Palladium-Catalyzed Oxidative C–H Alkoxycarbonylation of Arenes with Alkylcarbazates Directed by N-Heterocyclic Substituents

Article abstract of DOI:10.1002/ejoc.202000530

With alkyl carbazates as the green ester source, a novel palladium-catalyzed oxidative free radical carbonylative transformation of the C–H bond on aromatic rings to produce esters has been developed. Good yields of the corresponding products have been obtained with wide functional group tolerance and excellent regioselectivity. A variety of alkyl carbazates are found to be suitable reactants for the ortho-alkoxycarbonylation on the aromatic ring.

Full text of DOI:10.1002/ejoc.202000530

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Palladium-Catalysed Oxidative Nitrogen-Containing Heterocycle-
Directed C-H Alkoxycarbonylation of Arenes with Alkylcarbazates
Authors: Yogesh Kumar G.R and Noor Shahina Begum
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202000530
Link to VoR: https://doi.org/10.1002/ejoc.202000530
01/2020

10.1002/ejoc.202000530
European Journal of Organic Chemistry
FULL PAPER
Palladium-Catalysed Oxidative Nitrogen-Containing Heterocycle-
Directed C-H Alkoxycarbonylation of Arenes with Alkylcarbazates
Gujjenahalli Ramalingaiah Yogesh Kumar, Noor Shahina Begum*
Mr. Yogesh Kumar G.R, Prof. Dr. Noor Shahina Begum *
Department of Studies in Chemistry, Bangalore University,
Jnana Bharathi Campus, Bangalore- 560056,
Karnataka, India
E-mail: noorsb@bub.ernet.in
Supporting information for this article is given via a link at the end of the document. ((Please delete this text if not appropriate))
Abstract: With alkyl carbazates as the green ester source, a novel
palladium-catalyzed oxidative free radical carbonylative
carbonylation of aromatic C-H using alcohol and Mo(CO)6 as a
source of the ester group. Although many protocols to realize
the carbonylation by ligand-directed C−H activation strategy
have been documented.13 The discovery of new type of
surrogates that can be easily applied to the divergent and
expeditious synthesis of various carboxylic esters is still highly
demanded. In the past few years, carbazates are especially
promising alkoxycarbonyl surrogates since they are readily
available and operationally simple.14 In 2016, Gao et al.15a
developed iron-catalyzed oxidative addition of alkoxycarbonyl
radical to heterocyclic rings by using carbazates as the source of
ester group. In 2019, Xie et al15b reported metal-free C3-
alkoxycarbonylation of quinoxalin-2(1H)-ones with carbazates.
Encouraged by these results, we have investigated ortho-
alkoxycarbonylation of arenes with carbazates as the green
ester source via nitrogen-containing heterocycle-directed
palladium-catalyzed oxidative free radical (sp2)C−H activation.
We herein report results that demonstrate the facile approach to
the synthesis of alkoxycarbonyl-substituted arenes.
transformation of the C-H bond on aromatic rings to produce esters
has been developed. Good yields of the corresponding products
have been obtained with wide functional group tolerance and
excellent regioselectivity. A variety of alkyl carbazates are found to
be suitable reactants for the ortho-alkoxycarbonylation on the
aromatic ring.
Introduction
Esters are an important class of functional group compounds
and are indispensable materials of utmost relevance in the
manufacture of soaps, detergents, flavors, agrochemicals,
pharmaceuticals, plasticizers, etc. They are also used as
emulsifiers in the food and cosmetic industries.1 They can be
converted into diverse other functional groups such as
hydroxymethyl, carbonyl, amide, etc. 2 Direct introduction of the
ester group in to organic molecules undoubtedly represents a
very important strategy. The Heck carbonylation3 is a powerful
method for the synthesis of aryl carboxylic acid derivatives.
Palladium-catalyzed carbonylation of aryl halides, triflates and
tosylates using CO as the carbonyl source is a versatile and
reliable methodology,4,5 and widely used in the chemical industry.
However, the usage of toxic CO gas as the reagent and the
necessity to use high-pressure equipment has limited its
application for complex organic syntheses. Therefore, the
development of safe and facile CO-free strategies for
alkoxycarbonylation remains
chemists.
a challenge to the synthetic
During the past decade, a new strategy has been developed for
direct alkoxycarbonylation of arenes via transition metal (e.g.,
Ru, Rh, Cu, and Pd) mediated C-H bond functionalization
reaction of aromatic compounds through ligand-directed C-H
activation. In 2009, Guan and co-workers6 developed Rh-
catalyzed direct oxidative carbonylation of the aromatic C-H
bonds with CO and alcohol. Kochi7a and Liao7b independently
reported the alkoxycarbonylation of the aromatic C-H bonds with
alkyl chloroformates. The Yu and You groups utilized the diethyl
azodicarboxylate
(DEAD)
to
realize
the
ortho-
ethoxycarbonylation.8 Peng and co-workers9 also described
ethoxycarbonylation of the aromatic C-H bonds via C-C bond
cleavage of oxaziridine. In 2012, Zhou et al10 reported ortho-
alkoxycarbonylation using α-keto ester as an ester source. Li et
al11 utilized the potassium oxalate monoester for ortho-
ethoxycarbonylation of arenes. In 2016, Wang et al.12 developed
1
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10.1002/ejoc.202000530
European Journal of Organic Chemistry
FULL PAPER
Scheme 1. Metal-mediated alkoxycarbonylation reactions
Mn(OAc)3.2H2O (2 eq) and Mn(OAc)2 were used in this reaction,
the yield was not found to increase (Table 1, entries 11−12).
Furthermore, the influence of other Pd-catalysts such as
Pd(TFA)2, Pd(PPh3)4 and PdCl2 on the reaction was explored
(Table 1, entries 13-15). For instance, Pd(TFA)2 also produced
4a in a moderate yield of 60% (Table 1, entry 13). While Rh and
Ru-catalysts showed poor reactivity (Table 1, entries 16-17),
Pd(OAc)2 exhibited the highest activity yielding the desired
product 4a in a 90% yield (Table 1, entry 6). A variety of solvents
(Toluene, DMF, THF, EtOAc and DCE) were also screened for
the reaction (Table 1, entries 18–22) with CH3CN yielding the
best result, affording 4a in 90% yield (Table 1, entry 6).
Table 1. Optimization of the reaction conditions.
entry
catalyst
(10%)
activator
(equiv)
oxidant
(equiv)
solvent
Yield
(%)
10
1
Pd(OAc)2
AgNO3 (2)
TBHP (2)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Toulene
DMF
2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(TFA)2
Pd(PPh3)4
PdCl2
AgNO3 (2)
AgNO3 (2)
AgNO3 (2)
AgNO3 (2)
AgNO3 (2)
AgNO3 (1)
Ag2O (2)
TBPB (2)
K2S2O8 (2)
(NH4)2S2O8
Oxone (2)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
Oxone (1)
10
3
15
4
trace
65
5
6
90
7
80
8
40
9
AgOAc (2)
Ag2CO3 (2)
Mn(OAc)3.2H2O
Mn(OAc)2(2)
AgNO3 (2)
AgNO3 (2)
AgNO3 (2)
AgNO3 (2)
AgNO3 (2)
AgNO3 (2)
AgNO3 (2)
AgNO3 (2)
AgNO3 (2)
AgNO3 (2)
trace
trace
35
10
11
12
13
14
15
16
17
18
19
20
21
22
30
60
10
trace
15
RhCp*(OAc)2
[Ru(pcym)Cl2]2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
26
45
trace
trace
trace
28
THF
Results and Discussion
EtOAc
DCE
We commenced our investigations by examining the reaction
of 2-phenylpyridine (1a) with methyl carbazate (2a) as model
substrates. Intensive screening of oxidants was carried out by
employing acetonitrile as the solvent, AgNO3 as an activator and
Pd(OAc)2 as a catalyst at 90 °C. As can be seen from Table 1, it
emerges that TBHP, TBPB, K2S2O8 and (NH4)2S2O8 (2 eq) are
poor oxidants for the reaction and lead to the desired product 4a
in rather low yields (Table 1, entries 1-4). To our delight, Oxone
was found to be excellent yielding the desired ortho-
alkoxycarbonylation product 4a in an isolated yield of 65%
(Table 1, entry 5). Gratifyingly, the yield was found to increase to
90% when the amount of Oxone was decreased from 2.0 equiv
to 1.0 equiv. (Table 1, entry 6). Encouraged by this result, we
examined the efficacy of different silver salts such as Ag2O,
AgOAc, and Ag2CO3 but none of them led to improved yield
(Table 1, entries 7-10). When other activators such as
Reaction conditions: the reaction in each case was carried out at 90 °C by
employing 1a (0.64 mmol) and 2a (1.29 mmol) with other reagents and
oxidants as given.
With the optimized reaction conditions in hand, we proceeded
to investigate the scope of this reaction using various nitrogen-
containing heterocycle directed arenes with methyl carbazate 2a
(Table 2). The substrates with phenyl ring bearing both electron-
donating groups (Me, isopropyl and OMe) and electron-
withdrawing groups (fluoro, chloro, and bromo) afforded ortho-
alkoxycarbonylation products (Table 2, 3a-3k) in 74–95% yields.
The reaction of ortho-substituted 2-phenylpyrimidine gave the
corresponding product in good yield (3h, 86%). We then
examined the effect of the substituents on the heterocyclic rings.
2
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10.1002/ejoc.202000530
European Journal of Organic Chemistry
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The results revealed that when methyl and methoxy groups are
substituted at different positions in pyridine and pyrimidine rings,
the reactions led to moderate-to-good yields of products 3g–3m
(Table 2). Similar result was obtained from the 2-
phenylquinazoline 3n with the corresponding product being
isolated in a 70% yield. However, when the phenyl ring or the
heterocyclic pyridine or pyrimidine ring contained a nitro group
(NO2) or amino group (NH2), the corresponding product was
observed only in trace amounts.
Table 3. Alkoxycarbonylation with various alkyl carbazates.
Table 2. Alkoxycarbonylation of arenes with methyl carbazate.
Reaction conditions: 1a (0.64 mmol), 2 (1.29 mmol), Pd(OAc)2(10 mol%),
AgNO3 (2 equiv), Oxone (1 equiv) in acetonitrile 16 h at 90 °C. The values in
parentheses refer to isolated yields.
Scheme 2: Control experiment to examine radical-mediated pathway.
To understand the mechanism of the reaction some control
experiments were performed (Scheme 2). Notably, the addition
of a radical inhibitor TEMPO (2 equiv) to the alkoxycarbonylation
reaction did not result in the formation of methyl 2-(pyridine-2-
yl)benzoate (4a). This result suggested that the above reaction
follows a radical pathway.
Reaction conditions: 1 (0.64 mmol), 2a (1.29 mmol), Pd(OAc)2 (10 mol%),
AgNO3 (2 equiv), Oxone (1 equiv) in acetonitrile 16 h at 90 °C. The values in
parentheses refer to isolated yields.
At the same time, the 2-phenylpyridine underwent reactions
with various alkyl carbazates such as ethyl, isopropyl, butyl,
cyclobutyl, cyclohexyl carbazates, etc. under standard
conditions affording smoothly the corresponding products in 65–
92 % yields (Table 3, 4a- 4j). When N-Boc pyrrolidine carbazate
reacted with 2-phenylpyridine under standard conditions, the
desired product was isolated in 65 % yield (Table 3, entry 4i).
However, no desired product could be detected when benzyl
carbazate was employed
A plausible reaction pathway is proposed on the basis of the
above results and previous studies (Scheme 3).16a Firstly, a
cyclopalladated dimer intermediate I by a chelate-directed C-H
bond activation of 2-phenylpyridine (1a) with Pd(OAc)2 is
formed.10,12,16b The obtained palladacycle
I
reacts with
alkoxycarbonyl radical generated from the release of molecular
nitrogen of 2 with the aid of silver salt to provide intermediate II
in the presence of Oxone.16c-16h Finally, carbon-carbon bond
formation through a reductive elimination of II generated the
3
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10.1002/ejoc.202000530
European Journal of Organic Chemistry
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corresponding product 4 and regeneration of Pd(II) species for
Characterization data for all isolated products
the next run.
Methyl 5-methyl-2-(pyridin-2-yl)benzoate (3a): According to the
general procedure, a crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 3a (92%, 0.123 g) as white solid. IR (cm-1): 2958, 1723,
1296, 775. 1H NMR (DMSO-d6, 400 MHz): δ 8.55 (d, J = 4.4 Hz, 1H),
7.84 (d, J = 8.0 Hz, 1H), 7.62 – 7.56 (m, 2H), 7.43 (t, J = 8.0 Hz, 2H),
7.31 (t, J = 5.2 Hz, 1H), 3.54 (s, 3H), 2.38 (s, 3H). 13C{1H} NMR (CDCl3,
500 MHz): δ 169.62, 158.57, 149.05, 138.47, 138.09, 136.28, 131.86,
131.42, 130.31, 129.75, 122.72, 121.94, 52.32, 21.01. HRMS (ESI-TOF)
(m/z): [M + H]+ Calcd for C14H13NO2H 228.0946; Found 228.0949.
Scheme
3.
Plausible
mechanism
for
palladium-catalyzed
alkoxycarbonylation of aromatic C-H bond.
Methyl 5-fluoro-2-(pyridin-2-yl)benzoate (3b): According to the
general procedure, a crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 3b (78%, 0.105 g) as yellow oil. IR (cm-1): 2960, 1729,
1288. 1H NMR (DMSO-d6, 400 MHz): δ 8.56 (d, J = 4.4 Hz, 1H), 7.87 (t, J
= 8.0 Hz, 1H), 7.76 – 7.73 (m, 1H), 7.65 (d, J =8.0 Hz, 1H), 7.47 (t, J =
8.8 Hz, 2H), 7.36 – 7.33 (m, 1H), 3.57 (s, 3H). 13C{1H} NMR (CDCl3, 100
MHz): δ 168.36, 158.72, 154.08, 148.93, 140.98, 136.34, 131.29, 130.03,
129.80, 128.31, 122.64, 122.18, 52.18. 19F NMR (376 MHz, CDCl3): -
121.98 – -122.02 (m, 1F). HRMS (ESI-TOF) (m/z): [M + H]+ Calcd for
C13H10FNO2H 232.0696; Found 232.0698.
Methyl 2-chloro-3-fluoro-6-(pyridin-2-yl)benzoate (3c): According
to the general procedure, a crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 3c (76%, 0.1 g) as yellow oil. IR (cm-1):, cm-1): 2970, 1735,
1290. 1H NMR (DMSO-d6, 400 MHz): δ 8.59 (d, J = 4.4 Hz, 1H), 7.93 –
7.89 (m, 2H), 7.82 (s, 1H), 7.69 (d, J = 8.4 Hz, 1H), 7.42 (t, J = 5.6 Hz,
1H), 3.66 (s, 3H). 13C{1H} NMR (CDCl3, 500 MHz): δ 168.76, 164.59,
164.27, 162.26, 156.97, 135.07, 133.96, 132.33, 119.21, 117.83, 117.66,
116.39, 116.12, 52.50. 19F NMR (376 MHz, DMSO-d6): -185.75 – -108.77
(d, 1F). HRMS (ESI-TOF) (m/z): [M + H]+ Calcd for C13H9ClFNO2H
266.0384; Found 266.0384.
Conclusion
A novel and efficient approach for ortho-alkoxycarbonylation of
arenes by nitrogen-containing heterocycle-directed Pd-catalyzed
oxidative C-H insertion with alkyl carbazates has been
developed; different kinds of directing groups such as pyridine,
pyrimidine and quinazoline have been shown to be applicable.
Furthermore, the reaction is shown to occur with a variety of
carbazates such as methyl, ethyl, propyl, cyclobutyl and
cyclohexyl carbazates as sources of the alkoxycarbonyl group.
High regioselectivity and good functional group tolerance are
demonstrated.
Methyl 4-bromo-2-(pyridin-2-yl)benzoate (3d): According to the
general procedure, a crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 3d (75%, 0.1g) as yellow oil. IR (cm-1): 2960, 1731, 1289.
1H NMR (DMSO-d6, 400 MHz): δ 8.58 (d, J = 4.4 Hz, 1H), 7.90-7.87 (m,
2H), 7.72 (t, J = 8.8 Hz, 2H), 7.60 (d, J = 8.4 Hz, 1H), 7.38 (t, J = 5.2 Hz,
1H), 3.55 (s, 3H). 13C{1H} NMR (CDCl3, 500 MHz): δ 169.31, 158.61,
149.08, 140.93, 136.24, 131.25, 129.81, 128.41, 122.83, 122.18, 52.11.
HRMS (ESI-TOF) (m/z): [M + H]+ Calcd for C13H10BrNO2H 291.9973;
Found 291.9973
Experimental Section
Methyl 2-(pyrimidin-2-yl)benzoate (3e): According to the general
procedure,
a
crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 3e (86%, 0.12 g) as yellow oil. IR (cm-1): 2952, 1727, 1288.
1H NMR (CDCl3, 400 MHz): δ 8.80 (d, J = 4.8 Hz, 2H), 8.01 (d, J = 7.6 Hz,
1H), 7.72 (d, J = 7.6 Hz, 1H), 7.60 – 7.45 (m, 2H), 7.25 – 7.21 (m, 1H),
3.74 (s, 3H). 13C{1H} NMR (CDCl3, 100 MHz): δ 169.79, 165.64, 156.86,
138.03, 132.64, 130.85, 129.94, 129.64, 129.01, 119.10, 52.18. HRMS
(ESI-TOF) (m/z): [M + H]+ Calcd for C12H10N2O2H 215.0821; Found 215.
General procedure for Palladium-Catalyzed Oxidative Carbonylation
In a 25-mL screw-cap vial, a mixture of 2-phenylpyridine (0.64 mmol, 1
eq) and alkyl carbazates (1.29 mmol, 2 eq) in acetonitrile (5 mL) were
added AgNO3 (1.29 mmol, 2 equiv), Oxone (0.64 mmol, 1eq) and
Pd(OAc)2 (0.05 mmol, 10 mol%). The vial was capped and placed in a
pre-heated (90 °C) metal block for 16 h. After the completion of the
reaction (monitored by TLC), the reaction mixture was cooled to room
temperature, and diluted with water (20 mL) and extracted with ethyl
acetate (2 X 30 mL). The organic layer was washed with brine (20 mL),
dried over anhydrous Na2SO4 and concentrated under reduced pressure.
The crude products were purified by column chromatography on silica gel
to give the pure products.
Methyl 5-methoxy-2-(pyrimidin-2-yl)benzoate (3f): According to
the general procedure, a crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 3f (90%, 0.12 g) as yellow solid. Melting point: 76.3-77.5
ºC. IR (cm-1): 2954, 1718, 1227. 1H NMR (CDCl3, 400 MHz): δ 8.75 (d, J
= 4.8 Hz, 2H), 8.07 (d, J = 8.4 Hz, 1H), 7.17 – 7.15 (m, 2H), 7.09 – 7.06
(m, 1H), 3.88 (s, 3H), 3.76 (s, 3H). 13C{1H} NMR (CDCl3, 400 MHz): δ
170.09, 164.81, 160.72, 156.74, 134.62, 131.57, 129.81, 118.55, 116.24,
113.88, 55.59, 52.23. HRMS (ESI-TOF) (m/z): [M + H]+Calcd for
C13H12N2O3H 245.0926; Found 245.0927.
4
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10.1002/ejoc.202000530
European Journal of Organic Chemistry
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Methyl 2,4-dimethyl-6-(4-methylpyrimidin-2-yl)benzoate (3g):
According to the general procedure, a crude product was purified by
using column chromatography on silica gel (heptane/ethyl acetate = 8:2)
to give the pure products 3g (88%, 0.11 g) as white solid. Melting point:
80.2-83.6 ºC. IR (cm-1): 2950, 1726, 1580, 1419, 1255, 1078. 1H NMR
(DMSO-d6, 400 MHz): δ 8.68 (d, J = 5.2 Hz, 1H), 7.86 (s, 1H), 7.29 (d, J
= 5.2 Hz, 1H), 7.23 (s, 1H), 3.66 (s, 1H), 2.48 (s, 3H), 2.36 (s, 3H), 2.26
(s, 3H). 13C{1H} NMR (CDCl3, 400 MHz): δ 170.87, 166.73, 164.28,
156.47, 139.23, 136.05, 135.83, 132.81, 131.06, 127.73, 118.52, 51.76,
24.25, 21.19, 19.40. HRMS (ESI-TOF) (m/z): [M + H]+ Calcd for
C15H16N2O2H 257.129; Found 257.1293.
157.01, 130.81, 129.84, 129.30, 128.97, 52.15, 15.54. HRMS (ESI-TOF)
(m/z): [M + H]+ Calcd for C13H12N2O2H 229.0977; Found 229.0979.
Methyl 2-(5-methoxypyrimidin-2-yl)benzoate (3m): According to the
general procedure, a crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 3m (86%, 0.116 g) as yellow oil. IR (cm-1): 2947, 1726,
1561, 1415, 1283. 1H NMR (CDCl3, 400 MHz): δ 8.49 (d, J = 5.6 Hz, 1H),
8.05 (d, J = 7.6 Hz, 1H), 7.67 (d, J = 7.6 Hz, 1H), 7.58 – 7.48 (m, 2H),
6.65 (d, J = 6.0 Hz, 1H), 4.00 (s, 3H), 3.73 (s, 3H). 13C{1H} NMR (CDCl3,
500 MHz): δ 169.33, 165.23, 157.15, 137.89, 133.07, 130.67, 130.03,
129.68, 128.84, 106.40, 53.84, 52.20. HRMS (ESI-TOF) (m/z): [M +
H]+Calcd for C13H12N2O3H 245.0926; Found 245.0924.
Methyl
3-methyl-2-(4-methylpyrimidin-2-yl)benzoate
(3h):
According to the general procedure, a crude product was purified by
using column chromatography on silica gel (heptane/ethyl acetate = 8:2)
to give the pure products 3h (86%, 0.1 g) as yellow oil. IR (cm-1): 2950,
1726, 1580, 1419, 1255. 1H NMR (CDCl3, 400 MHz): δ 8.68 (d, J = 4.8
Hz, 1H), 7.87 (d, J = 7.6 Hz, 1H), 7.45 (d, J = 7.2 Hz, 1H), 7.37 (t, J = 8.0
Hz, 1H), 7.14 (t, J = 5.2 Hz, 1H), 3.63 (s, 3H), 2.59 (s, 1H), 2.14 (s, 3H).
13C{1H} NMR (CDCl3, 100 MHz): δ 167.52, 167.25, 166.88, 156.37,
140.33, 136.80, 134.29, 129.50, 128.23, 127.81, 118.43, 53.04, 24.23,
Methyl 2-(quinazolin-2-yl)benzoate (3n): According to the general
procedure,
a
crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 3n (70%, 0.09 g) as yellow oil. IR (cm-1): 2943, 1726, 1280.
1H NMR (CDCl3, 400 MHz): δ 9.45 (s, 1H), 8.17 (d, J = 7.2 Hz, 1H), 8.06
(d, J = 8.0 Hz, 1H), 7.97 – 7.91 (m, 2H), 7.77 (d, J = 7.6 Hz, 1H), 7.68 –
7.61 (m, 2H), 7.54 (t, J = 7.6 Hz, 1H), 3.73 (s, 3H). 13C{1H} NMR (CDCl3,
400 MHz): δ 169.87, 161.83, 159.97, 150.37, 138.44, 134.27, 132.98,
130.86, 130.33, 129.51, 129.04, 128.53, 127.72, 127.17, 123.28, 52.13.
HRMS (ESI-TOF) (m/z): [M + H]+ Calcd for C16H12N2O2H 265.0977;
Found 265.0977.
19.59. HRMS (ESI-TOF) (m/z): [M
243.1055; Found 243.1056.
+
H]+ Calcd for C14H14N2O2H
Methyl 5-isopropyl-2-(4-methoxypyrimidin-2-yl)benzoate (3i):
According to the general procedure, a crude product was purified by
using column chromatography on silica gel (heptane/ethyl acetate = 8:2)
to give the pure products 3i (95%, 0.12 g) as yellow oil. IR (cm-1): 2961,
1726, 1551, 1410. 1H NMR (CDCl3, 400 MHz): δ 8.47 (d, J = 5.2 Hz, 1H),
7.99 (d, J = 8.0 Hz, 1H), 7.50 (s, 1H), 7.41 (d, J = 8.0 Hz, 1H), 6.62 (d, J
= 6.0 Hz, 1H), 3.99 (s, 3H), 3.74 (s, 3H), 3.00 – 2.97 (m, 1H), 1.23 (d, J =
11.6 Hz, 6H). 13C{1H} NMR (CDCl3, 400 MHz): δ 170.63, 169.18, 164.97,
157.04, 150.80, 135.28, 133.12, 130.04, 128.70, 126.82, 106.04, 53.69,
52.06, 33.98, 23.71. HRMS (ESI-TOF) (m/z): [M + H]+ Calcd for
C16H18N2O3H 287.1396; Found 287.1399.
Methyl 2-(pyridin-2-yl)benzoate (4a): According to the general
procedure,
a
crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 4a (90%, 0.123 g) as yellow oil.IR (cm-1): 2950, 1726, 1287.
1H NMR (CDCl3, 400 MHz): δ 9.45 (s, 1H), 8.17 (d, J = 7.2 Hz, 1H), 8.06
(d, J = 8.0 Hz, 1H), 7.97 – 7.91 (m, 1H), 7.77 (d, J = 7.6 Hz, 2H), 7.67 –
7.61 (m, 1H), 7.52 (t, J = 7.06 Hz, 1H), 3.73 (s, 3H). 13C{1H} NMR (CDCl3,
500 MHz): δ 169.34, 158.61, 149.08, 140.93, 136.24, 131.25, 129.81,
128.41, 122.83, 122.18, 52.11. HRMS (ESI-TOF) (m/z): [M + H]+ Calcd
for C13H11NO2H 214.0868; Found 214.0867.
Methyl 5-fluoro-2-(pyrimidin-2-yl)benzoate (3j): According to the
general procedure, a crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 3j (78%, 0.103 g) as brown solid. Melting point: 82.1-83.5
ºC. IR (cm-1): 2987, 1717, 1446, 1290, 747. 1H NMR (CDCl3, 400 MHz): δ
8.78 (d, J = 4.4 Hz, 2H), 8.09 – 8.06 (m, 1H), 7.40 – 7.37 (m, 1H), 7.28 –
7.21 (m, 2H), 3.77 (s, 3H). 13C{1H} NMR (CDCl3, 500 MHz): δ 168.76,
164.59, 164.27, 162.26, 156.97, 135.07, 133.98, 133.33, 119.21, 117.83,
116.12, 52.50. 19F NMR (376 MHz, CDCl3): -110.92 – -110.98 (m, 1F).
HRMS (ESI-TOF) (m/z): [M + H]+ Calcd for C12H9FN2O2H 233.0726;
Found 233.0727
Ethyl 2-(pyridin-2-yl)benzoate (4b): According to the general
procedure,
a
crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 4b (85%, 0.13 g) as yellow oil. IR (cm-1): 2986, 1723, 1290,
752.1H NMR (DMSO-d6, 400 MHz): δ 8.56 (d, J = 4.4 Hz, 1H), 7.87 (t, J =
8.0 Hz, 1H), 7.67 – 7.60 (m, 4H), 7.54 – 7.50 (m, 1H), 7.35 (t, J = 5.2 Hz,
1H), 4.03 – 3.98 (q, 2H), 0.92 (t, J = 6.8 Hz, 3H). 13C {1H} NMR (CDCl3,
500 MHz): δ 168.86, 158.94, 149.06, 141.05, 136.25, 131.85, 131.14,
129.84, 128.36, 122.92, 122.07, 61.02, 13.88. HRMS (ESI-TOF) (m/z):
[M + H]+ Calcd for C14H13NO2H 228.1025; Found 228.1025.
Methyl 2,4-difluoro-6-(4-methylpyrimidin-2-yl)benzoate (3k):
According to the general procedure, a crude product was purified by
using column chromatography on silica gel (heptane/ethyl acetate = 8:2)
to give the pure products 3k (74%, 0.1 g) as yellow oil. IR (cm-1): 2958,
1739, 1572, 1381, 1267, 1123. 1H NMR (CDCl3, 400 MHz): δ 8.62 (d, J =
4.8 Hz, 1H), 7.90 (d, J = 9.2 Hz, 1H), 7.11 (d, J = 5.2 Hz, 1H), 6.99 – 6.94
(m, 1H), 3.88 (s, 3H), 2.56 (s, 1H). 13C{1H} NMR (CDCl3, 500 MHz): δ
167.28, 166.08, 164.16, 164.07, 156.77, 119.68, 112.76, 112.55, 106.01,
105.60, 52.64, 24.28.19F NMR (376 MHz, CDCl3): -107.6 – -107.62 (d,
Propyl 2-(pyridin-2-yl)benzoate (4c): According to the general
procedure,
a
crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 4c (88%, 0.14 g) as yellow oil. IR (cm-1): 2970, 1716, 1278,
1
752. H NMR (CDCl3, 400 MHz): δ 8.53 (d, J = 4.0 Hz, 1H), 7.83 (t, J =
7.2 Hz, 1H), 7.73 (t, J = 7.6 Hz, 1H), 7.54 (s, 2H), 7.45 (d, J = 6.8 Hz, 2H),
7.24 (d, J = 7.6 Hz, 1H), 4.03 (t, J = 6.4 Hz, 2H), 1.46 – 1.41 (m, 2H),
0.73 (t, J = 7.2 Hz, 3H). 13C{1H} NMR (CDCl3, 400 MHz): δ 168.88,
158.86, 149.02, 140.94, 136.17, 131.84, 131.01, 129.81, 129.77, 128.26,
122.82, 121.99, 66.66, 21.64, 10.27. HRMS (ESI-TOF) (m/z): [M + H]+
Calcd for C15H15NO2H 242.1181; Found 242.1183.
1F), 112.85 (t, 1F). HRMS (ESI-TOF) (m/z): [M
C13H10F2N2O2H 265.0710; Found 265.0711
+
H]+ Calcd for
Methyl 2-(5-methylpyrimidin-2-yl)benzoate (3l): According to the
general procedure, a crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 3l (84%, 0.11 g) as yellow oil. IR (cm-1): 2950, 1729, 1426,
1266. 1H NMR (CDCl3, 400 MHz): δ 8.63 (s, 2H), 7.98 (d, J = 8.0 Hz, 1H),
7.71 (d, J = 7.2 Hz, 1H), 7.58 (t, J = 7.6 Hz, 1H), 7.51 – 7.44 (m, 1H),
3.75 (s, 3H), 2.35 (s, 3H). 13C{1H} NMR (CDCl3, 400 MHz): δ 169.79,
Butyl 2-(pyridin-2-yl)benzoatec (4d): According to the general
procedure,
a
crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 4d (92%, 0.16 g) as yellow oil. IR (cm-1): 2958, 1716, 1278,
747. 1H NMR (DMSO-d6, 400 MHz): δ 8.56 (d, J = 4. Hz, 1H), 7.87 (d, J =
7.6 Hz, 1H), 7.66 – 7.61 (m, 3H), 7.53 – 7.50 (m, 1H), 7.35 (d, J = 6.8Hz,
1H), 3.96 (t, J = 6.8 Hz, 2H), 1.29 – 1.24 (m, 2H), 1.04-0.98 (m, 2H), 0.74
(t, J = 7.2 Hz, 3H). 13C{1H} NMR (CDCl3, 500 MHz): δ 168.99, 158.93,
5
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10.1002/ejoc.202000530
European Journal of Organic Chemistry
FULL PAPER
149.09, 140.98, 136.29, 131.87, 131.12, 129.91, 128.37, 122.94, 122.08,
64.98, 30.40, 19.05, 13.75. HRMS (ESI-TOF) (m/z): [M + H]+ Calcd for
C16H17NO2H 256.1338; Found 256.1339.
ºC. IR (cm-1): 2963, 1723, 1469, 1284, 1122, 752. 1H NMR (DMSO-d6,
400 MHz): δ 8.63 (d, J = 4.4 Hz, 1H), 7.82 (d, J = 7.6 Hz, 1H), 7.73 (t, J
= 8.0 Hz, 1H), 7.57-7.54 (m, 2H), 7.48-7.44 (m, 2H), 7.25-7.22 (m, 1H),
4.03– 3.94 (m, 2H), 3.54 (bs, 1H), 1.58 – 1.28 (m, 4H), 1.24 – 1.06 (m,
4H), 0.90 – 0.83 (m, 3H), 0.78 – 0.74 (m, 3H). 13C{1H} NMR(CDCl3, 400
MHz): δ 169.09, 158.85, 149.10, 140.89, 136.21, 131.86, 131.01, 129.91,
129.78, 128.28, 122.84, 122.00, 67.54, 38.53, 30.15, 28.90, 23.45, 22.91,
14.06, 10.94. HRMS (ESI-TOF) (m/z): [M + H]+ Calcd for C20H25NO2H
312.1964; Found 312.1963.
Isobutyl 2-(pyridin-2-yl)benzoate (4e): According to the general
procedure,
a
crude product was purified by using column
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 4e (90%, 0.15 g) as yellow oil. IR (cm-1): 2964, 1723, 1278,
752. 1H NMR (DMSO-d6, 400 MHz): δ 8.55 (s, 1H), 7.85 (t, J = 7.6 Hz,
1H), 7.64 – 7.60 (m, 4H), 7.53 – 7.49 (m, 1H), 7.34 – 7.31 (m, 1H), 3.73
(d, J = 6.4 Hz, 2H), 1.58 – 1.55 (m, 1H), 0.61 (d, J = 6.4 Hz, 6H). 13C{1H}
NMR (CDCl3, 400 MHz): δ 168.96, 158.81, 149.09, 140.90, 136.23,
131.85, 131.02, 129.86, 129.77, 128.28, 122.82, 122.04, 71.29, 27.44,
18.96. HRMS (ESI-TOF) (m/z): [M + H]+Calcd for C16H17NO2H 256.1338;
Found 256.1336.
Acknowledgements
We thank M. Srinivas (SFSL Bangalore) and Rajegowda for
technical assistance in recording the spectra of the compounds.
Isopropyl 2-(pyridin-2-yl)benzoate(4f): According to the general
procedure,
a
crude product was purified by using column
Keywords: C–H alkoxycarbonylation • Free radical reaction •
Carbazates •
chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 4f (84%, 0.13 g) as yellow oil. IR (cm-1): 2981, 1723, 1255,
752. 1H NMR (DMSO-d6, 400 MHz): δ 8.56 (d, J = 4.0 Hz, 1H), 7.86 (t, J
= 7.2 Hz, 1H), 7.65 – 7.59 (m, 4H), 7.52-7.48 (m, 1H), 7.36 – 7.33 (m,
1H), 4.87 – 4.81 (m, 1H), 0.96 (d, J = 6.0 Hz, 6H). 13C{1H} NMR (CDCl3,
500 MHz): δ 168.43, 159.07, 149.07, 140.93, 136.25, 130.98, 129.85,
129.79, 128.31, 123.00, 122.03, 68.58, 21.56. HRMS (ESI-TOF) (m/z):
[M + H]+ Calcd for C15H15NO2H 242.1181; Found 242.1181.
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a
crude product was purified by using column
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pure products 4g (86%, 0.14 g) as yellow oil.IR (cm-1): 2987, 1717, 1278,
752. 1H NMR (DMSO-d6, 400 MHz): δ 8.57 (d, J = 4.4 Hz, 1H), 7.89 –
7.85 (m, 1H), 7.67 – 7.59 (m, 4H), 7.53 – 7.49 (m, 1H), 7.38 – 7.35 (m,
1H), 4.91 – 4.87 (m, 1H), 2.12 – 2.08 (m, 2H), 1.71 – 1.67 (m, 2H), 1.59 –
1.48 (m, 2H). 13C{1H} NMR (CDCl3, 400 MHz): δ 167.82, 158.94, 148.93,
141.20, 136.13, 131.64, 131.08, 129.85, 129.78, 128.26, 122.89, 121.98,
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69.24, 29.93, 13.52. HRMS (ESI-TOF) (m/z): [M
C16H15NO2H 254.1181; Found 254.1181.
+
H]+Calcd for
Cyclohexyl 2-(pyridin-2-yl)benzoate (4h): According to the general
procedure, crude product was purified by using column
a
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chromatography on silica gel (heptane/ethyl acetate = 8:2) to give the
pure products 4h (88%, 0.15 g) as yellow oil. IR (cm-1): 2935, 1711, 1278,
747. 1H NMR (DMSO-d6, 400 MHz): δ 8.55 (d, J = 4.0 Hz, 1H), 7.86 (t, J
= 7.6 Hz, 1H), 7.66 – 7.59 (m, 4H), 7.53 – 7.49 (m, 1H), 7.36 – 7.33 (m,
1H), 4.68 – 4.64 (m, 1H), 1.58 – 1.44 (m, 2H), 1.37 – 1.26 (m, 3H), 1.24 –
1.18 (m, 2H), 1.13 – 1.08 (m, 3H). 13C{1H} NMR (CDCl3, 500 MHz): δ
168.21, 159.09, 149.08, 140.96, 136.24, 132.30, 130.98, 129.93, 129.86,
128.31, 123.06, 122.06, 73.55, 31.36, 25.32, 23.75. HRMS (ESI-TOF)
(m/z): [M + H]+ Calcd for C18H19NO2H 282.1494; Found 282.1493.
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Tert-butyl 3-((2-(pyridin-2-yl)benzoyl)oxy)pyrrolidine-1-carboxylate
(4i): According to the general procedure, a crude product was purified by
using column chromatography on silica gel (heptane/ethyl acetate = 7:3)
to give the pure products 4i (65%, 0.15 g) as yellow solid. Melting point:
82.3-85.6 ºC. IR (cm-1): 2978, 1717, 1715, 1278. 1H NMR (DMSO-d6, 400
MHz): δ 8.52 (d, J = 4.4 Hz, 1H), 7.88 (t, J = 7.6 Hz, 1H), 7.68 – 7.63 (m,
4H), 7.54 – 7.52 (m, 1H), 7.33 – 7.32 (m, 1H), 5.17 (s, 1H), 3.32 – 3.15
(m, 4H), 1.93 – 1.88 (m, 1H), 1.66 – 1.64 (m, 1H), 1.38 (s, 9H). 13C{1H}
NMR (CDCl3, 100 MHz): δ 168.36, 158.72, 154.08, 148.93, 140.98,
136.34, 131.29, 130.03, 129.80, 128.31, 122.64, 122.18, 79.35, 74.63,
51.43, 43.83, 30.39, 28.49. HRMS (ESI-TOF) (m/z): [M + H]+ Calcd for
C21H24N2O4H 369.1814; Found 369.1814.
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6
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European Journal of Organic Chemistry
FULL PAPER
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7
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European Journal of Organic Chemistry
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Entry for the Table of Contents
Table of contents
Key Topic: C–H alkoxycarbonylation, Free radical reaction,
Carbazates.
Gujjenahalli Ramalingaiah Yogesh
Kumar, Noor Shahina Begum*
Page No. – Page No.
Palladium-Catalysed Oxidative
Nitrogen-Containing Heterocycle-
Directed C-H Alkoxycarbonylation of
Arenes with Alkylcarbazates.
An environmentally benign protocol for the C-H alkoxycarbonylation of heterocycle-directed arene with alkyl carbazates as the
greenest ester source is described.
8
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