π Complexes in benzidine rearrangement

By the use of DFT calculations, the title rearrangement, Ph–NH–NH–Ph (1) → H₂N–C₆H₄–C₆H₄–NH₂ (2), was studied for the first time. Although it is a classical reaction (found in 1862), its mechanism is almost entirely unknown. There are three complexities associated with this mechanism. The first is the various rate orders for substituted hydrazobenzenes. The second is the product distribution. The third is the result of the kinetic isotope effect which is difficult to interpret. A reaction model, 1, (H₃O⁺)₂ and (H₂O)₁₀ was used to trace the reaction path. Two hydronium ions were included, because there are two nitrogen atoms in 1. In the paths of the main reaction, 1(H⁺)₂→2H⁺ + H⁺, transient intermediates were found. Through their conversion, the second product, diphenyline (3), was reached. For 1H⁺, only the Claisen shift path was found, and the π complex proposed by Dewar was not found. The absence is in accord with the kinetic result of Hammond and Shine. But the complex was revealed in the dimethoxyhydrazobenzene. Thus, while Dewar's π complex was ruled out in 1950, it has been revived by the present calculations.