Solvent-less mechanochemical asymmetric reactions in a ball mill utilizing polymer-supported Hayashi-Jørgensen catalyst: effect of polymer backbone and flexibility on its catalytic performance

Abstract

Fine chemical synthesis under solvent-free or solvent-less mechanochemical conditions are highly desirable from a green chemistry perspective. However, the inherent low contact efficiency between the catalyst and solid substrates often results in low reaction efficiency. Polymer-assisted grinding (POLAG) in a ball mill has been developed for the solvent-less organic synthesis, where polymers are used as additives to effectively disperse solid reactants. Specifically, polymer-supported catalysts have been shown to function as the POLAG additives to enhance catalytic performance. However, effects of the structures of the polymer-supported catalysts on their catalytic performance has not been fully investigated. Here, we prepared polymer-supported catalysts bearing Hayashi-Jørgensen catalyst with different polymer backbones and chemical structures of spacer monomers. In asymmetric Michael addition reaction in the presence of solid reactants with a ball mill, the polymer-supported catalyst exhibited a significantly higher turnover number compared to its small molecule counterpart. Additionally, a correlation between glass transition temperature of the polymer-supported catalyst and turnover frequency was confirmed, which suggested that the flexible polymer support facilitated the solid dispersion and boosted subsequent catalytic cycles.