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Anionic N-heterocyclic carbenes by decarboxylation of sydnone-4-carboxylates |
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| Authors: | Ana-Luiza Lücke Sascha Wiechmann Tyll Freese Martin Nieger Tamás Földes Imre Pápai Mimoza Gjikaj Arnold Adam Andreas Schmidt |
| Institution: | 1. Clausthal University of Technology, Institute of Organic Chemistry, Leibnizstraße 6, D-38678 Clausthal-Zellerfeld, Germany;2. University of Helsinki, Department of Chemistry, Laboratory of Inorganic Chemistry, P.O. Box 55, FIN-00014 University of Helsinki, Finland;3. Research Centre for Natural Sciences, Hungarian Academy of Sciences, Magyar Tudósok Körútja 2, H-1117 Budapest, XI, Hungary;4. Clausthal University of Technology, Institute of Inorganic and Analytical Chemistry, Paul-Ernst-Straße 4, D-38678 Clausthal-Zellerfeld, Germany |
| Abstract: | Unstable N-heterocyclic carbenes can be masked and stabilized as pseudo-cross-conjugated hetarenium-carboxylates which decarboxylate on warming. This study deals with the decarboxylation of carboxylates of mesoionic compounds to generate anionic N-heterocyclic carbenes. Lithium sydnone-4-carboxylates were therefore prepared via 4-bromosydnones by halogen-lithium exchange with nBuLi and subsequent treatment with carbon dioxide. Protonation gave the corresponding sydnone-4-carboxylic acids. Thermogravimetric measurements in addition to temperature dependent IR spectroscopy proved the decarboxylation of lithium sydnone-4-carboxylates and formation of the corresponding sydnone anions which can be represented as anionic N-heterocyclic carbenes. In DMSO-d6 solution, water favors the decarboxylation. Calculations have been performed to elucidate the mechanism of the decarboxylation in the absence and presence of water. |
| Keywords: | Mesoionic compound N-heterocyclic carbene Sydnone Decarboxylation IR spectroscopy |
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