How Mechanical and Chemical Features Affect the Green Synthesis of 1H-Pyrazoles in a Ball Mill

Abstract

This work investigated the chemical and mechanical factors that affect cyclocondensation reactions in a ball mill. Chemical characteristics such as the use or non-use of a catalyst, amount of catalyst and reactants, and product formation, as well as the yield and mechanical factors such as rotation frequency and the number, diameter, and material of the milling balls were evaluated. It was found that a rotation frequency of 450 rpm is efficient for energy transfer to the reactants because the conversion is higher at this rotation. The reaction was highly dependent on the time (3 min) and amount of p-TSA (p-toluenesulfonic acid) utilized as catalyst (10 mol %). Five steel balls of 10 mm were considered to be the ideal number for the efficient mixing of the particles. For this work, the ideal conditions determined were used for the green synthesis of a series of 1Hpyrazoles.

Greener Paal–Knorr Pyrrole Synthesis by Mechanical Activation

“…The best result (96 % conversion) was obtained with 10 mol‐% citric acid at 30 Hz. Independent of the catalyst amount, these results corroborate previous studies regarding the crucial role of the ball‐mill frequency on the reaction yield 32,33. Actually, increasing the frequency of the milling resulted in an increase in the kinetic energy, which resulted in a transfer of more effective energy to the reactants.…”

Base-catalysed, one-step mechanochemical conversion of chitin and shrimp shells into low molecular weight chitosan

“…Recently, mechanochemistry has been widely applied in chemical and material synthesis with the unique advantage of enabling efficient solid-state reactions and the ability to directly convert various recalcitrant feedstocks. [61][62][63][64][65][66][67][68][69][70][71][72][73][74] A series of studies transforming biomass in a solvent-free way have been demonstrated. For example, cellulose was converted to watersoluble oligomers by both mineral acids and solid acids in a ball milling process, 75,76 while lignin was decomposed similarly in the presence of a base.…”

Ball Milling for Heterocyclic Compounds Synthesis in Green Chemistry: A Review

“…Martins et al [43] investigated the mechanical factors that affect the synthesis of 1H-pyrazoles. They showed that the use of 10% of p-TSA as catalyst, five balls (10 mm) and rotation speed of 450 rpm considered the ideal conditions to achieve the pyrazole derivatives (Scheme 33) in three minutes.…”