Microwave-Assisted Oxidation of Side Chain Arenes by MagtrieveTM
COMMUNICATIONS
All the reactions were carried out under heteroge- Tesla 487C (80 MHz) spectrometer using TMS as an internal
TM
standard. GC/MS spectra were determined on GC/MS 5890
neous conditions using Magtrieve as an oxidant and
Series II Hewlett-Packard gas chromatograph equipped with
toluene as a medium, which was not oxidized under such
Ultra 2 (25 m  0.25 mm  0.25 mm) column with a Hewlett-
conditions. The microwave and conventional reactions
were conducted as described for the oxidation of 9H-
fluorene which was representative for the general
procedure of arene oxidations. The applied excess of
the oxidant (5:1 weight ratio) is required since only its
surface is reduced. The reactions were carried out
during the time listed in Table 1. Additional experi-
ments were run in solvent-free systems, but irradiation
Packard 5971 Series Mass Selective Detector.
General Method for the Oxidation
All the reactions were carried out according to the oxidation
procedure given for 9H-fluorene, which was representative of
the general procedure employed for microwave and conven-
tional conditions.
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of Magtrieve and substrate mixtures results in poor
yields (ca. 30% estimated by GC/MS) of the desired
compound due to sublimation of the starting material at
the very high temperature or ignition of the organic
materials.
a) Microwave process: 9H-Fluorene (6 mmol, 1.07 g) was
dissolved in toluene (30 mL), which was placed in a 100-mL
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round-bottom reaction flask. Then Magtrieve (5.34 g) was
In the experiments employing toluene as a solvent the added to the solution. Next the magnetically stirred suspension
carbonyl compounds were obtained with satisfactory was irradiated (450 W of microwave power) in an open vessel
at reflux using the multimode microwave reactor (Plazmatro-
yields. Additionally, microwave experiments were car-
nika, Poland) during 60 min. After completion of the process
ried out in a shorter time and with higher or comparable
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Magtrieve was isolated by a magnet, and the final solution
yields. Results of the experiments are listed in Table 1.
was evaporated under vacuum. The crude product was purified
In the conventional experiments, we did not observe
by crystallization to afford 9-fluorenone; yield: 0.67 g (57%).
such high yields as in the microwave system even though
the reaction time was three times longer (as in the case of
b) Conventional process: 9H-Fluorene (6 mmol, 1.07 g) was
dissolved in toluene (30 mL), which was placed in a 100-mL
9H-fluorene). In addition, the longer reaction times in
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round-bottom reaction flask. Then Magtrieve (5.34 g) was
the conventional experiments favor the creation of the added to the solution. Next the magnetically stirred suspension
side-products containing hydroxy groups in the aroma- was heated in a thermostatted bath in the vessel equipped with
tic ring (ca. 5% determined by GC/MS), which were not a reflux condenser during 180 min. After completion of the
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process Magtrieve was isolated by a magnet and the product
observed under microwave conditions.
were isolated and purified as described for microwave experi-
In conclusion, we have discovered a very efficient
ments to afford 9-fluorenone; yield: 0.56 g (48%).
method for the oxidation of side-chain aromatics to the
corresponding carbonyl compounds using the solid and
9
,10-Anthraquinodione: MS: m/z 209 (M 1, 15%), 208
(
(
(
M , 100%), 207 (13%), 180 (79%), 152 (48%), 151 (22%), 90
6%), 76 (29%), 75 (9%), 63 (6%), 50 (8%); IR (KBr): n 3437
w), 3426 (w), 3074 (w), 1680 (s), 1624 (m), 1592 (m), 1474 (m),
magnetically retrievable oxidant, chromium dioxide
TM
(
Magtrieve ). The advantages of described methodo-
logy focus in a simple reaction set-up and application of
commercially available reagents.
1
454 (m), 1333 (s), 1323 (m), 1286 (s), 1163 (w), 1094 (m), 912
À1
1
(m), 810 (m), 696 (s) cm ; H NMR (CDCl ): d 8.56 (m, 4H,
3
Finally, as shown in the experiments with the thermo- 1,4,5,8-Ar), 7.59 (m, 4H, 2,3,6,7-Ar).
vision camera, high product yields and short reaction
9-Fluorenone: MS: m/z 183 (M 1, 15%), 180 (M ,
1
00%), 152 (28%), 151 (13%), 126 (5%), 76 (13%), 75 (5%), 39
times can be attributed to the higher temperature of the
solid support (i.e., MagtrieveTM) during microwave
(1%); IR (KBr): n 3061 (w), 3041 (w), 3013 (w), 1715 (s), 1666
(
7
7
w), 1612 (s), 1600 (m), 1471 (w), 1451 (m), 1194 (m), 920 (m),
experiments in comparison with conventional processes,
which, in turn, can be the explanation of the higher
reaction rates in a number of reactions reported for
microwave protocols.
À1
1
36 (s) cm ; H NMR (CDCl ): d 7.6 (m, 2H, 1,8-Ar), 7.45 ±
3
.5 (m, 4H, 3,4,5,6-Ar), 7.26 (m, 2H, 2,7-Ar).
Benzophenone: MS: m/z 183 (M 1, 7%), 182 (M , 50%),
1
81 (5%), 105 (100%), 77 (47%), 51 (14%), 50 (4%); IR (KBr):
n 3291 (w), 3088 (w), 3067 (m), 1666 (s), 1595 (m), 1449 (m),
1
6
7
323 (m), 1281 (s), 1076 (w), 945 (m), 930 (m), 766 (m), 705 (s),
À1
1
39 (s) cm . H NMR (CDCl ): d 7.83 (m, 4H, o-Ar), 7.57 ±
3
Experimental Section
.46 (m, 6H, m, p-Ar).
1-Isobenzofuranone (phthalide): MS: m/z 135 (M 1,
All the chemicals were purchased from Aldrich and used as
received. The reactions were carried out in a multimode
microwave reactor with a continuous power regulation (Plaz-
matronika, Poland), which is equipped with a magnetic stirrer
and two inlets on the top and one side of the reactor. The inlets
allowed applying an upright condenser and introducing a fiber-
optic sensor (ReFlex, Nortech) which was used to control the
temperature during microwave experiments. IR spectra were
recorded on FT-IR BIORAD FTS-165 spectrophotometer as
3%), 134 (32%), 133 (11%), 105 (100%), 77 (41%), 76 (9%),
51(11%), 50(9%); IR (KBr): n 3491 (m), 2945 (w), 1767 (s),
1468 (m), 1440 (m), 1341 (m), 1219 (m), 1111 (m), 1062 (s), 1002
À1
1
(m), 743 (s) cm ; H NMR (CDCl ): d 7.5 ± 7.9 (m, 4H, Ar),
3
5.33 (s, 2H, CH ).
2
9-Xanthenone: MS: m/z 198 (M 2, 1%), 197 (M 1,
14%), 196 (M , 100%), 139 (27%), 84 (7%), 70 (4%), 63 (6%), 50
(4%); IR (KBr): n 3082 (w), 1660 (s), 1618 (s), 1608 (s), 1481
1
liquids on NaCl disks. H NMR spectra were collected on a
(s), 1451 (s), 1347 (s), 1333 (m), 1218 (w), 1209 (w), 936 (m), 809
Adv. Synth. Catal. 2003, 345, 1269 ± 1272
asc.wiley-vch.de
¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1271