Sandmeyer Reaction-百灵威

A radical mechanism, catalytic for copper(I) salt has been proposed for the Sandmeyer reaction, which converts an aryldiazonium halide/salt to an aryl halide or an aryl pseudohalide. The required aryldiazonium halide is usually prepared from arylamines (via diazotization) using either NaNO₂/hydrohalic acid or alkyl nitrites under anhydrous conditions. A single electron-transfer (SET) from the copper(I) catalyst to the diazonium, creates a diazonium radical and copper(II) halide. The diazonium radical readily releases nitrogen gas, forming an aryl radical, which then receives a halide from copper(II) halide, regenerating the copper(I)catalyst and forming the final aryl halide.

Lab Tips:

Both electron-donating and electron-withdrawing groups are tolerated on the aryldiazonium halide.
If the NaNO₂/hydrohalic acid route is chosen to synthesize the aryldiazonium halide, the counterion of the copper(I) salt must match the conjugate base of the hydrohalic acid, otherwise a mixture of aryl halide product is to be expected. ^[1]
The resulting aryldiazonium halide is then reacted in the same pot (not isolated first) with the copper(I) halide. ^[1]
Copper(I) bromide, chloride, or cyanide are used to obtain aryl chloride, aryl bromide, and aryl nitrile respectively. The formation of aryl iodides does not require the use of a copper (I) salt. Instead, potassium iodide is used. ^[1]
Note that phenols can also be made from aryldiazonium salts by heating with trifluoroacetic acid, aqueous sulfuric acid, or with an aqueous solution of copper salts (occasionally referred to as the Sandmeyer hydroxylation). ^[2,3,4]

_{1. Kürti, L., Czakó, B. (2005). Strategic Applications of Named Reactions in Organic Synthesis; Background and Detailed Mechanisms. Burlington, MA: Elsevier Academic Press.
2. Horning, D. E., Ross, D. A., Muchowski, J. M. Synthesis of phenols from diazonium tetrafluoroborates. Useful modification. Can. J. Chem. 1973, 51, 2347-2348.
3. Cohen, T., Dietz, A. G., Jr., Miser, J. R. A simple preparation of phenols from diazonium ions via the generation and oxidation of aryl radicals by copper salts. J. Org. Chem. 1977, 42, 2053-2058.
4. Hanson, P., Rowell, S. C., Walton, P. H., Timms, A. W. Promotion of Sandmeyer hydroxylation (homolytic hydroxydediazoniation) and hydrodediazoniation by chelation of the copper catalyst: bidentate ligands. Org. Biomol. Chem. 2004, 2, 1838-1855.}