Imidazole-Based Amine-Hydrogen-Bond Donor Cooperative Heterogeneous Organocatalysts

Bicontinuous polyHIPEs (PHs) containing imidazole groups as organocatalysts were prepared for the first time. The imidazole-bearing polymer, acting as an amine catalyst, failed to promote the Henry (nitroaldol) reaction between 4-nitrobenzaldehyde (4-NB) and nitromethane (NM) in organic solvents, including neat NM. Imidazole acted as a weak Lewis base and catalyzed the reaction only in water. To overcome this limitation, hydrogen-bond donor (HBD) groups were introduced into both phases of the high internal phase emulsions (HIPEs) to enable cooperative catalysis. This strategy enabled high catalytic conversion with 100% product selectivity in the Henry reaction and effective site isolation through the HIPE phases. The hydroxyl groups acted as strong HBDs, facilitating cooperative catalysis and leading to high conversions in methanol, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). In a control reaction, a bicontinuous polyHIPE bearing imidazole and benzyl chloride groups efficiently catalyzed the acetalization reaction in methanol, achieving very high conversions within a short reaction time. We hypothesize that the methylene chloride pendant group acted as a HBD, activating the carbonyl group of 4-NB via the −C–H···O interaction, yielding only acetal. The heterogeneous catalyst cooperatively catalyzed the formation of the acetal in methanol with a high conversion rate of 95%. In contrast, a polyHIPE synthesized using only vinylbenzyl chloride (VBC) exhibited low activity, suggesting that the presence of the amine group played a crucial cooperative role in the acetalization reaction, supporting the conclusion that the amine group is actively involved in the catalytic process. Although the methylene chloride pendant group is not a conventional hydrogen bond donor and is considered a very weak one, it enabled dominant solvent–solvent interactions in methanol that allowed this weak HBD to effectively activate 4-NB, contributing to the overall catalytic performance. The study also explored how the identity and the amine/HBDs affected catalytic performance.