Heterogeneous Suzuki-Miyaura coupling of heteroaryl ester: Via chemoselective C(acyl)-O bond activation

Article abstract of DOI:10.1039/c9ra02394a

A site-selective supported palladium nanoparticle catalyzed Suzuki-Miyaura cross-coupling reaction with heteroaryl esters and arylboronic acids as coupling partners was developed. This methodology provides a heterogeneous catalytic route for aryl ketone formation via C(acyl)-O bond activation of esters by successful suppression of the undesired decarbonylation phenomenon. The catalyst can be reused and shows high activity after eight cycles. The XPS analysis of the catalyst before and after the reaction suggested that the reaction might be performed via a Pd⁰/Pd^II catalytic cycle that began with Pd⁰.

Full text of DOI:10.1039/c9ra02394a

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Heterogeneous Suzuki–Miyaura coupling of
heteroaryl ester via chemoselective C(acyl)–O
bond activation†
Cite this: RSC Adv., 2019, 9, 17266
*
Hongpeng Ma, Chaolumen Bai and Yong-Sheng Bao
A site-selective supported palladium nanoparticle catalyzed Suzuki–Miyaura cross-coupling reaction
with heteroaryl esters and arylboronic acids as coupling partners was developed. This methodology
provides a heterogeneous catalytic route for aryl ketone formation via C(acyl)–O bond activation of
esters by successful suppression of the undesired decarbonylation phenomenon. The catalyst can be
reused and shows high activity after eight cycles. The XPS analysis of the catalyst before and after the
reaction suggested that the reaction might be performed via a Pd0/PdII catalytic cycle that began
with Pd0.
Received 30th March 2019
Accepted 27th May 2019
DOI: 10.1039/c9ra02394a
rsc.li/rsc-advances
Newman developed an NHC-based Pd catalyst which can catalyze
Suzuki–Miyaura coupling of phenyl esters (see Scheme 1d).7
Introduction
Although these homogeneous palladium catalysts offer high
selectivity and yields under relatively mild operating conditions,
but their industrial applicability is limited by the inherent
problem of catalyst separation from the product and its recycle.8
Moreover, the palladium residues in the product stream could
be a serious issue in the pharmaceutical industry.9 With these
considerations in mind, a wide variety of alternative methods
for heterogeneous Pd-catalyzed Suzuki–Miyaura reactions have
been introduced.10 But, to the best of our knowledge, there is no
report for heterogeneous Pd-catalyzed Suzuki–Miyaura reac-
tions using ester as coupling partners.
Transition metal-catalyzed Suzuki–Miyaura reactions have
emerged as a powerful and indispensable tool to forge linkages
between carbon atoms because of the inherent advantages of
organoboron reagents.1 Conventionally, halocarbon electro-
philes are the most applied coupling partners in Suzuki–
Miyaura reactions.2 However, the corrosive halide-containing
waste production does not meet the modern synthetic chem-
istry expectations with the growing interest in environmentally
friendly protocol development nowadays. Hence, expanding the
types of electrophiles that react via this pathway is thus an
ongoing goal in organic synthesis.
Recently, (hetero)aryl esters, due to their ubiquitous nature,
have gained signicant interest as new aryl-coupling partners via
nickel catalyzed C(aryl)–O cleavage (see Scheme 1a).3 Actually, the
bond dissociation energy (BDE) of the C(acyl)–O bond is lower than
the C(aryl)–O bond in (hetero)aryl esters.3h But, catalyzed by
transition-metal, there are challenges associated with C(acyl)–O
bond cleavage because of the decarbonylation phenomenon.4 In
2001, Yamamoto reported the rst example of palladium catalyzed
coupling reaction of aryl esters with organoboron compounds via
C(acyl)–O cleavage with carbonyl retention (see Scheme 1b).
However, the reaction was limited to electronically activated esters,
such as peruoroaliphatic carboxylic esters.5 Chatani reported the
palladium catalyzed coupling reaction of 2-pyridyl esters with
organoboron compounds (see Scheme 1c).6 More recently,
College of Chemistry and Environmental Science, Inner Mongolia Key Laboratory of
Green Catalysis, Inner Mongolia Normal University, Hohhot, 010022, China. E-mail:
sbbys197812@163.com; Tel: +86-471-4392442
† Electronic supplementary information (ESI) available: GC-MS analysis of model
reaction, characterization data for the products, 1H NMR and 13C NMR spectra of
the products. See DOI: 10.1039/c9ra02394a
Scheme 1 C–O activation of (hetero)aryl ester.
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Our previous work has conrmed that (hetero)aryl esters can
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Activity test
generate an activated acyl intermediate and perform amidation
reactions with tertiary amines or formamides to form amides
under supported palladium nanoparticles catalyzed condi-
tions.11 These results prompt us to explore the heterogeneous
palladium catalyzed Suzuki–Miyaura reaction of (hetero)aryl
esters. But one obstacle to establish this heterogeneous catal-
ysis is that the undesired Pd-catalyzed homocoupling of phe-
nylboronic acid could be easily facilitated under these reaction
conditions.12 Another is that in most of the cases, Suzuki–
Miyaura reaction need base condition which will give rise to
side reactions for ester. Therefore, the choose of catalyst with
moderate activity and (hetero)aryl ester with high selectivity is
vital.
Herein, we describe a nano-palladium catalyzed Suzuki–
Miyaura cross-coupling variant with heteroaryl ester and aryl-
boronic acid components as coupling partners. This newly
developed methodology provides a heterogeneous catalysis
route for C–C bond formation in a straightforward fashion,
which successfully suppresses the undesired decarbonylation
phenomenon attendant upon C(acyl)–O bond activation of
ester. In a series of supported PdNPs, Pd/g-Al2O3 catalyst with
a PdNPs mean diameter of 3.34 nm exhibited the best catalytic
activity and it maintained a relatively high activity aer several
recover cycles. The XPS analysis of the catalyst before and aer
reaction suggested that the reaction might be performed via
a Pd0/PdII catalytic cycle that began with Pd0.
The reaction in 25 mL over-dried reaction tube. The Suzuki–
Miyaura cross coupling reaction between 2-pyridyl 4-methyl-
benzoate 1a and phenylboronic acid 2a was used as the model
reaction. In a typical reaction catalyst (25 mg), 2-pyridyl 4-
methylbenzoate (21.3 mg, 0.10 mmol), phenylboronic acid
(24.4 mg, 0.20 mmol), K3PO4 (31.8 mg, 0.15 mmol), H2O (4.5 mL,
0.25 mmol), toluene (2 mL) were added into the reaction tube.
ꢀ
The resulting solution was stirred at 120 C for 48 h in an oil
bath. Aer cooling to room temperature, the mixture was
ltered, and the ltrate was evaporated in vacuo. The residue
was puried by ash column chromatography (silica gel, ethyl
acetate/petroleum ether ¼ 1 : 10 to 1 : 15 as an eluent) to afford
the desired ketone 3. All products were conrmed by compar-
1
ison with the previously reported H NMR and 13C NMR data.
Catalyst recycle experiment
Aer each reaction cycle, the solvent, substrate, and the prod-
ucts were removed by centrifugation; the separated catalyst was
washed thoroughly with 0.1 M NaOH ethanol solution (twice),
distilled water (4 times), and then washed twice with ethanol
followed by centrifugal separation and drying at 80 ꢀC for 12 h.
The recovered catalyst was used for the next cycle.
Hot ltration test
In a testing reaction 3 wt% Pd/g-Al2O3 (25 mg), 2-pyridyl 4-
methylbenzoate 1a (21.3 mg, 0.10 mmol), phenylboronic acid 2a
(24.4 mg, 0.20 mmol), K3PO4 (31.8 mg, 0.15 mmol), H2O (4.5 mL,
0.25 mmol), toluene (2 mL) were added into the reaction tube.
The hot ltration test is applied by ltering the reaction mixture
through a pre-heated Celite pad aer the reaction for 2 h. The
ltrate was detected by GC to obtain the conversion of 1a. Then
the ltered reaction solution continued to react for 2 h under
normal conditions, and was detected by GC again.
Experimental
Catalyst preparation
The PdNPs on the g-Al2O3 and other supports were prepared by
a low density lysine solution protected impregnation–reduction
method. For example, 3 wt% Pd/g-Al2O3 was prepared by the
following procedure: 0.97 g of g-Al2O3 powder was dispersed
into 50 mL of deionized water, followed by adding 28.2 mL of
0.01 M PdCl2 aqueous solution while magnetically stirring. One
milliliter of 0.03 M L-lysine was added with vigorous stirring.
Then 0.1 M NaOH aqueous solution was added into the mixture
to adjust the pH to 7. To this suspension, 4.5 mL of 0.35 M
NaBH4 solution was added dropwise in 10 min. The mixture was
aged for 24 h, and then the solid was separated, washed with
water (4 times) and ethanol (once), and dried at 80 ꢀC. The dried
solid was used directly as the catalyst.
Gram-scale synthesis
In a gram-scale reaction, catalyst (1.17 g), 2-pyridyl 4-methyl-
benzoate 1a (1.00 g, 4.69 mmol), phenylboronic acid 2a (1.15 g,
9.39 mmol), K3PO4 (1.49 g, 7.04 mmol), H2O (211 mL, 11.73
mmol) and toluene (60 mL) were charged in a 100 mL ovendried
round ask. The resulting solution was stirred at 120 ꢀC for 48 h
in an oil bath. Aer cooling to room temperature, the mixture
was ltered, and the ltrate was evaporated in vacuo. The
residue was puried by ash column chromatography (silica
gel, ethyl acetate/petroleum ether ¼ 1 : 10 as an eluent) to afford
the desired product 3aa (0.92 g, 84% yield).
Preparation of various esters (1a–s, 4a–h)
A mixture of carboxylic acid (10 mmol), pyridinol or phenol (10
mmol), DMAP (4-(dimethylamino)pyridine, 1 mmol) and 1-
ethyl-3-(3-dimethylaminopropyl)carbodiamide hydrochloride
Results and discussion
(EDC$HCl, 10 mmol) in THF (50 mL) was stirred overnight at Our initial attempt was executed toward the supported palla-
ꢀ
25 C. The resulting mixture was ltered, and the ltrate was dium nanoparticles catalyzed Suzuki–Miyaura cross-coupling
evaporated in vacuo. The residue was puried by ash column reaction using 2-pyridyl 4-methylbenzoate 1a and phenyl-
chromatography (silica gel, ethyl ether/petroleum ether ¼ 1 : 3 boronic acid 2a as the coupling partners. Using 3 wt% Pd/g-
to 1 : 10 as eluent), affording a corresponding esters 1a–s and Al2O3 as the catalyst, Cs2CO3 as base, and toluene as solvent,
4a–h.
aer 24 h of reuxing the desired ketone 3aa was obtained in
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54% yield (Table 1, entry 1). Meantime, along with the desired reaction conditions, and the results are presented in Table 2.
product only 9% yield of homocoupling byproduct of phenyl- Incorporation of electron-donating 3ca–3ga, electron-
boronic acid was isolated. This indicated that the heteroge- withdrawing 3ha–3na or naphthyl 3oa, groups on the esters
neous palladium catalyst is showed higher selectivity. Then, were tolerated. Compared with their para-isomers, 2-pyridyl 2-
various reaction parameters including catalyst, base, solvent, methylbenzoate 1c, 2-pyridyl 3-methylbenzoate 1d and 2-pyridyl
additive and temperature were screened. The PdNPs supported 3-nitrobenzoate 1m delivered a lower yield, which resulted from
on other oxide powders, including a-Al2O3, CeO2, TiO2 and the steric hindrance effect. Encouragingly, various heterocyclic
Fe3O4 were prepared by the impregnation–reduction method species could also be utilized 3pa–3ra and (1H-indol-2-
and applied to the model reaction (entries 2–5). Among them, yl)(phenyl)methanone 3pa was obtained quantitatively. The
the catalyst with 3 wt% Pd/g-Al2O3 exhibited the best perfor- ability to obtain high yields of indolyl aromatic ketones is
mance. The effect of solvent on the coupling reaction was rather a signicant advantage of this methodology. But when pyridin-
important, toluene proved to the best choice of solvent, whereas 2-yl propionate 1s react with phenylboronic acid 2a, no product
other solvents such as cumene and dioxane gave rise to slightly was detected. The result indicated that the acyl C–O bond
lower yields (entries 6–8). Base was also found to show signi- cleavage with the aid of palladium cannot tolerate aliphatic
cant inuence on the coupling reaction, and anhydrous K3PO4 substituents. The ease of activating this type of bond can be
showed the best efficiency for this transformation (entries 9– attributed to the stabilizing interaction of the arene p system
12). In sharp contrast, no reaction was occurred in the absence with palladium.14
of the base. It is noteworthy that with the additive of water,
Next, the scope of this reaction between 2-pyridyl 4-methyl-
higher conversion was achieved and the desired product was benzoate 1a with a variety of boronic acids was investigated and
obtained in 68% yield (entry 13). A small amount of water might the results are summarized in Table 3. Various functional
help dissolve the base into the reaction system. But only used groups including alkyl, ethoxy, uoro, bromo, triuoromethyl,
water as solvent is inadvisable because water-insoluble property thiomethyl and phenoxyl were compatible and the desired
of ester. We also screened the effect of different temperature on products were achieved in moderate to good yields. But the
the reaction (entries 14–16). We are pleased to nd that ortho-ethoxy phenyl boronic acid did not perform the reaction.
increasing the temperature to 120 ꢀC led to generate the highest The result indicated that the steric hindrance effect of boronic
yield of product, and further increasing the temperature of the acids is a determining factor of the coupling reaction (see 3ag).
reaction did not have any improvement on the yield. No product Interestingly, the Suzuki–Miyaura could be used to synthesize
was observed under otherwise identical conditions in the benzodioxole ketone 3an which is an important pharmaceutical
absence of Pd (entry 17).
intermediate albeit in lower yield.
The Suzuki–Miyaura cross-coupling of phenylboronic acid 2a
In order to verify that the ortho-directed effect of nitrogen on
with a set of 2-pyridyl esters was performed under the optimized C(acyl)–O bond cleavage of ester, pyridin-3-yl 2-methylbenzoate
Table 1 Optimization for the reaction conditionsa
Entry
Catalyst
Base
Solvent
Additive
Temp (ꢀC)
Yield (%)
1
2
3
4
5
6
7
8
3% Pd/g-Al2O3
3% Pd/a-Al2O3
3% Pd/CeO2
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Na2CO3
K2CO3
—
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
Toluene
Toluene
Toluene
Toluene
Toluene
Cumene
Dioxane
THF
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
—
—
—
—
—
—
—
—
—
—
—
—
H2O
H2O
H2O
H2O
H2O
100
100
100
100
100
100
100
100
100
100
100
100
100
110
120
130
120
54
25
38
49
37
35
20
NP
30
47
NP
58
68
73
88
70
NP
3% Pd/TiO2
3% Pd/Fe3O4
3% Pd/g-Al2O3
3% Pd/g-Al2O3
3% Pd/g-Al2O3
3% Pd/g-Al2O3
3% Pd/g-Al2O3
3% Pd/g-Al2O3
3% Pd/g-Al2O3
3% Pd/g-Al2O3
3% Pd/g-Al2O3
3% Pd/g-Al2O3
3% Pd/g-Al2O3
g-Al2O3
9
10
11
12
13
14
15
16
17
a
Reaction condition: 1a (0.10 mmol), catalyst (25 mg), Pd (7 mol%), phenylboronic acid 2a (2.0 equiv), base (1.5 equiv), H2O (2.5 equiv), solvent (2
mL), 120 ꢀC, 48 h.
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Table 2 Scope of Suzuki–Miyaura coupling with 2-pyridyl estersa
Table
3
Scope and limitations of Suzuki–Miyaura coupling with
arylboronic acidsa
a
Reaction conditions: 1a (0.10 mmol), 2 (2.0 equiv), 3 wt% Pd/g-Al2O3
(25 mg), Pd (7 mol%), K3PO4 (1.5 equiv), H2O (2.5 equiv), toluene (2
mL), 120 ꢀC, 48 h. Isolated yield.
was performed (entry 8). These results clearly show that the
pyridyl group serves as a directing group for cleavage of the acyl
C–O bond of the ester and that coordination of the nitrogen
atom to the catalyst is crucial for the catalyzed reaction. This
result is different from Chatani's work.
a
Reaction conditions: 1 (0.10 mmol), 2a (2.0 equiv), 3 wt% Pd/g-Al2O3
(25 mg), Pd (7 mol%), K3PO4 (1.5 equiv), H2O (2.5 equiv), toluene (2
mL), 120 ꢀC, 48 h. Isolated yield.
To investigate the practical application of this newly devel-
oped Suzuki–Miyaura cross-coupling reaction in organic
4a and pyridin-4-yl 2-methylbenzoate 4b were used to react with synthesis, we conducted a gram-scale reaction of 1a (1 g) with 2a
phenylboronic acid 2a (see Table 4, entries 1 and 2). Aer 24 h in the presence of 3 wt% Pd/g-Al2O3 catalyst, and isolated the
reaction, the desired ketone was not obtained, but instead desired product 3aa in 84% yield. As we can see, even though
phenyl 4-methyl benzoate 5aa was obtained in 48% and 44% the reaction scale was magnied up to 47 times, ideal synthet-
yield, respectively. These results conrmed the ortho-directed ically yields could be still obtained.
effect of nitrogen on C(acyl)–O bond cleavage of ester catalyzed
The recyclability of catalyst was examined in the reaction of
by palladium. The reason for generating phenyl ester is that 2-pyridyl 4-methylbenzoate 1a and phenylboronic acid 2a as
catalyzed by palladium, phenylboronic acid transferred to provided in the experimental section. As shown in Fig. 1, the
phenol11 in the presence of H2O which performed the trans- catalyst can be reused for eight cycles with only 13% decline of
esterication reaction with pyridin-2-yl 2-methylbenzoate 1a to activity aer the 8th recycle.
generate new ester catalyzed by K3PO4.15 Interestingly, when
To have a good knowledge of the information for the catalyst,
quinolin-8-yl 4-methylbenzoate 4c containing transannular the fresh and used (recovered aer 8th cycle) PdNPs on g-Al2O3
directing group was used as substrate, no reaction was detected catalysts were studied by transmission electron microscopy
(entry 3). When other heteroaryl benzoate containing ortho- (TEM), X-ray photoelectron spectroscopy (XPS) and so on. The
nitrogen, quinolin-2-yl 4-methylbenzoate 4d, 4f and pyrazin-2-yl transmission electron microscopic (TEM) analysis of the fresh
4-methylbenzoate 4e were employed instead of 1a, the Suzuki– and used catalysts are represented in Fig. 2. The PdNPs are
Miyaura cross-coupling reaction also proceed to afford the same distributed evenly on the g-Al2O3 surface, and the mean diam-
ketone 3aa but with the lower selectivity (entries 4–6). Under the eters of the PdNPs are 3.34 nm of fresh catalyst and 3.48 nm of
identical reaction conditions, the activated ester per- used catalyst, respectively. It does not cause obvious increase in
uorophenyl benzoate 4g only give the transesterication the average size of PdNPs aer eight runs.
product rather than desired ketone (entry 7). Not only that,
In order to understand the state of PdNPs supported on g-
when p-tolyl picolinate 4h was used as substrate, no reaction Al2O3, the catalysts before and aer reaction were tested by XPS
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Table 4 Scope and limitations of Suzuki–Miyaura coupling with other
(hetero)aryl estersa
Entry
1
Esters
Product
Yield (%)
48%
2
3
4
5
5aa
44%
NP
—
3aa
5aa
53%
39%
3aa
5aa
25%
36%
Fig. 2 (a and b) TEM images of fresh 3 wt% Pd/g-Al2O3 and used 3 wt%
Pd/g-Al2O3, respectively; (c and d) PdNPs size distributions of fresh
and used catalysts, respectively.
6
7
3aa
62%
32%
NP
catalyst and 335.80 eV and 341.00 eV of used catalyst respec-
tively. The binding energies of Pd0 in literature are 335.20 eV of
Pd 3d5/2 and 340.50 eV of Pd 3d3/2, respectively, whereas the
values of PdII are 336.70 eV of Pd 3d5/2 and 342.00 eV of Pd 3d3/2
,
respectively.13 It is clearly shown that Pd0 as the active center
completes the catalytic cycle.
8
—
The Brunauer–Emmett–Teller (BET) specic surface areas of
the catalysts were derived from N2 physical sorption data of the
samples using the BET model (Table 5). The g-Al2O3 is a kind of
support which has a large specic surface area, and it does not
cause obvious change in the specic surface area of these
catalysts aer loading a small amount of the PdNPs. The
amounts of Pd loading of the catalysts were derived from atomic
absorption spectrophotometer (AAS), and the Pd content of
fresh catalyst is approximately 3 wt%. We did note a slight
a
Reaction conditions: 4 (0.10 mmol), 2a (2.0 equiv), 3 wt% Pd/g-Al2O3
(25 mg), Pd (7 mol%), K3PO4 (1.5 equiv), H2O (2.5 equiv), toluene (2
mL), 120 ꢀC, 48 h. Isolated yield.
Fig. 1 Recyclability of the catalyst.
analysis. The XPS results of the catalysts conrm that palladium
exists in the metallic state on g-Al2O3 supports before and aer
reaction. As shown in Fig. 3, the binding energies of Pd 3d5/2
and Pd 3d3/2 electrons are 335.69 eV and 340.93 eV of fresh
Fig. 3 XPS spectra of fresh and used 3 wt% Pd/g-Al2O3.
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Initially, the nitrogen atom in 1a coordinates with Pd0 to form
intermediate I, and then Pd attacks the carbonyl carbon to give
ve-membered palladium ring intermediate II. C–O bond
cleavage takes place with re-formation of the carbonyl group
and acylpalladium III is generated. Transmetalation between III
and organoboron compounds affords intermediate IV.6,16
Finally, reductive elimination from IV yields ketone with
regeneration of active Pd0.
Table 5 Characterization results of BET, AAS of catalysts
Pd loading
(wt%)
Samples
SBET (m2 gꢁ1
)
g-Al2O3
3 wt% Pd/g-Al2O3 (fresh)
3 wt% Pd/g-Al2O3 (used)
161
166
159
—
3.02
2.89
Conclusions
Table 6 The results of hot filtration test
In summary, the results reported herein represent the rst
example of heterogeneous and site-selective nano-palladium
catalyzed Suzuki–Miyaura cross-coupling reaction with hetero-
aryl ester and arylboronic acids as coupling partners. The utility
of this newly developed protocol has been demonstrated by the
broad functional group tolerance and the application in the
synthesis of bioactive compounds. It is also shown that the
coordination of the nitrogen of the pyridyl group to the catalyst
is essential for the efficient reaction. This reaction involves the
heterogeneous catalytic generation of acylpalladium interme-
diates from esters by successful suppression of the undesired
decarbonylation phenomenon, and its application to aryl
ketone synthesis is achieved. The catalyst can be readily recov-
ered and reused for eight cycles with only 13% decline of activity
aer the 8th recycle. The XPS analysis of the catalyst before and
aer reaction suggested that the reaction might be performed
via a Pd0/PdII catalytic cycle that began with Pd0. The practicality
of this study may inspire further studies on heterogeneous
catalyzed C–O activation reactions.
Time (hour)
Conversiona (%)
0
2
0
42.41
42.81
4 (aer hot ltration)
a
GC data.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Scheme 2 Proposed reaction mechanism.
This research was nancially supported by the National Science
Foundation of China (21861030, 21462031), the Program for
Young Talents of Science and Technology in Universities of
Inner Mongolia Autonomous Region (NJYT-17-A22) and
Research Innovation Fund of Inner Mongolia Normal University
Graduate (CXJJS17090).
decrease in Pd content aer being cycled eight times (2.89%,
Table 5), which can decrease the catalytic activity on the basis of
available Pd on the support surface.
In order to prove whether the present reaction proceeded
with a heterogeneous catalyst or homogeneous catalyst, the hot
ltration test of the model reaction mixture was executed and
GC data are given in Table 6. The conversion rate of 1a indicated
that aer ltration the substrate turnover ceases. This is
intended to preclude the possibility of homogeneous catalysis.
This result was also proved by Hg(0) poisoning test (see ESI†).
To identify a byproduct, which is formed by the cleavage of
the C(acyl)–O bond, the model reaction solution was detected
with GCMS aer the end of the reaction (see the ESI†). The
GCMS data indicated that along with desired product 3aa
a small amount of biphenyl was obtained and that alkoxyl group
of ester eliminated to pyridin-2-ol. Based on the GC-MS analysis
results and the commonly accepted mechanism from the liter-
ature, the proposed reaction pathway is shown in Scheme 2.
Notes and references
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