Switchable Selectivity during Oxidation of Anilines in a Ball Mill

Abstract

A solvent-free method for the direct oxidation of anilines to the corresponding azo and azoxy homocoupling products by using a planetary ball mill was developed. Various oxidants and grinding auxiliaries were tested and a variety of substituted anilines were investigated. It was possible to form chemoselectively either azo, azoxy, or the nitro compounds from reaction of aromatic anilines. The selectivity of the solvent-free reaction is switchable by applying a combination of oxidant and grinding auxiliary. Furthermore, a comparison with other methods of energy input (microwave, classical heating, and ultrasound) highlighted the advantages of the ball mill approach and its high energy efficiency.

CHAPTER 10. Technical Implications of Organic Syntheses in Ball Mills

“…from light-weight natural mineral agate (r ¼ 2.7 g cm À3 ) to the heavier ZrO 2 (r ¼ 5.7 g cm À3 ). 62,70 However, there are also examples from the field of organic synthesis in which no influence was observed, which seems to indicate a product decomposition due to high energy. 60,66,72,73 Table 10.1 provides an overview of the materials for milling balls and milling vessels applicable for mechanochemical syntheses.…”

“…63 Most of the publications compare accomplishment of the respective reactions in a MBM and a PBM. 29,[61][62][63] Experimental results for a Suzuki-Miyaura cross-coupling reaction (Scheme 10.2) indicate that there is no difference between the reactions carried out with respect to yield and selectivity if reaction time and scale are on a comparable level. The only difference is the changed operating frequency ( Figure 10.6).…”

“…Although the kinetic energy provided by the propulsion system cannot be used completely as (thermal) activation energy due to dissipative losses, a correlation between the mass of the grinding bodies (m MB B r MB ) and the chemical yield has been described for some organic reactions in ball mills. 10,62,[69][70][71] In several cases a significant increase of yield was observed when the material is changed, e.g. from light-weight natural mineral agate (r ¼ 2.7 g cm À3 ) to the heavier ZrO 2 (r ¼ 5.7 g cm À3 ).…”

“…10,48,[57][58][59] However, studies comparing the effectiveness of the ball mill types in a similar reaction are seldom. Up to now, such comparative studies have been disclosed for the Zn/ZnCl 2 -mediated Pinacol coupling of aromatic ketones and aldehydes, 60 Suzuki-Miyaura cross-coupling with KF-Al 2 O 3 , 29 Knoevenagel condensation of aromatic aldehydes, 61 the oxidative homo-coupling of anilines, 62 and the CuI-catalyzed Glaser reaction of phenylacetylene. 63 Most of the publications compare accomplishment of the respective reactions in a MBM and a PBM.…”

Funding activities by the German Federal Environmental Foundation (Deutsche Bundesstiftung Umwelt) in the field of sustainable chemistry

“…The Stolle and Ondruschka group [24 -26] developed an elegant way for a solvent-free dehydrogenation synthesis in a planetary ball mill through direct oxidation of anilines to the corresponding azo and azoxy homocoupling products (Figure 2 ). This novel synthesis route without solvents shows some impressive advantages: it has a high energy effi ciency, the product is not contaminated Figure 2 Energy effi ciency for synthesis of azo dye from aniline with different methods of energy entry [24] .…”