Investigations of Thermally-Controlled Mechanochemical Milling Reactions

Abstract

Mechanochemical milling reactions gained a lot of attention lately as a green and highly efficient path towards various relevant materials. The control over the fundamental reaction parameters in milling procedure, such as temperature and pressure of the reactor, is still in infancy and the vast majority of milling reactions is done with controlling just the basic parameters such as frequency and milling media weight. We demonstrate here how the milling under controlled, prolonged and variable heating programs accomplished in a new milling reactor introduces a new level of mechanochemical reactivity beyond what can be achieved by conventional mechanochemical or solution procedures, and also reduces the time and energy costs of the milling process. The methodology is demonstrated on four varied systems: C–C bond forming Knoevenagel condensation, selective C–N bond formation for amide/urea synthesis, selective double-imine condensation, and solid-state formation of an archetypal open metal-organic framework, MOF-74. The potential of this methodology is best demonstrated on the one-pot selective synthesis of four complex products containing combinations of amide, amine or urea functionalities from the same and simple acyl azide and diamine reactants. Principal control over this enhanced reactivity and selectivity stemmed from the application of specific heating regimes to mechanochemical processing accomplished by a new, in-house developed mechanochemical reactor. As even the moderate increase in temperature strongly affects the selectivity and the rate of mechanochemical reactions, the results presented are in line with recent challenges of the accepted theories of mechanochemical reactivity.

Mechanochemical transformation of planar polyarenes to curved fused-ring systems

“…In this regard, in the past few years, various synthetic techniques such as direct mechanocatalysis [21][22][23][24][25] and liquid assisted grinding [26][27][28][29][30][31] are developed. New possibilities for heating [32][33][34] , cooling 35,36 and conducting gaseous 37,38 mechanochemical reactions are established. Furthermore, analytical techniques for in situ monitoring of the reactions are described [39][40][41][42][43][44][45] .…”

Mechanochemical Prebiotic Peptide Bond Formation**

“…Additionally, to study the effect of the temperature on the peptide bond formation, we applied a recently described temperature-controlled ball milling protocol. 45 For example, in a standard experiment, Gly (60 mg, 0.80 mmol) was milled with a five-fold molar excess of TiO2 (anatase), at 30 Hz for 18 h and temperatures ranging from room temperature to 130 °C, thus respecting practical thermal limits for prebiotic peptide bond formation, 18,19 while acknowledging the expected dilution of amino acids on mineral surfaces under prebiotic conditions (Figure 1d).…”

A guide to direct mechanocatalysis

“…3), who demonstrated that the major chemical product of the reaction between para-phenylenediamine (PDA) and para-nitrobenzoyl azide (PNBA) could be altered through control of the reaction temperature using a water-heated milling reactor. 50 If the reaction was run at elevated temperatures such as 80 1C, the resulting product featured urea-linkages while at lower temperatures, the generation of connecting amide bonds predominated. These results emphasized the potential of controlling the vessel temperature with the goal of directing the mechanochemical reaction towards the desired product.…”

“…3 Reaction between PDA and PNBA at different temperatures investigated by Uz ˇarevic ´group. 50 of the reaction vessel, the use of coolants such as liquid nitrogen during mechanochemical reactions is referred to as ''cryo milling''. 39 However, these conditions are not well suited for organic synthesis in the ball mill and few reports for the use of cryo milling in organic synthesis are available.…”