Palladium-catalyzed arylations in 4-pyrone systems: 2,6-diaryl-3,5-dibromo-4-pyrones and kojic acid

Article abstract of DOI:10.1007/s00706-015-1589-2

Palladium-catalyzed cross-coupling reactions of 2,6-diaryl-3,5-dibromo-4-pyrones with imidazole and benzimidazole in the presence of N,N?-dibutylbenzimidazol-2-ylidene as ligand resulted in the formation of the monosubstituted C-and N-arylated products. K

Full text of DOI:10.1007/s00706-015-1589-2

Monatsh Chem
DOI 10.1007/s00706-015-1589-2
ORIGINAL PAPER
Palladium-catalyzed arylations in 4-pyrone systems: 2,6-diaryl-
3
,5-dibromo-4-pyrones and kojic acid
1
1
1
1
Zarrin Ghasemi
•
Fatemeh Poorhossain Mejarshin
•
Zahra Fathi
•
Aziz Shahrisa
Received: 15 August 2015 / Accepted: 24 October 2015
Ó Springer-Verlag Wien 2016
Abstract Palladium-catalyzed cross-coupling reactions
of 2,6-diaryl-3,5-dibromo-4-pyrones with imidazole and
processes by activation of aromatic C–H bond and enabling
direct arylation reactions of aromatics and heteroaromatics
have greatly facilitated the synthesis of more complex
molecular systems from simpler ones [4–9]. Palladium
complexes of N-heterocyclic carbenes (NHC) are potential
catalytic species for the most challenging coupling reactions
[10–12]. In this area, the catalysts containing benzimidazole
ligand have been also well defined [13, 14]. Molecules
bearing aryl-heteroaryl bonds which are accessible through
above methodologies play a pivotal role in many of phar-
maceuticals [15], optoelectronic materials [16], liquid
crystals [17], and ligands for asymmetric catalysis [18].
0
benzimidazole in the presence of N,N -dibutylbenzimida-
zol-2-ylidene as ligand resulted in the formation of the
monosubstituted C- and N-arylated products. Kojic acid
was also treated with some aryl halides at the same con-
ditions, to afford C-6 arylated derivatives.
Graphical abstract
O
O
O
O
HO
HO
Ar
Cat. Ligand
+
Ar-Br
K CO / DMF / 130 o
C
2
3
4-Pyrones and their natural and bioactive derivatives [19, 20]
OH
OH
could be the central structural elements in such polyaromatic
systems [21, 22]. Kojic acid (5-hydroxy-2-(hydroxymethyl)-
4-pyrone) is a natural antioxidant and metabolic compound
Keywords Palladium-catalyzed arylations Á
that is produced by several species of fungi. Its derivatives
which are widelyrepresented in various biologically relevant
compounds have been the subject of several synthetic studies
C–H activation Á 2,6-Diaryl-4-pyrones Á Kojic acid Á
Cross couplings
[
23–25]. In connection with our studies directed toward the
synthesis of various heterocyclic derivatives of 4-pyrones
26, 27], we planned the direct arylation of 4-pyrones to
Introduction
[
afford substituted molecules.
Palladium-catalyzed cross-coupling reactions have arguably
become among the most powerful and versatile tools for
carbon–carbon bond formation [1–3]. Development of these
Due to the importance of C–H activation in the phe-
nolic compounds [28], herein we report the palladium-
catalyzed direct arylation reactions of kojic acid as a
natural OH containing heterocycle. On the other hand, we
have already reported the synthesis of 2,6-diaryl-3,5-di-
bromo-4-pyrones and their Pd catalyzed Heck reactions
with alkyl acrylates [29]. Because of the presence of
imidazole and benzimidazole nucleus in a large number of
naturally occurring as well as biologically active mole-
cules [30, 31], we decided to carry out the coupling
reactions of these azoles with above dibromides to access
new derivatives of 4-pyrones.
Electronic supplementary material The online version of this
article (doi:10.1007/s00706-015-1589-2) contains supplementary
material, which is available to authorized users.
&
Aziz Shahrisa
ashahrisa@yahoo.com
1
Department of Organic and Biochemistry, Faculty of
Chemistry, University of Tabriz, 5166614766 Tabriz, Iran
123

Z. Ghasemi et al.
Results and discussion
diphenyl-4-pyrone (2), which was prepared by bromination
of 2,6-diphenyl-4-pyrone with NBS/DMF [29], was chosen
as halide partner in the catalytic reactions with imidazole.
The results have been summarized in Table 2. As shown,
treatment of dibromide 2 with 2 eq imidazole in the pres-
ence of Pd or Cu based catalytic systems gave the two
monoarylated products 3a and 3b. In fact, the N–H group
We started our experiments by the metal catalyzed reactions
of kojic acid with aryl halides in the presence of a ligand and
a base. Table 1 shows the results of treatment of bro-
mobenzene with kojic acid at various conditions. Because
this 4-pyrone derivative is likely to react via phenolic OH
group (Ullman reaction) or C-6 carbon, we examined both
Pd(II)- and Cu(I)-based reactions in the presence of a ligand
as cocatalyst. As shown, the first reaction by employment of
and C
alyzed arylation of imidazole and benzimidazole [32, 33].
Once again, the use of Pd(OAc) with dibutylbenzimida-
–H are the most active positions for the metal cat-
2
2
2
.5 mol% palladium chloride as catalyst, 5 mol% triphen-
zolium bromide as cocatalyst was more beneficial (entry 1).
In all cases, the yields of two isomeric products were very
similar and no disubstituted product was detected. Benz-
imidazole also reacted with dibromide 2 under the
optimized conditions and the corresponding products 4a
and 4b were obtained (Fig. 2). In a similar manner and
under the same conditions, the substituted products 4c and
4d were synthesized by means of the reaction of 2,6-bis(p-
tolyl)-3,5-dibromo-4-pyrone [29] with benzimidazole.
In conclusion, we synthesized some new coupling
derivatives of 4-pyrones. Reaction of commercially avail-
able kojic acid with arylbromides in the presence of
ylphosphine as ligand and K CO in dry DMF, gave no
2
3
notable product even after 24 h at 130 °C (entry 1). The use
of N,N -dibutylbenzimidazolium bromide (DBBIB) instead
0
of PPh resulted in the formation of only C-phenylated
3
product 1a after 5 h in 68 % yield (entry 2). Application of
palladium acetate as catalyst under otherwise identical
conditions produced the higher yield (entry 3). Furthermore,
heating a mixture of reactants in an aqueous medium (H O:
2
1
,4-dioxane 1:1) at reflux produced only trace amount of 1a
entry 4). Treatment of kojic acid with 4-bromotoluene,
-bromochlorobenzene and 1-bromonaphthalene under the
(
4
optimized conditions (Table 1, entry 3) resulted in the rela-
Pd(OAc) , N,N’-dibutylbenzimidazolium bromide and a
2
ted C-6 arylated derivatives 1b–1d (Fig. 1). However,
4
base gave the C-6 arylated products. Coupling of imidazole
and benzimidazole with 2,6-diaryl-3,5-dibromo-4-pyrones
at the same conditions produced the C-3 monosubstituted
products.
-bromonitrobenzene, 4-bromoanisole, and 4-bromo-N,N-
dimethylaniline gave only the homocoupling products of
these arylbromides at the same conditions, and no cross-
coupling derivatives of 4-pyrone were observed. We also
tested copper promoted reactions of these substrates in the
presence of two ligands. But none of them leaded to any
product (Table 1, entries 5 and 6).
Experimental
We then directed our efforts to the bromo-4-pyrone
derivatives to evaluate their reactivity in direct reactions
with NH containing heteroaryls. 3,5-Dibromo-2,6-
Melting points were determined on a Electrothermal MEL-
TEMP apparatus (model 1202D). FT-IR spectra were
obtained with a Bruker Tensor 27 spectrometer. NMR
Table 1 Metal catalyzed reactions of kojic acid with bromobenzene
O
O
Br
HO
HO
Cat. Ligand
K CO / DMF / 130 C
+
O
o
O
2
3
OH
OH
1
a
Entry
Cat. (mol%)
Ligand (mol%)
PPh (5)
Time/h
Solvent
Yield/%
1
2
3
4
5
6
PdCl
2
2
(2.5)
(2.5)
3
24
5
DMF
DMF
DMF
Trace
68
PdCl
DBBIB (5)
DBBIB (5)
DBBIB (5)
L-Proline (10)
Pd(OAc)
Pd(OAc)
CuI (5)
2
2
(2.5)
(2.5)
5
76
24
24
24
H
2
O:dioxane (1:1)
DMF
CH CN
Trace
–
CuI (5)
1,10-Phenantroline (10)
CO
3
–
3
(2 mmol), 3 cm solvent
Conditions: kojic acid (2 mmol), aryl bromide (4 mmol), K
2
3
1
23

Palladium-catalyzed arylations in 4-pyrone systems: 2,6-diaryl-3,5-dibromo-4-pyrones and…
Fig. 1 C-6 arylated derivatives
of kojic acid
O
O
O
O
O
O
HO
HO
HO
OH
OH
OH
H C
Cl
3
1
b (74%)
1c (67%)
1d (72%)
Table 2 Metal catalyzed reaction of imidazole with 3,5-dibromo-2,6-diphenyl-4-pyrone (2)
O
N
O
O
O
HN
N
N
Br
Br
Cat. Ligand
Br
N
Br
Ph
N
+
+
K CO / DMF / 130 o
C
Ph
O
Ph
2
3
H
Ph
O
Ph
Ph
2
3a
3b
Entry
Cat. (mol%)
Ligand (mol%)
Time/h
3a/%
3b/%
1
2
3
4
5
Pd(OAc)
2
(2.5)
DBBIB (5)
DBBIB (5)
2
2
44
35
32
30
32
37
27
33
25
27
PdCl (2.5)
2
Pd(OAc)
CuI (5)
CuI (5)
2
(2.5)
PPh
L-Proline (10)
1,10-Phenantroline (10)
3
(5)
24
24
24
3
Conditions: dibromide 2 (2 mmol), imidazole (4 mmol), K
2
CO
3
(4 mmol), 3 cm DMF, 130 °C
General procedure for the synthesis of compounds
1a–1d
O
O
HN
O
O
Br
N
N
Br
N
A mixture of 0.284 g kojic acid (2 mmol, 2 eq), arylbro-
mide (4 mmol, 4 eq), 0.011 g palladium acetate
(0.05 mmol), 0.031 g dibutylbenzimidazolium bromide
R
R
R
R
(
0.1 mmol, 0.1 eq), 0.276 g K CO (2 mmol, 4 eq), and
2 3
4
4
b: R=H (36%)
d: R=CH (39%)
3
4
4
a: R=H (45%)
3 cm DMF was heated at 130 °C for 5 h. After cooling to
c: R=CH3 (42%)
3
3
25 °C, 30 cm
extracted with ethyl acetate (3 9 20 cm ). The organic
3
layers were combined, washed with 50 cm water, the
water was added and the mixture was
3
Fig. 2 Coupling of benzimidazole with 2,6-diaryl-3,5-dibromo-4-
pyrones
washed extracts were dried over Na SO , the dried extracts
4
2
were filtered, and the filtrate was concentrated under vac-
uum. Purification of the product was done by preparative
thin layer chromatography using acetone: n-hexane (1:3) to
obtain products 1a–1d.
spectra were recorded with a Bruker Spectrospin Avance
1
00 spectrometer operating at 400 MHz ( H NMR) and
4
1
00 MHz ( C NMR), chemical shifts are given in parts per
3
1
million (d/ppm) relative to solvent peaks as standards
3-Hydroxy-6-(hydroxymethyl)-2-phenyl-4H-pyran-4-one
1 13
CDCl3: 7.26 ppm ( H), 76 ppm ( C); DMSO-d6:
(
(1a, C12
H O )
10 4
1
13
.50 ppm ( H), 39.5 ppm ( C)) or tetramethylsilane at
2
Brown solid; yield 76 %; m.p.: 124–126 °C; FT-IR (KBr):
-
1 1
298 K. Elemental analysis was measured by Vario EL III
apparatus (Elementar Co.). Preparative thin layer chro-
vꢀ = 3300, 2923, 1647, 1615, 1572, 1215, 1108 cm ; H
NMR (400 MHz, DMSO-d ): d = 4.43 (d, 2H,
J = 4.1 Hz, benzylic CH ), 5.76 (s, 1H, benzylic OH),
6
matographies (PLC) were done with prepared glass-backed
2
plates (20 9 20 cm , 500 l) using silica gel (Merck
Kieselgel 60 PF254?366).
2
6.41 (s, 1H, pyrone H5), 7.44–7.51 (m, 3H, phenyl-H), 8.04
(d, 2H, J = 7.1 Hz, phenyl-H), 9.51 (s, 1H, pyrone-OH)
123

Z. Ghasemi et al.
1
3
ppm; C NMR (100 MHz, DMSO-d ): d = 59.6 (CH -
5-Bromo-2,6-diphenyl-3-(1H-imidazol-1-yl)-4H-pyran-4-
6
2
OH), 108.2, 126.8, 128.5, 129.3, 130.9, 142.3, 144.2,
one (3a, C H BrN O )
13
2
0
2 2
1
67.6, 174.2 (pyrone C = O) ppm.
Yellow solid; yield 44 %; m.p.: 172–175 °C; FT-IR (KBr):
-1 1
vꢀ = 1685, 1468, 1263, 1017 cm ; H NMR (400 MHz,
3
-Hydroxy-6-(hydroxymethyl)-2-(4-methylphenyl)-4H-
CDCl ): d = 7.23 (d, 2H, J = 8.7 Hz, Ar–H), 7.46–7.53
3
pyran-4-one (1b, C H O )
1
3 12 4
(
m, 4H, Ar–H), 7.56-7.63 (m, 4H, Ar–H), 7.94 (s, 1H, Im-
Brown solid; yield 74 %; m.p.: 142–144 °C; FT-IR (KBr):
1
H2), 8.25 (d, 2H, J = 7.7 Hz, Ar–H) ppm; C NMR
3
-
1
vꢀ = 3229, 2922, 1640, 1611, 1455, 1208, 1106, 797 cm
;
(
100 MHz, CDCl ): d = 97.6, 119.4, 126.2, 126.6, 127.2,
1
3
H NMR (400 MHz, DMSO-d ): d = 2.35 (s, 3H, CH ),
6
3
1
1
27.8, 127.9, 128.9, 129.4, 129.6, 130.7, 131.0, 132.1,
37.3, 167.8, 177.9 ppm.
4
.42 (s, 2H, benzylic CH ), 5.80 (s, 1H, benzylic OH), 6.39
2
(
s, 1H, pyrone-H5), 7.31 (d, 2H, J = 7.6 Hz, phenyl-H),
1
.93 (d, 2H, J = 7.6 Hz, phenyl-H) ppm; C NMR
3
7
5-Bromo-2,6-diphenyl-3-(1H-imidazol-2-yl)-4H-pyran-4-
(
100 MHz, DMSO-d ): d = 21.1, 59.8, 108.3, 126.9,
one (3b, C H BrN O )
20 13
6
2 2
1
28.2, 129.2, 139.3, 142.0, 144.7, 167.6, 174.3 ppm.
Yellow solid; yield 37 %; m.p.: 198–200 °C; FT-IR (KBr):
-1 1
vꢀ = 3564, 1691, 1587, 1564, 1260, 1022 cm ; H NMR
400 MHz, CDCl ): d = 7.03 (s, 1H, Im-H), 7.52–7.63 (m,
2
-(4-Chlorophenyl)-3-hydroxy-6-(hydroxymethyl)-4H-
(
3
pyran-4-one (1c, C H ClO )
1
2
9
4
9
H, Ar–H), 7.73 (s, 1H, Im-H), 8.21 (d, 2H, J = 7.9 Hz,
1
Brown solid; yield 67 %; m.p.: 173–175 °C; FT-IR (KBr):
vꢀ = 3300, 2923, 1688, 1558, 1226, 1132 cm ; H NMR
3
Ar–H) ppm; C NMR (100 MHz, CDCl ): d = 97.7,
-
1
1
3
119.7, 126.3, 127.2, 127.8, 128.0, 128.1, 129.4, 129.7,
(
400 MHz, DMSO-d ): d = 4.43 (s, 2H, benzylic CH ),
6
2
130.7, 131.2, 132.1, 137.5, 167.9, 177.9 ppm.
5
.79 (s, 1H, benzylic OH), 6.41 (s, 1H, pyrone-H5), 7.60
(
d, 2H, J = 6.9 Hz, phenyl-H), 8.06 (d, 2H, J = 6.9 Hz,
1
3-(1H-Benzimidazol-1-yl)-5-bromo-2,6-diphenyl-4H-
3
C
phenyl-H) ppm;
NMR (100 MHz, DMSO-d6):
pyran-4-one (4a, C H BrN O )
24
15
2 2
d = 59.3, 111.2, 128.4, 130.2, 131.4, 135.6, 141.5, 146.5,
Yellow solid; yield 45 %; m.p.: 126 °C (decomp.), FT-IR
-1 1
1
68.1, 173.2 ppm.
(KBr): vꢀ = 3068, 1682, 1553, 1448, 1027 cm ; H NMR
400 MHz, CDCl ): d = 6.72 (d, 1H, J = 8.2 Hz, Ar–H),
(
3
3
-Hydroxy-6-(hydroxymethyl)-2-(naphthalene-1-yl)-4H-
7
7
8
.14–7.17 (m, 1H, Ar–H), 7.31–7.35 (m, 1H, Ar–H),
.46–7.60 (m, 8H, Ar–H), 7.88 (d, 1H, J = 8.1 Hz, Ar–H),
.13 (dd, 2H, J = 7.4, 1.1 Hz, Ar–H), 8.24 (s, 1H,
pyran-4-one (1d, C H O )
1
6 12 4
Brown solid; yield 72 %; m.p.: 158–160 °C; FT-IR (KBr):
vꢀ = 3282, 3046, 2924, 1642, 1620, 1569, 1226,
1
3
benzimidazole-H2) ppm; C NMR (100 MHz, CDCl3):
-
1
1
1
132 cm
; H NMR (400 MHz, DMSO-d ): d = 4.37
6
d = 98.2, 111.7, 119.8, 122.7, 123.2, 126.0, 127.1, 127.5,
(
s, 2H, benzylic CH ), 5.73 (s, 1H, benzylic OH), 6.51 (s,
2
127.7, 127.8, 127.9, 128.2, 129.7, 131.3, 132.1, 142.6,
1
1
H, pyrone H5), 7.57–8.09 (m, 7H, naphthyl-H), 9.11 (s,
143.0, 166.9, 178.7 ppm.
1
H, pyrone-OH) ppm; C NMR (100 MHz, DMSO-d6):
3
d = 59.7 (CH ), 109.1, 125.2, 125.3, 126.3, 126.9, 127.9,
3-(1H-Benzimidazol-2-yl)-5-bromo-2,6-diphenyl-4H-
2
1
28.4, 128.6, 130.1, 130.3, 133.1, 142.9, 146.2, 168.1,
pyran-4-one (4b, C H BrN O )
2
4
15
2 2
1
74.1 ppm.
Yellow solid; yield 36 %; m.p.: 212 °C (decomp.); FT-IR
-
KBr): vꢀ = 3429, 1681, 1551, 1448, 1262, 804, 686 cm
;
1
(
1
H
NMR (400 MHz, CDCl3): d = 6.74 (d, 1H,
J = 8.0 Hz, Ar–H), 7.11–7.15 (m, 1H, Ar–H), 7.29–7.31
m, 1H, Ar–H), 7.49–7.67 (m, 8H, Ar–H), 7.88 (d, 1H,
J = 5.6 Hz, benzimidazole-H), 8.11 (s, 1H, benzimida-
General procedure for synthesis of compounds 3
and 4
(
A mixture of 2,6-diaryl-3,5-dibromo-4-pyrone (2 mmol,
1
3
zole-H)), 8.27 (d, 2H, J = 7.7 Hz, Ar–H) ppm; C NMR
2
eq), imidazole or benzimidazole (4 mmol, 4 eq), 0.011 g
(
100 MHz, CDCl ): d = 97.7, 110.5, 119.7, 122.2, 122.9,
3
palladium acetate (0.05 mmol), 0.031 g dibutylbenzimi-
125.3, 126.3, 127.2, 128.0, 128.2, 129.3, 130.1, 130.7,
dazolium bromide (0.1 mmol, 0.1 eq), 0.276 g K CO
2
3
3
132.1, 132.2, 138.2, 143.1, 143.3, 167.9, 177.9 ppm.
(
2 mmol, 4 eq), and 3 cm DMF was heated at 130 °C for
3
h. After cooling, 30 cm water was added and the mix-
2
3-(1H-Benzimidazol-1-yl)-2,6-bis(4-methylphenyl)-5-
3
ture was extracted with ethyl acetate (3 9 20 cm ). The
bromo-4H-pyran-4-one (4c, C H BrN O )
2
6
19
2 2
3
organic layers were combined, washed with 50 cm water,
Yellow solid; yield 42 %; m.p.: 131 °C (decomp.); FT-IR
-
1 1
the washed extracts were dried over Na SO , the dried
2
4
(KBr): vꢀ = 3067, 2923, 1688, 1631, 1444, 1082 cm ; H
NMR (400 MHz, CDCl ): d = 2.42 (s, 3H, CH ), 2.45 (s,
extracts were filtered, and the filtrate was dried over Na2-
3
3
SO and concentrated to dryness. The residue was purified
4
3H, CH ), 6.77 (d, 1H, J = 8.4 Hz, Ar–H), 7.14–7.18 (m,
3
by preparative thin layer chromatography using ethyl
1H, Ar–H), 7.26–7.40 (m, 5H, Ar–H), 7.45 (d, 2H,
acetate: n-hexane (2:5) to obtain products 3 and 4.
J = 8.0 Hz, Ar–H), 7.88 (d, 1H, J = 7.9 Hz, Ar–H),
1
23

Palladium-catalyzed arylations in 4-pyrone systems: 2,6-diaryl-3,5-dibromo-4-pyrones and…
8
.00 (d, 2H, J = 8.1 Hz, Ar–H), 8.20 (s, 1H, benzimidazol-
1
12. Viciu MS, Germaneau RF, Navarro-Fernandez O, Stevens ED,
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3
H2) ppm; C NMR (100 MHz, CDCl ): d = 20.7, 20.8,
3
¨
¨
1
3. Do gˇ an O, Demir S, Ozdemir I, C¸ etinkaya B (2011) Appl Orga-
nomet Chem 25:163
1
1
1
10.6, 111.8, 119.7, 122.6, 123.1, 123.3, 124.9, 127.1,
27.3, 128.2, 128.7, 128.9, 129.3, 129.7, 142.1, 142.6,
43.1, 143.2, 166.9, 177.7 ppm.
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3
-(1H-Benzimidazol-2-yl)-2,6-bis(4-methylphenyl)-5-
bromo-4H-pyran-4-one (4d, C H BrN O )
2 2
2
6
19
Yellow solid; yield 39 %; m.p.: 206 °C (decomp.); FT-IR
-
1 1
(
KBr): vꢀ = 3469, 2923, 1687, 1579, 1088 cm ; H NMR
400 MHz, CDCl ): d = 2.45 (s, 3H, CH ), 2.48 (s, 3H,
CH ), 6.76 (d, 1H, J = 7.9 Hz, Ar–H), 7.11–7.18 (m, 1H,
(
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1
2
4:7349
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3
3
3
9. Neupane P, Xia L, Lee YR (2014) Adv Synth Catal 356:2566
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Ar–H), 7.26–7.31 (m, 4H, Ar–H), 7.38 (d, 2H, J = 7.8 Hz,
Ar–H), 7.46 (d, 2H, J = 7.6 Hz, Ar–H), 7.92 (s, 1H, N–H),
4
1
3
8
.16 (d, 2H, J = 7.6 Hz, Ar–H) ppm;
C NMR
(
4
(
100 MHz, CDCl ): d = 20.8, 20.9, 96.9, 110.6, 119.6,
3
1
1
1
22.2, 122.9, 123.6, 127.1, 127.3, 128.5, 128.7, 128.9,
29.3, 129.7, 132.2, 138.2, 141.6, 143.2, 143.5, 167.9,
77.9 ppm.
1
22. Caturla F, Amat M, Reinoso RF, Calaf E (2006) Bioorg Med
Chem Lett 16:3605
2
2
2
2
3. Liu X, Xia W, Jiang Q, Xu Y, Yu P (2014) J Agric Food Chem
2:297
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Acknowledgments We are grateful to Research Affair of the
University of Tabriz for financial support.
6
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