Proton shuttle mediated by ionic liquid promotes aldol condensation

Abstract

Effective strategy is intensively desired to construct efficient and durable catalytic system for mild aldol condensation between aldehydes, considering the profound significance in synthesis of high-valued α, β-unsaturated aldehydes. Although series of mild catalytic systems using secondary amine-based catalysts have been proposed, the compatibility of high catalytic selectivity and durability still troubles the researchers in this field. Here, we developed a protonated diethanolamine ionic liquid catalyzed mild aldol reaction system with reasonable regulation of ion pair strength and ionic cluster microenvironment based on the deep insights into proton shuttle effects to improve catalytic selectivity and durability for the probed methacrolein synthesis. The interplay between the cation and anion mediated proton shuttle was elucidated by detailed kinetics derived from in situ Raman spectroscopy and theoretical calculations. Further experimental and characterization facts combined with molecular dynamic simulations identify the weak interaction of cation–anion pair and metastable worm-like cluster microenvironment facilitate proton shuttle and thus improve catalytic performance and durability. Consequently, the formation rate of methacrolein was enhanced by one fold, while overall side reaction rates were reduced by 27.5% compared to the conventional sec-amine catalyst. Additionally, this strategy also exhibited significant promotion in catalytic activity, selectivity, and durability in synthesis of the other representative α, β-unsaturated aldehydes. These findings will offer reference for the construction and modulation of ionic liquid catalyzed mild aldol reaction system utilizing proton shuttle effects.