The Acid-Catalysed Benzidine Rearrangements May Proceed via Cation Radicals Formed by Electron Transfer to a Proton from a Hydrazyl Nitrogen

Abstract

The acid-catalysed benzidine rearrangements are proposed to proceed via cation radical intermediates which arise by rapid electron transfer from a neutral nitrogen of the hydrazyl moiety to a proton. Consequently, both the concerted rearrangements, Table 1, benzidines, 4-methoxy- p-semidine formation, Eqns (2), (3) and o,o‘-biaryl-linked products, and non-concerted rearrangements, Table 2, diphenyline, o-semidine, Eqns (5), (6) and perhaps 4-chloro- p-semidine, may proceed via cation radical structures involving radical C – C bond formations and homolytic N – N bond cleavages. This is contrary to the current view of the Polar Transition State theory which postulates heterolytic bond formations and cleavages, Schemes 1, 3 and 4. Mono-substituted and 4,4'-disubstituted hydrazobenzenes are proposed to undergo oxidation of the most basic nitrogen as inferred from the estimated p Ka s as determined by the SPARC program. This can explain why of the two possible o-semidine rearrangement products, 2,N’ and N,2'-linked, the observed major o-semidine has the substituent para to the amino group, e.g. Eqn (5). However, if one of the para substituents is a halogen, it is the nitrogen para to the halogen which undergoes SET and oxidation. In these second-order acid reactions, this can be explained by protonation of the most basic nitrogen followed by SET from the hydrazyl nitrogen para to the halogen. In the case of 4,4'-disubstitution, the hydrazobenzene may not lose its substituents readily and undergoes a rapid one-electron reduction and consequent disproportionation reaction as shown in Eqn (8). Thus the kinetics, first-order in hydrazoarene and the same kinetic order in acid for both disproportionation and rearrangement, are explained.