Scalable Synthetic Strategy for Unsymmetrical Trisubstituted s-Triazines

Article abstract of DOI:10.1021/acs.orglett.1c01970

A scalable synthetic strategy to produce a large variety of unsymmetrical trisubstituted 1,3,5-triazines was developed. This protocol applied in situ formed acyl isocyanate from amide to react with amidine, introducing two substituents to the 1,3,5-triazinone ring with a low production cost and a simple workup procedure. The scalability of this method was demonstrated by translating a small-scale procedure to a multi-kilogram-scale synthesis. Chlorination and a further coupling reaction with various nucleophiles could provide unsymmetrical trisubstituted 1,3,5-triazines bearing diverse functional groups.

Full text of DOI:10.1021/acs.orglett.1c01970

pubs.acs.org/OrgLett
Letter
Scalable Synthetic Strategy for Unsymmetrical Trisubstituted
s‑Triazines
Helong Liang, Ganzhong Li, Lei Zhang, Gefei Wang, Mingyu Song, Heng Li, and Bingxin Yuan*
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ABSTRACT: A scalable synthetic strategy to produce a large
variety of unsymmetrical trisubstituted 1,3,5-triazines was devel-
oped. This protocol applied in situ formed acyl isocyanate from
amide to react with amidine, introducing two substituents to the
1,3,5-triazinone ring with a low production cost and a simple
workup procedure. The scalability of this method was demon-
strated by translating a small-scale procedure to a multi-kilogram-
scale synthesis. Chlorination and a further coupling reaction with
various nucleophiles could provide unsymmetrical trisubstituted 1,3,5-triazines bearing diverse functional groups.
1,3,5-Triazines and their derivatives based on nitrogen-
containing heterocycles are key structural features with wide-
ranging biological activities in drug discovery programs,
including antimicrobial, anticancer, antiviral, and antitubercu-
losis activities.1 The distinctive structure of s-triazines makes
them a remarkable synthon in material science,2 supra-
molecular chemistry,3 and coordination chemistry.4 Since the
year 2000, the strong electron acceptability and excellent
structural modifiability of the triazine moiety have made it an
exciting target, allowing an exponential increase in the usage of
its derivatives as electron transporters and hole-blocking
materials. A simple search of these compounds would provide
hundreds of articles and patents. Via modifying the
Figure 1. Synthetic strategies for unsymmetrical trisubstituted s-
triazines.
substituents on the triazine core to easily tune the mobility
and the LUMO energy for adjusted injection, s-triazine
derivatives were developed for fluorescence sensors,5 lumines-
cent liquid crystals,6 photovoltaic devices,7 and phosphor-
escent organic light-emitting diodes (OLEDs).8
from cheap and commercially available starting materials in a
straightforward manner.
Owing to the complex system of double bonds, acyl
isocyanate as a synthon was used to undergo cycloaddition
reactions with unsaturated bonds to synthesize a wide range of
heterocycles.11 We hypothesized that we could utilize the high
reactivity of acyl isocyanate to develop a convenient and
versatile method for the distinct construction of 1,3,5-triazin-
2(1H)-one, which is a precursor to 1,3,5-triazines (Figure 1,
bottom). Compared with the state of the art, such a strategy
for the unsymmetrical trisubstituted 1,3,5-triazine synthesis
Given the important and extensive applications of 1,3,5-
triazines, many efforts have been made to introduce different
substitutions on the triazine core unit and increase the diversity
of the compound libraries. A representative strategy to achieve
unsymmetrical trisubstituted s-triazines is via the multistep
sequential Pd-catalyzed coupling of cyanuric chloride with
various nucleophiles due to the different reactivities of C−Cl
bonds (Figure 1, top);9 however, multiple reaction steps and
the purification of each step led to tedious work and a low yield
of final product. Reaction condition screening also takes
laborious work to find the right combination of catalyst and
ligand. Concerning the introduction of different substituents
into a triazine core, the satisfactory synthesis of unsymmetrical
trisubstituted 1,3,5-triazines from readily available reagents is
still rare.10 To circumvent this limitation, we aimed to design a
ring construction strategy that is easily and efficiently achieved
Received: June 14, 2021
Published: July 14, 2021
https://doi.org/10.1021/acs.orglett.1c01970
© 2021 American Chemical Society
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could offer a few superior features: (1) a wide choice of
functional groups on the triazine ring according to need, (2) a
feasible separation and purification procedure due to the
avoidance of coupling byproducts resulting from multiple
reactive sites, (3) shortened reaction steps by introducing two
functional groups via a single step, and (4) a low production
cost by utilizing cheap starting materials. The disubstituted
1,3,5-triazinones could be efficiently converted into the desired
unsymmetrical trisubstituted 1,3,5-triazines via facile chlorina-
tion and coupling reactions. Herein we report the realization of
an unsymmetrical trisubstituted 1,3,5-triazine precursor via the
one-pot cyclization of acyl isocyanate with amidine. Our ring-
construction strategy offers good functional group compati-
bility and operational simplicity. A large variety of 1,3,5-
triazinones could be obtained by precipitation in water, and the
NMR spectrum showed good purity without any column
chromatography. We envision that this strategy could be the
key to opening new synthetic possibilities for the manufactur-
ing of OLED materials at more affordable price points.
Table 2. Substrate Scope of 1 with Aryl Amidines to
Synthesize Disubstituted 1,3,5-Triazin-2(1H)-ones
a b
,
As a model reaction for the initial investigations, benzoyl
isocyanate 1a was formed in situ by treating amide with oxalyl
chloride (COCl)2 followed by the addition of amidine 2a to
accomplish the one-pot cyclization and give 1,3,5-triazin-
2(1H)-one 3a as a desired product. In terms of solvents, xylene
was identified as optimal (Table 1, entries 1−3). Lowering the
a b
,
Table 1. Reaction Condition Screening
entry
1a (equiv)
solvent
T (°C)
time (h)
yield (%)
1
2
3
4
5
6
1.5
1.5
1.5
1.1
1.5
1.5
xylene
DMF
DMSO
xylene
xylene
xylene
130
130
130
130
130
100
12
12
12
12
8
80
25
35
65
70
65
12
a
Benzamide and oxalyl chloride reacted in solvent (3 mL) at 60 °C
for 2 h to form 1a (x equiv); then, 2a (0.6 mmol) was added to react
with the in situ formed 1a at x °C for y h. Isolated yields.
b
a
Amide and oxalyl chloride reacted in xylene (3 mL) at 60 °C for 2 h
to form 1 (0.9 mmol); then, 2 (0.6 mmol) was added to react with the
in situ formed 1 at 130 °C for 12 h. Isolated yields.
equivalent of 1a to 1.1 equiv led to a diminished product yield
(entry 4). Either less reaction time or a lower temperature
resulted in decreased yields as well (entries 5, 6). Finally, the
optimal reaction condition was determined without the usage
of any catalyst or additives.
With the optimal reaction conditions in hand, we continued
to explore the substrate scope of 1 with aryl amidines to
synthesize disubstituted 1,3,5-triazin-2(1H)-one (Table 2). A
large variety of aryl and alkyl isocyanates formed in situ from
amides underwent the reaction with amidines to furnish
moderate to excellent yields. Aryl amides bearing electron-
donating groups, such as methyl and methoxyl, and electron-
withdrawing groups, such as fluoro, chloro, bromo, and
trifluoromethyl, on the phenyl rings were transformed into
the respective isocyanates, which successfully reacted with
phenyl amidine to form disubstituted 1,3,5-triazin-2(1H)-ones
(3a−k). In terms of the chloro group on the phenyl ring of
b
amide, the para-site (3e) was more favorable compared with
meta and ortho sites (3f, 3g). In the case of the bromo group,
the meta-site was converted into the respective product 3i in
the highest yield. The steric effect of ortho substituents might
be the cause of lower yields, as seen in the case of 3g and 3j.
Note that these halogen groups offer a versatile functional
handle for further functionaliaztion, which is essential for drug
discoveries and material development. This protocol is also
applicable for amides bearing an extended π-framework (3l)
and S-heteroaryl group (3m). Aliphatic amides bearing primary
(3n) and secondary (3o, 3p) α-carbon atoms were transferred
into the desired product in good yields. Under the optimal
reaction conditions, we also screened various aryl amidines
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with phenylamide. The in situ formed benzoyl isocyanate 1a
from phenylamide could react with aryl amidines containing an
electron-efficient group, such as methoxyl (3q) and hydroxyl
(3r), and electron-deficient groups, such as chloro (3s, 3t),
bromo (3u), and nitro (3v), to furnish the target 1,3,5-triazin-
2(1H)-ones in good yields. Chloro groups on para or meta
sites of the phenyl ring did not show much effect on the
reaction. In addition, disubstituted 1,3,5-triazin-2(1H)-ones
deduced from pyridine amidines are accessible in moderate to
good yields (3w−y). The results indicated that the electronic
properties of pyridine ring were essential for the reaction.
We continued to explore the synthesis of dialkyl- and
alkylaryl-functionalized 1,3,5-triazin-2(1H)-ones by utilizing
alkyl amidines as substrates (Table 3). By using t-
Figure 2. Crystal structures of compound 3y (left, displacement
ellipsoids are drawn at the 50% probability level) and 5b (right,
displacement ellipsoids are drawn at the 50% probability level).
This protocol requires no catalyst or any other additive.
Once the reaction was completed, the reaction mixture was
concentrated under vacuum, then redissolved in a small
amount of DMF. The above DMF solution was dropped into
hot water, and the desired product would precipitate due to
the low solubility. After simple filtration, clean product could
be obtained as a solid. NMR showed that the purity of the
product was quite acceptable. Gratifyingly, this protocol was
successfully translated to a multi-kilogram-scale experiment.
Taking the reaction between 3-chlorobenzamide and phenyl-
amidine as an example (Scheme 1), compound 3f was
Table 3. Substrate Scope to Synthesize Dialkyl- and
a b
,
Alkylaryl-Substituted 1,3,5-Triazin-2(1H)-ones
Scheme 1. Multi-Kilogram-Scale Synthesis of 3f
synthesized in 70% yield (1.79 kg). It is worth noting that
this product yield is quite close to that of the small-scale trial
(Table 2, 72%).The scalability of this technology to synthesize
unsymmetrical disubstituted 1,3,5-triazin-2(1H)-ones is solid
evidence that our strategy might have great potential in
industrial manufacturing.
The ability to procure an ample amount of 1,3,5-triazin-
2(1H)-ones and introduce two different substituents in a single
step affords a unique opportunity to explore the synthesis of
unsymmetrical trisubstituted 1,3,5-triazines, as we expected.
The chlorination of 1,3,5-triazin-2(1H)-ones could be
accomplished by the treatment with POCl3. As shown in
Table 4, diaryl-, heteroaryl-, arylalkyl-, and dialkyl-substituted
1,3,5-triazin-2(1H)-ones were successfully transferred into the
corresponding 2-chloro-1,3,5-triazines. With regard to these
compounds, previous methods involved the Pd-catalyzed
Suzuki coupling reaction, Grignard reagents, or expensive
starting materials.12
The palladium-catalyzed cross-coupling reaction to forge
carbon−carbon bonds and carbon−heteroatom bonds is a
prominent synthetic tool to introduce the third functional
group onto the triazine core (Scheme 2). The 2-chloro-1,3,5-
triazine derivative 6a was taken as an example. The Pd-
catalyzed Suzuki coupling reaction of 6a with arylboronic
esters was realized to afford the desired product in excellent
yields (reactions 1 and 2). In addition, the amination reaction
a
Amide and oxalyl chloride reacted in xylene (3 mL) at 60 °C for 2 h
to form 1 (0.9 mmol); then, 4 (0.6 mmol) was added to react with the
b
in situ formed 1 at 130 °C for 12 h. Isolated yields.
butylcarbamidine as a reaction partner, phenylamide (5a)
and other aryl amides bearing an electron-rich group, such as
methoxyl (5b), and an electron-poor group, such as chloro (5c,
5d), on the phenyl ring led to alkylaryl-substituted 1,3,5-
triazin-2(1H)-ones in good yields. Amide with an S-heteroaryl
group, in this case, thiophene, could be converted into the
corresponding product 5e. Alkyl amides with primary and
secondary α-carbon atoms formed products 5f−h in moderate
yields. Propyl and isopropyl amidines could successfully react
with phenylamide andt-butylamide to form dialkyl- and
alkylaryl-substituted 1,3,5-triazin-2(1H)-ones (5i−l), respec-
tively.
The absolute configurations of the obtained 1,3,5-triazinones
3y and 5b were determined by X-ray crystallographic analysis
(Figure 2). According to the bond lengths and angles observed
in the analysis, the N−H bond on the triazinone ring was
identified to originate from the corresponding amidine.
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a
our method could greatly lower the production cost and
provide a new perspective to overcome challenges in the
industrial manufacturing of triazine-based organic luminescent
materials.
Table 4. Chlorination of 1,3,5-Triazin-2(1H)-ones
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01970.
Materials and methods, experimental procedures, useful
1
information, H NMR spectra, 13C NMR spectra, MS
data, crystal data, and structure refinement for 3y and 5b
(PDF)
a
Triazinone (0.2 mmol), POCl3 (3 equiv), Et3N (3 equiv), CH3CN
Accession Codes
(2 mL), 90 °C, 12 h.
CCDC 2080869−2080870 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Scheme 2. Functionalization of 2-Chloro-1,3,5-triazines to
Synthesize Unsymmetrical Trisubstituted 1,3,5-Triazines
AUTHOR INFORMATION
Corresponding Author
■
Bingxin Yuan − Green Catalysis Center, College of Chemistry,
Zhengzhou University, Zhengzhou, Henan 450001, China;
orcid.org/0000-0003-4580-366X; Email: bxyuan@
zzu.edu.cn
Authors
Helong Liang − Green Catalysis Center, College of Chemistry,
Zhengzhou University, Zhengzhou, Henan 450001, China
Ganzhong Li − Green Catalysis Center, College of Chemistry,
Zhengzhou University, Zhengzhou, Henan 450001, China
Lei Zhang − Green Catalysis Center, College of Chemistry,
Zhengzhou University, Zhengzhou, Henan 450001, China
Gefei Wang − Green Catalysis Center, College of Chemistry,
Zhengzhou University, Zhengzhou, Henan 450001, China
Mingyu Song − Green Catalysis Center, College of Chemistry,
Zhengzhou University, Zhengzhou, Henan 450001, China
Heng Li − Green Catalysis Center, College of Chemistry,
Zhengzhou University, Zhengzhou, Henan 450001, China;
orcid.org/0000-0001-8145-233X
of 6a with carbazole formed the corresponding unsymmetrical
trisubstituted 1,3,5-triazine in 94% yield (reaction 3). The
chloro group on the benzene serves as a synthetic handle for
further functionalization. These compounds have been
synthesized via a multiple-step coupling reaction and used as
intermediates in an organic material layer of an organic
electroluminescent device in patents.13
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01970
In summary, we have developed a novel synthetic strategy to
achieve unsymmetrical trisubstituted s-triazines. The unique
reactivity of the in situ formed isocyanate from amide allows
for cyclization with amidine and the successful transformation
into 1,3,5-triazin-2(1H)-ones while introducing two functional
groups (aryl or alkyl) to the core unit in a single step. The
disubstituted 1,3,5-triazin-2(1H)-ones could be purified via
simple precipitation in water and filtration, avoiding a multiple-
step coupling reaction and a tedious workup procedure.
Chlorization and the Pd-catalyzed coupling reaction with
various nucleophiles enable the synthesis of the desired
unsymmetrical trisubstituted 1,3,5-triazines with a wide range
of functional groups. Much to our delight, the 1,3,5-triazin-
2(1H)-ones could be produced on a multi-kilogram-scale,
ensuring the application of this protocol in chemical
manufacturing. Considering the rapid development of OLED
technology and the growing demand for low-cost materials,
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (no. 21801229) and Key-Area
Research and Development Program of Guangdong Province
(no. 2020B010188003).
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