Rapid and High-Yield Synthesis of [23]Crown Ether: Applied as a Wheel Component in the Formation of Pseudo[2]rotaxane and Synthesis of [2]Catenane with a Dibenzylammonium Dumbbell

Article abstract of DOI:10.1021/acs.joc.1c00674

A facile, rapid, and high yield synthesis of [23]crown ether (X23C7) has been developed from commercially available starting materials, in one step with good to excellent yield. The reaction is completed in 6 h under room temperature conditions, with the highest yield being 81%. The X23C7 macrocycle formed pseudo[2]rotaxane with a dibenzylammonium ion (DBA+) dumbbell, exhibiting strong association (Ka = 2.61 × 103 M-1). Consequently, a [2]catenane was synthesized from a DBA+-based diolefin terminated salt and X23C7 in 81% yield, using a threading-followed-ring-closing-metathesis approach.

Full text of DOI:10.1021/acs.joc.1c00674

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Note
Rapid and High-Yield Synthesis of [23]Crown Ether: Applied as a
Wheel Component in the Formation of Pseudo[2]rotaxane and
Synthesis of [2]Catenane with a Dibenzylammonium Dumbbell
Manisha Prakashni, Rasendra Shukla, and Suvankar Dasgupta*
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ABSTRACT: A facile, rapid, and high yield synthesis of [23]crown ether (X23C7) has been developed from commercially available
starting materials, in one step with good to excellent yield. The reaction is completed in 6 h under room temperature conditions,
with the highest yield being 81%. The X23C7 macrocycle formed pseudo[2]rotaxane with a dibenzylammonium ion (DBA+)
dumbbell, exhibiting strong association (Ka = 2.61 × 103 M−1). Consequently, a [2]catenane was synthesized from a DBA+-based
diolefin terminated salt and X23C7 in 81% yield, using a threading-followed-ring-closing-metathesis approach.
echanically interlocked molecules (MIMs)1 are of
considerable interest in the scientific community due
community. The [23]crown ethers utilized were 23C7H2 and
23C6H2, which neither possessed benzo-/naphtha- units nor
had 8 oxygen atoms. This provides the impetus to further
explore the utility of the supramolecular recognition motif,
DBA+-[23]crown ether analogues, in diverse fields. The
synthesis of 23C7H2 and 23C6H2 typically requires three to
four steps and involves the usage of a very expensive Grubbs
catalyst, which might limit the practical application of these
[23]crown ethers. Therefore, it is imperative to develop such a
derivative of [23]crown ether which is inexpensive, easily
synthesized, scalable and forms pseudo[2]rotaxane with the
DBA+ salt.
Herein, we report the synthesis of a [23]crown ether, o-
xylene-capped 23-crown-7-ether (X23C7), in single step from
commercially available starting materials, i.e., hexaethylene
glycol (HEG) and o-xylylene dibromide, in good to excellent
yield. As anticipated, the crown ether X23C7 formed
pseudo[2]rotaxane complex with the DBA·PF6 salt and
exhibited a high association constant value (Ka = 2.6 × 103
M−1). Subsequently, this affinity has been maneuvered for
constructing the [2]catenane molecule 1-H·PF6. The method-
ology involved threading-followed-by-ring-closing-metathesis,
M
to their broad range of applications as molecular switch based
molecular scale electronics,2 molecular actuators,3 molecular
elevators,4 smart surfaces,5 and controlled drug release.6 MIMs
have also been incorporated in metal−organic frameworks
(MOFs).7 Catenane8 and rotaxane9 happen to be the simplest
and fundamental class of MIMs, which are synthetically
obtained from the noncovalent template-directed synthesis.
Among the hydrogen bond templates,10 DBA+ (dibenzylam-
monium)-[24]crown ether (especially dibenzo-24-crown-8,
DB24C8) is one of the most widely used and prominent
complementary recognition pairs. DBA+-DB24C8 has been
extensively utilized to construct novel architectures,11
responsive polymers,12 responsive nanofibers,13 fluorescent
sensors,14 light-harvesting antennas,15 molecular elevators,4
molecular information ratchets,16 nanovalves,17 and other
stimuli-responsive molecular switches.18 The DBA+-DB24C8
recognition pair involves H-bonding ([N+−H···O], [C−H···
O]), and electrostatic ([N+···O]) interactions. The recognition
ability of DB24C8 is generally attributed to its [24]crown ether
constitution, which is believed to have the optimal ring size for
stable 1:1 pseudorotaxane formation with DBA+.19 Although
smaller crown ethers formed strong pseudorotaxane complexes
with dialkylammonium salts,20 for DBA+ face-to-face bonding
was observed21 and larger crown ethers formed weaker
complexes with DBA+.22
Received: March 21, 2021
Published: May 21, 2021
In 2012, however, Wu and Dasgupta reported23 that DBA+
could also form 1:1 pseudo[2]rotaxane complexes with
[23]crown ethers, much to the surprise of the scientific
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catalyzed by a Grubbs second generation catalyst. To the best
of our knowledge, no prior application of supramolecular
recognition motif, DBA+-[23]crown ether, has been reported
to date.
The optimized reaction conditions of X23C7 has been
obtained by varying the solvent, bases, catalyst, and their
equivalents (Table 1). Throughout, one equivalent of both
is much higher than the earlier reported isolated yield of
X23C7 (50%), prepared under heating conditions.25
The ability of X23C7 to form pseudo[2]rotaxane with DBA·
1
PF6 was investigated by H NMR spectroscopy and lower
resolution ESI-MS. Pseudo[2]rotaxane DBA⊂X23C7·PF6 was
obtained from the 1 mM equimolar mixture of DBA·PF6 and
X23C7 in CDCl3. The presence of complexed and
uncomplexed species corresponding to both X23C7 and
DBA·PF6 (Figure 1) suggests the pseudo[2]rotaxane formation
a
Table 1. Optimizing Conditions for X23C7
b
entry
solvent
base (equiv)
catalyst (equiv)
yield (%)
1
2
3
4
5
6
7
8
THF
DMF
ACN
ACN
ACN
ACN
ACN
ACN
ACN
THF
DMF
DMF
DMF
NaH (3)
NaH (3)
NaH (3)
NaH (3)
NaH (3)
NaH (3)
NaH (3)
NaH (3)
NaH (3)
NaH (3)
NaH (3)
KOH (2)
K2CO3 (10)
NIL
NIL
NIL
33
37
54
64
70
73
70
79
81
56
60
KI (0.1)
KI (0.2)
KI (0.3)
KPF6 (0.1)
KPF6 (0.2)
KPF6 (0.25)
KPF6 (0.2)
KPF6 (0.2)
NIL
9
10
11
12
13
c
−
c
1
NIL
−
Figure 1. H NMR (CDCl3, 400 MHz) spectra of (i) DBA·PF6, (ii)
a
b
c
100 mg scale of HEG has been used throughout. Isolated yield. No
1:1 equimolar mixture of DBA·PF6 and X23C7, and (iii) X23C7.
Dotted lines are drawn to indicate the presence of complexed and
uncomplexed species, highlighting the formation of pseudo[2]-
rotaxane DBA⊂X23C7·PF6.
product formation could be detected at room temperature, and
heating at 100 °C did not help.
HEG and o-xylylene dibromide has been used at room
temperature. X23C7 has been designed such that the starting
materials can undergo facile double O-alkylation at the reactive
benzyl sites. The yields in entries 1 and 2 are very much
comparable with that reported for DB24C8.24 However, as per
entry 3, acetonitrile (ACN) appears to be the best solvent
choice for X23C7 synthesis. Thus, for increasing the percent
yield, a template-directed approach has been investigated
primarily in the acetonitrile solvent system. To exploit the K+
template effect, KI (entries 4−6) and KPF6 (entries 7−9) have
been employed in the catalytic ratio. As can be seen from
entries 4−9, the percent yield substantially increases, with the
highest being 81% (entry 9). It is evident that KPF6 is more
effective than KI for the same equivalent. It is interesting to
note that KI, which is one of the routinely available chemicals
in laboratories, can also catalyze the reaction in pretty good
yields (up to 73%, entry 6). Increasing the percentage
equivalent of the catalyst from 10% to 20% results in higher
enhancements of the percent yield of isolated X23C7. Thus,
reactions with a 20% equivalent of KPF6 were attempted in dry
tetrahydrofuran (THF, entry 10) and dry dimethylformamide
(DMF, entry 11), which resulted in substantial increase of
yield to 56% and 60% respectively. Finally both KOH (entry
12) and K2CO3 (entry 13), expected to play the dual role of
base and template, were tried and found to be ineffective.
Ultimately, entry 9 was found to be the optimized reaction
condition, and the same has been employed for synthesizing
X23C7 on the gram scale. The yield however dropped to 78%
which can still be considered an excellent yield, as this yield has
been realized from commercially available chemicals. This yield
is slow on the NMR time scale. Moreover, a strong peak
corresponding to the 1:1 formation of the pseudo[2]rotaxane
could be detected in the nominal ESI-MS spectrum (Figure S1
in the Supporting Information). The peaks corresponding to
the −(OCH2CH2)− protons of X23C7 showed a broad peak
around 3.6 ppm, which on complexation with DBA·PF6
separated into six peaks of four protons each (Figure 1).
Due to the complexation, both deshielding and shielding was
observed for −(OCH2CH2)− protons, with the chemical shifts
ranging from 3.1 to 3.8 ppm. X23C7 is a symmetrical crown
ether, and post complexation the appearance of
−(OCH2CH2)− protons into six equal sets of four protons
indicates that the symmetry is still retained in the pseudo[2]-
rotaxane DBA⊂X23C7·PF6. The association constant value
was calculated using a single point NMR method and found to
be 2.61 × 103 M−1 for the 1:1 pseudo[2]rotaxane complex,
DBA⊂X23C7·PF6 (Figure S2). A comparison of association
constant values of varied cavity sized crown ethers with DBA+
in nonpolar media is available in the Supporting Information
(Table S1).
Obviously, the next logical step would be to harness the
utility of the DBA+-X23C7 supramolecular synthon in MIMs.
To achieve that, a diolefin-terminated linear thread comprising
the DBA+ moiety (7-H·PF6) has been prepared (Scheme 1).
As depicted in Scheme 1, p-hydroxy benzaldehyde was
alkylated with 11-bromo-1-undecene to give 2 in 97% yield.
Boc protection of p-hydroxy benzylamine gave 3 in 67% yield.
Compound 3 on alkylation with 11-bromo-1-undecene
produced 4 in 51% yield, which on Boc deprotection resulted
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Scheme 1. Synthesis of Diolefin Terminated Thread 7-H·PF6
in quantitative formation of the salt 5-H·Cl. Compound 2 was
condensed with 1 equiv of the salt 5-H·Cl in the presence of
triethylamine, which was subsequently reduced with sodium
borohydride to give 6 in 75% yield. On protonation with
HPF6, compound 7-H·PF6 was obtained in quantitative yield.
Subsequently, the salt 7-H·PF6 was stirred with 5 equiv of
X23C7 in dichloromethane to ensure complete conversion into
the pseudo[2]rotaxane 7-H⊂X23C7·PF6 (Scheme 2). This
Scheme 2. Synthesis of [2]Catenane 1-H·PF6
1
Figure 2. H NMR (CDCl3, 400 MHz) spectra of (i) 7-H·PF6, (ii)
[2]catenane 1-H·PF6, and (iii) X23C7. Dotted lines are drawn to
indicate the upfield and downfield shifts, typically observed in MIMs.
appearance of broad substituted double bond peak “e” (Figure
2) are another indication of the formation of a mixture of E
and Z isomers of the interlocked species 1-H·PF6.10d All of
these observations clearly indicate that 1-H·PF6 has a
[2]catenane topology, where X23C7 is encircling the DBA+
moiety. The full 1H NMR spectrum is available in the
Supporting Information.
Additional evidence has been obtained from the 2D NOESY
experiment, where strong NOE cross-peaks highlight the
spatial interactions between X23C7 protons and DBA+ protons
in 1-H·PF6 (Figure S3). The −(OCH2CH2) protons of X23C7
show a strong correlation with the benzylic protons “a” and
proximal benzene protons “b” of the DBA+ moiety of the
[2]catenane 1-H·PF6. Likewise, the NOE cross-peaks show a
strong correlation of the benzylic protons of “A” of X23C7
with the benzylic protons “a” and proximal benzene protons
“b” of the DBA+ moiety of the [2]catenane 1-H·PF6.
threaded species has been subjected to ring-closing-metathesis,
using a second generation Grubbs catalyst under refluxing
conditions. The desired [2]catenane 1-H·PF6 was isolated as a
mixture of E and Z isomers in excellent yield of 81%.
Moreover, the excess 3 equiv of X23C7 has been isolated as
well in pure form by column chromatography. This means that
effectively 2 equiv of X23C7 was required for the synthesis of
[2]catenane 1-H·PF6. The formation of [2]catenane, 1-H·PF6
can be confirmed from the stacked NMR spectra (Figure 2).
The observation of six equal sets of −(OCH2CH2)−
protons of [2]catenane 1-H·PF6 (Figure 2ii), completely
matches with that observed for the same set of protons of
DBA⊂X23C7·PF6 (Figure 1ii). The range of chemical shifts of
the −(OCH2CH2)− protons is in the same region as well, i.e.,
3.1−3.8 ppm. The downfield shift observed for benzylic
protons “a” of the DBA+ moiety (Figure 2ii) is also similar to
that observed for the same set of protons of the pseudo[2]-
rotaxane species, DBA⊂X23C7·PF6. Such a downfield shift and
splitting of benzylic protons of DBA+ moiety is typically
observed in interpenetrated geometries involving the crown
ethers. The upfield shifts of benzylic protons “A” (of X23C7)
in the [2]catenane 1-H·PF6 (Figure 2ii) again resembles the
complexed peak of the same set of protons of the
pseudo[2]rotaxane species, DBA⊂X23C7·PF6 (Figure 1ii).
The disappearance of terminal double bond peaks “c, d” and
Furthermore, HR ESI-MS of 1-H·PF6 showed a peak at m/z
= 890.6139, corresponding to the species [M−PF6]+ (Figure
3). The observed m/z value is extremely close to the calculated
mass of the species, which is 890.6141 (error 0.2 ppm). This
observation indicates the purity of the isolated [2]catenane, 1-
H·PF6. Next, we investigated the integrity of the 1-H·PF6 in
1
the polar solvent by recording its H NMR in DMSO-d6
1
solvent. The room temperature H NMR indicates complete
retention of the topology as well as confirmation (of X23C7
encircling the DBA+ moiety) of the [2]catenane. This
observation further corroborates the formation of an
1
interlocked geometry. Recording the H NMR in DMSO-d6
at increasing temperatures up to 353 K surprisingly did not
alter the spectrum at all (Figure S4). While cooling back to
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(25 mL) was heated to 80 °C in an oil bath under nitrogen
atmosphere for 12 h. The mixture was filtered to remove undissolved
excess K2CO3. The solvent was removed in vacuum to leave a residue
which was extracted with CHCl3. The organic layer was washed with
water and dried over anhydrous Na2SO4, and the solvent was then
removed under vacuum. The crude residue was purified by column
chromatography (silica gel, EA/hexane = 1:9) to give compound 2 as
1
a yellow oil (2.19 g, 97%). H NMR (400 MHz, CDCl3): δ ppm =
9.87 (s, 1H, CHO), 7.83 (d, J = 8.6 Hz, 2H, Ar), 6.99 (d, J = 8.6 Hz,
2H, Ar), 5.86−5.75 (m, 1H, C(H)=CH2), 5.01−4.91 (m, 2H, CH =
C(H2)), 4.03 (t, J = 6.52 Hz, 2H, CH2), 2.06−2.01 (m, 2H, CH2),
1.84−1.77 (m, 2H, CH2), 1.47−1.29 (br, 12H, CH2). 13C{1H} NMR
(100 MHz, CDCl3): δ ppm = 190.7, 164.2, 139.1, 131.9, 129.7, 114.7,
114.1, 68.4, 33.7, 29.47, 29.40, 29.3, 29.09, 29.05, 28.9, 25.9. HR MS
(ESI): m/z Calcd for C18H27O2 (M + H)+: 275.2006, found
275.2010.
Synthesis of tert-Butyl 4-Hydroxybenzylcarbamate (3). 4-
Hydroxybenzylamine (0.5 g, 4.06 mmol) was dissolved in AR grade
CHCl3 (25 mL) under nitrogen atmosphere. To this, triethylamine
(0.62 g, 6.09 mmol) was added and stirred for 5 min. This was
followed by addition of Boc2O (1.33 g, 6.09 mmol), and the reaction
mixture was stirred for 4 h. The solvent was removed under vacuum,
leaving the crude residue which was purified by column chromatog-
raphy (silica gel, EA/hexane = 2:8) to give compound 3 as a colorless
oil (0.61 g, 67%). 1H NMR (400 MHz, CDCl3): δ ppm = 7.14 (d, J =
7.9 Hz, 2H, Ar), 6.78 (d, J = 8.2 Hz, 2H, Ar), 5.23 (s, 1H, −OH),
4.78 (s, 1H, -NH), 4.23 (br, 2H, CH2), 1.45 (s, 9H, CH3). 13C{1H}
NMR (125 MHz): δ ppm = 156.0, 155.1, 130.7, 128.9, 115.4, 79.6,
44.1, 28.4. HR MS (ESI): m/z Calcd for C12H17NO3Na (M + H)+:
224.1281, found 224.1274.
Figure 3. HR ESI-MS spectrum of the [2]catenane 1-H·PF6. The
peak at m/z = 890.6139 corresponds to [M−PF6]+. The isotopic
distribution also confirms the species is a monopositive cation.
room temperature, the spectra remained the same. Thus, the
confirmation of X23C7 encircling the DBA+ moiety in 1-H·PF6
is the thermodynamically most stable conformation, even at
higher temperatures in a polar solvent system.
In conclusion, we have developed a facile, rapid, and high
yielding (81%) synthesis of crown ether X23C7 in a single step
from commercially available starting materials under room
temperature condition. The template used for the synthesis
involves readily available potassium-based salts such as KI and
KPF6. Gram scale synthesis has been achieved as well in 78%
yield, which indicates the efficacy of the methodology
developed. X23C7 has been found to form a pseudo[2]-
rotaxane complex with DBA·PF6, exhibiting a high association
constant value (Ka = 2.6 × 103 M−1). Following the footsteps
of DBA+-DB24C8, the supramolecular synthon DBA+-X23C7
has been employed for the synthesis of the [2]catenane 1-H·
PF6 by ring-closing-metathesis (RCM). The extremely good
yield (81%) of the isolated [2]catenane, 1-H·PF6, is very
encouraging for the onset of X23C7 in the field of MIMs.
Looking at the success of the supramolecular synthon DBA+-
DB24C8, the future of X23C7 seems bright. Since it is
established that the crown ether X23C7 can pass over the
benzene moieties, other axles like BPE2+,26 N-benzylanili-
nium,10f and benzimidazolium27 may well be exploited with
X23C7 for pseudo[2]rotaxane complex formation. X23C7 is
not only a cheaper alternative to DB24C8 but also a valuable
addition to the crown ether family for supramolecular and
MIM-based switch fabrication. The properties of X23C7, and
chemistry of other smaller crown ethers in supramolecular and
MIM systems, are currently under investigation in our
laboratory.
Synthesis of tert-Butyl 4-(Undec-10-en-1-yloxy)benzylcarbamate
(4). A mixture of compound 3 (0.6 g, 2.69 mmol), 11-bromo-1-
undecene (0.752 g, 3.23 mmol), and K2CO3 (1.12 g, 8.07 mmol) in
DMF (20 mL) was heated to 80 °C in an oil bath under nitrogen
atmosphere for 24 h. The mixture was filtered to remove undissolved
excess K2CO3. The solvent was removed in vacuum to leave a residue
which was extracted with CHCl3. The organic layer was washed with
water and dried over anhydrous Na2SO4, and the solvent was then
removed under vacuum. The crude residue was purified by column
chromatography (silica gel, EA/hexane = 1:9) to give compound 4 as
1
a yellow oil (0.513 g, 51%). H NMR (400 MHz, CDCl3): δ ppm =
7.19 (d, J = 8.3 Hz, 2H, Ar), 6.85 (d, J = 8.5 Hz, 2H, Ar), 5.86−5.75
(m, 1H, C(H)=CH2), 5.01−4.91 (m, 2H, CH = C(H2)), 4.76 (br,
1H, -NH), 4.23 (br, 2H, CH2), 3.92 (t, J = 6.5 Hz, 2H, CH2), 2.06−
2.01 (m, 2H, CH2), 1.79−1.72 (m, 2H, CH2), 1.45 (s, 9H, CH3),
1.37−1.29 (br, 12H, CH2). 13C{1H} NMR (100 MHz): δ ppm =
158.4, 155.8, 139.2, 130.8, 128.8, 114.5, 114.1, 79.3, 68.0, 44.2, 33.8,
29.4, 29.3, 29.1, 28.9, 28.4, 26.0. HR MS (ESI): m/z Calcd for
C23H37NO3Na (M + Na)+: 398.2666, found 398.2656.
Synthesis of (4-(Undec-10-en-1-yloxy)phenyl)methanaminium
chloride (5-H·Cl). Compound 4 (0.513 g, 1.37 mmol) was dissolved
in MeOH (50 mL, 0.03 M) under nitrogen. A volume of 30 mL of ∼2
M HCl was added under nitrogen and stirred for 1 h. The excess
reagent and solvent were removed under vacuum to obtain compound
EXPERIMENTAL SECTION
■
General Information. All reagents and starting materials were
bought from commercial suppliers and used without further
purification. Anhydrous dichloromethane (DCM) was obtained
from dry distillation of its analytical grade by refluxing over CaH2.
Anhydrous tetrahydrofuran (THF) was obtained by distilling its
analytical grade by refluxing over sodium-benzophenone. Anhydrous
acetonitrile and DMF were purchased from CDH. Freshly distilled
dry solvents were always used. Column chromatography was
performed on silica gel (100−200 mesh). Deuterated solvents
(Sigma-Aldrich) for NMR spectroscopic analyses were used as
received. All NMR spectra were recorded on a Bruker 400 MHz FT-
NMR spectrometer or Bruker Avance-III 500 MHz NMR
spectrometer. All chemical shifts are quoted in ppm with multiplicities
being denoted by s (singlet), d (doublet), t (triplet), q (quartet), m
(multiplet), and br (broad). Mass spectra were recorded in ESI mode
on a Maxis Impact instrument (Bruker).
1
5-H·Cl as a white amorphous solid in quantitative yield. H NMR
+
(500 MHz, DMSO): δ ppm = 8.19 (br, 3H, NH3 ), 7.36 (d, J = 8.4
Hz, 2H, Ar), 6.93 (d, J = 8.4 Hz, 2H, Ar), 5.80−5.72 (m, 1H, C(H)
=CH2), 4.98−4.90 (m, 2H, CH = C(H2)), 3.94−3.91 (br, 4H, CH2),
2.00−1.96 (m, 2H, CH2), 1.69−1.64 (m, 2H, CH2), 1.37−1.22 (br,
12H, CH2). 13C{1H} NMR (125 MHz): δ ppm = 159.2, 139.2, 130.9,
126.0, 115.7, 115.1, 114.9, 67.9, 61.2, 42.2, 33.5, 32.8, 29.3, 29.1, 28.8,
28.6, 25.8. HR MS (ESI): m/z Calcd for C18H30NO (M − Cl)+:
276.2322, found 276.2300.
Synthesis of 6. Compound 5-H·Cl (0.3 g, 0.96 mmol), compound
2 (0.26 g, 0.96 mmol), and triethylamine (0.097 g, 0.96 mmol) were
mixed in CH3CN (20 mL) under nitrogen. The suspension was
stirred for 45 min and then refluxed for another 4 h in an oil bath. The
reaction mixture was allowed to cool before removing the solvent
under vacuum. The white residue was dissolved in THF (20 mL) and
MeOH (20 mL) to which NaBH4 (0.24 g, 6.3 mmol) was added in
Synthesis of 4-(Undec-10-en-1-yloxy)benzaldehyde (2). A
mixture of 4-hydroxybenzaldehyde (1 g, 8.2 mmol), 11-bromo-1-
undecene (2.3 g, 9.8 mmol), and K2CO3 (3.4 g, 24.6 mmol) in DMF
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1
portions. After stirring for overnight, the solvent was removed under
vacuum, leaving a residue which was extracted by CHCl3. The organic
layer was washed with brine and dried over anhydrous Na2SO4, and
the solvent was then removed under vacuum leaving the crude residue
which was purified by column chromatography (silica gel, MeOH/
DCM = 1:9) to give compound 6 as light a yellow oil (0.384 g, 75%).
1H NMR (400 MHz, CDCl3): δ ppm = 7.16 (d, J = 8.4 Hz, 4H, Ar),
6.76 (d, J = 8.5 Hz, 4H, Ar), 5.76−5.66 (m, 2H, C(H)=CH2), 4.92−
4.82 (m, 4H, CH = C(H2)), 3.82 (t, J = 6.5 Hz, 4H, CH2), 3.62 (s,
4H, CH2), 1.97−1.92 (m, 4H, CH2), 1.69−1.62 (br, 4H, CH2), 1.34−
1.20 (br, 24H, CH2). 13C{1H} NMR (100 MHz, CDCl3): δ ppm =
158.4, 139.2, 130.3, 129.7, 114.4, 114.1, 68.0, 51.7, 33.8, 29.5, 29.4,
29.3, 29.1, 28.9, 26.0. HR MS (ESI): m/z Calcd for C36H56NO2 (M +
H)+: 534.4306, found 534.4302.
yellow color gum (0.123 g, 81%). H NMR (400 MHz, CDCl3): δ
ppm = 7.34 (br, 2H, Ar), 7.22 (d, J = 8.3 Hz, 4H, Ar), 7.16 (br, 2H,
Ar), 6.88 (d, J = 8.3 Hz, 4H, Ar), 5.26 (br, 2H, C(H)C(H)), 4.60
(s, 4H, CH2), 4.13 (br, 4H, CH2), 4.05 (br, 4H, CH2), 3.81 (br, 4H,
OCH2CH2), 3.73 (br, 4H, OCH2CH2), 3.60 (br, 4H, OCH2CH2),
3.40 (br, 4H, OCH2CH2), 3.25 (br, 4H, OCH2CH2), 3.14 (br, 4H,
OCH2CH2), 1.86 (br, 4H, CH2), 1.70 (br, 4H, CH2), 1.39 (br, 4H,
CH2), 1.24−1.16 (br, 20H, CH2). 13C{1H} NMR (100 MHz,
CDCl3): δ ppm = 159.6, 135.3, 131.5, 130.2, 129.8, 128.2, 123.7,
114.9, 114.8, 71.5, 71.0, 70.9, 70.7, 70.5, 67.5, 51.2, 32.4, 29.6, 29.4,
29.3, 29.2, 28.8, 28.7, 28.2, 25.5. HR MS (ESI): m/z Calcd for
C54H84NO9 (M − PF6)+: 890.6141, found 890.6139.
ASSOCIATED CONTENT
■
Synthesis of 7-H·PF6. Compound 6 (0.412 g, 0.77 mmol) was
dissolved in methanol (10 mL), and then 2 mL of HPF6 acid solution
(∼55 wt % in H2O) was added to precipitate out the salt 7-H·PF6.
The precipitate was filtered off, washed with water, and dried over
vacuum to give 7-H·PF6 as white amorphous solid in quantitative
yield. 1H NMR (500 MHz, CDCl3): δ ppm = 7.24 (br, 4H, Ar), 6.89
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.joc.1c00674.
Details on the characterization of all new compounds;
1H NMR, 13C NMR, 2D NOESY NMR, nominal ESI-
MS, and VT NMR spectra (PDF)
+
(d, J = 8.5 Hz, 4H, Ar), 6.59 (br, 2H, NH2 ), 5.85−5.76 (m, 2H,
C(H)=CH2), 5.00−4.91 (m, 4H, CH = C(H2)), 4.04 (s, 4H, CH2),
3.88 (t, J = 6.5 Hz, 4H, CH2), 2.05−2.01 (m, 4H, CH2), 1.76−1.68
(br, 4H, CH2), 1.40−1.29 (br, 24H, CH2). 13C{1H} NMR (125 MHz,
CDCl3): δ ppm = 160.5, 139.2, 131.3, 120.5, 115.4, 114.1, 68.1, 50.4,
33.8, 29.7, 29.5, 29.4, 29.3, 29.15, 29.13, 28.9, 26.0. HR MS (ESI): m/
z Calcd for C36H56NO2 (M-PF6)+: 534.4306, found 534.4306.
Optimized Procedure for the Synthesis of X23C7. NaH
(60%, 0.04 g, 1.064 mmol) was suspended in 5 mL of dry acetonitrile
(0.07 M) under nitrogen. To the suspended solution, hexaethylene
glycol (0.1 g, 0.354 mmol) was added in portions and stirred for 10
min. Next KPF6 (0.016 g, 0.0885 mmol) was added to the solution.
Subsequently, o-xylylenedibromide (0.093 g, 0.354 mmol) was added
to the reaction mixture and stirred for 6 h at room temperature. The
solvent was removed in vacuum to leave a residue which was extracted
with CHCl3. The organic layer was washed with water and dried over
anhydrous Na2SO4, and the solvent was then removed under vacuum.
The crude residue was purified by column chromatography (silica gel,
Acetone/Hexane = 1:3) to give X23C7 as a colorless oil (0.113 g,
81%). 1H NMR (400 MHz, CDCl3): δ ppm = 7.38 (br, 2H, Ar), 7.27
(br, 2H, Ar), 4.68 (s, 4H, CH2), 3.67 (br, 24H, OCH2CH2). 13C{1H}
NMR (100 MHz, CDCl3): δ ppm = 136.7, 129.0, 127.7, 71.1, 70.8,
70.74, 70.71, 70.6, 69.7. HR MS (ESI): m/z Calcd for C20H32O7Na
(M + Na)+: 407.2040, found 407.2043.
AUTHOR INFORMATION
■
Corresponding Author
Suvankar Dasgupta − Department of Chemistry, National
Institute of Technology Patna, Patna 800005, India;
orcid.org/0000-0003-2446-8268; Email: suvankar@
nitp.ac.in
Authors
Manisha Prakashni − Department of Chemistry, National
Institute of Technology Patna, Patna 800005, India
Rasendra Shukla − Department of Chemistry, National
Institute of Technology Patna, Patna 800005, India; Present
Address: R.S.: CSIR National Botanical Research
Institute, Lucknow −226001, UP, India.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.joc.1c00674
Notes
Optimized Procedure for the Synthesis of X23C7 on a Gram
Scale. NaH (0.44 g, 11 mmol) was suspended in 50 mL of dry
acetonitrile (0.07 M) under nitrogen. To the suspended solution,
hexaethylene glycol (1.02 g, 3.61 mmol) was added in portions and
stirred for 10 min. Next KPF6 (0.16 g, 0.90 mmol) was added to the
solution. Subsequently, o-xylylenedibromide (0.96 g, 3.63 mmol) was
added to the reaction mixture and stirred for 6 h at room temperature.
The solvent was removed in vacuum to leave a residue which was
extracted with CHCl3. The organic layer was washed with water and
dried over anhydrous Na2SO4, and the solvent was then removed
under vacuum. The crude residue was purified by column
chromatography (silica gel, acetone/hexane = 1:3) to give X23C7
as a colorless oil (1.08 g, 78%).
Synthesis of [2]Catenane, 1-H·PF6. Compound 7-H·PF6 (0.1 g,
0.148 mmol) and X23C7 (0.284 g, 0.74 mmol) were dissolved in 10
mL of DCM solvent and stirred for 24 h, and then the solvent was
removed under vacuum without heating. The residue (7-H⊂X23C7·
PF6) was dissolved in dry DCM (400 mL, 0.0003 M) under nitrogen
atmosphere. A second generation Grubbs catalyst (0.012 g, 0.0148
mmol) was added, and the resulting mixture was refluxed for 60 h in
an oil bath. The reaction mixture was cooled, followed by quenching
with ethyl vinyl ether. The excess solvent was removed in vacuum,
and the residue was subjected to column chromatography (silica gel:
acetone/hexane = 2:3 (v/v)) to isolate the excess (3 equiv) X23C7
(0.172 g), followed by another column chromatography (silica gel:
Acetone/CHCl3 = 5:95) to give the desired product 1-H·PF6 as a
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from DST-SERB (ECR/2016/001724) is
gratefully acknowledged. M.P. thanks NIT Patna for the
scholarship. R.S. thanks DST-SERB for the fellowship. We
thank NMR and Mass facility of IIT Patna, IISER Bhopal,
IISER Kolkata and IACS Kolkata for all analytical support.
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