Influence of Positive Charge on the NMR Parameters of Mono- and Diprotonated Forms of 4-Dimethylaminopyridine
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Influence of positive charge on the NMR parameters of mono- and diprotonated forms of 4-dimethylaminopyridine Alexander M. Genaev1, George E. Salnikov1, Konstantin Yu. Koltunov2,3* 1
N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Branch of the Russian Academy of Sciences, 9 Akademika Lavrentieva Ave., Novosibirsk 630090, Russia; e-mail: [email protected], [email protected] 2 Novosibirsk State University, 2 Pirogova St., Novosibirsk 630090, Russia; e-mail: [email protected] 3 Boreskov Institute of Catalysis, 5 Akademika Lavrentieva Ave., Novosibirsk 630090, Russia Translated from Khimiya Geterotsiklicheskikh Soedinenii, 2020, 56(7), 892–897
Submitted April 24, 2020 Accepted May 11, 2020
By the example of 4-dimethylaminopyridine and some of its derivatives taken as model nitrogen heterocycles with several basic centers, the correlation of the parameters of the NMR spectra with the direction (regioselectivity) and multiplicity of protonation of these compounds in CF3COOH–CD2Cl2, TfOH–CD2Cl2, and FSO3H–SbF5 acid systems was studied. The established dependence of the spectral characteristics on the value of the introduced positive charge will be useful in studying the reactivity of similar heterocyclic systems in the presence of acidic agents. Keywords: DMAP, super acids, NMR, N-protonation, quantum-chemical calculations, simulation of NMR spectra.
features of pyridine are mainly due to the presence of a lone electron pair on the nitrogen atom. Its blocking by protonation leads to the fact that the NMR parameters become similar to those of ''benzene''.6,7 Upon N-protonation, the spectra are also influenced by an increase in the electronegativity of the nitrogen atom and the appearance of an excess positive charge on the pyridine fragment.8 As far as we are aware, the latter factor has not been systematically studied. For a DMAP molecule (1) containing two nitrogen atoms, both N-mono- and N,N'-diprotonation is possible which thus allows for a significant variation in the charge on the pyridine ring.9 Due to the significant difference in pKa values of 9.70 and −9.28, corresponding to the generation of monocationic and dicationic forms 1а and 2а,9 it should be expected that exhaustive selective monoprotonation of DMAP (1) can occur in acids of medium strength, while diprotonation takes place in an excess of superacid (Scheme 1). A finer control of the charge on the pyridine ring can be achieved by varying the substituents on the charged N-1 atom in pyridinium cations, such as 1b,c discussed in this work. According to X-ray structural analysis data, the NMe2 group in molecule 1 has a planar structure,10 which does
The high basicity of the nitrogen atom determines the reactivity of N-heterocyclic compounds of the pyridine series in the medium of protic acids and superacids.1,2 NMR, along with theoretical calculations, occupies a leading place in the study of the structure and reactivity of the corresponding protonated forms.3 The present study investigated on the example of 4-dimethylaminop
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