The influence of crucial reaction parameters on Knoevenagel condensation in planetary ball mills was investigated. Rotation frequency (ν rot ), milling ball diameter (d MB ), milling ball filling degree (Φ MB ), and beaker size had obvious influences on yield. It was found that higher ν rot , lower d MB , milling beakers with larger diameter, and a Φ MB of ∼0.3 are advantageous for the reaction. Furthermore, the influence of the type of mill was investigated, including reactions performed in different planetary and mixer ball mills, in a stirred media mill, and with a mortar mill. Comparisons with the other solvent-free synthetic routes showed that ball milling is an effective way of performing the reaction with low energy intensity.
■ INTRODUCTIONThe use of mechanical stress for the accomplishment of mechanochemical reactions is now an established field of research represented by a huge number of reactions in different fields of organic, inorganic, and organometallic chemistry and materials science. 1 In all cases, mechanical stress is provided to the reactants, whereby the energy input can be realized in different ways. The easiest but imprecise way is to use a mortar and pestle. 2 Results that are more reliable can be obtained with different types of ball mills, such as planetary (PBMs) or mixer ball mills (MBMs). The results of organic reactions in ball mills are subject to several influencing parameters, especially the influence of rotation frequency (ν rot ), milling time (t), and number of milling balls (n MB ). 3 Although a large number of synthesis protocols have been published, parameters such as the milling ball diameter (d MB ) 3c,d,4 and beaker size 5 have received less attention. Furthermore, comparative studies on reactions in different types of mills are rare. 2,6 However, the kind of mill utilized can strongly influence the outcome of a reaction. For example, Schneider et al. investigated the Suzuki−Miyaura reaction in two types of PBMs and one MBM. 2 The reaction proceeded well in the PBMs at 300 min −1 , whereas in the MBM at the same frequency only low yields were observed. Wang and co-workers investigated pinacol couplings in an MBM and a mortar mill (MM). 6a Although mechanical energy is supplied in both mill types, higher yields of the pinacol coupling product were obtained in an MM.We have recently investigated the influence of several reaction parameters on the yield of a Knoevenagel condensation of vanillin (1a) and barbituric acid (2a) (Scheme 1) in a planetary ball mill aiming to close this knowledge gap, 5 including questions regarding the influence of beaker size, geometry, and the way the mechanical energy is provided. To this end, Knoevenagel condensation was performed using various tools for allocating mechanical stress.To investigate the influence of different milling and grinding apparatuses, three different types of PBMs, an MBM, a stirred media mill (SMM), an MM, and a mortar and pestle were used. The main differences between the PBMs are the diameter of the main disc (d MD ),...
