N‐Heterocyclic Olefin–Carbon Dioxide and –Sulfur Dioxide Adducts: Structures and Interesting Reactivity Patterns

Abstract

Depending on the amount of methanol present in solution, CO 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][OCO Me] and [EMMIm][OCO Me]. The reactivity pattern of representative 1-ethyl-3-methyl-NHO-CO adduct 4 has been investigated and compared with the corresponding NHC-CO zwitterion: The protonation of 4 with HX led to the imidazolium salts [NHO-CO H][X], which underwent decarboxylation to [EMMIm][X] in the presence of nucleophilic catalysts. NHO-CO zwitterion 4 can act as an efficient carboxylating agent towards CH acids such as acetonitrile. The [EMMIm] cyanoacetate and [EMMIm] cyanomalonate salts formed exemplify the first C-C bond-forming carboxylation reactions with NHO-activated CO . 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-CO Me][X]. The first NHO-SO adduct was synthesized and structurally characterized; it showed a similar reactivity pattern, which allowed the synthesis of imidazolium methyl sulfites upon reaction with methanol.

Dissolving Cellulose in 1,2,3-Triazolium- and Imidazolium-Based Ionic Liquids with Aromatic Anions

“…These, in turn, are very reactive and lead to all kinds of unwanted by-products. This effect has indeed been utilized for some applications, such as the chemisorption of carbon dioxide [40] or even the dissolution of elemental sulfur [41] in ILs due to the reaction with intrinsically present NHCs. In the latter case, around 50 mol-% of the cations in the IL reacted with sulfur to form the carbene adducts within 24 h at 25 • C, emphasizing the importance of this reaction pathway.…”

Pseudo Halide Chemistry in Ionic Liquids with Decomposable Anions

“…For synthesizing a set of different trialkylmethylammonium pseudohalides ( 1X and 2X , Scheme ) we first prepared trialkylmethylammonium methylcarbonates [Et 3 MeN][CO 3 Me] ( 1 ) and [ n Bu 3 MeN][CO 3 Me] ( 2 ) by a slightly changed literature protocol as illustrated in Scheme . Subsequently, the syntheses of quaternary ammonium pseudohalides was achieved in the reaction of [Et 3 MeN][CO 3 Me] ( 1 ) and [ n Bu 3 MeN][CO 3 Me] ( 2 ) with the corresponding trimethylsilyl pseudohalogen compound (Scheme ), a synthetic approach previously reported by Sundermeyer and co‐workers (see Supporting Information for further details).…”

“…In addition, long drying times at elevated temperatures are necessary to remove residual water or solvents, often resulting in darkening of the ionic liquid caused by traces of decomposition products. To avoid these problems, ionic liquids containing a methylcarbonatanion have proven to be promising starting materials (Scheme ). These ionic liquids are accessible by methylation of e.g.…”

CO2 Capture and in situ Catalytic Transformation

“…It has been verified that N -heterocyclic carbenes and N -heterocyclic olefins can react with CO 2 , forming the CO 2 adduct which can be used as “all-in-one” carboxylating agent (Zhou et al, 2008; Kelemen et al, 2014; Dong et al, 2015; Talapaneni et al, 2015; Finger et al, 2016; Saptal and Bhanage, 2016). For example, Tommasi group shows the CO 2 adduct 1-butyl-3-methylimidazolium-2-carboxylate and 1,3-dimethylimidazolium-2-carboxylate behaves as active CO 2 -carriers and reacted with CH 3 OH and acetophenone for the synthesis of methylcarbonate and benzoylacetate.…”