N‐Heterocyclic Olefin–Carbon Dioxide and –Sulfur Dioxide Adducts: Structures and Interesting Reactivity Patterns |
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Authors: | Dr. Lars H. Finger Jannick Guschlbauer Dr. Klaus Harms Prof. Dr. Jörg Sundermeyer |
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Affiliation: | Fachbereich Chemie and Materials Science Centre, Philipps-Universit?t Marburg, Marburg, Germany |
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Abstract: | Depending on the amount of methanol present in solution, CO2 adducts of N‐heterocyclic carbenes (NHCs) and N‐heterocyclic olefins (NHOs) have been found to be in fully reversible equilibrium with the corresponding methyl carbonate salts [EMIm][OCO2Me] and [EMMIm][OCO2Me]. The reactivity pattern of representative 1‐ethyl‐3‐methyl‐NHO–CO2 adduct 4 has been investigated and compared with the corresponding NHC–CO2 zwitterion: The protonation of 4 with HX led to the imidazolium salts [NHO–CO2H][X], which underwent decarboxylation to [EMMIm][X] in the presence of nucleophilic catalysts. NHO–CO2 zwitterion 4 can act as an efficient carboxylating agent towards CH acids such as acetonitrile. The [EMMIm] cyanoacetate and [EMMIm]2 cyanomalonate salts formed exemplify the first C?C bond‐forming carboxylation reactions with NHO‐activated CO2. The reaction of the free NHO with dimethyl carbonate selectively led to methoxycarbonylated NHO, which is a perfect precursor for the synthesis of functionalized ILs [NHO–CO2Me][X]. The first NHO‐SO2 adduct was synthesized and structurally characterized; it showed a similar reactivity pattern, which allowed the synthesis of imidazolium methyl sulfites upon reaction with methanol. |
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Keywords: | carbenes carbon dioxide fixation carboxylation heterocyclic olefins ionic liquids |
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