A guide to direct mechanocatalysis

Abstract

Direct mechanocatalysis (DM) describes solvent-free catalytic reactions that are initiated by mechanical forces in mechanochemical reactors such as ball mills. The distinctive feature of DM is that the milling materials, e.g. the milling balls themselves are the catalyst of the reaction. In this article we follow the historical evolution of this novel concept and give a guide to this emerging, powerful synthesis tool. Within this perspective we seek to highlight the impact of the relevant milling parameters, the nature of the catalyst and potential additives, the scope of reactions that are currently accessible by this method, and the thus far raised hypotheses on the underlying mechanisms of direct mechanochemical transformations.

Fig. 1. Timeline of mechanochemistry and “direct mechanocatalysis”. Blue: pre-modern time; red: important milestones of official recognition for mechanochemistry; yellow: major technological development; green: first reports of specific direct mechanocatalytic reactions.

Fig. 2. Influence of milling time, t_reaction, and milling ball diameter, d_MB, on the yield of the Knoevenagel condensation of vanillin and barbituric acid (equimolar ratio). Left: PBM Fritsch P7 premium line (V_MV = 0.045 L, n = 20 mmol, rpm = 800 min⁻¹). Right: PBM Fritsch P6 (V_MV = 0.25 L, n = 100 mmol, rpm = 650 min⁻¹).

Fig. 3. Reaction between PDA and PNBA at different temperatures investigated by Užarević group.

Fig. 4. Tuning of milling material for enhanced yield under direct mechanochemical Glaser coupling of phenylacetylenes.

Fig. 5. Development of yield over time for the DM Suzuki reaction between phenyliodide and phenylboronic acid with and without LAG conditions.

Fig. 6. Proposed reaction path of Glaser coupling through copper catalyzed DM (a) in situ Raman spectrum (b) scheme of proposed reaction path.