophilic substitution by the reductants, and (in the case of o-bromonitrobenzene) with a small amount of Ullmann-type coupling of the halide. Copper(I) oxide or the metal were the most effective of the copper species examined, and both dissolved during reaction. Unsaturated substituents, particularly o-NO2, greatly enhanced reducibility. α-Bromo-ketones readily underwent similar reduction. Fission occurred in the SCN group of o-nitrophenyl thiocyanate. Effective types of aromatic donors were phenols, amines, hydrazine derivatives, nitro-compounds, carboxylic esters, quinones, and pyridine N-oxide; dehydrogenation products of some representatives of the first three of these types were identified. Reactivity was also observed in solvents. The possibility of hydrogen transfer proceeding by homolytic mechanisms is discussed.