4650
J.-R. Wang et al. / Tetrahedron Letters 46 (2005) 4647–4651
also observed the formation of three other by-products
(molecular weights = 150, 148, and 132) with yields of
22.7%, 22.1%, and 16.4%, respectively. Thus, the aerobic
oxidation of indan catalyzed by NHPI was not selective
for the desired product.
NHPI was found to be a much better catalyst than
NHPI for the aerobic oxidation of benzylic compounds
to the carbonyl compounds in the ionic liquid.
Acknowledgments
It was then discovered that by using Py-NHPI as the
catalyst, indan could be exclusively oxidized to 1-inda-
none. The GC–MS yield was almost 100%, while the iso-
lated yield was 84%. It is worth noting that in the
previous study by Einhorn and co-workers, aerobic oxi-
dation of indan catalyzed by the NHPI type of catalyst
gave a yield of 77% in CH3CN.13 Furthermore, in 2003,
Alsters and co-workers reported that benzylic oxida-
tion of indan with dioxygen by metal/NHPI-catalyzed
co-oxidation with benzaldehyde in AcOH gave a yield
of 75% for 1-indanone.14
We thank NSFC (nos. 20332020 and 20472079) for the
financial support.
References and notes
1. Reviews: (a) Song, C. E. Chem.Commun. 2004, 1033–
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Deng, Y. Prog.Chem. 2003, 15, 222–241;(f) Dupont, J.;
de Souza, R. F.;Suarez, P. Chem.Rev. 2002, 102, 3667–
3692.
Some other benzylic compounds were also examined for
the aerobic oxidation (see Table 4). The GC–MS yields
ranged from 82% to 100%, while the isolated yields were
from 47% to 96%. Thus, the Py-NHPI catalyzed aerobic
oxidation of benzylic compounds in [bmim][PF6] is also
a synthetically valuable method. It is worth mentioning
that in all of the above reactions the ionic liquid
[bmim][PF6] can be recovered after each oxidation reac-
tion and then re-used in the next oxidation reaction.10
2. Howart, J. Tetrahedron Lett. 2000, 41, 6627–6629.
3. Farmer, V.;Welton, T. Green Chem. 2002, 4, 97–102.
4. Ansai, I. A.;Gree, R. Org.Lett. 2002, 4, 1507–1509.
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J.;Jacobs, P. A. Tetrahedron Lett. 2002, 43, 8107–8110.
6. Sun, H.;Harms, K.;Sundermeyer, J. J.Am.Chem.Soc.
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7. Reviews: (a) Ishii, Y.;Sakaguchi, S.;Iwahama, T. Adv.
Synth.Catal. 2001, 343, 393–427;(b) Sheldon, R. A.;
Arends, I. W. C. E. Adv.Synth.Catal. 2004, 346, 1051–
In summary, NHPI-catalyzed aerobic oxidations in the
ionic liquids were examined for the first time. Both
NHPI and its ionic derivative, Py-NHPI, were found
to have better performance in the ionic liquid than in
the conventional organic solvents for the aerobic oxida-
tion of N-alkylamides to imides. On the other hand, Py-
1071;(c) Tong, J.;Li, Z.;Xia, C.
96–110.
Prog.Chem. 2005, 17,
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Table 4. Aerobic oxidation of benzylic compounds catalyzed by Py-
NHPI in [bmim][PF6]a
Entry
Reactant
Product
Yieldb
O
1
100 (84)
O
2
3
82 (63)
9. Minisci, F.;Punta, C.;Recupero, F.;Fontana, F.;Pedulli,
G. F. J.Org.Chem. 2002, 67, 2671–2676.
10. Typical procedures for the aerobic oxidation of N-benzy-
lacetamide: a two-necked flask was charged with N-
benzylacetamide (149 mg, 1 mmol), Py-NHPI (16.7 mg,
0.05 mmol, 5 mol%), Co(PF6)2 (4.2 mg, 0.01 mmol,
1 mol%), and [bmim][PF6] (3 mL). The reaction mixture
was heated to 65 °C and stirred for 24 h under 1 atm of
O2. The resulting suspension was cooled to room temper-
ature and extracted with ether a few times. After concen-
tration of the ether solution, the residue was purified by
chromatography to afford the pure product (the yields
were shown in the previous Tables 1–4). To recycle the
ionic liquid, we added 10 mL of dichloromethane to the
remaining [bmim][PF6] and then washed the mixture with
water. The mixture was dried over anhydrous MgSO4.
After the solvent was removed [bmim][PF6] could be used
for the next reaction.
O
O
O
100 (86)
O
4
5
84 (47)
96 (96)
O
a Reaction conditions: Benzylic compound (3 mmol), Py-NHPI
(5 mol%), Co(PF6)2 (1 mol%), [bmim][PF6] (5 mL), O2 (1 atm),
65 °C, 24 h.
b Yields outside parentheses are GC–MC yields. Yields in parentheses
are isolated yields.
11. Py-NHPI: 1.62 g (10 mmol) of 5-methylisobenzofuran-
1,3-dione, 2.7 g (15 mmol) of NBS, and 0.1 g of (PhCOO)2