Electrochemical synthesis of 4-(dihydroxyphenylthio)-2H-chromen-2-one derivatives

Article abstract of DOI:10.1248/cpb.56.1562

The 4-(dihydroxyphenylthio)-2H-chromen-2-one derivatives have been synthesized by direct electrochemical oxidation of catechols in the presence of 4-mercaptocoumarin as a nucleophile in water/acetonitrile (50/50) solution, in a one-pot process, at carbon rod electrode, in an undivided cell and in constant current conditions, through an EC mechanism. The products are characterized by spectra data. Besides, the difference in electrochemical oxidation of catechol in the presence of 4-hydroxycoumarin and 4-mercaptocoumarin explained by computational structure, natural bond orbital (NBO) analysis and density functional theory (DFT: B3LYP/6-31G*//B3LYP/6-31G*) based methods, using the GAUSSIAN 98 package of programs.

Full text of DOI:10.1248/cpb.56.1562

1562
Chem. Pharm. Bull. 56(11) 1562—1566 (2008)
Vol. 56, No. 11
Electrochemical Synthesis of 4-(Dihydroxyphenylthio)-2H-chromen-2-one
Derivatives
Davood NEMATOLLAHI, ,a Javad AZIZIAN,b Mohsen SARGORDAN-ARANI,b Mahdi HESARI,c
*
b
Saeed JAMEH-BOZORGHI,d Abdolhamid ALIZADEH,e Lida FOTOUHI,f and Behrooz MIRZA
a Faculty of Chemistry, Bu-Ali-Sina University; c Department of Soil Science, Faculty of Agriculture, Bu Ali Sina
University; Hamadan, P. O. Box 65174, Iran: b Department of Chemistry, Science and Research Branch, Islamic Azad
University; Tehran, Iran: d Department of Chemistry, Faculty of Science, Arak Branch, Islamic Azad University; Arak, Iran:
e Department of Chemistry, Razi University; Kermanshah, P. O. Box 65174, Iran: and f Department of Chemistry, Faculty of
Science, Alzahra University; P. O. Box 1993891167, Tehran, Iran.
Received June 27, 2008; accepted August 11, 2008; published online August 12, 2008
The 4-(dihydroxyphenylthio)-2H-chromen-2-one derivatives have been synthesized by direct electrochemical
oxidation of catechols in the presence of 4-mercaptocoumarin as a nucleophile in water/acetonitrile (50/50) solu-
tion, in a one-pot process, at carbon rod electrode, in an undivided cell and in constant current conditions,
through an EC mechanism. The products are characterized by spectra data. Besides, the difference in electro-
chemical oxidation of catechol in the presence of 4-hydroxycoumarin and 4-mercaptocoumarin explained
by computational structure, natural bond orbital (NBO) analysis and density functional theory (DFT: B3LYP/6-
31G*//B3LYP/6-31G*) based methods, using the GAUSSIAN 98 package of programs.
Key words 4-mercpatocoumarin; electrochemical oxidation; catechol; B3LYP/6-31G*; 4-(3,4-dihydroxyphenylthio)-2H-
chromen-2-one
The number of natural and synthetic coumarin (2H- Results and Discussion
chromen-2-one) derivatives1) have been reported to exert
Electrochemical Studies Figure 1, curve a, shows the
notably antimicrobial2,3) as well as antifungal,4,5) tuberculo- voltammetric curve obtained for the oxidation of catechol
static6) and anti-human immunodeficiency virus (HIV)7,8) ac- (1 mM) in water/acetonitrile (50/50) solution containing
tivity. Moreover, the antibiotic novobiocin belongs to the hy- sodium acetate (cꢀ0.2 M) on a glassy carbon electrode. In the
droxycoumarin series. The importance of these compounds studied potential-range, a well defined voltammetric curve is
have motivated many workers to synthesize a number of obtained that has an anodic (A1) and the corresponding ca-
them, and numerous methods have been developed for their thodic (C1) peaks. These peaks are correspond to the oxida-
preparation.9—11) On the other hand, catechols are a promis- tion of catechol (1a) to o-benzoquinone (2a) and vice versa
ing group of compounds worthwhile for further investigation, within a quasi-reversible two electron process.14—21) The oxi-
which may lead to the discovery of selective acting, bio- dation of catechol (1a) in the presence of 4-mercapto-
degradable agrochemicals having high human, animal and coumarin (3) as a nucleophile was studied in some details.
plant compatibility.12,13) A literature survey reveals that, in Figure 1, curve b shows the cyclic voltammogram obtained
contrast to the widely studied, coumarin derivatives, no paper
has reported the synthesis of 4-(dihydroxyphenylthio)-2H-
chromen-2-one derivatives. Following our experiences in
electrochemical oxidation of catechols in the presence of nu-
cleophiles,14—21) we envisaged that synthesis of organic com-
pounds with both structures of catechol and coumarin might
cause an enhancement of pharmaceutical properties and me-
dicinal activities. This idea prompted us to investigate the
electrochemical oxidation of catechols in the presence of 4-
mercaptocoumarin as the nucleophile and we have discov-
ered an easy and one-pot electrochemical method for the
synthesis of 4-(dihydroxyphenylthio)-2H-chromen-2-one de-
rivatives (4a—d) in high yield and purity, using this environ-
mentally friendly method with high atom economy. In order
to study the feasibility of the electrochemical synthesis of 4-
(dihydroxyphenylthio)-2H-chromen-2-one derivatives (4a—
d) and mechanistic aspects of this conversions, electrochemi-
cal oxidation of catechols in the absence and presence of 4-
mercaptocoumarin was studied using cyclic voltammetry. As
well, in this work computational studies were used for quan-
titative answer to the question concerning the difference in
electrochemical oxidation of catechol in the presence of 4-
hydroxycoumarin and 4-mercaptocoumarin.
Fig. 1. Cyclic Voltammograms of (a) 1 mM Catechol (1a) in the Absence
of 4-Mercaptocoumarin (3), (b) 1 mM Catechol (1a) in the Presence of 1 mM
4-Mercaptocoumarin (3) and (c) 1 mM 4-Mercaptocouamrin (3) in the Ab-
sence of Catechol, at a Glassy Carbon Electrode in Water/Acetonitrile
(50/50) Solution Containing Sodium Acetate (cꢀ0.2 M)
Scan rate: 50 mV sꢁ1; tꢀ25ꢂ1 °C.
∗ To whom correspondence should be addressed. e-mail: nemat@basu.ac.ir
© 2008 Pharmaceutical Society of Japan

November 2008
1563
for a 1 mM solution of 1a in the presence of 1 mM of 4-mer- electrode surface, and ‘C’ represents a homogeneous chemi-
captocoumarin (3). In this case, the presence of the counter- cal reaction) (Chart 1).
part cathodic peak strongly depends on the sweep rate (Fig.
The increasing of the peak A1 current in the presence of 4-
2). Thus, for the highest sweep rate employed a well-defined mercaptocoumarin (3) (Fig. 1, curve b) is due to the oxida-
cathodic peak is observed. The peak current ratio (IpC1/IpA1) is tion of 4-mercaptocoumarin (3) which occurred at the same
lower than one. It is 0.15 for a sweep rate of 50 mV/s, and it potential as that observed for the oxidation of catechol. Also,
increases when the sweep rate increases. A similar situation the positive shift of the peak A1 in the presence of 4-mercap-
is observed when the 4-mercaptocoumarin (3) to catechol tocoumarin (3) (Fig. 1, curve b) that was enhanced during the
(1a) concentration ratio is decreased (Fig. 3). These indicate repetitive recycling of potential is probably due to the forma-
the reactivity of electrochemically generated o-benzoquinone tion of a thin film of product at the surface of the electrode,
(2a) toward 3. In this figure, curve c is the voltammogram of inhibiting, to a certain extent, the performance of the elec-
3.
trode process.14—21) The overoxidation of 4a was circum-
Diagnostic criteria of cyclic voltammetry and the spectro- vented during the preparative reaction because of the pres-
scopic data (IR, 1H-NMR, 13C-NMR and MS) of the isolated ence of the electron-withdrawing group.
products, indicated that the reaction mechanism of electro-
oxidation of catechol (1a) in the presence of 4-mercapto- dation of catechols in the presence of 4-hydroxycoumarin (6)
coumarin (3) is EC (‘E’ represents an electron transfer at the as a nucleophile.22) The results signified the formation of
benzofuran derivatives (coumestan derivatives) (9) via inter
and intramolecular Micheal addition reactions (ECEC mech-
anism) with consumption of four electrons per molecule of
catechols (Chart 2).
We have previously investigated the electrochemical oxi-
The significant difference in electrochemical oxidation of
catechols in the presence of 4-hydroxycoumarin (6) and 4-
mercaptocoumarin (3) is due to the structure of the formed
intermediate (7) in the case of 6 (Chart 3).
Comparison of the structure of 7 with 4a and 4d reveals
that in the compound 7, coumarin group acts as an electron-
donating group whereas in compounds 4a and 4d, mercapto-
coumarin group is an electron-withdrawing group. In this di-
rection, Fig. 4 presents cyclic voltammogram of product 4a
in comparison with catechol. A positive shift in half wave
Fig. 2. Typical Cyclic Voltammograms of 1 mM Catechol (1a) in the Pres-
ence of 1 mM 4-Mercaptocoumarin (3), at a Glassy Carbon Electrode, in
Water/Acetonitrile (50/50) Solution Containing Sodium Acetate (cꢀ0.2 M),
in Various Scan Rates
Scan rate from (a) to (d) are: 25, 35, 45 and 55 mV sꢁ1, respectively.
Chart 1
Fig. 3. Cyclic Voltammograms of 1 mM Catechol (1a) in the Presence of
(a) 1 mM 4-Mercaptocoumarin (3), (b) 2 mM 4-Mercaptocoumarin (3), at a
Glassy Carbon Electrode, in Water/Acetonitrile (50/50) Solution Containing
Sodium Acetate (cꢀ0.2 M)
Scan rate: 45 mV sꢁ1; tꢀ25ꢂ1 °C.
Chart 2

1564
Vol. 56, No. 11
potential (E1/2) of 4a is observed. E1/2 was achieved experi- ROꢁ is hard nucleophile whereas RSꢁ is soft nucleophile.23)
mentally as midpoint potential between the anodic and ca- Therefore, hydroxycoumarin reacted with its C atom (soft-
thodic peaks. E1/2 for the compound 4a and catechol are 0.16 nucleophile–soft-electrophile) whereas mercaptocoumarin
and 0.11 V, respectively (Fig. 4). This confirmed that mercap- reacted with S atom (softer-nucleophile).
tocoumarin group in compound 4a is an electron-withdraw-
ing group.
The electrooxidation of 4-methylcatechol (1d), 3-
methoxycatechol (1c) and 3-methylcatechol (1b) in the pres-
Also, as discussed above, in the electrochemical oxidation ence of 3 proceed in a similar way to that of 1a. The exis-
of catechols in the presence of 4-hydroxycoumarin (6), the tence of a methyl or methoxy group at the C-3 position of 1b
C-alkylation proceeds to form the intermediate 7, and sub- and 1c causes the o-benzoquinones derived from the oxida-
sequent O-cylization affords the benzofuran derivatives. tion of these catechols (2b, 2c) attacked by 3 from C-4 or C-
Whereas, in the presence of 4-mercaptorcoumarin (3), S- 5 positions to yield two types of product in each case which
1
alkylation proceeds to form the compound 4. This can be ex- has been confirmed by H-NMR results. So, we suggest that
plained using the hard and soft acids and bases (HSAB) con- o-benzoquinones 2b and 2c are attacked from two positions
cept. According to this concept, hard acids tend to interact by 3 leading to the formation of the two types of product in
with hard bases, while soft acids tend to interact strongly each case (isomers A and B) (Chart 4).
with soft bases. The carbon is a soft electrophile and the
A characteristic feature of the electrolysis is that low cur-
rent density is required. The current efficiency and yield of
product decrease with increasing current density. These ob-
servations can be explained by the occurrence of back reac-
tions, such as the reduction of o-benzoquinones (2) on the
platinum cathode and side reactions such as oxidation of nu-
cleophile and/or final product (4) during constant current
electrolysis in an undivided cell. In this work current density
1.5 mA/cm2 is preferred.
Chart 3
Computational Studies Here, the difference in electro-
chemical oxidation of catechol in the presence of 4-hydroxy-
coumarin (6) and 4-mercaptocoumarin (3) was explained by
computational structure, NBO (natural bond orbital) analysis
and density functional theory (DFT: B3LYP/6-31G*//
B3LYP/6-31G*) based methods, using the GAUSSIAN 98
package of programs. Structural parameters for the ground
state of compounds 1a, 7 and 4a, were calculated by
B3LYP/6-31G* level of theory. These results showed that in
the compound 7, catechol and the substituted rings have the
same direction and therefore electronic delocalization of
p→p* occurred between catechol and adjacent rings. But in
the compound 4a, rings are approximately orthogonal to-
gether (C1–S–C8 angle is 103 degree), and hence the elec-
tronic delocalization of p→p* is stopped and substituted
rings attract electrons of catechol ring (Figs. 5, 6).
Fig. 4. Cyclic Voltammograms of (a) 1 mM Catechol (1a), (b) 1 mM 4-(3,4-
Dihydroxyphenylthio)-2H-chromen-2-one (4a) at a Glassy Carbon Electrode
in Water/Acetonitrile (50/50) Solution Containing Sodium Acetate (cꢀ
0.2 M)
Based on the optimized ground state geometry using
B3LYP/6-31G* method, the NBO analysis of donor–acceptor
(bond–antibond) interactions revealed that the stabilization
energies (E2) associated with the electronic delocalization
Scan rate: 50 mV sꢁ1; tꢀ25ꢂ1 °C.
Chart 4

November 2008
1565
Fig. 5. Numbering Used for Structure of Compounds 1a, 7, and 4a
Fig. 6. B3LYP/6-31G* Calculated Structure of Compounds 7 and 4a
Table 1. NBO Stabilization Energies (E2) of Electronic Delocalization groups) against compound 1a and also showed that conjuga-
p→p* Molecular Orbital, Based on the B3LYP/6-31G* Calculated Geome-
tries
tion in the compound 7 is very greater than the compound
4a.
B3LYP/6-31G*
Experimental
Resonance energy
Reaction equipments are described in earlier papers.14—21) All chemicals
were reagent-grade materials. Sodium acetate, solvents and reagents were of
pro-analysis. These chemicals were used without further purification. 4-Mer-
captocoumarin (4-mercapto-2H-chromen-2-one) was prepared by the proce-
dure reported previously.24)
Compound 1a
Compound 7
Compound 4a
p
p
p
p
p
p
p
p
1–2→p*3–4
1–2→p*5–6
3–4→p*1–2
3–4→p*5–6
5–6→p*1–2
5–6→p*3–4
2–3→p*1–6
1–6→p*2–3
19.58
18.71
17.89
18.22
20.24
20.36
—
19.80
19.21
18.08
17.48
19.09
21.27
—
—
—
—
—
—
Electroorganic Synthesis A solution (ca. 80 ml) of sodium acetate so-
lution (cꢀ0.2 M) in water/acetonitrile (50/50) containing 1 mmol of cate-
chols (1a—d) and 1 mmol of 4-mercaptocoumarin (3) was electrolyzed in an
undivided cell equipped with graphite anode (an assembly of four rods with
30 cm2 area) and a large stainless steel gauze cathode at 25 °C under con-
stant-current density of 1.5 mA/cm2. The electrolysis was terminated when
the anodic peak that corresponds to the oxidation of catechol (1a—d) (A1 in
Fig. 1) in cyclic voltammetry disappears (after consumption of 2.9 F/mol,
current efficiency 69%). The process was interrupted during the electrolysis,
and the graphite anode was washed in acetone in order to reactivate it. At the
end of electrolysis, to achieve better precipitation, a few drops of acetic acid
were added to the solution and the cell was placed in a refrigerator
overnight. The solid precipitated was collected by filtration and was washed
several times with water.
—
18.54
18.98
0.96
—
—
—
—
Lp S→p*1–6
Table 2. NBO Calculated Energetic Gap of Frontier Molecular Orbital,
Based on the B3LYP/6-31G* Calculated Geometries
B3LYP/6-31G*
Energy
Compound 1a
Compound 7
Compound 4a
Characteristic of Products (4a—d). 4-(3,4-Dihydroxyphenylthio)-
2H-chromen-2-one (C15H10O4S) (4a) mp 245—247 °C (dec.). IR (KBr)
cmꢁ1: 3421, 3297, 3048, 1715, 1604, 1548, 1447, 1387, 1341, 1271, 1158,
943, 822, 759, 743, 652. 1H-NMR (300 MHz, acetone-d6) d: 5.52 (s, 1H, C9-
H), 7.08 (d, Jꢀ1.3 Hz, 2H, aromatic), 7.14 (t, Jꢀ1.1 Hz, 1H, aromatic), 7.41
(m, Jꢀ7.9 Hz, 2H, aromatic), 7.68 (t, Jꢀ8.1 Hz, 1H, aromatic), 7.91 (dd,
Jꢀ8.1, 1.6 Hz, 1H, aromatic) and 8.66 (broad, 2H, –OH). 13C-NMR
(75.4 MHz, DMSO-d6) d: 107.9, 115.1, 117.3, 117.4, 118.0, 122.8, 124.0,
124.6, 128.8, 132.9, 147.1, 148.7, 152.7, 158.6, 158.9. MS (EI): m/z (rela-
HOMO
LUMO
DE
ꢁ0.20665
0.09807
0.21472
ꢁ0.19811
ꢁ0.06221
0.13590
ꢁ0.21724
ꢁ0.07289
0.14435
p→p* molecular orbital decreased in the compound 7 to 4a.
NBO results show that the sum of p→p* resonance energy
in catechol ring of the compounds 7 and 4a are 114.93 and
38.48 kcal molꢁ1, respectively (Table 1).
In addition, NBO results are used to study frontier molec-
ular orbitals. Energetic gap of HOMO and LUMO for com-
pounds 1a, 7 and 4a showed that this gap of energy for the
compound 1a, 7 and 4a are 0.21472, 0.13590 and 0.14435,
respectively (Table 2).
tive intensity): 286 [M]ꢃ (71), 268 (58), 253 (32), 235 (48), 203 (25), 192
·
(13), 178 (24), 145 (59), 121 (52), 108 (81), 89 (93), 77 (71), 63 (66), 43
(100).
4-(3,4-Dihydroxy-5-methylphenylthio)-2H-chromen-2-one (4b) and 4-
(3,4-Dihydroxy-2-methylphenylthio)-2H-chromen-2-one (C16H12O4S)
(5b) (Isomers A and B) mp 257—261 °C (dec.). IR (KBr) cmꢁ1: 374,
2925, 2855, 1718, 1684, 1603, 1547, 1447, 1342, 1290, 1182, 1122, 1024,
943, 844, 820, 759, 743, 706. 1H-NMR (300 MHz, acetone-d6) d: 2.28 (s,
3H, methyl), 2.32 (s, 3H, methyl), 5.39 (s, 1H, C9-H), 5.57 (s, 1H, C9-H),
6.93 (m, Jꢀ8.6 Hz, 1H, aromatic), 7.06 (m, Jꢀ8.6 Hz, 1H, aromatic), 7.37
(m, Jꢀ9.1 Hz, 2H, aromatic), 7.43 (m, Jꢀ9.1 Hz, 2H, aromatic), 7.67 (m,
Decreased gap of energy for the compounds 7 and 4a
against the compound 1a revealed that the conjugated double
bonds in these molecules increased (aromatic substituted Jꢀ2.9 Hz, 2H, aromatic), 7.77 (m, Jꢀ2.9 Hz, 2H, aromatic), 7.89 (dd,

1566
Vol. 56, No. 11
Jꢀ8.4, 1.7 Hz, 1H, aromatic), 7.96 (dd, Jꢀ8.4, 1.7 Hz, 1H, aromatic). 13C-
NMR (75.4 MHz, acetone-d6) d: 10.8, 13.9, 107.5, 107.9, 114.3, 117.3,
120.2, 124.0, 124.1, 124.6, 128.8, 129.1, 130.2, 131.5, 132.8, 132.9, 133.0,
145.4, 146.1, 146.9 148.0, 152.7, 152.8, 157.8, 158.5, 158.6, 159.0, 167.5.
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MS (EI): m/z (relative intensity): 300 [M]ꢃ (100), 288 (31), 282 (51), 267
·
(41), 239 (11), 178 (70), 150 (36), 145 (47), 121 (45), 110 (26), 89 (59), 77
(20), 63 (33).
4-(3,4-Dihydroxy-5-methoxyphenylthio)-2H-chromen-2-one
(C16H12O5S) (4c) mp 228—230 °C (dec.). IR (KBr) cmꢁ1: 3438, 3176,
2927, 1673, 1599, 1544, 1505, 1343, 1309, 1239, 1196, 1175, 1107, 952,
843, 824, 762. 1H-NMR (300 MHz, acetone-d6) d: 3.90 (s, 3H, methoxy),
5.61 (s, 1H, C9-H), 6.86 (s, 2H, aromatic), 7.41 (m, Jꢀ8.8 Hz, 2H, aro-
matic), 7.70 (m, Jꢀ8.5 Hz, 1H, aromatic), 7.91 (dd, Jꢀ8.3, 1.6 Hz, 1H, aro-
matic) and 9.2 (broad, about 2H, –OH). 13C-NMR (75.4 MHz, acetone-d6) d:
56.3, 108.0, 111.4, 114.5, 117.2, 117.3, 118.0, 123.9, 124.6, 132.9, 137.4,
147.1, 149.7, 152.7, 158.5, 158.7. MS (EI): m/z (relative intensity): 316
[M]ꢃ (100), 283 (60), 178 (74), 145 (50), 121 (41), 89 (53), 63 (34).
·
4-(4,5-Dihydroxy-2-methylphenylthio)-2H-chromen-2-one
(C16H12O4S) (4d) mp 273—275 °C (dec.). IR (KBr) cmꢁ1: 3344, 1686,
1600, 1546, 1519, 1445, 1414, 1344, 1320, 1270, 1187, 1158, 950, 869,
841, 824,767, 743. 1H-NMR (300 MHz, acetone-d6) d: 2.29 (s, 3H, methyl),
5.41 (s, 1H, C9-H), 7.01 (s, 1H, aromatic), 7.10 (s, 1H, aromatic), 7.44 (m,
Jꢀ8.1 Hz, 2H, aromatic), 7.69 (t, Jꢀ9.2 Hz, 1H, aromatic), 7.94 (d,
Jꢀ8.4 Hz, 1H, aromatic), 8.5 (broad, 2H, –OH). 13C-NMR (75.4 MHz, ace-
tone-d6) d: 19.2, 107.3, 113.4, 117.2, 118.1, 118.7, 123.5, 124.2, 124.6,
132.9, 135,6, 145.0, 148.9, 152.9, 157.5, 158.5. MS (EI): m/z (relative inten-
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sity): 300 [M]ꢃ (38), 272 (8), 267 (16), 178 (24), 145 (30), 121 (44), 89
·
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(100), 77 (40), 63 (78), 39 (50).
Computational Details DFT (density functional theory) calculations
were carried out using B3LYP/6-31G*//B3LYP/6-31G*, level of theory with
the GAUSSIAN 98 package of programs25) implemented on a Pentium-PC
computer with a 2.6 GHz processor. Initial estimation of the structural
geometry of the compounds 1a, 7 and 4a was obtained by a molecular me-
chanic program PCMODEL (88.0),26) and for further optimization of geom-
etry, we used the PM3 method of the MOPAC 7.0 computer program.27,28)
The GAUSSIAN 98 package of programs were finally used to perform DFT
calculations at the B3LYP/6-31G* level. Energy-minimum molecular
geometries were located by minimizing energy, with respect to all geometri-
cal coordinates without imposing any symmetrical constraints. The nature of
the stationary points for the compounds 1a, 7 and 4a has been fixed by
means of the number of imaginary frequencies. For minimum state struc-
ture, only real frequency values, and in the transition-state, only single imag-
inary frequency values, were accepted.29,30) NBO analysis was then per-
formed at the B3LYP/6-31G* level by the NBO 3.1 program31,32) included in
the GAUSSIAN 98 package of programs.
Acknowledgements Financial support for this work by the Iran Na-
tional Science Foundation (INSF), Tehran, Iran, is gratefully acknowledged.
26) Serena Software, Box 3076, Bloomington, IN, U.S.A.
27) Stewart J. J. P., QCPE 581, Department of Chemistry, Indiana Univer-
sity, Bloomington, IN, U.S.A.
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