Mechanistic insight into the self-coupling of 5-hydroxymethyl furfural to C(12) fuel intermediate catalyzed by ionic liquids

DFT calculations have been carried out to obtain insight into the self-coupling of biomass-based 5-hydroxymethyl furfural (HMF) to C(12) fuel intermediate 5,5′-dihydroxymethyl furoin (DHMF) catalyzed by ionic liquids (ILs). It was found that acetate-based IL or thiazolium IL in combination with the additive Et(3)N show high catalytic performance, wherein N-heterocyclic carbons (NHCs) derived from the cations of ILs act as the nucleophiles and the protonated acetate anion or the [Et(3)NH](+) acts as the proton shuttle. The effectiveness of this catalysis is attributed to the proton-shared three-center-four-electron (3c-4e) bonds between HMF and HOAc or [Et(3)NH](+), which stabilize the transition states and the intermediates. In addition, the results of the calculations also confirm that the nucleophilicity and basicity of NHCs are key factors for the self-coupling reaction. These results rationalize the experimental findings and offer valuable insights into understanding the catalysis of ILs.