Studies of Organic Catalytic Reactions. VI. The Function of Pyridine and Copper in the Rosenmund-von Braun Reaction

The Rosenmund-von Braun reaction with aryl halide, with its Ar-X+CuCN-Base→Ar-CN+CuX-Base, has been studied in nitrobenzene in place of an organic base at 160–180°C. The free nitriles formed during the reaction were determined exactly by gas chromatography. In the use of bases with suitable coordination forces to CuCN, the reactivity of the base for the substitution was found to be in the order: prim.>sec.>tert. A steric hindrance of the base was observed, and the reactivity of the pyridine homologue followed the order: pyridine>quinoline>α-picoline>2, 6-lutidine. Ortho-substituted groups in the halobenzene seem to manifest no steric hindrance on the reaction. From the results of the kinetic study, it can be said that CuCN participates in the reaction essentially in the form of a Base(1 mol)-CuCN complex. The overall reaction is second-order, dc⁄dt=k[Complex][Ar-X]. The rate-determining step may be in the halogen exchange stage in the intervening complex, Base-CuCN-Ar-X, where the plane including CN-Cu-X and that of the benzene ring may be nearly perpendicular to each other. When an excess of CuCN over the base is used, the base is liberated from the Base-CuX complex, after the displacement, and then coordinates to the unreacted CuCN again.