Mechanistic investigation and DFT calculation of the new reaction between S-methylisothiosemicarbazide and methyl acetoacetate

Article abstract of DOI:10.1007/s11224-013-0223-3

A study on the synthesis and mechanistical aspects of formation of 3-methyl-5-oxo-3-pyrazolin-1-carboxamide (MOPC) starting from S-methylisothiosemicarbazide hydrogen iodide and methyl acetoacetate was performed. In the alkaline aqueous solution, the intermediate methyl acetoacetate S-methylisothiosemicarbazone undergoes substitution of CH ₃S^- anion by hydroxide anion, cyclization, carbanion formation, and elimination of methanol, thus yielding corresponding Na-enolate salt of pyrazol-5-one derivative. The structure of the compound obtained after protonation of the formed enolate salt was determined by means of spectroscopic techniques and single-crystal X-ray diffraction analysis. The mechanism of conversion of methyl acetoacetate S-methylisothiosemicarbazone into MOPC was investigated by means of the B3LYP functional, and it was found that the reaction is thermodynamically controlled.

Full text of DOI:10.1007/s11224-013-0223-3

Struct Chem
DOI 10.1007/s11224-013-0223-3
ORIGINAL RESEARCH
Mechanistic investigation and DFT calculation of the new reaction
between S-methylisothiosemicarbazide and methyl acetoacetate
•
Violeta Markovic Svetlana Markovic Ana Janicijevic
•
•
´
´
Marko V. Rodic Vukadin M. Leovac Nina Todorovic
´
´
•
•
•
´
Snezana Trifunovic Milan D. Joksovic
´
•
ˇ
´
´
Received: 6 December 2012 / Accepted: 28 January 2013
Ó Springer Science+Business Media New York 2013
Abstract A study on the synthesis and mechanistical
aspects of formation of 3-methyl-5-oxo-3-pyrazolin-1-car-
boxamide (MOPC) starting from S-methylisothiosemi-
carbazide hydrogen iodide and methyl acetoacetate was
performed. In the alkaline aqueous solution, the interme-
diate methyl acetoacetate S-methylisothiosemicarbazone
undergoes substitution of CH3S- anion by hydroxide
anion, cyclization, carbanion formation, and elimination of
methanol, thus yielding corresponding Na-enolate salt of
pyrazol-5-one derivative. The structure of the compound
obtained after protonation of the formed enolate salt was
determined by means of spectroscopic techniques and
single-crystal X-ray diffraction analysis. The mechanism of
conversion of methyl acetoacetate S-methylisothiosemi-
carbazone into MOPC was investigated by means of the
B3LYP functional, and it was found that the reaction is
thermodynamically controlled.
Keywords Pyrazolone ꢀ Reaction mechanism ꢀ DFT ꢀ
Thermodynamic and kinetic control
Introduction
Pyrazolones represent an important class of heterocyclic
compounds with a wide range of applications in both
chemistry and biology due to their favorable pharmaco-
logical properties. These characteristics caused an increas-
ing interest of medicinal chemists for this research area in
the last decades. For example, antipyrine (2,3-dimethyl-
1-phenylpyrazol-5-one) and its analogs aminopyrine,
dipyrone and propyphenazone are known for a long time as
antipyretic, analgesic, and anti-inflammatory substances
[1, 2]. A new pyrazol-5-one compound, edaravone (3-methyl-
1-phenyl-2-pyrazoline-5-one, RadicutÒ, Mitsubishi Tanabe
Pharma Corporation) has been presented as a strong free
radical scavenger and a promising drug for the treatment of
brain ischemia [3]. A number of other pyrazolone compounds
have been synthesized and found to exhibit diverse biologic
and pharmacological properties [4–7].
Electronic supplementary material The online version of this
article (doi:10.1007/s11224-013-0223-3) contains supplementary
material, which is available to authorized users.
´
´
´
´
V. Markovic ꢀ S. Markovic ꢀ A. Janicijevic ꢀ
´
M. D. Joksovic (&)
Department of Chemistry, Faculty of Science, University of
Kragujevac, R. Domanovica 12, 34000 Kragujevac, Serbia
e-mail: mjoksovic@kg.ac.rs
The synthesis of carboxamide derivatives of pyrazolones
starting from b-keto esters and semicarbazide hydrochloride
is the known way of preparation of this class of compounds
[8]. The study by microwave irradiation was carried out to
investigate the possibility of performing this reaction with-
out use of any solvent and in much shorter reaction time [9].
The reaction mechanism of the known synthesis of
3-methyl-5-oxo-3-pyrazolin-1-carboxamide (MOPC, 1) occurs
by initial regioselective attack of the least hindered and more
nucleophilic nitrogen atom of the semicarbazide hydrazine
´
M. V. Rodic ꢀ V. M. Leovac
Department of Chemistry Biochemistry and Environmental
Protection, Faculty of Science, University of Novi Sad,
´
Trg D. Obradovica 3, 21000 Novi Sad, Serbia
´
N. Todorovic
Institute for Chemistry, Technology and Metallurgy,
ˇ
Njegoseva 12, 11000 Belgrade, Serbia
´
S. Trifunovic
Faculty of Chemistry, University of Belgrade, Studentski trg 16,
P.O. Box 158, 11000 Belgrade, Serbia
123

Struct Chem
moiety on the ketone carbon atom of the b-keto ester
(Scheme 1) [10]. This claim is supported by identification of the
corresponding hydrazone intermediate. The regioselectivity of
the reaction is also a result of higher reactivity of ketone moiety
over ester [11]. The next step is an intramolecular cyclization
between the ester group and the second nitrogen atom of
hydrazine moiety to provide an equilibrium mixture of the
tautomeric forms of the compound 1.
amounts of methyl acetoacetate and S-methylisothiosemi-
carbazide hydrogen iodide in ethanol during 1 h. It is also
possible to synthesize this compound with a higher degree of
purity if the mixture is left standing for 48 h upon 1 h of
stirringatthe roomtemperature. The absence ofan ABsystem
in its 1H NMR spectrum for methylene protons excludes the
existence of the cyclic form. After the solvent was evaporated,
water was poured into the residue and the granulated NaOH
was added to the stirring mixture. Compound2b issensitive to
hydrolysis in water which results in the significant formation
of starting reactants as established by analyzing of 1H NMR
spectrum in D2O after 1 h of standing. Therefore, it is nec-
essary to add NaOH immediately after addition of water.
Owing to the dissolving of NaOH in water, the reaction
mixture becomes hot. Attempts to cool the flask led to the
decrease of the reaction yield, implying that this cyclization
reaction is very slow at the room temperature. The use of
granulated NaOH enabled slower dissolving and therefore
prolongation of heat evolving. Molar ratio of NaOH to 2b of
11:1 appeared to be the most effective. Lower molar ratio
resulted in prolongation of the reaction time and decrease of
the yield of enolate salt 16. As for concentration, 35–40 %
aqueous solution of NaOH gave the best results (Scheme 2).
The high purity of 16, formed few minutes upon the
addition of NaOH, does not require further purification.
Treatment of the enolate salt 16 (yield: 54 % relative to
S-methylisothiosemicarbazide hydrogen iodide) with 2 M
HCl resulted in the formation of the corresponding pro-
tonated form which instantly underlies tautomerisation
to 3-methyl-5-oxo-3-pyrazolin-1-carboxamide (MOPC, 1,
75 % yield relative to 16).
The reaction of S-alkylated isothiosemicarbazides with
ketone compounds proceeds in analogy with the reaction of
hydrazine derivatives giving S-alkylated thiosemicarba-
zones. In our previous research reaction, acetylacetone and
S-methylisothiosemicarbazide hydrogen iodide unexpect-
edly yielded 3-aminopyrazole derivative in an alkaline
medium [12]. Mechanism of the formation of this compound
was examined by means of NMR studies and DFT calcula-
tions. A cyclic intermediate 5-hydroxy-3,5-dimethyl-1-S-
methylisothiocarbamoyl-2-pyrazolinium iodide was formed
instead of acetylacetone mono-S-methylisothiosemicarba-
zone. In alkaline medium, it is subjected to ring opening
reaction forming the keto–imine form of acetylacetone
mono-S-methylisothiosemicarbazone. Herein, as a continu-
ation of the previous work, we have focused our attention on
performing a reaction between S-methylisothiosemicarba-
zide hydrogen iodide and methyl acetoacetate. Unlike the
former, this reaction resulted in the formation of 3-methyl-5-
oxo-3-pyrazolin-1-carboxamide (MOPC). In this paper, we
present a study on the synthesis and mechanistical aspects of
formation of this pyrazol-5-one derivative.
Results and discussion
2D NMR study
Synthesis of 3-methyl-5-oxo-3-pyrazolin-1-
carboxamide (MOPC, 1)
The structural confirmation of MOPC (1) was based on
elemental analysis, HRMS, one-dimensional (1D) and 2D
NMR techniques, and X-ray diffraction analysis. Detailed
assignation of NMR data is performed by means of 1H, 13C,
The precursor methyl acetoacetate S-methylisothiosemi-
carbazone (2b) was formed by refluxing of the equimolar
Scheme 1 Mechanism of
formation of 3-methyl-5-oxo-
3-pyrazolin-1-carboxamide
(MOPC, 1) from ethyl
acetoacetate and semicarbazide
123

Struct Chem
Scheme 2 Two different mechanistic pathways A and B for the
compounds 2a and b. Mechanism A: formation of a 3-aminopyrazole
(7) from 2a (operative mechanism [12]); there is no reaction from 2b
according to this mechanism. Mechanism B: formation of MOPC (1)
from 2b (operative mechanism, present work); there is no reaction
from 2a according to this mechanism
DEPT, HSQC, HMBC, NOESY, 1H–15N HSQC, and
1H–15N HMBC spectra (Figs. S1–S10, Supplementary data).
The position of the methyl group at 11.89 ppm can be
easily established according to correlations in HSQC
spectrum. In addition, NOESY spectrum displays a cross
peak between methyl protons and proton at C4. On the
other hand, HMBC spectrum shows two- and three-bond
correlations of methyl protons with C3 and C4, respec-
tively. The proton at C4 gives two two-bond cross peaks,
one with C3 atom and the other with the carbon atom of the
carbonyl group of the pyrazolone ring.
The absence of the expected correlation of N2–H can be
explained with the existence of annular tautomerism as
well as a high degree of tautomerisability of this nitrogen
atom. Indeed, after 2 weeks of standing in DMSO-d6
solution, NMR spectra became more complex and four
1
additional signals in H NMR attributed to methyl protons
were observed. Furthermore, NOESY spectrum reveals an
off-diagonal cross peak of N2–H with water present in the
1
deutero solvent. On the other hand, H–15N HMBC spec-
trum shows a correlation between methyl protons and
nitrogen atom at -233.3 ppm, which can only be assig-
nated to N2 atom. N1 atom at -193.7 ppm exhibits two
three-bond correlations with the proton at C4 and with one
of NH2 protons. The absence of correlation with the other
NH2 proton can be explained by the contribution of free
electron pair of this nitrogen atom to the resonance of the
amide group.
The assignation of the NH2 nitrogen atom was per-
1
formed according to the H–15N HSQC spectrum which
depicts a correlation of signals at 7.68 and 8.34 ppm with
nitrogen atom at -297.4 ppm. This implies that protons are
bonded to the same nitrogen atom: in this case, NH2 group
of the carboxamide moiety.
123

Struct Chem
˚
Crystallographic study
Table 1 Comparison of bond lengths (A) within pyrazole ring of
MOPC (1) with those found in 3-methyl-5-oxo-3-pyrazoline-N(1)-
substituted derivatives reported in literature
Molecular structures of the MOPC and atomic labeling
scheme are given in Fig. 1, while structural parameters are
listed in Tables S1, S2, and S3 (Supplementary data).
The molecule is highly planar and displacement from
mean plane for non-hydrogen atoms are in range 0.001 (2)
N1–
N2
N1–
C5
C5–
C4
C4–
C3
C3–
N2
References
1.376 1.420 1.408 1.367 1.337 [15]
(3) (2) (4) (3) (2)
1.378 1.414 1.412 1.375 1.338 [16]
(2) (2) (3) (3) (3)
1.400 1.392 1.425 1.357 1.355 [17]
(2) (2) (2) (3) (3)
1.380 1.408 1.403 1.367 1.343 [18]
(4) (4) (6) (4) (5)
1.376 1.394 1.412 1.359 1.348 [19]
(3) (4) (5) (5) (5)
1.387 1.400 1.418 1.356 1.345 [19]a
(3) (4) (5) (4) (4)
1.383 1.413 1.415 1.362 1.334 [20]
(3) (4) (4) (4) (4)
1.380 1.379 1.401 1.356 1.340 [21]
(2) (2) (2) (2) (2)
1.389 1.390 1.414 1.355 1.350 [22]
(3) (4) (3) (4) (4)
Minimum 1.376 1.379 1.401 1.355 1.343
(3) (2) (2) (4) (5)
Maximum 1.400 1.420 1.425 1.375 1.355
˚
to 0.040 (1) A with N1 atom deviating the most. Delo-
calization of electron density is evident in pyrazole ring
with bond lengths between those characteristic for pure
single or double bonds due to sp2 hybridization of atoms.
Formal double bond between Csp2–Csp2 can be ascribed to
C3–C4 distance, while other formally correspond to single
bonds [13, 14]. In order to corroborate that the main tau-
tomer in the solid state is MOPC, bond lengths within
pyrazole ring are compared with those found in other 3-
methyl-5-oxo-3-pyrazoline-N(1)-substituted derivatives
[15–22], and results are listed in Table 1. All bond lengths
are in good agreement with those found in structurally
related compounds, as N1–N2, N1–C5, C5–C4, C4–C3,
and C3–N2 distances in MOPC (1.368 (2), 1.400 (2),
˚
1.407 (2), 1.367 (2), and 1.334 (2) A, respectively) deviate
˚
not more than 0.01 A from the average values calculated
˚
from literature data (1.38, 1.40, 1.41, 1.36, and 1.34 A,
respectively).
(2)
(2)
(2)
(3)
(3)
Carboxamide group is coplanar with the pyrazole ring
with dihedral angle between N1/N2/C3/C4/C5 and O2/C7/
N3 being 3.1 (3)°. The trans orientation of N3 toward N2
is favored by intermolecular hydrogen bond N3–H3AꢀꢀꢀO1
which results in formation of a six-membered ring with
graph-set descriptor S(6) [23].
Average
MOPC
1.38
1.40
1.41
1.36
1.34
1.368 1.400 1.407 1.367 1.334 [This
(2) (2) (2) (2) (2) work]
a
Asymmetric unit contains two independent molecules
connects two neighboring molecules into centrosymmetric
dimer resulting in R22ð8Þ motif [23]. Furthermore, N2–
H2ꢀꢀꢀO1ii [symmetry code: (ii) x - 1, y, z] interaction
connects dimers laterally into highly planar 1D pattern,
which propagates along direction of a axis, as depicted in
Fig. 2. This hydrogen bond can serve as additional proof of
existence of hydrogen atoms bonded to N2 and correct
assignment of tautomeric form of 1. The arrangement of
All potential hydrogen bond donors and acceptors are
involved in hydrogen bonding. Strong N–HꢀꢀꢀO hydrogen
bonds determine the supramolecular architecture of the
MOPC (1) molecules, and their geometrical parameters are
given in Table S4 (Supplementary data). As expected,
amide groups in the crystal form head-to-head amide group
homosynthons [24]. Namely, hydrogen bond N3–
H3BꢀꢀꢀO2i [symmetry code: (i) –x ? 1, -y ? 1, -z]
Fig. 1 Molecular structure of MOPC (1): experimental (left) and
calculated (right). Ellipsoids are drawn at 50 % probability level.
Only one orientation of disordered methyl hydrogen atoms is shown
with site occupancy factors of 0.5. Calculated geometry corresponds
to the solvated molecule
123

Struct Chem
dimers in ribbon results in the hydrogen bonding ring
which is described by R46ð18Þ graph. The neighboring rib-
bons are connected via dipole–dipole interaction between
stacked carbonyl groups (Fig. 3). Geometry of this inter-
action can be classified as sheared antiparallel motif after
of 1 is surrounded with the molecules of the same kind) and
aqueous solution (where a molecule of 1 is surrounded with
the water molecules). Taking this fact into account, this
theoretic model was further employed to elucidate the
mechanism of transformation of the precursor 2b into 1.
Allen et al. [25] with O1–C5iii distance 3.184 (2) A
˚
Mechanism of formation of 3-methyl-5-oxo-3-
pyrazolin-1-carboxamide
[symmetry code: (iii) 2 - x, 1 - y, 1 - z], and C5–
O1ꢀꢀꢀC5iii and O1–C5ꢀꢀꢀO1iii angles 101.5 (1)° and
78.5 (1)°, respectively. Crystal packing is achieved by
layering ribbons which leads to high packing index of
71.1 % filled space.
In Scheme 2 according to the mechanism A, the keto–
imine tautomer 2a undergoes the further cyclization by
carbanionic mechanism forming corresponding 4-acetyl-
3(5)-amino-5(3)-methylpyrazole 7 [12]. In order to inves-
tigate a possible range of application of mechanism A,
herein, we performed a reaction between methyl acetoac-
etate and S-methylisothiosemicarbazide hydrogen iodide.
According to this mechanism, the expected 3-aminopyra-
zole derivative would have ester or carboxylic moiety
(upon alkaline hydrolysis) at C4 position. Opposite to our
expectations, formed intermediate methyl acetoacetate
S-methylisothiosemicarbazone (2b in Scheme 2) in alka-
line conditions yielded corresponding Na-enolate salt 16,
which upon the treatment with 2 M HCl formed 3-methyl-
5-oxo-3-pyrazolin-1-carboxamide (MOPC, 1).
The structure of 1 in the aqueous phase was calculated
by the B3LYP/6-311 ??G(d,p) level of theory in combi-
nation with the CPCM solvation model (Fig. 1). The
experimental and calculated geometrical parameters of 1
are presented in Tables S1, S2, and S3. Obviously, the
differences in geometrical parameters between the experi-
mental and calculated structures of 1 are very small. These
differences originate to a large degree from different
intermolecular interactions in the crystal (where a molecule
Our computational research showed that 2b can under-
take two different pathways in the basic environment.
Now, we will consider the mechanism B (Scheme 2),
which is in accord with the obtained experimental results.
The revealed transition states (TSs) are given in Fig. 4,
whereas all elementary steps of the reaction are presented
in Supplementary data via the optimized geometries of the
related participants.
Mechanism B begins with nucleophilic attack of OH- at
the electrophilic carbon-bearing CH3S group (natural bond
orbital [NBO] charge = 0.311). This step of the reaction
proceeds via transition state 8TS, where the new C–O bond
is being formed, while the C–S bond is being cleaved,
implying that the CH3S- anion is substituted by the
Fig. 2 Supramolecular arrangement of molecules in ribbon via
hydrogen bonds (a), crystal packing viewed perpendicular to bc
plane (b)
Fig. 3 Dipole–dipole interaction between stacked carbonyl groups.
Symmetry code: (iii) 2 - x, 1 - y, 1 - z
123

Struct Chem
hydroxide anion. This substitution leads to the formation of
intermediate 9 where hydrogen of hydroxyl group is
extremely acidic (NBO charge = 0.513). Thus, OH- from
the alkaline medium performs an abstraction of this proton.
This step of the reaction occurs spontaneously with the
energetic stabilization of the system, and the formation of
intermediate 10. As expected, in this intermediate nitrogen
bears notable negative charge (-0.617), whereas carbonyl
carbon is electrophilic (NBO charge = 0.852) allowing an
intramolecular nucleophilic attack. The system passes
through transition state 11TS, and the cyclic intermediate
12 is formed in this way. The NBO analysis of 12 revealed
single C–O bond in alkoxide. In accordance with this
finding, the oxygen bears strong negative charge (-0.916).
Thus, intramolecular abstraction of proton from the adja-
cent methylene group by the alkoxy group is expected.
This reaction step takes place via transition state 13TS,
yielding intermediate carbanion 14. This carbanion further
undergoes elimination of CH3OH, via transition state
15TS. 15TS is distinguished by the formation of the O–H
bond in methanol and cleavage of the C–O and O–H bonds
in intermediate 14 (Fig. 4). These alterations lead to the
formation of the carbanion 16, the existence of which was
confirmed by means of NMR spectra. Addition of HCl
invokes spontaneous abstraction of proton from hydronium
ion, giving the corresponding pyrazolone 1.
structures of the TSs appearing in mechanism A are pre-
sented in Fig. 5. This mechanism proceeds with the
abstraction of proton of the methylene group by OH-, via
transition state 3TS, leading to the formation of the cor-
responding carbanion 4. It further undergoes intramolecu-
lar cyclization by nucleophilic attack at the C-atom bearing
CH3S group. In transition state 5TS, the new C–C bond is
being formed, whereas the C–S bond is being cleaved,
implying that the CH3S- anion is eliminated in this step of
the reaction. The so-formed intermediate 6 further under-
goes tautomerisation, yielding thermodynamically more
favorable tautomeric equilibrium of corresponding 3-am-
inopyrazoles 7. However, experimental results show that
the reaction does not occur according to this pathway.
Keeping in mind this fact, our goal was to find out the
explanation by comparing two different mechanisms.
Comparison of mechanisms A and B
It is clear that acetylacetone mono-S-methylisothiosemi-
carbazone (2a) and methyl acetoacetate S-methylisothio-
semicarbazone (2b) conform to two different mechanisms:
A and B. In order to accomplish this challenging task we
investigated mechanism B starting from acetylacetone
mono-S-methylisothiosemicarbazone (Supplementary data).
Note that mechanism A for acetylacetone mono-S-methy-
lisothiosemicarbazone has already been investigated and
confirmed [12]. In Tables 2 and 3, all relevant relative free
energies concerning both mechanisms and both starting
compounds, as well as activation enthalpies and entropy
terms for operative mechanisms are listed.
The second path revealed by DFT calculations is
mechanism
A (Scheme 2, for compound 2b). The
Tables 2 and 3 reveal that all elementary steps are
enthalpy controlled, but not entropy controlled (DHàa [
-TDSàa). This implies that the rates of the reactions are
governed more by the bond strengths than by the issues of
orientation, trajectory, accessibility, etc. Table 2 shows
that the energetics concerning both compounds is very
similar. According to mechanism A, both reactions are
Fig. 4 Optimized geometries of the TSs figuring in mechanism B for
˚
compound 2b with crucial intramolecular distances (A) indicated
Fig. 5 Optimized geometries of the TSs figuring in mechanism A for
˚
compound 2b with crucial intramolecular distances (A) indicated
123

Struct Chem
Table 2 Activation parameters: enthalpies (DHàa), entropy terms (-TDSaà), and free energies (DGaà); and reaction free energies (DGr) in
elementary steps of mechanism A
Reaction steps
2a [12]
DHàa
2b
-TDSàa
DGàa
8.8
DGàr
-49.4
DGàa
DGr
-40.3
1. 2a, b ? HO- ? 3TS ? 4 ? H2O
2. 4 ? 5TS ? 6 ? CH3S-
3. Tautomerisation of 6–7
Overall reactiona
4.5
4.3
5.1
14.8
110.6
115.7
-72.4
-88.7
112.0
-75.3
-88.8
66.3
-210.6
71.8
-204.3
All energies are given in kJ/mol
a
In Tables 2 and 3 DGàa values for overall reactions were calculated relative to the corresponding RCs
Table 3 Activation parameters: enthalpies (DHàa), entropy terms (-TDSaà), and free energies (DGaà); and reaction free energies (DGr) in
elementary steps of mechanism B
Reaction steps
2a [12]
DGàa
2b
DGr
63.0
DHàa
-TDSàa
DGàa
DGàr
64.3
1. 2a, b ? HO- ? 8TS ? 9 ? CH3S-
2. 9 ? HO- ? 10 ? H2O
3. 10 ? 11TS ? 12
110.8
67.9
42.7
110.6
-80.5
9.9
-76.2
28.6
18.8
157.7
160.6
23.5
185.8
15.0
9.9
0.3
1.5
33.4
186.1
16.5
4. 12 ? 13TS ? 14
69.7
88.4
5. 14 ? 15TS ? 16 ? XH
6. 16 ? H3O? ? 1
-172.0
-189.3
-299.2
-211.3
-189.3
-295.5
Overall reaction
222.7
202.8
All energies are given in kJ/mol
Fig. 6 Energy profiles for mechanisms A and B. Energies were calculated relative to reactants. Solid lines represent operative mechanisms
exothermic and activation energies are moderately low.
The rate-determining step is elementary reaction 2. Taking
into account that step 1 is exothermic, the DGàa values for
the overall reactions are rather low. On the other hand,
mechanism B, in the case of both compounds, requires
significantly higher activation energies, whereas the reac-
tions are remarkably exothermic (Table 3). Some reaction
steps require significantly different activation barriers. This
particularly refers to step 5, where elimination of methanol
(for compound 2b) and methane (for compound 2a)
requires 16.5 and 160.6 kJ/mol, respectively. This differ-
ence can be attributed to much higher electronegativity of
oxygen over carbon, and to the very unfavorable orienta-
tion of the hydrogen of the hydroxyl group in regard to the
123

Struct Chem
carbon of the methyl group in 14 for compound 2a (Fig.
S24; see Fig. S15 for comparison). This hydrogen atom
takes such position to avoid the vicinity of partially posi-
tively charged hydrogens of the methyl and amino groups.
Experimental
Physical measurements
˚
The resulting intramolecular distance of 3.178 A between
Melting point was determined on a Mel-Temp capillary
melting points apparatus, model 1001 and is uncorrected.
Elemental (C, H, N, S) analysis of the samples was carried
out in the Center for Instrumental Analysis, Faculty of
Chemistry, Belgrade. IR spectrum was recorded on a Per-
kin Elmer Spectrum One FT-IR spectrometer with a KBr
disk, while HRMS analysis was performed on an Agilent
the reaction sites induces constrains for the formation of
15TS (for compound 2a). Taking into account that step 4 is
endothermic, it turns out that the DGàa value for the overall
reaction of transformation of acetylacetone mono-S-
methylisothiosemicarbazone according to mechanism B is
by around 20 kJ/mol higher in comparison to that of
methyl acetoacetate S-methylisothiosemicarbazone.
1
MSD TOF mass spectrometer. H- and 13C NMR spectra
In Fig. 6, the free energy profiles for transformation of
methyl acetoacetate S-methylisothiosemicarbazone and ace-
tylacetone mono-S-methylisothiosemicarbazone according to
both mechanisms A and B are depicted. In accordance with
experimental results, methyl acetoacetate S-methylisothiose-
micarbazone conforms to mechanism B. In this way, the
system surmounts higher activation barriers, but acquires
thermodynamically more stable product. On the other
hand, acetylacetone mono-S-methylisothiosemicarbazone
undergoes mechanism A. In this way, thermodynamically
less stable product is yielded, but energetically very unfavor-
able step, i.e., elimination of bad leaving group, methane,
is avoided. In other words, the reaction of methyl acetoacetate
S-methylisothiosemicarbazone is thermodynamically con-
trolled, whereas the reaction of acetylacetone mono-S-
methylisothiosemicarbazone is kinetically controlled.
were recorded on a Bruker Avance III 500 MHz spec-
trometer. The samples were recorded using CDCl3 and
DMSO-d6. The full assignments of all reported NMR sig-
nals were made by use of 1D and 2D NMR experiments.
X-ray crystallography
Single crystal of the compound was glued on a glass fiber
and mounted on a Gemini S diffractometer equipped using
Sapphire3 CCD detector (Agilent Technologies). Sealed
tube with Cu-target anode and graphite monochromator
˚
were used as source of X-radiation (Cu Ka, k = 1.5418 A).
Data were collected with x scan technique at room tem-
perature. Reflection intensities were integrated and cor-
rected for absorption (multi-scan) with CRYSALISPRO [26].
The structure was readily solved by direct methods
using SIR92 [27]. The full-matrix least-squares refinement
based on F2 was performed with SHELXL-97 [28] integrated
in SHELXLE graphical user interface [29]. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms bon-
ded to carbon atoms were introduced in idealized positions
and refined as riding with Uiso fixed as 1.2 or 1.5Ueq of the
parent atoms, while those bonded to nitrogen atoms were
determined from DF map and refined isotropically.
Hydrogen atoms belonging to the methyl group were found
to occupy two equivalent sites and were refined as ideally
disordered with 0.5 site occupancy factors. Crystallo-
graphic data are listed in Table S5 (Supplementary data).
Molecular graphics was plotted with ORTEP-3 [30] and
PLATON [31].
Conclusion
The reaction between S-methylisothiosemicarbazide
hydrogen iodide and methyl acetoacetate proceeds by the
formation of methyl acetoacetate S-methylisothiosemi-
carbazone. The exact structure of the final product was
determined by 1D and 2D NMR spectroscopy and single-
crystal X-ray diffraction analysis. The complete assigna-
tion of NMR data and positions of nitrogen atoms in the
structure of the compound was performed by means of
1
1H–15N HSQC and H–15N HMBC spectra. The mecha-
nism of transformation of methyl acetoacetate S-methy-
lisothiosemicarbazone was elucidated by density
functional theory, and compared to the previously repor-
ted mechanism of conversion of acetylacetone mono-
Crystallographic data for MOPC (1) have been depos-
ited with the Cambridge Crystallographic Data Centre as
Supplementary publication no. CCDC 902014. A copy of
this data can be obtained, free of charge, via www.ccdc.
cam.uk/data_request/cif, or by emailing data_request@ccdc.
cam.ac.uk.
S-methylisothiosemicarbazone into
a 3-aminopyrazole
derivative. It was concluded that the former reaction is
thermodynamically controlled. In the case of the latter
compound, the analogous mechanism would include
elimination of bad leaving group, methane (in contrast to
good leaving group, methanol), and because the reaction
of acetylacetone mono-S-methylisothiosemicarbazone is
kinetically controlled.
General procedure for the preparation of MOPC (1)
A mixture of thiosemicarbazide (3.00 g, 33.00 mmol) and
methyl iodide (5.68 g, 40.00 mmol) in ethanol (15 mL)
123

Struct Chem
was refluxed for 45 min. After cooling, white crystals of
S-methylisothiosemicarbazide hydrogen iodide were fil-
tered, washed with ethanol, and dried over CaCl2. Yield:
5.91 g (76 %). A mixture of S-methylisothiosemicarbazide
hydrogen iodide (2.80 g, 12.02 mmol), methyl acetoacetate
(1.42 g, 12.26 mmol), and ethanol (9 mL) in a 50-mL
round-bottomed flask was stirred for 1 h and left standing
for 48 h at the room temperature. Afterward, the solvent
was evaporated under reduced pressure giving S-methyli-
sothiosemicarbazone (2b), which is used in the next step
without further purification.
for the aqueous phase without any symmetry constraints.
The influence of water as solvent (e = 78.36) was estimated
by the conductor-like solvation model (CPCM) [36, 37].
TSs were searched by synchronous transit guided quasi-
Newton method [38]. The intrinsic reaction coordinates,
from the TSs down to the two lower energy structures,
were traced by the IRC routine in Gaussian to verify that
each saddle point is linked with corresponding reactant
complex (RC) and product complex (PC). The structure of
RC for each elementary reaction was obtained in the fol-
lowing way: the geometry which first results from pro-
gression backward along the reaction coordinate starting
from the TS was selected, and fully optimized without any
movement restriction. Similarly, the structure of PC for
each elementary reaction was determined by selecting the
geometry which first results from progression forward
along the reaction coordinate, and reoptimizing the selec-
ted geometry. The obtained geometries were verified, by
normal mode analysis, to be minima (no imaginary fre-
quencies) or maxima on the potential energy surface (one
imaginary frequency). NBO analysis [39] was performed
for all species.
1
Spectral data for 2b: H NMR (500 MHz, CDCl3): d
2.27 (s, 3H), 2.69 (s, 3H), 3.39 (s, 2H); 3.75 (s, 3H); 13C
NMR (125 MHz, CDCl3): d 14.91, 19.61, 43.63, 52.31,
156.15, 168.76, 173.87.
After the solvent was evaporated, water (15 mL) was
poured into the flask and the granulated NaOH (5.29 g,
13.22 mmol) was added immediately to the stirring mix-
ture. The reaction heat was evolved and the external
cooling was not applied. After 30 min of stirring, the
mixture was left standing at the room temperature for 24 h.
The formed enolate salt of pyrazol-5-one 16 was then fil-
tered off, dried over CaCl2. Yield: 1.05 g (54 %).
Spectral data for 16: 1H NMR (500 MHz, DMSO-d6): d
1.90 (s, 3H, CH3), 4.37 (s, 1H, H at C4), 6.73 (d, 1H,
J = 4.0 Hz, NH2), 9.49 (d, 1H, J = 4.0 Hz, NH2); 13C
NMR (125 MHz, DMSO-d6): d 15.03, 82.40, 151.09,
154.52, 166.28.
Acknowledgments The authors are grateful to the Ministry of
Education, Science and Technological Development of the Republic
of Serbia for financial support (Grant No. 172016).
The compound 16 was treated with 2 M HCl with stir-
ring. The solid was filtered off and dried over CaCl2 giving
the final compound MOPC (1). Yield: 0.68 g (75 %). The
suitable single crystals of 1 for single-crystal X-ray dif-
fraction analysis were obtained by recrystallization from
methanol.
References
1. Brogden RN (1986) Drugs 32:60–70
2. Chandrasekharan NV, Dai H, Roos KLT, Evanson NK, Tomsik J,
Elton TS, Simmons DL (2002) Proc Natl Acad Sci USA
99:13926–13931
3. Watanabe T, Tahara M, Todo S (2008) Cardiovasc Ther
26:101–114
4. Kimata A, Nakagawa H, Ohyama R, Fukuuchi T, Ohta S, Suzuki
T, Miyata N (2007) J Med Chem 50:5053–5056
5. Kakiuchi Y, Sasaki N, Satoh-Masuoka M, Murofushi H, Mura-
kami-Murofushi K (2004) Biochem Biophys Res Commun
320:1351–1358
Spectral data for 1: white powder; mp 175 °C; IR (KBr,
cm-1): 3309, 1708, 1643, 1556, 1386, 1350; 1H NMR
(500 MHz, DMSO-d6): d 2.11 (s, 3H, CH3), 5.06 (s, 1H, H
at C4), 7.68 (s, 1H, NH2), 8.34 (s, 1H, NH2), 12.21 (bs, 1H,
NH); 13C NMR (125 MHz, DMSO-d6): d 11.89 (CH3),
90.96 (C4), 149.43 (C3), 150.45 (C=O), 163.35 (C5). Anal.
Calcd. for C5H7N3O2 (141.13 g/mol): C, 42.55; H, 5.00; N,
29.77; Found: C, 42.39; H, 5.08; N, 29.50. HRMS (TOF,
m/z) [M ? H]?, Calcd. for C5H8N3O2: 142.13740; Found:
142.06110.
´
6. Brana MF, Gradillas A, Ovalles AG, Lopez B, Acero N, Llinares
F, Mun˜oz Mingarro D (2006) Bioorg Med Chem 14:9–16
7. Tripathy R, Ghose A, Singh J, Bacon ER, Angeles TS, Yang SX,
Albom MS, Aimone LD, Herman JL, Mallamo JP (2007) Bioorg
Med Chem Lett 17:1793–1798
8. Rajendran G (1999) Asian J Chem 11:153–161
9. Pal S, Mareddy J, Devi NS (2008) J Braz Chem Soc 19:
1207–1214
Computational method
10. Katritzky AR, Barczynski P, Ostercamp DL (1987) J Chem Soc
Perkin Trans 2:969–975
All calculations were carried out by the Gaussian 09 pro-
gram [32]. To provide the compatibility of the results of the
present investigation with those presented in our previous
paper [12], the calculations were performed at the B3LYP/
6-311??G(d,p) level of theory [33–35]. Geometry opti-
mizations for all species under investigation were achieved
11. Fox MA, Whitesell JK (2004) Nucleophilic addition and substi-
tution at carbonyl group, organic chemistry, 3rd edn. Jones and
Bartlett Publishers, Boston
´
´
´
12. Markovic S, Joksovic MD, Bombicz P, Leovac VM, Markovic V,
´
Joksovic Lj (2010) Tetrahedron 66:6205–6211
13. Llamas-Saiz AL, Foces-Foces C, Elguero J (1994) J Mol Struct
319:231–260
123

Struct Chem
14. Allen FH, Kennard O, Watson DG, Brammer L, Orpen AG,
Taylor R (1987) J Chem Soc Perkin Trans 2:S1–S19
29. Hu¨bschle CB, Sheldrick GM, Dittrich B (2011) J Appl Crystal-
logr 44:1281–1284
30. Farrugia LJ (1997) J Appl Crystallogr 30:565
´
15. Casas JS, Castan˜o MV, Castellano EE, Garcıa-Tasende MS,
Sanchez A, Sanjuan ML, Sordo J (2000) Eur J Inorg Chem
´
´
31. Spek AL (2009) Acta Crystallogr D 65:148–155
32. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA,
Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson
GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF,
Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K,
Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao
O, Nakai H, Vreven T, Montgomery JA Jr, Peralta JE, Ogliaro F,
Bearpark M, Heyd JJ, Brothers E, Kudin KN, Staroverov VN,
Kobayashi R, Normand J, Raghavachari K, Rendell A, Burant JC,
Iyengar SS, Tomasi J, Cossi M, Rega N, Millam JM, Klene M,
Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J, Gomperts
R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C,
Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth
GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD, Farkas
O, Foresman JB, Ortiz JV, Cioslowski J, Fox DJ (2009) Gaussian
09, Rev A1. Gaussian, Wallingford
33. Becke AD (1988) Phys Rev A 38:3098–3100
34. Lee C, Yang W, Parr RG (1988) Phys Rev B 37:785–789
35. Becke AD (1993) J Chem Phys 98:5648–5652
36. Barone V, Cossi M (1998) J Phys Chem A 102:1995–2001
37. Cossi M, Rega N, Scalmani G, Barone V (2003) J Comput Chem
24:669–681
38. Peng C, Ayala PY, Schlegel HB, Frisch MJ (1996) J Comput
Chem 17:49–56
39. Foster JP, Weinhold F (1980) J Am Chem Soc 102:7211–7218
83–89:2–9
16. Casas JS, Castellano EE, Ellena J, Garcıa-Tasende MS, Sanchez
´
A, Sordo J, Touceda A, Rodrıguez SV (2007) Polyhedron
´
´
26:4228–4238
17. Duan X-M, Fan M-L, Zheng P-W, Li J-S, Huang PM (2006) Acta
Crystallogr E 62:o2019
18. Sieler J, Kempe R, Hennig L, Becher J (1992) Z Kristallogr
198:313–314
19. Lorenzotti A, Marchetti F, Pettinari C, Pettinari R, Skelton BW,
White AH (2005) Inorg Chim Acta 358:3190–3200
20. Aleksanyan MS, Karapetyan AA, Oganisyan ASh, Struchkov
YuT (1997) J Struct Chem 38:989–992
21. Wang Q, Zhang Y, Wang R, Yang Y-L, Zhi F (2008) Acta
Crystallogr E 64:o1924
22. Foces-Foces C, Fontenas C, Elguero J, Sobrados I (1997) An
Quim 93:219
23. Etter MC (1990) Acc Chem Res 23:120–126
24. Desiraju G (1995) Angew Chem Int Ed 34:2311–2327
25. Allen FH, Baalham CA, Lommerse JPM, Raithby PR (1998) Acta
Crystallogr B 54:320–329
26. CrysAlisPro Software system, Version 1.171.35.19. Agilent
Technologies UK Ltd., Oxford
27. Altomare A, Cascarano G, Giacovazzo C, Gualardi A (1993)
J Appl Crystallogr 26:343–350
28. Sheldrick GM (2008) Acta Crystallogr A 64:112–122
123