TEMPO-mediated oxidation of primary alcohols to aldehydes under visible light and air

Article abstract of DOI:10.1002/cjoc.201300541

A homogeneous visible light photoredox TEMPO-mediated selective oxidation of primary alcohols to the corresponding carbonyl compounds was developed using molecular oxygen from air as the terminal oxidant. Ru(bpy)₃(PF ₆)₂ (bpy: bipyridyl) and Ir(dtb-bpy)(ppy) ₂(PF₆) (dtb-bpy: 4,4′-di-tert-butyl-2,2′- bipyridyl; ppy: 2-phenylpyridine) were used as the sensitizers. A homogeneous visible light photoredox TEMPO-mediated selective oxidation of primary alcohols to the corresponding carbonyl compounds was developed. Molecular oxygen from air was the terminal oxidant. Copyright

Full text of DOI:10.1002/cjoc.201300541

COMMUNICATION
DOI: 10.1002/cjoc.201300541
TEMPO-Mediated Oxidation of Primary Alcohols to Aldehydes
under Visible Light and Air
Dongwang Liu, Hongxia Zhou, Xiangyong Gu, Xiaoqin Shen, and Pixu Li*
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering,
and Materials Science, Soochow University, 199 RenAi Road, Suzhou, Jiangsu 215123, China
A homogeneous visible light photoredox TEMPO-mediated selective oxidation of primary alcohols to the cor-
responding carbonyl compounds was developed using molecular oxygen from air as the terminal oxidant.
Ru(bpy) (PF ) (bpy: bipyridyl) and Ir(dtb-bpy)(ppy) (PF ) (dtb-bpy: 4,4'-di-tert-butyl-2,2'-bipyridyl; ppy: 2-phe-
3
6 2
2
6
nylpyridine) were used as the sensitizers.
Keywords alcohol, aldehyde, photooxidation, TEMPO, molecular oxygen
[
11]
Introduction
cently.
TEMPO is believed to be oxidizable in
photoredox cycles sensitized by Ru/Ir polybipyridyl
complexes or organic dyes.
The selective oxidation of primary alcohols to the
corresponding aldehydes is one of the most important
and widely used reactions in organic synthesis. Nu-
merous selective reagents were developed for this trans-
[12]
Although it is well ac-
cepted that the real oxidant in most TEMPO catalyzed
oxidation processes is the oxammonium salt, to the best
of our knowledge, oxidizing TEMPO to the oxammo-
nium salt under homogeneous photoredox conditions
and using it for synthetically useful reactions has not
been reported. Herein, we report a selective areobic al-
cohol oxidation involving visible-light photoredox cycle
and TEMPO/oxammonium oxidation cycle (Figure 1).
[
1]
[
2]
formation, including pyridium chlorochromate (PCC),
[
3]
[4]
activated DMSO,
Dess-Martin periodinane,
and
[
5]
TEMPO/NaClO. However, many of the methods often
use stoichiometric expensive oxidants and generate a
large amount of waste. Aerobic oxidation has obvious
advantages over traditional oxidations because it uses
dioxygen as the terminal oxidant. However, molecular
oxygen is rarely utilized to directly oxidize substrates
because of its high activation energy. Inserting an elec-
tron-transfer mediator between the redox catalyst and
visible light
[Ru]2+*
HO
2
Ru]2+
[
2
O is the most common bio-mimic method to circum-
N
O
[
6]
vent the problem. The stable free radical 2,2',6,6'-
tetramethylpiperidine N-oxyl (TEMPO) was reported to
be an excellent electron-transfer mediator for aerobic
O2
O2
[Ru]+
[
5c,7]
oxidations of alcohols to carbonyl compounds.
interesting heterogeneous dye-sensitized TiO
An
O2
2
and
N
OH
TEMPO system was reported to selectively oxidize al-
cohols to the corresponding aldehyde under visible-light
N
O
[8]
irradiation and 0.1 MPa O
2
atmosphere. Another dye-
sensitized ZnO and TEMPO system used silver nitrate
[9]
RCHO
2
RCH OH
as the terminal oxidant for the same transformation.
Photooxidation of benzyl alcohol to aldehyde was
Figure 1 Visible-light photoredox aerobic alcohol oxidation.
[
10]
achieved using immobilized flavins without TEMPO.
It is well studied that dioxygen is easily activated
under photochemical conditions and participates in
various oxidation reactions. Many elegant visible-light
photoredox aerobic oxidations catalyzed by Ru/Ir poly-
bipyridyl complexes have appeared in the literature re-
Results and Discussion
Our investigation started with an open-vessel
p-methoxybenzyl alcohol oxidation catalyzed by
Ru(bpy) (PF ) and TEMPO under a 14 W household
3 6 2
*
E-mail: lipixu@suda.edu.cn; Tel.: 0086-0512-65880826
Received July 14, 2013; accepted September 26, 2013; published online November 4, 2013.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201300541 or from the author.
Chin. J. Chem. 2014, 32, 117—122
© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
117

Liu et al.
COMMUNICATION
compact fluorescent lamp as the visible-light source at
room temperature (Table 1). The first reaction only af-
forded 4% of the desired p-methoxybenzyl aldehyde
after 72 h (Entry 1). As TEMPO oxidation of alcohols is
often accelerated under basic conditions, a variety of
bases were screened (Entries 2－8). To our delight, the
addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
significantly improved the efficiency of the reaction.
p-Methoxybenzyl aldehyde was formed nearly quantita-
tively without observable over-oxidation to the corre-
sponding carboxylic acid. A survey of solvents showed
that MeCN was the optimal choice (Entries 9－12). The
oxidation did not occur or only occurred moderately in
(3) The reaciton occurred at room temperature. Visible
light was the driving force of the reaction with no
special lab equipment involved; (4) despite the relative-
ly slow reaction rate, the reaction is very high yielding;
(5) all of the reagents and catalysts involved in the pro-
cess are commercially available and inexpensive. There-
fore, it was decided to investigate this oxidation scope
on different primary alcohols. The results are summa-
rized in Table 2. Good yields were obtained when the
Table 2 Scope survey of Ru(II)/TEMPO catalyzed aerobic
a
oxidation
14 W CFL, air
the absence of either Ru(bpy)
3
(PF
6
)
2
(Entry 13), visible
Ru(bpy)3(PF6)2 (2 mol%)
TEMPO (20 mol%)
R
R
light (Entry 14), TEMPO (Entry 15), or air (Entry 16),
suggesting that the four elements were essential to the
reaction.
CH OH
CHO
2
DBU, MeCN, r.t.
b
Entry Alcohol
Aldehyde
Yield /%
a
OMe
OMe
Table 1 Optimization of aerobic oxidation of benzyl alcohol
1
89
1
4 W CFL, air
CH OH
CHO
OMe
2
Ru(bpy) (PF ) (2 mol%)
3
6 2
TEMPO (20 mol%)
MeO
CH OH
MeO
CHO
2
OMe
base, solvent, r.t.
2
3
4
70
91
81
b
Entry
Base
Solvent
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
Yield /%
CH OH
CHO
c
2
1
2
3
4
5
6
7
8
9
－
4
NMI
5
CH2OH
CHO
CHO
lutidine
colidine
DIEA
DABCO
2
2
CH OH
2
2
7
t
NaOBu
19
98
47
13
－
－
－
－
10
9
CH OH
CHO
CHO
2
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
5
6
79
10
DMSO
CH NO
Br
Br
11
3
2
CH OH
2
66(9)
1
1
1
1
2
3
4
5
6
H O
2
d
e
f
MeCN
MeCN
MeCN
MeCN
Br
Br
CH2OH
Br
Br
CHO
CHO
7
8
47(35)
52(19)
g
1
CH OH
2
a
Reaction conditions: p-methoxybenzyl alcohol (1 mmol),
Ru(bpy) (PF ) (2 mol%), TEMPO (20 mol%), DBU (1 equiv.),
3
6 2
Cl
Cl
solvent (10 mL), open to air at room temperature, irradiated under
b
CH OH
CHO
a 14 W CFL for 48 h. GC yield. The isolated yields of carbox-
2
9
1
54(15)
57(16)
c
d
ylic acid are in parentheses. Reacted for 72 h. No catalyst was
e
f
g
added. In the dark. No TEMPO was added. In a sealed reac-
CH OH
CHO
2
tion vessel under N atmosphere.
2
0
Cl
Cl
This aerobic oxidation of primary alcohols to alde-
hydes was very attractive for a number of reasons: (1)
the use of dioxygen as the terminal oxidant undoubtedly
has advantages to many traditional stoichiometric oxi-
dants; (2) compared to pure oxygen, air is safer and
more accessible; pure oxygen tanks are not often stored
in laboratories, thus making the reaction more practical;
1
1
CH OH
CHO 40
Reaction conditions: p-methoxybenzyl alcohol (1 mmol),
Ru(bpy) (PF (2 mol%), TEMPO (20 mol%), DBU (1 equiv.),
MeCN (10 mL), open to air at room temperature, irradiated under
a 14 W CFL for 48 h. GC yield. The isolated yields of carbox-
2
a
3
6 2
)
b
ylic acid are in parentheses.
118
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© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2014, 32, 117—122

TEMPO-Mediated Oxidation of Primary Alcohols to Aldehydes
benzyl alcohol has electron-donating groups attached to
the benzene ring (Entries 1－6). However, the alde-
hydes were formed in moderate yields when the ben-
zene rings were substituted with electron-withdrawing
groups. Moreover, significant over-oxidation occurred
to generate the corresponding carboxylic acids (Entries
over-oxidation to carboxylic acid was detected. To our
delight, changing the catalyst to Ir(dtb-bpy)(ppy) (PF
2
6
)
resulted in the formation of 4-bromobenzaldehyde in
nearly quantitative yield without observable over-
oxidation (Entry 4). Control reactions revealed that
visible light, iridium catalyst, and TEMPO were all es-
sential to the reaction. No reaction occurred without
either of them (Entries 5－7). Phase transfer agent
(TABCl) had some impact on the reaction. The yield
was lowered to 89% without TABCl (Entry 8). On the
other hand, the reaction was almost not affected by
eliminating either NaHCO or K CO (Entries 9－10).
7
－11). Aliphatic alcohols did not work well in the re-
action. Oxidation of octanol only afforded 40% of
octanal (Entry 12).
Although the visible-light TEMPO-mediated aerobic
oxidation of alcohol to aldehyde possesses many attrac-
tive features, the reaction under the current conditions
had a very limited scope of substrates. Only benzyl al-
cohols with neutral or electron-donating groups worked
well. Therefore, our effort in searching for a more selec-
tive reaction with wider substrate scope continued. In
the literature, many successful TEMPO-catalyzed selec-
tive oxidations were carried out in biphasic solvent sys-
3
2
3
However, the yield of the reaction was reduced to 62%
without any base (Entry 11). The yield went down fur-
ther to 22% when neither base nor TABCl was added
(
Entry 12). As a result, the optimized reaction condi-
tions were identified as using Ir(dtb-bpy)(ppy) (PF ) (2
2
6
mol%), TEMPO (20 mol%), TBACl (20 mol%) and
NaHCO (0.5 equiv.) in CH Cl /H O biphasic system
[
5a,13]
tems.
Because benzyl alcohols bearing electron-
3
2
2
2
withdrawing groups were more prone to over-oxidation,
we did the oxidation of 4-bromobenzyl alcohol with
under a 14 W CFL irradiation (Entry 5). An air balloon
was used to prevent the solvent from evaporating during
the course of the reaction.
Ru(bpy)
tylammonium chloride (TBACl, 20 mol%) in a biphasic
dichloromethane-aqueous NaHCO /K CO buffer sys-
3 6 2
(PF ) (2 mol%), TEMPO (20 mol%), tetrabu-
Thus, we have discovered a visible light promoted
Ir(III)/TEMPO-catalyzed aerobic oxidation of an alco-
hol to an aldehyde at near quantitative yield and with
excellent selectivity. We then turned our attentions to
investigating the scope of this reaction on different al-
cohols. The results are summarized in Table 4. When
the benzene rings of the benzyl alcohols were substi-
tuted with one weak electron-withdrawing group, high
yields of aldehydes were afforded with excellent selec-
tivities no matter where the substituent group was lo-
cated (Entries 1－6). The dihalogen-substituted benzyl
alcohol worked well unless the two substituent groups
were at ortho positions (Entries 7－9). The di-ortho-
substituted benzyl alcohol reacted very slowly. The GC
conversion for 2,6-difluorobenzyl alcohol was 79% af-
ter 96 h, and that of 2,6-dichlorobenzyl alcohol was
3
2
3
tem. The reaction system was open to air and irradiated
under a 14 W CFL for 24 h. Unfortunately, no conver-
sion of alcohol to aldehyde occurred (Table 3, Entry 1).
When the catalyst was switched to either Ir(ppy) or
3
rose bengal, a moderate yield of the aldehyde was ob-
served (Entries 2 and 3). The good news was that no
Table 3 Optimization of aerobic alcohol oxidation in biphasic
a
solvent system
14 W CFL, air
Cat. (2 mol%)
TEMPO (20 mol%)
Br
CH OH
Br
CHO
2
TBACl, NaHCO , K CO
3
3
2
CH Cl /H O=1/1
2
2
2
Entry Catalyst
Other conditions GC yield/%
6
0% after 96 h (Entries 10, 11). Nonetheless, no corre-
1
2
3
4
5
6
7
8
9
1
Ru(bpy)
Rose Bengal
Ir(ppy)
Ir(dtb-bpy)(ppy) (PF )
3
(PF
6
)
2
NR
15
sponding carboxylic acids were observed in either reac-
tion. It may be that the steric effects slowed down both
oxidation to aldehyde and over-oxidation to acid. How-
ever, the system did not work well with benzyl alcohols
bearing strong electron-withdrawing groups, such as
nitro or sulfonyl groups. Aldehydes were formed in
3
17
2
6
＞99
NR
No catalyst
Ir(dtb-bpy)(ppy) (PF ) No TEMPO
2
6
NR
NR
89
5
9% and 74% respectively along with around 20% of
Ir(dtb-bpy)(ppy) (PF ) No light
2
6
the corresponding carboxylic acids (Entries 12, 13). For
benzyl alcohols without substitution or substituted with
various electron-donating groups, the oxidation reac-
tions uniformly proceeded with excellent yields and
selectivities (Entries 14－22). Cinnamyl alcohol af-
forded cinnamaldehyde in quantitative yield (Entry 23).
The oxidation reaction worked with primary aliphatic
alcohols as well. 1-Octadecanol was quantitatively oxi-
dized to the corresponding aldehyde (Entry 24). Secon-
dary alcohols were also tested under the same reaction
conditions. Diphenylmethanol afforded diphenylketone
Ir(dtb-bpy)(ppy) (PF ) No TBACl
2
6
Ir(dtb-bpy)(ppy)
2
(PF
6
)
No K
2
CO
3
＞99
96
0
Ir(dtb-bpy)(ppy) (PF ) No NaHCO3
2 6
11
Ir(dtb-bpy)(ppy) (PF ) No base
62
2
6
1
a
2
Ir(dtb-bpy)(ppy) (PF ) No base or TBACl
22
2
6
Reaction conditions: p-bromobenzyl alcohol (0.5 mmol), cata-
lyst (2 mol%), TEMPO (20 mol%), TBACl (20 mol%), NaHCO3
0.5 equiv.), K CO (0.05 equiv.), CH Cl (5 mL), and H O (5
(
2
3
2
2
2
mL), open to air at room temperature, irradiated under a 14 W
CFL for 24 h.
Chin. J. Chem. 2014, 32, 117—122
© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
119

Liu et al.
COMMUNICATION
in 96% yield after 120 h (Entry 25). Aliphatic secondary
alcohol, such as cyclohexanol only resulted in 17%
yield of cyclohexanone after 120 h (Entry 26). Thus,
good selectivity could be achieved when primary and
secondary hydroxyl groups co-exist in the same mole-
cule. To demonstrate that the reaction is scalable, a re-
action of benzyl alcohol (10 mmol) was carried out. The
desired benzaldehyde was obtained in 92% GC yield
after 5 d under the optimized reaction conditions. No
over-oxidized benzoic acid was observed.
Continued
b
Entry
Aldehyde
Time/h
48
Yield /%
O
S
13
14
15
CHO
74 (20)
91
O
CHO
48
48
CHO
CHO
95
Table 4 Substrate scope survey of Ir(III)/TEMPO-catalyzed
aerobic alcohol oxidation
1
6
48
＞99
a
14 W CFL, air
Ir(dtb-bpy)(ppy) (PF ) (2 mol%)
2
6
R
R
17
18
MeO
CHO
48
72
97
TEMPO (20 mol%)
CH OH
CHO
2
TBACl, NaHCO , CH Cl /H O, r.t.
3
2
2
2
OMe
b
＞99
Entry
1
Aldehyde
Time/h
24
Yield /%
CHO
CHO
Br
CHO
＞99
1
2
9
0
48
48
＞99
＞99
Br
OMe
2
3
4
5
48
72
72
24
＞99
CHO
CHO
MeO
CHO
Br
Cl
97 (3)
85 (2)
＞99
OMe
OMe
Cl
MeO
MeO
CHO
CHO
2
1
2
72
48
＞99
CHO
2
F3CO
CHO
CHO
95 (1)
CHO
CF3
Cl
6
7
8
9
48
48
24
48
96
87 (1)
96 (5)
92 (＜1)
90 (11)
71
23
48
48
＞99
＞99
2
2
4
5
CH (CH ) CHO
3
2 16
Cl
F
CHO
CHO
O
c
120
120
96
Cl
O
2
6
17
CHO
Br
a
F
Reaction condition: p-bromobenzyl alcohol (1 mmol), Ir(dtb-
bpy)(ppy) (PF ) (2 mol%), TEMPO (20 mol%), TBACl (20
mol%), NaHCO (0.5 equiv.), CH Cl (10 mL), and H O (10 mL),
2
6
F
3
2
2
2
CHO
F
open to air at room temperature, irradiated under a 14 W CFL.
1
0
b
GC yield. The HPLC yields of carboxylic acid are in parenthe-
c
ses. HPLC yield.
Cl
CHO
Cl
Conclusions
1
1
96
47
In conclusion, we have developed a homogeneous
visible light photoredox TEMPO-mediated aerobic se-
lective oxidation of primary alcohols to aldehydes. Both
of the two reaction conditions utilize the most accessible,
1
2
O2N
CHO
120
59 (23)
120
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© 2014 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2014, 32, 117—122

TEMPO-Mediated Oxidation of Primary Alcohols to Aldehydes
virtually unlimited, and free molecular oxygen from air
as the terminal oxidant. The first set of reaction condi-
tions uses relatively inexpensive Ru(bpy) (PF ) as the
3 6 2
photo sensitizer. It worked well with benzyl alcohols
bearing electron-donating groups on the benzene ring.
The second set of oxidation conditions uses Ir(dtb-
Acknowledgement
Financial support is provided by National Natural
Science Foundation of China (No. 21102097), the Spe-
cialized Research Fund for the Doctoral Program of
Higher Education (No. 20103201120006), and Beijing
National Laboratory for Molecular Sciences (BNLMS).
2 6
bpy)(ppy) (PF ) as the photo sensitizer in a biphasic
solvent system. Excellent yields and selectivities were
achieved with various substituted benzyl alcohols.
Benzyl alcohols bearing strong electron-withdrawing
substituents, such as nitro and sulfonyl groups, on the
benzene ring did not work well. The reaction was ex-
tended to aliphatic primary alcohols. Preliminary results
showed that good selectivity could be achieved between
primary and secondary alcohols. Both oxidation condi-
tions were catalytic and occurred under very mild con-
ditions. No stoichiometric amounts of hazardous waste
were generated. They are green and practical alterna-
tives to the existing selective primary alcohol oxidation
methods.
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Ru(bpy) (PF ) (0.02 mmol). The reaction mixture was
placed 5 cm from a 14 W CFL bulb and stirred at room
temperature under air. Upon completion, as monitored
by GC, the reaction was quenched with ethyl acetate (10
mL) and water (10 mL). Layers were separated. The
aqueous layer was extracted with ethyl acetate (20 mL
×
2). The combined organic layers were washed with a
solution of NaI (250 mg) in 10% HCl (20 mL), satu-
rated Na aqueous solution (20 mL), and dried over
2 2 3
S O
anhydrous magnesium sulfate. Column chromatography
on silica gel eluted with petroleum ether (b.p. 30－60
℃
)/diethyl ether or petroleum ether (b.p. 30－60 ℃)/
ethyl acetate afforded the desired aldehyde.
Method B: To a solution of alcohol (1 mmol) and
biphenyl (0.25 mmol, internal standard) in CH
2 2
Cl (10
mL) was added TEMPO (0.2 mmol), 10 mL of an
4
3
8; (o) Hossain, M. M.; Shyu, S. G. Adv. Synth. Catal. 2010, 352,
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−
1
aqueous stock solution of NaHCO
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(0.05 mol•L ) and
−1
TBACl (0.02 mol•L ), and Ir(dtb-ppy)(ppy) (PF ) (0.02
2
6
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