= ball milling In an initial experiment, combining all three steps by milling 4-tolyl bromide (1a) with magnesium turnings under a CO2 atmosphere at 600 rpm for 90 min did not yield any 4-toluic acid (2a; Table , entry 2). Activating the magnesium turnings in a separate milling step prior to the addition of 1a generated acid 2a, albeit in only trace amounts (entry 3). Probably, this extra step enlarged surface areas and removed passivated metal surfaces, as previously suggested for flow-chemistry setups of magnesium insertions into aryl halide bonds and for the industrial scale production of Grignard reagents. Although many mechanochemical reactions are solvent-free, trace amounts of solvents often significantly affect the reaction rates. Such liquid-assisted grinding (LAG) conditions can be quantified by the parameter h [h = V (solvent in µL)/m (reagents in mg)]. Following this concept, 2 equiv. of THF were added to the reaction mixture (corresponding to h = 0.64 µL/mg). As a result, the yield of 2a increased to 25% (Table , entry 4). Probably, this positive effect of the additive THF was due to a stabilization of the organometallic intermediate by the Lewis basic ether as observed by Grignard himself, further investigated by Schlenk and Schlenk, and being well-established nowadays. Among various ethereal additives, only 2-methyltetrahydrofuran (2-MeTHF) performed as well as THF (Table and). In comparison to THF, 2-MeTHF offered several advantages including its production from furfural or levulinic acid derived from renewable biomass, its larger range of possible reaction temperatures at which it remains liquid, and its superior performance in various organometallic reactions. In the context of the study reported here, the finding by Kadam et al. was of particular interest, as they described 2-MeTHF as a superior alternative to diethyl ether and THF in Grignard reactions, highlighting its ability to suppress Wurtz couplings of benzyl halides. Furthermore, a recent study uncovered that its thermodynamic properties make 2-MeTHF a safer solvent for the formation of Grignard reagents than THF as it prevents thermal runaway reactions. Given the good results in the screening, 2-MeTHF was the preferred additive in the later discussed evaluation of the substrate scope.