The regioselectivity and synthetic mechanism of 1,2-benzimidazole squaraines: Combined experimental and theoretical studies

Article abstract of DOI:10.1039/c3ra43608j

In this paper, a synthetic method to produce 1,2-benzimidazole squaraines with a yield of up to 89% is developed. It is found that both strong organic and inorganic bases have a satisfactory catalytic activity for this reaction. Theoretical studies provide detailed explanations for the 1,2 versus 1,3 condensation regiochemistry of the squaraines. The experimental and theoretical studies agree well with each other, paving a practical way to efficiently synthesize 1,2-squaraines. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3ra43608j

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ARTICLE TYPE
Regioselectivity and Synthetic Mechanism of 1,2-
Benzimidazole Squaraines: Combined Experimental and Theoretical
Studies
Guomin Xia, Zhiwei Wu, Yanli Yuan, and Hongming Wang*
5
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
In this paper, a synthetic method to produce 1,2ꢀbenzimidazole squaraines with a yield of up to 89% is
developed. It is found that both strong organic and inorganic bases have satisfied catalyzed activity for
this reaction. Theoretical studies provide detailed explanations for the 1,2 versus 1,3 condensation
regiochemistry of the squaraines. Experimental and theoretical studies agree well with each other, paving
a practical way to efficiently synthesize 1,2ꢀsquaraines.
1
0
15
semisquaraine and identified the 1,3ꢀisomer as the reactive
intermediate in squaraine dye formation rather than the neutral
Introduction
1
,2ꢀisomer. Their investigations indicate that the base influences
Squaraine (SQ) dyes are the product of condensation of squaric
acid and two equivalents of a suitable electronꢀrich precursor
and be notable for their exceptionally high absorption coefficients
extending from the green to the nearꢀinfrared region. This has
prompted their exploitation in a number of technologically
the regioselectivity of 1,2ꢀ and 1,3ꢀsemisquaraine. Ronchi et al.
1
5
5
6
0
5
0
identified the main electronic factors in this reaction and changed
1
2
2
3
3
4
4
5
0
5
0
5
0
5
16
its conditions, increasing the yield of 1,2ꢀSQs up to 66%.
2,3
It is clear that the synthesis of 1,2ꢀSQs is of significant
challenge, and the yield as well as selectivity still remains
unsatisfactory. In this paper, we report a synthetic method to
produce 1,2ꢀbenzimidazole squaraines (3aꢀb) at a high yield. To
explore the reason for the high regioselectivity of 3aꢀb, the
overall reaction mechanisms were studied by density functional
theory (DFT) calculations as well. By combining the
experimental and computational findings, we are able to propose
a method of chemical regulation that changes the regioselectivity
of the product. This work sheds light on the factors influencing
reaction via the 1,2ꢀ versus the 1,3ꢀcondensation pathway, which
could provide guidelines for the synthesis of desired 1,2ꢀSQs in
the future.
4
5
relevant applications, including photoconductivity, data storage,
lightꢀemitting fieldꢀeffect transistors, solar cells, and fluorescent
6
7
8
histological probes. Since the pioneering work of Cohen in
9
1
959, SQs have also been used in the fields of bioorganic and
1
0
medicinal chemistry as well.
demonstrated that SQ is an effective pharmacophore for the
design of tyrosine phosphatase inhibitors.
The synthesis of squaraine dyes was first reported in the 1960s.
Then numerous reports describing the synthesis and properties of
SQs have appeared in recent decades. According to previous
investigations, SQs are the condensation product of two electronꢀ
rich derivatives (activated arenes, 2,4ꢀdimethylpyrrole and
benzothiazole) and squaric acid or their esters, which generally
affords the 1,3ꢀregioisomer as the main product and the 1,2ꢀ
isomer as a byꢀproduct. Modification of the reaction conditions
only slightly increases the regioselectivity toward 1,2ꢀSQ; the
reaction is regioselective and the major products are still the 1,3ꢀ
isomers. Although some studies indicate that 1,2ꢀSQ can be
For example, Xie et al.
1
1
1
2
1
3
6
5
Results and Discussion
Considering the most of synthesis of 1,3ꢀSQs in previous reports,
we planned to produce 1,3ꢀSQ (2a) by reacting benzimidazole
salt 1a with 3,4ꢀdiethyl squarate in the presence of a frequentlyꢀ
used base (triethylamine (Et3N)) in EtOH (Scheme 1), however,
synthesized through a twoꢀstep FriedelꢀCrafts acylation reaction 70 no products formed after heating under reflux for 12 h, and the 1a
between squaric acid chloride and bromoꢀsubstituted thiophene,
its direct synthesis from squaric acid or squarate have seldom
been reported. As we all know, 1,2ꢀSQs are considered to be
promising as sensitizers in organic photoconducting devices and
didn't react at all. Then, a fairly strong base was been used to
catalyze the reaction. As a result, the reaction mixture turned
from yellow to deep red instantly when diazabicycloundecene
(DBU) was been added, after 3h reflux, a dullꢀred product was
electronꢀacceptor components for construction of highꢀ 75 isolate with 68% yield and crystallized by the slow evaporation
[
14]
performance DꢀAꢀtype conjugated photovoltaic polymers, so a
facile and high selectivity route to synthesize 1,2ꢀSQs is urgently
required.
method. To our surprise, Xꢀray diffraction analysis of the product
showed that the red product was 1,2ꢀSQ 3a (Figure 1) instead of
1,3ꢀSQ 2a. Also, 3a was characterized by various spectroscopic
and analytical techniques: 1H and 13C NMR spectroscopy, FTꢀ
Ramaiah’s group achieved the selective synthesis of 1,2ꢀ and 1,3ꢀ
This journal is © The Royal Society of Chemistry [year]
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DOI: 10.1039/C3RA43608J
O
O
R
R
IR and elemental analysis.
R
O
O
N
EtOH
The results show that we should draw to the thought that the
basicity of the base has a large effect towards the products; this
prompted us to use a stronger base to verify this inference.
Sodium ethoxide (EtONa), as a common organic base, was used
to catalyze the condensation reaction between 1aꢀb and 3,4ꢀ
N
N
N
+
N
EtONa reflux3h
HO
OH
N
R
R
R
1
aꢀb
3
aꢀb trace
5
0
5
Scheme S2. The reaction between benzimidazole salt and squaric
diethyl squarate. As the way that was expected, red deposits 40 acid catalyzed byEtONa.
formed immediately as long as EtONa was been added, after 3h
Table 2. Reaction of Benzimidazole Salts (1aꢀb) and 3,4ꢀDiethyl Squarate
reflux, the reactions were completed to produce 3a and 3b in 89%
and 86% yield, respectively. Moreover, in these reactions, 1,3ꢀ
SQs (2a and 2b) were not been observed. However, when squaric
acid was reacted with 1aꢀb in EtOH solvent, no red product 1,2ꢀ
SQs formed after heating under reflux for 3h (Scheme 2). The
weaker base pyridine was also been used to catalyze the
condensation reactions between 1aꢀb and 3,4ꢀdiethyl squarate; the
mixture was always beige, and no red deposit formed in these
reactions, after heating under reflux for 12h, no 1,2ꢀ or 1,3ꢀSQs
formed, which is similar to the reaction using Et3N as the base.
under Various Inorganic Bases.
1
R
Base
Time (h) Product Yield (%)
1
2
3
4
5
6
7
8
9
PhCH2
NaHCO3
12
12
12
12
12
12
12
12
3
3a
3b
3a
3b
3a
3b
3a
3b
3a
3b
3a
trace
trace
trace
trace
trace
trace
trace
trace
76
CH3
NaHCO3
KHCO3
KHCO3
PhCH2
CH3
PhCH2
CH3
1
2
Na CO3
2
Na CO3
PhCH2
2
K CO3
R
R
O
CH3
K CO3
2
N
N
N
PhCH2 Cs CO3
2
N
N
10 CH3
11 PhCH
12 CH3
13 PhCH2
CH3
Cs CO3
3
3
73
81
t
3
12h
2
R
N
E
O
x
R
R
eflu
O
O
2
NaOH
NaOH
KOH
EtOH
r
2aꢀb
OEt+
2
3
75
3
b
N
R
EtO
O
O
R
D
R
B
3
3
3a
3b
85
80
U
N
N
N
1
aꢀb
1
4
KOH
N
a:R=PhCH
b:R=CH
2
R
R
3
3aꢀb
2
0
Scheme 1. The reaction of benzimidazole salt with 3,4ꢀdiethyl
squarate.
Table 1. Reaction of Benzimidazole Salts (1aꢀb) and 3,4ꢀDiethyl Squarate
catalyzed by various bases.
R
Base
Time (h) Product Yield (%)
1
2
3
4
5
6
7
8
PhCH2 pyridine
12
12
12
12
3
3
3
3
3a
3b
3a
3b
3a
3b
3a
3b
0
0
0
CH3
PhCH2
CH3
pyridine
Et N
3
Et N
3
0
4
5
Figure 1. ORTEP diagram of 3a.
PhCH2
CH3
DBU
DBU
68
61
89
86
Absorption spectra of 3a and 3b in selected solvents are presented
in Fig. 2A and 3A (see the ESI). The spectra indicate a solventꢀ
dependent change in the absorption maxima and intensity of 3a.
The absorption maximum changes considerably in methanol
compared with in other solvents. The absorption coefficient of 3a
in dichloromethane is larger than in any other solvent. Similar
results were also observed for 3b. In Fig. 2B, the absorption peak
at 524 nm decreases and that at 399 nm increases gradually as the
concentration of TFA increases, which implies a strong pH
dependence of the visible absorption spectra of 3a. 3a was also
investigated by timeꢀdependent density functional theory
(TDDFT) calculations. Fig. S5 shows the calculated ground state
structures and molecular orbitals of 3a. From TDDFT
calculations, the longꢀwavelength absorption (524 nm) is mainly
generated by the HOMO→LUMO transition. The shortꢀ
wavelength absorption (433 nm) corresponds to the promotion of
an electron from the HOMOꢀ2 to the LUMOꢀ1(see Figure. S5).
PhCH2 EtONa
EtONa
CH3
2
3
3
5
0
5
Based on the results above, we wanted to determine that whether
an inorganic base can be used to catalyze these reactions. The
yields of the reactions utilizing various inorganic bases are
presented in Table 2. It indicated that inorganic bases also can
catalyze this condensation reactions and the basicity of the base
worked as the same as organic bases: only trace amounts of 3a
and 3b were observed (inspected by TLC cromatography) when a
weak inorganic bases (NaHCO , KHCO , Na CO and K CO )
was been used after refluxing 12h; However, with 250%mol
strong base Cs CO , NaOH, KOH, the yield of 3a and 3b were
5
5
6
0
5
0
3
3
2
3
2
3
2
3
dramaticlly impromed towards to 76% and 73%, 81% and 75%,
5% and 80%, respectively.
8
2
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Figure 3. Absorption spectra of the 1,2ꢀSQ 3b in various solvents (A).
Absorption spectra change of 3a in acetone upon addition of different
ratio of trifluoroacetic acid (TFA) (B).
8
6
4
2
0000
0000
0000
0000
0
1
0
(
A)
To examine the different reaction activity of various bases
and their effect on the regioselectivity of this reaction, DFT
calculations were performed to explore the detailed reaction
mechanism[17] (Scheme S3). In the first step of this reaction, 1b
reacted with different bases (EtONa, DBU, Et3N, and pyridine)
to produce PRa complexes. Fig. 4A shows the energy profile for
this deprotonation step. The H1 atom is attacked by the bases
DMF
DMSO
acetone
dichloromethane
Methanol
ethanol
1
5
acetonitrile
(
EtONa, DBU, Et3N, and pyridine) to produce an INa (INa1,
3
50
400
450
500
550
600
650
700
Wavelength / nm
INa2, INa3, and INa4) complex. All of these complexes have
lower energies than the reactants (Fig. 2A). Because of the strong
electronꢀdonating ability of the lone pairs of electrons of the
bases, the H1 atom is then transferred from 1b to the base to
afford 1aꢀbase (INb) intermediates via transition states (TSa)
(Fig. 4A). Also, Fig. 4A shows that TSa1 has lower energy (ꢀ
25 18.69 kcal molꢀ1) than the reactant. However, the energy barriers
of TSa2, TSa3 and TSa4 are 7.42, 14.84 and 16.37 kcal molꢀ1,
respectively, which are higher than those of the reactants.
Subsequently, the products, PRa1, PRa2, PRa3 and PRa4 are
formed from the INb complexes. Fig. 4A reveals that PRa1 and
PRa2 are more stable than the reactants by 30.68 and 0.44 kcal
molꢀ1, respectively. However, the energies of PRa3 and Pra4 are
14.10 and 23.47 kcal molꢀ1 higher than those of the
corresponding reactants. Because of the low energy barriers and
exothermic characteristics, the reactions catalyze d by EtONa and
35 DBU proceed easily. For Et3N and pyridine, however, both
reactions are strongly endothermic with relatively high energy
barriers, it is understandable that these two reactions do not
proceed readily. In addition, EtONa has the highest activity
during all of the bases used. These conclusions are consistent
40 with the experimental results mentioned above where the reaction
catalyzed by EtONa gives a high yield of 1,2ꢀSQ (89%).
2
0
8
6
4
2
0000
0000
0000
0000
0
(
B)
3
a
3
3
3
3
3
3
a:TFA=5:1
a:TFA=2:1
a:TFA=1:1
a:TFA=1:2
a:TFA=1:2.5
a:TFA=1:3
3a:TFA=1:4
3
3
a:TFA=1:5
a:TFA=1:
∞
3
0
3
50
400
450
500
550
600
650 700
Wavelength / nm
Figure 2.Absorption spectra of the 1,2ꢀSQ 3a in various solvents (A).
Absorption spectra change of 3a in acetone upon addition of different
ratio of trifluoroacetic acid (TFA) (B).
5
1
1
1
40000
20000
00000
(
A)
DMF
8
6
4
2
0000
0000
0000
0000
0
DMSO
acetone
dichloromethane
methanol
ethanol
PRa4
2
1
0
0
0
A
TSa4
TSa3
TSa2
INb4
INb3
INb2
PRa3
acetonitrile
Re
INa3
PRa2
INa4
3
50
400
450
500
550
600
650
700
Wavelength / nm
INa2
-
10
1
1
1
1
60000
(
B)
-20
-30
-40
40000
20000
00000
TSa1
INa1
3
b
PRa1
3
3
3
3
3
3
3
3
3
b:TFA=5:1
b:TFA=2:1
b:TFA=1:1
b:TFA=1:2
b:TFA=1:2.5
b:TFA=1:3
b:TFA=1:4
b:TFA=1:5
INb1
80000
60000
40000
20000
0
b:TFA=1:
∞
350
400
450
500
550
600
650
700
Wavelength / nm
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5
4
3
2
1
0
0
0
0
0
0
TSe
TSc
40
B
A
30
TSes
20
TSd
10
TSb
TScs
0
PRa+PRb
INd
-
10
20
PRa+FS
-
3
b+EtOH
INC
-
10
20
PRb+ETOH
-
Figure 4. Energy profiles for (A) the deprotonation of
benzimidazole salt by base, and (B) the formation of PRb.
After deprotonation, the benzimidazole methylene (PRa)
was formed (Scheme S1 and Fig. 4A). Because of nucleophilicity
of PRa, it is able to undergo a 1,4ꢀconjugated attack at the
squarate (FS) double bond(Fig. 4B). The step involves a
transition state (TSb) that has a low active barrier (13.3 kcal molꢀ
5
0
5
0
5
3
0
B
C
Figure 5. Energy profiles for the formaction of 3b (pathways: IꢀA) and the
1
). An intermediate (INc) is then formed with an energy 2.31 kcal
structures of transition states(B:TSe ,C: TSes).
1
1
2
2
molꢀ1 above that of the reactants. The next step is a 1,5ꢀproton
shift between the methylene of PRa and enolate (see Fig. 4B and
Fig. S6 in the ESI). Finally, thermallyꢀ catalyzed elimination of
ethanol gives exclusively PRb. In fact, this step (TSc) has a very
high energy barrier (44.01 kcal molꢀ1). This barrier is too large to
enable the reaction to be conducted experimentally at ambient
temperature, so heating is required. Because the energy barriers
involve proton transfer, we considered that weather EtOH played
an important role in this process. When EtOH was introduced as
the proton transfer catalyst to assist in forming TScs (sixꢀmember
ring transition state), the energy barrier was reduced considerably
to 16.21 kcal molꢀ1 (Fig. 4B). This experiment also indicates that
the yield and reaction rate would be increased considerably in
ethanol compared with that in DMF catalyzed by DBU (Scheme
Because of its nucleophilicity, PRa can continue to attack
the C1 and C2 atoms of PRb through two reaction pathways: I-A
and I-B (see Scheme S1). In reaction pathway I-A, the C3 atom
of PRa attacks the C1 atom of PRb to produce an intermediate
INd. This process involves transition state TSd with an energy
barrier of 16.3 kcal mol (figure 5 and Figure. S8). After INd,
transition state TSe with an energy 37.58 kcal mol above that of
the reactants forms (Figure. S8). TSe involves a proton transfer
3
5
ꢀ
1
ꢀ
1
4
0
(
H8) between the O9 and C7 atoms, and leads to products 3b and
EtOH. When EtOH is introduced as a cocatalyst, the energy
barrier (TSe) decreases to 18.49 kcal mol .
ꢀ
1
5
4
0
0
TSg
3
).
TSh
A
O
O
30
20
10
0
R
R
R
R
TSi
O
O
O
O
N
N
N
N
DMF
TSf
+
DBU reflux
N
N
INe
R
R
INf
INg
1aꢀ1b
3aꢀ3b(30ꢀ36%)
PRa+PRb
Scheme 3. Reaction of Benzimidazole Salts (1aꢀb) and 3,4ꢀ
-10
Diethyl Squarate catalyzed by DBU in dry DMF.
-20
2
b+EtOH
4
5
B
C
Figure 6. Energy profiles for the formaction of 2b (pathways: IꢀB) and the
structures of transition states(B:TSg;C: TSgs).
5
0
4
| Journal Name, [year], [vol], 00–00
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As displayed in the figure 6 and Figure. S9, in reaction 55 determined in open capillaries. 1H NMR and 13C NMR spectra
pathway IꢀB, the C3 atom of PRa attacks the paraꢀC2 atom of
PRb to produce an intermediate INf via transition state TSf with
were measured on a Bruker AVANCE 400 spectrometer in
DMSOꢀd6 using TMS as an internal standard. The crystals for Xꢀ
ray diffraction analyses were grown in dichloromethane by
adding nꢀhexane to slow diffuse. The Xꢀray diffraction
experiment was carried out using Bruker SMART APEXꢀII
ꢀ
1
an energy barrier of 16.12 kcal mol . After INf, a transition state
TSg involving proton transfer (H8) from PRa to PRb with an
5
ꢀ
1
energy barrier of 45.12 kcal mol occurs. When EtOH is
introduced as a proton transfer catalyst to assist this step, the
6
0
Singleꢀcrystal diffractometer
.
ꢀ
1
energy barrier was reduced to 17.32 kcal mol . In the next step,
an OH group on the C2 atom transfers to the C1 atom. This
process proceeds via the transition state TSh, which is 36.1 kcal
Synthesis of 2-methyl benzoimidazole (a)
Phenylene diamine (0.3mol, 32.4g) and acetic acid (1mol,
1
1
2
0
5
0
ꢀ
1
mol higher in energy than the reactants. After TSh, INg is 65 60ml) were refluxed for 2h. Ice and KOH was added to PH=8~10
ꢀ
1
formed with an energy of 3.07 kcal mol above that of the
reactants. The last step for this pathway is proton transfer from
the OH group on C1 to O atom of the ethyoxyl. This process
results in product 2b via transition state TSi. The computed
and the light violet solid filtered out, then active carbon was used
and recrystallized from water to giving 2ꢀmethyl benzoimidazol
(
℃
a) as light yellow needle crystals (30g, yield 77%), M.p 176~178
.
21
ꢀ
1
activation barrier for TSi is 19.97 kcal mol . From the analyses
above, it can be concluded that pathway I-A is the favorable
reaction route, because its highest activation barrier (17.58 kcal
7
0
Synthesis of 1,3-dibenzyl-2-methyl-benzimidazolium chloride
(1a)
ꢀ
1
ꢀ1
mol ) is lower than that of channel I-B (36.1 kcal mol ). This
explains well the high regioselectivity for 1,2ꢀSQs (3a-b) in our
experiments.
To a solution of 2ꢀmethylbenzimidazole (1 g, 7.57 mmol) in
DMF (20 ml) was added benzyl chloride (2.66 ml, 22.7 mmol),
potassium carbonate (1.25 g, 9.08 mmol) and a catalytic amount
of tetraꢀnꢀbutylammonium bromide. The mixture was stirred at
room temperature for 24 h. The solution was filtered and the
solvent removed under reduced pressure. The residue was
recrystallized from ethanol to afford 1,3ꢀdibenzylꢀ2ꢀmethylꢀ
benzimidazolium chloride (1a) as colorless amorphous crystals
7
5
Conclusions
In summary, a method to prepare 1,2ꢀbenzimidazole squaraines
in high yield via one step reaction is developed. The yield of 1,2ꢀ
benzimidazole squaraines is up to 89%. The total reaction
mechanism is explored by DFT calculations. By combining
8
0
2
5
2
2
(0.8g, yield 27%). M.p 312~314 ℃ ; 1H NMR (400 MHz,
DMSOꢀd6δ): 7.92 (d, J = 1.67 Hz, 2H), 7.57 (d, J = 0.76 Hz, 2H),
5.81 (s, 4H), 7.36 (s, 10H)，2.99 (s, 3H); 13C NMR (400 MHz,
DMSOꢀd6 δ): 152.9 (1C), 134.6 (2C), 131.5 (2C) 129.4 (4C)
experimental and computational findings, we proposed
a
chemical regulation method to change the regioselectivity of the
product. Our experimental and theoretical studies agree well with
3
0
each other, paving a practical way to synthesize of 1,2ꢀsquaraines 85 128.8 (4C), 127.49 (2C), 126.8 (2C), 113.8 (2C), 48.8 (2C),
in high yield. Syntheses of other 1,2ꢀSQ complexes will be
reported in the future.
11.5(1C).
Synthesis of 1,2,3-trimethyl benzimidazolium iodide (1b)
In 250ml round bottomed flask, Na (0.04mol 0.9g) was dissolved
in 16ml ethanol, after this 2ꢀmethyl benzimidazole (0.04mol
5.2g)、100ml benzene and MeI (0.12mol 7.5ml) were added
successively, the mixtures were refluxed for 18h, then allowed to
cool to room temperature, and the solvent was removed under
reduced pressure. The crude product was purified by
recrystallization from 95% ethanol to afford 1,2,3ꢀtrimethyl
benzimidazolium iodide (1b) as colorless needle crystals (9.2g,
Experimental Section
9
0
Calculation detailed
3
4
4
5
0
5
Geometrical optimization, including transitionꢀstructure searches,
was carried out with the standard 6ꢀ311G(d,p) basis set by using
the threeꢀparameter hybrid functional developed by Becke in the
1
7
formulation implemented in the Gaussian 09 program (B3LYP),
9
5
1
8
which is slightly different from the original proposed by Becke.
23
yield 80%). M.p 258~259℃ ; 1H NMR (400 MHz, DMSOꢀd6
δ): 7.97 (dd, J = 5.97, 2.93 Hz, 2H), 7.62 (dd, J = 6.01, 2.83 Hz,
The transition states are ascertained by vibrational analysis with
only one imaginary frequency mode. In the case of TS, the
vibration associated with the imaginary frequency was checked to
correspond with a movement in the direction of the reaction
2
H), 3.99 (d, J = 14.04 Hz, 6H), 2.85 (s, 3H); 13C NMR (400
MHz, DMSOꢀd6 δ) ： 152.7 (1C), 131.8 (2C), 126.2 (2C),
00 113.1(2C), 32.2 (2C), 11.1(1C)
1
.
1
9
coordinate. The values of the relative energies, ꢁE, have been
calculated on the basis of the total energies of the stationary
points. The solvent effects have been considered using a
relatively simple selfꢀconsistent reaction field (SCRF) method,
General Procedure for the Synthesis of 1,2-Squaraines
All of the experiments described above were performed
according to the following general procedure. A mixture of
05 2mmol of squaric acid in 30ml anhydrous ethanol was refluxed at
2
0
based on the polarizable continuum model (PCM). The solvent
used in this calculation is ethanol.
1
1
00 °C for 3h under a nitrogen atmosphere, then the solvent was
5
0
General Remarks
removed under reduced pressure and an other 30ml steamed
anhydrous ethanol was added, the mixture was refluxed for
All starting chemicals and solvents were purchased from
commercial suppliers and used as received unless explicitly
stated. Absorption spectrometry was performed using Gold S54T
spectrophotometer of Lengguang Company. Melting points were
3
0min and the solvent was removed again, repeating this for at
1
10 least three times in order to get diethyl squarate generated
completely. The base (10mmol Organic base: EtONa、DBU、
This journal is © The Royal Society of Chemistry [year]
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Page 6 of 8
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DOI: 10.1039/C3RA43608J
Et N or pyridine; 5mmol Inorganic base: NaHCO 、KHCO 、
Notes and references
3
3
3
Na CO 、K CO 、Cs CO 、NaOH or KOH)、4mmol of the
a
b
2
3
2
3
2
3
Institute for Advanced Study, Nanchang University, Department of
benzimidazole salt and 15ml anhydrous ethanol was added, the
mixture was refluxed for 3h, then allowed to cool to room
temperature. The crude reaction mixture is inspected by TLC
cromatography. The products were determined by comparison
with the analitical samples of the various squaraines obtained by
Chemistry, Nanchang University, Nanchang, China, 330031. Fax: 0086-
791-3969552 Tel: 0086-791-3969552 E-mail:
Hongmingwang@ncu.edu.cn
6
6
0
5
5
†
Electronic Supplementary Information (ESI) available: Cartesian
coordinatesand calculated energies for all structures used in the quantum
chemical calculations. See DOI: 10.1039/b000000x/
1. (a) J. D. Park, S. Cohen, J. R. Lacher, J. Am. Chem. Soc. 2002, 84,
2919ꢀ2922; (b) J. J. McEwen, K. Wallace, J.Chem. Commun. 2009,
the procedures described in details in the following paragraph
.
6
339ꢀ6351.
1
0
Synthesis of 3,4-bis((1,3-dibenzyl-1H-benzo[d] imidazol-
(3H)-ylidene)methyl) cyclobut-3-ene-1,2-dione (3a)
2
. (a) F. Silvestri, M. D. Irwin, L. Beverina, A. Facchetti, G. A. Pagani, T.
J. Marks, J. Am. Chem. Soc. 2008, 130, 17640ꢀ17641; (b) G. D. Wei,
R. R. Lunt, K. Sun, S. Y. Wang, M. E. Thompson, S. R. Forrest,
Nano Lett. 2010, 10, 3555ꢀ3559; (c) U. Mayerhöfer, F. Wüthner,
Chem. Sci. 2012, 3, 1215ꢀ1220.
2
A mixture of squaric acid (2mmol，228mg) in 30ml anhydrous
ethanol was refluxed at 100 °C for 3 h under a nitrogen
atmosphere, then the solvent was removed under reduced
pressure and an other 30ml steamed anhydrous ethanol was
added, the mixture was refluxed for 30min and the solvent was
removed again, repeating this for at least three times in order to
made diethyl squarate generated as much as possible. In the
meantime Na (230mg, 10mmol) was dissolved in 10ml
anhydrous ethanol, then the solution of sodium ethoxide、1a（
70
75
80
3
4
. (a) A. Ajayaghosh, P. Chithra, R. Varghese, Angew. Chem. Int. Ed.
1
5
2
007, 46, 230 –233;( b) U. Mayerhöfer, F. Wüthner, Angew. Chem.
Int. Ed. 2012, 51, 5615 –5619; (c) S. Sreejith,; P. Carol, P. Chithra,
A. Ajayaghosh, J. Mater. Chem. 2008, 18, 264ꢀ274.
.(a) X. Zhang, J. Jie, W. Zhang, C. Zhang, L.Luo, Z. He, X. Zhang, W.
Zhang, C. Lee, S. Lee, Adv. Mater. 2008, 20, 2427–2432; (b) K. Y.
Law, Chem. Rev. 1993, 93, 449ꢀ486; (c) H. Choi, J.ꢀJ. Kim, K. Song,
J. Ko, M. K. Nazeeruddin, M. Grätzel, J. Mater. Chem. 2010, 20,
3280–3286.
. (a) A. Dualeh, J. H. Delcamp, M. K. Nazeeruddin, M. Grätzel, App.
Phys. Lett. 2012, 100, 173512; (b) M. Emmelius, G. Pawlowski, H.
Vollmann, Angew. Chem. Int. Ed. 1989, 28, 1445ꢀ1471; (c) R.
Petermann, M. Tian, S. Tatsuura, M. Furuki, Dyes and Pigments.
2003, 57, 43ꢀ54.
. E. C. P. Smits, S. Setayesh, T. D. Anthopoulos, M. Buechel, W.
Nijssen, R. Coehoorn, P. W. M. Blom, B. de. Boer, D. M.de. Leeuw,
Adv. Mater, 2007, 19, 734ꢀ738.
7 (a) A. Piechowski, G. Bird, D. Morel, E. Stogryn, J. Phys. Chem. 1984,
88, 934ꢀ950; (b) K. Liang, K.ꢀY. Law, D. G. Whitten, J. Phys.
Chem.1995, 99, 16704ꢀ16708; (c) J.ꢀH. Yum, P. Walter, S. Huber,
D. Rentsch, T. Geiger, F. Nuesch, F. D. Angelis, M. Grätzel, M. K.
Nazeeruddin, J. Am.Chem. Soc. 2007, 129, 10320ꢀ10321; (d) Y. Shi,
R. B. M. Hill, J.ꢀH. Yum, A. Dualeh, S. Barlow, M. Grätzel, S. R.
Marder, M. K. Nazeeruddin, Angew. Chem. Int. Ed. 2011, 50, 6619ꢀ
2
2
3
0
5
0
4
mmol, 1.39g）and 15ml anhydrous ethanol was added, the
5
6
mixtures turned to deep red at once and then was refluxed for 3h.
the solvent was removed under reduced pressure and 15ml
isopropanol was added to separate garnet solid out, the solution
was filtered and the residue was washed by isopropanol、ether、
petroleum ether alternately to give 3a as a garnet solid (1.25g,
yield 89%). M.p 217~218℃; 1H NMR (400 MHz, DMSOꢀd6δ):
8
9
9
5
0
5
7
7
.35 (dd, J = 4.88, 2.57 Hz, 4H), 7.17 (t, J = 6.53 Hz, 12H), 7.11ꢀ
.07 (m, 4H), 7.01 (d, J = 6.91 Hz, 8H), 5.42 (s, 8H), 4.70 (s,
2H); 13C NMR (400 MHz, DMSOꢀd6δ): 189.4 (2C), 170.9 (2C),
51.1 (2C), 136.6 (4C), 133.3 (4C), 129.1 (8C), 128.0 (8C), 127.2
4C), 123.3 (4C), 110.3 (4C), 65.9 (2C), 47.8 (4C); Anal. Calcd
for 3a: C, 81.95; H, 5.41; N, 7.96. Found: C, 81.73; H, 5.87; N,
.67
1
(
6
621; (e) U. Mayerhöfer, K. Deing, K. Gruβ, H. Braunschweig, K.
Meerholz, F. Wüthner, Angew. Chem. Int. Ed. 2009, 48, 8776ꢀ8779;
f) G. Wei, X. Xiao, S. Wang, J. D. Zimmerman, K. Sun, V. V. Diev,
7
.
3
5
(
Synthesis of 3,4-bis ((1,3-dimethyl-1H-benzo[d] imidaz ol-
(3H)-ylidene)methyl) cyclobut-3- ene - 1,2-dione (3b).
A mixture of squaric acid (2mmol，228mg) in anhydrous ethanol
was reacted as the same as the steps described above. In the
meantime Na (230mg, 10mmol) was dissolved in 10ml
M. E. Thompson, S. R. Forrest, Nano Lett. 2011, 11, 4261ꢀ4264; (g)
X. Xiao, G. Wei, S. Wang, J. D. Zimmerman, C. K. Renshaw, M. E.
Thompson, S. R. Forrest, Adv. Mater. 2012, 24, 1956ꢀ1960.
2
100
8
. (a) E. Arunkumar, C. C. Forbes, B. C. Noll, B. D. Smith, J. Am. Chem.
Soc. 2005, 127, 3288ꢀ3289; (b) U. Mayerhöfer, B. Fimmel, F.
Wüthner, Angew. Chem. Int. Ed. 2012, 51, 164ꢀ167; (c) J. M.
Baumes, J. J.Gassensmith, J. Giblin, J.ꢀJ. Lee, A. G. White, W. J.
Culligan, W. M. Leevy, M. Kuno, B. D. Smith, Nature Chem, 2010,
4
0
anhydrous ethanol. The solution of sodium ethoxide 、 1b （ 105
mmol, 1.15g）and 15ml anhydrous ethanol was added, the
4
2
, 1025ꢀ1030.
. (a) S. Cohen, J. R. Lacher, J. D. Park, J. Am. Chem. Soc. 1959, 81,
480ꢀ3481; (b) J. D. Park, S. Cohen, J. R. Lacher, J. Am. Chem. Soc.
1962, 84, 2919ꢀ2922; (c) S. Cohen, S. G. Cohen, J. Am. Chem. Soc.
966, 88, 1533ꢀ1536.
0. (a) M. B. Onaran, A. B. Comeau, C. T. Seto, J. Org. Chem. 2005, 70,
0792ꢀ10803; (b) Y. Xu, N. Yamamoto, D. I.Ruiz, D. S. Kubitz, K.
mixture turned from yellow to blood red immediately and was
refluxed for 3h, then allowed to cool to room temperature, the
solution was filtered and the residue was washed by ethanol、 110
ether、petroleum ether in turn to give a pure bright red solid
product 3b (684mg, yield 86%). M.P>300℃; 1H NMR (400
MHz, DMSOꢀd6δ): 7.39ꢀ7.34 (m,4H) , 7.22ꢀ7.18 (m, 4H),4.75ꢀ
9
1
3
4
5
1
1
D. Janda, Bioorg. Med. Chem. Lett. 2005, 15, 19, 4304ꢀ4307; (c) A.
Tevyashova, F. Sztaricskai, G. Batta, P. Herczegh, A. Jeney, Bioorg.
Med. Chem. Lett. 2004, 14, 18, 4783ꢀ4789; (d) J. R.Porter, S. C.
Archibald, K. Childs, D. Critchley, J. C. Head, J. M.Linsley, T. A.
Parton, M. K. Robinson, A. Shock, R. J. Taylor, G. J. Warrellow, R.
P. Alexander, B. Langham, Bioorg. Med. Chem. Lett. 2002, 12, 7,
1051ꢀ1054.
4
.72 (m, 2H), 3.67 (s, 12H); Anal. Calcd for 3b: C, 72.36; H, 115
5
0
5.53; N, 14.07. Found: C, 72.25; H, 5.64; N, 14.03.
Acknowledgments
1
20
1
1
1. J. Xie, A. B. Comeau, C. T. Seto, Org. Lett. 2004, 6, 83ꢀ86.
2. (a) A. T. Blomquist, E. A.L. Lancette, J. Am. Chem. Soc. 1961, 83,
The support of the National Natural Science Foundation
of China (10947171 and 21103082) is gratefully
acknowledged.
1
387–1391; (b) S. Cohen, S. G. Cohen, J. Am. Chem. Soc. 1966, 88,
5
5
1533–1536.
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DOI: 10.1039/C3RA43608J
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(d) A.
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K. Jacob, Liebigs Ann. Chem. 1966, 699, 153ꢀ167.
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1
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1
2
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4
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0
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3
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.
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View Article Online
DOI: 10.1039/C3RA43608J
R
R
O
O
N
N
N
N
R
R
3
2
3
3
2
3
3
R
R
R
O
O R
N
O
O
O
O
N
N
strong base: EtoNa
、
N
+
EtOH
+
DBU KOH NaOH
、
、
N
N
HO OH
EtO OEt
R
R
×
R
O
O
R
N
N
N
N
R
R