Nitrobenzene as hydrogen acceptor in Pd/C-catalyzed hydrogen transfer reaction

Article abstract of DOI:10.1016/j.tetlet.2005.05.095

Nitrobenzene was found to work as an efficient hydrogen acceptor in the oxidation of allylic alcohols to give the corresponding enones in high yields.

Full text of DOI:10.1016/j.tetlet.2005.05.095

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 4989–4991
Nitrobenzene as hydrogen acceptor in Pd/C-catalyzed
hydrogen transfer reaction
Takanori Tanaka, Hirotoshi Kawabata and Masahiko Hayashi*
Department of Chemistry, Faculty of Science, Kobe University, Nada, Kobe 657-8501, Japan
Received 6 May 2005; revised 19 May 2005; accepted 20 May 2005
Available online 9 June 2005
Abstract—Nitrobenzene was found to work as an efficient hydrogen acceptor in the oxidation of allylic alcohols to give the corre-
sponding enones in high yields.
Ó 2005 Elsevier Ltd. All rights reserved.
7
Oxidation of alcohols is one of the most important
1
transformation reactions in organic synthesis. Hydro-
carbazole derivative. In the latter case, even in the ab-
sence of nitrobenzene, the reaction proceeded; however,
the authors described that the presence of nitrobenzene
increased the yield. Both cases are aromatization reac-
tions. Therefore, this report is the first example of the
use of nitrobenzene as a hydrogen acceptor in Pd/C-cat-
alyzed oxidation reaction of alcohols.
gen transfer reaction between alcohols and a hydrogen
acceptor will afford the corresponding carbonyl and
hydrogenated compounds of the hydrogen acceptor. A
variety of metal complexes promote the hydrogen trans-
2
fer reaction. Among the many transition metals such as
ruthenium, rhodium, iridium and others, we focused our
attention on palladium complexes. Actually, we recently
reported the oxidation of allylic alcohols and benzylic
alcohols to the corresponding carbonyl compounds by
We examined the reaction of D-glucal 1 with a catalytic
amount of Pd/C in the presence of nitrobenzene. The
products were dehydrogenated enone (1,5-anhydrohex-
1-en-3-ulose) 2 and hydrogenated 2-deoxy-1,5-anhy-
dro-D-glucitol 3 (Scheme 1). The enone product
obtained by this reaction is very useful, because it can
be used as a substrate for glycosylation leading to the
3
the use of a Pd/C–ethylene system. Instead of this sys-
tem, Pd(OAc) –vinyl acetate was also effective for the
hydrogen transfer reaction.
2
8
There are some reports for transition metals-catalyzed
reduction of nitrobenzene derivatives to aniline deriva-
synthesis of 2-deoxy glycosides. Therefore, several
methods have been reported including PDC (pyridinium
dichromate) so far. Among those, in 1993, Czernecki
and his co-workers reported the oxidation of allylic
alcohol of D-glucal with a stoichiometric amount of
4
tives. In this letter, we report that nitrobenzene can
be used in hydrogen transfer reaction as an excellent
hydrogen acceptor. On the other hand, in 1987, Mura-
hashi et al. reported RuH (PPh ) -catalyzed oxidative
9
Pd(OAc) in aqueous DMF (1% H O). They disclosed
2
2
3 4
2
transformation of alcohols and aldehydes to esters and
5
lactones. To our knowledge, there have been two recent
that several attempts aiming at oxidation with a cata-
lytic amount of Pd(OAc) in the presence of reoxidants
such as copper acetate were unsuccessful.
2
reports in which nitrobenzene was used in Pd/C-cata-
lyzed reactions as a hydrogen acceptor. In 2002, Cossy
and Belotti reported the aromatization of enamines pro-
moted by Pd/C in the presence of nitrobenzene and
molecular sieves (MS) 4Aunder toluene reflux condi-
Ac2O
pyridine
OAc
O
6
20 weight%
OH
O
OAc
O
OAc
AcO
tions. The Nippon Steel Chemical group also disclosed
dehydrogenative aromatization of 1,2,3,4-tetrahydro-
1
0% Pd/C
+
OH
HO
PhNO2
CH3CN
AcO
O
Keywords: Oxidation; Hydrogen acceptor; Nitrobenzene.
*
1
2
3
Corresponding author. Tel.: +81 78 803 5687; fax: +81 78 803
688; e-mail: mhayashi@kobe-u.ac.jp
5
Scheme 1.
0
040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.05.095

4990
T. Tanaka et al. / Tetrahedron Letters 46 (2005) 4989–4991
a
2
Table 1. Oxidation of glycals (1, 4 and 6) by Pd/C–PhNO system
b
Entry
Glycals
PhNO
2
/equiv
Conditions
Products
Yield (2/3)
T/°C
t/h
1
2
3
4
OH
O
OH
None
0.17
0.33
1
50
50
50
50
16
48
48
48
88 (47/53)
100 (78/22)
99 (>98/<2)
100 (100/0)
OAc
O
HO
AcO
1
O
2
OAc
O
OH
O
AcO
HO
c
5
6
0.33
0.33
80
80
24
48
83
OH
O
4
5
HO
AcO
O
O
Me
Me
c
90
OH
6
O
7
a
b
c
3
All reactions were carried out in CH CN in the presence of 20 wt % of 10% Pd/C.
1
Combined yield. The ratio was determined by H NMR analysis.
Isolated yield after silica-gel column chromatography.
The ratio of dehydrogenated enone and hydrogenated
glucitol depended on the amount of nitrobenzene. Even
in the presence of 0.33 equiv of nitrobenzene, the forma-
tion of the hydrogenated product of glucal was only less
than 2% (Table 1, entry 3). These results clearly demon-
strate that nitrobenzene works as an efficient hydrogen
50 weight%
0% Pd/C
OH
OH
Me
H
Me
H
1
R
H
R
H
0
.33 eq PhNO2
CH3CN
H
80 °C, 3 days
H
HO
O
8
; R = Me 82%
1
0
acceptor. Nitrobenzene was much easier to be hydro-
gented than the double bond in D-glucal. Furthermore,
9; R = H 63%
Scheme 2.
1
equiv of nitrobenzene captures 3 equiv of hydrogen.
It is interesting that the complete conversion of nitro-
benzene to aniline was not observed. That is, when D-
glucal was treated with 20 wt % of 10% Pd/C in the pres-
ence of 0.33 equiv of nitrobenzene, nitrobenzene should
be consumed completely, and 0.33 equiv of aniline was
produced theoretically; however, about 30% nitroben-
zene remained. This surprising observation was ex-
plained by the spillover phenomenon. The hydrogen
atom on metal particles (Pd) may easily migrate to sup-
(Aldrich, 47.8 mg), nitrobenzene (54.4 lL, 0.53 mmol)
and CH CN (2 mL) was stirred at 50 °C for 48 h. After
3
confirmation of the completion of the reaction, pyridine
(0.8 mL) and acetic anhydride (0.6 mL) were added,
and the whole was stirred for 12 h at room tempera-
ture. The palladium precipitate was filtered through a
small amount of Celite and the filtrate was concen-
trated. Purification by silica-gel column chromatogra-
phy afforded the product (342.9 mg, 99%). The ratio
1
1
ports (carbon), which have a hydrogen acceptor site.
1
In the case of the use of nitromethane (0.33 equiv) instead
of nitrobenzene as hydrogen acceptor for this reaction,
the ratio of dehydrogenated enone and hydrogenated
glucitol (2:3) was 53:47 (92% yield). This means that nitro-
methane worked much less efficiently as hydrogen accep-
tor than nitrobenzene.
2:3 was determined as 98:2 by H NMR analysis. All
spectral data of the products were consistent with those
1
2
reported.
In conclusion, the present method has the following
characteristic features: (1) Secondary allylic alcohols
are oxidized to the corresponding enones in high yield.
(2) Nitrobenzene works as an efficient hydrogen accep-
tor. Three moles of hydrogen are captured by one mole
of nitrobenzene. (3) The reactions take place under mild
and environment-friendly conditions.
Not only D-glucal, but also D-galactal 4 and L-rhamnal
6
were converted to the corresponding enones (5 and 7)
under similar conditions (Table 1, entries 5 and 6). In
both cases, hydrogenated products of 4 and 6 were not
obtained. Furthermore, allylic alcohols of the Aring
in steroid compounds were also oxidized effectively to
afford testosterone 8 and 19-nortestosterone 9 in 82%
and 63% yield, respectively (Scheme 2).
Acknowledgements
The authors thank Dr. Hideki Amii for helpful discus-
sion. This work was supported by the Ministry of Edu-
cation, Science, Sports and Culture, Japan.
Atypical procedure is as follows ( Table 1, entry 3):
Amixture of
D-glucal (231 mg, 1.58 mmol), Pd/C

T. Tanaka et al. / Tetrahedron Letters 46 (2005) 4989–4991
4991
Supplementary data
S. Green Chem. 2000, 257–260; (f) Hayashi, M.; Kawa-
bata, H. J. Syn. Org. Chem., Jpn. 2002, 60, 137–144.
. Watanabe, Y.; Ohta, T.; Tsuji, Y.; Hiyoshi, T.; Tsuji, Y.
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cited therein.
. Murahashi, S.; Naota, T.; Ito, K.; Maeda, Y.; Taki, H. J.
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catalyzed reaction, Zhou, D. Y.; Koike, T.; Suetsugu, S.;
Onitsuka, K.; Takahashi, S. Inorg. Chim. Acta 2004, 357,
4
5
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2
005.05.095.
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