Substituent effect in photocatalytic oxidation of 2-oxo-1,2,3,4- tetrahydropyrimidines using TiO2 nanoparticles

Article abstract of DOI:10.1016/j.molcata.2011.12.026

The semiconductor-sensitized oxidation of various 1-, 4- and 5-substituted 2-oxo-1,2,3,4-tetrahydropyrimidines was carried out in acetonitrile using TiO₂ anatase nanoparticles. The aims of this study were to elucidate the effects of the nature

Full text of DOI:10.1016/j.molcata.2011.12.026

Journal of Molecular Catalysis A: Chemical 356 (2012) 46–52
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Journal of Molecular Catalysis A: Chemical
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Substituent effect in photocatalytic oxidation of
2-oxo-1,2,3,4-tetrahydropyrimidines using TiO2 nanoparticles
Hamid R. Memarain∗, Mahnaz Ranjbar
Catalysis Division, Department of Chemistry, University of Isfahan, 81746-73441 Isfahan, Islamic Republic of Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The semiconductor-sensitized oxidation of various 1-, 4- and 5-substituted 2-oxo-1,2,3,4-
tetrahydropyrimidines was carried out in acetonitrile using TiO2 anatase nanoparticles. The aims
of this study were to elucidate the effects of the nature of the substituents on the 1-, 4- and 5-positions
of the heterocyclic ring, the type of the photocatalyst and the nature of solvent on the rate of reaction.
The proposed electron-transfer mechanism is supported by the experimental results and also by the
computational studies.
Received 2 November 2011
Received in revised form
25 December 2011
Accepted 28 December 2011
Available online 5 January 2012
© 2012 Elsevier B.V. All rights reserved.
Dedicated to Prof. Dietrich Döpp on
occasion of his 75th birthday.
Keywords:
Biginelli
Electron transfer
TiO2 anatase nanoparticles
Photo-oxidation
Tetrahydropyrimidine
1. Introduction
electron-acceptor or electron-donor species present in the medium
[14]. The photogenerated holes are also able to accept an elec-
Photochemical oxidation (PCO) is a promising technology which
utilizes semiconductor powders; mainly metal oxides such as TiO2,
ZnO, WO3 suspended in solution or as supported thin film to
carry out a light-induced redox process for oxidation of various
chemicals [1,2]. TiO2 is the most extensively used semiconduc-
tor photocatalyst because of its photocatalytic activity, chemical
stability, non-toxicity and low cost. TiO2 has been widely used
for degradation of various contaminants such as azo dyes [3–5],
ammonia [6], cyanide [7], benzaton as a herbicide [8] or by photo-
oxidation of aryl alcohols [9], ␣,␤-dihydroxybenzyl derivatives
[10–12] or organic sulfides [13]. When TiO2 is subjected to UV
light (ꢀ ≥ 385 nm), the primary step following the light absorp-
tion is the excitation of an electron from the TiO2 valence band to
the conduction band producing an electron/hole pair (eCB−/hVB+),
which must be trapped somehow in order to avoid recombination
(Scheme 1).
tron from adsorbed water producing hydroxyl radical (when the
experiment is being carried out in aqueous media), or from the com-
pound to be oxidized which is present in the solution. Meanwhile,
oxygen dissolved in solution can scavenge the excited electrons,
limiting the electron–hole recombination by formation of super-
oxide radical (O2•−), which acts as an active oxidant or reductant
[15].
2-Oxo-1,2,3,4-tetrahydropyrimidines (THPMs) are important
compounds due to their biological activities such as antitubercu-
lar [16], antihypertensive [17], antibacterial [18], anticancer agents
[19], as calcium channel blockers [20] and inhibitors of the HIV
virus [21]. Different activities of these compounds are related to
the nature of the substituents located on the 4- and 5-positions of
the heterocyclic ring. Recently, we studied the efficient oxidation of
THPMs containing acetyl or carboethoxy groups on the 5-position
of the heterocyclic ring by potassium peroxydisulfate (K2S2O8) in
aqueous acetonitrile under thermal [22], sono-thermal [23,24] and
microwave [25] activations. According to the proposed mechanism
and considering the results obtained in these studies, we found out
that the removal of 4-hydrogen in the rate determining step occurs
by in situ hydroxyl radical, an active hydrogen-abstracting species,
formed in the reaction of potassium sulfate radical and water. This
The redox process takes place primarily on the surface of TiO2,
where the photogenerated electrons and holes may interact with
∗
Corresponding author. Tel.: +98 311 793 2707; fax: +98 311 668 9732.
E-mail address: memarian@sci.ui.ac.ir (H.R. Memarain).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.12.026

H.R. Memarain, M. Ranjbar / Journal of Molecular Catalysis A: Chemical 356 (2012) 46–52
47
hv
λ > 385 nm
TiO2 (e−CB + h+
)
TiO2
VB
.
h+VB + H2O (ads)
(ads) + H+
OH
.
−
e−CB + O2 (ads)
h+VB + comp.
O2 (ads)
e−
−
.
comp. +
.
Oxidation products
−H2O
.
H
−
.
OH (ads) + comp.
comp.
Scheme 1.
O
argument is supported by the influence of the steric and electronic
effects of the substituents attached to C4- and C5-atoms of the het-
erocyclic ring on the rate of this step. In continuation, we have
studied the light sensitivity of 5-carboethoxy- [26] and 5-acetyl-
THPMs [27] in the electron-transfer induced photo-oxidation in
chloroform. These results explain an electron-transfer process from
excited THPMs to CHCl3 and reconfirming the effect of the nature
of these substituents on the rate of oxidation. The results of pho-
tochemical studies lead us to extend our investigations concerning
the interaction of THPMs containing various substituents at 1-, 4-
and 5-positions of the heterocyclic ring toward photo-excited TiO2
anatase nanoparticles. The results should elucidate the effects of
the nature and steric hindrance of the substituent on the rate of
reaction.
O
Ph
Ph
H
H
CH3CH2O
H3C
N
hv
CH3CH2O
H3C
N
with cat.
without cat.
N
H
O
N
H
O
1a
2a
Scheme 2.
3. Results and discussion
3.1. Photocatalyst activity and optimization of the reaction
condition
In order to find a suitable form of TiO2 for the electron-transfer
induced oxidation of various THPMs, the solutions containing
0.01 mmol of ethyl 2-oxo-4-phenyl-1,2,3,4-tetrahydropyrimidin-
5-carboxylate (1a) as a test substrate in dry acetonitrile in the
absence of the photocatalyst or in the presence of 3.75 × 10−3 mmol
of each of the photocatalysts anatase, rutile, Degussa P-25 (a
mixture of 70% anatase and 30% rutile phases) and anatase nanopar-
ticles were irradiated until maximum conversion of 1a to 2a
(Scheme 2). The results shown in Fig. 1 indicated that the com-
plete conversion of 1a in shorter reaction time is observed using
TiO2 anatase nanoparticles, while irradiation in the presence of
TiO2 (anatase) and TiO2 (rutile) resulted in 50% and 15% conver-
sion of 1a, respectively, and Degussa P-25 causes 90% conversion
after the same irradiation time (3.75 h). Even though anatase should
be the most active form of TiO2, but some reports suggest that
the presence of rutile phase with anatase introduces wider pore
size distribution [29], this may be the cause of higher conversion
observed using Degussa P-25. The effect of the particle size on the
photocatalytic activity can be interpreted in terms of surface area.
Generally, the smaller the particle size, the larger the surface area
2. Experimental
The THPMs were prepared according to the known procedure
[28]. Titanium(IV) dioxide, anatase nanopowder (99.7%), anatase
powder (99.9+%) and rutile powder (99.9+%) were purchased from
Aldrich, and Degussa P-25 was purchased from Degussa, Germany.
Melting points were determined on a Stuart Scientific SMP2 appa-
ratus and are not corrected. IR spectra were recorded using KBr
pellets on a Jasco FT/IR-6300 spectrometer. The 1H and 13C NMR
spectra (DMSO–d6) were recorded on a Bruker DRX-300 Avance
and Bruker Avance III 400 spectrometers at 300, 400, 75.47, and
100.62 MHz. The 1H NMR spectra are reported as follows: chemical
shifts [multiplicity, number of protons, coupling constants J (Hz),
and assignment]. Mass spectra were obtained on Platform II Mass
Spectrometer from Micromass; EI mode at 70 eV. UV spectra were
taken (in CH3CN) with Shimadzu UV-160 spectrometer. All irradi-
ations were carried out using a 400 W high-pressure Hg lamp from
NARVA in Duran glass equipment, which was placed in a distance
of 20 cm from light source.
The TiO2 suspension (0.3 g TiO2 anatase nanoparticles in 10 mL
dry acetonitrile) was sonicated for 15 min to disperse TiO2 uni-
formly in the solution. 100 ␮L of this suspension was added to
solution of 0.01 mmol of THPM in 10 mL dry solvent, then the
solution was magnetically stirred in the dark until a homogenous
suspension was obtained. After that, the solution was irradiated
with stirring until total disappearance of THPM. It should be noted
that for the preparative scale, a solution of 0.1 mmol of THPMs and
12.5 × 10−2 mmol of TiO2 anatase nanoparticles in 100 mL acetoni-
trile was irradiated until total disappearance of THPM. The mixture
was centrifuged to remove TiO2 effectively. The solution was sep-
arated by careful filtration; the solid material was washed with
acetonitrile. Solvent was evaporated and the residue was recrys-
tallized from n-hexane/ethyl acetate (otherwise is mentioned) to
afford pure product. The known products were identified by com-
parison of their melting points and spectral data with those of the
authentic samples. The physical and the spectral data of the new
THPMs and the corresponding DHPMs are given as supplementary
material.
Fig. 1. The effect of TiO2-type on the rate of photo-oxidation of 1a after 3.75 h
photolysis.

48
H.R. Memarain, M. Ranjbar / Journal of Molecular Catalysis A: Chemical 356 (2012) 46–52
Y
Y
O
O
H
H
R2
R2
N
N
hv
H3C
N
O
H3C
N
O
R1
R1
2 : R1 = H
1 : R1 = H
4 : R1 = CH3
6 : R1 = C6H5
R2 = CH3, OCH3, OC2H5
3 : R1 = CH3
5 : R1 = C6H5
Fig. 2. The effect of the water content on photocatalytic oxidation of
1a [CH3CN/H2O: (1) 10:0; (2) 9.5:0.5; (3) 9:1; (4) 8.5:1.5; (5) 8:2];
TiO2 = 3.75 × 10−3 mmol.
Scheme 3.
nanoparticles in 100 mL acetonitrile were irradiated until total dis-
appearance of THPMs. The TiO2-sensitized oxidation of THPMs to
their corresponding DHPMs was confirmed by comparison of the
IR, UV, 1H NMR, 13C NMR and MS spectra of the products with those
of starting THPMs based on the following observations:
and the higher the expected activity [30,31]. The important point
of the presence of photocatalyst in this reaction is that 2a was
not formed in the absence of the photocatalyst even after 3.75 h
irradiation.
Since the amount of the photocatalyst has an important effect
on the rate of reaction, irradiation of 1a was carried out in dry
acetonitrile in the presence of various amount of TiO2 anatase
nanoparticles. The results showed that the progress of reac-
tion is influenced with increasing the amount of TiO2 anatase
nanoparticles up to an optimum loading of 12.5 × 10−3 mmol with
simultaneous decrease of the irradiation time to 1.66 h. It should
be noted that by further increasing the amount of TiO2 anatase
nanoparticles, the irradiation time did not decrease but a lower con-
version of 1a is observed. This may be possibly due to the screening
effect by increasing the amount of the photocatalyst in solution and
also due to self-quenching of the photocatalyst [32].
Due to solvent–solute interaction, especially in the electron-
transfer induced reactions, the nature of solvent can also affect the
rate of reaction. Therefore, irradiation of 1a as test substrate in the
presence of TiO2 anatase nanoparticles was carried out in air sat-
urated acetonitrile, ethanol and propan-2-ol. The results indicated
that after 3.75 h irradiation, complete conversion of 1a to 2a was
achieved in acetonitrile, whereas 45% conversion was observed in
propan-2-ol. 1a was obtained unchanged after the same irradiation
time when carrying the experiment in ethanol.
Many TiO2-sensitized photoreactions of organic compounds
are carried out in water. According to the proposed mechanism
(Scheme 1) the participation of hydroxyl radicals generated by
the adsorbed water on the photogenerated hole is responsible for
the oxidation process and also increasing the rate of reaction [15].
Based on our previous results concerning the oxidation of THPMs
by K2S2O8 in aqueous acetonitrile [22], the hydroxyl radical formed
by reaction of water with potassium sulfate radical (obtained from
thermal cleavage of K2S2O8), acts as oxidizing agent in this oxida-
tive process. Therefore, we have also carried out irradiation of 1a in
a mixture of CH3CN/H2O. The results presented in Fig. 2 indicated
that by increasing the amount of water a decrease of the rate of
conversion was observed.
Based on the above mentioned results and considering the opti-
mization conditions, for better comparison of the effect of different
substituents on the rate of reaction, the solutions of 0.01 mmol
of various 4- and 5-substituted THPMs (1a–q), N1-methyl substi-
tuted THPMs (3a–q), N1-phenyl substituted THPMs (5a,g,k) and
3.75 × 10−3 mmol of TiO2 anatase nanoparticles in 10 mL acetoni-
trile were irradiated until total disappearance of THPMs (Scheme 3).
The results are presented in Table 1.
I. A comparison of the IR spectra of the products 2a–q with those
of the reactants 1a–q showed a decrease in the intensity of the
NH vibration and a little shift to lower frequency, whereas in the
cases of N1-substituted THPMs, 3a–q and 5a,g,k, disappearance
of this vibration has been observed.
II. Due to the formation of 1-aza-1,3-diene (imino-diene) system
in the heterocyclic ring which is cross-conjugated with 4-aryl
substituent, this system benefits also from extended conjuga-
tion with the 2-CO ␲-bond and N-1 lone pair, therefore, the
bathochromic shift in the UV spectra of photoproducts has been
observed.
III. Analysis of 1H NMR spectra of all THPMs considered in this study
shows the lack of 3-NH and 4-H peaks due to being eliminated
upon oxidation. Formation of the imino-diene system causes a
shift of 1-NH peak in 2a–q and also 1-NCH3 peak in 4a–q to
lower fields due to the attachment to this conjugated system.
In consistent with these observations, due to anisotropic effect
of the semi-aromatic heterocyclic ring, the methyl group in 6-
position, especially in 2a–q is shifted downfield. Interestingly,
in 4a–q, no significant changes in the chemical shift of 6-CH3
compared with those in 3a–q has been observed. This is possibly
due to gauche interaction of both methyl groups in 1- and 6-
positions leading to an increase in the deviation of N-1 from
planarity and consequent diminished aromatic character of the
heterocyclic ring.
IV. Analysis of the 13C NMR spectra supported also the changes as
are also observed in the 1H NMR spectra.
3.2. Mechanistic aspects
The results obtained in the present study concerning effects of
the solvent, the presence of water and the nature of the substituents
on the 1-, 4- and 5-positions of the heterocyclic ring on the rate of
reaction are discussed separately.
The mechanism of TiO2-sensitized oxidation of many organic
compounds is illustrated in Scheme 1. As mentioned above, the
excitation of TiO2 by photons with energy equal to or larger than
its band gap energy leads to the promotion of an electron from the
valence band to the conduction band. This process results in the
occurrence of an electron in the conduction band and leaving a posi-
tive hole in the valence band. Without any electron or hole trapping
agent, recombination of eCB−/hVB+ occurs in a few nanoseconds and
loses the excitation e−nergy as heat [33]. Generally, molecular oxy-
gen can act as an eCB trapping agent, leading to the formation of
For a preparative work to obtain larger amount of oxida-
tion products for the spectroscopic characterization, the solutions
of 0.1 mmol of THPMs and 12.5 × 10−2 mmol of TiO2 anatase

H.R. Memarain, M. Ranjbar / Journal of Molecular Catalysis A: Chemical 356 (2012) 46–52
49
Table 1
Light induced oxidatiaon of THPM (1a–q; R1 = H), N1-methyl substituted THPM (3a–q; R1 = CH3) and N1-phenyl substituted THPM (5a,g,k; R1 = Ph).a
1
Y
R2
hꢁ/hb
3
Y
R2
hꢁ/hb
5
a
Y
R2
hꢁ/hb
8.75
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
H
4-Br
4-F
2-OCH3
3-OCH3
4-OCH3
4-NO2
H
4-Br
4-F
H
4-F
2-OCH3
3-OCH3
4-OCH3
4-NO2
H
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH3
OCH3
OCH3
CH3
CH3
3.75
2.5
3.5
3.5
4.25
3.0
7.5
3.0
2.5
3.5
3.17
1.75
3.08
3.08
2.5
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
H
4-Br
4-F
2-OCH3
3-OCH3
4-OCH3
4-NO2
H
4-Br
4-F
H
4-F
2-OCH3
3-OCH3
4-OCH3
4-NO2
H
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH2CH3
OCH3
OCH3
OCH3
CH3
CH3
2
2
H
OCH2CH3
2.5
3
3.25
2.5
4
g
4-NO2
H
OCH2CH3
CH3
16
2.25
2
2.5
2.5
1.5
2.75
2.67
2
k
6.5
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
5.5
4.17
5.17
3.42
Ph
Ph
a
A solution contains 0.01 mmol of THPMs and 3.75 × 10−3 mmol of TiO2 anatase nanoparticles in 10 mL acetonitrile.
b
Irradiation times (hours) are given after total disappearance of THPMs.
the superoxide anion radical. This process extends the lifetime of
effective volume of THPMs. This hinders facile electron donation
from THPMs to photogenerated holes. This is also supported by the
observed dimeric form in the crystal structure of THPMs, medi-
ated by the interaction between 3-NH and 2-CO of the heterocyclic
ring [34]. On the other hand, alcohols can compete with THPM
in adsorption at TiO2 surface via electron donation and under-
going oxidation [35,36]. It is expected that due to different sizes
of solvent molecules, the steric hindrance affects the attraction
between non-bonding electrons in the solvent molecule and photo-
generated holes; therefore, ethanol can be better adsorbed at TiO2
surface than propan-2-ol. Thus, the effective occupation of TiO2
holes and better solvation of THPMs by ethanol molecules result in
a faster oxidation of THPM in propan-2-ol than ethanol, which is
also observed in this study. In the aqueous media, H2O molecule is
typically adsorbed at TiO2 hVB+, generating a hydroxyl radical which
is a powerful oxidant [37]. It has been expected that by adding
water to the acetonitrile solution, the rate of reaction should be
increased. Surprisingly, the results shown in Fig. 2 indicated that by
increasing the water content, the oxidation rate is decreased. The
observed behavior of water in decreasing the rate of reaction in this
study is like those observed and discussed for ethanol and propan-
2-ol, namely the solvation of THPM molecules and competing with
THPMs by electron donation to photogenerated holes. The partic-
ipation of holes in the photocatalytic oxidation can be evaluated
using iodide ion acting as a scavenger and reacting with the valence
band holes to suppress the oxidation of compounds [14]. By adding
potassium iodide (KI) to the reaction mixture of 1a and TiO2 anatase
nanoparticles in acetonitrile:water (8.5:1.5) (KI is not soluble in
pure acetonitrile), the yield of oxidation is decreased approximately
to 10% after 3.75 h irradiation. As an evidence of the competitive
oxidation of I− to I2, colour of the reaction mixture became yellow.
The results presented in Table 1 do not clearly distinguish
between the effect of 5-carboethoxy and 5-carbomethoxy groups
(the ester groups) with the same substituent at 4-position of
the heterocyclic ring on the rate of reaction, whereas increasing
the rate of reaction is observed by replacing these ester groups
with the acetyl group. The results of electron-transfer induced
photo-oxidation of 5-carboethoxy- and 5-acetyl-THPMs in CHCl3
(electron transfer from excited THPMs to CHCl3) [26,27] indi-
cated that the presence of the acetyl group in 5-acetyl-THPMs
causes a bathochromic shift in the UV spectrum which results
in higher light absorbance and consequently an increase in the
rate of reaction compared with those of 5-carboethoxy derivatives.
But in the present study and in contrast to the electron-transfer
induced photo-oxidation in CHCl3 solution, the reaction starts
hVB+; therefore, the oxidation step is more likely to occur. In the
present study and based on the obtained results, we will propose
that the oxidation of THPMs should start by the donation of an elec-
tron from nitrogen lone pair (N1) to photogenerated hole producing
the 2-oxo-1,2,3,4-tetrahydropyrimidinyl radical cation (THPM+•),
in which the positive charge and the electron is delocalized as being
heteroallylic radical cation (Scheme 4). In the next step, the proton
detachment from this species leads to the formation of 2-oxo-
1,2,3-trihydropyrimidinyl radical (TrHPM•), as a simultaneously
stabilized allylic and benzylic radical. The second electron donation
and proton detachment accomplish the reaction by the forma-
tion of the final product, namely, 2-oxo-1, 2-dihydropyrimidines
(DHPMs).
The observed fast reaction in acetonitrile, as compared to that
in ethanol or propan-2-ol, indicates that THPM as an electron-
donor species should not be effectively solvated in acetonitrile,
which is a polar aprotic solvent, whereas in alcoholic solvents, due
to solvation phenomenon, hydrogen bonding can occur between
the NH and the OH groups which leads to enhancement of the
Y
Y
O
O
H
.
N
H
R2
N H
O
R2
H
O
N
+
h+
+
VB
(Step 1)
(Step 2)
H3C
H3C
N
R1
R1
.
THPM+
THPM
Y
Y
O
O
H
.
N
.
R2
H+
NH
O
R2
N H
O
+
H3C
N
R1
H3C
R1
.
THPM+
.
TrHPM
Y
Y
N
O
O
e−
H+
.
R2
N
H
O
R2
(Step 3)
H3C
N
O
H3C
N
R1
R1
.
DHPM
TrHPM
Scheme 4.

50
H.R. Memarain, M. Ranjbar / Journal of Molecular Catalysis A: Chemical 356 (2012) 46–52
Fig. 3. (a) UV spectra of 5-carboethoxy-NH- (1a), NCH3- (3a) and NC6H5-THPMs (5a) and (b) the corresponding 5-acetyl-THPMs (1k, 3k and 5k) in acetonitrile.
by electron donation from THMPs in the ground state to photo-
generated holes. Since under experimental conditions (irradiation
at ꢀ ≥ 280 nm), the light absorption by THPMs cannot be totally
excluded, therefore, due to higher reduction potential of ace-
tonitrile compared with chloroform, only very low conversion
of 5-acetyl-THPMs to the corresponding DHPMs is possible. This
means that a maximum conversion of THPMs occurs by electron-
transfer from THPMs in the ground state to photogenerated holes. A
comparison of the irradiation times of 5-carboethoxy-THPMs (a–g),
5-carbomethoxy-THPMs (h–j) and 5-acetyl-THPMs (k–p) shows
that, besides the effect of the nature of the 4-substituent, 5-acetyl-
THPMs are reacted faster than the corresponding ester derivatives
in the present study. Interesting results is the effect of introducing
a methyl or a phenyl group instead of the hydrogen in position
1 of the heterocyclic ring. While N1-methyl substituted THPMs
(3a,g,k,q) react faster than the N1-H derivatives (1a,g,k,q), N1-
phenyl substituted THPMs (5a,g,k) cause a decrease in the rate
of reaction. The UV spectra of 5-carboethoxy-NH-THPM (1a), 5-
carboethoxy-NCH3-THPM (3a) and 5-carboethoxy-NC6H5-THPM
(5a) and the corresponding 5-acetyl-THPMs (1k,3k,5k), as repre-
sentatives, are presented in Fig. 3. A bathochromic shift is observed
Fig. 4. Extinction time diagram obtained by direct irradiation of 1a, 3a and 5a in
acetonitrile.
by introducing the phenyl and methyl groups. This can be resulted
in higher light absorption of the N1-substituted THPMs. There-
fore, it is expected that if these compounds should be involved
in the photo-oxidation reaction by absorption of the light pho-
tons (direct excitation), the order of 3a > 5a > 1a should be obtained
for their photochemical reactivity. Irradiations of 1a, 3a and 5a in
Fig. 5. Reaction spectra of TiO2-sensitized oxidation of 1a, 3a and 5a in acetonitrile and the corresponding extinction time (ET) diagrams; time interval 10 min.

H.R. Memarain, M. Ranjbar / Journal of Molecular Catalysis A: Chemical 356 (2012) 46–52
51
R2
the absence of the photocatalyst have been followed by UV spec-
R1
troscopy. The slopes in the extinction time (ET) diagram for these
compounds (Fig. 4) derived from their UV reaction spectra indicate
that no changes in the UV spectra of 1a and 5a are observed after
50 min irradiation, whereas a bathochromic shift in the UV reac-
tion spectra of 3a has been observed due to the formation of the
corresponding product 4a.
The different behavior of 1-CH3 and 1-Ph groups on the rate of
TiO2-sensitized photo-oxidation reaction supports again our pro-
posal concerning the interaction of THPMs in the ground state
with the photogenerated holes. The photocatalytic oxidation reac-
tions of 1a, 3a and 5a have been followed by UV spectroscopy.
Bathochromic shift in the UV spectra and increased absorption have
been observed owing to the formation of the 4-amino-1-aza-1,3-
diene system. The corresponding plots of the ET diagram of these
reactions, illustrated in Fig. 5 compared with those presented in
Fig. 4, show clearly the effect of the presence of the photocatalyst
and comparative effect of hydrogen, methyl and phenyl substitu-
tions on the rate of reaction.
1
4
N
3
2
hN
H
hC
N
O
H
5
6
8
7
CH3
O
14
R3
R1 = H, CH3, Ph
R2 = Alkyl, Aryl, Heterocycle
Y
13
15
R2 =
16
12
11
R3 = CH3, Ph, OCH3, OC2H5, NHCOR
Fig. 6. General structures of 2-oxo-1,2,3,4-tetrahydropyrimidines (THPMs).
ii) The dihedral angles of 5-CO groups of the ester or the acetyl
moiety with respect to the C5 C6 bond.
iii) The dihedral angles formed by C12C11C4N3 atoms (the angle of
the aromatic ring on C4 with the THPM ring).
iv) The electron density of the heterocyclic and also aromatic ring
atoms and finally.
v) The additional effects of the substitution in the 1-position on the
above mentioned items.
3.3. Computational study
The observed comparative behavior of these compounds, espe-
cially the strong effect of the substitution in the 1-position of the
heterocyclic ring on the rate of photochemical oxidation reaction,
can be partially explained by the results obtained from ab initio cal-
culations. Computational study on the geometries of various 1-, 4-
and 5-substituted 2-oxo-1,2,3,4-tetrahydropyrimidines carried out
at the B3LYP/6-31++G(d,p) level of theory elucidates the electronic
and the steric effects of the substituents on the optimized struc-
tures of these compounds [38–41]. According to these results, the
heterocyclic ring appears as quasi-planar structure (boat confor-
mation), in which the aryl groups occupy a pseudo-axial position
(Fig. 6). In this structure, the C-4 and N-1 atoms are not in the same
plane as other remaining four atoms (C-2, N-3, C-5 and C-6). The
important points in the computational studies of various 1-, 4- and
5-substituted THPMs were to find the factors which influence the
following items:
The results explained that the above mentioned items were
influenced by the orientation of the aryl group attached to C-4 ring
toward the heterocyclic ring, especially the position of the addi-
tional substituent on the phenyl ring, the stereoeletronic effect of
the acetyl, carboethoxy and carbomethoxy groups located on C-5
and effectively by the substitution on N-1.
Recently, we reported the effect of the nature of the substituent
on the electrochemical oxidation of various substituted 2-oxo-
1,2,3,4-tetrahydropyrimidin-5-carboxamides [41]. These results
and the data obtained from computational studies of these com-
pounds [39] support the argument that the first electron donation
occurs from N-1 which followed the first proton detachment
forming 2-oxo-1,2,3-trihydropyrimidinyl radical (TrHPM•), as also
represented in Scheme 4. The stability of this intermediate and also
the value of the oxidation peak potential are dependent more on
i) The amounts of deviations of the C-4 and also N-1 centers from
ring planarity.
Fig. 7. The optimized structures of 1a, 3a and 5a (from left) obtained at the B3LYP/6-31++G(d,p) level of theory.

52
H.R. Memarain, M. Ranjbar / Journal of Molecular Catalysis A: Chemical 356 (2012) 46–52
the nature of C4 substitution, especially on the type and location of
the additional substituent on the aromatic ring at C4 position. The
important part of the photosensitized oxidation reaction of THPMs
is the adsorption of the substrate at the semiconductor surface, as
a key step in the electron-transfer process. The parameters, either
electronic or steric, especially the substitution in position 1, which
inhibits such adsorption, resulted in slower conversion of THPMs
to the corresponding DHPMs. The UV reaction spectra of TiO2 sen-
sitized oxidation of compounds 1a, 3a and 5a, as representative
substrates, and the corresponding extinction time diagram shown
in Fig. 5 indicate that the order of the oxidative ability of these
compounds is 3a > 1a > 5a. This explains that the enhanced reactiv-
ity of 3a compared with 1a is due to the positive inductive effect of
the methyl group, whereas the presence of the large and electron-
withdrawing phenyl group in 5a causes decreasing rate of reaction.
The influence of the steric hindrance on the lowering the rate of oxi-
dation reaction is also reported on the photo-oxidative dealkylation
of ␣-alkylbenzyl methyl ethers induced by TiO2 [42]. Computa-
tional studies carried out on some of these compounds support
this argument [40]. As representative, the characteristic data of the
computational studies of compounds 1a, 3a and 5a obtained from
their optimized structures are presented in Fig. 7. These results
explain that by changing the substitution in the 1-position from
hydrogen to methyl or phenyl, the dihedral angle of the aromatic
ring toward the heterocyclic ring (C12–C11–C4–N3), the dihedral
angle of 5-CO with respect to the C5 C6 bond (O8C7C5C6), the
dihedral angle formed by H(C)N1C6C10 atoms and also deviation
of the C4- and N1-atoms from ring planarity are affected. The
interesting point is that these dihedral angles depend on the type
of substituent in 1-position. This observation is possibly due to
increased gauche interaction of 6-CH3 with 1-H, 1-CH3 and 1-
C6H5.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molcata.2011.12.026.
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In conclusion, the present study reports photo-sensitized
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We are thankful to the Center of Excellence (Chemistry),
Research Council and Office of Graduate Studies of the University of
Isfahan for their financial support. The authors would like to thank
Professor M.H. Habibi for providing photocatalysts, and Professor
H. Sabzyan for discussion on the computational results.
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