A New Class of Organosuperbases,N‐Alkyl‐ and N‐Aryl‐1,3‐dialkyl‐4,5‐dimethylimidazol‐2‐ylidene Amines: Synthesis,Structure, pKBH+ Measurements,and Properties |
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Authors: | Roman A Kunetskiy Dr Svetlana M Polyakova Ji?í Vav?ík Dr Ivana Císa?ová Jaan Saame Eva Roos Nerut Dr Ivar Koppel Prof Ilmar A Koppel Dr Agnes Kütt Prof Ivo Leito Dr Ilya M Lyapkalo |
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Institution: | 1. Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nam. 2, 16610 Prague 6 (Czech Republic), Fax: (+420)?220‐183‐578;2. Department of Inorganic Chemistry, Charles University, Hlavova 2030, 12840 Prague 2 (Czech Republic);3. Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu (Estonia) |
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Abstract: | A series of stable organosuperbases, N‐alkyl‐ and N‐aryl‐1,3‐dialkyl‐4,5‐dimethylimidazol‐2‐ylidene amines, were efficiently synthesized from N,N′‐dialkylthioureas and 3‐hydroxy‐2‐butanone and their basicities were measured in acetonitrile. The derivatives with tert‐alkyl groups on the imino nitrogen were found to be more basic than the tBu P1 (pyrr) phosphazene base in acetonitrile. The origin of the high basicity of these compounds is discussed. In acetonitrile and in the gas phase, the basicity of the alkylimino derivatives depends on the size of the substituent at the imino group, which influences the degree of aromatization of the imidazole ring, as measured by 13C NMR chemical shifts or by the calculated ΔNICS(1) aromaticity parameters, as well as on solvation effects. If a wider range of imino‐substituents, including electron‐acceptor substituents, is treated in the analysis then the influence of aromatization is less predominant and the gas‐phase basicity becomes more dependent on the field‐inductive effect, polarizability, and resonance effects of the substituent. |
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Keywords: | amines aromaticity basicity imidazolylidene amines steric hindrance substituent effects superbases |
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