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Chemistry Letters Vol.36, No.12 (2007)
Oxidation of Secondary Benzylic Alcohols to Ketones by Activated
Carbon–Molecular Oxygen System
Yuki Sano, Takanori Tanaka, and Masahiko HayashiÃ
Department of Chemistry, Graduate School of Science, Kobe University, Rokkodai-cho, Nada, Kobe 657-8501
(Received September 11, 2007; CL-070993; E-mail: mhayashi@kobe-u.ac.jp)
A variety of benzylic alcohols were oxidized to the corre-
(O2) system. Also, we developed direct synthesis of 2-arylben-
zoxazoles,6 2-arylbenzimidazoles,6 and 2-arylbenzothiazoles7
by using the same activated carbon–O2 system. These situations
have inspired us to apply this activated carbon–O2 system to
the oxidation of alcohols, and we found activated carbon–O2
system was also effective for oxidation of some alcohols. Here,
we want to report oxidation of benzylic alcohols to the corre-
sponding ketones by activated carbon–O2 system.8
We first examined the effect of the amount of activated
carbon using fluorenol as a substrate. As shown in Table 1, the
presence of activated carbon is essential to promote oxidation.
When the reaction was carried out in the absence of activated
carbon, only 8% yield of fluorenone was obtained and 92% of
starting alcohol was recovered. The choice was the use of
50 wt % of activated carbon (81% yield), though the use of
100 wt % gave higher yield (93% yield).
sponding carbonyl compounds selectively in the presence of ac-
tivated carbon under molecular oxygen atmosphere. This proc-
ess does not require any metal oxides, thus this is environmental-
ly friendly and economical.
The transformation of alcohols into aldehydes and ketones is
one of the most fundamental reactions in organic synthesis.1 For
this purpose, some stoichiometric oxidizing agents such as chro-
mium and manganese oxides have been used so far. However,
these metal salts are usually toxic and hazardous, and they often
cause environmental problems. Therefore, catalytic process us-
ing oxygen or aqueous H2O2 as oxidizing agents by the aid of
less toxic metal complex is desirable from the viewpoints of
´
environmental concerns. Recently, Marko et al. reported an
efficient catalytic system consisting of CuCl/phenanthroline/
K2CO3/DBADH2 (1,2-bis(tert-butoxycarbonyl)hydrazine)2a and
TPAP (tetrapropylammonium perruthenate)/MS 4A2b using
oxygen or air as oxidant. Noyori and his co-workers developed
organic solvent- and halide-free oxidation of alcohol with aque-
ous H2O2 using Na2WO4 and phase-transfer catalyst (PTC) sys-
tem.2c With regard to a PdII catalyst process, Uemura et al. re-
ported Pd(OAc)2-catalyzed oxidation of alcohols by molecular
oxygen in the presence of MS 3A.2d A similar type of palladi-
um-catalyzed oxidation of alcohols was also reported by Peter-
son and Larock.2e
On the other hand, during the course of our study of chiral
Schiff base ligands in asymmetric reactions,3 in order to synthe-
size new type of chiral Schiff base ligands (Scheme 1), we faced
to necessity to develop new and efficient oxidation method for
benzylic alcohol after introduction of R1 group by Grignard re-
action, because treatment of phenolic compounds with chromi-
um oxide (CrO3) caused the production of undesired quinolic
compounds and their polymerized compounds which led the
low yield of desired ketonic compounds (31–35%).3a We recent-
ly reported oxidative aromatization of 9,10-dihydroanthracene,4
substituted pyrazoline,5 dihydropyridine,5 to the corresponding
aromatic compounds by activated carbon–molecular oxygen
Then, we examined the oxidation of a variety of benzylic
alcohols. Some of the obtained examples are summarized in
Table 2. As shown in Table 2, various benzylic alcohols were
converted to ketones. Nitrogen atom in pyridine and quinoline
was not oxidized to nitrogen oxide under the present conditions
(Entries 3–5). It was lucky for us, the oxidation step in our initial
purpose to prepare new chiral Schiff bases, was achieved
(Entries 7–10), for example, 3-tert-butyl-2-hydroxybenzhydrol
was effectively oxidized to ketones using activated carbon–O2
system. Unfortunately, however, even benzylic alcohols, some
alcohols, such as, (2-naphthyl)phenylmethanol, 1-(2-naphthyl)-
ethanol, and 1-(1-naphthyl)ethanol were unreactive to the oxida-
tion. This is maybe due to discord between substrate (alcohol)
character such as size and polarity of the molecule and micro-
pore character of the activated carbon such as pore size, surface
area, and oxygen functional group.
In conclusion, oxidation of alcohols to carbonyl compounds
proceeded selectively for benzylic alcohols. Some catalytic
Table 1. Effect of amount of activated carbon in oxidation of
fluorenola
Activated carbon
(Shirasagi KL)
O2
1) R1MgX
R1
Xylene
120 °C, 12 h
OH
O
2) H3O+
OHC
t-Bu
t-Bu
Yieldb/%
OH
OH OH
Entry
Activated carbon/wt %
1
2
3
0
50
100
8 (92)
81 (12)
93
R2
NH2
OH
R1
t-Bu
R1
R2
oxidation
N
OH
aAll reactions were carried out in xylene at 120 ꢀC for
12 h. bIsolated yield after silica-gel column chromato-
graphy. The values in the parentheses indicate the re-
covery of fluorenol.
t-Bu
O
OH
OH
Scheme 1. Synthesis of the chiral Schiff base.
Copyright Ó 2007 The Chemical Society of Japan
Sano, Yuki
