Photocatalytic organic transformation by layered double hydroxides: Highly efficient and selective oxidation of primary aromatic amines to their imines under ambient aerobic conditions

Article abstract of DOI:10.1039/c4cc01671h

We report the first application of layered double hydroxide as a photocatalyst in the transformation of primary aromatic amines to their corresponding imines with high efficiency and selectivity by using oxygen in an air atmosphere as a terminal oxidant u

Full text of DOI:10.1039/c4cc01671h

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Photocatalytic organic transformation by layered
double hydroxides: highly efficient and selective
oxidation of primary aromatic amines to their
imines under ambient aerobic conditions†
Cite this: Chem. Commun., 2014,
50, 6664
Received 5th March 2014,
Accepted 3rd May 2014
Xiu-Jie Yang,a Bin Chen,*b Xu-Bing Li,b Li-Qiang Zheng,*a Li-Zhu Wu*b and
DOI: 10.1039/c4cc01671h
Chen-Ho Tungab
www.rsc.org/chemcomm
We report the first application of layered double hydroxide as a
In this communication, we wish to report a highly efficient
photocatalyst in the transformation of primary aromatic amines to and selective oxidation of primary aromatic amines to imines
their corresponding imines with high efficiency and selectivity using layered double hydroxide (LDH) as a photocatalyst and
by using oxygen in an air atmosphere as a terminal oxidant under oxygen in air as an oxidant. Ruthenium(II),5a,b iridium(III),5b,c
light irradiation.
platinum(II),5d gold(II)5e complexes as well as organic dyes
including eosin Y,5f–h rose bengal, and Bodipy5i have been
Imine derivatives represent important intermediates for the involved in the photocatalytic reactions. However, the cost
synthesis of dyes, pharmaceuticals, agrochemicals and agri- and stability of metal complexes and organic dyes in the course
cultural chemicals.1 Over the past decades, a great deal of of irradiation remain a significant challenge. LDHs are well
efforts have been made to develop mild and practical oxidation known as hydrotalcite-like materials with a general formula
III
reactions for imine preparation.2 Whereas the traditional con- [MII
M
1Àx
x(OH)2]x+(AnÀ
)
x/nÁyH2O, where MII and MIII are diva-
densation of aldehydes and amines has been well-established, lent and trivalent metals and AnÀ is the exchangeable organic or
direct oxidation of amines to imines is of intense current inorganic anion.6 The attractive advantages of LDHs are: (1)
interest.3 In particular, the use of light and molecular oxygen they can be prepared in large quantities in a reliable and
to trigger the conversion of amines to imines has aroused much reproducible manner by precipitation of an aqueous solution
attention.4 For instance, Che et al.4a reported that a variety of of the corresponding metal salts; and (2) the atomic ratio between
secondary aromatic amines could be oxidized to imines in 90% the divalent and trivalent metal ions of the double layer can be
to 499% yields using singlet oxygen generated from oxygen and varied over a wide range without altering the structure of the
a porphyrin photosensitizer. Wang and Blechert4b presented the material. LDHs, particularly those bearing carbonate (CO32À) as a
aerobic oxidation of amines into imines in excellent yields by a compensating anion in the layers, have been widely used as supports
mesoporous graphite carbon nitrile (mpg-C3N4) photocatalyst. and/or confined reaction space,7a–c and even as catalysts7d–h for
Zhao et al.4c found a series of aromatic amines being trans- hydrogenation7d,e and oxidation7f reactions. However, few
formed into the corresponding imines under light irradiation, examples have been reported on the photocatalytic organic
8
´
air and TiO2 in acetonitrile. Despite the fact that considerable transformation by LDHs. In 2009, Garcıa and coworkers found
efficient methods have been developed for the oxidation of that ZnII/TiIV, ZnII/CeIV and ZnII/CrIII LDHs at different ZnII/metal
secondary amines to imines, little attention has been given to atomic ratios of LDHs have the ability to generate oxygen through
the oxidation of primary amines until recently,2 probably photocatalytic water splitting. Since then, the use of LDHs for
because the generated imines are intermediate products that photocatalytic hydrogen and oxygen evolution,9a–e carbon dioxide
are easily dehydrogenated to nitriles.
reduction,9f,g and even organic pollutant degradation,9h has been
developed. Here, we present our findings for the first time on the
highly efficient and selective photocatalytic oxidation of primary
aromatic amines to imines by LDHs (Scheme 1). With light
irradiation for 5 h, a simple, clean, recyclable and efficient
method is established for the virtually quantitative and selective
conversion of a variety of primary aromatic amines to their imines
under ambient aerobic conditions.
a Key Laboratory of Colloid and Interface Chemistry, Shandong University,
Ministry of Education, Jinan, Shandong 250100, P. R. China.
E-mail: lqzheng@sdu.edu.cn; Fax: +86-531-8856-4750; Tel: +86-531-8836-6062
b Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, The Chinese Academy of Science,
Beijing 100190, P. R. China. E-mail: chenbin@mail.ipc.ac.cn,
lzwu@mail.ipc.ac.cn; Fax: +86-10-8254-3580; Tel: +86-10-8254-3580
† Electronic supplementary information (ESI) available: Experimental details,
1H NMR spectra. See DOI: 10.1039/c4cc01671h
Considering the enhanced performance in energy conver-
sion,9a (ZnII/TiIV)LDH with a ZnII/TiIV ratio of 6 : 1 is selected as
6664 | Chem. Commun., 2014, 50, 6664--6667
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Table 1 Photocatalytic oxidation of aromatic aminesa
Entry Substrate
Product
t (h) Conv.b (%) Select.b (%)
1
2
3
4
5
6
7
8
9
R = H
R = H
5
5
5
5
5
5
5
5
5
100
99
96
99
98
100
98
99
97
97
99
93
96
95
91
91
90
R = p-Me
R = o-OMe
R = m-OMe
R = p-OMe
R = p-F
R = p-Cl
R = 3,4-diCl
R = p-Br
R = p-Me
R = o-OMe
R = m-OMe
R = p-OMe
R = p-F
R = p-Cl
R = 3,4-diCl
R = p-Br
Scheme 1
94
10
11
8
8
99
93
99
96
a
Reaction conditions: 0.2 mmol substrate, 20 mg (ZnII/TiIV)LDH, 5 mL
CH3CN, 1 atm air, 500 W high pressure Hg lamp (l 4 300 nm).
b
Determined by 1H NMR (CDCl3) using 1,3,5-trimethoxybenzene as
the internal standard.
To examine the composition effect on the photocatalytic reac-
tion, (ZnII/TiIV)LDH with a ZnII/TiIV ratio of 4 : 1 and 5 : 1,
respectively, was also prepared. It was found that (ZnII/TiIV)LDH
with a ratio of 6 : 1 shows the highest photocatalytic activity
(Table S2, ESI,† entries 1, 7 and 8), which may be due to its
higher crystallinity and purity than that of (ZnII/TiIV)LDH (5: 1)
Fig. 1 (a) HR-TEM image and TEM image (inset), (b) XPS spectra, (c)
powder XRD pattern, (d) FT-IR spectra of (ZnII/TiIV)LDH.
a photocatalyst, which was prepared according to the procedure and (ZnII/TiIV)LDH (4: 1) (as indicated in the XRD patterns in
in the literature.8 High-resolution transmission electron micro- Fig. S2, ESI†).9e In addition, the generality of LDHs as photo-
scopy (HR-TEM) images reveal that the (ZnII/TiIV)LDH has catalysts for the organic transformation was tested. We made a
platelike structure, and the lattice fringe is single crystalline sample of traditional (MgII/AlIII)LDH (Fig. S3, ESI†), and found
(Fig. 1a). X-ray photoelectron spectroscopy (XPS) shows the that it enabled the photocatalytic oxidation of benzylamine
binding energy of TiIV 2p3/2 and ZnII 2p3/2 in the (ZnII/TiIV)LDH smoothly in a good chemical yield (87%, Table S2, ESI,† entry 9).
shifted to the low energy as compared with that in TiO2 and
As shown in Table 1, the protocol can be used to convert a broad
ZnO, respectively (Fig. 1b), indicating the specific interaction range of primary aromatic amines into their corresponding imines
between TiIV and ZnII. X-ray diffraction (XRD) study revealed leaving various substituents unaffected. Using (ZnII/TiIV)LDH as the
long-range stacking order with the interplanar spacing of 2.5 Å photocatalyst and oxygen in air as the oxidant, good to excellent
(Fig. 1c). The absorption bands at 1654 cmÀ1, 1619 cmÀ1, 1508 cmÀ1
,
conversion and selectivity were obtained no matter electron-
1388 cmÀ1 and 1045 cmÀ1 in fourier transform infrared (FT-IR) donating (entries 1–5) or electron-withdrawing (entries 6–9)
spectroscopy suggested the presence of the CO32À anion, while groups were present on the benzene ring. When a heterocyclic
bands between 500–900 cmÀ1 are due to the lattice M–O and compound, 2-thiophene methylamine, was used as a substrate,
M–OH vibration (Fig. 1d).
the aerobic oxidation conversion and efficiency of the photo-
The photocatalytic aerobic oxidation was carried out at room catalyst are excellent.
temperature. 20 mg of (ZnII/TiIV)LDH was added into a 5 mL
In an effort to enhance the economic and environmental
solution of acetonitrile and a primary aromatic amine such attractiveness of the method, the reusability of the photo-
as benzylamine (0.2 mmol). The reaction mixture was then catalyst was assessed. After work up of an irradiated sample with
irradiated with a high-pressure mercury lamp (500 W) for 5 h under benzylamine as the substrate, the (ZnII/TiIV)LDH photocatalyst
an air atmosphere. After irradiation, the product was isolated was separated from the liquid by centrifugation and reused for
and identified by 1H NMR spectroscopy. Control experiments catalyzing the reaction of a second batch of benzylamine.
showed that the photocatalyst (ZnII/TiIV)LDH, air and light were Strikingly, the catalyst functioned as it did initially. After three
all essential for the effective oxidation of benzylamine (Table S2, additional runs with the same catalyst, a similar conversion
ESI,† entries 1–4) and the transformation cannot proceed and yield were achieved as in the first run (Fig. S4, ESI†).
effectively in the absence of light. Notably, the photocatalytic
Because LDHs are easy to isolate from the reaction system
activity of (ZnII/TiIV)LDH is better than TiO2 (P25) and ZnO (Table S2, for the next cycle, we further examined the reactivity of imines
ESI,† entries 1, 5 and 6), possibly due to the enhanced electron–hole generated in situ for the Ugi reaction.4a,10 Zhu and coworkers10
separation efficiency by superior dispersion of active TiO6 units.9e reported the first example of oxidative three-component Ugi-type
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À
ꢀ
photogenerated electrons to form O2
and benzylamine is
simultaneousÀly oxidized by the photogenerated hole. The
ꢀ
generated O2 abstracts a proton and a hydrogen atom from
the benzylamine radical cation to form benzenemethanimine
and H2O2. Successive addition of benzenemethanimine to
another molecule of benzylamine gives rise to the target imine
product. Meanwhile, the formed H2O2 intermediate can also
react with the benzylamine substrate (Table S2, ESI,† entry 12),
though the contribution of H2O2 to the reaction was proven to
be minor in the reaction system (Table S2, ESI,† entries 13 and
14). A trace amount of benzaldehyde product could be detected
in the acetonitrile solution owing to a small amount of H2O
formed. Nevertheless, the selectivity of the transformation of
primary aromatic amines to imines approaches 499%.
Scheme 2 One-pot synthesis of Ugi-type reaction starting from
benzylamine.
reaction by oxidation of amines with singlet oxygen, a key step
of which is the oxidation of benzylic amine to imine, followed
by reaction of the imine with isocyanide and carboxylic acid.
Che and coworkers4a used the in situ formed imine products
from porphyrin and singlet oxygen for application in the Ugi-
type reactions. Herein, the imine from primary benzylamine
and oxygen in air, (ZnII/TiIV)LDH under light irradiation was
refluxed with cinnamic acid and t-butylisocyanide in methanol
to obtain the Ugi product in 82% yield (Scheme 2), which is
comparable to those reported in the literature.4a
In summary, we have succeeded in the conversion of
primary aromatic amines to their imines for the first time by
using (ZnII/TiIV)LDH as a photocatalyst in the aerobic atmo-
sphere. The (ZnII/TiIV)LDH photocatalyst is stable, effective,
economical and reusable. In contrast to those reported in the
À
literature, O2 ꢀ generated from (ZnII/TiIV)LDH and oxygen in air
It is important to note that molecular oxygen is crucial for
the transformation of primary aromatic amines to imines.
When the reaction was performed in argon atmospheres, trace
amounts of photoproduct could be detected (Table S2, ESI,†
entry 3). It is accepted that singlet oxygen (1O2)4a,5i or the
has been demonstrated to be responsible for the oxidation of
aromatic amines to imines as well as the Ugi reaction. More
interestingly, the (ZnII/TiIV)LDH enables the transformation of
primary aromatic amines into their corresponding imines
efficiently and selectively, whereas leaving the substituents
unaffected. It is anticipated that the utilization of LDHs as
photocatalysts would be useful for cleaner, safer and cheaper
organic transformation.
We are grateful for financial support from the Ministry of
Science and Technology of China (2013CB834505 and
2013CB834804), the National Natural Science Foundation of
China (21390404, 91127017 and 91027041), and the Chinese
Academy of Sciences.
À
4b,d
ꢀ
superoxide radical anion (O2
)
is responsible for the con-
version of amines to imines. To figure out the active species Àof
ꢀ
oxygen in the current study, superoxide dismutase (SOD, O2
scavenger) was added to the photocatalytic aerobic oxidation
system containing (ZnII/TiIV)LDH and benzylamine under an air
atmosphere (this reaction was performed in water because SOD
is insoluble in acetonitrile). The yield of reaction was found to
decrease from 46%Àto 2% (Table S2, ESI,† entries 10 and 11),
ꢀ
suggesting that O2
plays a key role in the whole process.
Moreover, upon introduction of starch/KI/CH3COOH into the
system after 1 h irradiation, the color of solution changed from Notes and references
white to dark purple (Fig. S5, ESI†), which provides evidence of
the formation of H2O2 in the reaction system.
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