Base-free synthesis of 1,3,5-triazines via aerobic oxidation of alcohols and benzamidine over a recyclable OMS-2 catalyst

Article abstract of DOI:10.1016/j.catcom.2019.05.005

By using 4Na₂SO₄·2H₂O₂·NaCl as a reductant, manganese oxide octahedral molecular sieve (OMS-2)with enhanced surface area and mixed valence was prepared successfully. OMS-2 showed excellent catalytic ability towards aerobic oxidative synthesis of 1,3,5-triazines from benzyl alcohols and benzamidine. Methyl benzenes, DMF and DMSO could also be employed as substrates to react with benzamidine offering triazines under the heterogeneous conditions. The catalytic system features base-free conditions, broad substrate scope, high chemoselectivity, operational simplicity, catalyst recyclability and utilization of O₂ as the green oxidant.

Full text of DOI:10.1016/j.catcom.2019.05.005

Accepted Manuscript
Base-free synthesis of 1,3,5-triazines via aerobic oxidation of
alcohols and benzamidine over a recyclable OMS-2 catalyst
Jian Shen, Xu Meng
PII:
S1566-7367(19)30144-X
DOI:
https://doi.org/10.1016/j.catcom.2019.05.005
CATCOM 5698
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
4 April 2019
5 May 2019
6 May 2019
Please cite this article as: J. Shen and X. Meng, Base-free synthesis of 1,3,5-triazines
via aerobic oxidation of alcohols and benzamidine over a recyclable OMS-2 catalyst,
Catalysis Communications, https://doi.org/10.1016/j.catcom.2019.05.005
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ACCEPTED MANUSCRIPT
Base-free synthesis of 1,3,5-triazines via aerobic
oxidation of alcohols and benzamidine over a recyclable
OMS-2 catalyst
*, a
b
Jian Shen and Xu Meng
a
School of Materials and Chemical Engineering, Ningbo University of
Technology, Ningbo 315016, China. E-mail: 46676445@qq.com
b
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou
Research Institute of LICP, Lanzhou Institute of Chemical Physics (LICP),
Chinese Academy of Sciences, Lanzhou 730000, China.
Abstract: By using 4Na
2
SO
4
·2H
2
O ·NaCl as a reductant, manganese oxide
2
octahedral molecular sieve (OMS-2) with enhanced surface area and mixed
valence was prepared successfully. OMS-2 showed excellent catalytic ability
towards aerobic oxidative synthesis of 1,3,5-triazines from benzyl alcohols
and benzamidine. Methyl benzenes, DMF and DMSO could also be
employed as substrates to react with benzamidine offering triazines under
the heterogeneous conditions. The catalytic system features base-free
conditions, broad substrate scope, high chemoselectivity, operational
simplicity, catalyst recyclability and utilization of O as the green oxidant.
2
Key word: heterogeneous catalysis, oxidative cyclization, OMS-2, triazine,
benzamidine
1

ACCEPTED MANUSCRIPT
1. Introduction
1,3,5-triazines, as key skeletons of many important bioactive heterocyclic
compounds, frequently present in insecticides, fungicides, herbicides,
antimalarial, antitumor and antivirals.[1-3] Meanwhile, due to their chemical
properties, they are used in organic dyes, chelating ligands for synthesis of
organometallic materials, liquid crystals, electroluminescent materials and
fluorescent brighteners.[4-6] Generally, synthesis of 1,3,5-triazines involves
cyclotrimerization of nitriles, coupling of halogenated triazines over
transition-metal catalysts and specific multicomponent reactions.[7-9] In
recent years, amidines were found to be efficient substrates to react with
alcohols, aldehydes, amines, DMF, DMSO or toluene for synthesis of 1,3,5-
triazines via oxidative pathway.[10-15] However, from the perspective of
clean synthesis, very limited reports have been found on the applications of
non-noble metals as recyclable catalysts in 1,3,5-triazines synthesis under
base- and ligand-free conditions.
Manganese oxide octahedral molecular sieve (OMS-2) containing 2X2
and 1D tunnel structure with a pore size of 0.46 nm is composed by edge-
and corner-shared MnO octahedra.[16] Because of its excellent properties,
6
like catalysis, adsorption and semiconductivity, OMS-2 has been applied in
organic synthesis, environmental catalysis, separation and batteries.[17-21]
4+
3+
Especially, it generally exhibits mixed-valence of Mn (Mn , Mn and
2+
Mn ), which makes it superior heterogeneous catalyst and support in many
aerobic oxidations through tuning the ratio of mixed-valent manganese to
accelerate electron transfer during redox process.[22-26] Our group recently
developed methods for preparation of OMS-2 to further prepare supported
catalyst or directly use it as the catalyst in N-containing heterocycles
synthesis under ligand- and additive-free conditions. And, chemoselective
synthesis of 1,2-dihydro-1,3,5-triazine and quinazolines from alcohols and
N-arylamidines via establishment of certain catalytic systems was realized
(
Scheme 1, a and b).[27,28] Very recently, we prepared OMS-2 successfully
using 4Na SO ·2H ·NaCl as a reductant via reduction of KMnO (denoted
as OMS-2-SH-A). Synthesized OMS-2-SH-A which exhibited obviously
2
4
2
O
2
4
2
+
enhanced surface area and amounts of Mn compared with OMS-2 prepared
by conventional reflux method was found to be a highly efficient catalyst for
2

ACCEPTED MANUSCRIPT
oxidation of benzyl alcohols. As shown in Scheme S1 (see ESI), the
catalytic system could tolerate various substrates and excellent conversions
and yields were obtained when the reactions were carried out in toluene at
o
1
10 C under O
2
.
Scheme 1. OMS-2-based catalytic systems for the oxidative synthesis of
heterocycles using amidines as substrates.
With our continuous research interest in clean synthesis of heterocycles
over MnO
for synthesis of OMS-2 material using relatively safe and stable H
trapped reagent, 4Na SO ·2H ·NaCl, as the reductant via reduction of
x
-based catalysts,[29-31] we would like to describe a procedure
2
O
2
2
4
2
O
2
KMnO
4
. After full characterization, obtained OMS-2 catalyst shows larger
surface area and more amounts of Mn2 than OMS-2 synthesized by
conventional procedure. Importantly, as-synthesized material is an efficient
recyclable catalyst for the aerobic oxidative synthesis of 1,3,5-triazines from
benzamidine and alcohols via in-situ formed aldehydes. Furthermore,
toluene and its derivatives were directly employed as substrates replacing
alcohols to yield desired products over the synthesized catalyst (Scheme 1,
c).
+
3

ACCEPTED MANUSCRIPT
2
2
. Experimental
.1. Catalyst preparation
OMS-2-SH material was synthesized by the reduction of KMnO
mixture of KOAc (2.5 g, 25 mmol), acetic acid (2.5 mL) and
Na SO ·2H ·NaCl was stirred at room temperature to generate a buffer
solution. The solution of KMnO (3.25 g, 20 mmol) was added dropwisely
4
. Firstly, a
4
2
4
2
O
2
4
into the buffer solution with stirring at room temperature. Subsequently, the
o
brown mixture was kept stirring at 100 C for 24 h under air. The reaction
mixture was cooled down and filtrated followed by washing with deionized
o
water. Lastly, the obtained solid material was dried at 120 C for overnight
to yield the OMS-2-SH catalyst. The obtained OMS-2-SH catalysts were
named OMS-2-SH-A, OMS-2-SH-B, OMS-2-SH-C and OMS-2-SH-D,
when 4Na
into the buffer solution.
.2. Typical procedure for the synthesis of 1,3,5-triazine 3a
2
SO
4
·2H
2
O ·NaCl was added in 7, 5, 3.5 and 1.75 g respectively
2
2
OMS-2-SH-B (20 mg, 10 mol%), benzyl alcohol (0.3 mmol) and
benzamidine (0.5 mmol) were added into a Schlenk tube. Then, air was
removed and toluene (1 mL) was added by a syringe under O
2
atmosphere.
If substituted benzyl alcohol was liquid, it was added with toluene under O
2
balloon protection. If methyl benzene was used as substrate instead of
benzyl alcohol, it was added as solvent (1 mL) into the reaction tube by
syringe under O atmosphere. The mixture was stirred for required time at
2
certain temperature for 20 hours. The resulting mixture was cooled down,
filtered and washed with EtOAc, and concentrated under reduced pressure to
give the crude product. Finally, the product was purified by silica gel
chromatography to yield the pure product.
3. Results and discussion
Due to the excellent catalytic performance of OMS-2-SH-A in oxidation
of benzyl alcohol (Scheme S1, in ESI), we firstly used it as the catalyst to
optimize the reaction between benzyl alcohol and benzamidine in toluene
under reflux conditions (Table 1). Initially, the reaction results prove that
OMS-2-Re made by conventional method and its supported catalyst
Cu/OMS-2-Re both failed the reaction by the use of air as the oxidant (Table
4

ACCEPTED MANUSCRIPT
1, entries 1 and 2). Delightedly, 45% yield of 3a was isolated when OMS-2-
SH-A was used as the catalyst (Table 1, entry 3). According to changing the
oxidant, O was the best green oxidant and 85% yield of 3a was obtained
Table 1, entries 3-5). These results imply that the method for preparation of
2
(
catalyst has significant influence. In addition, it was found that decreasing
the reaction temperature was deleterious to the reaction. The precursors of
OMS-2 were also tested in the reaction under the standard conditions, while
the reactions did not occur at all (Table 1, entries 6 and 7). Subsequently,
different solvents were used in the reaction at their reflux temperatures and
only dimethyl carbonate (DMC), PhCl and MeNO
2
yielded 3a more or less
(Table 1, entries 8-15). Therefore, non-proton and non-polar solvents and
reaction temperature were important factors for the success of the oxidative
cyclization. Hence, the optimized reaction condition is as shown in Table 1,
entry 4.
Table 1. Optimization of reaction conditionsa
NH
NH2
Ph
catalyst (10 mol%)
N
N
Ph
OH
+
2
solvent, oxidant, reflux, 20 h
Ph
N
Ph
3a
Entry Catalyst
Solvent
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DMC
Oxidant
air
Yield of 3a (%)b
1
2
3
4
5
6
7
8
OMS-2-Re
0
Cu/OMS-2-Re
OMS-2-SH-A
OMS-2-SH-A
OMS-2-SH-A
air
0
air
45
O
H
O
O
O
2
85 (48c)
2
2
2
2
O
2
0
KMnO
MnSO
OMS-2-SH-A
4
0
4
0
28
5

ACCEPTED MANUSCRIPT
9
1
1
1
1
1
OMS-2-SH-A
OMS-2-SH-A
OMS-2-SH-A
OMS-2-SH-A
OMS-2-SH-A
OMS-2-SH-A
OMS-2-SH-A
EtOH
O
O
O
O
O
O
O
2
2
2
2
2
2
2
0
0
1
2
3
4
5
DEC
0
2-Me-THF
DMSO
0
0
chlorobenzene
dioxane
15
0
1
MeNO
2
40
a
Reaction conditions: benzyl alcohol (0.3 mmol), phenyl amidine (0.5 mmol), catalyst
10 mol%; for OMS-2-based catalyst, 20 mg was used), solvent (1 mL), 20 h, under air or
(
O
b
c
o
2
balloon. Isolated yields. At 90 C.
To further optimize the catalytic reaction, the catalysts synthesized using
different amounts of 4Na
2
SO
4
·2H
2
O ·NaCl were investigated (Figure 1). It
2
was found that OMS-2-SH-B showed the best catalytic ability (95% yield of
2
3
a) and it exhibited the second largest surface area (114 m /g). However,
2
OMS-2-SH-D with the highest surface area (220 m /g) only offered 15%
yield of 1,3,5-triazine 3a under the optimal conditions. Characterization of
catalysts via X-ray diffraction (XRD) demonstrates that OMS-2-SH-A,
OMS-2-SH-B, and OMS-2-SH-C all possessed typical and pure
cryptomelane phase (JCPDS 29-1020) while OMS-2-SH-D possessed
birnessite phase (JCPDS 80–1098, Scheme S2, in ESI). So, the results from
XRD imply that too less amounts of 4Na
to reduce KMnO and ordered cryptomelane phase and MnO
form, which makes OMS-2-SH-D the largest surface area. The added
loadings of 4Na SO ·2H ·NaCl not only affected the surface areas, but
2
SO
4 2
·2H O
2
·NaCl was not enough
4
6
units cannot
2
4
2
O
2
also crystallinity of the final catalysts, such as OMS-2-SH-B has the largest
surface area and relatively weaker crystallinity. Compared with OMS-2-Re
2
(
surface area is 67 m /g), catalysts prepared involving 4Na
2
SO
4
·2H
2
O ·NaCl
2
all show much larger surface area and stronger crystallinity (Table S1, in
ESI). From our experimental results of 3a synthesis (Figure 1), it is believed
that large surface area usually means more exposed surface reactive sites
6

ACCEPTED MANUSCRIPT
which are critical in catalysis and different crystal phases of solid catalysts
always influence the catalytic activity.[32]
Figure 1. The influence of surface area of the catalysts on the catalytic
performance.
Characterization of as-synthesized catalysts was also performed by SEM
and TEM. From the SEM images, we found that added loadings of
4Na
2
SO
4
·2H
2
O ·NaCl could significantly affect the morphologies of
2
catalysts. Obviously, OMS-2-SH-B shows longer, thinner and more uniform,
while OMS-2-SH-A is thicker and OMS-2-SH-C is shorter (Scheme S3, in
ESI). In addition, compared with OMS-2-Re possessing short rod-like
morphology, OMS-2-SH-B possesses long fiber-like morphology (Scheme
S5, in ESI). Especially, OMS-2-SH-D does not have fiber-like morphology
and is composed by small flakes (Scheme S3 and S12, in ESI). Similarly,
difference of morphologies of as-synthesized catalysts was confirmed by
TEM images (Scheme S4, in ESI). From the analysis of EDX, there are not
Na ions existed in catalysts prepared by 4Na
ions are detected clearly, which indicates that manganese oxide was formed
by MnO unites with K ions in tunnel to stabilize the structure (Scheme S6,
2
SO
4
·2H
2
O ·NaCl and only K
2
6
in ESI). These results from morphologies are in agreement with the
information of XRD very well.
7

ACCEPTED MANUSCRIPT
Subsequently, the surface chemistry of obtained catalysts was studied by
X-ray photoelectron spectroscopy (XPS). The catalysts synthesized with
4
Na
O 1s (Scheme S8 and Table S3, in ESI). On the other hand, all catalysts
synthesized with 4Na SO ·2H ·NaCl show mixed-valence of Mn as the
2
SO
4
·2H
2
O ·NaCl possess very similar oxygen species from analysis of
2
2
4
2
O
2
same as OMS-2-Re (Scheme S7, in ESI). It is obvious to find out that the
method of catalyst preparation and the added loadings of
4
Na
2
SO
4
·2H
2
O
2
·NaCl affect the ratio of mixed-valence of manganese.
SO ·2H ·NaCl all
Specifically, OMS-2 catalysts prepared with 4Na
2
4
2
O
2
possess lower amounts of Mn4+ than OMS-2-Re does. Among as-
synthesized OMS-2 in this study, OMS-2-SH-B possesses the lowest
4+
2+
amounts of Mn and highest amounts of Mn (Table S2, in ESI). Low
amounts of Mn in OMS-2 materials generally imply high concentration of
oxygen vacancy defects which can adsorb, active and desorb the substrates
in the reaction.[33] Therefore, low amounts of Mn and high concentration
of oxygen vacancy defects of OMS-2-SH-B might be important factors in
catalytic performance.
4+
4+
With the optimal catalyst (OMS-2-SH-B) in hand, a series of substituted
benzyl alcohols were employed to react with benzamidine to examine the
tolerance of the catalytic system (Table 2). Generally, electronic effect did
not affect the reactions, while steric effect influenced the reaction
significantly (Table 2, 3g). Benzyl alcohols which can be oxidized to
corresponding aldehydes all participated in the reaction smoothly and led to
1,2,3-triazines in good to excellent yields under the optimal conditions.
Moreover, cinnamyl alcohol and the less-reactive heterocyclic alcohols
could be tolerated successfully and gave corresponding functional 1,3,5-
triazines in high yields (Table 2, 3n, 3o and 3p). Cyano-substituted benzyl
alcohol that has two reactive functional groups was reported to react with
benzamidine yielding 1,2,4-triazole with an aldehyde group through copper-
catalyzed oxidative cyclization.[34,35] Interestingly, under our Mn-based
catalytic system, desired 1,3,5-triazine with an untouchable cyano was
obtained in excellent yields, which implies that the present reaction
conditions have superior chemoselevtivity (Scheme 2).
8

ACCEPTED MANUSCRIPT
Table 2. Scope of the reactiona
NH
NH2
Ph
OMS-2-SH (10 mol%)
N
N
R
OH
Ph
+
2
toluene
R
N
Ph
o
1
10 C, O , 20 h
2
Ph
Ph
N
N
N
N
N
N
N
Ph
N
Ph
N
Ph
Et
Me
3
b, 88%
3c, 89%
Ph
3d, 88%
Ph
N
Ph
N
N
N
N
N
Me
Me
Me
N
Ph
N
Ph
N
Ph
Me
3g, 55%
3
e, 90%
3
f, 91%
Ph
Ph
Ph
N
N
N
N
N
N
Cl
Cl
Cl
N
Ph
N
Ph
N
Ph
Cl
Br
3
h, 95%
3
i, 93%
3j, 90%
Ph
Ph
N
Ph
N
N
N
N
N
N
Ph
N
Ph
N
Ph
F
F3C
3k, 90%
3l, 92%
3m, 90%
Ph
Ph
Ph
N
N
N
N
N
N
S
O
N
Ph
N
Ph
Ph
N
Ph
3o, 83%
3p, 78%
3n, 91%
a
Reaction conditions: OMS-2-SH-B (20 mg, 10 mol%), alcohol (0.3 mmol), phenyl
o
amidine (0.5 mmol), toluene (1 mL), 110 C, 20 h, O balloon, isolated yields.
2
9

ACCEPTED MANUSCRIPT
Ph
NH
NH2
N
N
OMS-2-SH (10 mol%)
toluene
OH
N
N
Ph
+
+
2
O
NC
10 oC, O2, 20 h
HN
N
1
NC
chemoselective reaction
3r, 95% yield
not observed
Scheme 2. The chemoselectivity of the catalytic system between two
functional groups of benzyl alcohol under the optimized reaction conditions.
For further expanding the scope of the reaction, aliphatic alcohols were
tried in the reaction. However, corresponding cyclic alkanes substituted
1,3,5-triazines were not observed at all and triphenyl-substituted 1,3,5-
triazines were isolated in good yields under the optimal conditions (Scheme
S9). Additionally, aliphatic alcohols, like cyclohexanemethanol and
cyclohexenemethanol, did not react with OMS-2-SH-B and all alcohols were
recovered after the reactions. These experimental facts indicate that toluene
which was oxidized by OMS-2-SH-B proceeded into the reaction. In this
way, some simple methyl benzenes were selected to directly react with
benzamidine under the optimal conditions. As expected, electron-rich or
electron-neutral toluenes proceeded the reactions very well and
corresponding 1,3,5-triazines were obtained in good yields (Table 3, entries
1
-4). However, electron-poor toluenes were difficult to oxidize over OMS-2-
SH-B and very poor yields of final products were obtained (Table 3, entries
and 6).
5
Table 3. The scope of the reaction directly using substituted toluene as the
substrates.
Ph
NH
N
N
Me
OMS-2-SH (10 mol%)
NH2
R
+
2
N
Ph
o
1
10 C, O , 24 h
R
2
Entry
R =
H
Compound
Isolated yield %
1
2
3a
3d
82
71
4-Me
10

ACCEPTED MANUSCRIPT
3
4
5
6
4-Et
3c
3e
3h
3i
70
75
35
38
3,4-diMe
4-Cl
3-Cl
a
Reaction conditions: OMS-2-SH-B (20 mg, 10 mol%), substituted toluene (1 mL),
o
phenyl amidine (0.5 mmol), 110 C, 24 h, O
2
balloon, isolated yields.
Finally, inspired by the results of Jiang`s and Zhang`s work,[11,13] one-
carbon suppliers, such as DMF and DMSO, were used in the reactions to
form biphenyl-substituted 1,3,5-triazines 3q (Scheme S10). The reactions
using DMF or DMSO replacing alcohols proceeded smoothly and provided
desired products in good yields, which offers a simple and straightforward
pathway on synthesis.
In order to investigate the stability of the optimal catalyst (OMS-2-SH-B),
it was recycled and reused for 5 consecutive times with the reaction of
benzyl alcohol and benzamidine under the optimized conditions. As shown
in Figure 2, the catalyst shows very stable activity and could be reused for
many times. After the first run, the reaction solution after removal of solid
catalyst was analyzed by ICP-MS and the results imply that Mn and K were
below 10 ug/L respectively. In addition, XRD patterns show that the catalyst
retrieved after five recycles maintains cryptomelane phase very well
(Scheme S11, in ESI).
11

ACCEPTED MANUSCRIPT
Figure 2. The recycling experiments.
. Conclusion
In summary, we developed a mixed-valent OMS-2 catalyst with enhanced
4
surface area and high concentration of oxygen vacancy defects. The obtained
catalyst exhibits excellent oxidative ability for oxidation of benzyl alcohols
and catalytic performance in aerobic oxidative synthesis of 1,3,5-triazines. A
broad array of benzyl alcohols and electron-rich toluenes were both tolerated
in the reactions and offered 1,3,5-triazines in good to excellent yields. The
catalytic system which is chemoselective, recyclable and base-/ligand-free
provides a sustainable, simple and straightforward way to synthesize N-
containing heterocycles.
Acknowledgements
We gratefully acknowledge the National Natural Science Foundation of
China (Grant No. 21606136 and 21403256) and the Youth Innovation
Promotion Association CAS (Grant No. 2018456).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
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Highlights
1. Heterogeneous synthesis of triazines was achieved using OMS-2 as the
catalyst.
2
3
4
. Various substituted toluenes can be employed directly as the substrates.
. A broad of starting materials can be tolerated under the catalytic system.
. The catalytic system shows good chemoselectivity.
17

Figure 1

Figure 2