- Carbon protonation of 2,4,6-triaminopyrimidines: Synthesis, NMR studies, and theoretical calculations
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Several C(5)-substituted 2,4,6-triaminopyrimidine derivatives and their HBF4 salts were synthesized to study the carbon protonation of the pyrimidine ring. NMR investigations in DMSO-d6 prove experimentally that, in addition to the usual protonation at N(1), the compounds can be protonated at C(5) as well. We present several new stable cationic σ-complexes in the pyrimidine series, where C(5) protonation predominates over N(1) protonation. Quantum chemical calculations using the B3LYP/cc-pVDZ method were utilized in the gas phase and also in DMSO solvent with the polarized continuum model (PCM) method to rationalize the observed protonation behavior. Results of the calculations accord with the experimental observations and prove that combined steric and electronic effects are responsible for the observed C(5) protonation and for σ-complex stability. We demonstrate that C(5) protonation is a general feature of the 2,4,6-triaminopyrimidine system.
- Nemeth, Balazs,Weber, Csaba,Veszpremi, Tamas,Gati, Tamas,Demeter, Adam
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