Ru(PPh3)(OH)-salen complex: a designer catalyst for chemoselective aerobic oxidation of primary alcohols

Article abstract of DOI:10.1016/j.tetlet.2009.02.169

Based on the analysis of the mechanism of aerobic oxidation of alcohols using Ru(NO)-salen catalyst, we designed a new complex, Ru(PPh₃)(OH)-salen 3, which was proved to be an excellent catalyst for chemoselective aerobic oxidation of primary alcohols to the aldehydes in the presence of secondary alcohols under ambient and non-irradiated conditions. Complex 3 was also successfully applied to the oxidation of 1-phenyl-1,n-diols to the lactols or the n-hydroxy aldehyde. It is of note that selective oxidation of primary alcohols was achieved even in the presence of activated secondary alcohols.

Full text of DOI:10.1016/j.tetlet.2009.02.169

Tetrahedron Letters 50 (2009) 3432–3435
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Ru(PPh3)(OH)-salen complex: a designer catalyst for chemoselective aerobic
oxidation of primary alcohols
*
Hirotaka Mizoguchi, Tatsuya Uchida, Kohichi Ishida, Tsutomu Katsuki
Department of Chemistry, Faculty of Science, Graduate School, Kyushu University, Hakozaki, Higashi-Ku, Fukuoka 812-8581, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Based on the analysis of the mechanism of aerobic oxidation of alcohols using Ru(NO)-salen catalyst, we
designed a new complex, Ru(PPh3)(OH)-salen 3, which was proved to be an excellent catalyst for chemo-
selective aerobic oxidation of primary alcohols to the aldehydes in the presence of secondary alcohols
under ambient and non-irradiated conditions. Complex 3 was also successfully applied to the oxidation
of 1-phenyl-1,n-diols to the lactols or the n-hydroxy aldehyde. It is of note that selective oxidation of pri-
mary alcohols was achieved even in the presence of activated secondary alcohols.
Received 15 January 2009
Revised 19 February 2009
Accepted 23 February 2009
Available online 26 February 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Aldehydes are useful synthetic intermediates, and the oxidation
(dehydrogenation) of primary alcohols is the most conventional
method for their synthesis. However, most alcohol substrates are
polyols, such as diols and triols, and the chemoselective oxidation
of primary alcohols is of tremendous importance for organic syn-
thesis.1 Although a variety of efficient methods have been devel-
oped for this purpose, most of them require a stoichiometric
oxidant which generates undesirable waste materials as the
coproduct.1–5 The ecological sustainability of the dehydrogenation
reaction primarily depends on the terminal electron acceptor and
the physical properties of the coproduct derived from the accep-
tor.6,7,1c Among various electron acceptors, molecular oxygen is
the most atom efficient, and the accompanying coproduct is water
or hydrogen peroxide. In particular, molecular oxygen in air is
abundant, safe, and ubiquitous.8 Therefore, aerobic oxidations of
alcohols using homogeneous6 or heterogeneous7,1c transition me-
tal catalysts have been developed, but most of these reactions re-
quire the addition of a base, heating, or pressured oxygen. These
reactions can be applied generally to the oxidation of both primary
and secondary alcohols, but the reports on these reactions gener-
ally do not refer to the chemoselectivity between the two oxida-
tions, with the exception of the following. Ishii et al. reported a
catalytic version of the Oshima method6a using molecular oxygen
as the terminal oxidant.6g Sheldon et al. reported that a CuBr2/
(Bipy)/TEMPO/air system (Bipy=2,20-bipyridine, TEMPO=2,2,6,6-
tetramethylpiperidinyl-1-oxy) catalyzed selective oxidation of pri-
mary alcohols in the presence of secondary alcohols under ambient
conditions, though oxidation of aliphatic alcohols was slow at
these conditions.9 Sekar et al. also reported that activated primary
alcohols can be selectively oxidized using a CuCl/DABCO/TEMPO/
O2 system (DABCO=1,4-diazabicyclo [2.2.2]octane).10 On the other
hand, Mizuno and co-workers demonstrated that the Al2O3-sup-
ported Ru hydroxide-catalyzed oxidation of 4-(1-hydroxy-ethyl)-
benzyl alcohol using O2 at 83 °C gives 4-(1-hydroxyethyl)
benzaldehyde with 94% selectivity at >99% conversion, while the
intermolecular competitive reaction between the primary and sec-
ondary benzylic alcohols proceeded with moderate selectivity.11
Still, the development of an efficient catalyst for chemoselective
oxidation between primary and secondary, especially activated
secondary, alcohols is a challenging task. Recently, we revealed
that Ru(NO)-salen complexes efficiently catalyze aerobic oxidation
of alcohols in air under ambient and visible light-irradiated condi-
tions.12,13 Among the various Ru(NO)-salen complexes, 1 and 2
efficiently catalyze the selective oxidation of primary alcohols in
kprim
R1
R1
OH
O
prim. alcohol
a
1 or 2 (2 mol%),
+
+
hν, benzene, RT, Air
ksec
R2 OH
R2
O
sec. alcohol
b
R1, R2=n-C8H17
R1, R2=Ph,
,
1:a/b=100/0 (12 h)
1:a/b=100/0 ( 5 h)
R1=n-C8H17, R2=Ph, 1:a/b=100/29, (kprim:ksec=12:1)
R1=n-C8H17, R2=Ph, 2:a/b=100/6.5, (kprim:ksec= >30:1), at 10ºC
X
Ru
Y
N
O
N
O
R
R
1: X=NO, Y=Cl, R=t-Bu
2: X=NO, Y=OH, R=t-Hex
3: X=PPh3, Y=OH, R=t-Bu
R
R
* Corresponding author. Tel.: +81 92 642 2586.
E-mail address: katsuscc@chem.kyushu-univ.jp (T. Katsuki).
Scheme 1.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.169

H. Mizoguchi et al. / Tetrahedron Letters 50 (2009) 3432–3435
3433
the presence of secondary alcohols (Scheme 1).13 In particular,
complex 2 selectively oxidizes aliphatic primary alcohols even in
the presence of activated secondary alcohols: for example, the rel-
ative oxidation rate between 1-decanol and phenethyl alcohol is
>30, while the relative rate in the oxidation using 1 as the catalyst
is 12.13d Under these conditions, both aliphatic and activated pri-
mary alcohols are oxidized quantitatively to the corresponding
aldehydes.13 However, the reactions need visible light irradiation,
though available light can also be used as the light source.
Photo-activation of an asymmetric metal catalyst is a scientifically
interesting technique, but irradiation is accompanied by heat evo-
lution, which might make reaction control difficult, especially on a
large scale. From a practical point of view, there is still a strong de-
mand for the development of a new catalyst that promotes the
chemoselective oxidation of primary alcohols without photo-irra-
diation or any additive.
Table 1
Aerobic oxidation of alcohols in the presence of the Ru(PPh3)(OH)-salen complex 3 as
the catalysta
Entry
Mono-ol
Time (h)
Conv.b (%)
Yieldb (%)
1c
2d
3c
4d
5c
6d
7c
Benzyl alcohol
Benzyl alcohol
1-Decanol
1.5
1.5
3
3
4
>99
>99
98
97
96
97
97 (93)e
97
1-Decanol
97
96
95
20
1-Phenylethanol
1-Phenylethanol
2-Decanol
4
24
93
22
a
Reaction was run in CDCl3 (1.0 mL) with substrate (0.5 mmol) and 3 (10 lmol)
at room temperature under air.
b
Determined by 1H NMR analysis (400 MHz) using phenanthrene as an internal
standard.
c
d
e
The reaction was carried out under light shielding conditions.
The reaction was carried out under visible light irradiation.
Isolated yield.
The catalytic cycle, including single electron transfer (SET),
intramolecular hydrogen atom transfer (HAT), and product/sub-
strate exchange steps, has been proposed for the Ru(NO)-salen-cat-
alyzed aerobic oxidation of alcohols under irradiation, based on the
results of the kinetics and kinetic isotope effect studies (Scheme
2).12d,13e It is of note that the rate-determining step (RDS) of this
catalytic cycle depends on the nature of the apical ligand (X). For
example, the SET step is the RDS when X is an electron-withdraw-
ing Cl group, while the HAT step is RDS when X is a donating OH
group. Moreover, the oxidation with the Ru-salen(OH) complex is
slower but more chemoselective than that with the Ru-salen(Cl)
complex. We speculated that the coordination of a donating ligand
reduces the oxidation potential of the Ru(III) ion and the reactivity
of the cationic phenol oxygen radical. The oxidation with complex
1 or 2 is commenced by photo-dissociation of the apical NO group
and coordination of alcohol.12d We expected that the substitution
of the hydroxo ligand on the transition metal ion with an alcohol
might occur and, therefore, that the Ru(III)(L)(OH)-salen complex
(L = a donating ligand) would be a desired catalyst for the aerobic
oxidation of alcohols under non-irradiated conditions. Hence, we
investigated the oxidation catalysis of the Ru(PPh3)(OH)-salen
complex 3.14,15 Herein, we communicate the aerobic oxidation of
various alcohols using 3 as a catalyst under ambient and non-irra-
diated conditions (Table 1).
well in the dark or under irradiation (entries 1 and 2). The reaction
of non-activated primary alcohol, 1-decanol, also proceeded
smoothly, irrespective of the reaction conditions (entries 3 and
4). Against our expectation, catalytic activity of 3 was higher than
that of 1, and the reaction of the activated secondary alcohol also
proceeded smoothly and gave the ketone in good yields over a
slightly extended reaction time (entries 5 and 6). The oxidation
of the non-activated secondary alcohol was much slower (entry
7). Over-oxidation of the aldehyde to carboxylic acid was not ob-
served during the oxidation of primary alcohols. Although 3 oxi-
dized both non-activated primary and activated secondary
alcohols efficiently, we examined chemoselectivity in the alcohol
oxidation using complex 3 under non-irradiated conditions.
Thus, we first examined the relative reaction rate between 1-
decanol and 1-phenylethanol in various solvents at room temper-
ature (Table 2, entries 1–4). To our delight, high selectivity [initial
reaction ratio (=IRR) > 50] was obtained in the reaction using chlo-
roform as the solvent. Use of other solvents slightly reduced the
chemoselectivity. The other competitive oxidation reactions also
proceeded with high IRR values greater than 50 under the same
conditions (entries 5 and 6). Competitive oxidation between non-
activated primary and secondary alcohols or activated primary
and secondary alcohols also proceeded with IRR values greater
than 50 (entries 7 and 8). This high chemoselectivity may be attrib-
uted to the greater ease of the coordination of primary alcohols,
compared to that of secondary alcohols.16
We first examined the oxidation of benzyl alcohol using com-
plex 3 as the catalyst. As expected, the reaction proceeded equally
Encouraged by these results, we further examined the oxidation
of 1-phenyl-1,n-diols (Table 3). The reaction of 1-phenylpropan-
1,3-diol was slow. Although 3-hydroxyaldehyde was obtained as
the major product at 42% conversion, a complex mixture was pro-
duced as the reaction time was extended due to the undesired side
reactions (entry 1). On the other hand, oxidation of the other 1,n-
diols (n = 4 and 5) gave the corresponding lactols exclusively,
although the formation of small amounts (64% yields) of dicar-
bonyl compounds was detected (entries 2 and 3). The reaction of
1,6-diol also gave the aldehyde with high selectivity (aldehyde/
keto-aldehyde=>50) at 38% conversion, and the hydroxy aldehyde
was the major product even at the extended reaction time (entries
4 and 5). Formation of a hydroxy ketone was not detected in these
reactions (entries 1–5). It is of note that the reaction with the
Ru(O@PPh3)(OH)-salen complex showed modest regioselectivity
(entry 6), suggesting that the PPh3 ligand does not dissociate from
the ruthenium ion during the oxidation.
NO
Ru
X
N
O
N
Ru(NO)-salen
O
RCH2OH
NO
P1=H, P2=non
or
hν
P1=non, P2=H
H
H
H
R
R
H
OP1
IVN
O2, hν
OH
IIIN
O•
OP2
N
N
O
SET
Ru
X
Ru
O
O
O
X
RDS (X=Cl)
RCHO
RCH2OH
H
H
R
H
R
OP1
-H2O2
HAT
O
O•
OP2
N
O
IIIN
N
O
IIIN
Ru
X
Ru
X
O
O
In conclusion, we synthesized the Ru(PPh3)(OH)-salen complex
3, based on the analysis of the mechanism of the (ON)Ru(salen)-
catalyzed oxidation. This complex was found to be excellent cata-
lyst for the primary selective aerobic oxidation of alcohols to the
aldehydes under non-irradiated conditions. Oxidation of 1-phe-
RDS (X=OH)
Scheme 2. Proposed mechanism for Ru(NO)-salen-catalyzed aerobic oxidation of
alcohols. The salen ligand is omitted for clarity, except for donor atoms and
ethylene carbons.

3434
H. Mizoguchi et al. / Tetrahedron Letters 50 (2009) 3432–3435
Table 2
Primary selective aerobic oxidation of alcohols using complex 3 as the catalysta
Entry
Solvent
Alcohols
Yield (%)b RCHO/RCOR0
IRRc
1
CHCl3
CH2Cl2
Hexane
AcOEt
CHCl3
CHCl3
CHCl3
CHCl3
1-Decanol/1-phenylethanol
1-Decanol/1-phenylethanol
1-Decanol/1-phenylethanol
1-Decanol/1-phenylethanol
1-Decanol/1-(4-methoxy-phenyl)ethanol
1-Decanol/1-(4-chlorophenyl)ethanol
1-Decanol/2-decanol
93/8
>50
50
34
2d
3d
4d
5d
6d
7
85/9e
90/11e
94/12e
78/5
30
>50
>50
>50
>50
80/4
>99/1
99/2
8
Benzyl alcohol/1-phenylethanol
a
Reaction was run in solvent (5.0 mL) with alcohols (0.5 mmol each) and complex 3 (10.0 lmol) at room temperature under air.
b
c
Determined by 1H NMR analysis using phenanthrene as the internal standard.
IRR value was calculated based on the yields of aldehydes and ketones at ca. 20% conversion: IRR = (% yield of aldehyde)/(% yield of ketone) ꢀ 100.
d
e
The reaction was run on a 0.1 mmol scale.
Determined by GLC analysis using bicyclohexyl as the internal standard.
Table 3
Ru-salen complex 3-catalyzed intramolecular selective aerobic oxidation of diolsa
Entry
1
Diols
OH
Product
Time (h)
5
Conv.b (%)
42
Yieldb (%)
41
OH
Ph
Ph
OH
OH
O
O
OH
OH
2
3
4
5
5
6
1
2
>99
99
96 (4)c
Ph
OH
OH
Ph
Ph
OH
95 (4)c
O
Ph
OH
OH
OH
OH
38
38 (<0.5)c
73 (5)c
OH
OH
OH
O
O
O
Ph
Ph
Ph
Ph
Ph
Ph
OH
OH
78
6d,e
6
>99
51 (23)c
a
Reaction was run in CDCl3 (5.0 mL) with 0.5 mmol of substrate and complex 3 (10.0
Determined by 1H NMR analysis using phenanthrene as the internal standard.
Yield of keto-aldehyde.
The reaction was run on a 0.1 mmol scale.
Using 2 mol% of Ru(O@PPh3)(OH)-salen complex as a catalyst.
lmol) at room temperature under air.
b
c
d
e
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nyl-1,n-diols also proceeded to give the corresponding lactols or n-
hydroxy aldehyde in good yields.
The typical experimental procedure for the aerobic oxidation of
alcohols using 3 as the catalyst is as follows, First, to a 2.0 M solu-
tion of Ru(PPh3)(OH)-salen complex in CHCl3 (5 mL) was added
benzyl alcohol (54.0 mg, 0.5 mmol). The mixture was stirred for
1.5 h under ambient conditions, concentrated by rotary evapora-
tion, and chromatographed on silica gel (hexane/AcOEt = 5:1) to
give benzaldehyde (50.8 mg, 93%).
Acknowledgment
Financial support (Specially Promoted Research 18002011 and
the Global COE Program, ‘Science for Future Molecular Systems’)
from a Grant-in-Aid for Scientific Research from MEXT, Japan, is
gratefully acknowledged.
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15. Complex
3 could not be synthesized from Ru(PPh3)3Cl2 according to the
reported procedure (Ref. 14); however, it was synthesized from
Ru(NO)Cl3(PPh)2 with a slight modification of Sessoli’s procedure (Ref. 14g):
Salen-H2 (110 mg, 0.2 mmol) and Ru(NO)Cl3(PPh)2 (175 mg, 0.26 mmol) were
suspended into 20 mL of EtOH. To the suspension was added 0.2 mL of
ethyldiisopropylamine. The mixture was heated at reflux. After 24 h, the
solvent was removed by rotary evaporation. The resulting residue was
chromatographed on silica gel (CH2Cl2/EtOH/AcOEt = 20:1:2) to afford
Ru(PPh3)(OH)-salen complex 3 (34.5 mg, 19%).
16. In 1H NMR analysis, the
a- and b-protons of 1-decanol and of 1-phenyletanol
appear at 3.65 and 1.57 ppm and at 4.89 and 1.50 ppm, respectively. When
complex 3 (1 equiv) was added to the CDCl3 solution of these alcohols (1 equiv
each) under nitrogen atmosphere, all the signals of the alcohols were
broadened, and the signals at 3.65 and 1.57 shifted upfield by ca. 0.01 and
0.02 ppm after 3 h and by 0.02 and 0.03 ppm after 7 h. No more shift was
observed, even if the measuring time was extended. On the other hand, the
signals at 4.89 and 1.50 ppm did not shift. These 1H NMR data suggest that the
coordination of 1-deacnol to ruthenium ion is kinetically and
thermodynamically more favorable than that of 1-phenylethanol. It should
be noted that the aforementioned NMR data depend on the catalyst used.
Freshly prepared catalyst was used for this experiment.
6049–6052.
ꢁ
14. Ru(PPh3)2-salen and Ru(PPh3)(X)-salen complexes [X = Cl, OAc, Cl, ClO4, NO3
]
prepared from Ru(PPh3)3Cl2 have been used as the catalyst for the coupling of
carbon dioxide and epoxides, aldehyde olefination, cyclopropanation, and
amidation of silyl enol ether: (a) Murray, K. S.; Bergen, A. M.; West, B. O. Aust. J.