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On the Question of Stability, Conjugation, and "Aromaticity" in Imidazol-2-ylidenes and Their Silicon Analogs

Summary: On the Question of Stability, Conjugation, and "Aromaticity" in
Imidazol-2-ylidenes and Their Silicon Analogs
Christoph Heinemann, Thomas Mu1ller,§ Yitzhak Apeloig,*,§ and
Helmut Schwarz*,
Contribution from the Institut fušr Organische Chemie der Technischen UniVersitašt Berlin,
Strasse des 17. Juni 135, D-10623 Berlin, Germany, and Department of Chemistry,
Technion-Israel Institute of Technology, Haifa 32000, Israel
ReceiVed July 14, 1995X
Abstract: Thermodynamic, structural, and magnetic criteria, the properties of the charge distributions, and low-
energy ionization processes are theoretically analyzed to learn about the role of -electron delocalization in recently
synthesized stable singlet carbenes (Arduengo et al. J. Am. Chem. Soc. 1991, 113, 361) and silylenes (Denk et al. J.
Am. Chem. Soc. 1994, 116, 2691) of the imidazol-2-ylidene type and also in related model systems. The different
approaches show consistently that cyclic electron delocalization does indeed occur in the CdC unsaturated imidazol-
2-ylidene systems, in particular with respect to the corresponding C-C saturated imidazolin-2-ylidenes. However,
the conclusion regarding the degree of conjugation and aromaticity depends on the criteria used, being quite small
according to the "atoms-in-molecules" charge analysis but more significant according to the energetic and the magnetic
properties. According to all criteria, the aromatic character of imidazol-2-ylidenes is less pronounced compared to
benzene or the imidazolium cation. -Electron resonance is found to be less extensive in the silylenes compared to
their carbene analogs.


Source: Apeloig, Yitzhak - Department of Chemistry, Technion, Israel Institute of Technology


Collections: Chemistry