H3PW12O40-catalysed alkylation of arenes and diveratrylmethanes: Convenient routes to triarylmethanes and to symmetrical and unsymmetrical 9,10-diaryl-2,3,6,7-tetramethoxyanthracenes

Article abstract of DOI:10.1002/ejoc.201001267

An efficient method for the synthesis of triarylmethanes and difurylarylmethanes through solventless reactions between aldehydes and arenes in the presence of H₃PW₁₂O₄₀ as a reusable catalyst under thermal and microwave irradiation conditions has been developed. H₃PW₁₂O₄₀-catalysed one-pot consecutive Friedel-Crafts reactions between veratrole and aldehydes were also applied as a convenient protocol for the preparation of symmetrical 9,10-diaryl-2,3,6,7- tetramethoxyanthracenes.

Full text of DOI:10.1002/ejoc.201001267

FULL PAPER
DOI: 10.1002/ejoc.201001267
H3PW12O40-Catalysed Alkylation of Arenes and Diveratrylmethanes:
Convenient Routes to Triarylmethanes and to Symmetrical and Unsymmetrical
9,10-Diaryl-2,3,6,7-tetramethoxyanthracenes
Iraj Mohammadpoor-Baltork,*[a] Majid Moghadam,[a] Shahram Tangestaninejad,[a]
Valiollah Mirkhani,[a] Kazem Mohammadiannejad-Abbasabadi,[a] and Hamid R. Khavasi[b]
Keywords: C–C coupling / Carbocycles / Arenes / Green chemistry / Triarylmethanes
An efficient method for the synthesis of triarylmethanes and
difurylarylmethanes through solventless reactions between
the preparation of symmetrical 9,10-diaryl-2,3,6,7-tetrameth-
oxyanthracenes. The conversion of aldehydes into their cor-
aldehydes and arenes in the presence of H3PW12O40 as a re- responding acylals in the presence of H3PW12O40 and the
usable catalyst under thermal and microwave irradiation
conditions has been developed. H3PW12O40-catalysed one-
pot consecutive Friedel–Crafts reactions between veratrole
and aldehydes were also applied as a convenient protocol for
one-pot reactions of diveratrylmethanes with these acylals
were also used for the synthesis of symmetrical and unsym-
metrical 9,10-diaryl-2,3,6,7-tetramethoxyanthracenes.
Hashmi et al.[15] have recently shown that gold com-
pounds can be used as efficient catalysts in synthetic or-
ganic chemistry under mild conditions. In this respect, both
this group[16] and Nair et al.[17] have reported the synthesis
of triheteroarylmethanes in high yields through AuCl3- or
AuCl3/AgOTf-catalysed reactions between carbonyl com-
pounds and electron-rich arenes.
Introduction
Triarylmethanes (TRAMs) are of great significance, due
to their wide variety of applications in synthetic, medicinal
and industrial chemistry and as protecting groups.[1] So far,
many methods based on C–C bond formation have been
reported for their synthesis. Vicarious nucleophilic substitu-
tion of hydrogen[2] and addition of arylboronic acids to al-
dehydes catalysed by cationic PdII/bipyridine,[3] as well as
Friedel–Crafts alkylation of arenes with aldehydes catalysed
variously by trifluoromethanesulfonic acid or trifluoro-
acetic acid,[4] Cu(OTf)2,[5] sulfuric acid,[6] InCl3/chlorometh-
Anthracene and its derivatives are an important class of
polyaromatic hydrocarbons.[18] These compounds have been
seriously assessed in nonlinear optical applications such as
organic light-emitting diodes,[19] organic field-effect transis-
tors[20] and fluorescent chemosensors.[21] Anthracenes have
also been used in Diels–Alder reactions.[22] These com-
pounds are generally prepared through Friedel–Crafts reac-
tions,[6,11,23] aromatic cyclodehydration,[24] Haworth reac-
tions,[25] Lewis-acid-induced Bradshare-type reactions from
diveratrylmethanes,[26] Elbs reactions,[27] twofold Suzuki–
Miyaura cross-coupling reactions,[28] coupling reactions be-
tween zirconacyclopentadienes and dihalonaphthalenes,[29]
cyclization of diacetates with TfOH,[30] H2SO4-catalysed re-
actions between benzocrown ethers and aldehydes,[31] and
addition of Grignard reagents or aryllithiums to anthra-
quinones followed by reduction of the resulting diols.[32]
The use of Keggin-type heteropolyacids as homogeneous
or heterogeneous catalysts in organic synthesis has been de-
veloped as a result of their several advantages, such as envi-
ronmental compatibility, reusability, non-corrosiveness and
relative lack of disposal problems, which make them eco-
nomically and environmentally attractive. Heteropolyacids,
especially H3PW12O40 (PWA), have attracted great atten-
tion, because of their high acid strengths and selectivity
properties.[33] The application of these solid acid catalysts
[10]
ylsilane,[7] [Ir(COD)Cl]2-SnCl4,[8] Yb(OTf)3,[9] FeCl3 and
ZnBr2/SiO2/AcBr[11] have been reported for the synthesis of
TRAMs. These compounds are also synthesized through
the alkylation of electron-rich arenes with (3-indolyl)me-
thanamines in the presence of chiral Brønsted acids,[12a]
double Friedel–Crafts reactions of indoles and 2-formylbi-
phenyls catalysed by chiral phosphoric acids,[12b] alkylation
of α-(3-indolyl)benzylamines with N-methylindole catalysed
by Brønsted acids,[12c] palladium-catalysed direct arylation
of aryl(azaaryl)methanes with aryl halides,[13a] Suzuki–Mi-
yaura coupling of diarylmethyl carbonates with arylboronic
acids[13b] and by benzylation/[3+3] cyclocondensation in the
presence of FeCl3.[14]
[a] Catalysis Division, Department of Chemistry,
University of Isfahan, Isfahan 81746-73441, Iran
Fax: +98-311-6689732
E-mail: imbaltork@sci.ui.ac.ir
[b] Department of Chemistry, Shahid Beheshti University,
Tehran 19839-63113, Iran
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201001267.
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under solventless conditions provides even more benign natives to traditional reactions in organic solvents both
processes.[34]
from the economical and from the synthetic point of view.
In continuation of our research into the application of Increasing the amount of veratrole, the temperature or the
heteropolyacids in useful synthetic organic transforma- amount of catalyst did not improve the yield (Table 1, En-
tions,[35] here we wish to disclose our results in: i) one-pot tries 6–8), whereas reduction of each of these parameters
syntheses of triarylmethanes and difurylarylmethanes led to a reduction in product yield (Table 1, Entries 9–11).
through PWA-catalysed reactions between arenes and alde-
Reactions between various aldehydes and veratrole in the
hydes, ii) the direct preparation of symmetrical 9,10-diaryl- presence of PWA under the optimized conditions were ex-
2,3,6,7-tetramethoxyanthracenes through reactions between amined (Scheme 1) and the results are summarized in
veratrole and aldehydes in the presence of PWA, and iii) a Table 2. The electronic properties of the substituents on the
convenient route to symmetrical and unsymmetrical 9,10- aromatic aldehydes have a significant influence on the reac-
diaryl-2,3,6,7-tetramethoxyanthracenes through PWA-cata- tion times and yields. Benzaldehyde and aldehydes bearing
lysed Friedel–Crafts alkylations of diveratrylmethanes electron-withdrawing groups, such as nitro, chloro, cyano,
(DVMs) with acylals.
formyl and 3-methoxy,[36] and also methyl as a weakly elec-
tron-donating group, afforded the corresponding products
in high yields (Table 2, Entries 1–12 and 17). In contrast,
aldehydes containing strongly electron-donating groups,
such as 4-methoxy and 4-hydroxy groups, failed to produce
the products (Table 2, Entries 18, 19). This is attributed to
the reduced electrophilicity of the aldehyde group as a result
of the electron-rich nature of the phenyl ring to which the
aldehyde is attached. It is noteworthy that aldehydes con-
taining both electron-donating and electron-withdrawing
groups also remained unreacted in the reaction mixture
(Table 2, Entries 20, 21). These results show that electron-
donating substituents have more pronounced effects than
electron-withdrawing ones.
Results and Discussion
Solventless Syntheses of Triarylmethanes and
Difurylarylmethanes Under Thermal and Microwave
Irradiation Conditions
Initially, the reaction parameters – such as solvent, molar
ratio of veratrole to aldehyde, the amount of catalyst and
temperature – were optimized in the reaction between 4-
chlorobenzaldehyde and veratrole (1) as a model under
thermal conditions. The model reaction was examined in
various solvents as well as under solventless conditions in
the presence of PWA (Table 1, Entries 1–5). The highest
yield of the corresponding DVM was obtained with 4-chlo-
robenzaldehyde (1 mmol), veratrole (3 mmol) and PWA
(0.07 mmol) at 75 °C under solventless conditions (Table 1,
Entry 5). These conditions are promising as an essential
facet of “green chemistry” and are of great interest as alter-
Scheme 1. PWA-catalysed syntheses of triarylmethanes.
The reactivities of 2-naphthaldehyde (2m) and polycyclic
aldehydes such as anthracene-9-carbaldehyde and phenan-
threne-9-carbaldehyde were also examined. 2-Naphth-
aldehyde afforded the corresponding DVM 3m in 84% yield
(Table 2, Entry 13), whereas anthracene-9-carbaldehyde
and phenanthrene-9-carbaldehyde failed to give the desired
products and were recovered unchanged, possibly because
of steric effects. To the best of our knowledge, the synthesis
of fluorinated diveratrylmethanes has not been reported
previously. We succeeded in preparing several fluorinated
triarylmethanes (Table 2, Entries 8–10). Aliphatic aldehydes
were also converted into their corresponding diveratrylme-
thane derivatives at room temperature in excellent yields
(Table 2, Entries 14–16).
It is important to note that excellent chemoselectivity
was observed in the reaction between terephthalaldehyde
(2l) and veratrole (1) under thermal conditions. With a 1:2.5
molar ratio of 2l to 1, only one formyl moiety reacted selec-
tively and the aldehyde 3l was isolated in 81% yield,
whereas with a 1:7 molar ratio of terephthalaldehyde to ver-
atrole, both formyl groups reacted and the tetrakis(veratryl)
adduct 4 was obtained in 78% yield (Scheme 2). Reactions
between anisole derivatives and various aldehydes under the
same conditions were also investigated and the correspond-
Table 1. Optimization of the reaction conditions for the PWA-cata-
lysed synthesis of DVMs.
Entry PWA
Veratrole Aldehyde Solvent
T
Time Yield[a]
[mol-%] [mmol]
[mmol]
[°C]
CH3CN reflux
EtOH reflux
CH2Cl2 reflux
[min]
[%]
1
2
3
4
5
6
7
8
7
7
7
7
7
7
7
9
7
7
5
3
3
3
3
3
3.5
3
3
2.5
3
1
1
1
1
1
1
1
1
1
1
1
60
60
60
60
25
25
25
25
25
25
25
50
12
25
55
93
93
93
93
71
74
77
CHCl3
–
reflux
75
[b]
–
–
–
–
–
–
75
90
75
75
60
75
9
10
11
3
[a] Isolated yield. [b] Solventless.
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Triarylmethanes and 9,10-Diaryl-2,3,6,7-tetramethoxyanthracenes
Table 2. PWA-catalysed syntheses of DVMs under thermal and microwave irradiation conditions.
[a] Reaction conditions: veratrole (3 mmol), aldehyde (1 mmol), PWA (7 mol-%), 75 °C. [b] Reaction conditions: veratrole (3 mmol), alde-
hyde (1 mmol), PWA (4 mol-%), applied power: 150 W, 75 °C. [c] Isolated yield. [d] Veratrole (1, 2.5 mmol), terephthalaldehyde (2l,
1 mmol). [e] At room temperature.
ing TRAMs were obtained in excellent yields (Table 3). The many trials on the model reaction. The best results were
reaction between anisole and terephthalaldehyde failed to obtained with a 3:1:0.04 veratrole/aldehyde/PWA molar ra-
produce the corresponding triarylmethane 8, and only the tio and with an applied power of 150 W at 75 °C under
tetrakis(anisyl) adduct 9 was obtained, in 87% yield under solventless conditions. Under these optimized conditions, a
thermal conditions (Scheme 2).
wide range of diveratrylmethanes and triarylmethanes were
The efficiency of this method for the synthesis of difuryl- synthesized in excellent yields and in very short reaction
arylmethanes was also explored. As can be seen in Table 4, times in the presence of smaller amounts of catalyst than
the difurylarylmethanes were obtained in excellent yields on under thermal conditions (see Tables 2, 3 and 4). When
treatment of 2,5-dimethylfuran (10) with aldehydes.
anisole and veratrole were treated with terephthalaldehyde
Organic reactions assisted by microwave irradiation have (2l) under microwave irradiation conditions, different re-
attracted considerable attention in recent years. The main sults in terms of selectivity were obtained. The reaction be-
benefits of performing the reactions under microwave irra- tween anisole and terephthalaldehyde (2l) produced the
diation are much improved reaction rates, higher yields and tetrakis(anisyl) adduct 9 in 91% isolated yield (Scheme 3),
formation of cleaner products. A key advantage of modern whereas with veratrole only one formyl group of terephthal-
scientific microwave apparatus is the ability to control reac- aldehyde contributed and the corresponding diveratryl-
tion parameters such as temperature, pressure and reaction methane 3l was isolated in 88% yield.
times very specifically.[37] Microwave irradiation had not
The reusability of the catalyst was also investigated, and
previously been employed for the synthesis of TRAMs, and it was observed that it could be reused at least five times
so microwave-assisted syntheses of these compounds were under thermal or microwave irradiation conditions without
investigated. Optimal conditions were determined after any significant decrease in its activity.
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Scheme 2. Investigation of selectivity in the reactions of veratrole and anisole with terephthalaldehyde under thermal conditions.
Table 3. PWA-catalysed syntheses of TRAMs under thermal and microwave irradiation conditions.
Entry
Compound
ArCHO
Product
Thermal[a] (MW[b]
)
Time [min]
Yield[c] [%]
M.p. [°C]
1
2
3
4
5
6
7
8
X = Z = H, Y = Me (5a)
X = Z = H, Y = Me (5a)
X = Z = H, Y = Me (5a)
X = Z = H, Y = Me (5a)
X = Z = H, Y = Cl (5b)
X = Z = H, Y = Cl (5b)
X = Z = H, Y = Cl (5b)
Y = H, X = Z = OMe (5c)
Y = H, X = Z = OMe (5c)
Y = H, X = Z = OMe (5c)
4-ClC6H4CHO (2e)
4-MeC6H4CHO (2b)
4-NCC6H4CHO (2k)
2-naphthaldehyde (2m)
4-NO2C6H4CHO (2v)
2-ClC6H4CHO (2d)
PhCHO (2a)
4-BrC6H4CHO (2g)
4-ClC6H4CHO (2e)
2-naphthaldehyde (2m)
6a
6b
6c
6d
6e
6f
35 (4)
40 (4)
35 (4)
40 (5)
50 (5)
55 (5)
60 (5)
15 (3)
15 (3)
15 (3)
93 (95)
92 (93)
92 (95)
86 (92)
88 (93)
90 (94)
78 (85)
94 (95)
93 (95)
92 (94)
153–155
119–120
132–133
138–139
168–170
138–140
138–139.5
149–151
164–165
173–174
6g
6h
6i
9
10
6j
[a] Reaction conditions: arene (3 mmol), aldehyde (1 mmol), PWA (7 mol-%), 75 °C. [b] Reaction conditions: arene (3 mmol), aldehyde
(1 mmol), PWA (4 mol-%), MW irradiation (150 W), 75 °C. [c] Isolated yield.
Table 4. PWA-catalysed syntheses of difurylarylmethanes under
thermal and microwave irradiation conditions.
One-Pot Consecutive Preparations of Symmetrical 9,10-
Diaryl-2,3,6,7-tetramethoxyanthracenes
The development of organic transformations in fewer
steps increases the reaction yields and selectivity, decreases
the reaction times, by-products, waste and costs, and saves
on energy and resources. In the course of the last two dec-
ades a great deal of attention has therefore been paid to the
design of one-pot and modern strategies in organic trans-
formations.[38] In this context, we focused on the synthesis
of symmetrical 9,10-diaryl-2,3,6,7-tetramethoxyanthracenes
through Friedel–Crafts reactions between veratrole and al-
dehydes. Our initial investigations showed that PWA is able
to catalyse these reactions. It was also found that addition
of Ac2O to the reaction mixture promotes the product for-
mation, so a one-pot two-step strategy was postulated. In
this, the first step included the diveratrylmethane synthesis
Entry ArCHO
Prod. Thermal[a] (MW[b]
Time [min] Yield[c] [%]
)
M.p. [°C]
111–113
1
2,4-(Cl)2C6H3CHO
(2f)
4-NCC6H4CHO (2k) 11b 40 (3)
40 (3)
90 (92)
11a
2
3
89 (91)
91 (94)
98–99
101–102
4-NO2C6H4CHO
40 (3)
11c
(2r)
[a] Reaction conditions: 2,5-dimethylfuran (3 mmol), aldehyde
(1 mmol), PWA (1.5 mol-%), 75 °C. [b] Reaction conditions: 2,5-
dimethylfuran (3 mmol), aldehyde (1 mmol), PWA (1.5 mol-%),
MW irradiation (150 W), 75 °C. [c] Isolated yield.
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Triarylmethanes and 9,10-Diaryl-2,3,6,7-tetramethoxyanthracenes
Scheme 3. Investigation of selectivity in the reactions of veratrole and anisole with terephthalaldehyde under MW irradiation conditions.
according to Table 2 and in the second step the reaction tages of this protocol (Table 5, Entry 1). Halogenated alde-
mixture was treated, without any product isolation, with an hydes were also converted into their corresponding anthrac-
aldehyde and Ac2O in the presence of catalytic amounts of enes with good yields and selectivities (Table 5, Entries 2–8).
PWA. To determine the optimal conditions for the second Another noteworthy advantage of the method was shown in
step, the preparation of the anthracene 12a was chosen as the reaction between 2-naphthaldehyde (2m) and veratrole, in
a model. In the second step, the best yield was obtained which the corresponding 2,3,6,7-tetramethoxy-9,10-di-
with aldehyde (2 mmol), Ac2O (4 mmol) and PWA (2 mol- naphthylanthracene (12i) was selectively prepared in 48%
%) in glacial AcOH (2 mL) at 75 °C.
yield (Table 5, Entry 9). It should be noted that although al-
The scope and limitations of the described procedure dehydes such as 4-methylbenzaldehyde (2b), 3-nitrobenzalde-
were investigated in reactions between aldehydes and vera- hyde (2c) and 4-cyanobenzaldehyde (2k) were highly reactive
trole. Under the optimized conditions, several symmetrical in the first step, they nevertheless failed to produce the corre-
9,10-diaryl-2,3,6,7-tetramethoxyanthracenes were success- sponding anthracenes (Table 5, Entries 10–12).
fully synthesized in good yields. As illustrated in Table 5,
It is noteworthy that these conditions are only suitable
the preparation of 2,3,6,7-tetramethoxy-9,10-diphenyl- for syntheses of symmetrical 9,10-diaryl-2,3,6,7-tetrameth-
anthracene (12a) in 73% yield is one of the notable advan- oxyanthracenes. In the first step, veratrole reacts with the
Table 5. PWA-catalysed one-pot syntheses of symmetrical 9,10-diaryl-2,3,6,7-tetramethoxyanthracenes.
Entry
ArCHO
Time [min], first step
Product
Yield[a] [%]
M.p. [°C]
1
2
3
4
5
6
7
8
PhCHO (2a)
40
30
25
25
30
30
40
40
40
40
25
25
12a
12b
12c
12d
12e
12f
12g
12h
12i
73
56
61
70
63
65
58
65
48
–
306–308
166–167.5
219–220
327–329
249–251
Ͼ330
211–213
238–239
288–290
–
2-ClC6H4CHO (2d)
3-ClC6H4CHO (2v)
4-ClC6H4CHO (2e)
3-BrC6H4CHO (2w)
4-BrC6H4CHO (2g)
3-FC6H4CHO (2h)
4-FC6H4CHO (2i)
2-naphthaldehyde (2m)
4-MeC6H4CHO (2b)
3-NO2C6H4CHO (2c)
4-NCC6H4CHO (2k)
9
10
11
12
12j
12k
12l
–
–
–
–
[a] Isolated yield.
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aldehyde in a 3:1 molar ratio, and some veratrole remains more reactive than the parent aldehydes in these reactions.
in the reaction mixture in addition to the produced DVM. The acylal 13a was therefore first prepared by treatment of
In the second step, the added aldehyde can react with the benzaldehyde (2a) with Ac2O. Compound 13a (1.5 mmol)
DVM to produce the anthracene derivative. In addition, the was then treated with the DVM 3a (1 mmol) in the presence
added aldehyde also reacts with the remaining veratrole to of PWA (7 mol-%) in AcOH as the solvent.
afford the DVM, which further reacts with the aldehyde to
Analysis of the crude product revealed that in addition
give an anthracene derivative. If the same aldehyde is used to the desired anthracene 12a, some benzaldehyde 2a was
in these two steps, only one kind of anthracene derivative produced by deprotection of acylal 13a. In order to reduce
is formed, and this is symmetrical. If the aldehydes used for the extent of deprotection of 13a and to increase the yield
the two steps are not the same, a mixture of anthracene of the corresponding anthracene, this reaction was repeated
derivatives is formed in low yields and the isolation and in the presence of smaller amounts of PWA. It was found
purification of the products are therefore very difficult.
that 2 mol-% PWA afforded the best result.
With the knowledge of the critical role of Ac2O, a one-
pot synthetic methodology for the preparation of 9,10-di-
aryl-2,3,6,7-tetramethoxyanthracenes was designed. Firstly,
the acylals 13a–h were prepared by treatment of the alde-
hydes (1.5 mmol) with Ac2O (4 mmol) in the presence of
PWA (2 mol-%) at 60 °C for 30 min under solventless con-
ditions. The DVM of choice (1 mmol) and glacial AcOH
One-Pot Syntheses of Symmetrical and Unsymmetrical
9,10-Diaryl-2,3,6,7-tetramethoxyanthracenes with
Diveratrylmethanes and Acylals as Key Precursors
In order to demonstrate the efficiency and applicability
of this method further, syntheses of symmetrical and un-
symmetrical 9,10-diaryl-2,3,6,7-tetramethoxyanthracenes
through reactions between DVMs and aldehydes in the
presence of PWA were also investigated. Optimization of
the reaction conditions was carried out in the reaction be-
tween the DVM 3a and benzaldehyde (2a). Initial experi-
ments showed that addition of Ac2O to the reaction mix-
ture improved this reaction. The best results were obtained
with a DVM/aldehyde/Ac2O molar ratio of 1:1.5:4 in the
presence of PWA in glacial acetic acid at 60 °C. Under these
conditions, several 9,10-diarylanthracenes were prepared as
illustrated in Table 6. Drawbacks such as low yields and the
need for large amounts of PWA encouraged us to find a
more efficient strategy for the synthesis of target molecules
via DVMs. On consideration of the role of Ac2O in these
reactions, it was postulated that aldehydes are initially con-
verted into their corresponding acylals, which are probably
Table 7. One-pot two-step syntheses of symmetrical and unsymmet-
rical 9,10-diaryl-2,3,6,7-tetramethoxyanthracenes through reac-
tions between DVMs and acylals in the presence of PWA.[a]
Table 6. Syntheses of 9,10-diaryl-2,3,6,7-tetramethoxyanthracenes
through reactions between DVMs and aldehydes.[a]
Entry ArЈCH ArCHO
Product Time
[(Ve)2] [min]
Yield[b]
[%]
1
2
3
4
5
6
7
3a
3a
3e
3e
3c
3f
PhCHO (2a)
4-BrC6H4CHO (2g)
PhCHO (2a)
4-ClC6H4CHO (2e)
PhCHO (2a)
PhCHO (2a)
12a
12m
12n
12d
12q
12r
90
90
90
90
120
90
90
59
37
58
46
NR
38
49
3i
PhCHO (2a)
12o
[a] First-step reaction conditions: aldehyde (1.5 mmol), Ac2O
(4 mmol), PWA (2 mol-%), 60 °C, 30 min. Second-step reaction
conditions: DVM (1 mmol), AcOH (glc., 2 mL), 60 °C, 60 min.
[b] Isolated yield.
[a] Reaction conditions: DVM (1 mmol), aldehyde (1.5 mmol),
Ac2O (4 mmol), PWA (201 mg, 7 mol-%) and AcOH (glc., 2 mL)
at 60 °C. [b] Isolated yield.
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Triarylmethanes and 9,10-Diaryl-2,3,6,7-tetramethoxyanthracenes
(2 mL) were added and the resulting mixture was stirred at
The applicability of this protocol was also confirmed
60 °C for a further 1 h. As can be seen in Table 7, various when the tetrakis(veratryl) adduct 4 reacted with 2-[bis-
symmetrical and unsymmetrical 9,10-diaryl-2,3,6,7-tetra- (acetoxy)methyl]naphthaline 13h through both diveratryl-
methoxyanthracenes were successfully synthesized under methane moieties to afford the adduct 14 in 81% yield
these conditions. The condensation reactions between acyl- (Scheme 4).
als and DVMs containing halogenated aryl, phenyl and
naphthyl groups afforded the corresponding anthracenes diarylanthracenes is proposed in Scheme 5. The acylal A is
for the first time and in good yields. first produced through the reaction between the aldehyde
A plausible mechanism for the formation of the 9,10-
Comparison of the results shown in Tables 6 and 7 shows and Ac2O in the presence of PWA. The produced acylal
that the syntheses of anthracene derivatives via the acylal undergoes two successive Friedel–Crafts reactions with the
intermediates are superior in terms of yield and amounts of DVM B in the presence of the catalyst to give the intermedi-
the catalyst.
ates C and D. The intermediate D is then converted into E.
Finally, aromatization of E under air in the presence of cat-
alyst affords the corresponding 9,10-diarylanthracene F as
Scheme 4. Synthesis of the adduct 14 through the one-pot consecu-
tive reaction between the DVM 4 and the acylal 13h.
Figure 1. Crystal structure of 12g.
Scheme 5. Plausible mechanism for the synthesis of 9,10-diarylanthracenes.
Eur. J. Org. Chem. 2011, 1357–1366
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1363

I. Mohammadpoor-Baltork et al.
FULL PAPER
OCH3), 3.77 (s, 6 H, OCH3) ppm. 13C NMR (125 MHz, CDCl3):
δ = 149.16, 148.44, 148.04, 146.68, 135.35, 135.17, 129.12, 124.05,
a desired product and releases the catalyst for the next run.
In order to confirm this point, O2 was bubbled into the
reaction mixture of benzaldehyde (2a) and veratrole (1,
Table 5, Entry 1). The results showed that the reaction was
accelerated several times and the reaction time was reduced
from 90 min to 30 min.[39]
121.51, 121.39, 112.72, 111.26, 55.91, 55.59 ppm. FT-IR (KBr): ν
˜
= 3084, 3003, 2947, 1589, 1516, 1463, 1342, 1028, 916, 869, 736
cm–1. MS (70 eV, EI): m/z (%) = 410.12 (25.86) [M + 1]+, 409.11
(100) [M]+, 378.06 (46.26), 287.18 (40.23), 226.13 (9.99), 152.10
(7.33), 139.08 (6.90), 77.02 (14.01). C23H23NO6 (409.15): calcd. C
67.47, H 5.66, N 3.42; found C 67.34, H 5.64, N 3.42.
The structures of the products were deduced from their
1
IR, mass, H NMR and 13C NMR spectra and their ele-
Typical Procedure for Synthesis of DVMs and TRAMs Under Mi-
crowave Irradiation Conditions: See Table 3, Entry 4; 4-methylani-
sole (5a, 366 mg, 3 mmol), 2-naphthaldehyde (2m, 156 mg, 1 mmol)
and PWA (4 mol-%) were mixed and exposed to MW irradiation
(150 W) for 5 min. The reaction temperature was monitored with
an IR optical sensor. At the end of the reaction, the mixture was
allowed to cool to room temperature. The workup was performed
as described for the thermal method and the pure product 6d was
obtained in 92% yield; m.p. 138–139 °C. 1H NMR (500 MHz,
CDCl3): δ = 7.84–7.86 (m, 1 H, 6-H naphthyl), 7.78 (d, J = 8.78 Hz,
1 H, 4-H naphthyl), 7.74–7.78 (m, 1 H, 7-H naphthyl), 7.47 (d, J
= 3.24 Hz, 1 H, 5-H naphthyl), 7.45 (d, J = 3.26 Hz, 1 H, 8-H
naphthyl), 7.44 (s, 1 H, 1-H naphthyl), 7.34 (dd, J = 7.4, J =
1.71 Hz, 1 H, 3-H naphthyl), 7.07 (dd, J = 8.22, J = 1.96 Hz, 2 H,
4-H naphthyl), 6.85 (d, J = 8.24 Hz, 2 H, 3-H anisyl), 6.38 (s, 1 H,
Ar3CH), 3.71 (s, 6 H, OCH3), 2.24 (s, 6 H, CH3) ppm. 13C NMR
(125 MHz, CDCl3): δ = 155.79, 142.41, 133.90, 132.66, 132.53,
131.40, 129.68, 129.14, 128.33, 128.08, 127.94, 127.79, 127.64,
mental analyses. The structure of the product 12g was also
confirmed unambiguously by single-crystal X-ray analysis
(see the Supporting Information and Figure 1).[40]
Conclusions
In summary, we have developed an efficient and green
method for the solventless synthesis of triarylmethanes and
difurylarylmethanes under thermal and microwave irradia-
tion conditions in the presence of PWA as a reusable cata-
lyst. One-pot reactions between veratrole and aldehydes
were successfully used for the synthesis of symmetrical 9,10-
diaryl-2,3,6,7-tetramethoxyanthracenes. Moreover, the one-
pot consecutive protocol has been used for the synthesis of
symmetrical and unsymmetrical 9,10-diaryl-2,3,6,7-tetra-
methoxyanthracenes in good yields from acylals and DVMs
as key precursors.
125.94, 125.51, 111.39, 56.44, 43.61, 21.24 ppm. FT-IR (KBr): ν =
˜
3049, 2935, 1068, 1492, 1436, 1363, 1234, 1105, 1035, 804, 742
cm–1. MS (70 eV, EI): m/z (%) = 384.15 (9.55) [M + 2]+, 383.14
(56.50) [M + 1]+, 382.04 (97.56) [M]+, 367.07 (47.97), 351.02
(81.30), 335.07 (16.67), 319.06 (18.70), 289.03 (10.87), 260.14
(33.33), 245.11 (32.11), 229.16 (19.21), 215.18 (12.30), 152.24
(29.27), 140.96 (100), 135.08 (60.98), 122.07 (47.97), 104.96 (73.98),
91.03 (20.43), 77.05 (17.99). C27H26O2 (382.19): calcd. C 84.78, H
6.85; found C 84.31, H 6.86.
Experimental Section
General Information: The chemicals used in this work were pur-
chased from Fluka and Merck. Progress of reactions was moni-
tored by TLC (0.25 μm pre-coated silica gel plates). Melting point
were determined with a Stuart Scientific SMP2 apparatus and are
uncorrected. 1H and 13C NMR (500 and 125 MHz) spectra were
recorded with a Bruker AC 500 spectrometer. FT-IR spectra were
The syntheses of the tetrakis(veratryl) adduct 4, the tetrakis(anisyl)
adduct 9 and the difurylarylmethanes 11a–c were carried out, both
under thermal and under microwave irradiation conditions, by the
same methods as described for the synthesis of DVMs and
TRAMs.
recorded over the 400–4000 cm–1 range with
a Nicolet-Im-
pact 400D instrument. Elemental analysis was carried out with a
LECO, CHNS-932 instrument. Mass spectra were recorded with a
Platform II spectrometer from Micromass. EI mode at 70 eV. The
microwave system used in these experiments includes the following
items: Micro-SYNTH labstation, complete with glass door, dual
magnetron system with pyramid-shaped diffuser, 1000 W delivered
power, exhaust system, magnetic stirrer, “quality pressure” sensor
for flammable organic solvents, ATCFO fibre optic system for
automatic temperature control.
Typical Procedure for the Synthesis of Symmetrical 9,10-Diaryl-
2,3,6,7-tetramethoxyanthracenes: See Table 5, Entry 6; In the first
step, a mixture of veratrole (1, 414 mg, 3 mmol), 4-bromobenzalde-
hyde (185 mg, 1 mmol) and PWA (7 mol-%) was stirred at 75 °C
for 30 min. The progress of the reaction was monitored by TLC
(eluent: n-hexane/EtOAc 4:1). 4-Bromobenzaldehyde (370 mg,
2 mmol), PWA (2 mol-%), Ac2O (4 mmol) and glacial AcOH
(2 mL) were then added and the mixture was stirred at 75 °C for
90 min. The reaction mixture was allowed to cool to room tempera-
ture, CH2Cl2 (3ϫ 5 mL) was added, and PWA was separated by
simple filtration. The organic layer was neutralized with saturated
Typical Procedure for Synthesis of DVMs and TRAMs Under Ther-
mal Conditions (Table 2, Entry 3): Veratrole (1, 414 mg, 3 mmol), 3-
nitrobenzaldehyde (2c, 151 mg, 1 mmol) and PWA (7 mol-%) were
mixed in a 25 mL flask and stirred at 75 °C for 35 min. The pro- aqueous NaHCO3 and dried with Na2SO4. The solvent was evapo-
gress of the reaction was monitored by TLC (eluent: n-hexane/
EtOAc 4:1). At the end of the reaction, the mixture was allowed to
cool to room temperature. CH2Cl2 (2ϫ 5 mL) was added and the
catalyst was separated by simple filtration. The solvent was evapo-
rated and the resulting mixture was recrystallized from EtOH. The
pure product 3c was obtained as a yellow solid in 93% yield; m.p.
rated and the residue was purified with a short column of silica gel
(eluent: n-hexane/EtOAc 4:1) to provide the pure anthracene 12f in
1
65% yield; m.p. Ͼ330 °C. H NMR (500 MHz, CDCl3): δ = 7.75
(d, J = 8.2 Hz, 4 H, 2,6-H phenyl), 7.37 (d, J = 8.25 Hz, 4 H, 3,5-
H phenyl), 6.76 (s, 4 H, 1,4,5,8-H anthracene), 3.76 (s, 12 H, OCH3)
ppm. 13C NMR (125 MHz, CDCl3): δ = 149.20, 138.81, 132.79,
154–155.5 °C. 1H NMR (500 MHz, CDCl3): δ = 8.05–8.08 (m, 1 132.05, 131.88, 125.73, 121.73, 103.66, 55.64 ppm. FT-IR (KBr): ν
˜
H, 4-H 3-nitrophenyl), 8.00 (s, 1 H, 2-H 3-nitrophenyl), 7.44–7.46
(m, 2 H, 5,6-H 3-nitrophenyl), 6.81 (d, J = 8.25 Hz, 2 H, 5-H vera-
tryl), 6.65 (d, J = 1.95 Hz, 2 H, 2-H veratryl), 6.57 (dd, J = 8.25,
J = 1.95 Hz, 2 H, 6-H veratryl), 5.53 (s, 1 H, Ar3CH), 3.86 (s, 6 H,
= 2933, 2827, 1635, 1527, 1492, 1435, 1240, 1207, 1124, 1012, 974,
898, 844, 754, 582 cm–1. MS (70 eV, EI): m/z (%) = 610.00 (54.86)
[M + 4]+, 608.00 (100) [M + 2]+, 606.02 (52.78) [M]+. C30H24Br2O4
(606): calcd. C 59.23, H 3.98; found C 59.17, H 4.02.
1364
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Eur. J. Org. Chem. 2011, 1357–1366

Triarylmethanes and 9,10-Diaryl-2,3,6,7-tetramethoxyanthracenes
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Typical Procedure for One-Pot Syntheses of Symmetrical and
Unsymmetrical 9,10-Diaryl-2,3,6,7-tetramethoxyanthracenes: See
Table 7, Entry 6;
A mixture of benzaldehyde (2a, 159 mg,
1.5 mmol), Ac2O (408 mg, 4 mmol) and PWA (2 mol-%) was stirred
at 60 °C for 30 min to afford the acylal 13a. Afterward, the DVM
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by column chromatography on silica gel (eluent: n-hexane/EtOAc
4:1) to afford the pure anthracene 12p in 67% yield; m.p. 219–
221 °C. 1H NMR (500 MHz, CDCl3): δ = 8.08 (d, J = 8.0 Hz, 1
H, 3-H naphthyl), 8.04 (d, J = 8.05 Hz, 1 H, 4-H naphthyl), 8.00
(s, 1 H, 1-H naphthyl), 7.95 (d, J = 7.2 Hz, 1 H, 8-H naphthyl),
7.59–7.65 (m, 5 H, phenyl), 7.51–7.59 (m, 3 H, 5,6,7-H naphthyl),
6.86 (s, 4 H, 1,4,5,8-H anthracene), 3.74 (s, 6 H, OCH3), 3.65 (s, 6
H, OCH3) ppm. 13C NMR (125 MHz, CDCl3): δ = 149.04, 148.92,
139.88, 137.41, 133.80, 133.33, 132.78, 131.08, 130.00, 129.46,
128.72, 128.30, 128.16, 127.95, 127.46, 126.31, 126.13, 126.06,
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The adduct 14 was prepared by the same method, through conden-
sation of the acylal 13h and the adduct 4 in the presence of PWA.
Firstly, a mixture of 2-naphthaldehyde (2m, 312 mg, 2 mmol),
Ac2O (612 mg, 6 mmol) and PWA (2 mol-%) was stirred at 60 °C
for 30 min to give the acylal 13h. The tetrakis(veratryl) adduct 4
(325.5 mg, 0.5 mmol) and glacial AcOH (3 mL) were then added
to the reaction mixture. The resulting mixture was stirred at 60 °C
for 90 min. The workup was performed as mentioned and the pure
adduct 14 was obtained in 81% yield; m.p. Ͼ330 °C. 1H NMR
(500 MHz, CDCl3): δ = 8.00–8.17 (m, 8 H, naphthyl), 7.86 (s, 4 H,
2,3,5,6-H terephthaloyl), 7.63–7.71 (m, 6 H, naphthyl), 7.16 (s, 4
H, 4,4Ј,5,5Ј-H anthracene), 6.96 (s, 4 H, 1,1Ј,8,8Ј-H anthracene),
3.72 (s, 12 H, OMe), 3.82 (s, 12 H, OMe) ppm. 13C NMR
(125 MHz, CDCl3): δ = 149.05, 149.00, 139.08, 137.28, 133.78,
133.13, 132.98, 132.82, 131.63, 130.88, 129.99, 129.38, 128.40,
128.17, 127.98, 126.38, 126.25, 125.96, 104.23, 103.94, 55.65,
[19]
[20]
[21]
54.97 ppm. FT-IR (KBr): ν = 2947, 2827, 1629, 1527, 1492, 1435,
˜
1238, 1205, 1132, 1112, 1006, 997, 898, 848, 748 cm–1. C62H50O8
(922.35): calcd. C 80.67, H 5.46; found C 82.58, H 5.47.
Supporting Information (see footnote on the first page of this arti-
cle): General experimental procedures and characterization data
1
along with copies of H and 13C NMR spectra.
Acknowledgments
The authors thank the Centre of Excellence of Chemistry and Re-
search Council of the University of Isfahan for financial support
for this work.
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Received: September 9, 2010
Published Online: January 12, 2011
1366
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 1357–1366