Multi-gram syntheses of four crown ethers using K+ as templating agent

Article abstract of DOI:10.1016/j.tet.2015.11.055

Dibenzo-30-crown-10 (DB30C10, 1c), dibenzo-24-crown-8 (DB24C8, 1a), 4-carbomethoxydibenzo-24crown-8 (1b) and a new crown ether, 4-carbomethoxydibenzo-30-crown-10 (1d), were synthesized by a simple, high yielding, three-step method. Potassium ions from the highly soluble KPF₆ were employed to template the cyclization step, which resulted in very high isolated yields (80-90%).

Full text of DOI:10.1016/j.tet.2015.11.055

Tetrahedron xxx (2015) 1e4
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Multi-gram syntheses of four crown ethers using Kþ as templating
agent
*
Hanlie R. Wessels, Harry W. Gibson
Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA, 24061, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Dibenzo-30-crown-10 (DB30C10, 1c), dibenzo-24-crown-8 (DB24C8, 1a), 4-carbomethoxydibenzo-
24crown-8 (1b) and a new crown ether, 4-carbomethoxydibenzo-30-crown-10 (1d), were synthesized by
a simple, high yielding, three-step method. Potassium ions from the highly soluble KPF6 were employed
to template the cyclization step, which resulted in very high isolated yields (80e90%).
Ó 2015 Elsevier Ltd. All rights reserved.
Received 16 October 2015
Received in revised form 20 November 2015
Accepted 23 November 2015
Available online xxx
Keywords:
Crown ethers
Templation
Supramolecular chemistry
Improved synthesis
1. Introduction
reaction mixture contained at least 6% of the desired crown.13a To
our knowledge, the highest cyclization yield that has been obtained
DB30C10 and DB24C8 are useful components in the construc-
tion of mechanically interlocked molecules such as pseudorotax-
anes,1 rotaxanes,1d,2 catenanes,2c,d,3 molecular shuttles,2b,4 and
other molecular machines.2b,5 These crown ethers also have great
utility as components in potentiometric sensors,6 in selective
complexation and extraction of ions,7 in polymers,8 and other
stimuli-responsive materials.9 DB30C10 has anticoccidial activity.10
Functionalized DB24C8 has been used extensively in more com-
plicated supramolecular devices,1,4,5,11 supramolecular polymers,12
and self-healing supramolecular gels.9b
DB30C10 is commercially available at w$285/gram. Our re-
search requires multiple grams of the material, which makes pur-
chasing it cost prohibitive. A careful search of the literature
revealed that most authors to date used the method published by
Pederson in 1967 (Scheme 1).13 Pedersen did not isolate the
DB30C10 completely in his initial report, but claimed that his
using Pedersen’s method to date is 25%.13b
None of the reports in the literature takes advantage of the
fact that the final cyclization step in the syntheses of DB30C10
and DB24C8 can be templated with potassium ion, as was
previously demonstrated in the syntheses of the cis-dicarbo-
methoxy derivatives.14 Association constants for complexation
between DB24C8 and potassium ion in acetonitrile as measured
by conductivity and solubility average 7.6 (ꢀ1.9)ꢁ103 Mꢂ1 15
.
Similarly association constants for DB30C10 with Kþ average
4.3ꢁ104 Mꢂ1 15d,16
.
Given these association constants, we
thought it reasonable to suppose that potassium would be an
effective templation agent for formation of DB24C8 and
DB30C10.
K2CO3 is often employed as the base in such cyclizations;
however, its solubility in the organic solvents normally used is quite
low, minimizing the effect of templation. Furthermore, the
Scheme 1. DB30C10 synthesis as outlined by Pedersen.
* Corresponding author. E-mail address: hwgibson@vt.edu (H.W. Gibson).
http://dx.doi.org/10.1016/j.tet.2015.11.055
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Wessels, H. R.; Gibson, H. W., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.11.055

2
H.R. Wessels, H.W. Gibson / Tetrahedron xxx (2015) 1e4
synthesis that gave the highest yield for DB30C1013b uses a benzyl
protecting group to make the diphenolic intermediate shown in
Scheme 1. Since this crown ether is symmetrical, the use of a pro-
tecting group is wholly unnecessary. We designed the synthetic
route depicted in Scheme 2 to take advantage of templation by
potassium ion in the form of the highly organic-soluble hexa-
fluorophosphate and the symmetry of the desired DB30C10. Our
route closely resembles that reported by Bruns et al. for the syn-
thesis of 1,5-dinaptho-38-crown-10 (28 or 33%, syringe pump ad-
dition),17 but with the exception that they did not use potassium
ion templation in the form of KPF6.
using KPF6 as template, the Kþ was removed by simply washing
a DCM solution with water three times.14b This was not the case
with the parent DB30C10, as extensive water washing did not
remove the Kþ. Eventually the template was removed by washing
with pyridinium hydrochloride solution to displace the Kþ. The
solution turned yellow, indicating the formation of a charge
transfer complex between DB30C10 and pyridinium hydrochlo-
ride. The solution was then washed with 10% (w/v) NaOH, which
led to the deprotonation of the pyridinium salt and subsequent
decomplexation. The resulting solid was reasonably pure, but had
a slight brown color that was removed by treatment with de-
colorizing carbon, followed by recrystallization from MeOH to
yield DB30C10 as a colorless solid in 86% yield. (The discoloration
was only observed in one of the five batches prepared in this
fashion. The recrystallization step was therefore omitted in the
other batches).
The other crown ethers were prepared in a similar fashion. The
carbomethoxy crown ethers 1b and 1d were washed with NaHCO3,
instead of 10% NaOH, to prevent hydrolysis of the esters. Both esters
contained residual pyridine (1H NMR), which was removed by
drying under high vacuum or by passing the compound through
a cation exchange resin.
The synthesis is amenable to multi-gram preparation of product,
as up to 5 g can be produced in a single reaction without undue
difficulty, whereas multigram preparation is impractical for
untemplated cyclizations. Note that, though syringe pump addition
or high dilution is unnecessary, some attention should be paid to
concentration. In more concentrated solutions (67 mM) some lin-
ear oligomers were formed. This led to a decrease in the yield of 1c
from 86% to 65%.
Scheme 2. Improved syntheses of crown ethers.
Since the templation of the reaction between ditosylate and
catechol does not produce significant by-products (save for olig-
omers at high concentration), we tested the efficacy of the tem-
plation by attempting a one-pot preparation of the crown ethers.
Typically this method produces very poor yields (6e15%), for ex-
ample, the one pot preparation of bis(m-phenylene)-32-crown-10
and its derivatives.18 The one-pot procedure was attempted for
DB24C8. Tri(ethylene glycol) ditosylate, catechol, KPF6, and K2CO3
were combined and allowed to reflux in MeCN for several days.
DB24C8 was easily isolated from the mixture by column chro-
matography in 36% yield. Although a yield of 43% has been re-
ported in the one-step synthesis of DB24C8, that synthesis
required much harsher conditions such as boiling DMSO and
KOH.19
Knowing that the cis-dicarbomethoxy derivatives of both
DB30C1014b and DB24C814a can be templated with potassium, and
that, unlike the dicarbomethoxy crown derivatives, the unsub-
stituted and monocarbomethoxy derivatives cannot form regio-
isomers, we saw no reason that DB24C8 and the 4-carbomethoxy
derivatives of DB30C10 and DB24C8 could not be synthesized by
the same route.
Indeed, this synthetic route requires only a single chromato-
graphic step after the tosylation of 2a and 2b. The cyclization step
essentially yields a single product: the crown ethers 1aed, which
can easily be purified by washing or by ion exchange resin. In
contrast, most untemplated cyclizations require two chromatog-
raphy columns, or chromatography followed by recrystallization, to
yield crown ethers of satisfactory purity.
3. Conclusion
2. Results and discussion
DB30C10, DB24C8 and their corresponding 4-carbomethoxy
derivatives were prepared by a facile, three step synthesis
from relatively inexpensive starting materials using KPF6 to
template the cyclizations. The isolated yields of the final cycli-
zation steps ranged from 80 to 90%. It is believed that the high
yield and the lack of significant by-products is a result of the
ethyleneoxy arms of the ditosylates (3a or 3b) ‘wrapping
around’ the Kþ ion. With the tosylated ends of the ethylene
glycol arms positioned close together, the entropic barrier to
ring closure by reaction with the catecholate anion is reduced.
This arrangement favors ring closure over polymerization, the
typical competing process in macrocyclizations of this kind. The
method does not require special purification or syringe pump
addition. The syntheses require only a single chromatographic
purification of the ditosylates. Isolation of the crown ether is
accomplished by washing or by treatment with ion exchange
resin. This simple, economical, high yielding synthesis provides
access to multiple grams of these valuable supramolecular
building blocks.
Known diols 2a3a and 2b11c were prepared via Williamson
ether synthesis with the monotosylate of the appropriate glycol
and catechol, in MeCN with K2CO3 as base. Tosylate 3a3a was
formed by dropwise addition of tosyl chloride in DCM to a solution
of diol 2a in DCM at 0 ꢃC. Likewise tosylate 3b11a was prepared by
adding a solution of tosyl chloride in DCM to a cold solution of 2b,
trimethylamine and DMAP. DB30C10 was prepared by Williamson
ether synthesis of ditosylate 3b with catechol. K2CO3 was used as
the base, and 1.2 equiv KPF6 were added as a templating agent by
simple mixing without the use of a syringe pump. The concen-
trations of the two components were 44 mM. After four days at
reflux, the crude solution’s 1H NMR spectrum revealed no trace of
the starting material and indicated almost exclusively the complex
between DB30C10 and Kþ [as compared to the 1H NMR of a sample
of commercial DB30C10 mixed with KPF6 (Supplementary data,
Fig. S13)].
Removing the Kþ from DB30C10 proved unexpectedly chal-
lenging. When the diester derivative of DB30C10 was prepared
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H.R. Wessels, H.W. Gibson / Tetrahedron xxx (2015) 1e4
3
4. Experimental
4.1. General
595.2753 (found), 595.2749 (calculated for
0.6 ppm.
C30H42O12), error
4.4. 4-Carbomethoxydibenzo-24-crown-8 (1b)
Compounds 2a3a, 2b11c, 3a3a and 3b11a were prepared according
to literature procedures in yields of 88, 94, 62 and 71%, respectively.
The diols were sufficiently pure to proceed, but the ditosylates were
purified by column chromatography on silica (6:1 DCM:EtOAc). THF
was dried by distillation over Na/benzophenone. DCM was dried by
distillation over CaH, and pyridine was dried by distillation over
molecular sieves. All other chemicals were used as received.
Melting points were measured with a Mel-Temp II device in cap-
illary tubes and are uncorrected. 1H, 19F and 13C NMR spectra were
obtained at ambient temperature on JOEL Eclipse Plus 500 MHz,
Varian Unity Plus or Varian MR 400 MHz spectrometers. 1H NMR
and 13C spectra are corrected relative to residual solvent peaks.
High-resolution mass spectra were obtained with an Agilent 6220
LCMS ESITOF spectrometer. Ion exchange was performed with
Amberlite GC-120, strongly acid sulfonated cation exchange resin
Ditosylate 3a (2.40 g, 3.50 mmol), methyl 3,4-dihydroxybenzoate
(0.59 g, 3.5 mmol) and KPF6 (0.78 g, 4.2 mmol) were dissolved in
MeCN (110 mL). The solution was degassed by bubbling N2 (g)
through it. The flask was wrapped in aluminum foil to exclude light,
and K2CO3 (1.96 g, 14.0 mmol) was added. The mixture was heated
at reflux under N2 (g) for 5 days, allowed to cool to room temper-
ature and the solids were filtered. The solvent was removed by ro-
tary evaporation and the residue was dissolved in DCM. The solution
was washed with pyridinium hydrochloride (12ꢁ15 mL), water
(15 mL), NaHCO3 (satd) (4ꢁ15 mL), and water (1ꢁ15 mL), dried over
MgSO4, filtered, and the solvent was removed by rotary evaporation
to yield a white solid (1.61 g, 90%), mp 86.0e87.5 ꢃC. Lit. yield 40%
(12.5 g by pseudo-high dilution in 3.5 L of DMF using K2CO3 as base
and syringe pump addition of the two reactants), mp 83e85 ꢃC.20
The analogous ethyl ester was reportedly prepared in 56% yield
(2.6 g) by slow (12 h) addition of the ditosylate in 500 mL of MeCN to
a mixture of K2CO3 and the catechol derivative and in 1 L of
(RSOꢂ Naþ) 100e200 mesh. Column chromatography was per-
_
formed with silica gel, 40e63
m
m, 60 A from Sorbent Technologies,
or Flash Alumina-N from Agela Technologies.
MeCN.11a 1H NMR (400 MHz, CDCl3)
d
7.64 (dd, J¼8, 2 Hz, 1H), 7.51
4.2. Dibenzo-30-crown-10 (1c)
(d, J¼2 Hz, 1H), 6.90e6.86 (m, 4H), 6.84 (d, J¼8 Hz, 2H), 4.21e4.17
(m, 4H), 4.16e4.13 (m, 4H), 3.95e3.90 (m, 8H), 3.87 (s, 3H), 3.84 (s,
Ditosylate 3b (7.52 g, 9.75 mmol), catechol (7.07 g, 9.75 mmol)
and KPF6 (2.15 g, 11.7 mmol) were dissolved in MeCN (220 mL). N2
(g) was bubbled through the solution for 20 min. The flask was
wrapped in aluminum foil, and K2CO3 (8.08 g, 59 mmol) was added.
The mixture was allowed to reflux under N2 (g) for 4 days, allowed
to cool to room temperature and evaporated under reduced pres-
sure. The residue was partitioned between DCM and water. The
DCM layer was washed with water 1ꢁ, pyridinium hydrochloride
solution (1:1:1 pyridine: conc. HCl: water) (5ꢁ20 mL), 10% (w/v)
NaOH (20 mL), and water (1ꢁ 20 mL). The organic layer was dried
over Na2SO4, filtered, and evaporated in vacuo to yield a white solid,
(4.50 g, 86%), mp 102.0e103.6 ꢃC; lit. mp 106.0e107.5 ꢃC.13a 1H NMR
4H), 3.83 (s, 4H). 13C NMR (101 MHz, CDCl3)
d 166.8, 152.9, 148.95,
148.93, 148.3, 123.9, 122.9, 121.4, 114.3, 114.1, 112.0, 71.5, 71.4, 71.3,
69.95, 69.8, 69.6, 69.5, 69.4, 69.39, 69.32, 51.9. ESITOF: (MþH)þ m/z
507.2238 (found), 507.2225 (calculated for
2.6 ppm.
C26H34O10), error
4.5. Dibenzo-24-crown-8 (1a)
Ditosylate 3a (2.23 g, 3.2 mmol), catechol (0.36 g, 3.2 mmol) and
KPF6 (0.72 g, 3.9 mmol) were dissolved in MeCN (100 mL). Ar (g)
was bubbled through the solution for 10 min. The flask was
wrapped in aluminum foil to exclude light, and K2CO3 (1.78 g,
13 mmol) was added. The mixture was heated at reflux under Ar (g)
for 3 days, allowed to cool to room temperature and the solids were
filtered. The solvent was removed by rotary evaporation and the
purple residue was partitioned between DCM and water. The or-
ganic layer was washed with 1:1:1 pyridine: HCl: H2O (10ꢁ20 mL),
water (1ꢁ20 mL), 10% (w/v) NaOH (3ꢁ20 mL) and water (1ꢁ20 mL).
The purple solution turned bright red when it was washed with the
pyridinium$HCl solution and back to purple again with the NaOH
washings. The organic layer was dried over MgSO4 and activated
carbon was added. The solution was filtered through CeliteÒ, fil-
tered again and the solvent was removed. The residual pyridine was
removed by drying under high vacuum to yield a white solid (1.27 g,
87%), mp 100e102 ꢃC. Lit. yield 43% (NaOH/DMSO), mp
(400 MHz, CDCl3)
d
6.89e6.81 (m, 8H), 4.10 (t, J¼4 Hz, 8H), 3.82 (t,
J¼4 Hz, 8H), 3.75e3.69 (m, 8H), 3.66e3.64 (m, 8H). 13C NMR
(101 MHz, CDCl3) d 149.1, 121.6, 114.6, 70.9, 70.7, 69.8, 69.2. ESI-TOF:
(MþH)þ m/z 537.2691 (found), 537.2694 (calculated for C28H40O10),
error ꢂ0.5 ppm.
4.3. 4-Carbomethoxydibenzo-30-crown-10 (1d)
Ditosylate 3b (1.30 g, 1.70 mmol), methyl 3,4-dihydroxybenzoate
(0.28 g, 1.7 mmol) and KPF6 (0.38 g, 2.1 mmol) were dissolved in
MeCN (100 mL). The solution was degassed with N2 (g) and the flask
was wrapped in aluminum foil to exclude light. K2CO3 (0.96 g,
7.0 mmol) was added and the mixture was heated at reflux for 7
days, allowed to cool to room temperature and filtered to remove
the solids. The solvent was removed by rotary evaporation. The
residue was dissolved in DCM and washed with pyridinium hy-
drochloride (1:1:1 pyridine: conc. HCl: water) (12ꢁ15 mL), water
(15 mL), NaHCO3 (satd) (4ꢁ15 mL), and water (1ꢁ15 mL). The or-
ganic layer was dried over MgSO4, filtered, and the solvent was
removed by rotary evaporation. The white solid still had some
pyridinium peaks in the 1H NMR spectrum. The solid was dissolved
in acetone and passed through an ion exchange resin. The solvent
was removed to yield a white solid (0.80 g, 80%), mp 76e78 ꢃC. 1H
102e103 ꢃC.19,21 1H NMR (400 MHz CDCl3, Fig. S10)
d 6.88 (m, 8H),
4.15 (t, J¼4 Hz, 8H), 3.92 (t, J¼4 Hz, 8H), 3.83 (s, 8H). 13C NMR
(101 MHz, CDCl3)
d 149.1, 121.5, 114.2, 71.4, 70.1, 69.5 ESITOF:
(MþH)þ m/z 449.2171 (found), 449.2174 (calculated for C24H32O8),
error 0.3 ppm.
4.6. One step synthesis of dibenzo-24-crown-8
Catechol (1.01 g, 9.10 mmol), tri(ethylene glycol) ditosylate
(4.16 g, 9.10 mmol) and KPF6 (6.66 g, 36.0 mmol) were dissolved in
MeCN (120 mL). N2 (g) was bubbled through the solution for
20 min. The flask was wrapped in aluminum foil to exclude light,
and K2CO3 (5.03 g, 36 mmol) was added. The mixture was heated at
reflux under N2 (g). After one week, the solution was allowed to
cool to room temperature. The solvent was removed by rotary
evaporation and the solid was partitioned between DCM and water
NMR (400 MHz, CDCl3)
d
7.64 (dd, J¼8, 2 Hz, 1H), 7.52 (d, J¼2 Hz,
1H), 6.88 (d, J¼2 Hz, 4H), 6.85 (d, J¼8 Hz, 1H), 4.19 (t, J¼4 Hz, 4H),
4.14 (t, J¼4 Hz, 4H), 3.92e3.86 (m, 11H), 3.80e3.75 (m, 8H),
3.70e3.66 (m, 8H). 13C NMR (101 MHz, CDCl3)
d 166.8, 152.9, 149.1,
148.3, 123.9, 122.9, 121.5, 114.7, 114.6, 112.4, 71.02, 70.98, 70.91, 70.7,
69.8, 69.6, 69.5, 69.18, 69.16, 69.15, 68.9, 51.9. ESI-TOF: (MþH)þ m/z
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4
H.R. Wessels, H.W. Gibson / Tetrahedron xxx (2015) 1e4
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(15 mL). The DCM layer was washed with water (3ꢁ15 mL), dried
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CDCl3)
d 7.01e6.96 (m, 4H), 4.24e4.20 (m, 4H), 3.80e3.76 (m, 4H),
3.73 (s, 4H). 13C NMR (101 MHz, CDCl3)
70.0, 69.0.
d 150.1, 123.0, 118.1, 71.2,
Author contributions
The authors have given approval to the final version of the
manuscript.
Competing financial interests
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The authors declare no competing financial interests.
Acknowledgements
We thank Dr. M. Arunachalan and Dr. D. V. Schoonover for
valuable discussions during the development of this work. We also
thank the National Science Foundation (CHE-1106899 and CHE-
1507553) and the Virginia Tech Chemistry Department for funding
this research.
Supplementary data
Supplementary data related to this article can be found in the
online version, at http://dx.doi.org/10.1016/j.tet.2015.11.055.
References and notes
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Please cite this article in press as: Wessels, H. R.; Gibson, H. W., Tetrahedron (2015), http://dx.doi.org/10.1016/j.tet.2015.11.055