An efficient synthesis of 4,4′,5,5′-tetraiododibenzo-24-crown-8 and its highly conjugated derivatives

Article abstract of DOI:10.1016/j.tetlet.2005.10.147

4,4′,5,5′-Tetraiododibenzo-24-crown-8 (9), a practical building block, was prepared under efficient and mild reaction conditions starting from the simple starting material, catechol (1). Highly conjugated 4,4′,5,5′-tetraethynyldibenzo-24-crown-8 (10a,b) w

Full text of DOI:10.1016/j.tetlet.2005.10.147

Tetrahedron
Letters
Tetrahedron Letters 47 (2006) 233–237
An efficient synthesis of 4,40,5,50-tetraiododibenzo-24-crown-8
and its highly conjugated derivatives
Joshua J. Pak,* Jaime L. Mayo and Endrit Shurdha
Department of Chemistry, Idaho State University, Campus Box 8023, Pocatello, ID 83209-8023, USA
Received 26 August 2005; revised 26 October 2005; accepted 27 October 2005
Available online 16 November 2005
Abstract—4,40,5,50-Tetraiododibenzo-24-crown-8 (9), a practical building block, was prepared under efficient and mild reaction con-
ditions starting from the simple starting material, catechol (1). Highly conjugated 4,40,5,50-tetraethynyldibenzo-24-crown-8 (10a,b)
were prepared via a Sonogashira coupling reaction from tetraiodocrown ether 9. These highly conjugated crown ethers form com-
plexes in CD2Cl2 with dibenzylammonium hexafluorophosphate in a 1:1 ratio. Emission spectrum of pseudorotaxane 11 shows a
dramatic shift from the non-complexed precursor.
Ó 2005 Elsevier Ltd. All rights reserved.
1. Introduction
arylhalides and alkynes remain challenging. For exam-
ple, Sonogashira coupling reactions between 4,5-dibromo-
veratrole and terminal alkynes often require high
temperatures and extended reaction periods. In this letter,
we are focusing on the preparation of highly conjugated
crown ethers containing ortho-bisethynylbenzene moie-
ties. Herein, we present an efficient preparation of
4,40,5,50-tetraiododibenzo-24-crown-8 ether (9) starting
from simple catechol followed by Pd catalyzed alkyne
coupling reactions to afford highly conjugated 24-
crown-8 ethers containing ortho-bisethynylbenzene
subunits.
There is a continuing interest in the preparation of func-
tionalized dibenzo-24-crown-8 ether (DB24C8) deriva-
tives.1 Since Pedersenꢀs discovery2 of crown ethers over
three decades ago, interactions between crown ethers
and guest entities, such as alkylammonium (RNH3
+
)
ions, have generated much interest.3 Specifically,
DB24C8 derivatives bind with various cationic species
such as alkyl ammonium ions. These self-assembly
systems possess interesting properties. For example,
DB24C8s have been used in the preparation of mole-
cular receptors,4 polymers,5 (pseudo)rotaxanes,6 caten-
anes,7 switches,8 and surface-bound materials.9
2. Results and discussion
Recent synthetic advances in transition metal catalyzed
C–C coupling reactions, such as the Sonogashira, Suzu-
ki, and Stille coupling reactions, have provided efficient
routes for the preparation of highly conjugated phenyl-
acetylene containing compounds.10 In particular, ortho-
bisethynylbenzene moieties have been studied as integral
parts of light-harvesting systems,11 subunits of dehy-
drobenzoannulenes,12 highly conjugated polymers,13
and organic high-spin magnetism.14 Despite recent
advances, such coupling reactions between electron-rich
In order to construct 4,40,5,50-tetraiododibenzo-24-
crown-8 (9), we first prepared 4,5-diiodocatechol (4). A
conventional strategy employs the diiodination of verat-
role (5) with mercuric acetate and iodine followed by
demethylation using BBr3 to afford catechol 4 in high
yield (Scheme 1).15 Although this is a very efficient meth-
od, BBr3 is highly corrosive and often limits incorpora-
tion of a wide variety of functional groups. As an
alternative, we endeavored to prepare 4 directly from
catechol (1) using a variety of iodonium sources,
without much success. However, when catechol was
protected as a ketal, compound 2 underwent an iodin-
ation reaction analogous to that for veratrole (5) to give
diiodoketal 3 in good yield. Upon the preparation of
diiodoketal 3, we were able to obtain catechol 4 in
Keywords: Crown ether; Pseudorotaxane; Self-assembly; Alkyne;
Conjugation.
*
Corresponding author. Tel.: +1 208 282 2612; fax: +1 208 282
4373; e-mail: pakjosh@isu.edu
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.10.147

234
J. J. Pak et al. / Tetrahedron Letters 47 (2006) 233–237
OH
OH
I
P2O5
Hg(OAc)2
O
O
O
O
1
0.8
0.6
0.4
0.2
0
Acetone
37%
I2, CH2Cl2
86%
I
9
3
1
2
AcOH
cat. HCl
reflux
10a
10b
94%
OMe
OMe
I
I
OMe
OMe
BBr3
I
I
OH
OH
Hg(OAc)2
CHCl3, -78 oC
84%
I2, CH2Cl2
91%
6
5
4
Scheme 1. Preparation of 4,5-diiodocatechol.
86% yield employing milder deprotection conditions.
Deprotection was most effective providing 94% yield
when a solution of compound 3 in acetic acid in the
presence of catalytic amount of HCl was gently heated
under reflux.
200
250
300
nm
350
400
Figure 1. Absorption spectra of DB24C8 9 and 10a,b.
Upon the preparation of catechol 4, tetraiodo crown
ether derivative 9 was prepared following the standard
crown ether procedure as shown in Scheme 2.16
Catechol 4 was first reacted with triethylene glycol
monotosylate17 in basic solution to provide bisglycol 7
in 61% yield. Ditosylation of bisglycol 7 provided
ditosylate 8 in 80% yield. Macrocycle 9 was then
prepared using pseudo-dilution conditions in the pres-
ence of Cs2CO3 in high yield. Further conversion into
the tetraethynyl derivatives by Sonogashira coupling
reaction gave highly conjugated DB24C8 species 10a,b
(Table 1). The Pd coupling reaction yields are quite
good (75–81%) considering that we are performing four
simultaneous coupling reactions, which equates to
above 95% conversion per coupling reaction.
tion spectra of compounds 9 and 10a,b exhibit the char-
acteristic pattern associated with the ortho-disubstituted
catechols. As expected, addition of alkynes to the
DB24C8 core shifted k max 1 and k max 2 by approxi-
mately 40 and 50 nm, respectively, from their precursor
9 as depicted in Table 1. The general trend is also
observed in compound 10b as 4-tert-buylphenylacetyl-
enes are coupled shifting peaks an additional 32 and
27 nm, respectively.
The interaction between macrocycle 10b and the dibenz-
ylammonium hexafluorophosphate salt in CD2Cl2 at
room temperature results in a 1:1 complex as expected
(Scheme 3). The 1H NMR spectra suggest the formation
of pseudorotaxane 11 host–guest complex as indicated
by clear upfield shifts shown in Table 2. The most
noticeable differences are in the ethylene moieties of
the crown ether, where the 3.86 ppm resonance shifts
to 3.58 ppm. Furthermore, the singlet representing the
aromatic resonance of DB24C8 shifts 0.1 ppm as the
complex forms. Based on the previous studies of similar
systems, the observed chemical shifts and the absence of
resonances from 10b is consistent with the formation of
pseudorotaxane.6a
The DB24C8 10a,b derivatives were characterized by 1H
NMR and UV–vis spectroscopy. The extended p-conju-
gation of these macrocycles were verified by the
expected absorptions as shown in Figure 1. The absorp-
Table 1. Yields of Pd catalyzed coupling reactions and selected peaks
(k max 1 and 2 in nm) from the absorption spectra of 9 and 10a,b
Compounds
Pd coupling yields
k max 1
k max 2
9
—
81%
75%
219
252
284
244
292
319
Surprising evidence of the complexation between crown
ether 10b and the ammonium ion is also provided by the
comparison of fluorescence spectra of 10b and 11 as
10a (R = TMS)
10b (R = 4-tBu-Ph)
O
O
O
O
O
O
O
O
O
O
OTs
OH
I
O
O
OH
OH
I
I
O
O
OTs
OTs
I
I
OH
OH
TsCl, Et3N
acetone, K2Co3
65%
CH2Cl2, DMAP
80%
I
7
4
8
R
R
R
R
R
O
O
O
O
O
O
O
I
I
O
O
I
O
O
O
PdCl2(PPh3)2
Cs2CO3
CuI, Et3N
CH3CN
CH3CN
85%
O
O
I
O
O
9
10
R = TMS (10a), 4-tBu-Ph (10b)
Scheme 2. Preparation of DB24C8 10a,b from 4,5-diiodocatechol (4).

J. J. Pak et al. / Tetrahedron Letters 47 (2006) 233–237
235
t-Bu
t-Bu
NH2
t-Bu
t-Bu
PF6
O
O
O
O
O
O
O
O
N
H
PF6
O
O
O
O
O
O 2
O
O
t-Bu
t-Bu
10b
11
t-Bu
t-Bu
Scheme 3. Formation of pseudorotaxane 11.
Table 2. Selected 1H NMR resonances (ppm) of DB24C8 10b and
complex 11
3. Experimental
3.1. 5,6-Diiodo-2,2-dimethylbenzo[1,3]dixole (3)
10b
11
D (10bꢀ11)
4.20
3.95
3.86
7.23
4.14
3.85
3.58
6.93
0.06
0.10
0.28
0.10
In a round bottom flask, ketal 2 (2.0 g, 13.3 mmol), I2
(7.44 g, 29.3 mmol), and Hg(OAc)2 (9.34 g, 29.3 mmol)
were dissolved in CH2Cl2 (250 ml). The reaction mixture
was stirred at room temperature and monitored by
TLC. Upon completion, the reaction mixture was fil-
tered and washed with an aqueous solution of sodium
thiosulfate and extracted with CH2Cl2. The CH2Cl2
solution was dried with magnesium sulfate and concen-
trated to give diiodoketal 3 as a light orange solid in
200
180
160
140
120
100
80
10b
1
11
86% yield (4.59 g). Mp 143–145 °C. H NMR (CDCl3,
300 MHz) d 7.22 (s, 2H), 1.66 (s, 6H,). 13C NMR
(CDCl3, 75 MHz) d 26.1, 95.7, 118.8, 120.2, 149.0. MS
(m/z); 276 (MꢀI, 100), 261 (MꢀIꢀCH3, 100), 134
(Mꢀ2IꢀCH3, 24), IR (cmꢀ1) 2980, 1480, 1228. Anal.
Calcd for C9H8I2O2 (401.861): C, 26.89; H, 2.01. Found:
C, 26.95; H, 1.92.
10b UV-Vis
60
3.2. 4,5-Diiodocatechol (4)
40
In a round bottom flask equipped with a reflux con-
denser, diiodoketal 3 (250 mg, 0.622 mmol) was dis-
solved in AcOH (15 ml), H2O (5 ml), and 6 M HCl
(2 ml). The reaction mixture was heated to gentle reflux
for 2 h. The reaction mixture was concentrated and
redissolved in ethylacetate. The solution was filtered
through a plug of silica gel using ethylacetate. Upon
20
0
200
300
400
500
600
700
800
900
nm
Figure 2. Fluorescence spectra of compound 10b and complex 11.
concentration, diiodocatechol (4) as
a solid was
shown in Figure 2. When excited with a 325 nm laser,
the macrocycles 10b and 11 provided distinctively differ-
ent fluorescence emissions. The fluorescence spectrum of
complex 11 shows bathochromic shift of approximately
50 nm from compound 10b as the result of the complex-
ation. Currently, we are examining various cations,
which might provide even larger changes in fluorescence
emission for the purpose of optical switching.
obtained in 94% yield (211 mg). Spectroscopic data are
consistent with the literature values.17 1H NMR (CDCl3,
300 MHz) d 7.35 (s, 2H) 1.76 (s, 2H). 13C NMR (CDCl3,
75 MHz) d 96.2, 125.8, 144.5. MS (m/z) 361 (M+, 0.96),
236 (MꢀI, 30.2), 109 (Mꢀ2I, 14.9).
3.3. 4,5-Diiodo-1,2-bis[2-[2-(2-hydroxyethoxy)ethoxy]-
ethoxy]benzene (7)
In conclusion, we have developed an efficient synthetic
method to prepare 4,40,5,50-tetraiododibenzo-24-crown-
8 (9) and its highly conjugated derivatives. Further stud-
ies exploring the optical behavior of these compounds
and their complexes will be reported in due course.
In a round bottom flask, diiodocatechol 4 (2.00 g,
5.52 mmol), triethylene glycol monotosylate18 (4.13 g,
12.1 mmol), and K2CO3 (1.68 g, 12.1 mmol) were added
and dissolved in dry acetone (100 ml) and refluxed
overnight. The reaction mixture was concentrated and

236
J. J. Pak et al. / Tetrahedron Letters 47 (2006) 233–237
filtered through a plug of silica gel with a MeOH/EtOAc
(1:4) mixture. Column chromatography with MeOH/
EtOAc (1:4) mixture yielded a yellowish viscous oil in
3.6. 4,40,5,50-Tetra(trimethylsilylethynyl)dibenzo-24-
crown-8 ether (10a)
1
61% yield (2.10 g). H NMR (CDCl3, 300 MHz) d 7.31
In a pressure microwave vessel, compound 9 (100 mg,
0.11 mmol), CuI (40 mg, 0.21 mmol), and PdCl2(PPh3)2
(15 mg, 0.021 mmol) were added in degassed Et3N/
MeCN (1:9, 10 ml) under N2. Trimethylsilylacetylene
(0.15 ml, 1.05 mmol) was added via syringe under N2
and capped. The reaction mixture was irradiated by
microwave 3 h at 85 °C. The crude mixture was concen-
trated and chromatographed on silica gel using an
EtOAc/MeCN/hexane (3:3:4). A pale yellow solid was
isolated in 81% yield (132 mg) as pure compound. Mp
150–152 °C. 1H NMR (CDCl3, 300 MHz) d 6.89 (s,
4H), 4.11 (t, J = 2.4 Hz, 8H), 3.90 (t, J = 2.4, 8H),
3.81 (s, 8H), 0.26 (s, 36H). 13C NMR (CDCl3,
75 MHz) d 0.0, 69.2, 69.5, 71.3, 96.7, 103.2, 115.9,
119.0, 148.5. MS (m/z) 855.3 (M+Na+, 100), 759.2
(MꢀTMS, 60). IR (cmꢀ1) 2958, 1247, 833, 737. Anal.
Calcd for C44H64O8Si4ÆC4H8O2: C, 62.57; H, 7.88.
Found: C, 62.89; H, 7.99.
(s, 2H), 4.12 (t, J = 5.7, 4H), 3.86 (t, J = 5.7 Hz, 4H),
3.74–3.83 (m, 12H), 3.61 (t, J = 3.6 Hz, 4H), 2.96 (s,
2H). 13C NMR (CDCl3, 75 MHz) d 61.7, 69.0, 69.5,
70.3, 70.8, 72.6, 96.8, 124.5, 149.3. MS (m/z) 626 (M+,
58.3), 500 (MꢀI, 17.8), 493 (MꢀC6H13O3, 22.6), 388
(MꢀC10H22O6, 100), 266 (Mꢀ2IꢀC4H10O3, 43.7). IR
(cmꢀ1) 3422, 2873, 2360, 1490, 1245, 1122. Anal. Calcd
for C18H28I2O8 (626.987): C, 34.52; H, 4.51. Found: C,
34.39; H, 4.22.
3.4. 4,5-Diiodo-1,2-bis[2-[2-[2-[[(4-tosyl)sulfonyl]oxy]-
ethoxy]ethoxy]ethoxy]benzene (8)
In a round bottom flask, diiodo catechol bisglycol 7
(9.78 g, 15.6 mmol), DMAP (20 mg) and Et3N
(8.0 ml) were dissolved in CH2Cl2 (100 ml) and the
resulting solution was cooled to 0 °C. Via a dropping
funnel, p-toluenesulfonyl chloride (6.53 g, 34.2 mmol)
in CH2Cl2 (50 ml) was added over 2 h. After the addi-
tion, the reaction mixture was allowed to stir overnight
at room temperature. The reaction mixture was con-
centrated and chromatographed over silica gel using
MeOH/EtOAc (1:9). Compound 8 was isolated as a
yellow viscous oil in 80% yield (11.6 g). 1H NMR
(CDCl3, 300 MHz) d 7.806 (dd, J = 8.1, 13.7 Hz, 8H),
7.31 (s, 2H), 4.15 (t, J = 4.8 Hz, 4H), 4.09 (t,
J = 4.0 Hz, 4H), 3.80 (t, J = 4.0 Hz, 4H), 3.72–3.52
(m, 12H), 2.44 (s, 6H). 13C NMR (CDCl3, 75 MHz)
d 21.6, 68.7, 69.1, 69.2, 69.6, 70.8, 96.7, 124.6, 127.9,
129.8, 132.9, 144.8, 149.5. MS (m/z) 934 (M+, 0.02),
648 (MꢀC14H16, 1.00), 564 (MꢀC8H9O3S, 1.36), 388
(MꢀC10H22O6, 100). IR (cmꢀ1) 2949, 1356, 1174,
910, 727. Anal. Calcd for C32H40I2O12S2ÆC4H8O2: C,
42.28; H, 4.73. Found C32H40I2O12S2ÆC4H8O2: C,
42.09; H, 4.59.
3.7. 4,40,5,50-Tetra(4-tert-butylphenylethynyl)dibenzo-24-
crown-8 ether (10b)
In a pressure microwave vessel, compound 9 (50.0 mg,
0.053 mmol), CuI (3.0 mg, 0.016 mmol), and PdCl2-
(PPh3)2 (4.0 mg, 0.0032 mmol) were added in degassed
Et3N/MeCN (1:9, 10 ml) under N2. 4-tert-Butylphenyl-
acetylene (41 mg, 0.26 mmol) was added via syringe
under N2 and capped. The reaction mixture was irradi-
ated by microwave 1.5 h at 90 °C. The crude mixture
was concentrated and chromatographed on silica gel
using EtOAc/MeCN/hexane (3:3:4). A pale yellow
solid was isolated in 75% yield (42 mg). Dec. 196 °C.
1H NMR (CDCl3, 300 MHz) d 7.51 (d, J = 8.4 Hz,
8H), 7.37 (d, J = 8.7 Hz, 8H), 7.03 (s, 4H) 4.19 (t,
J = 3.9 Hz, 8H), 3.94 (t, J = 3.9 Hz, 8H), 3.86 (s, 8H),
1.34 (s, 36H). 13C NMR (CDCl3, 75 MHz) d 31.4,
35.0, 69.6, 69.9, 71.6, 88.0, 92.6, 116.1, 119.4, 120.7,
125.5, 131.5, 148.8, 151.5. HRMS Calcd for C72H80O8Æ
1095.5751. Found 1095.5752. IR (cmꢀ1) 2958, 1511,
1248, 665.
3.5. 4,40,5,50-Tetraiododibenzo-24-crown-8 ether (9)
In a round bottom flask equipped with a condenser,
Cs2CO3 (14.1 g, 43.2 mmol) was mixed with MeCN
(334 ml) and refluxed under N2. Compound 8 (8.1 g,
4. Pseudorotaxane 11
8.64 mmol)
and
diiodocatechol
(4)
(3.124 g,
8.635 mmol) were dissolved in 50 ml of MeCN and
added drop wise via syringe pump over 24 h. The reac-
tion mixture was heated under reflux for additional
two days. The reaction mixture was concentrated and
run through a plug of silica gel using an EtOAc/
MeCN/hexane (3:3:4) mixture. White solid was isolated
without further purification in 85% (7.0 g). Mp 183–
184 °C. 1H NMR (CDCl3, 300 MHz) d 7.25 (s, 4H),
4.08 (t, J = 3.6 Hz, 8H), 3.88 (d, J = 3.9 Hz, 8H), 3.78
(s, 8H). 13C NMR (CDCl3, 75 MHz) d 69.8, 71.5, 96.8,
124.1, 149.6. MS (m/z) 952.0 (M+, 2.31), 826.1 (MꢀI,
3.32), 698.1 (Mꢀ2IꢀH, 4.20), 572.1 (Mꢀ3IꢀH, 23.8).
IR (cmꢀ1) 3054, 2986, 2305, 1494, 1232, 896, 705. Anal.
Calcd for C24H28I4O8ÆCH3CN (992.822): C, 31.44; H,
3.15. Found: C, 31.45; H, 3.18.
Compound 10b (5.0 mg, 0.0047 mmol) was dissolved
in CD2Cl2 with dibenzylammonium hexafluorophos-
phate salt (3.0 mg, 0.0087 mmol) and sonicated for
30 min. The reaction mixture was filtered through
cotton plug to eliminate any undissolved starting
materials. The resulting solution was used without fur-
1
ther purification to obtain proton NMR spectrum. H
NMR (CD2Cl2, 300 MHz) d 7.53–7.28 (m, 18H), 6.93
(s, 4H), 4.70 (t, J = 5.4 Hz, 4H), 4.21 (s, 4H), 4.18–4.
10 (m, 8H), 3.85–3.82 (m, 8H), 3.58 (s, 8H), 2.47 (br s,
2H) 1.35 (s, 36H). 13C NMR (CD2Cl2, 75 MHz) d
31.4, 35.3, 52.1, 68.9, 70.7, 71.3, 87.7, 93.4, 115.7,
120.0, 120.5, 126.2, 129.4, 129.6, 130.0, 130.2, 131.7,
147.8, 152.6.

J. J. Pak et al. / Tetrahedron Letters 47 (2006) 233–237
237
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Acknowledgements
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Authors thank Idaho State University and the National
Science Foundation (ESP-0132626) for generous finan-
cial support of this research. E.S. and J.L.M. thank
ISU Undergraduate Research Council for the under-
graduate travel grants.
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Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.tetlet.2005.10.147.
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