Merging the catechol-TiO2 complex photocatalyst with TEMPO for selective aerobic oxidation of amines into imines

Article abstract of DOI:10.1039/c8cy01096j

Catechol and its derivatives are model compounds used to investigate the binding dynamics and photo-induced interfacial charge transfer events between an organic molecule and TiO₂. However, the practical potential of this process has received l

Full text of DOI:10.1039/c8cy01096j

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Merging
the
Catechol-TiO2
Complex
Photocatalysis with TEMPO for Selective Aerobic
Oxidation of Amines into Imines†
Ji-Long Shi,a Huimin Hao,a Xia Li, a and Xianjun Lang*a
a
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China
*To whom correspondence should be addressed (X. J. Lang). E-mail: xianjunlang@whu.edu.cn
† TEMPO, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl
Keywords: catechol • surface complex • titanium dioxide • visible light photocatalysis •
oxidation of amines
Abstract: Catechol and its derivatives are model compounds to investigate the binding dynamics
and photo-induced interfacial charge transfer events between an organic molecule and TiO2.
However, the practical potential of this process has received little attention in part due to the
conceived instability of these molecules on TiO2 under light irradiation. Herein, we present a new
paradigm for selective oxidation of amines into imines with atmosphere O2 by exploring the
surface complexation of catechol derivatives on TiO2 that give rise to visible-light-induced events
of charge transfer. Our success depends on the merger of visible light photocatalysis of catechol
surface complexed TiO2 with TEMPO catalysis. With 5 mol% of TEMPO as a co-catalyst and
catechol-TiO2 complex (containing 0.8 mol% of catechol) as a photocatalyst, amines can be
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efficiently oxidized into imines with atmosphere O2 under irradiation of blue light emitting diodes.
Our work suggests that a suitable co-catalyst is the key to significantly expand the dimension of
visible light photocatalysis for selective chemical transformations.
1. Introduction
Photo-induced charge transfer occurring at the interface between an organic molecule and
TiO2 underpins a variety of energy and environmental applications and therefore receives much
attention.1-3 The experimental and theoretical challenges during the investigation on these intricate
and complicated reaction networks necessitates model compounds to simplify the overall process,
consequently giving a unambiguous picture on the reaction details. Because of their simple
molecular structures, catechol and its derivatives turn out to be excellent model molecules to study
the interfacial charge transfer and binding dynamics occurrence on the surface of TiO2. Especially,
much effort has been directed towards the understanding the binding and charge transfer dynamics
of these organic molecules on TiO2, armed with a series of spectroscopic4, 5, computational tools6,
7 or combined tools8. In most of these cases, catechol and its derivatives function as a convenient
choice to mimic the behavior of complex dye molecules on TiO2 which is only of fundamental
value. To achieve the application of catechol-TiO2 complex under realistic experimental
conditions is a quite challenging undertaking.
However, decades of intensive effort to interpret the fundamental aspect of catechol-TiO2
complex did pay off to guide the development of system to apply it into real photocatalytic
operation. Several works emerged to prove the ability of catechol-TiO2 in implementing visible
light photocatalytic generation of H2 from sacrificial agents.9-11 In addition, the photocatalytic
reduction of Cr(VI) to Cr(III) was also disclosed.11 One recent work even suggests the possibility
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for photoelectrochemically oxidation of H2O to O2 with the same complex via an oxidation of
catechol to semiquinone step.12 Quinones, abundant in nature, are the core components for bio-
inspired oxidation catalysis.13, 14 Thus, we want to develop a paradigm for visible-light-induced
selective aerobic oxidation with the ortho-quinone catalysis from oxidatively quenching of photo-
excited catechol-TiO2 complex. However, one primary concern is the stability of surface
complexed catechols to sustain the photocatalytic process, since they are quite fragile molecules
that usually need to be protected from oxidative damages and light irradiation for the purpose of
storage.
For the generation of H2 from sacrificial agents or the reduction of Cr(VI), the reducing
agents are in excess which leads the overall reaction environment to be a reductive one that can
make sure the tenacity of catechol-TiO2 complex photocatalyst to a large extent. On the contrary,
for the selective aerobic oxidation, O2 is supplied in excess to achieve the target results which in
turn engenders a predominantly oxidative circumstance. In effect, the photodegradation of catechol
on TiO2 surface under oxidative conditions was well-established by several previous reports.15, 16
Therefore, the maintenance of intactness of catechol-TiO2 complex is an issue of major
consideration that needs to be resolved. Moreover, the selective aerobic oxidation amines into
17
imines was a very popular reaction in demonstrating the activity of ZnIn2S4 or NH2-MIL-125(Ti)
metal organic framework18. Particularly, visible light activity of NH2-MIL-125(Ti) was attributed
to the ligand-to-metal charge transfer between 2-aminoterephthalate and Ti which was quite
similar to the catechol-TiO2 complex. Since quinone catalysis have a proven track record for the
aerobic oxidation of amines,19 we selected this particular reaction to demonstrate our effort in
transforming catechol-TiO2 complex into a functional photocatalyst for selective aerobic oxidation.
Our success depends on the merger of visible light photocatalysis with TEMPO catalysis to devise
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selective chemical transformations,20 a special case of the concept of cooperative photocatalysis
advanced recently by us21 and others22 .
2. Experimental
2.1 Light source
The light-emitting diodes (LEDs) were supplied by Shenzhen Qilai Optoelectronics Science
and Technology Co., LTD., China. The reaction was usually irradiated with blue LEDs. In detail,
four LED lamps were fastened around a magnetic stirrer in a circle in which the light irradiation
was centered on the 10 mL Pyrex vessel. For blue LEDs, the irradiating wavelength range is 460
± 10 nm. Thus, the possible heating of the reaction medium by infrared light and possible leaking
of UV light are completely excluded.
2.2 UV-Visible spectra
The UV-visible absorption spectra of the solid samples were recorded on a Shimadzu UV-
2700 UV-VIS spectrophotometer with a diffuse reflectance measurement accessary. The UV-
visible absorption spectra of the solutions were also carried out with the same equipments with a
normal operating mode.
2.3 PXRD
The phase composition of the anatase TiO2 sample was identified by powder X-ray diffraction
(PXRD) using a Bruker D8 Advances diffractometer.
2.4 BET
The specific surface areas were determined by N2 physisorption by using an ASAP automated
system and the Brunauer-Emmet-Teller (BET) method. Each sample was degassed under vacuum
(<1×10-5bar) in the Micromertics system at 120℃ for 12 h prior to N2 physisorption.
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2.5 ESR
The ESR experiments were carried out on an electron spin resonance (ESR) spectrometer
(JEOL, JES-FA200). The catechol-TiO2 complex photocatalyst under the standard conditions was
irradiated in the light chamber of the ESR spectrometer irradiated by a high-pressure Hg lamp with
a filter to select the band of 460 nm visible light.
2.6 GC and GC-MS
The gas chromatography (GC) quantitative measurements of conversions of substrates and
selectivities of products were made on a gas chromatograph (Agilent 7890B) equipped with a
flame ionization detector (FID) using high pure N2 as the carrier gas and an Agilent J & W DB-5
capillary column (30 m × 0.32 mm × 0.25 μm, 19091J-413).
GC-FID standard analysis conditions: injector temperature 250 °C, detector temperature
280 °C, column temperature program: 50 °C (hold 1 min) raised up to 280 °C (hold 2 min) at a
rate of 20 °C/min. The results were all obtained using a split mode with a split ratio is 30:1.
Gas chromatography–mass spectrometry (GC–MS) analysis was performed on a Shimadzu
GC 2010 gas chromatograph equipped with a Shimadzu GCMS-QP2010 Ultra electron ionization
mass spectrometer using a Restek (Rxi®-5Sil MS) capillary column (30 m × 0.25 mm × 0.25 μm)
with high pure He as the carrier gas.
2.7 General procedure for the selective oxidation of amine into imine with air
In a typical reaction, 50 mg TiO2, 2.4×10-3 mmol of CAT D, 0.3 mmol of primary
benzylamine and 0.015 mmol of TEMPO were added to 1 mL of CH3CN in a 10 mL pyrex vessel.
For the oxidation of secondary amines, 0.2 mmol of secondary amines, 50 mg TiO2, 2.4×10-3 mmol
of benzenediol, and 0.015 mmol of TEMPO were added to the reaction vessel. For 5 min of
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ultrasonication, then the reaction mixture was stirred for 1h in dark to reach adsorption equilibrium.
The 10 mL reaction Pyrex vessel was capped with butyl rubber septum. Then a rubber hole puncher
was used to punch a hole in the rubber septum to connect with air to supply the oxygen demand in
the experimental process. The reaction mixture was magnetically stirred at 1500 rpm and
illuminated with blue LEDs in an air-conditioned room with additional electronic fan cooling to
warrant the reaction temperature constantly at room temperature. At the end of reaction, the
catechol-TiO2 particles were separated from the reaction mixture by centrifugation and the
products were quantitatively analyzed by gas chromatography (GC) equipped with a flame
ionization detector (FID) using chlorobenzene as the internal standard. The structures of products
were confirmed by comparison with the retention time with authentic samples and further
confirmed by gas chromatography–mass spectrometry (GC-MS).
3. Results and discussion
Table 1. Different benzenediols complexed with TiO2 merged with TEMPO on the visible-light-
induced selective oxidation of benzylamine with air [a]
Entry
1
Benzenediol
Conv.[%][b]
55
Sel.[%][b]
90
1,2-Benzenediol
2
3
1,3-Benzenediol
1,4-Benzenediol
26
16
89
86
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[a] Reaction conditions: 0.3 mmol of benzylamine, 2.4×10-3 mmol of benzenediol, 50 mg of TiO2,
0.015 mmol of TEMPO, 3 W blue LED irradiation, 1 mL of CH3CN, 45 min. [b] Determined by
GC-FID using chlorobenzene as the internal standard, conversion of benzylamine, selectivity of
N-benzylidenebenzylamine.
The binding of catechol onto TiO2 occurs very quickly without the assistance of extra
conditions. We developed a technique by introducing precise amount of solution of catechol into
the TiO2 suspension to afford catechol-TiO2 complex to conduct the selective oxidation of
benzylamine with O2 directly in the presence of TEMPO. In this fashion, thorough investigation
to correlate different benzenediols with photocatalytic activity under visible light irradiation. All
three benzenediols including 1,2-benzenediol (CAT A), 1,3-benzenediol and 1,4-benzenediol
were screened to assess their influence on the selective oxidation of amines with O2 in air (eq. 1).
We were very pleased to find that three benzenediols can enhance the selective oxidation in all
cases (entries 1-3, Table 1), but catechol gave the best results (entry 1, Table 1). CAT A binds
with the Brønsted acid sites of TiO2 through bidentate mononuclear linkage; whilst the linkage for
the other benzenediols is bidentate binuclear one. Surface complex interaction between
benzenediols and anatase TiO2 decreased from CAT A to 1,3-benzenediol and 1,4-benzenediol.23
This is the same order of conversions of benzylamines, ascribing to visible light absorption of
benzenediol-TiO2 complex. There was previous a large π system, graphene, complexed with TiO2
as a photocatalyst for the selective aerobic oxidation of amines.24 It is quite remarkable to find out
that a small molecule like CAT A can be so efficient in implementing selective aerobic oxidation
on TiO2 under visible light irradiation.
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Table 2. Different substituted catechol complexed with TiO2 merged with TEMPO on the
visible-light-induced selective oxidation of benzylamine with air [a]
Entry
1
Catechols
Conv.[%][b]
55
Sel.[%][b]
90
CAT A
2
3
4
5
6
7
CAT B
CAT C
CAT D
CAT E
CAT F
CAT G
48
57
68
49
61
36
94
98
98
93
94
84
[a] Reaction conditions: 0.3 mmol of benzylamine, 2.4×10-3 mmol of catechols, 50 mg of TiO2,
0.015 mmol of TEMPO, 3 W blue LED irradiation, 1 mL of CH3CN, 45 min. [b] Determined by
GC-FID using chlorobenzene as the internal standard, conversion of benzylamine, selectivity of
N-benzylidenebenzylamine.
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Encouraged by the knowledge that CAT A delivers the best result for visible-light-induced
aerobic oxidation of benzylamine, we studied the influence of substituted groups of catechols on
the photocatalytic selective aerobic oxidation of benzylamines (Table 2). There are two positions
in the phenyl ring of catechol that can be occupied by substituted group, i.e. the position 3 and the
position 4. First, we used an electron donating group of -OCH3 and electron withdrawing group of
-CN to be in the position 3 of catechols, CAT B and CAT C, -OCH3 giving slight worse result
(entry 2, Table 2) and -CN giving almost the same results (Entry 3, Table 2) in comparison with
that of catechol (entry 1, Table 2). However, if the -CN group were anchored in the position 4 of
catechol, CAT D, evident increase in the conversion of benzylamine was observed (entry 4, Table
2). Nevertheless, another two electron withdrawing groups like -COOH and -CHO at the position
4 of catechol, CAT E and CAT F gave varied results (entries 5 and 6, Table 2), one is negative
and the other is positive. This might be attributed to the high tendency of -COOH to adsorb onto
TiO2 without forming visible light absorbing surface complex. Another electron donating group t-
Bu at the position 4 of catechol, CAT G, gave significantly low conversion of benzylamine. Apart
from the electronic effect, very bulky t-Bu might play a role in preventing the interaction of
catechol radical cation with TEMPO. Finally, we found that CAT D give the best results for the
selective aerobic oxidation under blue LED irradiation. We will use it to carry on further
investigation to the mechanistic insights of merging CAT D-TiO2 complex photocatalysis with
TEMPO for the visible-light-induced selective aerobic oxidation of benzylamine. The selective
aerobic oxidation of benzylamines into imines on TiO2 under UV irradiation was reported
previously.25 Here, the reaction was achieved in higher concentration of amines, higher
selectivities for imine products, much shorter reaction time and under milder irradiation of visible
light and quite different reaction mechanism.
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The formation of catechol-TiO2 complex through bidentate mononuclear linkage was
unequivocal according to ATR-FTIR and UV-visible absorbance (Figures S1 and S2). This mode
of adsorption of catechol on anatase TiO2 nanoparticles has been directly evidenced by combined
solid state NMR and DFT calculation.26 Moreover, there is little difference between CAT A- and
CAT D-TiO2 complexes. Benzylamine, TEMPO, CAT A, and CAT D does not absorb any visible
light, implying catechol-TiO2 complex was an authentic visible-light photocatalyst (Figure S3).
We subtracted the absorption of TiO2 to get visible light absorption of the surface complex (Figure
S3, a) which is in significant overlap with the light emitting spectrum of blue LED (Figure S3, b),
giving credence to the second-best results amongst the LEDs tested (Table S1). Next, we
performed a plethora of control experiments to elucidate the nature of the selective aerobic
oxidation (Table S2). It was found that CAT D, TiO2 and TEMPO is an integral component of the
visible light photocatalysis. Without one, two or three of them, the reaction can be stopped or
significantly hampered (entries 1-7, Table S2). Benzylamine can directly form surface complex
with TiO2 that can lead to self-catalyzation under λ>420 nm visible light irradiation.27 However,
the control experiments unequivocally suggests that reaction induced by the surface complexation
between benzylamine and TiO2 (entry 1 vs. entry 2, Table S2) but induced by catechol-TiO2
complex photocatalyst under blue LED irradiation. Besides, none of the individual component can
absorb visible light (Figure S4). Without light irradiation, the reaction stopped (entries 8, Table
S2), suggesting it is a true photocatalytic reaction. Control experiments regarding the nature
reactive oxygen species indicated that superoxide but not singlet oxygen was involved in affording
the imine product (entries 9-12, Table S2). Furthermore, we thoroughly studied the impact of the
initial O2 pressure and found that it had moderate influence on the aerobic oxidation of
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benzylamine (Table S3). Therefore, atmosphere O2 was an ideal oxidant in the present system for
the visible-light-induced selective oxidation of benzylamine.
The reaction proceeded well in different inert organic solvents (Table S4); TiO2 was of
great importance in prompting the oxidation reaction (Table S5). These results suggest that the
reaction conditions are quite general and further proves TiO2 is an integral component of the
complex as a photocatalyst. With the replacing of the anatase TiO2, with rutile and Aeroxide P25
TiO2, 1% and 19% conversions of benzylamine were obtained in 45 min, much inferior to that of
the anatase TiO2. Next, we carried out a battery of kinetics studies to shed more light on the
reaction mechanism. It was uncovered that the visible-light-induced selective oxidation of
benzylamine follows zero-order reaction (Figure 1, a), with a reaction rate constant kH of 0.00525
molꞏL-1ꞏmin-1. The oxidation of benzyl-α,α-d2-amine conforms to the same reaction order and a
reaction rate constant kD of was obtained as 0.00261 molꞏL-1ꞏmin-1. Thus, the kinetics isotope
effect value kH/kD is 2.01, suggesting the Cα-H abstraction by TEMPO partially be involved in the
rate determining step. To determine the nature of benzylamine radical after abstraction of Cα-H,
Hammett plot for the oxidation kinetics of para-substituted benzylamines (Figure S5) was
retrieved by correlating the electronic effect of para-substituted groups with log (kX/kH) to get an
almost flat linear relationship (Figure 1, a), suggesting a neutral free radical participated in the
reaction processes.
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Figure 1. Reaction kinetics studies on merging the visible light photocatalysis of CAT D-TiO2
complex with TEMPO using air as the oxidant (a) the selective oxidation of benzylamine (a); (b)
the selective oxidation of benzyl-α,α-d2-amine; (c) Hammett plot for the oxidation of para-
substituted benzylamines. (a) 0.3 mmol of benzylamine, (b) 0.3 mmol of benzyl-α,α-d2-amine, (c)
0.3 mmol of para-substituted benzylamine. Reaction conditions: 2.4×10-3mmol of CAT D, 50 mg
of TiO2, 0.015 mmol of TEMPO, 3 W blue LED irradiation, 1 mL CH3CN.
Without adding TEMPO into the photocatalytic system, we do observe N-centered free
radical with ESR, indicating N radical cations was involved in the product formation. However,
after adding TEMPO, the reaction pathways shifted divergently. TEMPO was intensively studied
for the oxidation of alcohols. The application of it in the oxidation of amines was quite exiguous.
Only several reports appeared in literature such as selective aerobic oxidation of primary amines
to imines28, 29, oximes30 and nitriles31 with TEMPO and a transition metal catalyst. We previously
disclosed the cooperative photocatalysis of dye-sensitized TiO2 and TEMPO for visible-light-
induced selective aerobic oxidation of amines to imines.32 Besides the practical purpose, the
oxidation amines to imines can be viewed as an analogue for oxidation of alcohols,33 offering new
perspective for mechanistic implications. To further distinguish the role of TEMPO in the
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photocatalytic process, we used TEMPO as a stoichiometric oxidant after replacing the atmosphere
O2 with N2. Indeed, we found that imine can be afforded accompanied by generation of TEMPOH
as the other product (eq. 2). TEMPOH can be rapidly re-oxidized back to TEMPO by superoxide
(O2ꞏ-), a well-accepted practice in biological or chemical studies.34, 35 By the same token, we
hypothesized that the same catalytic amount of TEMPOH should be considered the equivalent of
TEMPO. Thus, we used 5 mol% of TEMPOH to start out the investigation for the visible-light-
induced aerobic oxidation of benzylamine and found almost the same results as TEMPO as the
starting agent (eq. 3).
Based on the results above mentioned results including condition screenings, control
experiments, kinetics studies, etc., a plausible reaction mechanism has been proposed in Figure 2.
CAT D belongs to type II sensitizer during the electron injecting to the conduction band of anatase
TiO2. In addition, this type of sensitizer can lead to new broad absorption, accounting for all the
visible light absorption in the present case. In this fashion, CAT D surface complex does not
experience the excited state but rather directly injects electron to the conduction band of TiO2
under blue LED irradiation. Simultaneously, radical cation will be generated at the CAT D surface
complex which is under oxidative stress that need to be quickly released to safeguard its integrality.
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Figure 2. Plausible mechanism for the visible-light-induced selective aerobic oxidation of
amines by catechol-TiO2 complex photocatalysis merged with TEMPO
Hence, the catalytic cycle of TEMPO plays a such pivotal role in quenching the radical
cation state of CAT D surface complex to refurbish its ground state. TEMPO+ will be created to
commence the oxidation of benzylamine, producing benzylamine radical whose deprotonation
resulting in the formation of benzaldimine. Before adding TEMPO, the mechanism should be quite
similar to Au25/TiO2 photocatalyst in which the formation of imine is governed by single electron
transfer.36 We could use ESR to track the presence of N-centered free radicals (Figure S6). In
contrast, with the involvement TEMPO, direct two-electron transfer pathway takes control of the
oxidation of amine. The coupling between benzaldimine and benzylamine. In our previous report
for the photocatalytic aerobic oxidation of sulfides, TEMPO was reestablished from TEMPO+. 37
Currently, Cα-H of benzylamine was involved, resulting in the formation of TEMPOH which will
be restore to TEMPO by O2ꞏ- formed by the reduction of O2 by conduction band electron.
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Figure 3. The ESR spectra recorded during the selective oxidaiton of benzylamine by CAT D-
ꞏ-
TiO2 photocatalysis (a) spin trapping of superoxide radicals (O2 ) with DMPO (5,5-dimethyl-1-
pyrroline N-oxide), (b) TEMPO
We have confirmed that singlet oxygen was of negligent involvement but O2ꞏ- was an
overwhelmingly critical factor in the formation of imine through control experiments of using
CD3CN as a solvent and p-BQ quenching. To support this argument in a further step, we adopted
in situ ESR technique to verify the free radicals that got involved in the reaction process (Figure
3). We can clearly observe the characteristics of DMPO trapped O2ꞏ- and the signal increased with
the progress of time (Figure 3, a), indicating the accumulation of DMPO trapped O2ꞏ-. Thus, it was
constantly produced during the visible-light-induced photocatalytic process. Because TEMPO is
an ESR active specie, we followed the signal of it during the progress of reaction. The ESR signal
persists in the 10 min of irradiation, but the strength of the signal decreased (Figure 3, b),
suggesting it was in part transformed into ESR inactive species, in agreement with the proposed
mechanism in Figure 2.
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Table 3. Visible-light-induced selective oxidation of amines into imines with air by CAT D-TiO2
complex photocatalysis merged with TEMPO [a]
Conv.
[%] [b]
Sel. [%]
Entry
Substrate
Product
T(h)
1.2
3.5
1.0
1.0
1.0
1.0
1.0
1.0
1.0
[b]
1
2[c]
3
98
95
95
98
90
92
90
90
90
98
98
98
97
97
98
96
95
97
4
5
6
N
7
Cl
Cl
8
9
10
1.5
94
98
O
O
N
11
12
13
1.5
1.2
1.5
97
94
91
96
92
97
O
O
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[a] Reaction conditions: 0.3 mmol of primary amine, 2.4×10-3 mmol of CAT D, 50 mg of TiO2,
0.015 mmol of TEMPO, 3 W blue LED irradiation, 1 mL of CH3CN. [b] Determined by GC-FID
using chlorobenzene as the internal standard, conversion of amine, selectivity of corresponding
imine. [c] 1.2 mmol of primary amine, 4.8×10-3 mmol of CAT D, 100 mg of TiO2, 0.06 mmol of
TEMPO, 3 W blue LED irradiation, 1 mL of CH3CN, 3.5 h.
After the clarification of mechanistic insights, we then evaluated a series of primary amines
to probe the reaction scope and to generate preliminary information to further verify the
mechanism (eq. 4). The scope for the selective aerobic oxidation of primary amines by merging
the visible light photocatalysis of CAT D-TiO2 complex (PXRD characterization, see Figure S7)
and TEMPO was listed in Table 3. We first demonstrated that our reaction system can be carried
out in very high concentration of amine and scaled up to the gram level of imine product (entry 1,
Table 3). The CAT D-TiO2 complex photocatalyst was recycled. And 80% yield for N-
benzylidenebenzylamine was achieved with adding CAT D. Importantly, we increased
benzylamine and TEMPO catalyst to 4 times and CAT D-TiO2 complex to twice, 0.103 g of N-
benzylidenebenzylamine can be afforded as 88% yield of imine product (entry 2, Table 3) in which
the initial concentration of benzylamine is 1.2 molꞏL-1 and turnover number in terms of CAT D
is 250, quite remarkable results for photocatalysis based on TiO2. Para-substituted benzylamines
with electron-donating or electron-withdrawing substitutes underwent the selective oxidation into
imines with air with high conversions and selectivities (entries 3-8, Table 3). Importantly, the
needed time to achieve comparable conversions for these different benzylamines were almost the
same which is in agreement with the Hammett plot result and the proposed mechanism. This
reaction pattern holds for the oxidation of meta-substituted benzylamine (entry 9, Table 3) and
breaks for ortho-substituted benzylamine (entry 10, Table 3) in which longer time was needed. We
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attributed this effect to the steric hindrance of the substitutes during the interaction with CAT D
surface complex or TEMPO. Di-substituted benzylamine (entry 11, Table 3) also needed longer
time to finish the oxidation reaction. Heteroatomatic methylamines such as 2-
pyridinemethylamine and 2-thiophenemethylamine underwent the visible light photocatalytic
aerobic oxidation into imines effectively (entries 12 and 13, Table 3).
4. Conclusion
In summary, we have developed a visible light photocatalyst system based on the catechol
surface complexed TiO2 for the selective oxidation of amines into imines with air with the
guidance many years of fundamental research of the interfacial interaction between catechol and
TiO2. The key to our success is the merging the catechol-TiO2 complex photocatalysis with
TEMPO catalysis to make the photocatalyst durable under aerobic oxidative conditions and blue
LED irradiation. We found that O2ꞏ- was essential reactive oxygen species in the oxidation process.
Moreover, the degradation role of O2ꞏ- could be sufficiently contained by TEMPOH to regenerate
TEMPO which in turn results in high selectivities for imine products through the orchestration of
the photocatalytic and catalytic cycles.
Acknowledgment
Financial support from the National Natural Science Foundation of China (grant numbers
21503086 and 21773173), the Fundamental Research Funds for the Central Universities (grant
number 2042018kf0212) and the start-up fund of Wuhan University is gratefully acknowledged.
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Table of contents entry
Merging the catechol-TiO2 complex photocatalysis with TEMPO
catalysis can successfully implement the selective oxidation of amines
into imines with air under blue LED irradiation.