Selective aerobic oxidation of amines to imines by TiO2 photocatalysis in water

Article abstract of DOI:10.1039/c3cc41407h

Selective oxidation of benzylic amines to imines with atmospheric O ₂ is achieved in water on TiO₂ under UV irradiation. The afforded imine could be easily separated and TiO₂ can be recycled by filtration. To scale up, gra

Full text of DOI:10.1039/c3cc41407h

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Selective aerobic oxidation of amines to imines by
TiO photocatalysis in water†
2
Cite this: Chem. Commun., 2013,
4
9, 5034
Ning Li, Xianjun Lang, Wanhong Ma, Hongwei Ji, Chuncheng Chen* and
Jincai Zhao
Received 23rd February 2013,
Accepted 11th April 2013
DOI: 10.1039/c3cc41407h
www.rsc.org/chemcomm
13
Selective oxidation of benzylic amines to imines with atmospheric WO /TiO (with selectivities of 41–62%) at 50% conversion of
3
2
O
2
is achieved in water on TiO
imine could be easily separated and TiO
filtration. To scale up, gram-scale imine products can be obtained. key to ensuring high selectivity, bestowing guidance in designing
2
under UV irradiation. The afforded benzyl alcohols under UV irradiation in water. In both cases, the
2
can be recycled by weak adsorption ability of products on the surface of TiO is the
2
14
selective photocatalytic aerobic oxidations in water. However, the
The selective photocatalytic oxidation of organic compounds has
received increased attention, since it could endow room temperature
synthesis with O
particular, the selective oxidation of amines to imines, which
represents one of the most important functional-group transfor-
mations, has been attracting special interest from the catalysis
research community. We recently reported the photocatalytic
selective aerobic oxidation of benzylamines to imines on TiO in
higher selectivity is at the cost of lower activity of these specially
12,13
designed TiO
2
photocatalysts.
It is a more challenging task to
1
2
under renewable solar light irradiation. In
achieve high selectivity in water without influencing the high activity
of photocatalysts. Thus, we intend to utilize the instinct physico-
chemical properties between amines (miscible) and imines (non-
miscible) in water to avoid overoxidation of the formed products to
2
2
achieve high selectivity, whilst the most active TiO could be directly
2
selected as a photocatalyst to ensure high efficiency.
3
,4
CH
g-C
complexes
3
CN. Since then, different genres of photocatalysts such as
Herein, we report our recent findings on the selective oxidation
5
6
7
8
3 4
N ,
Nb
2
,10
O
5
,
Au/TiO
2
, Ru containing MOF, Ru and Rh
have been developed to implement this reaction.
2
of amines to imines in water with atmospheric O on commercial
9
benchmark Degussa P25 TiO exposed to >300 nm UV irradiation.
2
In these aforementioned photocatalytic systems, organic solvents
were always employed as the reaction media.
The oxidation reaction could be run at a much higher rate than that
in CH CN, suggesting the prompting factor of water for the
3
Water is a more economic and environmentally viable solvent for
the organic reaction. Moreover, some organic reactions in water could
exhibit special reactivity or selectivity due to the unique physicochem-
ical properties of water. However, not only for amines, literature
reports about the selective oxidation of organic compounds in water
oxidation process. More intriguingly, we found that TiO could be
aggregated along with the accumulation of imines (Fig. S1, ESI†).
Hence, by B u¨ chner filtration, imines could be easily separated
2
2
without tedious work-ups; TiO could be facilely recycled without
centrifugation. The reaction was scaled up to a gram-scale imine
synthesis, which is one of the best reported results for the selective
oxidation by photocatalysis in water.
with O
2
by TiO
2
photocatalysis have been quite sporadic so far. This is
ꢀ
in part because of the formation of active oxygen species such as OH
ꢀ
À
ꢀ
+
and O
2
/HO
2
radicals derived from the oxidation of H
2
Obyhvb and
The time plot for the oxidation of benzylamine to imine in water
À
the reduction of O by e
under aqueous conditions, which could
2
cb
with 1 atm of air on Degussa P25 TiO is shown in Fig. 1. After
2
induce the overoxidation of organic compounds, even ultimately to
>300 nm UV irradiation for 1 h, 32% of benzylamine was converted,
11
CO and H O. Thus, to access high selectivity in water, one of the
2
2
and 87% of them was found to be transformed to imine with a
primary tasks is to avoid the overoxidation of the targeted products.
Despite the scarcity, there were encouraging results of photo-
catalytic synthesis in water that are worth highlighting. Selective
aerobic oxidation of benzyl alcohols to benzaldehydes was reported
3
selectivity comparable to that in CH
result considering the fact that complete degradation of organic
compounds into CO and H O usually occurs under aqueous
3
CN. This is a quite satisfactory
2
2
conditions on TiO photocatalysts. In addition, the high selectivity
2
12
2
on a home-made rutile TiO (with selectivities of 45–74%) or
at the early stage indicates that the oxidation of amines to products
other than desirable intermediates (aldehyde, see below) and over-
oxidation of the intermediates are not significant in water. After a
prolonged irradiation time of 9 h, the selectivity decreased to some
Key Laboratory of Photochemistry, Beijing National Laboratory for Molecular
Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190,
China. E-mail: ccchen@iccas.ac.cn
3
extent comparable to photocatalytic oxidation of amine in CH CN as
studied earlier. The selectivity for imine was 63% at benzylamine
†
Electronic supplementary information (ESI) available: Experimental details and
3,4
Fig. S1–S4. See DOI: 10.1039/c3cc41407h
5
034 Chem. Commun., 2013, 49, 5034--5036
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Table 1 Selective aerobic oxidation of benzylic amines to imines in water
photocatalyzed by TiO
2
a
a
b
b
Conv. Select. Conv. Select.
Entry Substrate
Product
(%)
32
22
37
22
35
(%)
87
85
80
72
94
(%)
81
81
82
65
85
(%)
63
55
61
63
83
1
2
3
4
5
Fig. 1 Conversion of amine (–’–), yield of imine (–m–) and selectivity (–J–)
against irradiation time for the photocatalytic aerobic oxidation of benzylamine
on TiO
2 2 2
. Reaction conditions: benzylamine (0.1 mmol), H O (2 mL), TiO (Degussa
P25, 10 mg), Hg lamp (100 W), >300 nm, air (1 atm).
conversion of 83%. The decrease in selectivity with irradiation time is
understandable as the free radical intermediates, which are famous
in the aqueous photocatalytic system, might attack the formed
imines to destroy them to some extent.
6
7
8
9
35
24
79
90
73
70
67
66
65
Motivated by this result, we expanded the scope of the substrates
for the oxidation of amines to imines in water by TiO photocatalysis
2
25
33
40
79
56
94
72
59
82
with 1 atm of air as the terminal oxidant. Table 1 summarizes the
photocatalytic oxidation of the benzylamines to the corresponding
imines at two different conversions corresponding to reaction times of
52
76
55
1
and 9 h, respectively. The selectively oxidative coupling reactions
1
1
0
1
proceeded smoothly in water for all the benzylamine derivatives, and
formation of the desired imine products was in moderate to high
selectivities (entries 1–10): 56–94% at low conversions under irradia-
31
—
94
—
86
—
c
d
tion for 1 h; 52–83% at high conversions (after 9 h). Methyl substituted 12
51
benzylamines at o-, m-, p-positions of the phenyl ring (Table 1,
entries 2–4) could all be transformed to corresponding imines with
slight differences in conversions and selectivity. Bulky groups such as
Reaction conditions: amine (0.1 mmol), H
0 mg), Hg lamp (100 W), >300 nm, air (1 atm). Conversions and
selectivities were determined by GC analysis using bromobenzene as
2 2
O (2 mL), TiO (Degussa P25,
1
a
b
c
t-butyl at the p-position of the phenyl ring did not decrease the reaction internal standard. 1 h. 9 h. Amine (5 mmol), H
2 2
O (0.15 L), TiO
(
Degussa P25, 0.5 g), 15 h, for a more detailed experimental procedure
rate (entry 5). It was found that electronic effects associated with
electron donating substituents (CH – and CH O–) and electron with-
d
see ESI. Isolated yield by filtration.
3
3
drawing substituents (F– and Cl–) on the phenyl ring had little effect
on the reaction rate and product selectivity of the oxidation reaction.
For example, not much difference in the conversions of the para-
substituted benzylamines was observed, and similar selectivity for the
imines was also obtained (entries 1, 3, 5–8), indicating that the
photocatalytic selective oxidation of amines to imines in water is not
sensitive to substituents and has high functional-group tolerance. It is
noteworthy that the oxidative coupling of halo-substituted benzyl-
amineswasfoundtobewithgoodselectivity(entries7–10),asthehalo-
substituted positions along with the imine functionality are useful for
further transformations. The amine substrate with a bulky aromatic
ring such as naphenyl could also be subjected to the photocatalytic
oxidation conditions with excellent selectivity of 94% at 31% conver-
sion and moderate selectivity of 55% at 86% conversion (entry 11).
The imine product could be easily precipitated from water as a
Fig. 2 Comparison of the photocatalytic aerobic oxidation rates of 2,4-dichloro-
benzylic amine when the reaction is carried out in water and in CH
3
CN. Reaction
conditions: amine (0.1 mmol), solvent (2 mL), TiO
lamp (100 W), air (1 atm).
2
(Degussa P25, 10 mg), Hg
white solid in the photocatalytic aerobic oxidation of 2,4-dichloro- After 1 h of irradiation, 40% of amine was converted in water,
benzylamine with a high selectivity of 76% at 82% conversion (entry whereas the conversion was less than 5% of amine for the reaction
1
0) which is comparable with the result in CH CN. Thus it was in CH CN; after 9 h, 82% of amine was converted in water, whereas
3
3
selected as the substrate to investigate the role of water in the only 20% of amine was converted in CH CN, indicating that the
3
photocatalytic coupling of amines. Fig. 2 illustrates a detailed oxidation of amine is significantly accelerated by water. Adding a
1
5
2e,16
comparison between different reaction rates in water and CH
3
CN. reasonable amount of water, using an acidic catalyst
or employ-
17
The reaction in water proceeded significantly faster than that in ing water as a solvent has been observed to facilitate the aerobic
the organic solvent, particularly at the beginning of the irradiation. oxidation of amines to imines in the thermally catalytic systems.
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photocatalysis. Besides providing a more environment friendly
and economic solvent alternative, the photocatalytic selective
oxidation of amine occurs much more rapidly in water than that
(
(
1)
2)
in CH CN. Our work also presents an excellent example that the
3
immiscibility of the products with water can be used to avoid
overoxidation of the products and to realize the easy separation
of the product and the recycling of the photocatalyst.
3
,4
According to our previous results in CH
tion of benzylamine to imine on TiO in water under UV irradiation
should follow a two-step process: benzaldehyde is first generated by
the oxygenation of benzylamine with O (eqn (1)); nucleophilic
3
CN, the aerobic oxida-
2
Financial support from 973 project (2010CB933503,
2
2
2
013CB632405), from NSFC (No. 21137004, 21273245 and
1277147) and CAS is gratefully acknowledged.
addition of the formed benzaldehyde by benzylamine affords the
final imine (eqn (2)). This pathway should hold true in aqueous
systems. In the first step, photocatalytic oxidation of amine is a series Notes and references
of electron transfer and proton transfer processes (dehydrogenation
of the amine and the reduction of oxidant O ). The presence of water
2
would greatly accelerate these proton concerted electron transfer
reactions by providing more protons and by forming hydrogen bonds
2
on the surface of TiO . For example, we recently discovered that the
1
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3
4
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systems. Intuitively, employing water as the solvent could have a
detrimental effect shifting the equilibrium backward in eqn (2). In
addition, the protonation of the amino groups would hinder the
3
4
5
6
nucleophilic reaction. However, based on the pK
4.66), only 2% of the amino groups are protonated. It is possible for
the nucleophilic reaction to occur.
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is the recyclability of TiO . We observed that the photocatalyst
b
of benzylamine
(
2
could aggregate along with the formation of imine to allow for easy
separation of both the product and the photocatalyst (Fig. S1, ESI†).
Because the newly formed CQN bonds have less polarity than the
7
8
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–
NH
2
groups in the substrates, the imine products exhibit much
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4
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1
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2
the current aerobic oxidation protocol by TiO photocatalysis by
using 0.69 g benzylamine in 150 mL water with 0.5 g TiO . After
2
separation of the reacted suspensions by simple B u¨ chner filtra-
tion, the substrate and product distribution in the filtrate and the
solid was carefully analysed using GC. The unreacted amine was
the dominant organic species in the filtrate, whereas the imine
was enriched in the solid (Fig. S2 and S3, ESI†). The GC analysis
1
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the imine formation. In another experiment, we filtered out the
solid every 5 h of irradiation. After separating the products using
1
1
1
1
2
3
0 mL acetonitrile, TiO was put back into the filtrate for the
2
further conversion of the unreacted amine. Three such runs
afforded 0.32 g (51% isolated yield) of imine (see ESI† for details).
Both the GC and NMR revealed that the isolated product is imine
with purity higher than 95% (Fig. S4, ESI†).
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In summary, water can be used as the ideal medium for
scalable synthesis of imines with good selectivities by TiO
2
5
036 Chem. Commun., 2013, 49, 5034--5036
This journal is c The Royal Society of Chemistry 2013