High reactivity of triazolinediones as superelectrophiles in polar reactions: A DFT study

The mechanism of the polar reaction between the superelectrophile PTAD (2) and diene (6b) in the presence of THF has theoretically been studied at the MPWB1K/6-311G** level. Our investigations reveal that this reaction takes place through a stepwise mechanism via formation of intermediate (7b) with a strong zwitterionic character. The first step of the reaction, which is associated with the nucleophilic attack of diene (6b) on PTAD (2), presents complete endo selectivity. Formation of the expected formal [4+2] cycloadduct (8b) quickly takes place through a ring closure process which is much favoured both energetically and geometrically. However, formation of (8b) is a reversible process. Interestingly, intermediate (7b) undergoes a 1,7-hydrogen shift process yielding the major ene adduct (9b), which is thermodynamically very stable. Analysis of relative Gibb free energies suggests that while the expected formal [4+2] cycloadduct (8b) is obtained under kinetic control, the experimentally observed ene adduct (9b) is formed under thermodynamic control. Finally, analysis of the DFT reactivity indices of the reagents accounts for the high reactivity of PTAD (2) acting as a superelectrophile, and the asynchronicity found in the C-N single bond formation as a consequence of the asymmetric nucleophilic activation of diene system of (6b).