Perhaps due in no small part to its historical mystique, mechanochemistry has been shrowded in relative obscurity for quite some time. The successes of mechanochemistry in organic synthesis have often been curtailed by a fundamentally limited mechanistic understanding. Lately, however, the community has made great strides towards understanding the fundamentals, as well as large steps forward regarding industrially significant scale-up via twin-screw extrusion. A variety of recent work has traded in some of mechanochemistry's mystique for simple, straight-forward chemical guidelines. This exchange eases the concerns of a would-be practitioner of mechanochemistry, interested in having another tool in their lab. Simultaneously, pressures on institutions and researchers regarding lab safety, environmental awareness, productivity, and so on, continue to make mechanochemistry particularly attractive. The demand has never been higher for a methodology addressing these pressures, and at the same time the barriers to adopting mechanochemistry have never been lower. That being said, there still remains some mystique guarding the “rules” of mechanochemistry. For those interested in solving these fundamental questions, there remain plenty of high-value targets, and several of these will feature throughout this review.

Herein, we report on the solvent-free Sonogashira reaction utilizing high speed ball milling. Sonogashira coupling of a variety of para substituted aryl halides were performed with trimethylsilylacetylene or phenylacetylene. We observed that iodo and bromo substituted aromatics successfully undergo Sonogashira coupling. However, chloro and fluoro substituted aryl compounds were unreactive. Conducting the coupling reaction in the absence of copper iodide led to low yields. Alternately, if the reaction is conducted with a copper ball in a copper vial in lieu of copper iodide, the coupling product is observed in high yields. This demonstrates the first report on the use of the vial and ball material as a catalyst in a ball milled chemical reaction.

Mechanochemistry continues to reveal new possibilities in chemistry including the opportunity for “greening” reactions. Nevertheless, a clear understanding of the energetic transformations within mechanochemical systems remains elusive. We employed a uniquely modified ball mill and strategically chosen Diels–Alder reactions to evaluate the role of several ball-milling variables. This revealed three different energetic regions that we believe are defining characteristics of most, if not all, mechanochemical reactors. Relative to the locations of a given ball mill's regions, activation energy determines whether a reaction is energetically easy (Region I), challenging (Region II), or unreasonable (Region III) in a given timeframe. It is in Region II, that great sensitivity to mechanochemical conditions such as vial material and oscillation frequency emerge. Our unique modifications granted control of reaction vessel temperature, which in turn allowed control of the locations of Regions I, II, and III for our mill. Taken together, these results suggest envisioning vibratory mills (and likely other mechanochemical methodologies) as molecular-collision facilitating devices that act upon molecules occupying a thermally-derived energy distribution. This unifies ball-milling energetics with solution-reaction energetics via a common tie to the Arrhenius equation, but gives mechanochemistry the unique opportunity to influence either half of the equation. In light of this, we discuss a strategy for translating solvent-based reaction conditions to ball milling conditions. Lastly, we posit that the extra control via frequency factor grants mechanochemistry the potential for greater selectivity than conventional solution reactions.

Herein we report the first copper vial catalysed CuAAC reaction. Under solvent-free mechanochemical conditions the reaction is complete in as little as 15 minutes and the triazole product isolated straight from the reaction vial with no further purification.

Using a mechanically driven Diels–Alder reaction we were able to characterize the chemical energetics of a SPEX 8000M mixer/mill. Our results demonstrate that the conditions produced by this type of mill are similar to those produced when performing the same reaction at 90 °C in solution. Discrete element models and in situ temperature logging were used to analyse the energetics of this system. These models indicate that the yields obtained using a SPEX 8000M mill are best correlated to the velocity of the media and number of non-zero force collisions.