Chemistry Letters 2002
583
Table 2. N2O Oxidation of Various Substrates
Figure 1. Proposed mechanism for selective oxidation of 9,10-
dihydroanthracene.
acid, anthracene (III) was recovered in 94% yield even after
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exposure to the vigorous conditions of 200 C for 20 h. Although
the remarkable effect of sulfuric acid as the appropriate additive
for the oxidative aromatization could not be clearly explained yet,
similar effects by several metal sulfates were observed in the N2O
oxidation catalyzed by the oxo-ruthenium complex. The total
yield of the benzylic oxidation and aromatization of 9,10-
dihydroanthracene was remarkably improved in the presence of
the several metal sulfates: 97% yield with CuSO4 and 93% yield
with FeSO4 vs. 74% yield without any additive. The results that
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copper chloride or acetate is not effective on the total yield
suggested that sulfate anions should work to enhance the
oxidation reaction. Further investigations of this phenomenon
and the detailsof the reaction mechanism are currently under way.
References
1
a) Y.-C. Kao, K. R. Korzekwa, C. A. Laughton, and S. Chen, Eur. J.
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4
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6
D. L. Wertz, M. F. Sisemore, M. Selke, J. Driscoll, and J. S. Valentine, J.
Am. Chem. Soc., 120, 5331 (2001).
For a review of dehydrogenative aromatization, see: P. P. Fu and R. G.
Harvey, Chem. Rev., 78, 317 (1978).
a) T. Aida, Chourinkai SaishinGijutsu, 4, 72 (2000). b) M. P. Doyle, J. W.
Terpstra, C. H. Winter, and J. H. Griffin, J. Mol. Catal., 26, 259 (1984).
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M. A. Lockwood, T. J. Blubaugh, A. M. Collier, S. Lovell, and J. M.
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therein.
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Purchased from Air Liquid Japan Ltd.: 99.999% purity.
a) J. T. Groves and J. S. Roman, J. Am. Chem. Soc., 117, 5594 (1995). b)
K. Hashimoto, Y. Kitaichi, H. Tanaka, T. Ikeno, and T. Yamada, Chem.
Lett., 2001, 922. c) T. Yamada, K. Hashimoto, Y. Kitaichi, K. Suzuki, and
thracene in high yields (Entries 4, 6, and 8). Although the reaction
rate of the nitrous oxide oxidation of 9-ethyl-9,10-dihydroan-
thracene under reaction conditions A and B decreased, 10 mol%
of the ruthenium catalyst was loaded to complete the reaction
(
Entries 9 and 10). Under conditions A, the corresponding ketone
was obtained in 47% yield along with 42% yield of anthraqui-
none, while 9-ethylanthracene was selectively obtained under
reaction conditions B.
7
8
9
The reaction pathways to produce both the aromatized
product and the benzylic oxidation product were explained as
follows (Figure 1). In the first step, 9,10-dihydroanthracene (I)
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4
T. Ikeno, Chem. Lett., 2001, 268. The liberated N2 gas in N O
should be oxidized into 9-hydroxy-9,10-dihydroanthracene (II)
2
epoxidation was detected by GCanalysis to be ca. 70% of the ideal
amount. d) T. Yamada, K. Suzuki, K. Hashimoto, and T. Ikeno, Chem.
Lett., 1999, 1043.
as the common intermediate both for anthracene (III) and
anthraquinone (IV). Under reaction conditions A, further
oxidation catalyzed by the oxo-ruthenium complex would then
proceed to similarly afford the corresponding ketone on another
benzylic carbon, and anthraquinone (IV) was finally obtained in
high selectivity. On the other hand, in the presence of a strong acid
10 Nitrous oxide oxidation of benzene to phenol using heterogeneous
catalysts is reported: references are cited in ref. 9d.
1
1 K. Imagawa, T. Nagata, T. Yamada, and T. Mukaiyama, Chem. Lett.,
994, 527.
2 H. Ohtake, T. Higuchi, and M. Hirobe, J. Am. Chem. Soc., 114, 10660
1
1
(
the conditions B) protonation to II followed by dehydration
(1992).
13 S. Banfi, A. Maiocchi, A. Moggi, F. Montanari, and S. Quici, J. Chem.
Soc., Chem. Commun., 1990, 1794.
could proceed to selectively afford the aromatized product,
anthracene (III). It should be mentioned that the oxidation of the
resulting anthracene (III) was inhibited under conditions B.
Anthracene (III) was oxidized toanthraquinone (IV) in 53%yield
under vigorous conditions, e.g., at 160 C with a 5 mol%
ruthenium-complex catalyst. However, in the presence of sulfuric
1
4 D. R. Boyd, N. D. Sharma, D. J. Stevenson, J. Chima, D. J. Gray, and H.
Dalton, Tetrahedron Lett., 31, 3887 (1991).
1
5 The total yield of the benzylic oxidation and aromatization of 9,10-
dihydroanthracene was 20% with CuCl and 71% with Cu(AcO) .
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