Synthesis of 1,3,5-triazines via Cu(OAc)2-catalyzed aerobic oxidative coupling of alcohols and amidine hydrochlorides

Article abstract of DOI:10.1039/c5ob00724k

Cu(OAc)₂ was found to be an efficient catalyst for dehydrogenative synthesis of 1,3,5-triazine derivatives via oxidative coupling reaction of amidine hydrochlorides and alcohols in air. Both aromatic and aliphatic alcohols can be involved in the reaction and thirty-three products were obtained with good to excellent yields. Moreover, the use of a ligand, strong base and organic oxidant is unnecessary.

Full text of DOI:10.1039/c5ob00724k

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Journal Name
RSCPublishing
ARTICLE
Synthesis of 1,3,5-Triazines via Cu(OAc)2-Catalyzed
Aerobic Oxidative Coupling of Alcohols and Amidine
hydrochlorides
Cite this: DOI: 10.1039/x0xx00000x
Qing You, Fei Wang, Chaoting Wu, Tianchao Shi, Dewen Min, Huajun Chen, Wu
Zhang*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
Abstract: Cu(OAc)2 was found to be an efficient catalyst for dehydrogenative synthesis of
1,3,5ꢀtriazine derivatives via oxidative coupling reaction of amidine hydrochlorides and
alcohols in air. Both aromatic and aliphatic alcohols can be involved in and thirtyꢀthree
products were obtained with good to excellent yields. Moreover, the use of ligand, strong base
and organic oxidant is unnecessary.
www.rsc.org/
Introduction
1,3,5ꢀtriazines are very important skeletons of many
biologically active compounds, such as herbicides, insecticides,
fungicides, anticarcinogens and antivirals (Figure 1).1 In
addition, they are also widely used as organic dyestuff, liquid
crystal, flourescent brightener, electroluminescent materials, gas
generating agent, solid propellant and pyrotechnics.2–3 In the
Figure 1. Parts of Products of 1,3,5ꢀTriazines
Previous report:
past decades,
metalꢀcatalyzed
a
few approaches, including transition
coupling reaction of halogenated
1,3,5ꢀtriazines,4 the cycotrimerizations of nitriles5 and
multicomponent reaction of cyanamid, 1,2,4ꢀtriazole and
triethyl orthoformate have been developed for the synthesis of
1,3,5ꢀtriazine derivatives.6 Alternatively, these compounds
were also obtained through the pathways based on the use of
available amidines as substrates, for example, domino
heterocyclization of isothiocyanates with aryl amidines,7
cesium carbonateꢀpromoted cyclodehydrogenation of amidine
hydrochloride with aromatic aldehyde.8 However, these
methods suffer from limitations of operating difficulties or
tough reaction conditions involving strong base, microwave,
noble metal catalyst and organic oxidant. Recently, Zhang et al.
This work:
Scheme 1. Synthesis of 1,3,5ꢀTriazines
reported
new
strategies
for
the
synthesis
of
2,4,6ꢀtriarylꢀ1,3,5ꢀtriazines based on rutheniumꢀcatalyzed
reaction of aryl methanols and amidines.9 The disadvantages of
the strategy lies in noble metal rutheniumꢀbased catalysts and
limitation of substrate suitability. As a result, more robust and
efficient method for the synthesis of 1,3,5ꢀtriazine is still
extremely in demand, especially involving the synthesis carried
out under gentle conditions with inexpensive catalysts.
Copper catalysts with features of low cost and environmental
friendliness were frequently utilized to synthesize novel
heterocyclic compounds.10–12 For example, Nagasawa et al.
have reported that Cu(OTf)2 exhibited impressive catalytic
ability in intramolecular oxidative C–N coupling reaction and
intermolecular amino cyclization.13 Our previous findings also
showed that copperꢀbased catalysts are attractive in the catalytic
formation of carbon–heteroatom bonds (Scheme 1).14 Herein,
we report the synthesis of 1,3,5ꢀtriazine derivatives directly by
oxidative coupling of amidine hydrochlorides and alcohols in
air with Cu(OAc)2 as a catalyst. Compared with the relevant
reported reactions for the synthesis of 1,3,5ꢀtriazine
derivative,8,9 our method is more efficient as evidenced by the
fact that both aryl methanols and alkyl alcohols are utilizable as
starting reactants in the oxidative coupling reaction.
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Table 1. Optimization of the Reaction Conditionsa
from screening of the copper catalysts, ligands and solvents are
summarized in Table 1. Given efficient catalytic activity and
good recyclability of CuO nanoparticles,15 they were evaluated
in the first place and the result indicates that the reaction of 1a
with 2a in the presence of 5 mol% of CuO nanoparticles at 110
°C for 24
h
in toluene affords the production of
2,4,6ꢀtriphenylꢀ1,3,5ꢀtriazine (3aa) in 85% yield (Table 1, entry
1). However, the transformation of CuO nanoparticles into
Cu(II) in the reaction limits their further uses. Therefore,
different copper catalysts such as CuCl2, Cu(NO3)2, Cu(OAc)2,
Cu(OTf)2 and Cu(acac)2 were detected, and Cu(OAc)2 stood out
from the rest with 88% yield (Table 1, entries 2–6). Decreasing
the amount of catalyst to 5% resulted in much lower yield
(Table 1, entry 7). Increasing the amount of catalyst to 15%
resulted in slightly higher yield (Table 1, entry 8). To our
delight, we discovered that none of ligand wasnecessary (Table
1, entry 9). The yields of reactions performed in the presence of
different bases such as NaHCO3, K2CO3 and Na2CO3 are
essentially the same (Table 1, entries 9–11). Notably, base
plays a vital role in the oxidative coupling reaction as the
reaction cannot proceed without it (Table 1, entry 12). Na2CO3
was selected as base in the following due to its moderate basic
strength and lowꢀcost. Unfortunately, yields decreased
remarkablly when DMF and DMSO were used as solvents, and
a new product was detected (Table 1, entries 13 and 14).
Lowering temperature to 80 oC, the yield reduced rapidly
(Table 1, entries 15 and 16). With the reduction of reaction time
to 18, 12 and 6 h, reduced yields of 80%, 70% and 55% were
obtained, respectively (Table 1, entries 17–19).
entry
catalyst
(mol%)
ligand solvent
base
yield
(%)b
85
1
2
nanoCuO (5) Phen
CuCl2 (10) Phen
toluene
toluene
K2CO3
K2CO3
75
3
4
5
Cu(NO3)2
(10)
Phen
Phen
Phen
toluene
toluene
toluene
K2CO3
K2CO3
K2CO3
70
88
80
Cu(OAc)2
(10)
Cu(acac)2
(10)
6
7
8
Cu(OTf)2 (10) Phen
Cu(OAc)2 (5) Phen
Cu(OAc)2
(15)
toluene
toluene
toluene
K2CO3
K2CO3
K2CO3
85
60
89
Phen
9
Cu(OAc)2
(10)
toluene
K2CO3
88
87
88
_
_
10
Cu(OAc)2
(10)
toluene NaHCO3
_
_
11
Cu(OAc)2
(10)
toluene
toluene
DMF
Na2CO3
_
12
Cu(OAc)2
(10)
_
_
_
_
_
_
_
_
With the optimized reaction conditions presented in entry 11 of
Table 1 in hand, a variety of primary alcohols and amidine
hydrochlorides with different substituents were exploited as
starting reactants to the synthesis of 1,3,5ꢀtriazine derivatives,
and representative results are listed in Figure 2. Several
functional groups, such as methoxy, chloro, fluoro and nitro
were wellꢀtolerated. No significant substitute effect was
observed on aromatic alcohols, excellent yields were obtained
13
Cu(OAc)2
(10)
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
45
55
30
60
55
70
80
14
Cu(OAc)2
(10)
DMSO
toluene
toluene
toluene
toluene
toluene
15c
16d
17e
18f
19g
Cu(OAc)2
(10)
Cu(OAc)2
(10)
for
alcohols
with
both
electronꢀdonating
and
electronꢀwithdrawing substituents on the paraꢀposition of aryl
ring (Figure 2, 3ba, 3ea, 3ha, 3ia, 3la and 3ma). Relatively,
low yields were obtained when alcohols were substituted with
electronꢀwithdrawing substituents such as Br and NO2, for
example, the presence of a nitro group reduced yield to 46%
(Figure 2, 3ja and 3ka). Besides, the steric hindrance has
obvious effect on the reaction. For example, when methyl
substituted benzyl alcohols were used, para and meta
substituted substances gave higher yields than those with ortho
substituents (Figure 2, 3ea, 3fa and 3ga). Apparent substitute
effect was observed on amidine hydrochlorides. The reaction
Cu(OAc)2
(10)
Cu(OAc)2
(10)
Cu(OAc)2
(10)
aReaction conditions: benzamidine hydrochloride (1 mmol),
benzyl alcohol (0.6 mmol), Cu(OAc)2 (10 mol %), Na2CO3 (1
mmol), toluene (2.5 mL) under reflux in air for 24 h. bIsolated
yield. Reaction temperature: c80 oC, d100 oC. Reaction time: e6
h, f12 h, g18 h.
favored
and 4ꢀ(trifluoromethyl)benzamidine hydrochloride 2d, afforded
1,3,5ꢀtriazines in excellent yields (Figure 2, 3ab 3mb 3ad
1 with both 4ꢀmethylbenzamidine hydrochloride 2b
–
,
,
Results and discussion
3bd and 3id), while 2c with 4ꢀBr on the phenyl ring led to
1,3,5ꢀtriazines in moderate yields (Figure 2, 3ic and 3lc). To
our delight, good results were obtained as primary aliphatic
alcohols such as methanol, ethanol, nꢀbutyl alcohol and
We started with the optimization of the oxidative coupling
conditions exploiting benzyl alcohol (1a) and benzamidine
hydrochloride (2a) as the model substrates. The results obtained
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Cl
N
Cl
Cl
N
N
N
N
N
N
N
N
N
N
N
3da, 55%
3aa
3ba
3ca
, 88%
, 88%
, 85%
CF3
aReaction conditions: benzamidine hydrochloride (1 mmol),
benzyl alcohol (0.6 mmol), Cu(OAc)2 (10 mol %), Na2CO3 (1
mmol), toluene (2.5 mL) under reflux in air for 24 h.
N
N
N
N
N
N
N
N
N
N
N
N
3ga
, 85%
NO2
3ea
, 88%
O
3fa
, 47%
Br
3ha
Figure 2. Scope of the Synthesis of 1,3,5ꢀTriazinesa
, 87%
isobutanol were tested (Figure 2, 3qa–3ta, 3qb and 3rb). In
addition, isopropyl alcohol and secꢀbutyl alcohol were tested
successively and the results showed that 3ra are the
majorproduct, in accordance with the product from ethanol.
However, the reaction of alkyl amidines such as acetamidine
with benzylic alcohol failed to yield the desired product may be
due to its boiling point was lower than our reaction
temperature16.
N
N
N
N
N
N
N
N
N
N
N
N
3ia
3la, 92%
, 90%
F
3ka
, 46%
3ja
, 52%
Cl
N
N
N
Cl
N
N
N
N
N
N
N
N
Furthermore, the reaction between benzamidine hydrochlorides
N
N
N
(
2a and 2b) and DMF (or DMSO) was investigated respectively
3pa
, 97%
3ma
, 95%
3na, 90%
3oa, 97%
Cl
N
(Scheme 2). The results showed that 3qa 3qb and
,
2,4ꢀbis(4ꢀ(trifluoromethyl)phenyl)ꢀ1,3,5ꢀtriazine were obtained
in good to excellent yields via aerobic copperꢀcatalyzed
cyclization of amidines with DMF or DMSO, a oneꢀcarbon
supplier. Comparing with Zhang’s report about CuIꢀcatalyzed
reaction of amidine with DMF in oxygen,17 more straight and
simple route was presented.
N
N
N
N
N
N
N
N
3ab
, 87%
3bb
, 96%
3eb
, 86%
F
CF3
NH
N
N
R=H, 50% yield
R=CH3, 46% yield
R=CF3, 73% yield
O
S
Cu(OAc)2 H2O (10 mol%)
Na2CO3 (2 equiv) air 110 o
NH2
HCl
N
+
C
R
R
R
N
N
N
N
N
N
N
NH
NH2
N
N
Cu(OAc)2 H2O (10 mol%)
Na2CO3 (2 equiv) air 110 o
N
N
R=H, 60% yield
R=CH3, 63% yield
R=CF3, 89% yield
HCl
+
O
N
N
C
3mb
, 94%
3fb, 66%
3hb
, 87%
R
R
R
O
Scheme 2. Formation of 1,3,5ꢀTriazines from DMSO/DMF and
Benzamidine hydrochlorides
N
N
N
On the basis of great progress about metal catalyzed
N
N
N
N
dehydrogenation formation of aldehyde from alcohol,18
a
N
N
F3C
Br
CF3
possible
pathway
for
the
formation
of
Br
Br
Br
3ad
, 98%
3ic, 50%
3lc
, 66%
2,4,6ꢀtrisubstitutedꢀ1,3,5ꢀtriazines was proposed (Scheme 3).
Initially, aldehyde was formed by Cu(OAc)2 catalyzed alcohol
oxidation in air.18a Then, the reaction of aldehyde and amidine
was initiated and promoted by Cu(OAc)2 and air oxidation,
giving rise to 1,3,5ꢀtriazines. The role of base is to neutralize
hydrochloride of the amidine. The mechanism of the reaction is
similar to that reported in the literature.8,9
O
Cl
N
N
N
N
N
N
F3C
CF3
F3C
CF3
3bd
, 88%
3id, 75%
Conclusions
In summary, an efficient and simple method for the preparation
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gel using petroleum ether/EtOAc (100:1) as an eluent to give
the corresponding products 3aa–3rb.
Notes and references
Key Laboratory of Functional Molecular Solids, Ministry of Education,
Anhui Laboratory of MoleculeꢀBased Materials, College of Chemistry
and Materials Science, Anhui Normal University, Wuhu 241000,
People’s Republic of China. Eꢀmail: zhangwu@mail.ahnu.edu.cn
Tel: +86ꢀ553ꢀ3883513; fax: +86ꢀ553ꢀ3869310
†
Electronic Supplementary Information (ESI) available: Catalyst
characterization, analytic data, images of 1H and 13C NMR of all products
and other electronic format. See DOI: 10.1039/b000000x/
1
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benzamidine hydrochlorides and alcohols exploiting Cu(OAc)2
as catalyst. The obvious benefit of this approach is its tolerance
to a broad range of both aromatic and fatty alcohols. In
addition, the facile reaction atmosphere of air and no need of
ligands make this approach highly practical for industrial
manufacture. Further studies for the construction of diverse
heterocyclic compounds relying on present strategy are ongoing
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