J. M. Kremsner, C. O. Kappe
FULL PAPER
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rotor. The aqueous reaction mixture was extracted with diethyl
ether (4×20 mL). The combined ether fractions were then dried
with MgSO4 and after removal of the solvent the product was puri-
fied by dry column flash chromatography with dichloromethane as
an eluent to furnish 0.93 g (64%) of indole 7 as a colorless solid
(HPLC purity: Ͼ99%). m.p. 101–102 °C (ref.[44] m.p. 105.5–
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1
106.5 °C). H NMR (360 MHz, DMSO): δ = 2.13 (s, 3 H), 2.29 (s,
3 H), 6.97–6.88 (m, 2 H), 7.19 (d, J = 7.9 Hz, 1 H), 7.33 (d, J =
7.6 Hz, 1 H), 10.6 (s, 1 H) ppm. MS (pos. APCI): m/z = 146.2 [M
+ 1]+ (M = 145.21).
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Rearrangement of Pinacol (8): Pinacol (8; 360 mg, 3.05 mmol) was
added to the NaCl solution (0.03 ) in vessel 1. The reaction mix-
ture was heated to 270 °C by employment of a 7 min linear heating
ramp and was then irradiated for an additional 30 min at 270 °C.
After having been cooled to 40 °C by an air-flow (20 min), the ves-
sel at position 1 was removed from the rotor. A solution of 2-(2,4-
dinitrophenyl)hydrazine (700 mg, 3.53 mmol), H2SO4 (concd.,
4 mL), and methanol (20 mL; CAUTION: exothermic reaction of
H2SO4 with methanol!) was added to the aqueous reaction mixture.
The precipitate was filtered off, washed with aqueous NaHCO3
(5%) and distilled water, and placed in a drying oven (50 °C) over-
night to provide 647.6 mg (76%) (HPLC purity: Ͼ99%) of the cor-
responding hydrazone as an orange solid. m.p. 119–120 °C. 1H
NMR (360 MHz, CDCl3): δ = 1.25 (s, 9 H), 2.05 (s, 3 H), 7.98 (d,
J = 9.7 Hz, 1 H), 8.33 (d, J = 9.7 Hz, 1 H), 9.16 (d, J = 2.4 Hz, 1
H), 11.03 (s, 1 H) ppm. MS (pos. APCI): m/z = 281.1 [M + 1]+ (M
= 280.29).
[9]
[10]
[11]
[12]
[13]
Diels-Alder Reaction of 2,3-Dimethylbutadiene (10) and Acryloni-
trile (11): 2,3-Dimethylbutadiene (10, 1.31 g, 1.80 mL, 15.9 mmol)
and acrylonitrile (11, 0.40 g, 0.50 mL, 7.60 mmol) were added to
the NaCl solution (0.03 ) in vessel 1. The reaction mixture was
heated to 295 °C by employment of an 8 min linear heating ramp
and was then irradiated at 295 °C for an additional 20 min. After
having been cooled to 40 °C by an air-flow (20 min), the vessel at
position 1 was removed from the rotor. The aqueous reaction mix-
ture was extracted with diethyl ether (5×20 mL). The combined
ether fractions were then re-extracted with cold water and dried
with MgSO4. After removal of the solvent, the product was purified
by dry column flash chromatography with dichloromethane as an
eluent and iodine on silica gel for detection, which produced
664 mg (65%) of cycloadduct 12 as a yellowish oil (HPLC purity:
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1
Ͼ99%). H NMR (360 MHz, CDCl3): δ = 1.63 (s, 6 H), 1.81–2.26
(m, 6 H) 2.78–2.80 (m, 1 H).[45] MS (pos. APCI): m/z = 136.1 [M
+ 1]+ (M = 135.21).
Acknowledgments
[14]
[15]
The group of C. R. Strauss developed a prototype multimode
microwave batch reactor (MBR) for performing organic syn-
thesis and in the 1990s studied a variety of organic chemical
transformations in microwave-heated water as solvent in the
200–260 °C temperature range. This reactor is currently not a
commercial product for these high temperature/pressure speci-
fications. See ref.[33,34] for details.
The majority of microwave-assisted organic reactions today are
performed in so-called single-mode reactors, which have a pres-
sure limit of 20 bars, thereby limiting the reaction temperature
for water as solvent to 200 °C. See refs.[11,16] for more details.
B. Hayes, Microwave Synthesis, CEM Publishing, Matthews,
NW, 2002.
E. Neas, M. Collins, Introduction to Microwave Sample Prepa-
ration: Theory and Practice (Eds.: H. M. Kingston, L. B. Jas-
sie), American Chemical Society, Washington, DC, 1988.
We gratefully acknowledge support for this work by the Austrian
Research Promotion Agency (FFG, Projects 807587 and 809716)
and would like to thank Anton Paar GmbH for providing micro-
wave instrumentation and technical support.
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© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2005, 3672–3679