Michael addition of ethyl anthranilate and phenyl monothioanthranilate to acetylenic esters: experimental and theoretical results

Article abstract of DOI:10.1007/s11224-021-01735-9

The reaction of ethyl anthranilate with DMAD and with methyl propiolate in dichloromethane in the presence of ethylaluminium dichloride as catalyst at room temperature gives a mixture of the respective E- and Z-intermediate derivatives which do not cyclize even on refluxing for several hours. On carrying out the reaction in refluxing methanol in the absence of the catalyst, only one stereochemical isomer is obtained which is Z-intermediate indicating conversion of less stable isomer into the thermodynamically more stable derivative under these conditions. The resulting product did not cyclize even on refluxing in methanol in the presence of montmorillonite K10 clay catalyst. The reaction of phenyl monothioanthranilate with DMAD and with methyl propiolate gives similar results. A theoretical investigation of the reaction of ethyl anthranilate with methyl propiolate at the B3LYP/6-31 + G(d) level reveals that it occurs in five steps.

Full text of DOI:10.1007/s11224-021-01735-9

Structural Chemistry
https://doi.org/10.1007/s11224-021-01735-9
ORIGINAL RESEARCH
Michael addition of ethyl anthranilate and phenyl
monothioanthranilate to acetylenic esters: experimental
and theoretical results
Pratibha Sharma1 & Raakhi Gupta1 & Raj K. Bansal1
Received: 21 December 2020 /Accepted: 15 January 2021
#
The Author(s), under exclusive licence to Springer Science+Business Media, LLC part of Springer Nature 2021
Abstract
The reaction of ethyl anthranilate with DMAD and with methyl propiolate in dichloromethane in the presence of ethylaluminium
dichloride as catalyst at room temperature gives a mixture of the respective E- and Z-intermediate derivatives which do not
cyclize even on refluxing for several hours. On carrying out the reaction in refluxing methanol in the absence of the catalyst, only
one stereochemical isomer is obtained which is Z-intermediate indicating conversion of less stable isomer into the thermody-
namically more stable derivative under these conditions. The resulting product did not cyclize even on refluxing in methanol in
the presence of montmorillonite K10 clay catalyst. The reaction of phenyl monothioanthranilate with DMAD and with methyl
propiolate gives similar results. A theoretical investigation of the reaction of ethyl anthranilate with methyl propiolate at the
B3LYP/6-31 + G(d) level reveals that it occurs in five steps.
.
.
.
.
.
Keywords Michael addition Ethyl anthranilate Phenyl monothioanthranilate DMAD Methyl propiolate DFT calculations
Introduction
The reaction of aromatic amines with DMAD under ther-
mal condition has been reported to give 4(1H)-quinoline de-
rivatives [22]. Similarly, ethyl anthranilate on refluxing with
DMAD in methanol followed by heating under vacuum at ~
250 °C gave 8-carbethoxy-2-carbomethoxy-4-(1H)-quinolone
(Scheme 1) [23].
We were motivated to reinvestigate the above reaction exper-
imentally and theoretically with three objectives: to isolate the
initially formed E- or/and Z-intermediate(s) by carrying out the
reaction in a less polar solvent, to attempt accomplish it under
milder conditions with the use of a catalyst, if necessary and to
know the reason of regioselectivity in cyclization. Furthermore,
we carried out a similar reaction of phenyl monothioanthranilate
with methyl propiolate and with DMAD under similar conditions
to observe difference, if any, in the mode of cyclization. We
expected that theoretical investigation would reveal the reason
of the cyclization occurring at high temperature.
The Michael reaction though discovered in the year 1887 [1, 2]
continues to attract attention of the chemists owing to its versa-
tility in organic synthesis [3–6]. The aza-Michael reaction involv-
ing addition of amines and related nitrogen compounds makes a
variety of β–amino carbonyl compounds accessible, which are
important synthons for obtaining bioactive natural motifs [7–10]
and chiral auxiliaries [11–13]. Michael addition of amines to
acetylenecarboxylic acid derivatives has been studied for its
mechanistic [14–16] and synthetic [17–19] aspects. We recently
reported our experimental and theoretical results about the addi-
tion of primary and secondary amines to DMAD [20] and maleic
anhydride [21]. In the former case, we succeeded for the first
time in isolating and characterizing the intermediate, dimethyl
2-(N-benzylamino)butane-1,4-dicarboxylate generated from the
reaction of benzylamine with DMAD, thereby confirming the
reaction mechanism predicted by theoretical calculations [20].
Experimental details
* Raakhi Gupta
raakhi27@yahoo.com
General
1
Commercially available ethyl anthranilate, methyl propiolate,
and DMAD were purchased from Merck and used without
Department of Chemistry, IIS (deemed to be University),
Jaipur 302020, India

Struct Chem
O
Scheme 1 Reaction of ethyl
anthranilate with DMAD
MeO2C
CO2Me
H
C
C
MeOH
2500C
+
NH2
CO2Et
reflux
vacuum
N
H
CO2Et
CO2Me
NH
CO2Et
CO2Me
CO2Me
1
2b
3b
4b
further purification. Ethylaluminium dichloride (1.8 M in tol-
uene) solution was purchased from Sigma-Aldrich. Phenyl
monothioanthranilate was prepared according to the literature
method [24]. IR spectra were recorded on Brucker spectrom-
eter in KBr pellet. NMR spectra were recorded in CDCl3 on
A similar procedure was followed for the reaction of ethyl
anthranilate (13.5 mmol, 2234mg, 2 mL) with DMAD (13.5
mmol, 1965mg, 1.7 mL) in the presence of ethylaluminium
dichloride as catalyst.
1
Jeol-400 MHz spectrometer, H NMR at a frequency of
Reaction of ethyl anthranilate with DMAD/methyl propiolate
in methanol in the absence/presence of montmorillonite K10
clay catalyst
399.78 MHz, and 13C NMR at a frequency of 100.53 MHz
using TMS as the internal reference. The C, H, N elemental
analyses were done on a FLASH Ea 1112 series CHN
analyzer.
Ethyl anthranilate (7.0 mmol, 1170 mg, 1 mL,) was dissolved
in methanol (5 mL) and taken in a 25 mL RB flask at r.t.. To
this was added 1 equiv. of DMAD (6.7 mmol, 959 mg, 830
μL) dissolved in 8 mL of methanol dropwise at r.t. with con-
tinuous stirring. The reaction mixture was then refluxed for 2
h. The progress of the reaction was monitored by TLC (sol-
vent: pet ether 60–80 °C:EtOAc: 7:3 v/v). After the reaction
was complete, the reaction mixture was concentrated and a
few drops of diethyl ether were added and kept in refrigerator.
A viscous mass was obtained, which was separated and dried
under vacuum.
Similar procedure was followed for the reaction of ethyl
anthranilate (7.0 mmol, 1170 mg, 1.0 mL) with methyl
propiolate (6.7 mmol, 567 mg, 0.6 mL).
The above experiments were repeated under similar condi-
tions in the presence of montmorillonite K10 clay (10 mol%).
Reaction of ethyl anthranilate with methyl propiolate
in methylene chloride in presence of ethylaluminium
dichloride as catalyst
Methyl propiolate (3.4 mmol, 284 mg, 0.3 mL) was dissolved
in CH2Cl2 (10 mL) taken in a 25 mL RB flask at r.t. under
nitrogen atmosphere. To this solution was added EtAlCl2 (1.8
mmol, 217mg, 190 μL of 1.8 M solution in toluene) when
yellow to orangish brown colour developed. After stirring it
for about 30 min. at r.t., a solution of ethyl anthranilate (3.4
mmol, 558 mg, 0.5 mL) in CH2Cl2 (2 mL) was added
dropwise and the reaction mixture was stirred at r.t. for 36 h,
progress of the reaction being monitored by TLC (solvent: pet
ether 60-80oC EtOAc: 3:1 v/v). After completion of the reac-
tion, the reaction mixture was quenched with distilled water
(30 mL) followed by the addition of NH4OH until it became
neutral. The organic layer was separated with the separatory
funnel. The aqueous layer was extracted with CH2Cl2 (3 × 5
mL). The CH2Cl2 extracts were mixed, dried over anhydrous
sodium sulfate and the solution was concentrated to ~2 mL.
The crude product on column chromatography over silica gel-
eluent: pet ether 60–80 °C EtOAc: 9:1 v/v afforded a white
crystalline product.
Reaction of phenyl monothioanthranilate with DMAD/methyl
propiolate in methanol
Phenyl monothioanthranilate (0.40 mmol, 100 mg) was dis-
solved in methanol (5 mL) taken in a 25 mL RB flask at r.t..
To this was added 1 equiv. of DMAD (4.3 mmol, 620 mg,
0.53 mL) dissolved in 10 mL of methanol dropwise at room
temperature with continuous stirring. The reaction mixture
1
Scheme 2 Reaction of ethyl
anthranilate with methyl
propiolate and with DMAD in the
presence of catalyst
MeO2C
CO2Me
1
H
6'
2
6'
CO2Me
2
H
1'
5'
4'
1'
2'
NH
C
C
EtAlCl2 (10 mol%)
r.t. CH2Cl2
5'
4'
+
+
NH2
CO2Et
C
2'
C
3
R
3
3'
CO2Et
NH
R
R
3'
CO2Et
2
5a,b
3a,b
1
a
b
H COOMe
R

Struct Chem
The intrinsic reaction coordinate (IRC) calculations [26,
27] starting from the transition structure were carried out at
the same theory level to confirm its relation to the respective
reactants and the intermediate/product. The total enthalpy was
calculated by adding thermal corrections to the sum of the
electronic and thermal enthalpy. The free energy ΔG at a tem-
perature of 298.15 K was calculated as follows:
Scheme 3 Reaction of ethyl anthranilate with methyl propiolate and with
DMAD in methanol
was then refluxed for 6 h. The progress of the reaction was
monitored by TLC (solvent: pet ether 60-80oC:EtOAc: 8:2
v/v). After the reaction was complete, solvent was removed
under vacuum and the residue was distilled under reduced
pressure.
ΔG ¼ ΔH−TΔS
Similar procedure was followed for the reaction with meth-
yl propiolate, but in this case, product was isolated by column
chromatography and solvent was removed under vacuum.
The above experiments were repeated under similar condi-
tions in the presence of montmorillonite K10 clay (10 mol%).
ΔH = relative enthalpy
ΔS = relative entropy
T = 298.15 K
Computational methods
Results and discussion
Gaussian 16 suite of programs was used for all calcu-
lations [25].
Experimental results
Geometries of the reactants, transition structures, interme-
diates, and the products involved in the model reaction were
optimized in the gas phase at the B3LYP/6-31 + G(d) level of
theory. Frequency calculations were done at the same level to
characterize the energy minimum or the first saddle point in
the presence of no imaginary and only one imaginary frequen-
cy, respectively.
Ethyl anthranilate did not react with DMAD in dichlorometh-
ane even on refluxing for several hours. In view of many
reports about the use of organometallic complexes as catalysts
for the addition of aromatic amines to different Michael ac-
ceptors [28, 29], ethyl anthranilate was reacted with methyl
propiolate (2a) and with DMAD (2b) in the presence of
EtAlCl2 as catalyst in dichloromethane at r.t. when a smooth
Table 1 The 1H NMR chemical shift values of 3b reported earlier [23] and the values obtained in present investigations. aOriginally
reported on the scale of τ
1
MeO2C
6'
2
H
1'
C
5'
4'
2'
C
3
NH
CO2Me
3'
CO2Et
3b
1H NMR Chemical shifts (δ )
Present studies
(CDCl3)
11.42
Reported earliera
(CDCl3)
11.71
N-H, 1H
Ph, 4H
6.64 - 7.32
5.58
7.01-8.38
5.47
H-2, 1H
OCH2CH3,2H
C-1 OMe, 3H
C-3 OMe, 3H
OCH2CH3,
3H
4.41
4.68
3.88
3.89
3.81
3.82
1.41
1.43

Struct Chem
1
Scheme 4 Reaction of phenyl
monothioanthranilate with methyl
propiolate and with DMAD in
methanol
MeO2C
CO2Me
6'
2
C
H
1'
MeOH
5'
4'
+
2'
NH
NH2
C
3
reflux
R
3'
R
CSPh
O
CSPh
O
6
2a,b
7a,b
R= H, CO2Me
reaction occurred to afford white crystalline products in mod-
erate yields (Scheme 2).
The reaction did not proceed further even on prolonged
refluxing under the above conditions. Furthermore, no change
occurred on refluxing in toluene also.
These results are in accordance with our earlier report wherein
we described in detail the results of the Michael reaction of
primary and secondary amines with DMAD; secondary amines
give a single product whereas reaction with a primary amine
leads to the formation of a mixture of the Z- and E-isomers, the
former being formed as the major product [20].On investigating
the mechanism of isomerization of the initially formed E-isomer
to the Z-isomer theoretically, it was found that the proton on the
nitrogen atom is reversibly transferred to the C2 atom through a
bridge formed by a second molecule of the amine followed by
successive rotation about the C2–C3 bond and transfer of the
proton back to the N atom [20].
As revealed by 1H NMR spectra, in each case a mixture of
Z- and E- products is formed, which, however, could not be
separated, but it could be possible to determine the relative
percentages of the two isomers in each case on the basis of the
relative intensities of the 1H NMR signals of the vinylic pro-
ton(s). The percentage of the Z-isomer having intramolecular
hydrogen bonding between the N-H proton and the ester
group is much higher, which may be attributed to its greater
thermodynamic stability.
All the products could be well characterized on the basis of
the IR, 1H NMR and 13C spectral studies. In the IR spectra, an
Scheme 5 Model reaction of ethyl anthranilate with methyl propiolate computed at the B3LYP/6-31 + G(d) level

Struct Chem
1
2a
11a (Int.2- E)
8a(TS1)
12a (TS3)
9a (Int.1)
13a (Int.3- Z)
10a (TS2)
Fig. 1 Geometries of different species involved in the reaction of ethyl
anthranilate with methyl propiolate optimized at the B3LYP/6-31 + G(d) level
absorption band of moderate intensity at ~ 3250–3450 cm-1
results due to the N–H stretching (st.) vibration. Besides, in-
tense absorption bands in the regions of 1615–1735 cm-1
(C=O st.) and 1100–1270 cm-1 confirm the ester groups.
1
4a
In the H NMR spectrum, the characteristic chemical shifts
and the multiplicities of the signals resulting due to the vinylic
Fig. 1 (continued)
protons (=C–H) confirmed the presence of the Z- and E-isomers
1
in each case. For example, in the H NMR spectrum of the

Struct Chem
Table 2 Total enthalpy and entropy of different species computed at the
B3LYP/6-31 + G(d) level
It may be noted that the chemical shift values obtain-
ed in the present investigation accord well with the
values reported earlier.
Entry
Total enthalpy in au
Entropy(calK-1 mol-1)
Clays have been extensively used as catalysts in a variety
of organic reactions including Michael additions [30, 31]. A
montomorillonite K10 clay catalyzed Michael type addition of
aniline derivatives to cinnamaldehyde followed by intramo-
lecular cyclization was reported [32]. In another report, trisub-
stituted pyridines were obtained from a montomorillonite K10
clay catalyzed three-component reaction of enaminones, β-
ketoesters/1,3-diketones, and ammonium acetate in boiling
isopropyl alcohol [33]. In view of this, an equimolar solution
of ethyl anthranilate and methyl propiolate in methanol was
refluxed in the presence of montomorillonite K10 (10 mol%);
the reaction, however, did not proceed further after the forma-
tion of 3a as revealed by the TLC.
On refluxing an equimolar solution of phenyl
monothioanthranilate (6) and methyl propiolate/DMAD in
methanol, a single product (7a,b) was formed in each case
(Scheme 4).
In this case also, initially formed product did not cyclize
even on refluxing in methanol in the presence of
montomorillonite K10 for prolonged period.
1
− 554.428224
− 305.059551
− 859.443772
− 859.444182
− 859.437333
− 859.554599
− 859.478704
− 859.537430
− 743.939056
− 115.614075
105.507
78.643
2a
8a (TS1)
9a (Int.1)
10a (TS2)
11a (Int.2-E)
12a (TS3)
13a (Int.3-Z)
4a
144.796
143.877
141.758
141.715
140.751
139.401
116.599
56.858
MeOH
product obtained from the reaction of ethyl anthranilate with
methyl propiolate, a doublet at δ 2.59 (H2) and a double doublet
(dd) at δ 4.20 ppm (H3) with a large coupling constant (3JHH
=
13.6 Hz) confirm the E-isomer (5a), whereas another set of a
doublet at δ 2.56 (H2) and a dd at δ 4.70 (H3) with a compara-
tively smaller coupling constant (3JHH = 8.0 Hz) is in accordance
with the Z-isomer (3a). It is noteworthy that the NMR signal for
the H2 is much upfield than that for the H3, which may be
attributed to the +M effect of the nitrogen lone pair. Another
distinguishing feature of the 1H NMR spectra of the Z- and E-
isomers is the chemical shift of the N–H proton: in the former, it
is much downfield (δ ~ 12 ppm) and broadened due to the
intramolecular hydrogen bonding, whereas in the latter, it is up-
field (δ ~ 11 ppm) and sharper. The detailed physical and spectral
data are given in the Supplementary Material.
The products so formed could be well characterized on the
basis of IR and NMR spectral studies.
Theoretical results
With a view to investigate the mechanism of the reaction, we
computed a model reaction of ethyl anthranilate with methyl
propiolate at the DFT level (Scheme 5).
On refluxing an equimolar solution of ethyl anthranilate
and methyl propiolate/DMAD in methanol, a single product
(3a,b) was formed in each case, as reported earlier [23], indi-
cating complete change of the E-isomer into the Z-isomer
(Scheme 3).
Scanning of the potential energy surface reveals that the
reaction occurs in five steps.
The first step involving the attack of ethyl anthranilate on
methyl propiolate leads to the formation of a zwitterionic in-
termediate (9a Int. 1). It is followed by a 1,3-prototropic shift
of the NH proton to form the E Int.2 (11a). The latter has
nitrogen lone pair conjugated with the C=C bond due to which
C2-C3 bond acquires a single bond character and rotation
about it becomes possible. Thus in the third step, the E inter-
mediate Int.2 changes into the Z Int.3. As will be seen later, it
is a high energy path, but much greater thermodynamic sta-
bility of the Z Int.3 (13a) makes it possible. The next step
The products could be well characterized on the basis of IR
and NMR studies.
As mentioned earlier, the product 3b formed from the re-
action of ethyl anthranilate with DMAD was reported by
1
George and co-worker [23]. The H NMR chemical shift
values of the compound reported earlier [23] and the values
obtained by us are given in Table 1.
Table 3 Thermodynamic data of different steps of Michael addition of ethyl anthranilate to methyl propiolate
Step
ΔH# (kcal mol-1) [a]
ΔS# (cal mol-1 K-1) [b]
ΔG# (kcal mol-1) [c]
ΔHo (kcal mol-1) [d]
ΔGo (kcal mol-1) [f]
1
27.61
4.30
47.22
-
− 39.35
− 2.11
− 0.42
-
39.34
4.93
47.51
-
27.35
39.27
2
− 68.88
10.37
− 68.14
11.03
3
Overall reaction energy
− 41.02
-37.82

Struct Chem
Fig. 2 Energy profile diagram
followed by addition of ethyl
anthranilat with methyl propiolate
involves intramolecular cyclization to form Int.4(15a); the
transition structure (14a) involved in this cyclization, howev-
er, could not be located. The intermediate Int.4 splits off a
CH3OH molecule to give the final product 4-(1H)-quinolone
derivative (4a).
free energy barrier of steps 1 and 3. Thus, apparently, it ap-
pears that the third step is the rate determining, but in practice,
the first step will be rate determining. To initiate the first step,
whose activation free energy is high, ΔG# (39.34 kcal mol-1),
external energy is required. This is in consonance with the
experimental results, where the reaction takes place on
refluxing the reaction mixture in methanol.
Optimized geometries
The optimized geometries of different species involved in the
reaction of ethyl enthranilate with methyl propiolate are pre-
sented in Fig 1.
Conclusions
The polarity of the solvent plays a crucial role in the Michael
reaction of ethyl anthranilate with acetylenic esters. The reac-
tion in a less polar solvent, namely dichloromethane, occurs
only on refluxing in the presence of an organoaluminium cat-
alyst to yield a mixture of the E- and Z-intermediate deriva-
tives. However, on refluxing the reactants in methanol, reac-
tion occurs without the aid of a catalyst forming only the
thermodynamically more stable Z-isomer; thus, the initially
formed E-intermediate changing into the Z-isomer under these
conditions, but the latter does not cyclize even on refluxing the
reactants in methanol in the presence of montomorillonite
K10 catalyst.
Energetics
The thermodynamic data of different species and the relative
enthalpies and free energies of different steps of the aza-
Michael addition of ethyl anthranilate to methyl propiolate
are presented in Tables 2 and 3 respectively.
The energy profile diagram depicting the enthalpy and free
energy changes in the reaction of ethyl anthranilate 1with
methyl propiolate 2a is presented in Fig. 2.
Overall, the reaction is exergonic, ΔG0 being − 37.82 kcal
mol. It will provide sufficient energy to overcome the high

Struct Chem
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Similar results are obtained on reacting phenyl
monothioanthranilate with methyl propiolate and with
DMAD under similar conditions.
A theoretical investigation of the model reaction of ethyl
anthranilate with methyl propiolate at the DFT level rational-
izes the experimental results, particularly the high temperature
to accomplish the reaction.
Supplementary Information The online version contains supplementary
material available at https://doi.org/10.1007/s11224-021-01735-9.
Funding This study is funded by the IIS (deemed to be University),
Jaipur (India).
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Data availability All data generated or analyzed during this study are includ-
ed in this published article (and its supplementary information files).
Compliance with ethical standards
Competing interests The authors declare no competing interests.
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