Imidazolium Hydrogen Carbonates versus Imidazolium Carboxylates as Organic Precatalysts for N-Heterocyclic Carbene Catalyzed Reactions

Abstract

Imidazolium-2-carboxylates (NHC-CO(2) adducts, 3) and (benz)imidazolium hydrogen carbonates ([NHC(H)][HCO(3)], 4) were independently employed as organic precatalysts for various molecular N-heterocyclic carbene (NHC) catalyzed reactions. NHC-CO(2) adducts were obtained by carboxylation in THF of related free NHCs (2), while the synthesis of [NHC(H)][HCO(3)] precursors was directly achieved by anion metathesis of imidazolium halides (1) using potassium hydrogen carbonate (KHCO(3)) in methanolic solution, without the need for the prior preparation of free carbenes. Thermogravimetric analysis (TGA) and TGA coupled with mass spectrometry (TGA-MS) of most [NHC(H)][HCO(3)] precursors 4 showed a degradation profile in stages, with either a concomitant or a stepwise release of H(2)O and CO(2), between 108 and 280 °C, depending on the nature of the azolium and substituents. In solution, NHC generation from both [NHC(H)][HCO(3)] salts and NHC-CO(2) adducts could be achieved at room temperature, most likely by a simple solvation effect. Both types of precursors proved efficient for organocatalyzed molecular reactions, including cyanosilylation, benzoin condensation, and transesterification reactions. The catalytic efficiencies of NHC-CO(2) adducts 3 were found to be approximately 3 times higher than those of their [NHC(H)][HCO(3)] counterparts 4.

Poly(ionic liquid)s based on imidazolium hydrogen carbonate monomer units as recyclable polymer-supported N -heterocyclic carbenes: Use in organocatalysis

“…Three different organocatalyzed reactions of molecular chemistry were implemented, namely, transesterification, benzoin condensation and cyanosilylation of aldehydes, known as being efficiently catalyzed by both molecular NHCs and ([NHC(H)][HCO 3 ]) salt precursors …”

“…In addition, progressive hydrolysis of NHC‐CO 2 adducts, forming imidazol(in)ium hydrogen carbonates, denoted as [NHC(H)][HCO 3 ], has been reported . However, we have shown that these salts can serve as a genuine source of NHCs under very mild conditions, formally by a loss of “H 2 CO 3 ” (H 2 O +CO 2 ) . Synthesis of [NHC(H)][HCO 3 ] precursors can be readily achieved in one pot, by anion exchange from commercial imidazol(in)ium halides ([NHC(H)][X]; X = Br or Cl), in the presence of KHCO 3 in methanol.…”

Efficient Domino Hydroformylation/Benzoin Condensation: Highly Selective Synthesis of α‐Hydroxy Ketones

“…Therefore, more stable NHC adducts with haloalkanes,17 alcohols,18 silver,19 CO 2 ,20 and H 2 CO 3 21 have been developed to liberate NHCs on demand under base‐free conditions. Among these masked NHCs, zwitterionic (dihydro)imidazolium‐2‐carboxylates ((H 2 )IMes‐CO 2 ) gain much attention owing to their accessibility, good stability, and easy generation of the respective free NHCs 13b. 14b, 20c…”

NHC in Imidazolium Acetate Ionic Liquids: Actual or Potential Presence?

“…Among them hydrogen carbonate and hydroxide are particularly important. As regards 1,3-dialkylimidazolium hydrogen carbonates, it is reported that these ionic liquids are in equilibrium with the corresponding NHC-CO 2 adducts (vide infra), demonstrating the possibility to afford NHCs (see for example: Fèvre et al, 2012; Zhao et al, 2017) by releasing of carbon dioxide due to high temperature effect or, at room temperature, using a solvent favoring the carbene generation (as THF or toluene).…”