Recyclable Supported Carbene Catalysts for High-Yielding Self-Condensation of Furaldehydes into C10 and C12 Furoins

Two highly efficient and recyclable heterogeneous azolium catalyst systems, one grafted (g) onto the inorganic oxide (Silica) and the other onto the organic polymer [Merrifield's peptide or chloromethylated polystyrene (PS) resin], have been developed and employed to catalyze quantitative self-coupling (umpolung condensation) reactions of furfural and 5-hydroxymethylfurfural (HMF) into C-10 and C-12 furoins, respectively. Supported benzimidazolium ([BI]) salts bearing a long-chain alkyl substituent (i.e., C-12 dodecyl) on the azolium nitrogen atom, upon activation with a suitable base to generate the corresponding N-heterocyclic carbene (NHC) catalyst, are found to be far more effective catalysts for furaldehyde self-coupling reactions than the analogous catalysts carrying a short-chain alkyl substituent (i.e., C-1 methyl). Thus, supported NHC catalysts generated in situ from Silica-g[BI]-C-12 or PS-g[BI]-C-12-benzyl/base afford the C-10 and C-12 furoins in about 97% and 94% yield, respectively. By adopting a catalyst recycling procedure that involves activation of the precatalyst with a base to generate the NHC catalyst, catalysis in conversion of furaldehydes into furoins, and recycle of the catalyst by quenching the reaction with HCl to convert the catalyst back to the precatalyst, excellent recyclability has been achieved without loss of the catalytic activity after 10 cycles by maintaining essentially a constant furoin yield of 96-97% for all 10 cycles performed with both supported catalyst systems.