Mechanism of Direct Electrophilic Aromatic Amination: an Electrophile is Found by Quantum-Chemical Study

Direct amination is an extremely valuable reaction, allowing for the one-step preparation of aromatic amines. However, its mechanism was poorly studied. Here, for the first time, using quantum chemical calculations, we have shown that direct amination of arenes by hydrazoic acid follows the classical S _E Ar mechanism with aminodiazonium cation H ₂ N ₃ ⁺ as electrophile. The peculiarity of H ₂ N ₃ ⁺ electronic structure has been described using our novel method for tracing the molecular orbitals. The located stationary points and transition states allowed us to define direct amination as S _E Ar reaction, which rate is determined by early transition state between π- and σ-complexes. Considering the calculated reaction constant ρ and an early transition state, we placed direct amination of arenes by HN ₃ somewhere in-between nitration and halogenations. Our results explain the accumulated experimental data and open a prospect for the development of the new aminating agents working in milder conditions.