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In situ monitoring and mechanism of the mechanochemical Knoevenagel reaction

Traditional organic synthesis methods have been developed to be very successful, but as natural resources are becoming more and more limited there is constant pursuit for developing more efficient and more eco-friendly synthetic procedures. Mechanochemical milling reactions emerged as a new “green” alternative to traditional solution synthesis, [1] and have been already reported for various organic reactions with clear benefits in the reaction rate and yields.[2] The Knoevenagel condensation is an important C–C bond forming reaction for synthesizing α, β-unsaturated structures from different aldehydes/ketones and carbon acid compounds. The general procedure for Knoevenagel reaction in solution include the presence of a weak base and higher temperatures of reaction mixture.[3] While the milling reactions provided higher yields and shorter reaction periods, [4] their mechanisms remain largely unknown, which limits their use and industrial applicability. Here we present simultaneous in situ synchrotron powder X-ray and Raman monitoring of mechanochemical synthesis of 5-aryliden barbiturates 5-8 by a condensation reaction between aromatic aldehyde 1-4 and barbituric acid carried out in a ball mill. In situ monitoring by these two complementary techniques enabled a direct insight both into the reactivity at molecular level and the evolution of crystalline phases during the milling reactions. Also, it revealed a cause of the inhibiting action of water in the reaction. These results highlight again the importance of in situ techniques for elucidating the mechanisms of organic milling reactions and provide a new entry for their understanding.

In situ, mechanochemical, organic reaction, Knoevenagel, Raman, X-ray

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