Cross Dehydrogenative Coupling Reactions by Ball Milling - Ball Milling Towards Green Synthesis: Applications, Projects, Challenges - page 105

Figure 5.1 Influence of frequency (v) and milling time (t) on the yield of 3a.

Reactions were carried out with 1a (1.0 mmol), 2a (2.0 mmol), DDQ
(1.0 mmol), silica gel (0.5 g) and two stainless steel balls (d ¼ 8.0 mm)
(see Table 5.1). Isolated yields based on 1a.
Reproduced from ref. 13, Copyright (2011), with permission from

American Chemical Society.

5.2.4.2 Grinding Auxiliary
The grinding auxiliary can have a huge impact on the ball milling reaction,
especially for grinding liquid substances. It is necessary to adsorb the sub-
strates with a suitable grinding auxiliary, or the reaction may result in a
mud-like mixture. Such mixtures would have different mechanical prop-
erties, which prevent inelastic interaction between balls and substances and
result in inecient energy transfer. Various grinding auxiliaries were exam-
ined in respect of the above three types of CDC reactions. Silica gel was found
to be the most effective. It might act as both grinding aid and absorbent in the
reaction. In the absence of grinding auxiliary, the formation of mud-like
mixtures would lead to a poor yield (Table 5.4, entry 5). However, adding a
relatively great amount of grinding auxiliary also decreased the yield dra-
matically owing to the dilution of reactant concentrations (Table 5.4, entry 2).

5.2.4.3 Scalability
The issue of scalability in mechanosynthesis has not yet been broadly
addressed. Indeed, a common perception is that there are still diculties
in scaling up such mechanical-power promoted reactions.
Nonetheless, efforts have been made to examine the scalability of these
CDC reactions under ball milling conditions. The results show that this
method was amenable to relatively larger scale preparation, which provides