Hydration of phenylacetylene on sulfonated carbon materials: Active site and intrinsic catalytic activity

Article abstract of DOI:10.1039/c8ra07966h

A series of sulfonated carbon materials (sulfonated glucose-derived carbon, carbon nanotubes, activated carbon and ordered mesoporous carbon, denoted as Sglu, SCNT, SAC and SCMK, respectively) were synthesized and applied as acid catalysts in phenylacetyl

Full text of DOI:10.1039/c8ra07966h

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PAPER
Synthesis of 4-trifluoromethyl 2-pyrones and
pyridones through the Brønsted base-catalyzed
Pechmann-type reaction with cyclic 1,3-diones†
Cite this: Org. Biomol. Chem., 2018,
16, 9440
Weitao Yan, Ruo Wang, Tesen Zhang, Hongtao Deng, Jian Chen, Wei Wu and
Zhiqiang Weng
*
An efficient method for the synthesis of 4-trifluoromethyl 2-pyrones through the Brønsted base-cata-
lyzed Pechmann-type reaction of cyclic 1,3-diones with ethyl 4,4,4-trifluoroacetoacetate is described. In
the presence of 2-dimethylamino pyridine (2-DMAP) as a catalyst, the resulting 4-trifluoromethyl
2-pyrones are formed in good to excellent yields. Additionally, the reaction also provides 4-trifluoro-
methyl 2-pyridones by using the easily available NH4OAc as a source of NH3.
Received 31st October 2018,
Accepted 27th November 2018
DOI: 10.1039/c8ob02701c
rsc.li/obc
anes, starting from the reaction of (α-furoyl)methyl-
triphenylphosphonium bromide with methyl 2-perfluoro-
Introduction
The syntheses of 2-pyrones and their derivatives have been alkynoates (Scheme 1a).20,21 Zanatta and co-workers reported a
widely studied because of their intrinsic biological and self-condensation reaction of 5-aryl-5-methoxy-3-(trifluoro-
pharmacological activities such as antifungal, antibiotic, cyto- methyl)penta-2,4-dienenitriles to afford 6-aryl-4-trifluoro-
toxic, neurotoxic and phytotoxic activities (Fig. 1).1,2 In methyl-2H-pyran-2-ones (Scheme 1b).22 However, convenient
addition, 2-pyrones also serve as versatile starting materials for and more efficient methods to construct 4-trifluoromethylated
the synthesis of key intermediates in synthetic organic chem- 2-pyrones by direct reactions with simple starting materials are
istry as well as in medicinal chemistry.3,4 As a consequence, extremely attractive.
significant progress has been made on the synthesis of
2-pyrones and their derivatives by the conventional approaches popular and successful method for the synthesis of coumarins
or by using a transition metal-catalysed reaction.5–9
because of its preparative simplicity and requirement of in-
The Pechmann reaction has emerged as a particularly
The introduction of fluorine or fluorinated groups into bio- expensive starting material.23,24 The reaction is carried out by
logically active molecules can modify the lipophilicity, meta- simply mixing the phenol with the β-keto ester in the presence
bolic profile and/or receptor-binding affinity of lead of an acid as a catalyst and it is often highly efficient and easy
compounds.10–13 The incorporation of the trifluoromethyl to implement. The reaction mechanism involves three steps,
group (–CF3) into pyrones to generate trifluoromethylated i.e. electrophilic aromatic substitution, transesterification, and
pyrones is expected to modify the biological activity of the dehydration. This strategy has been widely applied to the syn-
parent pyrones due to the increased electrophilicity of the
–CF3 group. Therefore, significant effort has been focused on
introducing the –CF3 group into the pyrones.14–19 Although the
synthetic methods towards 3- or 6-trifluoromethylated
2-pyrones are highly documented, fewer examples focus on the
regiocontrolled synthesis of their 4-trifluoromethylated
isomers. Cao and co-workers reported the synthesis of 4-per-
fluoroalkyl-6-(α-furyl)-6-phyranones by hydrolysis of phosphor-
State Key Laboratory of Photocatalysis on Energy and Environment, College of
Chemistry, Fuzhou University, Fuzhou 350108, China. E-mail: zweng@fzu.edu.cn;
Fax: +86 594 22866121; Tel: +86 594 22866121
†Electronic supplementary information (ESI) available: Copies of 1H NMR, 13C
NMR, 19F NMR of synthesized products. CCDC 1872006 and 1870545. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8ob02701c
Fig. 1 Representative examples of bioactive 2-pyrone derivatives.
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may undergo the Pechmann-type reaction with ethyl 4,4,4-tri-
fluoroacetoacetate to afford 4-trifluoromethyl 2-pyrone deriva-
tives under Brønsted base-catalyzed conditions (Scheme 1c).
With this understanding and our interest in the synthesis of
perfluoroalkylated heteroaromatic molecules,29–33 we herein
report a general and practical procedure for the synthesis of
4-trifluoromethyl 2-pyrone and 2-pyridone derivatives.
Results and discussion
To test the feasibility of our envisioned method, the reaction
of cyclohexane-1,3-dione (1a) with ethyl 4,4,4-trifluoroaceto-
acetate (2) was carried out under classic Pechmann-type reaction
Scheme 1 Methods for the preparation of 4-trifluoromethyl 2-pyrone
derivatives.
thesis of variously substituted coumarin derivatives, starting
from phenols with β-keto esters.
1,3-Diketones are useful intermediates in organic synthesis,
especially for the preparation of some biologically active com-
pounds.25,26 In view of the fact of the keto–enol tautomerism
in 1,3-diketone compounds,27,28 we envisioned that 1,3-diones
Fig. 2 ORTEP diagrams of 3a (left) and 4e (right) with thermal ellipsoids
at the 40% probability level.
Table 1 Optimization of the synthesis of the 4-trifluoromethyl 2-pyrone derivative (3a)a
Entry
[cat.] (20 mol%)
Solvent
Temperature (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
H2SO4
H2SO4
p-TsOH
FeF3
TiCl4
AlCl3
NEt3
Pyridine
2-DMAP
4-DMAP
2-DMAP
2-DMAP
2-DMAP
2-DMAP
2-DMAP
2-DMAP
2-DMAP
2-DMAP
2-DMAP
2-DMAP
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
Nitrobenzene
DMSO
DMF
DCE
DMAc
Diglyme
NMP
DCE
DCE
DCE
60
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
100
80
16
24
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
16
8
0
<1
1
12
3
5
47
69
73
38
27
53
99
0
66
39
74c
70
35
74
9
10
11
12
13
14
15
16
17
18
19
20
DCE
120
a Reaction conditions: 1a (0.30 mmol), 2 (0.45 mmol, 1.5 equiv.), solvent (1.0 mL), N2. b Yields were determined by 19F NMR analysis of the crude
reaction mixture with PhOCF3 as an internal standard. p-TsOH = p-toluenesulfonic acid; 4-DMAP = 4-dimethylamino pyridine; 2-DMAP = 2-di-
methylamino pyridine; DCE = 1,2-dichloroethane; DMAc = N,N-dimethylacetamide. c The reaction was conducted under an air atmosphere.
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conditions [nitrobenzene, H2SO4 (20 mol%)]34 at 60 °C by 19F NMR (Table 1, entry 3). Similarly, other Lewis acids,
(Table 1). We observed that 2 remained unreacted (Table 1, such as FeF3, TiCl4, and AlCl3, provided product 3a in a poor
entry 1), even at 120 °C after a reaction time of 24 h and even yield (Table 1, entries 4–6).
though these were the most efficient conditions for the
Interestingly, the further attempt produced a fruitful
Pechmann reaction of the phenol with the β-keto ester output, with the expected product 3a being generated in 47%
(Table 1, entry 2). Although p-TsOH was also reported as an yield, when NEt3 was used as the catalyst in nitrobenzene at
efficient promoter of the Pechmann reaction,35 under our con- 120 °C (Table 1, entry 7). Delighted by this result, we per-
ditions, only a trace amount of product 3a could be monitored formed further optimization studies to establish the best reac-
Table 2 Synthesis of 4-trifluoromethyl 2-pyronesa
Entry
1
1,3-Dione substrates
Products
Yieldb (%)
99
2
3
4
99
99
99
5
99
6
7
8
43
84
—
No desired product
a Reaction conditions: 1 (0.30 mmol), 2 (0.45 mmol, 1.5 equiv.), 2-DMAP (0.060 mmol, 20 mmol%), DCE (1.0 mL), 120 °C, 16 h, N2. b Isolated
yields.
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tion conditions. Other organic bases such as pyridine,
2-DMAP, and 4-DMAP were also found to be effective in
driving this transformation, and afforded 3a in 69, 73 and 38%
yields, respectively (Table 1, entries 8–10). 2-DMAP was there-
fore established as the preferred catalyst for this transform-
ation. In addition, the structure of 3a was verified by X-ray crys-
tallographic analysis (Fig. 2). After screening various common
organic solvents, we found that the reaction proceeded most
efficiently in DCE, which provided product 3a in 99% yield
(Table 1, entry 13). The reactions that were performed in
DMSO, DMF, diglyme and NMP produced diminished product
yields (Table 1, entries 11, 12, 15 and 16), whereas other sol-
vents such as DMAc did not provide the desirable product for
this reaction (Table 1, entry 14). Thus, DCE was selected as the
preferred solvent for the subsequent experiments as a result of
its efficiency in the reaction and ease of handling during the
workup procedure. Notably, product 3a was formed in a lower
yield when the reaction was carried out under an air atmo-
sphere (entry 17). Reactions performed at lower temperatures
(100 or 80 °C) produced lower yields (70% and 35% yields,
respectively; Table 1, entries 18 and 19). Furthermore, reducing
the reaction time to 8 h resulted in incomplete conversion
(74% yield; Table 1, entry 20).
Scheme 2 Mechanistic experiments.
Table 3 Synthesis of 4-trifluoromethyl 2-pyridonesa,b
With the optimized reaction conditions in hand [i.e.,
2-DMAP (20 mol%) in 1,2-dichloroethane at 120 °C for 16 h],
the Pechmann-type reaction of ethyl 4,4,4-trifluoroacetoacetate
2 with a variety of cyclic 1,3-diones 1 was examined (Table 2).
Substituted 1,3-cyclohexanediones that contain a methyl (1b),
dimethyl (1c), and phenyl group (1e) at the C-5 position took
part in the reaction to effectively afford the desired 4-trifluoro-
methyl 2-pyrone products 3b, 3c, and 3e all in nearly quantitat-
ive yields (Table 2, entries 2, 3, and 5). The reaction of 4,4-di-
methylcyclohexane-1,3-dione (1d) with 2 led to the formation
of a mixture containing two regioisomers 3d and 3d′ in a 1 : 3
ratio with an overall yield of 99% (Table 2, entry 4). The triplet
at δ 2.93 ppm in 3d, corresponding to the resonance of C-8-H
protons, appeared downfield compared to aliphatic hydrogen
atoms because of the deshielding effect of the pyrone function-
a Reaction conditions: 1 (0.30 mmol), 2 (0.45 mmol, 1.5 equiv.),
NH4OAc (0.90–1.50 mmol), 2-DMAP (0.060 mmol, 20 mmol%), DCE
(3.0 mL), 140 °C, 48 h, N2. b Isolated yields.
ality. Importantly, heterocyclic derivatives of 1,3-diones such other conceivable scenarios cannot be ruled out at this
as pyran-3,5-dione 1f smoothly underwent the reaction with 2 moment, we propose a mechanistic rationale of this reaction
to give the desired product 3f in 43% yield (Table 2, entry 6). to be nucleophilic addition, dehydration, and enolization, fol-
In addition to 1,3-cyclohexanediones, 1,3-cyclopentanedione lowed
1g was also reactive under these reaction conditions and deli- (Scheme 1c).
vered the desired product 3g in a good yield (84%; Table 2,
Encouraged by these results, we next explored the possi-
by
intramolecular
nucleophilic
substitution
entry 7). Unfortunately, the reaction of an acyclic 1,3-dione 1h bility for the synthesis of the pyridone analogues, a related
with 2 failed to provide any discernible pyrone products class of compounds having wide applications from agriculture
(Table 2, entry 8).
to medicine.36–39 The reaction of the 1,3-dione substrates 1
To examine whether any reactive species was involved in the with 2 and NH4OAc (source of NH3) in the presence of
cyclization, the progress of the reaction was monitored by 19F 20 mol% of 2-DMAP in DCE at 140 °C for 48 h resulted in the
NMR (see the ESI†). The results revealed that a nucleophilic formation of the corresponding 4-trifluoromethyl 2-pyridines
addition product (I) is formed in 99% NMR yield during the in moderate to excellent yields (Table 3). In contrast to its
reaction of 1 with 2 in the presence of 2-DMAP (20 mol%) at pyrone analogues, the pyridone product 4d was obtained as a
60 °C (Scheme 2). Intermediate I underwent further reaction to single regioisomer in 47% yield, implying that the reaction
afford the desired product 3a in 70% NMR yield. These results outcome is sensitive to the steric properties of the plausible
provide support for the possible role of the nucleophilic pyrone intermediate 3d′. The structure of 4e was unambigu-
addition species I as an intermediate in the reaction. Although ously assigned by X-ray crystallography (Fig. 2).
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Two possible reaction pathways for the formation of pyri- thesis of pyridones although we could not rule out path B
dones were considered (Scheme 3); either a base-mediated (Scheme 3).
Pechmann-type reaction of cyclic 1,3-dione 1a with ethyl 4,4,4-
To demonstrate the scalability and practicability of this pro-
trifluoroacetoacetate 2, affording 2-pyrone 3a, which, upon cedure, 1.12 g of cyclohexane-1,3-dione (1a) was reacted with
amination, led to the desired pyridone 4a (path A), or, alterna- 2.76 g of ethyl 4,4,4-trifluoroacetoacetate (2) in DCE at 120 °C
tively, 1,3-dione 1a mono-aminated with NH4OAc furnishing for 20 h (Scheme 4). The corresponding 4-trifluoromethyl
3-iminocyclohexanone 5, which, by reacting with 2, resulted in 2-pyrone 3a was isolated in 56% yield (1.30 g).
the formation of 4a (path B).
To demonstrate the high synthetic value of the obtained
To distinguish these pathways, a series of experiments were 4-trifluoromethyl 2-pyrones, we further investigated a variety of
rationally designed and performed. Firstly, the possible for- chemical transformations with compound 3a as a model sub-
mation of 3-iminocyclohexanone 5 as an intermediate of path strate (Scheme 5). Addition of dimethyl acetylenedicarboxylate
B was tested. Indeed, under the standard reaction conditions, (DMAD) to 3a yielded the benzene derivative 6 in 72% yield.
the reaction of 3-iminocyclohexanone 5 with 2 led to the for- Treatment of 3a with p-toluidine in ethanol led to the isolation
mation of 4a in a low yield (16%). Path A, in contrast, was sup- of N-tolyl-substituted pyridin-2(1H)-one 7 in 80% yield.
ported by the formation of 4a (76% NMR yield) resulting from
the amination of 2-pyrone 3a with NH4OAc. These results
clearly demonstrate that path A involving the formation of
2-pyrone probably is favored during the aforementioned syn-
In summary, we reported a Pechmann-type reaction of cyclic
Conclusions
1,3-diones with ethyl 4,4,4-trifluoroacetoacetate for the syn-
thesis of 4-trifluoromethyl 2-pyrones. This Brønsted base-cata-
lysed reaction proceeded well to furnish the desired 4-trifluoro-
methyl 2-pyrones in good to excellent yields. The method also
gave access to a number of different 4-trifluoromethyl 2-pyri-
dones by using the easily available NH4OAc as a source of
NH3. We anticipate that this protocol will be useful for the syn-
thesis of 4-trifluoromethyl 2-pyrones and pyridones that could
find further applications as biologically active compounds.
Scheme 3 Proposed reaction pathways for the formation of 4a.
Experimental
General remarks
1H NMR, 19F NMR and 13C NMR spectra were recorded using a
1
Bruker AVIII 400 spectrometer. H NMR and 13C NMR chemi-
cal shifts were reported in parts per million (ppm) downfield
from tetramethylsilane and 19F NMR chemical shifts were
determined relative to CFCl3 as the external standard and low
field is positive. Coupling constants (J) are reported in hertz
Scheme 4 Scalability of the synthesis of 3a.
(Hz). The residual solvent peak was used as an internal refer-
1
ence: H NMR (chloroform δ 7.26) and 13C NMR (chloroform
δ 77.0). The following abbreviations were used to explain the
multiplicities: s = singlet, d = doublet, t = triplet, q = quartet,
m = multiplet, br = broad. HRMS were obtained on a Thermo
Exactive Plus LC-MS system. Reagents were obtained from
commercial sources. Solvents were freshly dried and degassed
according to the published procedures prior to use.
General procedure for the synthesis of 4-trifluoromethyl
2-pyrones
In a glove box filled with nitrogen, to an oven-dried 5 mL
pressure tube equipped with a stir bar was added 1,3-diones 1
(0.30 mmol), ethyl 4,4,4-trifluoroacetoacetate 2 (0.45 mmol,
1.5 equiv.), 2-dimethylaminopyridine (0.060 mmol, 0.20 equiv.),
and 1,2-dichloroethane (1.0 mL). The tube was sealed with a
Scheme 5 Derivatizations of 3a.
Teflon screw cap and the solution was stirred at 120 °C for
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16 h. The reaction mixture was cooled to room temperature, 11 J. Hu and T. Umemoto, Synthetic Organofluorine Chemistry,
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2-pyridones
In a glove box filled with nitrogen, to an oven-dried 5 mL
pressure tube equipped with
a stir bar was added 1
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(0.30 mmol), NH4OAc (0.90–1.50 mmol, 3.0–5.0 equiv.), ethyl
4,4,4-trifluoroacetoacetate 2 (0.45 mmol, 1.5 equiv.), 2-di-
methylaminopyridine (0.060 mmol, 0.20 equiv.), and 1,2-
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