Photocatalytic aerobic oxidation of amines to imines on BiVO4 under visible light irradiation

Article abstract of DOI:10.1039/c4cc07097f

BiVO₄ was found to be an efficient photocatalyst under visible light irradiation for selective oxidation of amines to imines with high activity (99% conversion) and selectivity (up to 99%) using oxygen as an oxidant.

Full text of DOI:10.1039/c4cc07097f

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: C. Li, B. Yuan, R.
Chong, B. zhang, J. Li and Y. Liu, Chem. Commun., 2014, DOI: 10.1039/C4CC07097F.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/chemcomm

View Article Online
DOI: 10.1039/C4CC07097F
Page 1 of 5
ChemComm
ChemComm
RSC
COMMUNICATION
Photocatalytic Aerobic oxidation of amine to imine on
BiVO4 with visible light
Bo Yuan, Ruifeng Chong, Bao Zhang, Jun Li, Yan Liu* and Can Li*
amines to prepare imine with the different substituted groups.11
BiVO4 was found to be an efficient photocatalyst under
visible light irradiation for selective oxidation of amines to
imines with high activity (99% conversion) and selectivity (up
to 99%) using oxygen as oxidant.
Shishido and Tanaka et al found that Nb2O5 was another efficient
photocatalyst for aerobic oxidation of various amines to imines.12
However, similar to TiO2, it’s an ultraviolet responding catalyst
(light absorption edge of Nb2O5 is 390 nm). Other photocatalyst such
as mpg-C3N4 (mesoporous graphite carbon nitride),3 Au/TiO2,13 Ru
containing MOF,14 Ru and Rh complexes,15 CdS16 and LDH17 are
also reported to implement the oxidation of amine. However, most of
catalytic systems are not efficient for secondary amine oxidation.
Over the years, intensive attention has been devoted to the use of
light as a driving force for chemical reactions1 because it can easily
perform chemical transformation with “green” synthetic route. The
exploring of low-cost and stable catalytic system with high catalytic
efficiency is always a major goal in this fascinating area. Ruthenium
and iridium complexes have been proven to be available visible-light
photoredox catalysts in the cycloadditions as well as carbon-carbon
and carbon-heteroatom bond formations.2 However, the stability in
long term operation and inclusion of precious metal restrict their
practical application.3 Inorganic semiconductors are promising
photocatalytic materials due to their availabilities, chemical
stabilities and diversity.4 Among them, titanium dioxide has been
widely used for the transformation of organic compounds such as
alcohol oxidation5 and nitrobenzene reduction6. However, TiO2
responds only to UV light and tends to be slightly selective for many
oxidation reactions. Therefore, exploring suitable visible-light
photocatalysts to achieve excellent selectivity is highly desirable in
synthetic chemistry.
BiVO4, as a visible-light-responsive photocatalyst, has been
demonstrated
to
be
active
in
photocatalytic18
and
photoelectrochemical water oxidation19. In this communication, we
report our finding that BiVO4 can be used as an efficient
photocatalyst on a series of amine oxidation to prepare the
corresponding imine with high activity and selectivity under mild
conditions. Moreover, we found that the exposed surfaces of both
(110) facet and (040) facet in BiVO4 play an important role in the
photocatalytic activity of amine oxidation. The control experiment
and mechanistic studies suggested that the amine was oxidized
directly involving holes generated in BiVO4 irradiated with visible
light and using O2 as an electron acceptor to react with proton.
N-t-butylbenzylamine 1a was chosen as the model substrate,
because it could supply a synthetically useful imine product and was
proved to be a tough substrate in the photocatalytic oxidation by
Imine is an important synthetic intermediate because of its
versatile applications in medicines or biologically active nitrogen
containing organic compounds.7 Generally, its synthesis relies on the
condensation of amine and carbonyl compound. Efforts have been
devoted to direct oxidation of amine to its corresponding imine.8
Although some oxidation procedures with stoichiometric amount of
11a
either TiO2 or Nb2O512a. Bi containing salts were employed as the
photocatalysts and a Xe light (λ>420 nm) was used as the source of
visible light. Photocatalytic activity and selectivity of 1a oxidation
over various Bi containing salts catalysts are summarized in Table 1.
Bi2MoO6, Bi2WO6 and BiVO4 are able to oxidize benzylamine to
dehydrogenated imine. The primary results show that Bi2MoO6
exhibits the highest oxidation ability but moderate selectivity (Table
1, entry 1). Bi2WO6 has low activity and selectivity in the model
photocatalytic oxidation (Table 1, entry 2). BiVO4 shows excellent
selectivity although the activity is poor (Table 1, entry 3). Over the
past decades, exposed crystal planes of inorganic semiconductor-
based photocatalyts have been verified to have great influence on
catalytic activity. Recently, this group revealed that (040) and (110)
facets of BiVO4 play an important role in the water oxidation
oxidants
such
as
2-iodoxybenzoic
acid
or
N-tert-
butylphenylsulfinimidoyl chloride have been developed, a catalytic
system using molecular oxygen as oxidant is an ideal way
concerning green chemistry.9 Several transition metal based catalytic
systems have been established for this purpose, but only limited
kinds of amines are oxidized to imines with expensive metals under
relatively high temperature.10 Very recently, people tried to utilize
semiconductor as photocatalyst for amine transformation. Zhao et al
reported a photocatalytic oxidation of primary amines on TiO2 to
prepare homo-condensed imine. Although the high selectivities were
achieved, the catalytic system was not available for the secondary
through spatial separation of the photogenerated electrons and
20
holes.18b,
Inspired by this finding, we envisioned that catalytic
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 1

View Article Online
DOI: 10.1039/C4CC07097F
ChemComm
Page 2 of 5
COMMUNICATION
Journal Name
ability of BiVO4 in the oxidation of amine might be improved
through fine tuning of the facets. We found that BiVO4 crystals with
different ratios of (040) and (110) facets could be hydrothermally
synthesized by treating with different pH values (Figure S3). It is
worthy to note that all batches of BiVO4 have almost same UV-vis
diffuse reflectance, particle size and BET surface area (Figure S2, S4,
S5). To our delight, BiVO4 with almost 1:1 ratio of the peak
intensity of XRD referring to (040) and (110) facets exhibits the
highest activity in the amine photo-oxidation with 94% selectivity.
Entry
Substrate
Reaction
time/h
Conversion
Selectivity
(%) b
(%) b
1
2
6
99
99
7
97
97
93
91
3
4
10
7
99
90
Table 1: Oxidation of N-t-butylbenzylamine over various
photocatalysts a
5
6
7
16
18
20
96
96
97
93
95
91
Entry Catalyst
I(110)/I(040)
Conversion
(%) b
Selectivity
(%) b
8
9
14
16
96
95
>99
92
1
2
Bi2MoO6
Bi2WO6
72
29
16
37
40
81
49
38
97
73
53
94
90
93
94
93
94
91
3
BiVO4-1
BiVO4-2c
BiVO4-3c
BiVO4-4c
BiVO4-5c
BiVO4-6c
BiVO4-4c
10 e
11 e
12 e
16
12
20
89
93
85
89
86
89
4
1.86d
1.30d
1.17d
1.48d
1.63d
5
6
7
13 c
14 c
15 c
17
13
20
92
92
90
92
90
95
8
9 e
a
Reaction conditions: λ>420 nm, catalyst (100 mg), N-t-butylbenzylamine (0.1
mmol), CH3CN as a solvent (8 mL), irradiation time : 5 hours, oxygen balloon (1
atm); b Determined by gas chromatography analysis using 1,4-diisopropylbenzene
16 c
21
48
91
79
90
92
c
as the internal standard; Related to different batches of catalyst, BiVO4-1
prepared by calcination, BiVO4-2, 3, 4, 5, 6 prepared by a hydrothermal method at
160 oC for 12 hours; d BiVO4-2,3,4,5,6 samples prepared by hydrothermal at pH
values of 2.26, 2.51, 3.11, 3.64, 4.52 ; e irradiation time : 7 hours .
17 d
a
Reaction conditions: λ>420 nm, BiVO4-4 (100 mg), amine (0.1 mmol), CH3CN
as a solvent (8 mL), oxygen balloon (1 atm); b Determined by gas chromatography
analysis using 1,4-diisopropyl benzene as the internal standard; c BiVO4-4 (20 mg);
d BiVO4-4 (100 mg), amine (1.5 mmol); e BiVO4-4 (20 mg), amine (0.35 mmol);
The oxidation of various amines including primary, secondary
amine derivatives were also examined, and the results are listed in
Table 2. Dibenzyl amine exhibits high reactivity and selectivity
(Table 2, entry 1). To verify the oxidative activity of our catalyst
system, we investigated different substituted N-t-butyl amines which
were demonstrated to have poor reactivities in the other
photocatalytic systems.11a, 12a We found that the high activity of this
photocatalytic system can be extended for the oxidation of different
substituted N-t-butyl amines. As listed in Table 2, variation of the
substituent on the different positions of phenyl moiety of the amine
has little impact on the selectivities of the reaction (Table 2, entries
2-9). Various substituted N-t-butyl imine could be obtained with
high conversion (93-97%) and selectivities (90-99%). However, the
oxidation of amine carrying the electron-donating group on the
phenyl (Table 2, entries 3 and 4) proceeded more efficiently than
those amines with electron-withdrawing substituted group (Table 2
entries 5-9). Moreover, higher reaction rates for para-substituted
substrates relative to the meta and ortho isomers reveal the presence
of a steric effect (Table 2, entries 5 VS 6-7). Primary amine could
also undergo oxidative coupling to the corresponding homo-imine
with high conversion and selectivity (Table 2, entries 10-12). To
prove the practicality of this reaction system, the reactions were
After the reaction, solid BiOV4 could be easily separated after a
simple filtration and reused. We did not observe significant
deterioration of activity and selectivity in the recovered catalyst even
after seven times of use (Figure S6).
100
90
80
70
N-tert-butyl
dibenzyl
60
50
40
30
20
10
0
0
1
2
3
4
5
6
7
Time/h
carried out with lower catalyst loadings (Table 2 entries 12-15) and Figure 1: The comparison of the reaction conversion between the N-
larger scale of substrates (Table 2 entry 16). Prolonging the reaction benzyl and N-tert-butyl benzylamine
time, high conversions and selectivities could also be achieved.
In the photo-oxidations of amines catalyzed by TiO2 and Nb2O5,
Table 2: Oxidation of amine over photocatalyst a
the process which substrates adsorbed onto metal oxides to form
amide species is regarded as the prerequisite. Because of the steric
hindrance around the nitrogen atom, the bulky N-tert-butyl
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

View Article Online
DOI: 10.1039/C4CC07097F
Page 3 of 5
Journal Name
ChemComm
COMMUNICATION
derivatives have much slower reactivities than N-benzyl derivatives
In summary, we found that BiVO4 acted as an efficient
in those reaction systems. In our case, we also make a comparison of photocatalyst with visible light for the oxidation of a series of
the reaction rates between the N-benzyl and N-tert-butyl amines, forming the corresponding imines using O2 as oxidant.
benzylamine. The results were described in the figure 1. The slight Importantly, various bulky N-tert-butyl substituted benzylamine,
difference of the reactivities among these two substrates indicated which were proven to be a disfavoured type of substrates reported in
that the photooxidation of amine catalyzed by BiVO4 might have a the literature11a, 12a, could also be oxidized with high selectivities (up
different course to produce the corresponding imines.
to 99%).
To understand the mechanism of the present catalytic system, a
series of kinetic experiments of para-substituted N-tert-butyl
benzylamines (MeO, Me, H, Cl, and F groups) were conducted in
the photocatalytic oxidation. The slope of the linear plot giving a ρ
value close to zero indicates that the rate-determining step of this
reaction doesn’t involve the Cα-H activation (Figure 2).
We acknowledge the financial support from the National Nature
Science Foundation of China (NSFC Grant No. 21322202) and the
Basic Research Program of China (973 Program: 2014CB239403).
Notes and references
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, Dalian 116023, China.
E-mail: yanliu503@dicp.ac.cn, canli@dicp.ac.cn
† Electronic Supplementary Information (ESI) available: [Experimental
details and Figures S1-S6]. See DOI: 10.1039/c000000x/
1.5
1.0
1. (a) H. Kisch, Angew. Chem. Int. Ed., 2013, 52, 812-847; (b) G.
Palmisano, V. Augugliaro, M. Pagliaro and L. Palmisano, Chem.
Commun., 2007, 3425-3437; (c) G. Palmisano, E. Garcia-Lopez, G.
Marci, V. Loddo, S. Yurdakal, V. Augugliaro and L. Palmisano, Chem.
Commun., 2010, 46, 7074-7089; (d) D. Ravelli, M. Fagnoni and A.
Albini, Chem. Soc. Rev., 2013, 42, 97-113; (e) S. Linic, P. Christopher
and D. B. Ingram, Nature Mater., 2011, 10, 911-921.
2. (a) J. Du, K. L. Skubi, D. M. Schultz and T. P. Yoon, Science, 2014, 344,
392-396; (b) D. A. Nicewicz and D. W. MacMillan, Science, 2008, 322,
77-80; (c) A. McNally, C. K. Prier and D. W. C. MacMillan, Science,
2011, 334, 1114-1117; (d) M. T. Pirnot, D. A. Rankic, D. B. Martin and
D. W. MacMillan, Science, 2013, 339, 1593-1596; (e) D. A. Nagib and
D. W. MacMillan, Nature, 2011, 480, 224-228; (f) A. G. Condie, J. C.
González-Gómez and C. R. J. Stephenson, J. Am. Chem. Soc., 2010, 132,
1464-1465; (g) J. D. Nguyen, E. M. D'Amato, J. M. Narayanam and C. R.
Stephenson, Nature Chem., 2012, 4, 854-859.
p-CH
p-CH O
p-H
0.5
0.0
3
p-F
3
p-Cl
-0.5
-1.0
-1.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4

Figure 2: Hammett plots for the oxidation of substituted secondary
amines with oxygen using BiVO4 catalyst. Hammett plots were
obtained from a ratio of conversion with reaction time of 60 min; the
reaction condition was given in Table 2
3. F. Z. Su, S. C. Mathew, L. Möhlmann, M. Antonietti, X. C. Wang and S.
Blechert, Angew. Chem. Int. Ed., 2011, 50, 657-660.
4. X. J. Lang, X. D. Chen and J. C. Zhao, Chem. Soc. Rev., 2014, 43, 473-
486.
5. (a) Q. Wang, M. Zhang, C. C. Chen, W. H. Ma and J. C. Zhao, Angew.
Chem. Int. Ed., 2010, 49, 7976-7979; (b) S. Yurdakal, G. Palmisano, V.
Loddo, V. Augugliaro and L. Palmisano, J. Am. Chem. Soc., 2008, 130,
1568-1569.
6. (a) W. Y. Ahn, S. A. Sheeley, T. Rajh and D. M. Cropek, Appl. Catal. B,
2007, 74, 103-110; (b) O. V. Makarova, T. Rajh and M. C. Thurnauer,
Environ. Sci. Technol., 2000, 34, 4797-4803.
7. R. D. Patil and S. Adimurthy, Asian J. Org. Chem., 2013, 2, 726-744.
8. M. Largeron, Eur. J. Org. Chem., 2013, 5225-5235.
Based on the above results, we propose a possible reaction
mechanism as shown in figure 3: electron (e-) and hole (h+) pairs are
generated by the irradiation of BiVO4. The photogenerated electron
reduces molecular oxygen to produce activated oxygen species. At
the same time, amine probably loses an electron, thus forming the
carbocationic-radical type intermediate. The activated oxygen
species react with protons which are from the hole oxidation of
amine.
Figure 3: The proposed mechanism of photooxidation of amine
9. (a) K. C. Nicolaou, C. J. N. Mathison and T. Montagnon, Angew. Chem.
Int. Ed., 2003, 42, 4077-4082; (b) K. C. Nicolaou, C. J. N. Mathison and
T. Montagnon, J. Am. Chem. Soc., 2004, 126, 5192-5201.
catalyzed by BiVO4.
10. S. I. Murahashi, Angew. Chem. Int. Ed., 1995, 34, 2443-2465.
11. (a) X. J. Lang, H. W. Ji, C. C. Chen, W. H. Ma and J. C. Zhao, Angew.
Chem. Int. Ed., 2011, 50, 3934-3937; (b) N. Li, X. J. Lang, W. H. Ma, H.
W. Ji, C. C. Chen and J. C. Zhao, Chem. Commun., 2013, 49, 5034-5036;
(c) X. J. Lang, W. H. Ma, Y. B. Zhao, C. C. Chen, H. W. Ji and J. C.
Zhao, Chem. Eur. J., 2012, 18, 2624-2631.
12. (a) S. Furukawa, Y. Ohno, T. Shishido, K. Teramura and T. Tanaka, ACS
Catal., 2011, 1, 1150-1153; (b) S. Furukawa, Y. Ohno, T. Shishido, K.
Teramura and T. Tanaka, J. Phy. Chem. C, 2013, 117, 442-450.
13. S. I. Naya, K. Kimura and H. Tada, ACS Catal., 2013, 3, 10-13.
14. Z. Xie, C. Wang, K. E. deKrafft and W. Lin, J. Am. Chem. Soc., 2011,
133, 2056-2059.
15. (a) H. A. Ho, K. Manna and A. D. Sadow, Angew. Chem. Int. Ed., 2012,
51, 8607-8610; (b) M. Rueping, C. Vila, A. Szadkowska, R. M. Koenigs
and J. Fronert, ACS Catal., 2012, 2, 2810-2815.
16. W. W. Zhao, C. B. Liu, L. M. Cao, X. G. Yin, H. L. Xu and B. Zhang,
RSC Adv., 2013, 3, 22944-22948.
In order to confirm the mechanism, a reaction using AgNO3
instead of O2 as acceptor of photogenerated electron was carried out.
The reaction shows even higher activity (See Figure S7). It
demonstrates that photogenerated hole is an oxidative active site and
O2 is a capture of photogenerated electron.
17. X. J. Yang, B. Chen, X. B. Li, L. Q. Zheng, L. Z. Wu and C. H. Tung,
Chem. Commun., 2014, 50, 6664-6667.
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 3

View Article Online
DOI: 10.1039/C4CC07097F
ChemComm
Page 4 of 5
COMMUNICATION
Journal Name
18. (a) A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253-278; (b) D. E.
Wang, H. F. Jiang, X. Zong, Q. Xu, Y. Ma, G. L. Li and C. Li, Chem.
Eur. J., 2011, 17, 1275-1282; (c) G. C. Xi and J. H. Ye, Chem. Commun.,
2010, 46, 1893-1895.
19. W. J. Jo, J. W. Jang, K. J. Kong, H. J. Kang, J. Y. Kim, H. Jun, K. P. S.
Parmar and J. S. Lee, Angew. Chem. Int. Ed., 2012, 51, 3147-3151.
20. R. G. Li, F. X. Zhang, D. E. Wang, J. X. Yang, M. R. Li, J. Zhu, X. Zhou,
H. X. Han and C. Li, Nat. Commun., 2013, 4, 1432.
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 5 of 5
ChemComm
View Article Online
DOI: 10.1039/C4CC07097F
BiVO4 was found to be an efficient photocatalyst for selective oxidation of amines to
imines with high conversion and selectivity.