(NO)Ru(salen)-Catalyzed Aerobic Oxidation of o-Hydroxybenzyl Alcohol Derivatives

Article abstract of DOI:10.1055/s-2003-41411

Various salicylaldehyde derivatives were prepared from the corresponding alcohols in good to excellent yields by ecologically benign aerobic oxidation using ruthenium salen nitrosyl complex 1 as catalyst under visible light-irradiation.

Full text of DOI:10.1055/s-2003-41411

1868
LETTER
(NO)Ru(salen)-Catalyzed Aerobic Oxidation of o-Hydroxybenzyl Alcohol
Derivatives
(N
O)Ru(salen)-Ca
y
talyzed
A
ero
a
bic
O
xidationof
o
-Hydroxybenz
a
yl
A
lcohol
s
D
erivative
h
s
iro, Akimasa Mitsuishi, Ryo Irie, Tsutomu Katsuki*
Department of Chemistry, Faculty of Science, Graduate School, Kyushu University 33, CREST, Science and Technology (JST), Hakozaki,
Higashi-ku, Fukuoka 812-8581, Japan
Fax +81(92)6422607; E-mail: katsuscc@mbox.nc.kyuhsu-u.ac.jp
Received 8 July 2003
for the preparation, but its utility is reduced due to cum-
Abstract: Various salicylaldehyde derivatives were prepared from
bersome preparation of such oxidants and product isola-
the corresponding alcohols in good to excellent yields by ecologi-
cally benign aerobic oxidation using ruthenium salen nitrosyl com-
tion.5 DMSO oxidizes benzyl alcohols selectively,
however, strongly acidic condition is required for this re-
action.6 On the other hand, zirconium propoxide has been
reported to catalyze oxidation of salicyl alcohol in the
presence of t-butyl hydroperoxide.7 Recently, molecular
oxygen that is the most atom-efficient and ecologically
benign oxidant has been demonstrated to serve as the ter-
minal oxidant for alcohol oxidation,8 and oxidation of
benzylic alcohols to the corresponding aldehydes using
molecular oxygen have also been effected by using vari-
ous catalysts.8,9 However, their application to the prepara-
tion of salicylaldehyde derivatives is rather limited.10 We
have recently found that complex 1 is an efficient catalyst
for aerobic oxidation of primary alcohols including ben-
zyl alcohols to the corresponding aldehydes under visible
light-irradiation (Scheme 1).11
plex 1 as catalyst under visible light-irradiation.
Key words: aerobic oxidation, oxygen, ruthenium salen complex,
salicylaldehyde
Optically active tri- and tetra-dentate Schiff bases (I and
II) including salicylaldimine substructure have been
widely used as chiral auxiliaries for various metal-mediat-
ed asymmetric reactions (Figure 1).1 Extensive studies on
the catalysis of the complexes bearing these Schiff base
ligands have disclosed that asymmetric induction by the
complexes is strongly affected by the natures of the ligand
substituents (A, B, C, and D).2,3 Thus, many optically ac-
tive ligands of this class have been synthesized, but there
is still room for further improvement in their synthesis.
hν, air, toluene
C
D
C
C
p-XC6H4CH2OH
p-XC6H4CH=O
*
*
*
*
N
OH
N
N
X= H, CH3, NO2: 100%
NO
Ru
Cl
N
O
N
O
B
OH
B
OH HO
B
t-Bu
Bu-t
*
A
*
A
A
*
I
II
Bu-t
t-Bu
1
A, B, C, D: substituents
* = central or axial chirality
Scheme 1
Figure 1
Recently, we needed to convert of bis-alcohol 2 to bis-al-
dehyde 3 in the course of our study on salen chemistry, but
conventional methods for oxidation of benzyl alcohols
were inefficient for this conversion (Figure 2). Out of ne-
cessity, we examined aerobic oxidation with 1 as the cat-
alyst and found that the desired oxidation proceeded
Schiff bases (I and II) are synthesized from the corre-
sponding salicylaldehydes and aminoalcohols or di-
amines, and salicylaldehydes are generally prepared by
ortho-directed formylation of phenol derivatives or by ox-
idation of o-hydroxybenzyl alcohol derivatives. So far,
many oxidants have been developed for oxidation of ben-
zylic alcohols and some of them have been successfully
applied to the preparation of salicylaldehyde derivatives.
DDQ and MnO2 are widely used for this purpose, but sto-
ichiometric or more than stoichiometric amount of oxi-
dant is required.4 Cr(VI)-based reagents can also be used
SYNLETT 2003, No. 12, pp 1868–1870
Advanced online publication: 28.08.2003
DOI: 10.1055/s-2003-41411; Art ID: U12903st.pdf
© Georg Thieme Verlag Stuttgart · New York
2
9
.0
9
.2
0
0
3
Figure 2 2Æ 3 Reagents and conditions: a) DDQ, 1,4-dioxane: tra-
ce; (b) vinyl acetate, Pd/C, CH3CN:12 no reaction; c) 1, air, hn,
CHCl3: 82%

LETTER
(NO)Ru(salen)-Catalyzed Aerobic Oxidation of o-Hydroxybenzyl Alcohol Derivatives
1869
smoothly. This result prompted us to examine the scope of pling, we also examined the oxidation of compound 10.
this aerobic oxidation of o-hydroxybenzyl alcohol and its Again, the benzylic alcohol was selectively oxidized and
derivatives (Table 1).
no coupling product was detected (entry 6).
It is noteworthy that this aerobic oxidation proceeded
smoothly in various solvents such as diethyl ether, ethyl
acetate, halocarbons, and aromatic solvents (benzene,
toluene, and chlorobenzene), except for acetonirile. Use
of diethyl ether is beneficial in case the product is rather
volatile (Table 1, entry 2). Otherwise, the solvent of
choice should be varied according to the solubility of the
substrates.
Table 1 Aerobic Oxidation of Hydroxylated Benzylic Alcohol De-
rivatives
Entry
Alcohol
Solvent
EtOAc
Et2O
Time (h)
Yield (%)a,b
1
2
3
4
5
6
7
4
6
16
11
72
8
83
100
92
7
EtOAc
EtOAc
EtOAc
EtOAc
Toluene
Typical experimental procedure is exemplified by the ox-
idation of 2-hydroxy-4-nitorobenzyl alcohol (7): the alco-
hol 7 (16.9 mg, 0.1 mmol) and 1 (1.4 mg, 2 mol%) were
dissolved in EtOAc (1.0 mL). The solution was stirred un-
der irradiation with a halogen lamp (75 W) in air for 72
hours at room temperature. The reaction mixture was con-
centrated, and the residue was purified by column chro-
matography on silica gel (CH2Cl2:acetone = 4:1) to give
2-hydroxy-4-nitrobenzaldehyde (15.4 mg, 92%). In con-
clusion, aerobic oxidation of o-hydroxybenzyl alcohol de-
rivatives was efficiently performed under visible light-
irradiation in the presence of Ru(salen) complex 1.
8
86
9
7
86
10
11
13
21
92
74
a Values are isolated yields after purification by column chromatogra-
phy.
b As evidenced by 1H NMR analyses of the crude products, all the re-
actions proceeded quantitatively and no by-product formation was de-
tected.
Oxidation of compound 4 with DDQ provided the desired
aldehyde in various yields together with the undesired
coupling product 513 and the oxidation with MnO2 gave 5
(27%) as the major product (Figure 3).14 However, aero-
bic oxidation with 1 as the catalyst gave the aldehyde in
83% yield as the single product (entry 1). Oxidation of
structurally simple o-hydroxybenzyl alcohol and its deriv-
atives also gave the corresponding aldehydes as the sole
products, respectively (entries 2–4). It is noteworthy that,
though oxidation of 2-hydroxy-4-nitrobenzyl alcohol re-
quired a prolonged reaction time, no overoxidation to car-
boxylic acid was observed (entry 3). The present aerobic
oxidation oxidizes primary alcohols selectively in the
presence of secondary alcohols.11 As expected, the oxida-
tion of dibenzylic alcohol 9 gave the corresponding alde-
hyde and oxidation of secondary benzylic alcohols was
not detected until the primary alcohol was consumed com-
pletely. Under the present conditions, 2-naphthol (11) un-
derwent oxidative coupling to afford BINOL (entry 7).15
Although oxidation of compound 8 showed that the oxi-
dation of benzylic alcohol is faster than the oxidative cou-
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Synlett 2003, No. 12, 1868–1870 © Thieme Stuttgart · New York