title: Carbon protonation of 2,4,6-triaminopyrimidines: synthesis, NMR studies, and theoretical calculations.
abstract: Several C5-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 N1, the compounds can be protonated at C5 as well. We present several new stable cationic sigma-complexes in the pyrimidine series, where C5 protonation predominates over N1 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 C5 protonation and for sigma-complex stability. We demonstrate that C5 protonation is a general feature of the 2,4,6-triaminopyrimidine system.
authors.affiliation: Department of Inorganic Chemistry, Budapest University of Technology and Economics, Gellért tér 4, H-1521 Budapest, Hungary.
authors.forename: Balázs
authors.initials: B
authors.lastname: Németh
authors.forename: Csaba
authors.initials: C
authors.lastname: Wéber
authors.forename: Tamás
authors.initials: T
authors.lastname: Veszprémi
authors.forename: Tamás
authors.initials: T
authors.lastname: Gáti
authors.forename: Adám
authors.initials: A
authors.lastname: Demeter
mesh_terms: D002244:Carbon / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods* / Q000592:standards; D015394:Molecular Structure; D011522:Protons; D011743:Pyrimidines / Q000138:chemical synthesis* / Q000737:chemistry*; D011789:Quantum Theory; D012015:Reference Standards; D012680:Sensitivity and Specificity
chemical_list: D011522:Protons; D011743:Pyrimidines; C012070:2,4,6-triaminopyrimidine; D002244:Carbon; C030986:pyrimidine