Visible-light-induced selective photocatalytic aerobic oxidation of amines into imines on TiO2

Article abstract of DOI:10.1002/chem.201102779

Imines are important intermediates for the synthesis of fine chemicals, pharmaceuticals, and agricultural chemicals. Selective oxidation of amines into their corresponding imines with dioxygen is one of the most-fundamental chemical transformations. Herein, we report the oxidation of a series of benzylic amines into their corresponding imines with atmospheric dioxygen as the oxidant on a surface of anatase TiO₂ under visible-light irradiation (λ>420 nm). The visible-light response of this system was caused by the formation of a surface complex through the adsorption of a benzylic amine onto the surface of TiO₂. From the analysis of products of specially designed benzylic amines, we demonstrated that a highly selective oxygenation reaction proceeds via an oxygen-transfer mechanism to afford the corresponding carbonyl compound, whose further condensation with an amine would generate the final imine product. We found that when primary benzylic amines (13 examples), were chosen as the substrates, moderate to excellent selectivities for the imine products were achieved (ca. 38-94%) in moderate to excellent conversion rates (ca. 44-95%). When secondary benzylic amines (15 examples) were chosen as the substrates, both the corresponding imines and aldehydes were detected as the main products with moderate to high conversion rates (ca. 18-100%) and lower selectivities for the imine products (ca. 14-69%). When tribenzylamine was chosen as the substrate, imine (27%), dibenzylamine (24%), and benzaldehyde products (39%) were obtained in a conversion of 50%. This report can be viewed as a prototypical system for the activation of C-H bonds adjacent to heteroatoms such as N, O, and S atoms, and oxofuctionalization with air or dioxygen as the terminal oxidant under visible-light irradiation using TiO₂ as the photocatalyst. Go green! Visible-light irradiation of a surface complex of benzylic amines adsorbed onto a TiO₂ surface induces charge-transfer; an oxygen-transfer mechanism affords the corresponding carbonyl compounds and condensation with an amine generates the imine product. Copyright

Full text of DOI:10.1002/chem.201102779

DOI: 10.1002/chem.201102779
Visible-Light-Induced Selective Photocatalytic Aerobic Oxidation of Amines
into Imines on TiO2
Xianjun Lang, Wanhong Ma, Yubao Zhao, Chuncheng Chen, Hongwei Ji, and
Jincai Zhao*[a]
Abstract: Imines are important inter-
mediates for the synthesis of fine
chemicals, pharmaceuticals, and agri-
cultural chemicals. Selective oxidation
of amines into their corresponding
imines with dioxygen is one of the
most-fundamental chemical transfor-
mations. Herein, we report the oxida-
tion of a series of benzylic amines into
their corresponding imines with atmos-
pheric dioxygen as the oxidant on a
surface of anatase TiO2 under visible-
light irradiation (l>420 nm). The visi-
ble-light response of this system was
caused by the formation of a surface
complex through the adsorption of a
benzylic amine onto the surface of
TiO2. From the analysis of products of
specially designed benzylic amines, we
demonstrated that a highly selective
oxygenation reaction proceeds via an
oxygen-transfer mechanism to afford
the corresponding carbonyl compound,
whose further condensation with an
amine would generate the final imine
product. We found that when primary
benzylic amines (13 examples), were
chosen as the substrates, moderate to
excellent selectivities for the imine
products were achieved (ca. 38–94%)
in moderate to excellent conversion
rates (ca. 44–95%). When secondary
benzylic amines (15 examples) were
chosen as the substrates, both the cor-
responding imines and aldehydes were
detected as the main products with
moderate to high conversion rates (ca.
18–100%) and lower selectivities for
the imine products (ca. 14–69%).
When tribenzylamine was chosen as
the substrate, imine (27%), dibenzyl-
AHCTUNGTRENGNaUN mine (24%), and benzaldehyde prod-
ucts (39%) were obtained in a conver-
sion of 50%. This report can be viewed
as a prototypical system for the activa-
À
tion of C H bonds adjacent to hetero-
atoms such as N, O, and S atoms, and
oxofuctionalization with air or dioxy-
gen as the terminal oxidant under visi-
ble-light irradiation using TiO2 as the
photocatalyst.
Keywords: amines · charge trans-
fer · imines · oxidation · photocatal-
ysis · titanium
Introduction
tive enough to perform organic syntheses at higher concen-
trations under visible-light irradiation.
TiO2 is one of the most-attractive photocatalytic materials
owing to its low cost, excellent chemical and photochemical
stability, and nontoxicity.[1] However, because the band gap
of TiO2 is 3.2 eV (anatase), the TiO2 photocatalyst can only
be activated by UV-light irradiation (l<385 nm). To utilize
the visible range of the solar spectrum, bulk doping with
nonmetals such as H, B, C, N, and S atoms have attracted
much attention.[2] However, such doping sites could act as
recombination centers for the photoexcited charge-carriers.
Thus, the visible-light activities of those TiO2-based
Over the last few years, many efforts have been devoted
to organic redox-transformation reactions using TiO2 photo-
catalysis.[3] However, to date, most of the reported reactions
for the synthetic transformations using TiO2 photocatalysts
were carried out under UV irradiation and were usually as-
sociated with low selectivity.[4] Performing visible-light-in-
duced selective transformations by photocatalysts is a chal-
lenge that has attracted much attention, as shown by some
special homogenous and heterogeneous systems.[5] Recent
discoveries demonstrated that surface modification of TiO2
with noble-metal complexes or nanoparticles rather than
bulk doping might be a better strategy in the pursuit of new
visible-light-responsive photocatalysts that could prompt the
design of efficient redox reactions under visible-light irradia-
tion. In particular, ruthenium-containing dyes[6] and gold
nanoparticles[7] supported on TiO2 have been successfully
used for the reduction of nitrobenzene and the oxidation of
alcohols under visible-light irradiation. However, to drive a
photoredox reaction with a noble-metal-free organic dye as
the antenna molecule is more appealing, provided that the
dye is resilient under the photocatalytic conditions.
photocatACHTUNGTRENNUNGaACHTUNGTRENNUNGlysts are relatively low. They are typically used for
the degradation of very dilute pollutants and are not effec-
[a] Dr. X. Lang, Prof. Dr. W. Ma, Dr. Y. Zhao, Prof. Dr. C. Chen,
Dr. H. Ji, Prof. Dr. J. Zhao
Key Laboratory of Photochemistry
Beijing National Laboratory for Molecular Sciences
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P. R. China)
Fax : (+86) 10-82616495
E-mail: jczhao@iccas.ac.cn
Indeed, we successfully used a commercially available
dye, alizarin red, as the antenna molecule to absorb visible
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102779.
2624
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 2624 – 2631

FULL PAPER
light and drive the redox cycle of 2,2,6,6-tetramethylpiperi-
dinyloxyl (TEMPO) to realize the oxidation of alcohols into
their corresponding carbonyl compounds.[8] TEMPO is es-
sential in the system to prevent degradation of the dye[9] and
autooxidation of the substrates. However, this system is lim-
ited in substrate scope because TEMPO is only an efficient
co-catalyst for the oxidation of alcohols. If the substrates
themselves could act as the antenna to absorb visible light,
the assembly could be simplified significantly. However, in
general, organic substrates do not absorb visible light.
Hence, they cannot play the role of the antenna molecule to
initiate visible-light reactions directly. However, some color-
less substrates adsorbed onto TiO2 would form a surface
complex that could absorb visible light and so initiate elec-
tron-transfer and the ensuing reactions.[10] This property can
be further explored to achieve a more-straightforward solu-
tion for visible-light-induced organic transformations.
Recently, we discovered that oxygen transfer from dioxy-
gen to the substrates dominates the aerobic oxidation of al-
cohols[11] and amines[12] on a TiO2 surface in inert organic
solvents under UV irradiation to afford high selectivities for
the target products. The unique reaction of electrons in the
conduction band of TiO2 plays a dual role of activating the
dioxygen and preventing the unselective autooxidation pro-
cess. Merging these new discoveries into one unified photo-
catalytic system would be a powerful approach for highly se-
lective oxidation reactions under visible-light irradiation;
i.e., under visible-light irradiation, the photoinduced charge-
transfer of the surface complex formed by adsorption of the
substrates onto a TiO2 surface would produce an oxidized
substrate radical and an electron in the conduction band of
TiO2, whereas the unique reaction of an electron in the con-
duction band guarantees a high selectivity for the oxidation
reaction.
Scheme 1. Proposed mechanism for the formation of imines by the oxida-
tion of amines on a TiO2 surface under visible-light irradiation.
such nascent aldehydes by the unreacted amines will thus
yield the corresponding imines (step5). The photoreaction
process does not involve the hole in the valence band of
TiO2, which will help us to further reveal or confirm the
mode of activation and the reaction of dioxygen on the TiO2
surface.
Results and Discussion
To test our hypothesis, benzylamine was chosen as the
probe substrate to carry out the reaction (Table 1). First, we
investigated whether there was visible-light absorption after
the adsorption of benzylamine on the surface of anatase
TiO2, a prerequisite for a visible-light-induced reaction.
From the UV/Vis spectra (Figure 1), we found that the ad-
sorption of benzylamine onto the surface of TiO2 extended
the absorption spectrum of TiO2 into the visible-light range,
which provides the possibility of selective oxidation of ben-
zylamine into an imine with dioxygen under l>420 nm irra-
diation (Table 1).
In control experiments, the reaction did not proceed with-
out a photocatalyst (Table 1, entry 1), dioxygen (Table 1,
entry 2), or visible-light irradiation (Table 1, entry 3). With
1 atm of air as the oxidant, TiO2 as the photocatalyst,
CH3CN as the solvent, and irradiation at l>420 nm, the re-
action proceeded smoothly to form the imine product with
high selectivity (98%) and a conversion of 54% (Table 1,
entry 4). With 2 atm of pure dioxygen as the oxidant, the re-
action rate accelerated slightly (Table 1, entry 5) without
having any influence on the selectivity. As atmospheric di-
oxygen is the best and most abundant oxidant, all of the fol-
lowing reactions were carried out in 1 atm air as the oxidant
unless otherwise stated.
Imines are important intermediates for the synthesis of
fine chemicals, pharmaceuticals, and agricultural chemi-
cals.[13] Herein, we report the oxidation of a series of benzyl-
ic amines into their corresponding imines using atmosphere
dioxygen as the oxidant on TiO2 under visible-light irradia-
tion (l>420 nm). We envisage that this highly selective for-
mation of imines under visible-light irradiation follows the
mechanism in Scheme 1. The adsorption of benzylamine on
TiO2 would form a surface complex,[14] which plays the role
of the antenna to absorb visible light (step 1). Upon visible-
light irradiation, separation of the photogenerated charges
occurs, the holes localize on the oxidized substrate, and elec-
trons localize within the TiO2 lattice. The deprotonation of
the oxidized substrate generates a neutral carbon-centered
radical (step 2). After that, the reaction mechanism should
follow the same path as that under UV irradiation to pro-
duce the aldehyde. The interaction between dioxygen and
the photogenerated free-radical and the electron in the con-
duction band of TiO2 will finish the electron-transfer be-
tween the dioxygen and the substrate radical and the
titaniumACHTUNGTRENNUNG
(III) center (step 3).[11,12] The regeneration of the
surface titanium(VI) sites of TiO2 completes the photocata-
lytic oxidation cycle (step 4). The nucleophilic attack on
Chem. Eur. J. 2012, 18, 2624 – 2631
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2625

J. Zhao et al.
Table 1. The photocatalytic oxidation of benzylamine into an imine with
dioxygen on a TiO2 surface under visible-light irradiation.[a]
cates that the formation of a surface complex is essential for
the photoreaction to proceed under visible-light irradiation.
The influence of the solvent on the reaction was also investi-
gated (Table 1, entries 4, 10–14). The reaction proceeded
well in all of the inert organic solvents examined with little
influence on the selectivity. The solvent did show an influ-
ence on the rate of reaction, and CH3CN was found to be
the best solvent among those examined. According to our
proposed mechanism (Scheme 1), a condensation reaction
(step 5) takes place to afford the imine. Thus, the polarity of
CH3CN should contribute to the formation of the imine.
Note that DMF is the most polar solvent but competitive
adsorption between the solvent and the substrate might
result in the sluggish conversion rate observed (Table 1,
entry 11).
Based on the preliminary results summarized in Table 1,
we are quite optimistic about improving the efficiency of
this photocatalysis reaction, by, for example, using anatase
TiO2 that has an even-larger surface area as the photocata-
lyst, using dioxygen to replace atmospheric air as the termi-
nal oxidant, using other wide-band-gap metal-oxide semi-
conductors that have more-negative conduction bands than
anatase TiO2 as the photocatalyst, surface modification of
the photocatalyst, etc.
To demonstrate the scope of this system, the photocatalyt-
ic oxidation of a variety of amine substrates was investigat-
ed. The results of the oxidation of various primary benzylic
amines on a TiO2 surface with 1 atm air in CH3CN under
visible-light irradiation (l>420 nm) are summarized in
Table 2. The oxidation of benzylamine and its derivatives
proceeded efficiently under visible-light irradiation with
high conversion and high selectivity of their corresponding
imines (Table 2, entries 1–7). The substituents on the ben-
zene ring had a slight influence on the reaction rate and
only a small influence on selectivity. The relatively low se-
lectivity for the oxidation of 3,4-(methylenedioxy)benzyl-
Entry
Photocatalyst
–
Solvent
Conv.
[mol%][b]
Select.
G
ACHTUNGTRENNUNG
1
2
3
4
5
6
7
8
9
10
11
12
13
14
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH2Cl2
DMF
0
0
0
–
–
–
[c]
TiO2
TiO2
[d]
TiO2
TiO2
54
76
35
38
18
43
38
35
45
52
42
98
98
98
98
98
98
97
97
98
94
98
[e]
TiO2 (P25)[f]
TiO2 (6008C)[g]
TiO2 (7008C)[g]
TiO2 (F)[g]
TiO2
TiO2
TiO2
TiO2
TiO2
PhCH3
EtOAc
BTF
[a] Reaction conditions: benzylamine (0.2 mmol), solvent (5 mL), air
(1 atm), TiO2 (anatase, 50 mg; Alfa Aesar, 130 m2 gÀ1), 4h, Xe lamp
(300 W, cutoff wavelengths below 420 nm). [b] Determined by GC analy-
sis using bromobenzene as the internal standard. [c] Under an Ar atmos-
phere (1 atm). [d] 408C thermal reaction without irradiation. [e] O2
(2 atm). [f] Degussa P25. [g] Calcinated at the respective temperatures or
by treatment of hydrofluoric acid (see the Supporting Information).
CH3CN=acetonitrile, DMF=N,N-dimethylformamide, PhCH3 =toluene,
EtOAc=ethyl acetate, BTF=trifluorotoluene.
AHCTUNGTREGaNNNU mine (Table 2, entry 8) into its corresponding imines might
be caused by the activation of the substituents and an ensu-
ing degradation process under these photocatalytic condi-
tions. This system can tolerate the presence of heteroatoms
(such as O, S, and N) on the aromatic ring (Table 2, en-
tries 9–11), which often have detrimental effects on the ac-
tivity of metal-complex catalysts, although a slight drop in
selectivity was observed in comparison with those substrates
without heteroatoms in the aromatic ring (Table 2, en-
tries 1–7). When 1-phenylethylamine and its derivative were
tested in the oxidation reaction (Table 2, entries 12 and 13),
moderate selectivities for the corresponding imines were
achieved with the formation of small amounts of corre-
sponding acetophenone byproducts, thus suggesting that the
oxygen-transfer from dioxygen to the substrates occurs as
predicted (Scheme 1). The relatively low selectivity for the
imine when 1-phenylethylamine and its derivatives were
used as the substrates can be partially ascribed to the rela-
tive difficulty of the condensation reaction between the
newly generated acetophenones and 1-phenylethylamine. As
a result, for those substrates, only moderate selectivities
Figure 1. Adsorption spectra of TiO2 and benzylamine adsorbed onto a
TiO2 surface (for experimental details, see the Supporting Information).
The photocatalytic activities of Degussa P25 TiO2 and dif-
ferent calcined TiO2 samples were also investigated
(Table 1, entries 6–8). Degussa P25 TiO2 usually has the
highest activity for reactions under UV irradiation. Howev-
er, the reaction rates in this reaction were in the order
TiO2 >TiO2 (6008C)>TiO2 (Degussa P25)>TiO2 (7008C),
which correlates well with the surface areas of these photo-
catalysts (see the Supporting Information, Table S1). How-
ever, treatment of the surface of TiO2 with hydrofluoric acid
to partially occupy the interaction sites of TiO2 led to a de-
cease in the reaction rate (Table 1, entry 9); this result indi-
2626
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 2624 – 2631

Photocatalytic Aerobic Oxidation of Amines into Imines on TiO2
FULL PAPER
Table 2. Aerobic oxidation of primary benzylic amines photocatalyzed by TiO2 under visible-light irACHTUNGTRENNNUG ACHTUNGTREGdNUNN iCAHTNUGTRENaNUGN -
aldehyde and the amine, which
is in agreement with the pro-
posed mechanism in Scheme 1,
in which an aldehyde inter-
mediate is present in the forma-
tion of the imine.
ACHTUNGTRENNUNG
tion.[a]
Entry
Substrate
Product
t [h]
Conv.
A
ACHTUNGTRENNUNG
1
2
10
6
The formation of imine in-
volves a selective oxyxgenation
step and a further condensation
step. The overall process can be
simply summarized in Equa-
tion (1). Along this line, the for-
mation of imines from secon-
dary benzylic amines should
proceed according to Equa-
tion (2). As the newly generat-
ed benzylamine could be fur-
ther transformed following
Equation (1). Thus, the oxygen-
ation products, the correspond-
ing aldehydes, should be ob-
served for the oxidation of
these substrates, acting as a cru-
cial mechanistic evidence for
Scheme 1. The results of the ox-
idation of a series of secondary
benzylic amines are summar-
ized in Table 3 (entries 1–5).
Significant amounts of benzal-
dehyde or its derivatives were
observed for all these substrates
(Table 3, entries 1–14), which is
a clear indication that an oxy-
genation process still dominates
the formation of imines under
visible-light irradiation.
78
90
94
94
3
10
4
5
6
5.5
6.5
7
95
84
83
93
88
90
7
8
9
14
6
92
90
77
91
70
86
12
10
11
9
5
77
68
80
38
12[d]
5
5
44
49
53
49
13[d]
For asymmetric dibenzylic
amines (Table 3, entries 6 and
7), in addition to the two corre-
sponding benzaldehydes, four
imines products were observed
for each substrate; this result
[a] Reaction conditions: amine (0.2 mmol), CH3CN (5 mL), air (1 atm), TiO2 (anatase, 50 mg; Alfa Aesar,
130 m2gÀ1), Xe lamp (300 W, cutoff wavelengths below 420 nm). [b] Determined by GC analysis using bromo-
benzene as the internal standard. [c] Selectivity=yield/conversion. [d] About 3% of acetophenones were
found in the product.
(44%, 49%) were achieved at a moderate conversion rate
(53%, 49%).
The reaction profile for the oxidation of benzylamine is
shown in Figure 2. From the experimental results, we can
conclude that the reaction obeys zero-order kinetics. At rel-
atively low conversion rates, the benzylamine could be
transformed into the corresponding imine with excellent se-
lectivity. However, with a prolonged reaction time and a
higher conversion of benzylamine, a slight decrease in the
selectivity for the imine would occur and small amounts of
benzaldehyde appeared in the products. This result is be-
cause the increased consumption of benzylamine in the solu-
tion could not ensure the full condensation of the nascent
further confirms the sole involvement of an oxygenation
step leading to the final product distribution rather than the
involvement of an an oxidative dehydrogenation process.
When N-benzyl-1-phenylethylamine was chosen as the sub-
strate (Table 3, entry 8), benzaldehyde, acetophenone, and
imine products were all observed; these products were pre-
Chem. Eur. J. 2012, 18, 2624 – 2631
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2627

J. Zhao et al.
39%, 24%, and 27%, respectively. The formation of the
corresponding imine from tribenzylamine also proceeded
through an oxygenation step followed by condensation
which agrees well with the mechanism in Scheme 1.
As the product formation is governed by the proposed
mechanism in Scheme 1, it is reasonable to obtain excellent
selectivities and relatively low conversion rate (Table 1). It
is for the same reason that we observed very good selectivi-
ty and very high conversation rates for the reactions shown
in Table 2 (except for a few cases that might be caused by
instability of the imine product, such as Table 2, entry 11),
even though they are slight lower than the results in Table 1.
Furthermore, the relationship of conversion versus selectivi-
ty allows us to understand the results for the substrates in
Table 3 according to the mechanism in Scheme 1. As a
result, we can only achieve moderate conversion rates and
moderate selectivities for the imines products in Table 3.
Figure 2. Reaction profiles for the oxidation of benzylamine on a TiO2
surface under visible-light irradiation. Conversion of benzylamine (&),
selectivity for the imine (*), selectivity for benzaldehyde (~). Reaction
conditions: benzylamine (0.2 mmol), CH3CN (5 mL), TiO2 (anatase,
50 mg; Alfa Aesar, 130 m2 gÀ1), Xe lamp (300 W, cutoff below 420 nm),
air (1 atm).
sumably formed by the activation of the benzylic carbon–hy-
drogen atoms on both sides of the nitrogen atom. The oxida-
tion of N-benzyl-4-methoxyaniline (Table 3, entry 9), in
which only one benzylic carbon atom is present, afforded
benzaldehyde and 4-methoxyaniline products; the difficulty
of the condensation reaction between benzaldehyde and 4-
methoxyaniline prevented the formation of the imine.
As an aliphatic amine could be further partially degraded
under these conditions, benzaldehyde was present in excess.
Because the corresponding aliphatic radicals lack the re-
À
Conclusion
We have discovered that a series of benzylic amines ad-
sorbed onto the surface of TiO2 could extend light absorp-
tion into the visible range. This feature can be used for the
aerobic oxidation of amines into their corresponding imines
under visible-light irradiation in inert solvents. In addition
to the synthetic significance, this finding gave us a unique
opportunity to understand the concerted interactions of di-
oxygen with photogenerated free-radicals and electrons on
the conduction band of TiO2, excluding the influence of
holes in the valence band of TiO2 under UV irradiation,
which give us a clearer picture of the activation of dioxygen
and its influence on the selectivity of the products in TiO2
photocatalysis. The formation of imines proceeds via an oxy-
genation pathway rather than an oxidative-dehydrogenation
process; in this oxygenation pathway, oxygen-transfer from
dioxygen to the substrates is the main oxidation step under
visible-light irradiation. These findings will deepen our un-
derstanding of the role of dioxygen in TiO2 photocatalysis
and will offer a general mechanistic framework for aerobic-
oxidation reactions involving semiconductor photocatalysis.
Furthermore, new heteroatom-containing substrates might
be oxofunctionalized in dioxygen or in air on TiO2 surfaces
under visible-light irradiation.
quired stability under the photocatalytic conditions, Ca
H
activation of the aliphatic arm only affords benzylamine,
which can be further oxidized according to Equation (1) to
À
produce the other imine. However, Ca H activation is pre-
ferred at the benzyl group and this pathway affords the ma-
jority of the observed products, i.e. benzaldehyde and the
imine (formed by condensation between benzaldehyde and
either benzylamine or an aliphatic amine; Table 3, en-
À
tries 10–12). For substrates without a Ca H bond on the ali-
À
phatic arm (Table 3, entry 13), only the Ca H bond on the
benzyl arm can be activated to selectively generate benz-
ACHTUNGTRENNUNGaldehyde and one imine, which is formed from a coupling
reaction between benzaldehyde and tert-butylamine. We
used a cyclopropyl group as the intramolecular radical trap
to verify the radical nature of this reaction (Table 3,
entry 14); only benzaldehyde and N-benzylidenebenzyla-
mine were detected as the final products and no cyclopropyl
groups were found intact, thereby suggesting that the reac-
tion proceeds through a radical intermediate. The oxidation
of 1,2,3,4-tetraACHTUNGTRENNUNGhydroquinoline afforded 3,4-dihydro-quino-
line with a relative high selectivity (68%; Table 3, entry 15).
According to Scheme 1, when tribenzylamine was chosen
as the substrate, the product profiles should follow Equa-
tion (3). Benzaldehyde, dibenzylamine, and N-benzylidene-
benzylamine were the main products with selectivities of
2628
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 2624 – 2631

Photocatalytic Aerobic Oxidation of Amines into Imines on TiO2
FULL PAPER
Table 3. Aerobic oxidation of secondary and tertiary benzylic amines on a TiO2 surface under visible-light irradiation.[a]
Entry
Substrate
t [h]
Conv.
[mol%][b]
Products
N
ACHTUNGTRENNUNG
1
2
8
8
81
66
3
8
44
4
5
8
8
75
70
6
5
79
7
8
5
8
74
62
9
10
11
5
4
4
18
67
36
12
13
4
4
63
69
14
15
4
5
100
100
Chem. Eur. J. 2012, 18, 2624 – 2631
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2629

J. Zhao et al.
Table 3. (Continued)
Entry
Substrate
t [h]
Conv.
[mol%][b]
Products
ACHTUNGTRENNUNG
[mol%][c]
U
16
8
50
[a] Reaction conditions: amine (0.1 mmol), CH3CN (5 mL), air (1 atm), TiO2 (anatase, 50 mg; Alfa Aesar, 130 m2 gÀ1), Xe lamp (300 W, cutoff wave-
lengths below 420 nm). [b] Determined by GC analysis using bromobenzene as the internal standard. [c] Determined by GC analysis using bromoben-
zene as the internal standard; values in parentheses show the selectivity for the desired product (yield/conversion).
157–170; d) S. Yurdakal, G. Palmisano, V. Loddo, O. Alagçz, V. Au-
gugliaro, L. Palmisano, Green Chem. 2009, 11, 510–516; e) G. Palm-
Experimental Section
isano, E. Garcia-Lopez, G. Marci, V. Loddo, S. Yurdakal, V. Augu-
gliaro, L. Palmisano, Chem. Commun. 2010, 46, 7074–7089; f) V.
Augugliaro, L. Palmisano, ChemSusChem 2010, 3, 1135–1138; g) V.
Materials and instruments: All reagents and solvents were obtained com-
mercially and used without further purification. GC measurements were
made on a Hitachi GC 3900 equipped with a standard FID detector and
Augugliaro, T. Caronna, V. Loddo, G. Marcꢂ, G. Palmisano, L. Palm-
a DM-5 amine capillary column (5% polysilarylene, 95% polydimethyl-
isano, S. Yurdakal, Chem. Eur. J. 2008, 14, 4640–4646; h) D. Tsuka-
siloxane, 30 m, 0.5 mmꢁ0.25 mm, Dikma) using highly pure N2 as the car-
moto, M. Ikeda, Y. Shiraishi, T. Hara, N. Ichikuni, S. Tanaka, T.
rier gas.
Hirai, Chem. Eur. J. 2011, 17, 9816–9824.
Standard analytical conditions: Injector temperature: 2508C, FID detec-
tor temperature: 3008C, column temperature program: 808C (hold
[4] a) M. A. Fox, M. T. Dulay, Chem. Rev. 1993, 93, 341–357; b) A. Mal-
dotti, A. Molinari, R. Amadelli, Chem. Rev. 2002, 102, 3811–3836.
3 min) to 2808C (hold 5 min) at 208CminÀ1. GC-MS analysis (Thermo-
[5] Recent reports on visible-light-induced organic synthesis: For homo-
Finingan Trace 2000/Trace DSQ) was carried out with a DB-5MS capilla-
ry column (30 m, Dikma) and highly pure He as the carrier gas.
geneous systems, see: a) T. P. Yoon, M. A. Ischay, J. Du, Nat. Chem.
2010, 2, 527–532; b) A. Berlicka, B. Kçnig, Photochem. Photobiol.
Sci. 2010, 9, 1359–1366; c) J. M. R. Narayanam, C. R. J. Stephenson,
Chem. Soc. Rev. 2011, 40, 102–113, and the references therein. For
g-C3N4 photocatalysts, see: d) F. Z. Su, S. C. Mathew, G. Lipner, X.
Fu, M. Antonietti, S. Blechert, X. Wang, J. Am. Chem. Soc. 2010,
132, 16299–16301; e) X. Chen, J. Zhang, X. Fu, M. Antonietti, X.
Wang, J. Am. Chem. Soc. 2009, 131, 11658–11659; f) F. Z. Su, S. C.
Typical procedure for the oxidation of amines: The photocatalytic reac-
tions were carried out under irradiation by a Xe lamp (300 W, PLS-SXE
300, Beijing Trusttech Co.), equipped with a cutoff filter to filter out light
below 420 nm and a broad band filter to cutoff light above 750 nm, with
magnetic stirring in a 10 mL Pyrex glass bottle. The bottle was sealed
with a rubber stopper wrapped in aluminum foil and opened to air
through a syringe needle. A typical reaction system consisted of the sub-
Mathew, L. Mohlmann, M. Antonietti, X. C. Wang, S. Blechert,
strate (0.2 mmol) and TiO2 (anatase, 50 mg) in CH3CN (5 mL). The tem-
Angew. Chem. 2011, 123, 683–686; Angew. Chem. Int. Ed. 2011, 50,
perature of the reaction medium was around 408C. After filtering
657–660; g) P. F. Zhang, Y. Wang, J. Yao, C. M. Wang, C. Yan, M.
through a Millipore filter to remove the TiO2 powder, the reaction mix-
ture was analyzed by GC using bromobenzene as the internal standard.
The products were confirmed by comparison of retention times with au-
Antonietti, H. R. Li, Adv. Synth. Catal. 2011, 353, 1447–1451. For
plasmonic photocatalysts, see: h) H. Y. Zhu, X. B. Ke, X. Z. Yang, S.
Sarina, H. W. Liu, Angew. Chem. 2010, 122, 9851–9855; Angew.
Chem. Int. Ed. 2010, 49, 9657–9661; i) X. Chen, Z. F. Zheng, X. B.
Ke, E. Jaatinen, T. F. Xie, D. J. Wang, C. Guo, J. C. Zhao, H. Y. Zhu,
Green Chem. 2010, 12, 414–419; j) K. Mori, M. Kawashima, M.
Che, H. Yamashita, Angew. Chem. 2010, 122, 8780–8783; Angew.
Chem. Int. Ed. 2010, 49, 8598–8601; k) Y. Ide, M. Matsuoka, M.
Ogawa, J. Am. Chem. Soc. 2010, 132, 16762–16764; l) P. Christo-
pher, H. Xin, S. Linic, Nat. Chem. 2011, 3, 467–472; m) Z. Zheng,
B. Huang, X. Qin, X. Zhang, Y. Dai, M.-H. Whangbo, J. Mater.
Chem. 2011, 21, 9079–9087.
thentic samples and by GC-MS analysis.
Acknowledgements
Financial support from the National Basic Research Program of China
(973 project No. 2010CB933503), the National Natural Science Founda-
tion of China (NSFC, Nos. 20920102034, 21077110 and 20877076), and
the Chinese Academy of Sciences (CAS) are gratefully acknowledged.
[6] S. Fꢃldner, R. Mild, H. I. Siegmund, J. A. Schroeder, M. Gruber, B.
Kçnig, Green Chem. 2010, 12, 400–406.
[7] S.-i. Naya, A. Inoue, H. Tada, J. Am. Chem. Soc. 2010, 132, 6292–
6293.
[1] a) X. Chen, S. S. Mao, Chem. Rev. 2007, 107, 2891–2959; b) A. L.
Linsebigler, G. Q. Lu, J. T. Yates, Chem. Rev. 1995, 95, 735–758;
c) A. Fujishima, X. T. Zhang, D. A. Tryk, Surf. Sci. Rep. 2008, 63,
515–582.
[8] M. Zhang, C. C. Chen, W. H. Ma, J. C. Zhao, Angew. Chem. 2008,
120, 9876–9879; Angew. Chem. Int. Ed. 2008, 47, 9730–9733.
[9] An organic dye can be easily photodegraded on TiO2 under visible-
light irradiation: C. C. Chen, W. H. Ma, J. C. Zhao, Chem. Soc. Rev.
2010, 39, 4206–4219.
[10] For environmental remediation: a) T. Lana-Villarreal, A. Rodes,
J. M. Pꢄrez, R. Gꢅmez, J. Am. Chem. Soc. 2005, 127, 12601–12611;
b) S. Kim, W. Choi, J. Phy.Chem. B 2005, 109, 5143–5149; c) N. M.
Dimitrijevic, E. Rozhkova, T. Rajh, J. Am. Chem. Soc. 2009, 131,
2893–2899. For generation of H2: d) G. Kim, W. Choi, Appl. Catal.
B 2010, 100, 77–83; For organic synthesis: e) F. Parrino, A. Ramak-
rishnan, H. Kisch, Angew. Chem. 2008, 120, 7215–7217; Angew.
Chem. Int. Ed. 2008, 47, 7107–7109; f) S. Higashimoto, N. Kitao, N.
Yoshida, T. Sakura, M. Azuma, H. Ohue, Y. Sakata, J. Catal. 2009,
266, 279–285.
[2] a) X. B. Chen, L. Liu, P. Y. Yu, S. S. Mao, Science 2011, 331, 746–
750; b) R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science
2001, 293, 269–271; c) S. Sakthivel, H. Kisch, Angew. Chem. 2003,
115, 5057–5060; Angew. Chem. Int. Ed. 2003, 42, 4908–4911; d) W.
Zhao, W. H. Ma, C. C. Chen, J. C. Zhao, Z. G. Shuai, J. Am. Chem.
Soc. 2004, 126, 4782–4783; e) T. Ohno, T. Mitsui, M. Matsumura,
Chem. Lett. 2003, 32, 364–365; f) C. C. Chen, W. H. Ma, J. C. Zhao,
Curr. Org. Chem. 2010, 14, 630–644; g) Q. Wang, C. C. Chen, W. H.
Ma, H. Y. Zhu, J. C. Zhao, Chem. Eur. J. 2009, 15, 4765–4769.
[3] a) G. Palmisano, V. Augugliaro, M. Pagliaro, L. Palmisano, Chem.
Commun. 2007, 3425–3437; b) S. Yurdakal, G. Palmisano, V. Loddo,
V. Augugliaro, L. Palmisano, J. Am. Chem. Soc. 2008, 130, 1568–
1569; c) Y. Shiraishi, T. Hirai, J. Photochem. Photobiol. C 2008, 9,
2630
www.chemeurj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 2624 – 2631

Photocatalytic Aerobic Oxidation of Amines into Imines on TiO2
FULL PAPER
[11] a) M. Zhang, Q. Wang, C. C. Chen, L. Zang, W. H. Ma, J. C. Zhao,
Angew. Chem. 2009, 121, 6197–6200; Angew. Chem. Int. Ed. 2009,
48, 6081–6084; b) Q. Wang, M. Zhang, C. C. Chen, W. H. Ma, J. C.
Zhao, Angew. Chem. 2010, 122, 8148–8151; Angew. Chem. Int. Ed.
2010, 49, 7976–7979.
[12] X. J. Lang, H. W. Ji, C. C. Chen, W. H. Ma, J. C. Zhao, Angew.
Chem. 2011, 123, 4020–4023; Angew. Chem. Int. Ed. 2011, 50, 3934–
3937.
[13] a) H. Sun, F. Z. Su, J. Ni, Y. Cao, H. Y. He, K. N. Fan, Angew.
Chem. 2009, 121, 4454–4457; Angew. Chem. Int. Ed. 2009, 48, 4390–
4393; b) B. Gnanaprakasam, J. Zhang, D. Milstein, Angew. Chem.
2010, 122, 1510–1513; Angew. Chem. Int. Ed. 2010, 49, 1468–1471;
c) J. S. M. Samec, A. H. Ell, J. E. Backvall, Chem. Eur. J. 2005, 11,
2327–2334; d) O. Rios-Bernꢆ, S. O. Flores, I. Cordova, M. A. Valen-
zuela, Tetrahedron Lett. 2010, 51, 2730–2733; e) A. Hakki, R. Dil-
lert, D. Bahnemann, Catal. Today 2009, 144, 154–159; f) Y. Shir-
aishi, M. Ikeda, D. Tsukamoto, S. Tanaka, T. Hirai, Chem. Commun.
2011, 47, 4811–4813; g) M. A. Fox, J. N. Younathan, Tetrahedron
1986, 42, 6285–6291; h) H. Wang, R. E. Partch, Y. Li, J. Org. Chem.
1997, 62, 5222–5225; i) Y. Shiraishi, Y. Sugano, S. Tanaka, T. Hirai,
Angew. Chem. 2010, 122, 1700–1704; Angew. Chem. Int. Ed. 2010,
49, 1656–1660. j) S. Furukawa, Y. Ohno, T. Shishido, K. Teramura,
T. Tanaka, ACS Catal. 2011, 1, 1150–1153.
[14] The adsorption mode of amines on TiO2 has been well established
by several physicochemical methods: a) L. F. Liao, C. F. Lien, D. L.
Shieh, F. C. Chen, J. L. Lin, Phys. Chem. Chem. Phys. 2002, 4, 4584–
4589; b) E. Farfan-Arribas, R. J. Madix, J. Phys. Chem. B 2003, 107,
3225–3233; c) N. Nakayama, T. Hayashi, Colloids Surf. A 2008, 317,
543–550; d) S. C. Li, U. Diebold, J. Am. Chem. Soc. 2010, 132, 64–
66.
Received: July 21, 2011
Revised: September 23, 2011
Published online: January 23, 2012
Chem. Eur. J. 2012, 18, 2624 – 2631
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2631