Kolbe Nitrile Synthesis
Synthesis of Nitriles
Rosenmund-von Braun Reaction
Aryl nitriles can be prepared by the cyanation of aryl halides with an excess of copper(I) cyanide in a polar high-boiling solvent such as DMF, nitrobenzene, or pyridine at reflux temperature.
Mechanism of the Rosenmund-von Braun Reaction
The mechanism probably involves the formation of a Cu(III) species through oxidative addition of the aryl halide. Subsequent reductive elimination then leads to the product:
The excess of copper cyanide and the use of a polar, high-boiling point solvent makes the purification of the products difficult. In addition, elevated temperatures (up to 200°C) lower the functional group tolerance. The use of alkali metal cyanides or cyanation reagents such as cyanohydrins, a catalytic amount of copper(I) iodide and kalium iodide, allows a mild, catalytic cyanation of various aryl bromides.
If aryl iodides, sodium cyanide and copper(I) iodide are used, a simple mechanism similar to that of an Ullmann-type reaction can be proposed:
Reactions with aryl bromides and added alkali metal iodides involve additional equilibria in which aryl bromides give the more reactive aryl iodides:
For example, the formation of a copper(III) species and the use of cyanohydrins is discussed by H.-J. Christeau (Chem. Eur. J., 2005, 11, 2483. DOI).
Copper-Catalyzed Domino Halide Exchange-Cyanation of Aryl Bromides
J. Zanon, A. Klapars, S. L. Buchwald, J. Am. Chem. Soc., 2003, 125, 2890-2891.