Sustainability and efficiency assessment of vanillin allylation: in solution versus ball-milling

Allylation of phenols, a widely used reaction in multistep synthetic pathways, was herein investigated using mechanochemistry. This synthesis was first optimized on vanillin by varying key parameters including both chemical (e.g., stoichiometry, reaction time) and mechanical (e.g., rotational speed, material, size and number of beads, liquid additive) conditions, leading to the isolation of allylated vanillin at the gram scale in excellent yield (95%). The optimized procedure was also successfully implemented to another bio-based phenol of interest, ethyl ferulate (92% isolated yield). The environmental impact of these procedures was compared with more classical in-solution protocols by calculating E factors. When work-up solvents were not taken into account, E factor (sEF) clearly indicated the superiority of the ball-milling approach over the solution-based procedure, underscoring the capacity of ball-mills to drastically reduce the need for "reaction" solvents. On the other hand, when work-up solvents were taken into consideration, E factors (cEF) were in favor of the solvent-based approach, which could be explained by the solvent quantities required to recover the reaction mixture from the ball-mill reactor. Overall, these results highlight (i) the great potential of mechanochemistry to enable the development of both efficient and waste-less allylation of lignin-derived phenolic synthons, and (ii) the need to study higher-scale and continuous mechanochemical processes, such as by using extruders, to further improve efficiency and sustainability of such mechanochemical processes.