Anionic N-heterocyclic carbenes by decarboxylation of sydnone-4-carboxylates

作者：Luecke Ana Luiza; Wiechmann Sascha; Freese Tyll; Nieger Martin; Foeldes Tamas; Papai Imre; Gjikaj Mimoza; Adam Arnold; Schmidt Andreas^*

来源：Tetrahedron, 2018, 74(16): 2092-2099.

DOI：10.1016/j.tet.2018.03.019

• Mesoionic compound
• N-heterocyclic carbene
• Sydnone
• Decarboxylation
• IR spectroscopy

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.

• 出版日期2018-4-19