is in clear contrast to the application of ball mills in particle
refinement processes.21
were placed into the milling beaker (ZrO2, V = 45 ml). Finally,
the beaker was sealed with a PTFE-gasket and the ZrO2-lid.
Two similar loaded beakers are mounted into the planetary ball
mill (Pulverisette 7 classic line, Fritsch GmbH, Idar-Oberstein,
Germany). The parameters, rotations per minute and milling
time, were set-up and the milling process was started. After the
milling process, the beakers were opened and the milling balls
were removed. The reaction mixture was transferred into a glass
tube and subsequently demineralized water (3 ml) as well as ethyl
acetate (5 ml) were added. The organic layer was separated and
the composition was analyzed by GC-FID respective GC-MSD.
Conclusion
As shown, the solvent-free oxidative dehydrogenation of g-
terpinene (1) to p-cymene (2) initiated by grinding in a planetary
ball mill is a very time-efficient method. After 5 min and in the
presence of alumina as a grinding auxiliary, with KMnO4 as
the oxidant quantitative yields were obtained. Other auxiliary
materials like silica or other metal oxides can also be applied. The
employment of montmorillonite causes moderate yields without
the addition of any further oxidizing agent. The assessment of
R
other oxidants like p-benzoquinone, NaIO4 or Oxoneꢀ revealed
Acknowledgements
high conversions, accompanied by excellent selectivities. Widely-
used oxidation agents like K2Cr2O7 or H2O2 (as urea-H2O2-
adduct) are less effective. Finally, KMnO4 and NaIO4 are the best
oxidants regarding cost, toxicity, and work-up of the reaction
mixture. Nevertheless, an excess of these oxidants has to be
appliedtoreceive quantitative yields. The assessment ofvariables
like rotation frequency or number of milling balls, revealed
a significant influence on the outcome of the reaction. Thus,
variation of these parameters and of the reaction time allows
fine-tuning of the reaction conditions, achieving high yields and
excellent selectivities. As optimum milling parameters t = 5 min
at vrot = 800 rpm (13.3 Hz) with 6 milling balls were identified.
This work is funded by the German Federal Environmental
Foundation (DFG; grant No. 27281-31).
Notes and references
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◦
◦
to 280 C (hold 3 min); injector temperature: 250 C; detector
temperature: 280 ◦C. GC-MS: HP 5, 30 m ¥ 0.32 mm ¥ 0.25 mm,
7 psi He; program: 55 ◦C (hold 1 min), 5 K min-1 up to 150 ◦C,
20 K min-1 up to 280 ◦C (hold 5 min); injector temperature:
280 ◦C, EI (70 eV).
All product yields reported herein are GC-determined yields
and are comparable with the isolated ones. Nevertheless, the
reported yields were corrected by means of different FID-
sensitivity for substrate and product. The reported yields are
mean values from at least two independent experimental runs.
The grinding beakers and the milling balls were purified with
10 wt% hydrochloric acid in ethanol for complete removal of
residual Mn-species.
Reaction procedure
A milling beaker was equipped with 6 milling balls (ZrO2, d:
15 mm). Afterwards, the grinding auxiliary (3.8 g), g-terpinene
(1; 2 mmol, 272 mg), the oxidant (6 mmol), and H2O (0.1 g)
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The Royal Society of Chemistry 2010
Green Chem., 2010, 12, 1288–1294 | 1293
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