Selectivity-tunable oxidation of tetrahydro-β-carboline over an OMS-2 composite catalyst: preparation and catalytic performance

Article abstract of DOI:10.1039/d1dt00168j

Controlling the reaction selectivity of organic transformations without losing high conversion is always a challenge in catalytic processes. In this work, a H₃PO₄·12WO₃/OMS-2 nanocomposite catalyst ([PW]-OMS-2) was prepared through the oxidation of a Mn(ii) salt with sodium phosphotungstate by KMnO₄. Comprehensive characterization indicates that different Mn²⁺precursors significantly affected the crystalline phase and morphology of the as-synthesized catalysts and only MnSO₄·H₂O as the precursor could lead to a cryptomelane phase. Moreover, [PW]-OMS-2 demonstrated excellent catalytic activity toward aerobic oxidative dehydrogenation of tetrahydro-β-carbolines due to mixed crystalline phases, enhanced surface areas, rich surface oxygen vacancies and labile lattice oxygen species. In particular, β-carbolines and 3,4-dihydro-β-carbolines could be obtained from tetrahydro-β-carbolines with very high selectivity (up to 99%) over [PW]-OMS-2viatuning the reaction solvent and temperature. Under the present catalytic system, scalable synthesis of a β-carboline was achieved and the composite catalyst showed good stability and recyclability. This work not only clarified the structure-activity relationship of the catalyst, but also provided a practical pathway to achieve flexible, controllable synthesis of functional N-heterocycles.

Full text of DOI:10.1039/d1dt00168j

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Selectivity-tunable oxidation of tetrahydro-
β-carboline over an OMS-2 composite catalyst:
preparation and catalytic performance†
Cite this: Dalton Trans., 2021, 50,
3682
Xiuru Bi,a,b Luyao Tao,a,b Nan Yao,a Mingxia Gou,a Gexin Chen,a Xu Meng *a and
Peiqing Zhao*a
Controlling the reaction selectivity of organic transformations without losing high conversion is always a
challenge in catalytic processes. In this work, a H3PO4·12WO3/OMS-2 nanocomposite catalyst ([PW]-
OMS-2) was prepared through the oxidation of a Mn(II) salt with sodium phosphotungstate by KMnO4.
Comprehensive characterization indicates that different Mn2+ precursors significantly affected the crystal-
line phase and morphology of the as-synthesized catalysts and only MnSO4·H2O as the precursor could
lead to a cryptomelane phase. Moreover, [PW]-OMS-2 demonstrated excellent catalytic activity toward
aerobic oxidative dehydrogenation of tetrahydro-β-carbolines due to mixed crystalline phases, enhanced
surface areas, rich surface oxygen vacancies and labile lattice oxygen species. In particular, β-carbolines
and 3,4-dihydro-β-carbolines could be obtained from tetrahydro-β-carbolines with very high selectivity
(up to 99%) over [PW]-OMS-2 via tuning the reaction solvent and temperature. Under the present catalytic
system, scalable synthesis of a β-carboline was achieved and the composite catalyst showed good stability
and recyclability. This work not only clarified the structure–activity relationship of the catalyst, but also
provided a practical pathway to achieve flexible, controllable synthesis of functional N-heterocycles.
Received 18th January 2021,
Accepted 12th February 2021
DOI: 10.1039/d1dt00168j
rsc.li/dalton
Pictet–Spengler reaction between a tryptamine derivative and
an aldehyde or carboxylic acid followed by oxidative dehydro-
Introduction
The β-carboline skeleton is one of the most intriguing indole genation to provide the corresponding β-carbolines and 3,4-
alkaloids which are widely present in marine organisms, dihydro-β-carbolines.21–23 According to the report, the tra-
plants, microorganisms, and fungi.1,2 In particular, aromatic ditional process of aerobic oxidative dehydrogenation is
β-carboline compounds have attracted much attention due to usually carried out by heating with palladium on carbon24 or
their excellent biological and pharmaceutical activities, such toxic sulfur,5,22 or treating with strong stoichiometric oxidants
as antimalarial,3 antibacterial,4 antitumor,5 and anti-HIV such as SeO2,25 MnO2,26 and KMnO4 at high temperature
20
activities.3,6 Besides, some β-carboline derivatives can be used over
a prolonged time. Besides, dichlorodicianoquinone
as potential therapeutic agents for neurodegenerative dis- (DDQ),27 trichloroisocyanuric acid (TCCA),28 (diacetoxyiodo)
orders, such as Alzheimer’s and Parkinson’s disease,7 because benzene PhI(OAc)2,21 N-chlorosuccinimide (NCS),29,30 and
of the inhibition of acetylcholinesterase8 and human mono- 2-iodoxybenzoic acid (IBX)31 have also been used to perform
amine oxidase.9 Besides β-carbolines, 3,4-dihydro-β-carbolines oxidative dehydrogenation, but these methods suffer from
can be used as exclusive intermediates for the synthesis of drawbacks such as the use of excess reagents, generation of
pharmaceutical molecules via further modification.10 In view stoichiometric waste, poor yields and a lack of practicability.
of the important pharmacological properties of β-carbolines, In particular, there are no reports on selectivity-tunable oxi-
many synthetic methods have been developed so far.11–20 One dative synthesis of β-carbolines and 3,4-dihydro-β-carbolines to
classical approach to generate tetrahydro-β-carbolines is date. Therefore, it is of great practical significance to develop
an efficient, high chemoselectivity, and environmentally
friendly method for syntheses of β-carbolines and 3,4-dihydro-
β-carbolines.
aState Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research
Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of
Manganese oxide octahedral molecular sieve (OMS-2) is a
Sciences, Lanzhou 730000, China. E-mail: xumeng@licp.cas.cn, zhaopq@licp.cas.cn
cryptomelane-type microporous nanostructural manganese
oxide, which is formed of 2 × 2 edge-shared MnO6 octahedral
chains and composed of corner-connected MnO6 octahedral to
bUniversity of Chinese Academy of Sciences, Beijing 100049, China
†Electronic supplementary information (ESI) available: Experimental procedures
and data. See DOI: 10.1039/d1dt00168j
3682 | Dalton Trans., 2021, 50, 3682–3692
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form 4.6 × 4.6 Å tunnels.32–35 Manganese is present in OMS-2 million relative to the residual solvent peaks CDCl3 and
as Mn2+, Mn3+, and Mn4+, and K+ exists in the tunnels to (CD3)2SO. Chemical shifts of 1H NMR spectra were recorded
balance the whole valence state and stabilize the tunnel relative to TMS (δ 0.00) or residual deuterated solvents (CDCl3:
structure.36–39 Due to its unique structural properties, redox δ 7.26; (CD3)2SO: δ 2.50). Chemical shifts of 13C NMR spectra
ability, adsorption, ion-exchange, and semiconductivity, were recorded relative to solvent resonance (CDCl3: δ 77.16;
OMS-2 has been widely applied to environmental catalysis, (CD3)2SO: δ 39.52).
electrode materials, semiconductors, separation, organic syn-
thesis and many other fields.40–46 Because of abundant oxygen Catalyst preparation
vacancy and mixed-valence Mn ions on the surface, OMS-2 is
All the OMS-2-based composites (2 mol% of dopant to the
amount of KMnO4) in the present research were prepared via
able to readily activate O2 or H2O2 used as a green oxidant in
clean synthesis of organic molecules.47 In the past years, our
the reaction between Mn2+ and potassium permanganate
group has focused on the fabrication and modification of
with a reflux method. Taking MnSO4·H2O as a Mn2+ precur-
materials involving OMS-2 and their catalytic application in
sor, the typical procedure was as follows: in a round bottom
flask, MnSO4·H2O (8.80 g, 52 mmol) and sodium phospho-
the clean synthesis of heterocycles.48–51 To effectively improve
the selectivity and conversion of organic synthesis, the devel-
tungstate (2.18 g, 0.74 mmol) were dissolved in 30 mL of de-
opment of new catalysts, modification of pathways and tuning
ionized water followed by the addition of 3 mL of HNO3 at
catalytic systems are meaningful.
room temperature. Then, 100 mL of KMnO4 (5.89 g,
Very recently, we have prepared an OMS-2-based nano-
37 mmol) solution was added dropwise into the above solu-
composite doped with sodium phosphowolframate for the first
tion under vigorous stirring at room temperature. After
time and applied it to the aerobic oxidative dehydrogenation
the resulting solution was refluxed at 100 °C for 24 h,
the resulting solid was collected by filtration, washed
of N-heterocycles.48 Sufficient characterization indicated that
the catalyst had newly generated mixed crystal phases of
with deionized water, dried in air at 80 °C for 8 h, and
H3PO4·12WO3 and cryptomelane by doping of 2 mol% sodium
ground into a powder to afford the OMS-2-based nano-
phosphowolframate, which was critical for the superior cata-
composite which was labeled as [PW]-OMS-2. When MnCl2,
lytic performance. Kinetic studies showed that the presence of
Mn(NO3)2, Mn(Ac)2, MnCO3, and anhydrous MnSO4 were
the mixed crystal could significantly increase the initial reac-
used as precursors, the catalytic materials prepared were
tion rate and decrease the apparent activation energy of oxi-
labeled as [PW]-OMS-2-Cl, [PW]-OMS-2-NO, [PW]-OMS-2-Ac,
dation reaction. To develop a new synthetic strategy for 3,4-
[PW]-OMS-2-CO, and [PW]-OMS-2-SO, respectively.
dihydro-β-carboline and β-carboline compounds, we attempted
to selectively oxidize tetrahydro-β-carbolines over OMS-2 doped
with sodium phosphotungstate.
General procedure for [PW]-OMS-2-catalyzed 3,4-dihydro-
β-carboline synthesis
OMS-2 materials are usually prepared via the oxidation reac-
Typically, a mixture of [PW]-OMS-2 (20 mg, 13 mol%), tetrahy-
tion between Mn2+ and KMnO4. Different Mn2+ precursors can
dro-β-carbolines (0.2 mmol), and 1 mL mixture solvent of
have various effects on the physicochemical properties of the
acetonitrile/toluene (v/v = 1/1) in a reaction tube was magneti-
catalyst, for example, the crystal phase, surface area, and redox
cally stirred at 80 °C for 8 h in air. The reaction process was
monitored by thin layer chromatography (TLC). After com-
properties, and consequently affect the catalytic perform-
ance.52 In this work, we investigated the effects of Mn2+ precur-
pletion of the reaction, the reaction mixture was diluted with
sors on the preparation of OMS-2 doped with 2 mol% sodium
ethyl acetate and filtered. The filtrate was removed using a
phosphotungstate to obtain the optimal hybrid catalyst with a
rotary evaporator at reduced pressure, and then the pure
synergistic effect of mixed phases. With the as-synthesized
product was obtained by silica gel column chromatography
OMS-2 nanocomposite, selectivity-tunable oxidative dehydro-
(petroleum/ethyl acetate = 5/1 as the eluent). The composition
genation of tetrahydro-β-carbolines was realized for the first
and structure of target products were determined by 1H and
time through tuning the catalytic system.
13C NMR spectroscopy.
General procedure for [PW]-OMS-2-catalyzed β-carboline
synthesis
Experimental
General information
[PW]-OMS-2 (20 mg, 13 mol%), tetrahydro-β-carboline com-
All chemicals used here were of analytical grade, were pur- pounds (0.2 mmol), and 1.0 mL o-dichlorobenzene were added
chased from commercial suppliers and were used without into a reaction tube. The mixture was stirred at 130 °C under
further purification. All experiments were carried out under air air for the indicated time. After completion of the reaction, the
or under an O2 balloon. Standard column chromatography was reaction mixture was cooled to ambient temperature. Then,
performed on 200–300 mesh silica gel (obtained from Merck) the reaction mixture was diluted with ethyl acetate and filtered.
using flash column chromatography techniques. 1H and 13C The resulting filtrate was concentrated under reduced pressure
NMR spectra for compound characterization were recorded on to give the crude product which was purified by silica gel
WNMR-1 500. Chemical shift values are given in parts per chromatography to yield the pure product.
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Catalyst characterization
OMS-2-CO had the same ε-MnO2 phase (JCPDS file #30-820).
The sizes of the as-prepared catalyst materials with different
precursors were calculated from the line broadening of XRD
peaks using Scherrer’s formula (d = 0.9λ/β cos θ). The as-calcu-
lated crystal sizes of [PW]-OMS-2-Cl, [PW]-OMS-2-NO, [PW]-
OMS-2-Ac, [PW]-OMS-2-CO, [PW]-OMS-2-SO, and [PW]-OMS-2
were 13.5, 10.2, 12.1, 11.7, 10.3, and 9.1 nm, respectively.
These observations suggested that Mn2+ precursors directly
affected the crystalline structure. Among the tested Mn(II)
salts, just manganese sulfate (with or without crystal water)
was able to afford cryptomelane-type OMS-2.
The XRD patterns of the as-prepared catalysts were collected
using powder X-ray diffraction with an X-Pert PRO X-ray diffr-
actometer using Cu Kα radiation (λ = 0.15406 nm). The data of
2θ from 5 to 80° were collected with a step size of 0.065°. X-ray
photoelectron spectroscopy (XPS) analysis was conducted on a
Kratos AXIS Ultra DLD high-performance electron spectro-
meter operated at a pass energy of 30 eV with Al Kα (λ = 1486.7
eV) as an exciting X-ray source. The binding energy was cali-
brated with the contaminant carbon value of 284.8 eV. The
specific surface areas of the catalysts were determined from N2
adsorption/desorption analysis at 76 K using an ASAP 2020 M
analyzer. The samples were first pretreated at 300 °C for
4 hours before measurements. The specific surface areas were
determined utilizing the BET model for the calculation of
specific surface areas. The morphology was observed with
FE-SEM (Quanta 400FEG) and HRTEM (Tecnai G2 F20 S-Twin
200 kV).
Subsequently, the BET surface areas and porosities of the
[PW]-OMS-2 materials were determined by N2 adsorption–de-
sorption, and the results are presented in Table 1. Compared
with original OMS-2, it is obvious to summarize that the
dopant of sodium phosphotungstate significantly enhanced
the BET surface area. The influence of surface area on the cata-
lytic materials prepared with different Mn2+ precursors was
different. Among them, the as-synthesized materials with the
ε-MnO2 crystal phase had similar specific surface areas
(188–224 m2
g
−1). Although [PW]-OMS-2-SO and [PW]-
Results and discussion
Characterization of [PW]-OMS-2 composite catalysts
OMS-2 had similar mixed crystal phases of cryptomelane and
phosphotungstic acid, the latter (204 m2 g−1) was twice as
large as the former (102 m2 g−1). The remarkable difference in
surface areas between [PW]-OMS-2-SO and [PW]-OMS-2 further
indicated the notable effect of Mn2+ precursors for the syn-
thesis of the catalyst. Mn2+ precursors not only influenced the
crystal structure of the products, but also altered their textural
properties. Generally, the catalytic performance of catalytic
materials is closely related to their surface areas, and a higher
surface area tends to expose more catalytically active sites to
enhance the catalytic performance.
In our previous study, the oxygen and metal species on the
surface of the catalyst have a vital influence on the catalytic
performance in oxidation reactions.47–51 Therefore, X-ray
photoelectron spectroscopy was employed to analyze the
surface elemental species (Fig. 2 and 3, Tables 1 and 2).
[PW]-OMS-2-Cl, [PW]-OMS-2-NO, [PW]-OMS-2-Ac and [PW]-
OMS-2-CO with ε-MnO2 phases all showed similar contents
of different valences of manganese. Due to the influence of
OMS-2 composite catalysts ([PW]-OMS-2) were synthesized and
obtained by the use of different Mn(II) salts, including MnCl2,
Mn(NO3)2, Mn(Ac)2, MnCO3, MnSO4·H2O and anhydrous
MnSO4, with sodium phosphotungstate as the dopant. XRD
patterns of the as-prepared catalysts derived from different
Mn2+ precursors are depicted in Fig. 1. From the analysis of
XRD spectra, it was found that the catalysts prepared from
MnSO4 or MnSO4·H2O were well crystallized with the crypto-
melane structure, characteristic of the octahedral molecular
sieve material (JCPDS file #29-1020). In addition, [PW]-OMS-2
also showed the mixed crystal phase of phosphotungstic acid
(H3O40PW12·xH2O, JCPDS file #38-178). When anhydrous
MnSO4 was used as the precursor, the crystal type of phospho-
tungstic acid was more obvious. However, the diffraction peaks
of [PW]-OMS-2-Cl, [PW]-OMS-2-NO, [PW]-OMS-2-Ac, and [PW]-
Table 1 Textural properties of OMS-2 and the [PW]-OMS-2 catalysts
Surface area
Pore volume
Pore size
(nm)
Catalyst
(m2 g−1
)
(cm3 g−1
)
AOSa
OMS-2
67
188
193
207
224
102
204
0.37
0.51
0.56
0.39
0.35
0.34
0.59
28
11
12
8
6
13
13
3.83b
3.39
3.36
3.28
3.40
3.39
3.65
[PW]-OMS-2-Cl
[PW]-OMS-2-NO
[PW]-OMS-2-Ac
[PW]-OMS-2-CO
[PW]-OMS-2-SO
[PW]-OMS-2
a Average oxidation state (AOS) of Mn was calculated according to the
binding energy difference (ΔE) of Mn 3s in XPS through an empirical
formula, i.e. AOS = 8.956–1.126 × ΔE. ΔE: the splitting width (Δ) of the
XPS spectra in the Mn 3s region. b AOS determination by the magnetic
method.53
Fig. 1 XRD patterns of the [PW]-OMS-2 catalysts synthesized using
different Mn2+ precursors.
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Table 2 The O 1s XPS of the as-synthesized materials
Binding energy of O 1s
(eV)
Oxygen species (%)
Catalyst
Surface O
Lattice O
Surface O
Lattice O
OMS-2
531.12
531.08
531.17
531.36
531.18
531.21
531.43
530.03
529.66
529.72
529.83
529.68
529.79
529.94
33.6
38.1
29.6
33.5
29.8
32.2
19.6
66.4
61.9
70.4
66.5
70.2
67.8
80.4
[PW]-OMS-2-Cl
[PW]-OMS-2-NO
[PW]-OMS-2-Ac
[PW]-OMS-2-CO
[PW]-OMS-2-SO
[PW]-OMS-2
Fig. 2 The Mn 3s XPS patterns of the as-synthesized materials.
Fig. 4 SEM images of (a) [PW]-OMS-2-Cl, (b) [PW]-OMS-2-NO, (c)
[PW]-OMS-2-Ac, (d) [PW]-OMS-2-CO, (e) [PW]-OMS-2-SO, and (f)
[PW]-OMS-2.
The effect of different Mn2+ precursors on MnO2 crystal
growth was also observed with SEM (Fig. 4) and TEM (Fig. 5).
SEM diagrams showed that [PW]-OMS-2-Cl, [PW]-OMS-2-NO,
[PW]-OMS-2-Ac, and [PW]-OMS-2-CO were composed of nano-
particles and some clusters. However, both [PW]-OMS-2-SO
and [PW]-OMS-2 showed a typical uniform nanorod mor-
Fig. 3 The O 1s XPS patterns of the as-synthesized materials.
complex multiplet splitting, binding energy values overlapping
and the loss of the satellite peak, it is hard to calculate the
valence of manganese from Mn 2p. Instead, the average oxi-
dation state (AOS) can more accurately reflect the oxidation
state of the Mn species.54 Through the analysis of the Mn 3s
XPS spectra, the average oxidation state (AOS) of Mn was calcu-
lated based on the following formula: AOS = 8.956–1.126 × ΔE,
where ΔE is the binding energy difference between the doublet
Mn 3s peaks as shown in Fig. 2. Compared with the original
OMS-2, the AOS of [PW]-OMS-2 materials decreased to
different degrees, while only [PW]-OMS-2 showed a slight
decrease. Next, the O 1s XPS spectra were fitted into two peaks
including surface adsorbed unsaturated oxygen species with
high binding energy and saturated lattice oxygen with low
binding energy (Fig. 3 and Table 2). We can see an obvious
shift of the lattice oxygen peak on comparing [PW]-OMS-2 with
the other OMS-2-based materials and original OMS-2, which
means that there are weaker interactions between Mn and O
atoms and the reactivity of lattice oxygen improved. Moreover,
[PW]-OMS-2 showed the highest content of lattice oxygen
species (80.4%) compared with others.
Fig. 5 TEM images of (a) [PW]-OMS-2-Cl, (b) [PW]-OMS-2-NO, (c)
[PW]-OMS-2-Ac, (d) [PW]-OMS-2-CO, (e) [PW]-OMS-2-SO, and (f)
[PW]-OMS-2.
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1). When the raw material and the dopant of the catalyst, such
as H3PO4·12WO3, KMnO4 and MnSO4·H2O, were added
respectively to the reaction system as the catalyst, unsatisfac-
tory conversions and selectivities of 2a and 3a were observed
(Table 3, entries 2–4). The catalytic effectiveness of the OMS-2
and H3PO4·12WO3 mixture is similar to that of single OMS-2.
That is possibly because simple physical mixing did not
change the structure and enhance the performance of OMS-2
(Table 3, entries 5 and 6). Subsequently, the original OMS-2
and [PW]-OMS-2 catalytic materials prepared from different
precursors were used in the catalytic system to test their
activity (Table 3, entries 7–12). Among them, the reaction cata-
lyzed by OMS-2 achieved 72% conversion, 57% selectivity of
2a, and 43% selectivity of 3a. For the catalytic material pre-
pared with MnCO3 as the precursor, the substrate conversion
was 79%, and the main product was 3,4-dihydro-β-carboline 2a
(Table 3, entry 9). In the presence of [PW]-OMS-2-Ac, no de-
hydrogenation products were obtained in spite of high conver-
sion of the substrate (Table 3, entry 10). The rest of the cata-
lysts using different Mn2+ precursors all showed moderate or
good substrate conversion, and the main dehydrogenation
product was β-carboline 3a (Table 3, entries 7, 8 and 11). In
particular, [PW]-OMS-2 facilitated the reaction with the highest
catalytic activity and 3a was obtained at ∼100% conversion
with 99% selectivity (Table 3, entry 12). These experimental
results indicated that only cryptomelane-type OMS-2 catalysts
with mixed crystal phases could give 100% conversion of 1a
with excellent selectivity of 3a compared with ε-MnO2 type cat-
alysts. Meanwhile, we can conclude that more abundant
oxygen vacancy on the catalyst, like [PW]-OMS-2, led to high
conversion in the oxidation (Table 3, entry 6 vs. entry 12).
Moreover, appropriate mixed crystal phases of OMS-2 and
phosphotungstic acid and surface area could influence the
selectivity of the reaction (Table 3, entry 11 vs. entry 12).
Fig. 6 Element mapping of [PW]-OMS-2.
phology that was the same as original OMS-2 (Fig. S1 in the
ESI†). These results on morphology mean that with anhydrous
MnSO4 or MnSO4·H2O as the Mn2+ precursor, sodium phos-
photungstate doping did not change the structure and mor-
phology of OMS-2-based composites. From the TEM images
(Fig. 4), it was found that [PW]-OMS-2-Cl, [PW]-OMS-2-NO,
[PW]-OMS-2-Ac, and [PW]-OMS-2-CO all had lamellar struc-
tures that were the same as that of a typical ε-MnO2
material.55,56 For [PW]-OMS-2-SO, a irregular surface of OMS-2
nanorods could be observed, which might be phosphotungstic
acid generated in situ from sodium phosphotungstate co-
ordinated with MnO6 uniting on the nanorods (Fig. 5e).
However, [PW]-OMS-2 had a relatively smooth surface as
observed from the TEM images (Fig. 5f). These observations
were in accordance with the XRD patterns of [PW]-OMS-2-SO
and [PW]-OMS-2, and the former had much stronger diffrac-
tion peaks of phosphotungstic acid than the latter (Fig. 1).
Lastly, [PW]-OMS-2 was selected as a sample for EDX ana-
lysis because it had the mixed crystal phases with the largest
surface area and the most potential catalytic ability based on
XPS analysis (Fig. 6). EDX results illustrated that the doped
elements (P and W) were evenly distributed on the catalyst
surface. However, no Na ions existed on the sample (Table S1
in the ESI†). Taking the aforementioned characterization into
account, phosphotungstic acid formed in situ from the
decomposition of sodium phosphotungstate coordinated with
OMS-2 nanorods to generate the H3PO4·12WO3/OMS-2 nano-
composite material.
So far, we determined that [PW]-OMS-2 was the optimal
catalyst. Then, we further optimized the reaction conditions.
We tried to use oxygen, hydrogen peroxide and air as oxidants,
respectively, and the results showed that the oxidative dehydro-
genation reaction had the highest conversion and selectivity in
air (Table 3, entries 12–14). The results showed that the reac-
tion was much worse in N2 compared with that in air, which
means that the reaction was aerobic oxidation (Table 3, entry
15). Next, a lot of polar solvents were examined to further
improve the yield of 3a at 130 °C with [PW]-OMS-2 (Table 3,
entries 16–18). It was found that o-dichlorobenzene gave the
highest conversion of 1a (∼100%) and selectivity of 3a (99%)
within 8 h (Table 3, entry 12). Under the same reaction con-
Catalytic activity of [PW]-OMS-2 composite materials
In order to examine the activity of the as-synthesized catalysts ditions, we decreased the reaction temperature to 80 °C, and
using different precursors for selective oxidative dehydrogena- the results showed that the conversion of 1a was reduced to
tion of tetrahydro-β-carbolines, we initially tested different 85% and the selectivity of 3a was 66% (Table 3, entry 19).
[PW]-OMS-2 materials with 1-phenyl-tetrahydro-β-carboline as When using some solvents with relatively low boiling points in
the substrate. Firstly, a blank experiment was performed to the catalytic system, the selectivity of the reaction was changed
test whether the substrate was converted without any catalyst. and the main product was 3,4-dihydro-β-carboline 2a rather
The reaction tube containing 1 mL o-dichlorobenzene as the than 3a. Acetonitrile gave the highest selectivity of 2a (82%)
reaction solvent and 0.2 mmol substrates was stirred at 130 °C but 58% conversion of 1a, while toluene gave the highest con-
for 8 h, and there was no substrate conversion (Table 3, entry version of 1a (∼100%) but medium selectivity of 2a (68%)
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Table 3 Optimization of the reaction conditionsa
Entry
Catalyst
Temp. (°C)
Solvent
Oxidant
Con. (%)
Sel. of 2a (%)
Sel. of 3a (%)
1
—
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
130
80
o-Dichlorobenzene
o-Dichlorobenzene
o-Dichlorobenzene
o-Dichlorobenzene
o-Dichlorobenzene
o-Dichlorobenzene
o-Dichlorobenzene
o-Dichlorobenzene
o-Dichlorobenzene
o-Dichlorobenzene
o-Dichlorobenzene
o-Dichlorobenzene
o-Dichlorobenzene
o-Dichlorobenzene
o-Dichlorobenzene
DMF
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
O2
N.R.
26
42
N.R.
72
69
97
∼100
79
∼100
∼100
∼100
82
∼100
35
∼100
∼100
83
85
87
—
32
29
—
57
54
11
6
95
—
12
—
24
49
47
55
18
78
34
79
73
82
68
99
—
68
71
—
43
46
89
94
5
—
88
>99
76
51
53
45
82
22
66
21
27
18
32
—
2b
3c
H3PO4·12WO3
KMnO4
MnSO4·H2O
OMS-2
4d
5
6
7
8
H3PO4·12WO3/OMS-2
[PW]-OMS-2-Cl
[PW]-OMS-2-NO
[PW]-OMS-2-CO
[PW]-OMS-2-Ac
[PW]-OMS-2-SO
[PW]-OMS-2
[PW]-OMS-2
[PW]-OMS-2
[PW]-OMS-2
[PW]-OMS-2
[PW]-OMS-2
[PW]-OMS-2
[PW]-OMS-2
[PW]-OMS-2
[PW]-OMS-2
[PW]-OMS-2
[PW]-OMS-2
[PW]-OMS-2
9
10
11
12
13e
14f
15g
16
17
18
19
20
21
22
23
24h
H2O2
N2
Air
Air
Air
Air
Air
Air
Air
Air
Air
DMSO
Chlorobenzene
o-Dichlorobenzene
DMC
1,4-Dioxane
Acetonitrile
Toluene
Acetonitrile: methylbenzene
80
80
80
80
89
58
∼100
95
80
a Reaction conditions: substrate (0.2 mmol), [PW]-OMS-2 (20 mg, 13 mol%), solvent 1 mL, air (1 atm), 80 °C, 8 h. “—” means there is no corres-
ponding product obtained. b H3PO4·12WO3 (1.5 mg, 2.6 mol%). c KMnO4 (4.1 mg, 13 mol%). d MnSO4·H2O (4.4 mg, 13 mol%). e O2 balloon.
f H2O2 (50 μL). g N2 balloon. h Methylbenzene: acetonitrile = 0.5 mL: 0.5 mL.
(Table 3, entries 20–24). Therefore, the mixed solvent of aceto- acetonitrile into 0.5 mL toluene as a mixed solvent in the reac-
nitrile and toluene probably could efficiently transform 1a and tion, the desired dehydrogenation product 2a was obtained
give 2a in high yield, because toluene was suitable for the con- with 99% selectivity with a 95% conversion of 1a (Table 3,
version of 1a while acetonitrile was superior for the partial oxi- entry 24). As a result of the optimization, the two best reaction
dative dehydrogenation. As expected, when we added 0.5 mL conditions were as follows: (a) the use of [PW]-OMS-2 as the
Table 4 Conversion and selectivity comparison of this work with those of other reported catalytic systems
Reaction conditions
Reaction temperature (°C) Reaction time (h) Yield of 2a (%) Yield of 3a (%) Ref.
a
[PW]-OMS-2(13 mol%)
130
80
100
25
25
25
160
120
25
8
8
8
—
94
90
—
—
—
—
—
—
—
>99
—
—
88
60
70
70
52
95
85
This work
This work
[PW]-OMS-2(13 mol%)
I2 (50 mol%), H2O2 (1.0 eq.)23
23
10
31
31
13
56
57
58
Trichloroisocyanuric acid (0.7 eq.)10
2-Iodoxybenzoic acid (2.5 eq.), TBAB (0.5 eq.)31
Dess–Martin periodinane (5.5 eq.)31
Ag2CO3 (2.0 eq.), Li2CO3 (2.0 eq.)13
I2 (1.5 eq.), TFA (1.0 eq.)57
2
13
24
16
24
18
0.5
CuBr2 (20 mol%), DBU (2.0 eq.)58
N-Chlorosuccinimide (2.1 eq.), TEA (2.5 eq.)59
25
a “—” means there is no corresponding product obtained.
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catalyst at 130 °C in o-dichlorobenzene for obtaining complete (Table 1, entry 24). Then, we compared our catalytic selectivity
dehydrogenation product β-carboline 3a (Table 1, entry 12); with the previous reports on oxidative dehydrogenation of
and (b) the use of [PW]-OMS-2 as the catalyst at 80 °C in a 1-aryl-tetrahydro-β-carboline (Table 4). To date, there is no
mixed solvent of acetonitrile/toluene (v/v = 1/1) for obtaining report that can realize a selectivity-tunable reaction process for
partial dehydrogenation product 3,4-dihydro-β-carboline 2a dehydrogenation of tetrahydro-β-carbolines. The comparison
Table 5 Scope of oxidative dehydrogenation to obtain 3,4-dihydro-1-aryl-β-carbolinesa
Entry
1
Substrate
Product
Con. (%)
95
Sel. (%)
99
TONb
7.0
2
3
4
91
96
86
89
99
99
6.7
7.1
6.3
5
6
7
84
79
6.2
6.0
7.2
81
94
>99
>99
8
9
88
93
98
84
6.5
6.8
10
11
95
78
89
92
7.0
5.7
a Reaction conditions: substrate (0.2 mmol), [PW]-OMS-2 (20 mg), toluene: acetonitrile = 0.5 mL: 0.5 mL, air (1 atm), 80 °C, 8 h. b Turn over
number, TON = moles of tetrahydro-β-carbolines converted/moles of catalyst. Formula weight of OMS-2 (KMn8O16·nH2O) is 734.6 g·mol−1 (exclud-
ing the water).60
3688 | Dalton Trans., 2021, 50, 3682–3692
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Paper
results showed that the advantages of the present work
With the optimized conditions in hand, we turned our
included low catalyst dosage, no stoichiometric additives and attention to explore the scope and generality of this selectivity-
oxidants, and especially the high selectivity towards the target tunable oxidative dehydrogenation of tetrahydro-β-carbolines
product.
over [PW]-OMS-2. Firstly, substrates 1a–1k were employed with
Table 6 Scope of oxidative dehydrogenation to obtain 1-aryl-β-carbolinesa
Entry
1
Substrate
Product
Con. (%)
Sel. (%)
>99
TONb
7.2
∼100
2
3
>99
>99
96
98
7.2
7.2
4
>99
>99
92
>99
84
7.2
7.2
6.8
7.0
7.2
6.4
6.8
6.2
5
6
69
7
95
98
8
>99
87
>99
88
9
10
11
93
99
84
94
a Reaction conditions: substrate (0.2 mmol), [PW]-OMS-2 (20 mg), 1,2-dichlorobenzene (1 mL), air (1 atm), 130 °C, 8 h. b Turn over number, TON
= moles of tetrahydro-β-carbolines converted/moles of catalyst. The formula weight of OMS-2 (KMn8O16·nH2O) is 734.6 g mol−1 (excluding the
water).60
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a mixed solvent of acetonitrile/toluene under the optimized
conditions for the synthesis of 3,4-dihydro-β-carbolines. As
depicted in Table 5, there was no obvious electronic effect of
the substituted groups found and a series of substrates were
tolerated in the partial oxidative dehydrogenation reaction
system. The corresponding 3,4-dihydro-β-carbolines 2a–2k
were successfully formed with good conversion (78–99%) and
selectivity (79–99%). Tetrahydro-β-carbolines substituted with
heterocycles made the reaction proceed smoothly, although 1k
gave a relatively low conversion (Table 5, 1k). Next, we ran the
dehydrogenation reaction with substrates 1a–1k in o-dichloro-
benzene with high temperature over [PW]-OMS-2 for selective
obtainment of β-carbolines (Table 6). As expected, β-carbolines
with a wide range of substituents were provided smoothly with
very high selectivity ranging from 88% to 99%. Unfortunately,
1f just gave a medium selectivity for β-carboline 2f (Table 6,
1f). Remarkably, halogen, methyl, methoxy, naphthyl, pyridyl,
quinolyl, and thienyl groups of these desired β-carboline pro-
ducts would be useful handles for further modification to gain
other pharmacologically active molecules.
Scheme 1 Gram–scale reaction.
changes of the retrieved catalyst were measured by XRD, which
confirms that the mixed phases of cryptomelane/phospho-
tungstic acid and crystallinity were retained after five recycles
(see ESI Fig. S3†). Importantly, we attempted to realize the syn-
thesis of β-carbolines on a gram-scale reaction with [PW]-
OMS-2 and obtained 98% yield of the desired product, which
indicates that the heterogeneous catalytic oxidation system is a
potential option for practical application (Scheme 1).
Discussion of the mechanism
Based on our experimental results and previous works,48 we
proposed the possible reaction mechanism that follows the
Mars–van Krevelen mechanism (Fig. 8).61 Firstly, the substrate
tetrahydro-β-carboline adsorbed on the catalyst metal Mnn+
center and the electron of nitrogen in saturated cycloalkane to
active site Mnn+. Then, the formed amine radical species could
be subjected to C–H dehydrogenation. The two H atoms on N–
H and C–H groups combined with one O atom on the surface
of the catalyst to form an H2O molecule. At the same time, the
active site Mnn+ was reduced to Mn(n−1)+. After desorption of
the partial dehydrogenation product 1-aryl-3,4-dihydro-
β-carboline and H2O, the original surface adsorption oxygen
formed a surface oxygen vacancy. During the process, O2 in
the atmosphere was necessary to reoxidize the reduced cata-
lyst, replenish the oxygen vacancies and restore the catalytic
activity. Thereafter, the imine of the partial dehydrogenation
product was transformed to tautomeric cyclic imine and
through secondary dehydrogenation the final dehydrogenation
product 1-aryl-β-carboline was obtained. The latter catalytic
oxidation process is the same as the first step.
The stability and recyclability of [PW]-OMS-2
The recyclability of the catalyst was also studied via the cata-
lytic synthesis of β-carbolines. To ensure the reliability of the
catalyst stability test, we correspondingly reduced the amount
of catalyst and controlled the reaction at a low conversion
(other conditions were maintained under standard reaction
conditions). 1-Phenyl-tetrahydro-β-carboline 1a (0.2 mmol,
50 mg) and [PW]-OMS-2 (3.2 mol%, 5 mg) were placed in a
reaction tube, and 1,2-dichlorobenzene (1 mL) was added.
Then, the reaction mixture was stirred at 130 °C for 8 h under
air. After the reaction, the reaction mixture was cooled and
separated by filtration. The used catalyst was collected by fil-
tration and washed with ethyl acetate and ethanol. Then, the
retrieved catalyst was placed in an oven and dried at 100 °C for
8 h for the next use. To ensure that the amount of catalyst was
sufficient for the next cycle, we carried out parallel reactions.
The filtrate was concentrated with a rotary evaporator and the
product was purified and obtained by silica gel chromato-
graphy. The recycling test showed that [PW]-OMS-2 could be
reused at least 5 consecutive times with excellent selectivity
(97%–99%) and stable conversion (Fig. 7). Subsequently, the
Fig. 7 The recycling experiment.
Fig. 8 The proposed reaction mechanism.
3690 | Dalton Trans., 2021, 50, 3682–3692
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The proposed reaction mechanism was verified by experi-
ments. The conversion of the substrate under the nitrogen
atmosphere decreased remarkably (35%, Table 3, entry 15)
after 8 h, which may be due to a lack of oxygen for re-oxidation
of the reduced Mn species. During this process, the labile
lattice oxygen was consumed. Additionally, the XPS character-
ization of [PW]-OMS-2 revealed the amount of lattice oxygen
that decreased after reaction under N2 (Fig. S5, ESI†). On the
other hand, under the standard conditions (Table 3, entry 12),
the intermediate 1-phenyl-3,4-dihydro-β-carbolines almost
completely converted to the complete oxidation product
1-phenyl-β-carbolines within 3 hours. These results are consist-
ent with the abovementioned mechanism.
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Conflicts of interest
There are no conflicts to declare.
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We gratefully acknowledge the National Natural Science 20 Y. Ling, C. Xu, L. Luo, J. Cao, J. Feng, Y. Xue, Q. Zhu, C. Ju,
Foundation of China (Grant No. 21403256 and 21573261), the
Youth Innovation Promotion Association CAS (2018456) and
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