Synthesis of new mixed-bistriarylmethanes and novel 3,4-dihydropyrimidin-2(1H)one derivatives

Article abstract of DOI:10.1039/C8NJ05845H

Firstly, triarylmethanes bearing formyl groups (TRAM-CHOs) have been synthesized by the reaction of arenes with aromatic dialdehydes catalyzed by sulfuric acid immobilized on SiO₂ under solvent-free grinding conditions in good yields and short grinding time. Secondly, condensation of TRAM-CHOs with several arenes proceeded rapidly within a few minutes to afford the new corresponding mixed-bistriarylmethanes in excellent yields. TRAM-CHOs were also found to be valuable precursors for the Biginelli reaction to synthesize a variety of 3,4-dihydropyrimidin-2(1H)-one-triarylmethane hybrid derivatives.

Full text of DOI:10.1039/C8NJ05845H

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Synthesis of new mixed-bistriarylmethanes and
novel 3,4-dihydropyrimidin-2(1H)one derivatives†
Cite this:DOI: 10.1039/c8nj05845h
Kazem Mohammadiannejad, *a Reza Ranjbar-Karimi*b and Farzaneh Haghighatb
Firstly, triarylmethanes bearing formyl groups (TRAM-CHOs) have been synthesized by the reaction of
arenes with aromatic dialdehydes catalyzed by sulfuric acid immobilized on SiO2 under solvent-free grinding
conditions in good yields and short grinding time. Secondly, condensation of TRAM-CHOs with several
arenes proceeded rapidly within a few minutes to afford the new corresponding mixed-bistriarylmethanes
in excellent yields. TRAM-CHOs were also found to be valuable precursors for the Biginelli reaction to
synthesize a variety of 3,4-dihydropyrimidin-2(1H)-one-triarylmethane hybrid derivatives.
Received 17th November 2018,
Accepted 13th March 2019
DOI: 10.1039/c8nj05845h
rsc.li/njc
presence of In(OTf)3 under grinding conditions,13 palladium-
catalyzed decarboxylative cross-coupling reactions of benzoic
Introduction
Triarylmethane (TRAM) scaffolds are structural units of several acids with diarylmethyl iodides,14 LDA mediated C–H arylation
dyes, therapeutic agents, and natural products. Owing to their of diarylmethanes with fluoroarenes,15 rhodium-catalyzed
intrinsic optical and electrical properties, they have been widely enantioselective arylation of diarylmethylamines with aryl-
used in the design of liquid crystals, conducting polymers, boroxines,16 Pd-catalyzed arylation of diarylmethyl phenyl sulfones
photochromic agents, and optically active devices.1 Hence, with aryl boronic acids,17 arylative desulfonation of diarylmethyl
various protocols based on C–C bond formation have been phenyl sulfones with arenes and heteroarenes in the presence of
developed for convenient access to them.1b,c Classic syntheses Sc(OTf)3,18 and alkylation of acetanilides by benzhydrol derivatives
of TRAMs rely on the Friedel–Crafts alkylation of arenes with using a catalytic amount of AgOTf.19 Recently, Zhang et al. have
2
¨
aldehydes or diraylmethanols catalyzed by Bronsted acids,
used the palladium-catalyzed Suzuki-coupling reaction of
Lewis acids,3 iodine,4 phosphine,5 or ionic liquids.6 In situ 1,1-diarylmethyl-trimethylammonium triflates with arylboronic
oxidation of benzyl alcohols followed by the Friedel–Crafts acids for the preparation of triarylmethane derivatives.20 TRAMs
alkylation of anisole derivatives in the presence of BF3ÁOEt2, have shown great potential in functionalization reactions,2d,18 and
offers another way to obtain a novel series of diversely functiona- as synthons in the synthesis of acridines,1b anthracenes,2b, j,3j,r
lized TRAMs.7 Vicarious nucleophilic substitution of hydrogen,8 indenes,21 tetraarylmethanes,22 and supramolecular frame-
benzylation/[3+3] cyclocondensation reaction catalyzed by FeCl3,9 works.23 However, the synthesis of functionalized TRAMs and
Baeyer condensation of aldehydes and N,N-dimethylaniline by discovery of new synthetic aspects for them still remains in
atomized sodium in THF,10 as well as transition metal-catalyzed continuous demand.
cross-coupling reactions,11 are powerful alternative approaches
On the other hand, 3,4-dihydropyrimidin-2(1H)-ones (DHPMs)
for the effective formation of TRAMs. Moreover, various sym- exhibit a variety of biological activities, including antitumor, anti-
metrical and/or unsymmetrical TRAMs have been obtained hypertensive, anti-bacterial, anti-microbial, anti-inflammatory,
via Friedel–Crafts type alkylation of arenes with the in situ antimalarial, and anti-HIV.24 DHPMs and their sulfur analogues
generated alkoxyaluminium intermediates,12 Friedel–Crafts are generally prepared through the Biginelli reaction, including
alkylation of arenes with o-hydroxy bisbenzylic alcohols in the simple three-component condensation reaction of b-dicarbonyl
compounds with aldehydes and urea or thiourea in the presence
of acid catalysts. In recent years, interest in the Biginelli reaction
has been renewed and accordingly several synthetic procedures
have been developed for the preparation of DHPMs under
a NMR Laboratory, Faculty of Science, Vali-e-Asr University of Rafsanjan,
Rafsanjan 77176, Islamic Republic of Iran. E-mail: kmmohammadi@gmail.com;
Fax: +98-343-131-2148; Tel: +98-343-131-2148
b Department of Chemistry, Vali-e-Asr University of Rafsanjan, Rafsanjan 77176,
Islamic Republic of Iran. E-mail: r.ranjbarkarimi@vru.ac.ir;
Fax: +98-343-131-2429; Tel: +98-343-131-2429
classical or modern conditions, using different Lewis acids,
25,26
¨
Bronsted acids, ionic liquids (ILs), and solid acids.
In
continuation of our ongoing research involving TRAMs,2h–j,3r
we wish to describe the SiO2–H2SO4 catalyzed synthesis of
triarylmethanes containing a formyl group under grinding
† Electronic supplementary information (ESI) available: General experimental
procedures and spectral data for all the compounds. See DOI: 10.1039/
c8nj05845h
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conditions, and their use for construction of new mixed-
bistriarylmethanes, and novel derivatives of 3,4-dihydropyrimidin-
2(1H)-ones containing a triarylmethane moiety.
Results and discussion
Synthesis of triarylmethanes bearing a formyl group
Scheme 2 Solvent-free synthesis of formylated-TRAM 6c under thermal
conditions.
There are few procedures available for the synthesis of triaryl-
methanes bearing a formyl group (henceforth called formylated-
triarylmethanes) (Scheme 1).2h–j,3r,19,27,28 These methods, however,
suffer from one or more drawbacks including very limited sub-
strate scope, prolonged reaction times, low yields of products,
harsh reaction conditions, and use of toxic chemicals, and
synthetic precursors, and/or expensive catalytic systems. To
overcome some of these deficiencies, we embarked on our
study by choosing 4-methylanisol 4b and terephthalaldehyde
5a as model reactants for optimization of the Friedel–Crafts
alkylation reaction in the presence of H2SO4 immobilized on
SiO2. It was found that the solvent, molar ratio of reactants,
amount of catalyst, and temperature had a tremendous effect
on the reaction yield. Under the optimized reaction conditions,
4b (2.5 mmol) reacted within 10 minutes at 75 1C and under
solvent-free conditions, with 5a (1 mmol) in the presence of
SiO2–H2SO4 (150 mg) to furnish the corresponding triaryl-
methane 6c in 91% yield (Scheme 2). However, the applicability
of this method is limited by a crucial drawback. In the course of
Scheme 3 The effect of reaction time on the yield of product 6f in the
solvent-free reaction of 4b with 5b.
our studies, we found that TRAM-CHO was obtained from this
reaction as the sole product if the progress of the reaction was
monitored carefully by TLC to inhibit the formation of an
adduct compound. For instance, product 6d was isolated in
84% yield after heating a mixture of 4b and isophthaldehyde 5b
for 12 min under optimal conditions. Further stirring the
reaction mixture for 30 min resulted in the formation of 6d,
as well as adduct 12, respectively, in 53% and 21% yields
(Scheme 3). However, this event complicates the purification
and isolation process of the TRAM product.
These results led us to postulate that formation of adduct 12
can be inhibited by decreasing the interactions of in situ
produced TRAM 6d with the remaining arene 4b under milder
reaction conditions. In the past few decades, mechanical
organic synthesis based on neat grinding and liquid-assisted
grinding (LAG) has been widely reported as a useful alternative
to the solvent-based organic synthesis approaches.29 Therefore,
grinding of reactants in a mortar with a pestle at room
temperature and in the presence of a catalytic amount of
SiO2–H2SO4 was proposed as a potential solution. Herein,
efficiency of the reaction is affected by the molar ratio of
reactants, the amount of the catalyst, grinding auxiliary, and
reaction time. For optimizing the experimental conditions, the
effects of these parameters were examined on the model
reaction, and the results are summarized in Table 1. Initially,
4-methylanisole 4b and 5a in various molar ratios were ground
together in a mortar in the presence of SiO2–H2SO4 (300 mg)
(Table 1, entries 1–4). Increasing the molar ratio of 4b : 5a up to
2.25 : 1, showed an improved effect on the yield of product 6c.
However, using higher 4b : 5a ratios resulted in lower yields,
due to the formation of the adduct compound (Table 1, entry 4).
As can be seen, simple grinding of 4-methylanisole (2.25 mmol)
and terephthalaldehyde (1 mmol) at room temperature for
5 min in the presence of SiO2–H2SO4 (300 mg) afforded 6c in
96% yield (Table 1, entry 3). Under similar conditions, the use
of 200, 250, and 350 mg of H2SO4 immobilized on SiO2 led to
Scheme 1 Routes to the synthesis of TRAM-CHO derivatives.
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Table 1 Optimization of the reaction conditions for SiO2–H2SO4 catalyzed
synthesis of formylated-triarylmethane 6c under solvent-free grinding
conditions
Table 2 Synthesis of formylated-triarylmethanes 6a–h under solvent-
free grinding conditionsa
Time Yieldb Mp
Entry Ar–H (4)
5
Product 6 (min) (%)
(1C)
4b
5a
SiO2–H2SO4 Auxiliary Time
Yielda
(min) (%)
Entry (mmol) (mmol) (mg)
(10 mg)
1
2
3
4
5
6
7
8
1,2-(MeO)2C6H4 (4a)
5a 6a
5b 6b
5a 6c
5b 6d
5a 6e
5a 6f
5a 6g
5a 6h
10
10
5
6
8
91 128–129
87 93–94
96 144–146
91 93–95
93 162–164
89 117–119
83 Oil
1,2-(MeO)2C6H4 (4a)
4-Methylanisole (4b)
4-Methylanisole (4c)
4-Chloroanisole (4d)
4-Bromoanisole (4e)
Anisole (4f)
1
2
3
4
5
6
7
8
2.0
2.2
2.25
2.5
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
2.25
1
1
1
1
1
1
1
1
1
1
1
1
1
1
300
300
300
300
200
200
250
250
350
300
300
300
300
300
—
—
—
—
—
—
—
—
5
5
5
5
5
10
5
10
5
5
5
54
87
96
83
32
50
67
83
96
96
96
96
73
65
8
10
15
10
10
20
o-xylene (4g)
43,67c Oil
9
10
11
2-Bromothiophene (4h) 5a 6i
Phenol (4i) 5a 6j
4-tert-Butylphenol (4j) 5a 6k
87 Oil
53 Oil
68 191–193
9
—
10
11
12
13
14
MgSO4
Na2SO4
NaCl
SiO2
Sand
a
Reaction conditions: arene (2.25 mmol), aldehyde (1 mmol), SiO2–H2SO4
b
5
5
5
(300 mg), room temperature, and simple grinding. Isolated yields.
c
Xylene (3.5 mmol), aldehyde (1 mmol), SiO2–H2SO4 (350 mg), room
temperature, and simple grinding.
a
Isolated yield.
that the reactions remarkably avoid the formation of the adduct
compounds. o-Xylene 4g as a less-activated arene was also sub-
mitted to the reaction with 5a under optimal conditions, and
their respective TRAM-CHO 6h was isolated in 43% yield. To
enhance the yield of the reaction, running the reaction using
reactants in higher molar ratios and further amounts of catalyst is
suggested. For example, when o-xylene (3.5 mmol) was ground
manually with 5a (1 mmol), and SiO2–H2SO4 (350 mg) for
15 minutes, the target product 6h was achieved in 67% yield
(Table 2, entry 8). The compatibility of the method for the
synthesis of bis(heteroaryl)methylbenzaldehyde derivatives was
also confirmed. For instance, the reaction of 2-bromothiophene
4h with terephthalaldehyde under optimal conditions furnished
product 6i in high yield (Table 2, entry 9). Our next investigation
revealed a moderate reactivity for phenols in the reaction with
terephthalaldehyde 5a. Treatment of phenol 4i with 5a for
10 minutes under grinding conditions gave TRAM 6j in 53%
yield. The liquid-assisted grinding (LAG) technique is known as
an effective method for improving the rate of reaction, yield of
products, and selectivity compared to typical solution-based
methods.30 Grinding a mixture of 4-tert-butylphenol 4j (2.25 mmol)
and 5a (1 mmol) in the presence of SiO2–H2SO4 (300 mg) and three
drops of EtOH for 15 minutes furnished the corresponding TRAM
6k in 68% yield.
the formation of 6c in 32%, 67%, and 96% yields, respectively
(Table 1, entries 5, 7 and 9), and reducing the amount of
catalyst requires longer grinding times for completion
(Table 1, entries 6 and 8). The effect of the grinding auxiliary
was also explored on the efficiency of the model reaction
(Table 1, entries 10–14). The use of sand and silica gel showed
an inhibitory effect on the formation of 6c, but addition of
MgSO4Á7H2O, Na2SO4, and NaCl did not show a considerable
effect on the productivity of the reaction (Table 1, entries 10–14).
The optimum reaction conditions for the synthesis of 6c were
found to be 4b (2.25 mmol), 5a (1 mmol), SiO2–H2SO4 (300 mg),
and room temperature (Table 1, entry 3).
Having in hand these operational conditions, the reactions
of various electron-rich arenes 4a–f with aromatic dialdehydes
5a or 5b were examined under grinding conditions to determine
the generality and scope of this method. All reactants were
converted to the corresponding formylated triarylmethanes 6a–g
in yields ranging from 83 to 96%, in short grinding times
(Scheme 4, and Table 2, entries 1–7). It is noteworthy to mention
Grinding-assisted synthesis of mixed-bistriarylmethane
derivatives
The condensation reaction of aromatic dialdehydes with arenes
or heteroarenes in at least 1 : 4 molar ratio is a common synthetic
route to bistriaryl- and bis(diheretoaryl)aryl methanes.2b,h–j,3b,h,j,r,11a
To the best of our knowledge, there has been no report on the
synthesis of mixed-triarylmethane compounds until now. We
identified formylated-triarylmethanes 6 as the ideal precursors
Scheme 4 Chemical structures of the synthesized TRAM-CHO derivatives.
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Scheme 6 Synthesis of xanthene 14 under LAG conditions.
Scheme 5 Chemical structures of synthesized mixed-bistriarylmethanes.
for the synthesis of mixed-bistriarylmethane derivatives. Thus,
the efficiency of the above mentioned procedure to design and
synthesize mixed-bistriarylmethanes was explored and the
results are illustrated in Scheme 5. As expected, treatment of
TRAM-CHOs 6c–f (1 mmol) with veratrole 4a (2.3 mmol) in the
presence of SiO2–H2SO4 (300 mg) at rt for 8 minutes grinding
Scheme 7 Synthesis of xanthene 14 using 4-(14H-dibenzo[a,j]xanthen-14-yl)-
benzaldehyde 6l under thermal conditions.
However, the described method is generally clean, mild, and
time gave the corresponding adducts 13a–d in excellent yields. efficient for the chemoselective synthesis of the desired mixed-
This method was also compatible to synthesize mixed-bistriaryl- bistriarylmethanes.
methanes 13e–g by the reaction of 6c with, respectively, anisole
Synthesis of 3,4-dihydropyrimidin-2(1H)-one derivatives
bearing a triarylmethane moiety
4f, 4-chloroanisole 4d, and 2-bromothiophen 4h in 88–93%
isolated yields within 10 to 12 minute grinding times. Further-
more, TRAM 6k reacted efficiently with 2-bromothiophene 4h Triarylmethane derivatives containing N-based heterocycles are well-
under grinding conditions to afford the adduct 13h in high yield. known to possess various biological activities.1c,14,37 Di(uracilyl)-
Xanthene derivatives constitute an important class of dyes arylmethanes, manufactured from the reaction of 1-alkyluracil
which have a wide spectrum of biological activities.31 These derivatives and arylaldehydes catalyzed by HBr/AcOH, have been
heterocyclic compounds are also being used in laser techno- reported as novel supramolecular architectures.38 On the other
logy,32 dye sensitized solar cells (DSSC),33 organic light-emitting hand, Montagut et al. succeeded in synthesizing several families
devices (OLEDs),34 and bioimaging processes.35 However, of hydrophobic and oleophobic leuco dyes by introducing a
14-aryl-14-H-dibenzo[a,j]xanthene derivatives have been 1,3,5-triazine core into the structure of TRAMs.39 Therefore, the
synthesized via the condensation reaction of 2-naphthol 4k availability of TRAMs including nitrogen-containing heterocycles is
with aromatic aldehydes in the presence of acid catalysts.36 expected to provide a unique opportunity for the discovery of novel
When a neat mixture of 6c (1 mmol) and 2-naphthol properties and applications for them.
(2.25 mmol) was ground in the presence of SiO2–H2SO4
However, we thought molecular hybridization of TRAMs and
(300 mg) for 10 minutes, neither mixed-bistriarylmethane 3,4-dihydropyrimidin-2(1H)-ones can be achieved via submit-
13i nor xanthene 14 was formed. Interestingly, the addition ting TRAM-CHO derivatives 6a–h to the Biginelli reaction with
of three drops of ethanol to the mixture of 6c (1 mmol) and ethylacetoacetate 15 and urea 16a. SiO2–H2SO4 has been proved
2-naphthol (2.1 mmol) prior to its grinding in the presence of to be an efficient promoter for the Biginelli reaction under
catalyst (300 mg) for 8 minutes allowed us to isolate xanthene solvent-free conditions.40 Surprisingly, our initial trials showed
14 as the sole product in 96% yield (Scheme 6). Starting from the efficiency of this catalytic system in the condensation of
2-naphthol and terephthalaldehyde 5a we also performed the formylated-TRAM 5c with ethylacetoacetate and urea. Given
synthesis of compound 14, following the reactions depicted in this reaction as a model, the effect of various parameters
Scheme 7. Anyway, the presence of two chromophoric systems, including solvent, reaction temperature, stoichiometric ratios
a xanthene core and a triarylmethane unit, in compound 14 of reactants, and the loading amount of catalyst were examined.
may offer special photophysical properties. Moreover, this In the best conditions, product 17c was obtained in the highest
strategy can be used for the synthesis of other analogues of yield from the solvent-free reaction of 6c (1 mmol) with 15
compound 14.
(1.1 mmol) and 16a (1.5 mmol), in the presence of SiO2–H2SO4
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Table
3
Synthesis of 3,4-dihydropyrimidin-2(1H)-one-triarylmethane
Table 3 (continued)
hybrid derivativesa
Time Yieldb Mp
Entry
6
16 Product 17
(min) (%)
(1C)
Entry
1
6
16 Product 17
10
6c 16b
90
54
161–163
6a 16a
6b 16a
50
60
94
83
221–223
a
Reaction conditions: TRAM-CHO 6 (1 mmol), ethylacetoacetate 15
(1.1 mmol), urea or thiourea 16 (1.5 mmol), SiO2–H2SO4 (100 mg), and
b
95 1C. Isolated yields.
2
3
107–109
205–207
(100 mg) at 95 1C for 50 minutes (Table 3, entry 3). Once
the feasibility of molecular hybridization of TRAM-CHOs with
3,4-dihydropyrimidin-2(1H)-one derivatives was proved, we
explored the scope of the reaction substrates with different
TRAM-CHOs using the above mentioned conditions, and the
results are illustrated in Table 3. Accordingly, the reaction of
TRAM-CHOs 6a, 6e, 6f, and 6h with 15, and urea after allotted
times, gave the corresponding products 17a and 17e–g, in
excellent yields (Table 3, entries 1 and 5–7). TRAM-CHO 6b
also reacted smoothly with 15 and urea to afford DHPM–TRAM
hybrid derivative 17b in 83% yield (Table 3, entry 2). Compared
with 6b, only a trace amount of TRAM-CHO 6d was transformed
to the product 17d, when 6d was treated with 15 and urea for
90 minutes under optimal conditions (Table 3, entry 4).
As a further application, this protocol was also examined
to manufacture sulfur analogues of 17a and 17c. Aiming at
this objective, 3,4-dihydropyrimidin-2(1H)-thiones-triarylmethane
hybrid derivatives 17h and 17i were prepared in high yields, when
the reaction of their respective aldehydes (6a and 6c) with 15, and
thiourea 16b was carried out under the optimal conditions
(Table 3, entries 8 and 9). Likewise, stirring a mixture of 6c
(1 mmol), 15 (1.1 mmol), N-allylthiourea 16c (1.5 mmol), and
SiO2–H2SO4 (100 mg) at 95 1C for 90 minutes resulted in the
formation of product 17j, as the N-substituted analogue of 17i,
in 54% yield (Table 3, entry 10).
6c 16a
50
90
96
4
6d 16a
Trace
—
5
6
6e 16a
6f 16a
40
55
95
94
144–146
161–163
7
6h 16a
60
93
216–218
Conclusions
8
9
6a 16b
6c 16b
75
70
91
92
173–175
203–205
Friedel–Crafts alkylaion reaction of arenes with aromatic
dialdehydes under grinding conditions and in the presence of
SiO2–H2SO4 as a catalyst, has been developed for the efficient
synthesis of formylated-triarylmethanes. Compared to the
corresponding solution-based reactions, this method offers
advantages including simplicity and cleanness, mild reaction
conditions, short grinding times, lower energy consumption,
wider range of substrate scope, and excellent chemoselectivity.
Under these conditions, a variety of new mixed-bistriarylmethanes
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were synthesized starting from formylated-triarylmethane SiO2–H2SO4 (300 mg) was ground in a mortar with a pestle at room
derivatives and different arenes. Formylated-triarylmethanes temperature for 8 minutes, and left to harden for about 2 hours. The
were demonstrated to be valuable precursors for the prepara- progress of the reaction was monitored by TLC (eluent: petroleum
tion of various 3,4-dihydropyrimidin-2(1H)-(thi)one derivatives ether/EtOAc 1 : 1). Then, the resultant mixture was transferred into a
containing a triarylmethane moiety in high to excellent yields. flask and washed twice with CHCl3 (2 Â 5 mL), and the catalyst was
separated by simple filtration. The solvent was evaporated and the
residue was purified by recrystallization from EtOH or by flash
column chromatography on silica gel; elution with 1 : 1 petroleum
ether/EtOAc or 10 : 2 petroleum ether/MeOH. Pure mixed-
Experimental section
General considerations
bistriarylmethane 13a was obtained in 95% yield. Mp 91–
93 1C. H NMR (500 MHz, CDCl3): d 6.99 (d, J = 8.2 Hz, 2H),
All materials were obtained from commercial sources and were
used as received. The progress of the reactions was monitored
by TLC analysis using polyester sheets pre-coated with silica
gel-60 and fluorescent indicator (F-252), commercially available
from Merck company. Melting points were determined using a
Stuart SMP2 apparatus and are uncorrected. FT-IR spectra were
recorded as KBr pellets using a Nicolet-Impact 400D spectro-
photometer. 1H, 13C, and APT NMR spectra, as well as the H–H
COSY spectrum of compound 17j were acquired on a Varian
UNITYInova 500 MHz spectrometer.
1
6.96 (dd, J1 = 8.3 Hz, J2 = 1.8 Hz, 4H), 6.89 (d, J = 8.2 Hz, 2H), 6.82
(t, J = 7.7 Hz, 4H), 6.69 (d, J = 1.7 Hz, 2H), 6.57 (dd, J1 = 8.3 Hz, J2 =
1.8 Hz, 4H), 6.53 (d, J = 1.7 Hz, 2H), 6.05 (s, 1H, Ar3CH), 5.40 (s, 1H,
Ar3CH), 3.69 (s, 6H, OMe), 3.61 (s, 6H, OMe), 3.58 (s, 6H, OMe), 2.08
(s, 6H, Me) ppm. 13C NMR (125 MHz, DMSO-d6): d 155.18, 148.94,
147.66, 142.16, 141.83, 137.21, 132.13, 130.44, 129.22, 129.01,
128.78, 128.08, 121.46, 113.48, 112.14, 111.64, 56.07, 55.92, 55.79,
55.08, 42.38, 20.84 ppm. FT-IR (KBr): n~ 3028, 2933, 2834, 1606,
1500, 1462, 1414, 1242, 1183, 1139, 1108, 807, 638 cmÀ1
.
Preparation of silica-supported sulfuric acid
Typical procedure for the synthesis of 3,4-dihydropyrimidin-2(1H)-
Sulfuric acid immobilized on silica gel (SiO2–H2SO4) was prepared
according to the literature.41 Silica gel (10 g, 200–300 mesh) was
dispersed in dry Et2O (100 mL). Under slow stirring, concentrated
H2SO4 (3.2 mL, 98%) was added dropwise to the resulting mixture
over 10 minutes, and stirring continued at rt for a further 10 min.
After removal of the volatiles, the resulting solid powder was dried
overnight at 60 1C.
(thi)one-triarylmethane hybrid derivatives (Table 3, entry 3)
A 25 mL round-bottom flask equipped with a magnetic stir bar was
charged with 4-(bis(2-methoxy-5-methylphenyl)methyl)benzaldehyde
6c (360.2 mg, 1 mmol), ethylacetoacetate 15 (143.2 mg, 1.1 mmol),
urea 16a (90.1 mg, 1.5 mmol), and SiO2–H2SO4 (100 mg). The
resulting mixture was stirred at 95 1C, for 50 minutes. The
reaction progress was checked by TLC (n-hexane/EtOAc 3 : 1 as
eluent). Upon completion, EtOH (2 Â 5 mL) was added to the
reaction mixture and the catalyst was filtered off. The volatiles of
the resulting organic solution were evaporated and the residue
was washed twice by water (2 Â 10 mL). The crude product was
purified from EtOH to obtain pure product 17c in 96% yield.
White powder. Mp 221–223 1C. 1H NMR (500 MHz, CDCl3) d 8,47
(bs, 1H, NH), 7.22 (d, J = 8.1 Hz, 2H), 7.04 (d, J = 8.0 Hz, 2H),
6.77 (d, J = 8.2 Hz, d, 2H), 6.64 (d, J = 2.05 Hz, 2H), 6.56 (dd, J1 =
8.2 Hz, J2 = 2 Hz, 2H), 5.76 (bs, 1H, NH), 5.40 (s, 1H), 5.37 (d, J =
2.8 Hz, 1H), 4.03–4.13 (m, 2H, CH2CH3), 3.85 (s, 6H, OMe), 3.76
(s, 6H, OMe), 2.33 (s, 3H, Me), 1.14 (t, J = 7.2 Hz, 3H, Me) ppm.
13C NMR (125 MHz, CDCl3) d 165.64, 153.49, 148.75, 147.49,
146.39, 144.09, 141.69, 136.50, 136.48, 129.56, 126.51, 121.35,
112.70, 110.83, 101.27, 59.93, 55.84, 55.82, 55.80, 55.59, 55.37,
18.57, 14.17 ppm. FT-IR (KBr) n~ 3230, 3108, 3017, 1935, 2835,
Typical procedure for the synthesis of formylated-triarylmethanes
under grinding conditions (Table 2, entry 3)
In an open mortar, a mixture of 4-methylanisole 4b (280.8 mg,
2.3 mmol), terephthalaldehyde 5a (134.0 mg, 1 mmol), and
SiO2–H2SO4 (300 mg) was ground together using a pestle at
room temperature for 5 minutes (formation of a green and non-
sticky mixture can be considered as a trustable endpoint of the
reaction), and left to harden for about 2 hours. The progress of
the reaction was monitored by TLC (n-hexane/EtOAc 10 : 4).
Then, the resultant mixture was transferred into a flask and
washed twice with ethanol (2 Â 5 mL), and the catalyst was
separated by simple filtration. The crude product was purified
by recrystallization from EtOH, to provide product 6c as a white
solid in 96% yield. Mp 144–146 1C. 1H NMR (500 MHz, CDCl3) d
9.97 (s, 1H, CHO), 7.76 (d, J = 8 Hz, 2H), 7.22 (d, J = 7.8 Hz, 2H),
7.03 (d, J = 7.3 Hz, 2H), 6.79 (d, J = 8.1 Hz, 2H), 6.59 (s, 2H),
6.20 (s, 1H), 3.66 (s, 6H, OMe), 2.20 (s, 6H, Me) ppm. 13C NMR
(125 MHz, CDCl3) d 192.2, 155.2, 152.2, 134.3, 131.2, 131.0,
129.8, 129.5, 129.4, 128.0, 111.90, 55.8, 43.6, 20.7 ppm. FT-IR
(KBr): n~ 2939, 2833, 1700, 1602, 1573, 1497, 1463, 1386, 1241,
1701, 1644, 1511, 1462, 1262, 1225, 1139, 1091, 1027, 768 cmÀ1
.
Conflicts of interest
There are no conflicts to declare.
1109, 1034, 808 cmÀ1
.
Typical procedure for the synthesis of mixed-bistriarylmethanes
under grinding conditions (Scheme 5, 13a)
Acknowledgements
A mixture of 4-(bis(2-methoxy-5-methylphenyl)methyl)benzaldehyde We are grateful to the research council of the Vali-e-Asr Uni-
6c (360.2 mg, 1 mmol), veratrole 4a (317.6 mg, 2.3 mmol), and versity of Rafsanjan for financial support of this work.
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