Copper(I) Bromide

Copper(I) bromide (cuprous bromide, CuBr) is a diamagnetic copper(I) halide with a polymeric structure similar to that of zinc sulfide. In organic synthesis it is used as a reagent and as a catalyst, either on its own or as a complex with nitrogen or sulfur ligands.

Sandmeyer bromination. In the Sandmeyer reaction, CuBr converts arenediazonium salts into aryl bromides. The diazonium salt is formed from an aromatic amine with sodium nitrite in hydrobromic acid and is then treated with CuBr, which replaces the diazonium group with bromine. The method gives substituted aryl bromides from the corresponding anilines. A catalytic version uses copper(I) and copper(II) with 1,10-phenanthroline as the ligand.

Organocopper reagents. The dimethyl sulfide complex of copper(I) bromide, CuBr·SMe₂, is a widely used copper source for organocopper reagents. For example, with an organolithium reagent it gives a lithium diorganocuprate.

Atom transfer radical polymerization. With nitrogen ligands, CuBr is used as the activator in copper-catalysed atom transfer radical polymerization (ATRP). The copper(I) complex takes the halogen atom from an alkyl halide initiator, which forms a propagating radical and the copper(II) halide complex. The copper(II) complex returns the halogen to the growing chain. The exchange is reversible. In a controlled ATRP the molecular weight of the polymer increases linearly with conversion, and the method gives block and graft copolymers and polymers with functional end groups.

Cross-dehydrogenative coupling. With tert-butyl hydroperoxide as the oxidant, CuBr catalyses cross-dehydrogenative couplings, in which a C–H bond next to nitrogen couples directly with a C–H bond of the partner. N-Aryl tetrahydroisoquinolines couple in this way with nucleophiles such as terminal alkynes, nitroalkanes, malonates and indoles.