Ball milling in organic synthesis: solutions and challenges.

During the last decade numerous protocols have been published using the method of ball milling for synthesis all over the field of organic chemistry. However, compared to other methods leaving their marks on the road to sustainable synthesis (e.g. microwave, ultrasound, ionic liquids) chemistry in ball mills is rather underrepresented in the knowledge of organic chemists. Especially, in the last three years the interest in this technique raised continuously, culminating in several high-quality synthetic procedures covering the whole range of organic synthesis. Thus, the present tutorial review will be focused on the highlights using this method of energy transfer and energy dissipation. The central aim is to motivate researchers to take notice of ball mills as chemical reactors, implementing this technique in everyday laboratory use and thus, pave the ground for future activities in this interdisciplinary field of research.

Fast, ligand- and solvent-free synthesis of 1,4-substituted buta-1,3-diynes by Cu-catalyzed homocoupling of terminal alkynes in a ball mill.

A method for the Glaser coupling reaction of alkynes by using a vibration ball mill has been developed. The procedure avoids the use of ligands and solvents during the reaction. Aryl- and alkyl-substituted terminal alkynes undergo homocoupling if coground with KF-Al(2)O(3) and CuI as a milling auxiliary and catalyst. Furthermore, an alternative protocol has been developed incorporating 1,4-diazabicyclo[2.2.2]octane (DABCO) as an additional base allowing the use of KF-Al(2)O(3) with a lower KF loading. Besides Cu salts, the homocoupling of phenylacetylene is also catalyzed by Ni or Co salts, as well as by PdCl(2). TMS-protected phenylacetylene could be directly converted into the homocoupling product after in situ deprotection of the alkyne by fluoride-initiated removal of the trimethylsilyl group.

Fate of monoterpenes in near-critical water and supercritical alcohols assisted by microwave irradiation.

The rearrangement of alpha- and beta-pinene was studied under microwave irradiation in near-critical water and supercritical lower aliphatic alcohols, with the aim of identifying the pathway of alpha- and beta-pinene isomerization. Generally, two pathways occur, pyrolysis on the one and acidolysis on the other hand, whereby acidolysis is predominant in the case of near-critical water and the second pathway is favored for experiments employing supercritical alcohols. The different behavior of these two structurally related solvents is attributed to the increased availability of protons if water is heated to 270 degrees C and 80 bar, thus enhancing the autoprotolysis of water. The application of alcohols instead furnished rearrangement products clearly attributed to thermal pyrolysis route by the formation of radical reaction intermediates.

Solvent-free and time-efficient Suzuki-Miyaura reaction in a ball mill: the solid reagent system KF-Al(2)O(3) under inspection.

Although a plethora of synthetic procedures mediated by KF-loaded aluminas is available in the literature, there is almost no data concerning the influence of parameters such as alumina modification or KF-loading on experimental results. Hence, the Pd-catalyzed, solvent-free Suzuki-Miyaura reaction was chosen as model reaction to investigate the effect of the above mentioned parameters on the results of coupling reactions. The results from ball milling experiments led to the conclusion that self-prepared and commercially available KF-Al2O 3differ in water content. The higher the residual water content, the higher are the product yields.

An alternative solvent-free synthesis of nopinone under ball-milling conditions: Investigation of reaction parameters.

A new method for the oxidative cleavage of ß-pinene (1) yielding nopinone (2) with potassium permanganate as oxidant under solvent-free conditions was established. The reaction was performed in a conventional ball mill with use of a grinding auxiliary. The auxiliary has the ability to sorb liquid reactants such as 1 on its surface to make liquid(s) accessible for mechanical impact. Different reaction parameters and technical variables were assessed concerning their influence on yield and selectivity of 2. Different chemical parameters such as oxidants, grinding auxiliaries, and quantities were investigated. Also tuning parameters including milling time, rotation frequency, and number of milling balls were explored.

Pd on porous glass: a versatile and easily recyclable catalyst for Suzuki and Heck reactions.

The catalytic activity of Pd supported on porous glass was studied for both Suzuki and Heck reactions under aerobic conditions, with particular focus on the Suzuki coupling. The reactions were carried out in water under microwave irradiation. The effects of the catalyst preparation process (calcination time and temperature), as well as the base, substrate, and boron compound used on the coupling reaction were investigated in relation to the reusability of the catalyst. Various bases promote the Suzuki coupling of phenylboronic acid with bromophenol very effectively resulting in quantitative conversion and excellent selectivity for the coupling product. However, most bases lead to deactivation of the catalyst after the first reaction cycle and the catalyst must be reactivated before reuse. Therefore, excellent conversions and selectivities for individual reactions are not sufficient to conclude if the chosen conditions are suitable for a given reaction, but resuits from recycling studies have to be considered also.