Thermally Regulated Recyclable Carbene Catalysts for Upgrading of Biomass Furaldehydes

Abstract

The upgrading of biomass furaldehydesfurfural (FF) and 5-alkyl (R) substituted furfurals (R = Me, MF; CH2OH, HMF; CH2OMe, MMF)into C10–14 furoins was achieved by the selective umpolung self-coupling reaction catalyzed by polystyrene (PS) supported azolium salts paired with an acetate counterion, which allows a thermal equilibrium between the dormant ion pair and the active N-heterocyclic carbene (NHC) states. By regulating the equilibrium with heating to generate sufficient amounts of the active form of the catalyst for catalysis and cooling to deactivate the catalyst for recycling or recovery, such heterogeneous NHC organocatalysis can be performed without stoichiometric quantities of base for NHC catalyst generation and acid for catalyst quenching or recycling. Among several azolium structures investigated, a supported benzimidazolium coupled with a dodecyl chain achieved the highest furoin yields for the self-coupling reactions of FF (>95%) and HMF (>88%). This supported catalyst bearing the long alkyl chain also showed superior recyclability when exposed to air upon filtration, experiencing only a net decrease of ≤5% in yield over five cycles of FF self-coupling. In comparison, HMF self-coupling is less effective in both product yield and catalyst recyclability due to unproductive acid–base interactions between the acetate counterion and the HMF hydroxy proton. This detrimental effect can be overcome by use of substrates MF or MMF, an industrially relevant intermediate in the production of furan-based polyesters.

Imidazolium Based Fluorous N‐Heterocyclic Carbenes as Effective and Recyclable Organocatalysts for Redox Esterification

“…Up to now, the effort focused on the application of separable (pre)catalysts was limited to benzoin condensation or Stetter reaction. Chen et al published benzoin self‐coupling of furfural catalyzed by polystyrene or inorganic silica supported azolium salts paired with an acetate counterion. Poly(ionic liquid)s based on imidazolium hydrogen carbonate were studied as precatalysts for benzoin condensation of benzaldehyde .…”

Organocatalytic Upgrading of Biomass Derived Building Blocks

“…Based on their previous studies, [23] Chen and coworkers described a polystyrene (PS) supported benzimidazolium salt bearing a dodecyl alkyl chain as effective and recyclable catalytic system to promote the umpolung self-coupling reaction of furaldehydes for the synthesis of chain-extended C 10-14 furoins 5 (Scheme 5). [24] Furoins are useful precursors of high-quality jet-fuels and find direct application as building blocks for the synthesis of polyesters and polyurethane materials.…”

“…Based on their previous studies, [23] Chen and coworkers described a polystyrene (PS) supported benzimidazolium salt bearing a dodecyl alkyl chain as effective and recyclable catalytic system to promote the umpolung self‐coupling reaction of furaldehydes for the synthesis of chain‐extended C 10–14 furoins 5 (Scheme 5). [24] …”

C−H Activation Based Functionalization of Furfural Derivatives

“…Several authors also studied the self-condensation of HMF to generate the 5,5'-dihydroxymethylfuroin 66, which is a potential fuel intermediate. [132][133][134][135][136][137][138][139] Several catalysts were tested, including ionic liquid carbenes, and lyases (Scheme 45).…”