Capto-Dative Stabilization by Thermal Oxidation of 2-Oxo-1,2,3,4-tetrahydropyrimidines

Article abstract of DOI:10.1071/CH16101

Various 4,6-diaryl substituted 2-oxo-1,2,3,4-tetrahydropyrimidines (THPMs) were oxidized to 2-oxo-1,2-dihydropyrimidines (DHPMs) by potassium peroxydisulfate (PPS) in aqueous acetonitrile solution under thermal conditions. Based on the proposed oxidation

Full text of DOI:10.1071/CH16101

CSIRO PUBLISHING
Aust. J. Chem.
Full Paper
http://dx.doi.org/10.1071/CH16101
Capto-Dative Stabilization by Thermal Oxidation of
2-Oxo-1,2,3,4-tetrahydropyrimidines*
A C
,
A
A
Hamid R. Memarian,
and Giuseppe Bruno
Esmael Sanchooli, Hadi Amiri Rudbari,
B
A
Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Iran.
B
Department of Chemical Sciences, University of Messina, Via F. Stango d’Alcontres 31,
98166 Messina, Italy.
C
Corresponding author. Email: memarian@sci.ui.ac.ir
Various 4,6-diaryl substituted 2-oxo-1,2,3,4-tetrahydropyrimidines (THPMs) were oxidized to 2-oxo-1,2-dihydropy-
rimidines (DHPMs) by potassium peroxydisulfate (PPS) in aqueous acetonitrile solution under thermal conditions. Based
on the proposed oxidation reaction mechanism by way of a radical, a capto-dative stabilized radical intermediate, among
two possible formed double benzylic/allylic radical centres, governs the type of product formed. Whereas the electron-
donating nature of the additional methoxy-substituent enhances the rate of oxidation, its attachment to the radical
intermediate decreases the radical stability, simultaneously causing the shift of the radical centre to the capto-dative
stabilized benzylic radical centre. The data of the density functional theory computational studies concerning the bond
lengths to the radical centres and Mulliken population analysis support the results of the experimental work.
Manuscript received: 21 February 2016.
Manuscript accepted: 22 February 2016.
Published online: 23 March 2016.
heterocyclic ring on the rate of thermal oxidation,[19–21] photooxi-
dation,[22–24] and voltammetric studies (Scheme 1).[25,26] The
results of these studies indicate that the electronic nature of the
4-substituent has more effect on the rate of oxidative processes
compared with those of the 5-substituent.
Introduction
2-Oxo-1,2,3,4-tertahydropyrimidines (THPMs) or dihydro-
pyrimidinones, also known as Biginelli compounds,[1] and the
corresponding oxidized derivatives, namely 2-oxo-1,2-dihydro-
pyrimidines (DHPMs) or 2-pyrimidinones, display various
biological and pharmacological activities.[2–6] 5-Unsubstituted
THPMs, known as Biginelli-like compounds, and the corre-
sponding dehydro-compounds also exhibit pharmacological
activities.[7,8] In contrast to various methods for the synthesis of
5-substituted THPMs,[9] there are few reports on the synthesis of
5-unsubstituted THPMs. These compounds are normally syn-
thesized via a one-pot cyclo-condensation of an aldehyde, an
enolizable carbonyl compound, and urea or thiourea in the
presence of a suitable catalyst,[10] or they are obtained by con-
densation of urea or thiourea with chalcones.[11]
The readily available peroxydisulfate ion (S2O28ꢀ) alone or in
the presence of a metal ion co-catalyst is an excellent and
versatile oxidizing agent of a variety of organic and inorganic
compounds.[12–15] Thermal or photochemical O–O bond cleavage
of this ion leads to the formation of two sulfate radical ions, which
are able to abstract hydrogen or to accept an electron from a
hydrogen-donating molecule or electron-donating species, respec-
tively, present in the medium.[16–18]
Based on the proposed mechanism for the thermal oxidation
of 5-carboethoxy- and 5-acetyl-THPMs by potassium peroxy-
disulfate (PPS) in aqueous acetonitrile solution,[19,20] the
hydrogen attached to the C4-atom of the heterocyclic ring is
removed in the rate determining step by an in situ formed
hydroxyl radical. This hydrogen removal process results in the
formation of benzylic- and allylic-radical intermediates, namely,
the trihydropyrimidinyl radical (TrHPMꢁ). Owing to the greater
stability of the benzylic radical intermediate, further removal of a
second hydrogen atom from the 3-NH position results in the
formation of DHPMs (Scheme 2). Introducing the hydroxyl
radical as an active species in the oxidation reactions by PPS is
also supported by the oxidation of folic acid[15] or hydroxymethy-
lation of quinoxalines in methanol,[16] known as the Minisci
reaction.
In continuation to these works, we extended our investiga-
tions on 5-unsubstituted THPMs, in which two different aryl
groups are located at the 4- and 6-positions of the heterocyclic
ring. It is expected that in the new system involving these
4,6-diaryl-THPMs, due to the absence of any substitution at
the 5-position: i) the steric hindrance caused by the substituent in
5-substituted THPMs is omitted and ii) the presence of the aryl
In the course of our studies concerning the chemistry of
THPMs, we have investigated the steric and electronic effects
of the substituent at the 4- (various aryl groups) and 5-positions
(5-acetyl, 5-carboethoxy, and 5-carboxamide groups) of the
*Dedicated to Professor Dr Dietrich Do¨pp, University of Duisburg-Essen, Germany, on the occasion of his 80th birthday.
Journal compilation Ó CSIRO 2016
www.publish.csiro.au/journals/ajc

B
H. R. Memarian et al.
O
C
O
C
Ar
Ar
H
1
Results and Discussion
H
Under optimized reaction conditions a mixture of 0.23 mmol of
1a–g and 0.23 mmol K2S2O8 (mol ratio 1 : 1) in 6 mL of CH3CN/
H2O (5/1) was reacted in a preheated oil bath at 808C. The
progress of the reaction was followed by TLC until total dis-
appearance of 1a–g (Scheme 4). The results are presented in
Table 1.
4
N
N
3
2
R
R
5
[O]
6
N
H
O
O
H3C
N
H3C
H
R ꢀ CH3, OC2H5, NHAr
Scheme 1. 2-Oxo-1,2,3,4-tetrahydropyrimidines.
The results presented in Table 1 indicate that by oxidation of
the parent molecule 1a and removal of two hydrogens, the
compound 2a (4-Ph/6-Ph, the same as 3a) is formed. While for
the oxidation of 1b–g with a methoxy group at the ortho-, meta-,
or para-positions of the 4- or 6-aryl rings, independent of the
location of the substituted aryl ring, either at the 4- or at
the 6-position of the heterocyclic ring, a sole product with the
methoxy-substituted aryl ring attached to the C-atom of the
heterocyclic ring neighbouring the NH moiety is formed. This
Δ
K2S2O8
2KOSO3
KHSO4 ꢂ OH
2KOSO3 ꢂ H2O
O
C
O
C
O
C
Ar
H
Ar
Ar
H
H
H
N
R
N
R
ꢂOH
N
R
was confirmed by the observation of the same IR, 1H NMR, 13
C
ꢁH2O
N
H
NMR, and UV spectra of the products 3b, 3c, and 3d formed by
the oxidation of 1b or 1e, 1c or 1f, and 1d or 1g, respectively (the
spectra are presented in the Supplementary Material). The
characterization of the products 2a and 3b–d was achieved by
comparison of their IR, 1H NMR, 13C NMR, and UV data with
those of the starting materials 1a–g as follows:
N
H
H3C
O
H3C
O
N
H
O
H3C
TrHPM (benzylic/allylic)
O
TrHPM (allylic)
THPM
Ar
C
N
ꢁH
R
1. A comparison of the IR spectra showed a decrease in the
intensity of the NH vibration with a small shift to lower
frequency. Due to the oxidation of the heterocyclic ring and
formation of a semi-aromatic heterocyclic ring, the carbonyl
stretching is shifted to higher frequency.
N
H
H3C
O
DHPM
1
2. A comparison of the H NMR spectra with those of the
Scheme 2. Benzylic/allylic and allylic radical intermediates involved in
starting materials 1a–g showed the lack of the resonances of
4-CH and one of the NH protons and simultaneous shift of
another NH resonance to lower field due to oxidation of the
heterocyclic ring.
the oxidative process.
Ar1
4
3. A comparison of the 13C NMR spectra showed an increase of
peaks in the aromatic region.
H
N
2
3
5
6
4. A comparison of the UV spectra (Table 2) showed a bath-
ochromic shift in the UV spectra of the products due to
oxidation of the heterocyclic ring and formation of the amino
conjugated dienone system.
Ar2
O
N
1
4-Ar1/6-Ar2
H
ꢁH
Ar1
Ar1
Ar1
4
H
H
O
H
H
H
H
H
·
·
N
ꢂOH
N
N
5
6
3
Mechanistic Studies
·
ꢁH2O
2
Since monitoring by TLC is not a confident way to explain the
electronic effect of the substituent at the 4- or 6-positions of the
heterocyclic ring on the rate of reaction, especially in the case of
reactions having short reaction times (the present study), the
reactions of 1b and 1e with a p-methoxyphenyl substituent at
the 4- and 6-position of the heterocyclic ring, respectively, were
followed by UV spectroscopy. The slopes in the extinction
versus time diagrams (ET diagrams)[27] at 380 nm for these
compounds (Fig. 1) derived from their UV reaction spectra
indicate that compound 1b has a comparatively higher reaction
rate constant (7.2 ꢂ 10ꢀ3 mol Lꢀ1 sꢀ1) than 1e (6.0 ꢂ 10ꢀ3 mol
O
N
H
Ar2
Ar2
O
N
H
N
H
Ar2
1
·
TrHPM
·
Ar1
6
THPM
ꢁH
H
H
5
N
2
1
4
Ar2
N
3
O
6-Ar1/4-Ar2
Scheme 3. Involvement of two different benzylic/allylic radical inter-
mediates in the oxidative process.
L
ꢀ1 sꢀ1). These data are supported by the observed experimental
reaction times of 15 and 20 min for the total disappearance of 1b
and 1e, respectively.
group instead of a CH3 group on the 6-position should influence
the electron density of the heterocyclic ring, depending on the
electronic nature of the additional substitutent on the aryl rings.
Therefore, the oxidation of the new compounds by PPS should
pass through two different benzylic/allylic radical inter-
mediates, which may result in the formation of two different
DHPMs, namely 4-Ar1/6-Ar2 and/or 6-Ar1/4-Ar2 (Scheme 3).
The observed electronic effect in the shortening of the reaction
time for total disappearance of 1b compared with that of 1e by
PPS in the present study can be explained by considering the
reaction mechanism presented in Schemes 2 and 3. Based on the
proposed mechanism, hydrogen removal from the 4-position of
the heterocyclic ring in the rate determining step results in the

Capto-Dative Radical Stabilization
C
Ar1
4
Ar1
Ar1
H
6
H
H
H
5
H
4
H
N
3
N
2
5
N
2
3
5
4
1
80
ꢃC
and/or
ꢂK2S2O8
6
6
CH3CN/H2O
2
O
Ar2
N
1
O
Ar2
Ar2
N
3
N
H
O
1
H
(ꢁ
H
H)
(−HH)
2
3
1
Scheme 4. Oxidation of 4,6-diaryl-2-oxo-1,2,3,4-tetrahydropyrimidines.
Table 1. Oxidation of 4,6-diaryl-THPMs (1a–g) by PPS under thermal conditions to 2-oxo-1,2-dihydropyrimidines (DHPMs)A
THPM
Ar1
Ar2
DHPM
Ar1
Ar2
TimeB [min]
YieldC [%]
1a
1b
1c
1d
1e
1f
C6H5
C6H5
2a
3b
3c
3d
3b
3c
3d
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
40
15
15
26
20
15
21
78
82
77
87
81
62
88
4-CH3OC6H4
3-CH3OC6H4
2-CH3OC6H4
C6H5
C6H5
4-CH3OC6H4
3-CH3OC6H4
2-CH3OC6H4
4-CH3OC6H4
3-CH3OC6H4
2-CH3OC6H4
C6H5
C6H5
4-CH3OC6H4
3-CH3OC6H4
2-CH3OC6H4
C6H5
1g
C6H5
A0.23 mmol THPMs, 0.23 mmol PPS in 6 mL CH3CN/H2O at 808C.
BTimes are given after total disappearance of THPMs.
CIsolated yield after recrystallization.
Table 2. Comparison of the UV-absorption (lmax, nm) of 1a–g with those of the products in chloroform solution
THPM
lmax (log e)
DHPM
lmax (log e)
1a
1b
1c
1d
1e
1f
350 (sh, 2.56), 278.8 (3.77), 246 (3.90)
355 (sh, 2.53), 279.4 (3.80), 243.6 (3.79)
300 (sh, 3.27), 283.2 (3.58), 245 (3.81)
300 (sh, 3.29), 275 (3.76), 245 (3.24)
355 (sh, 2.78), 280.6 (3.74), 245.4 (3.80)
335 (sh, 2.04), 277.2 (3.71), 250 (3.22)
300 (sh, 3.55), 281.4 (3.66), 250 (3.48)
2a
360 (sh, 3.78), 346.2 (3.92), 280.2 (4.00), 263.4 (3.95), 253.4 (3.97)
360 (4.07), 346.2 (4.15), 290 (sh, 3.97), 248.2 (4.05)
3b
3c
375 (sh, 3.81), 356.5 (4.05), 330 (4.03), 298 (4.04), 257 (4.04)
380 (sh, 3.88), 361.5 (4.06), 336 (4.00), 300 (sh, 3.89), 257 (3.99)
360 (4.04), 346.2 (4.18), 290 (sh, 3.97), 249.6 (4.04)
3d
3bA
3cB
3dC
375 (sh, 3.78), 354.5 (4.03), 330.5 (4.01), 297.5 (3.99), 257.5 (4.00)
380 (sh, 3.92), 363 (4.09), 335 (4.00), 300 (sh, 3.91), 257 (4.00)
1g
AThe product is obtained by oxidation of 1e.
BThe product is obtained by oxidation of 1f.
CThe product is obtained by oxidation of 1g.
0.10
0.08
0.7
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.6
0.5
0.4
0.3
0.2
0.1
0
y ꢀ 0.006x ꢂ 0.0249
0.06
1e
1b
0.04
y ꢀ 0.0072x ꢂ 0.0134
0.02
0
250
300
350
400
0
4
8
12
250
300
350
400
λ [nm]
Time [min]
λ [nm]
Fig. 1. Reaction spectra of thermal oxidation of 1b (left) and 1e (middle) by PPS in acetonitrile/water and the corresponding extinction time (ET) diagrams;
Time interval 10 min.
formation of a double benzylic/allylic TrHPMꢁ intermediate.
Further removal of the second hydrogen from this intermediate
causes the formation of the DHPM molecules, namely, 4-Ar1/
6-Ar2 and/or 6-Ar1/4-Ar2. The methoxy group at the ortho/para-or
meta-position of the 4-aryl ring behaves as a s-donor or
s-acceptor group due to the attachment of the 4-aryl ring with
the sp3 C-atom of the heterocyclic ring, respectively. While due
to the conjugation of the 6-aryl ring with the C6=C5 double bond,

D
H. R. Memarian et al.
+
CH3
CH3
CH3
+
Table 3. The oxidation peak potential (Ep), versus
·
·
·
· O
ꢂO
ꢂO
ferrocene redox potential (0.583 V), obtained from cyclic
voltammograms in acetonitrile for THPMs (1a–g)
·
·
δ
·
OH
Compound
Ep [V]
OH
H
H
H
H
1a
1b
1c
1d
1e
1f
1.274
1.154
1.204
1.248
1.165
1.245
1.181
H
H
O
H
H
H
·
δ
N
N
N
N
H
N
H
N
H
Ph
Ph
O
Ph
O
1b
OCH3
OCH3
OCH3
1g
theory, the presence of an electron-donating substituent on the
olefinic C=C double bond causes an increase of both HOMO and
LUMO levels, while the connection of the C=C double bond with
the electron-withdrawing groups decreases these levels.[28]
Keeping these effects in mind, the stability of a radical centre
is dependent on its possible interactions with the attached
a-substituents. These interactions may be delocalization of the
single electron over the attached conjugating p-system, hyper-
conjugation with the attached alkyl substituent, or interaction
with the attached electron-withdrawing or electron-donating
substituents.[29] Such interactions with a-substituents, which
result in the stability of the radical centres, are rationalized in
term of one-electron molecular orbital (OEMO) theory.[29]
The concept of capto-dative radical stabilization, in which
the synergetic effect of the electron-donor (dative) and the
electron-acceptor (capto) substituent on the same radical centre
leads to an enhanced stabilization compared with that expected
from the sum of the stabilization energies of the separate
substituents, has been proposed by Viehe and co-workers.[30]
The origin of this extra stabilization is easily explained using a
perturbational molecular orbital (PMO) approach.[31] Simulta-
neous interaction of the singly occupied molecular orbital
(SOMO) of the radical centre with the p-acceptor or p-donor
results in the enhanced stability of this single electron.[32,33]
Based on the proposed mechanism, the hydrogen in the
4-position is removed by the in situ formed hydroxyl radical
in the rate determining step under formation of the TrHPMꢁ
intermediate. There are two types of substituted THPMs consi-
dered in the present study. In the first type (compounds 1b, 1c,
and 1d), the methoxy-substituted aryl ring is attached to the
C4-atom of the heterocyclic ring, whereas in the second type
(compounds 1e, 1f, and 1g), the substituted aryl ring is attached
to the C6-atom. Upon delocalization of the single electron in the
two different TrHPMꢁ intermediates, the two double benzylic/
allylic radicals are converted into each other (Scheme 6).
Further removal of the second hydrogen atom from one of the
N1- or N3-positions in these intermediates results in the forma-
tion of the final DHPM product, namely, 4-Ph/6-Ar or 4-Ar/
6-Ph. The important decision factor in the second hydrogen
removal is reasonably the stability of one of these radical
intermediates as a more stable and preferred candidate involved
in the second hydrogen removal process. In both these radical
intermediates, the radical centre is attached to the nitrogen
atom in the heterocyclic ring. Based on the mentioned OEMO
theory,[29] the radical centre can be stabilized by interaction with
the nitrogen lone pair, as an electron-donating substituent. The
important point on increasing the stability of the radical centre is
additional interaction either with the methoxy-substituted aryl
ring, again as an electron-releasing substituent, or interaction
H
H
·
H
O
H
O
H
H
O
·
N
N
ꢁH2O
N
ꢁH
·
N
H
N
H
Ph
Ph
N
Ph
More stable
Less stable
3b
Scheme 5. Contribution of the oxygen lone pair to facilitate the 4-H
removal by hydroxyl radical.
this group at the mentioned positions of the 6-aryl ring behaves
as a p-donor or p-acceptor, respectively, if the 6-aryl ring can be
orientated co-planar to the C6=C5 double bond of the hetero-
cyclic ring. The balance of the negative inductive effects and the
positive electron-pushing resonance effects of the methoxy
group at the para-position of the phenyl ring at the C4-position
of the heterocyclic ring (compound 1b), makes this substituent
an electron-donating species. This causes an electron population
on the aromatic carbon atom attached to the C4-atom of the
heterocyclic ring (Scheme 5). Therefore, as expected, the
homolytic hydrogen removal from 4-CH by a hydroxyl radical
in the rate determining step will be facilitated compared with
that in compound 1a containing the normal phenyl ring at this
position. This is illustrated in Scheme 5.
Cyclic voltammetric measurements of compounds considered
in the present study elucidate the steric and electronic effects of
the methoxy substituent on the oxidation peak potentials
(Table 3). A comparison of the measured oxidation peak poten-
tials shows clearly that the location of the methoxy group at the
right position (para-position, compound 1b), which does not
sterically hinder the conjugation of its lone pairs with the phenyl
ring, facilitates the electron detachmentprocess. Incontrasttothis
observation, due to the steric hindrance of the lone pairs of this
group at the 2-position (ortho-position, compound 1d) to conju-
gate, the group’s electron donation effect on the lowering of the
oxidation peak potential is retarded. In this case only an upward
orientation of the methyl group allows the lone pairs of the
oxygen atom to conjugate with the phenyl ring. These data also
explain why all methoxy-substituted THPMs (1b–g) are oxidized
faster than the parent compound 1a, which supports the obtained
oxidation times in the experimental work.
The surprising results obtained in the present study are that the
sameproducts 3b, 3c, and 3dare observedupon oxidation of1b or
1e, 1c or 1f, and 1d or 1g, respectively. This can be explained by
the influence of the electronic effect of the additional substituent
on the electron-donating ability of the substituted aromatic ring
and on the stability of the radical intermediate involved in this
oxidative reaction. According tofrontiermolecularorbital (FMO)

Capto-Dative Radical Stabilization
E
Ph
H
4
··
·
N
3
··
6
N
Ar
O
1
Ph
H
Ph
H
H
··
··
·
H
Ph
ꢂOH
4
N
N
·
3
··
N
4
··
3
ꢁH2O
5
N
6
O
Ar
N
O
Ar
1
2
P
h
H
N
H
6
H
1
Ar
O
4
H
··
N
4-Ph/6-Ar
3
·
··
6
N
O
Ar
4-Ph/6-Ar
1
·
H
_
H
and/or
180ꢃ rotation
Ar ꢀ CH3OC6H4
of view
Ar
4
H
··
·
N
P
h
3
··
6
N
1
H
6
O
Ph
1
5
N
Ar
H
2
Ar
H
··
H
N
3
4
H
··
Ar
O
4
N
·
N
·
3
ꢂOH
··
··
N
N
6
ꢁH2O
P
h
O
Ph
O
1
4-Ar/6-Ph
H
Ar
H
4
H
··
4-Ar/6-Ph
N
3
·
··
6
P
h
N
O
1
H
Scheme 6. Hydrogen abstraction by the hydroxyl radical leading to the possible formation of two different
products.
with the phenyl ring, as an electron-withdrawing substituent.
The concept of the capto-dative effect explains that the radical
centre attached to the phenyl ring, as a capto-dative stabilized
radical centre, is more stable than the other radical attached to
the methoxy-substituted aryl ring.[32,33] This means that simul-
taneous interactions of the radical centre with the nitrogen lone
pair (electron donor, dative) and the phenyl ring (electron
acceptor, capto) support the suggestion that the capto-dative
stabilized radical centre is the preferred radical intermediate
involved in the second hydrogen removal, therefore, the forma-
tion of 4-Ph/6-Ar DHPMs is expected.
shorter than the bond length with the substituted aryl ring. The
radical centre in 1aꢁ is a double benzylic/allylic radical either on
the C4- or C6-atoms and is delocalized with the phenyl rings at
the 4- or 6-positions of the heterocyclic ring, therefore, the C4–
˚
C8 and the C6–C14 bond lengths are 1.4574 A. This is illustrated
in Fig. 2. Since the radical centre obtained by the hydrogen
removal from the remaining THPMs (1b–g) is more delocalized
with the phenyl ring than with the substituted aryl ring, the
C–Cphenyl bond length is shorter than C–Caryl bond length. In a
true allylic radical and in the absence of any additional stabili-
zation or delocalization of the single electron, which is con-
sidered as a symmetrical allylic radical due to the delocalization
of the single electron over three allylic carbon atoms, the
electron density on both ends of the radical is equal to 0.5 in the
C2 molecular orbital and both of the C1–C2 and C2–C3 bonds
have the same partial double-bond character (Scheme 7). Such a
situation is observed in the case of 1aꢁ, which is bonded to the
phenyl ring at both ends of the delocalized radical, as a
symmetrical double benzylic and allylic radical centre in the
present study. In 1aꢁ, both C6–C5 and C5–C4 bonds have equal
Computational Studies
Density functional theory (DFT) at the B3LYP/6–31þþG(d,p)
level using the Gaussian98 package[34] was applied to study
structural, electronic, and bonding characteristics of THPMs,
the TrHPMꢁ intermediate, and the corresponding DHPM
products considered in the present study. The numerical results
of the DFT study concerning the selected bond lengths and the
dihedral angles related to the orientation of the aryl rings
towards the heterocyclic ring are extracted from the optimized
structures and are listed in Table 4. A comparison of the bond
lengths in the TrHPMꢁ intermediate, especially the bond length
of the C-radical centre with the substituted and unsubstituted
aryl ring, indicates that due to effective delocalization of the
single electron towards the phenyl ring, the bond length is
˚
distances of 1.3932 A and the Mulliken spin densities on the C4-
and C6-atoms are 0.3946. This situation has been changed by
attachment of the carbon atoms of the conjugated radical centre
to the phenyl and methoxy-substituted phenyl rings (aryl ring).
In this case an asymmetrical double benzylic radical interme-
diate is formed, which is supported by the observed different

F
H. R. Memarian et al.
ꢁ
˚
Table 4. Bond lengths (A) and dihedral angles (8) of 4,6-diaryl-THPMs, trihydropyrimidyl radical (TrHPM ) and DHPMs obtained by using the
B3LYP/6–3111G(d,p) calculations
X
11
17
11
11
12
X
10
9
12
13
12
13
18
19
16
15
10
9
X
10
13
14
8
8
4
8
1a•: X
1b• : X
1c•: X
1d•: X
1e•: X
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
H, Y
OMe, Y
OMe, Y
OMe, Y
H, Y
ꢀ
H
9
H
4
-
ꢀ
ꢀ
ꢀ
H
H
H
1a: X ꢀ H, Y ꢀ H
H
H
H
4
3-
1b: X ꢀ 4-OMe, Y ꢀ H
1c: X ꢀ 3-OMe, Y ꢀ H
1d: X ꢀ 2-OMe, Y ꢀ H-
1e: X ꢀ H, Y ꢀ 4-OMe
1f: X ꢀ H, Y ꢀ 3-OMe
1g: X ꢀ H, Y ꢀ 2-OMe
2a: Y
ꢀ H
3b: X ꢀ 4
3c: X ꢀ 3
3d: X ꢀ 2
-
OMe
5
6
N
N3
2
5
6
6
3
5
5
6
N3
2
N
4
2-
3e
3f
ꢀ
ꢀ
3b: Y
3c: Y
ꢀ
ꢀ 3-
4
-
OMe
OMe
OMe
1
-
OMe
19
16
19
14
ꢀ
4-
OMe
2
17
16
4
13
10
2
-
OMe
1f•: X
ꢀ
H, Y
ꢀ
3-OMe
3g
O
7
ꢀ 3d: Y ꢀ 2-
18
17
N1
H
O
N
18
17
O
7
19
18
O
7
N1
H
1g•: X
ꢀ
H, Y
ꢀ
2
-
OMe
N
7
12
11
1
14
8
14
3
H
15
15
15
9
Y
16
Y
Y
Comp.
N1–C2
C2–N3
N3–C4
C4–C5
C5–C6
N1–C6
C2–O7
C4–C8
C6–C14
b1A
b2B
g1C
g2D
1a
1aꢁ
1.3889
1.3823
1.4183
1.3876
1.3813
1.4191
1.3878
1.3816
1.4189
1.3952
1.3856
1.4161
1.3874
1.3828
1.4191
1.3867
1.3822
1.4189
1.3896
1.3769
1.4159
1.3680
1.3823
1.3747
1.3714
1.3831
1.3762
1.3685
1.3827
1.3766
1.3656
1.3770
1.3773
1.3693
1.3813
1.3762
1.3716
1.3828
1.3764
1.3703
1.3843
1.3771
1.4680
1.4051
1.3207
1.4726
1.4064
1.3219
1.4687
1.4054
1.3214
1.4737
1.4042
1.3213
1.4698
1.4069
1.3219
1.4711
1.4060
1.3215
1.4685
1.4029
1.3213
1.5093
1.3932
1.4306
1.5125
1.3908
1.4301
1.5094
1.3923
1.4314
1.5049
1.3975
1.4286
1.5090
1.3964
1.4299
1.5115
1.3941
1.4318
1.5116
1.3900
1.4289
1.3462
1.3932
1.3768
1.3469
1.3964
1.3760
1.3461
1.3937
1.3738
1.3469
1.3911
1.3781
1.3467
1.3901
1.3758
1.3464
1.3923
1.3741
1.3476
1.3999
1.3783
1.4026
1.4051
1.3629
1.4017
1.4073
1.3627
1.4024
1.4058
1.3623
1.4029
1.4042
1.3600
1.4026
1.4059
1.3628
1.4012
1.4054
1.3622
1.4037
1.4039
1.3601
1.2290
1.2248
1.2245
1.2289
1.2254
1.2247
1.2295
1.2252
1.2244
1.2291
1.2264
1.2255
1.2294
1.2253
1.2247
1.2288
1.2250
1.2245
1.2295
1.2266
1.2256
1.5312
1.4574
1.4932
1.5246
1.4574
1.4907
1.5327
1.4582
1.4908
1.5357
1.4614
1.4918
1.5322
1.4545
1.4911
1.5273
1.4567
1.4904
1.5266
1.4589
1.4917
1.4837
1.4574
1.4862
1.4835
1.4544
1.4758
1.4835
1.4569
1.4806
1.4839
1.4580
1.4835
1.4814
1.4585
1.4767
1.4842
1.4587
1.4804
1.4867
1.4602
1.4831
ꢀ52.1
71.2
157.4
180.0
67.5
ꢀ37.5
ꢀ21.1
0.0
140.2
157.4
21. 2
0.0
2a
180.0
1b
1bꢁ
3b
1c
ꢀ54.35
ꢀ21.3
16.1
37.7
ꢀ140.8
ꢀ158.0
143.5
155.7
18.9
ꢀ162.7
72.4
ꢀ35.2
ꢀ37.5
ꢀ20.7
38.2
ꢀ51.3
ꢀ22.8
ꢀ15.4
68.5
140.5
1cꢁ
155.4
164.2
ꢀ167.3
ꢀ159.3
ꢀ161.9
67.4
157.8
3c
141.1
1d
1dꢁ
3d
1e
38.1
ꢀ139.8
ꢀ156.6
ꢀ147.9
140.5
23.5
21.4
17.5
33.9
ꢀ56.4
ꢀ19.7
ꢀ16.6
ꢀ53.9
ꢀ20.2
ꢀ14.4
ꢀ54.0
ꢀ21.7
ꢀ15.6
ꢀ37.8
ꢀ23.5
ꢀ35.7
38.6
1eꢁ
158.7
163.1
67.9
155.2
3e 5 3b
1f
1fꢁ
143.6
ꢀ139.7
ꢀ155.3
ꢀ140.4
ꢀ138.4
ꢀ161.4
ꢀ147.4
156.8
164.7
67.6
21.6
3f ¼ 3c
38.2
1g
1gꢁ
44.6
155.5
163.5
19.8
3g 5 3d
33.9
AN3–C4–C8–C9 dihedral angle; BC4–C5–C8–C9 dihedral angle; CN1–C6–C14–C15 dihedral angle; DC5–C6–C14–C15 dihedral angle.
Mulliken spin densities on the C4- and C6-atoms and the
different double bond characters of the C6–C5 and C5–C4 bonds
shown in Fig. 2. The greater Mulliken spin density of 0.4040 on
the C6-atom in 1bꢁ compared with 0.3973 on the C4-atom of this
radical and also the greater Mulliken spin density of 0.3946 on
the C4-atom in 1eꢁ compared with 0.3922 on the C6-atom of this
radical and different partial double-bond character of the C6–C5
and C5–C4 bonds support our argument that due to the electron-
withdrawing character of the phenyl ring, the benzylic radical
centre with the phenyl ring as a part of the capto-dative stabi-
lized radical centre is more stabilized, therefore, the elimination
of the hydrogen atom from the neighbouring NH moiety to this
radical centre leads to the formation of 4-Ph/6-Ar DHPMs
(Scheme 6).
The sum of the Mulliken spin densities on both benzylic
radical parts, namely, on the C4, C9, C11, and C13 atoms (Part I),
and on the C6, C15, C17, and C19 atoms (part II) in radicals 1aꢁ,
1bꢁ, and 1eꢁ are 0.7607 and 0.7607, 0.7080 and 0.8073, and
0.7879 and 0.6969, respectively. A comparison of these data
indicates that in the case of the symmetrical benzylic radical 1aꢁ,
the sum of the spin densities on both parts is equal to 0.7607,
whereas for unsymmetrical benzylic radicals 1bꢁ and 1eꢁ the
single electron is more delocalized over the phenyl ring than
over the p-methoxyphenyl ring, which displays a greater sum of
the spin density on the benzylic radical attached to the phenyl
group than the benzylic radical attached to the p-methoxyphenyl
group. These results support the argument that the behaviour of
the phenyl group as an electron-withdrawing member of the
capto-dative radical involved preferable removal of the second
hydrogen.
The structure of 3d obtained by the oxidation of 1d was
confirmed by X-ray diffraction analysis. This crystal structure
presented in Fig. 3 indicates that the 2-methoxyphenyl substituent
is located on the 6-position of the heterocyclic ring next to the NH
moiety.
Conclusion
Thermal oxidation of some methoxy-substituted 4,6-diaryl-
THPMs was achieved using PPS in aqueous acetonitrile solu-
tion. The results indicate that a capto-dative stabilized radical
intermediate among two possibly formed double benzylic/
allylic radical centres governs the type of product formed. The
data of the DFT computational studies concerning the bond
lengths to the radical centres and Mulliken population analysis
support the results of the experimental work.
Experimental
The 4,6-diaryl-substituted THPMs considered for this study
were synthesized according to the reported procedure.[35]
Melting points were determined on a Stuart Scientific SMP2
apparatus and were not corrected. IR spectra were recorded

Capto-Dative Radical Stabilization
G
0.1352
11
12
11
12
13
1.3
10
9
13
10
8
0.1305
0.1004
9
1.5093H
8
1.5312
H
1.4574
0.3946
H
4
ꢁ0.1919
932
N
3
5
6
1.3462
4
N
3
19
16
2
0.3946
0.1004
5
1.3932
18
17
2
19
N
H
O
6
7
1
1.4837
14
18
O
7
N
H
1
14
1.4574
15
15
17
0.1352
0.1304
16
1a
1a•
OCH3
OCH3
11
0.0973
12
11
10
12
13
10
9
13
0.0830
0.1304
8
8
9
1.5125H
1.5246
1.3908
0.3973
H
1.4574
ꢁ
0.1887
H
4
N
3
4
N
5
3
0.4040
1.3469
5
1.3964
2
19
2
19
6
0.1091
18
6
N
O
18
17
N
H
O
7
1
7
1
14
1.4544
14
1.4835
15
H
17
0.1494
15
0.1448
16
16
1b
12
1b•
11
0.1479
11
10
13
12
10
8
9
9
13
0.1407
H
0.1047
8
1.5090
1.5322
H
4
1.4545
0.3946
H
1.3964
N
3
5
ꢁ0.1885
1.3467
4
2
19
6
N
3
0.3922
0.1032
5
1.3901
14
18
O
N
H
7
19
2
1.48
15
1
14
6
18
N
H
O
1
7
14
1.4585
15
17
H3CO
0.0977
H3CO
16
17
0.1038
16
1e
1e•
Fig. 2. Comparison of the bond lengths and the Mulliken spin densities in the symmetric (1aꢁ) and asymmetric (1bꢁ and 1eꢁ)
double benzylic/allylic radical intermediates.
2
2
2
2
·
carried out in a conventional three electrode system at room
temperature. A silver electrode, a large area Pt plate (99.99 %),
and a glass carbon electrode (GCE) were used as reference,
counter, and working electrodes, respectively.
3
1
1
3
·
1
3
3
1
1
·
·
·
1
2
2
Scheme 7. Single electron delocalization in a symmetrical allylic radical.
Oxidation of 4,6-Diaryl-THPMs by PPS
A mixture of 0.23 mmol 1a–g and 0.23 mmol K2S2O8 (mol ratio
1 : 1) in 6 mL CH3CN/H2O (5/1) was reacted in a preheated oil
bath at 808C for the time given in Table 1. The progress of the
reaction was followed by TLC until total disappearance of 1a–g.
The solvent was evaporated under reduced pressure, water was
added, and the mixture was extracted with CHCl3. The organic
layer was dried over MgSO4. After evaporation of the solvent,
the residue was recrystallized from n-hexane/ethyl acetate or
washed with n-hexane.
using KBr pellets on a Jasco FT/IR-6300 spectrometer. The 1H
and 13C NMR spectra (DMSO-d6) were recorded on a Bruker
Avance III 400 spectrometer at 400 and 100 MHz. The 1H NMR
spectra are reported as follows: chemical shifts, [multiplicity,
number of protons, coupling constants J (Hz), and assignment].
UV spectra (in CHCl3) were measured with a Shimadzu UV-160
spectrometer.
Cyclic Voltammetry (CV) Measurements
4,6-Diphenyl-2-oxo-1,2-dihydropyrimidine (2a)
Solutions with concentrations of 1 mM of THPMs and 50 mM of
tetrabutylammonium perchlorate in acetonitrile were prepared.
The CV measurements were performed using a SAMA 500
Potentio/Galvanostat. All electrochemical experiments were
Mp 236–2378C, recrystallized from n-hexane/ethyl acetate. nmax
(KBr)/cmꢀ1 3285, 3006, 2895, 2741, 1625, 1577, 1490, 1456,
1420, 1336, 993, 763, 684. dH (400 MHz, DMSO-d6) 7.54–7.60

H
H. R. Memarian et al.
nm (log e) 380 (sh, 3.88), 361.5 (4.06), 336 (4.00), 300 (sh, 3.89),
257 (3.99).
6-(4-Methoxyphenyl)-4-phenyl-2-oxo-1,2-
dihydropyrimidine (3e = 3b)
Mp 256–2588C, recrystallized from n-hexane/ethyl acetate. dH
(400 MHz, DMSO-d6) 3.86 (s, 3H, OCH3), 7.10 (d, J 8.8, 2H,
Ar-H), 7.49 (s, 1H, 5-H), 7.53–7.61 (m, 3H, Ar-H), 8.14–8.18
(m, 4H, Ar-H), 11.94 (s, 1H, N1-H). dC (100 MHz, DMSO-d6)
55.44, 99.13, 114.18, 127.50, 128.76, 129.36, 131.35, 162.07.
6
5
1
lmax (CHCl3)/nm (log e) 360 (4.04), 346.2 (4.18), 290 (sh, 3.97),
249.6 (4.04).
4
2
6-(3-Methoxyphenyl)-4-phenyl-2-oxo-1,2-
dihydropyrimidine (3f = 3c)
3
Mp 220–2218C, recrystallized from n-hexane/ethyl acetate. dH
(400 MHz, DMSO-d6) 3.87 (s, 3H, OCH3), 7.15–7.17 (m, 1H,
Ar-H), 7.48 (t, J 8.0, 1H, Ar-H), 7.56–7.60 (m, 6H, Ar-H), 8.18
(br s, 2H, Ar-H), 12.09 (s, 1H, N1-H). dC (100 MHz, DMSO-d6)
55.34, 101.36, 112.34, 117.51, 119.92, 127.60, 128.79, 1129.93,
131.49, 159.64. lmax (CHCl3)/nm (log e) 375 (sh, 3.78), 354.5
(4.03), 330.5 (4.01), 297.5 (3.99), 257.5 (4.00).
Fig. 3. The structure of compound 3d showing the presence of the
substituted aryl ring next to the NH moiety.
(m, 7H, Ar-H and 5-H), 8.16–8.18 (m, 4H), 12.07 (s, 1H, N1-H).
dC (100 MHz, DMSO-d6) 100.40, 127.55, 128.79, 131.45,
134.68, 159.93. lmax (CHCl3)/nm (log e) 360 (sh, 3.78), 346.2
(3.92), 280.2 (4.00), 263.4 (3.95), 253.4 (3.97).
6-(2-Methoxyphenyl)-4-phenyl-2-oxo-1,2-
dihydropyrimidine (3g = 3d)
Mp 240–2418C, washed with n-hexane. dH (400 MHz, DMSO-
d6) 3.87 (s, 3H, OCH3), 7.11 (t, J 7.6, 1H, Ar-H), 7.21 (d, J 8.4,
2H, Ar-H), 7.53–7.59 (m, 5H, Ar-H), 8.11 (br s, 2H, Ar-H),
11.94 (s, 1H, N1-H). dC (100 MHz, DMSO-d6) 55.75, 102.01,
111.95, 120.55, 127.52, 128.81, 130.27, 131.49, 132.39, 156.98.
6-(4-Methoxyphenyl)-4-phenyl-2-oxo-1,2-
dihydropyrimidine (3b)
M. p. 254–2568C, recrystallized from n-hexane/ethyl acetate.
lmax (KBr)/cmꢀ1 3271, 3099, 2999, 2903, 2833, 1611, 1577,
1513, 1458, 1424, 1395, 1338, 1261, 1172, 1028, 991,
822 cmꢀ1. dH (400 MHz, DMSO-d6) 3.86 (s, 3H, OCH3), 7.10
(d, J 8.8, 2H, Ar-H), 7.49 (s, 1H, 5-H), 7.53–7.61 (m, 3H, Ar-H),
8.14–8.18 (m, 4H, Ar-H), 11.94 (s, 1H, N1-H). dC (100 MHz,
DMSO-d6) 55.44, 99.08, 114.18, 127.50, 128.76, 129.36,
131.35, 162.07. lmax (CHCl3)/nm (log e) 360 (4.07), 346.2
(4.15), 290 (sh, 3.97), 248.2 (4.05).
l
300 (sh, 3.91), 257 (4.00).
max (CHCl3)/nm (log e) 380 (sh, 3.92), 363 (4.09), 335 (4.00),
Crystallographic Data
CCDC 1434836 contains the supplementary crystallographic
data for compound 3d. This data can be obtained free of charge
via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK, fax: (þ44) 1223-336-033, or email:
deposit@ccdc.cam.ac.uk.
6-(3-Methoxyphenyl)-4-phenyl-2-oxo-1,2-
dihydropyrimidine (3c)
Supplementary Material
Mp 221–2228C, recrystallized from n-hexane/ethyl acetate.
lmax (KBr)/cmꢀ1 3285, 3092, 3000, 2900, 2826, 1628, 1592,
1539, 1498, 1460, 1384, 1346, 1266, 1182, 1042, 915, 843, 773.
dH (400 MHz, DMSO-d6) 3.87 (s, 3H, OCH3), 7.15–7.17 (m,
1H, Ar-H), 7.48 (t, J 8.0, 1H, Ar-H), 7.56–7.60 (m, 6H, Ar-H),
8.18 (br s, 2H, Ar-H), 12.09 (s, 1H, N1-H). dC (100 MHz,
DMSO-d6) 55.34, 112.34, 117.52, 119.92, 127.60, 128.80,
129.93, 131.50, 159.55. lmax (CHCl3)/nm (log e) 375 (sh, 3.81),
356.5 (4.05), 330 (4.03), 298 (4.04), 257 (4.04).
FT-IR, 1H NMR, and 13C NMR spectra of the oxidation products
are available on the Journal’s website.
Acknowledgements
The authors are thankful to the Research Council and Office of Graduate
Studies of the University of Isfahan for their financial support.
References
[1] P. Biginelli, Gazz. Chim. Ital. 1893, 23, 360.
6-(2-Methoxyphenyl)-4-phenyl-2-oxo-1,2-
dihydropyrimidine (3d)
[2] B. Lagu, D. Tian, G. Chiu, D. Nagarathram, J. Fang, Q. Shen,
C. Forray, R. W. Ransom, R. S. L. Chang, K. P. Vyas,
K. Zhang, C. Gluchowski, Bioorg. Med. Chem. Lett. 2000, 10, 175.
doi:10.1016/S0960-894X(99)00653-8
[3] S. Bartolini, A. Mai, M. Artico, N. Paesano, D. Rotill, C. Spadafora,
G. Sbardella, J. Med. Chem. 2005, 48, 6776. doi:10.1021/JM0507330
[4] R. H. Tale, A. H. Rodge, G. D. Hatnapure, A. P. Keche, Bioorg. Med.
Chem. Lett. 2011, 21, 4648. doi:10.1016/J.BMCL.2011.03.062
[5] J. P. Wan, Y. Pan, Mini Rev. Med. Chem. 2012, 12, 337. doi:10.2174/
138955712799829267
Mp 239–2408C, washed with n-hexane. lmax (KBr)/cmꢀ1 3194,
3116, 3072, 3000, 2888, 1636, 1598, 1581, 1532, 1461, 1394,
1287, 1248, 1182, 1017, 762. dH (400 MHz, DMSO-d6) 3.86 (s,
3H, OCH3), 7.11 (t, J 7.6, 1H, Ar-H), 7.21 (d, J 8.4, 2H, Ar-H),
7.53–7.59 (m, 5H, Ar-H), 8.11 (br s, 2H, Ar-H), 11.94 (s, 1H,
N1-H). dC (100 MHz, DMSO-d6) 55.75, 102.0, 111.95, 120.55,
127.52, 128.81, 130.27, 131.49, 132.39, 156.99. lmax (CHCl3)/

Capto-Dative Radical Stabilization
I
[6] R. Sharma, S. S. Jadav, S. Yasmin, S. Bhatia, H. Khalilullah, M. J.
Ahsan, Med. Chem. Res. 2015, 24, 636. doi:10.1007/S00044-014-
1146-2
[7] K. Venkateshwarlu, G. C. Rao, V. Reddy, Y. N. Reddy, J. Iran, Chem.
Soc. 2014, 11, 1619. doi:10.1007/S13738-014-0438-2
[8] V. Pathak, H. K. Maurya, S. Sharma, K. K. Srivastava, A. Gupta,
Bioorg. Med. Chem. Lett. 2014, 24, 2892. doi:10.1016/J.BMCL.2014.
04.094
[24] H. R. Memarian, M. Soleymani, H. Sabzyan, J. Iran, Chem. Soc. 2012,
9, 805. doi:10.1007/S13738-012-0097-0
[25] H. R. Memarian, M. Soleymani, H. Sabzyan, M. Bagherzadeh,
H. Ahmadi, J. Phys. Chem. A 2011, 115, 8264. doi:10.1021/JP2014435
[26] H. R. Memarian, M. Ranjbar, H. Sabzyan, A. Kiani, C. R. Chim. 2012,
15, 1001. doi:10.1016/J.CRCI.2012.09.009
[27] The absolute term ‘extinction’ is a German word and has now been
replaced with ‘absorbance’, but the phrase ‘extinction time’ is still
being used.
[9] C. O. Kappe, Acc. Chem. Res. 2000, 33, 879. doi:10.1021/AR000048H
[10] Z. T. Wang, L. W. Xu, C.-G. Xia, H.-Q. Wang, Tetrahedron Lett. 2004,
45, 7951. doi:10.1016/J.TETLET.2004.08.107
[28] I. Fleming, Frontier Orbitals and Organic Chemical Reactions 1976
(John Wiley: London).
[11] M. Kidwai, P. Misra, Synth. Commun. 1999, 29, 3237. doi:10.1080/
00397919908085949
[12] R. G. Bacon, W. J. Hanna, D. Stewart, J. Chem. Soc. C 1966, 1384.
doi:10.1039/J39660001384
[13] Y. M. Li, X. H. Wei, X. A. Li, S. D. Yang, Chem. Commun. 2013, 49,
[29] F. Bernardi, N. D. Epiotis, W. Cherry, H. B. Schlegel, M. H. Whangbo,
S. Wolfe, J. Am. Chem. Soc. 1976, 98, 469. doi:10.1021/JA00418A024
[30] H. G. Viehe, R. Merenyi, L. Stella, Z. Janousek, Angew. Chem. 1979,
18, 917. doi:10.1002/ANIE.197909171
[31] T. Albright, J. Burdett, M.-H. Whangbo, Orbital Interactions in
11701. doi:10.1039/C3CC47287F
Chemistry 1985 (Wiley: New York, NY).
[14] M. J. A. Abualreish, Am. J. Chem. 2012, 2, 214. doi:10.5923/
J.CHEMISTRY.20120204.05
[32] D. Crans, T. Clark, P. V. R. Schleyer, Tetrahedron Lett. 1980, 21,
3681. doi:10.1016/S0040-4039(00)78744-2
[15] H. Ouyang, L. Wang, J. Tian, A. Liang, Z. Jiang, Food Anal. Methods
2013, 6, 76. doi:10.1007/S12161-012-9416-4
[33] R. Sustmann, H.-G. Korth, Adv. Phys. Org. Chem. 1991, 26, 131.
doi:10.1016/S0065-3160(08)60045-3
[16] Y. Liu, B. Jiang, W. Zhang, Z. Xu, J. Org. Chem. 2013, 78, 966.
doi:10.1021/JO302450F
[17] S. Yuan, P. Liao, A. N. Alshawabkeh, Environ. Sci. Technol. 2014, 48,
656. doi:10.1021/ES404535Q
[18] A. R. Khataee, Environ. Technol. 2010, 31, 73. doi:10.1080/
09593330903358302
[19] H. R. Memarian, A. Farhadi, J. Iran, Chem. Soc. 2009, 6, 638.
doi:10.1007/BF03246543
[20] H. R. Memarian, A. Farhadi, H. Sabzyan, Ultrason. Sonochem. 2010,
[34] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery, Jr, R. E.
Stratmann, J. C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels,
K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi,
R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford,
J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui, K. Morokuma,
D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman,
J. Ciolowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J. Fox,
T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez,
M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong,
J. L. Andres, M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian
98 1998 (Gaussian Inc.: Pittsburg, PA).
17, 579. doi:10.1016/J.ULTSONCH.2009.11.014
[21] H. R. Memarian, M. Soleymani, Ultrason. Sonochem. 2011, 18, 745.
doi:10.1016/J.ULTSONCH.2010.10.006
[22] H. R. Memarian, A. Farhadi, H. Sabzyan, M. Soleymani, J. Photochem.
Photobiol. Chem. 2010, 209, 95. doi:10.1016/J.JPHOTOCHEM.2009.
10.012
[35] H. R. Memarian, E. Sanchooli, Magn. Reson. Chem. 2016, 54, 178.
doi:10.1002/MRC.4360
[23] H. R. Memarian, A. Farhadi, Monatsh. Chem. 2009, 140, 1217.
doi:10.1007/S00706-009-0156-0