Rotaxane with multiple functional groups

Article abstract of DOI:10.1021/jo502235z

High-yield syntheses of Cu(II)- and Ni(II)-templated [2]pseudorotaxane precursors (CuPRT and NiPRT, respectively) were achieved by threading bis(azide)bis(amide)-2,2-bipyridine axle into a bis(amide)tris(amine) macrocycle. Single-crystal X-ray structural analysis of CuPRT revealed complete threading of the axle fragment into the wheel cavity, where strong aromatic π-π stacking interactions between two parallel arene moieties of the wheel and the pyridyl unit of axle are operative in addition to metal ion templation. Attachment of a newly developed bulky stopper molecule with a terminal alkyne to CuPRT via a Cu(I)-catalyzed azide-alkyne cycloaddition reaction failed as a result of dethreading of the azide-terminated axle under the reaction conditions. However, the synthesis of a metal-free [2]rotaxane containing triazole with other functionalities in the axle was achieved in ～45% yield upon coupling between azide-terminated NiPRT and the alkyne-terminated stopper. The [2]rotaxane was characterized by mass spectrometry, 1D and 2D NMR (COSY, DOSY, and ROESY) experiments. Comparative solution-state NMR studies of the [2]rotaxane in its unprotonated and protonated states were carried out to locate the position of the wheel on the axle of the metal-free [2]rotaxane. Furthermore, a variable-temperature ¹H NMR study in DMSO-d₆ of [2]rotaxane supported the kinetic inertness of the interlocked structure, where the newly developed stopper prevents dethreading of the 30-membered wheel from the axle.

Full text of DOI:10.1021/jo502235z

Article
pubs.acs.org/joc
[2]Rotaxane with Multiple Functional Groups
Subrata Saha, Saikat Santra, Bidyut Akhuli, and Pradyut Ghosh*
Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Kolkata 700
032, India
S
* Supporting Information
ABSTRACT: High-yield syntheses of Cu(II)- and Ni(II)-
templated [2]pseudorotaxane precursors (CuPRT and NiPRT,
respectively) were achieved by threading bis(azide)bis(amide)-
2,2′-bipyridine axle into a bis(amide)tris(amine) macrocycle.
Single-crystal X-ray structural analysis of CuPRT revealed
complete threading of the axle fragment into the wheel cavity,
where strong aromatic π−π stacking interactions between two
parallel arene moieties of the wheel and the pyridyl unit of axle
are operative in addition to metal ion templation. Attachment
of a newly developed bulky stopper molecule with a terminal
alkyne to CuPRT via a Cu(I)-catalyzed azide−alkyne cyclo-
addition reaction failed as a result of dethreading of the azide-terminated axle under the reaction conditions. However, the
synthesis of a metal-free [2]rotaxane containing triazole with other functionalities in the axle was achieved in ∼45% yield upon
coupling between azide-terminated NiPRT and the alkyne-terminated stopper. The [2]rotaxane was characterized by mass
spectrometry, 1D and 2D NMR (COSY, DOSY, and ROESY) experiments. Comparative solution-state NMR studies of the
[2]rotaxane in its unprotonated and protonated states were carried out to locate the position of the wheel on the axle of the
1
metal-free [2]rotaxane. Furthermore, a variable-temperature H NMR study in DMSO-d6 of [2]rotaxane supported the kinetic
inertness of the interlocked structure, where the newly developed stopper prevents dethreading of the 30-membered wheel from
the axle.
such as amide, tris(amine), bipyridyl, triazole, and ester, which
are potential sites for metal coordination, hydrogen bonding,
anion binding and π−π stacking interactions. Furthermore, a
new stopper unit has also been obtained that can resist
dethreading of the 30-membered macrocycle wheel even at
elevated temperature.
INTRODUCTION
■
In general, rotaxanes have attracted more attention than other
interlocked systems in studies of various non-covalent
interactions and their potential applications in molecular
devices and nanotechnology.1−15 To obtain desirable properties
in rotaxanes, various functional groups such as metal-chelating
bipyridine units, amine/imine coordinating sites, paraquat,
polyether, squaraine, amide, triazole/triazolium, and porphyrin
have been introduced in the wheel, axle, or stopper
component(s).16−30 Judicial incorporation of different func-
tional groups in such interlocked molecules has resulted in
various highly complicated and well-defined mechanically
interlocked systems, such as molecular shuttles,31−44 molecular
elevators,45 molecular information ratchets,46,47 and so on.
Thus, interlocked molecules with multiple functional groups are
in demand for the development of various new molecular
machineries. The integration of porphyrin and 1,10-phenan-
throline into the wheel component results in an adaptable
[3]rotaxane receptor,48 while an axle containing both amide
and triazolium functionalities in rotaxanes exhibits anion
recognition properties.25 Furthermore, cation-induced motion
of a molecular shuttle has been observed in a multistation
rotaxane with an amide/amine axle and a polyether/amine-
containing wheel.49 Rotaxanes with amide functionalities in the
wheel/axle components are promising for hydrogen-bonding
and anion coordination properties.50,51 Herein we report a new
[2]rotaxane whose backbone contains various functionalities
RESULTS AND DISCUSSION
■
Strategy and Design Aspects. Different types of template
strategies have been developed for effective syntheses of
rotaxanes.10,25,52−63 Transition metal ion-templated threading is
one of the popular strategies for interlocked molecular
systems.28,64−67 In this direction, copper(I)-templated threaded
molecules have been mostly explored by Sauvage and co-
workers.68−70 Our recent studies have shown that Cu(II) can
act as an excellent template for high-yielding syntheses of
threaded heteroleptic complexes (pseudorotaxanes) with “3 +
2” donor sets of a bis(amide)tris(amine) wheel and bidentate
chelating axle.71−73 In the present study, our obvious choice of
wheel was a bis(amide)tris(amine) macrocycle (BATAMC).
On the other hand, a 2,2′-bipyridine unit was chosen as the
primary functionality in the axle design, which can be
functionalized with bis(amide) and bis(azide) groups to
introduce multiple functionalities in the axle component
Received: September 29, 2014
Published: October 29, 2014
© 2014 American Chemical Society
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(BAzBABPy) (Scheme 1). Thus, “3 + 2” threading between
BATAMC and BAzBABPy templated by Cu(II) or Ni(II)
5,5′-dimethyl-2,2′-bipyridine (Scheme 1A). First, 5,5′-dimethyl-
2,2′-bipyridine was oxidized with potassium dichromate in
sulfuric acid to give 2,2′-bipyridyl-5,5′-dicarboxylic acid
(DABPy), which was reacted with thionyl chloride to
synthesize the corresponding diacid chloride. Finally, the
reaction of 2,2′-bipyridyl-5,5′-dicarbonyl chloride with 2-
aminoethyl azide afforded the axle precursor BAzBABPy via
amide bond formation. This bis(azide)-terminated linear-type
molecule BAzBABPy can act as an axle fragment toward the
synthesis of M(II) (M = Cu, Ni)-templated [2]-
pseudorotaxanes by “3 + 2” heteroleptic threaded complex
formation (Scheme 1B). These two [2]pseudorotaxanes
(CuPRT and NiPRT) were synthesized upon reaction of
BATAMC (1 equiv) with BAzBABPy (1 equiv) in the presence
of Cu(ClO4)2 or Ni(ClO4)2 (1 equiv), respectively, in
CH3OH/CH2Cl2 solution at room temperature. The ESI-MS
spectra of CuPRT and NiPRT showed prominent peaks at m/z
1072.97 and 1068.08 corresponding to [CuPRT−ClO4]+ and
[NiPRT−ClO4]+, respectively. Thus, ESI-MS of both CuPRT
and NiPRT clearly indicated 1:1:1 complexation of the wheel,
axle, and M(II) template ion in the [2]pseudorotaxanes.
Furthermore, the formation of the heteroleptic threaded
complex between BATAMC and BAzBABPy was confirmed
by a single-crystal X-ray diffraction study of CuPRT (Figure 1).
In the complex, the Cu(II) center is pentacoordinated, with
three coordination sites attached to the NH sites of BATAMC
and the other two coordinated to the nitrogen atoms of the
bidentate chelate of BAzBABPy. The Cu−N bond lengths
around the Cu(II) center in CuPRT are in the range of 1.987−
2.184 Å. Two parallel arene moieties of BATAMC and the
pyridyl unit of BAzBABPy are within the π−π stacking
interaction distance. The interaction distances between the
centroids of the arene units of BATAMC and the centroid of
the pyridyl ring of BAzBABPy are 3.474 and 3.494 Å (Figure
1a). The space-filling model of CuPRT clearly shows complete
threading of the BAzBABPy into the macrocyclic cavity of
BATAMC with the azide terminals exposed outside the wheel
in the [2]pseudorotaxane structure. Thus, the two azide
terminals are accessible for further functionalization.
Scheme 1. (A) Synthetic Scheme for the Axle Fragment
BAzBABPy [(i) SOCl2, 80 °C; (ii) 2-Aminoethyl Azide, 0
°C]; (B) Synthetic Route for the [2]Pseudorotaxanes
CuPRT and NiPRT
might result in heteroleptic complexes with pseudorotaxane
architecture. In that case, the terminal azide functionalities in
the axle fragment of the [2]pseudorotaxane could be utilized as
a precursor for the synthesis of a [2]rotaxane via a copper(I)-
catalyzed azide−alkyne cycloaddition reaction (“click” chem-
istry) with an alkyne-terminated stopper.
Synthesis of [2]Pseudorotaxane Precursors. The axle
fragment, bis(azide)bis(amide)-2,2′-bipyridine (BAzBABPy)
was synthesized in good yield in three steps starting from
Synthesis of the [2]Rotaxane. Attempts were made to
synthesize the metal-free [2]rotaxane using azide-terminated
Figure 1. Single-crystal X-ray structure of azide-terminated [2]pseudorotaxane CuPRT using (a) ball-and-stick and (b) space-filling models. H
atoms, counteranions, and solvent molecules have been omitted for clarity.
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Scheme 2. (A) Synthesis of STP [(i) Ethanol, H2SO4, Reflux, 12 h; (ii) 3,5-Di-tert-butylbenzyl Bromide, CH3CN, Reflux, 24 h;
(iii) LiOH, HCl, THF/H2O, RT, 24 h; (iv) Propargyl Bromide, TBAF, THF, RT, 8 h]; (B) Synthetic Route for the Metal-Free
[2]Rotaxane RTX
[2]pseudorotaxane precursors CuPRT and NiPRT by attaching
stoppers at both the ends of the axle fragment. Traditional
bulky stoppers, such as triphenylmethyl (trityl) units are the
most popular stoppers and have been used extensively in the
last few decades.74−76 To achieve potential functional diversity
in the interlocked molecules, we synthesized a new bulky
molecule having ester functionality and a di-tert-butylbenzyl-
substituted aromatic platform as a new stopper unit (Scheme
2A). The stopper was synthesized in good yield in four steps
starting from 3,5-dihydroxybenzoic acid. Esterification of 3,5-
dihydroxybenzoic acid generated 3,5-dihydroxybenzoic acid
ethyl ester (L1), which upon reaction with 3,5-di-tert-
butylbenzyl bromide in the presence of K2CO3 in dry
CH3CN afforded ethyl 3,5-bis(3,5-di-tert-butylbenzyloxy)-
benzoate (L2). Then L2 was hydrolyzed to give 3,5-bis(3,5-
di-tert-butylbenzyloxy)benzoic acid (L3) in LiOH in 1:1:1
THF/CH3OH/H2O. Finally, alkyne terminal stopper prop-2-
ynyl 3,5-bis(3,5-di-tert-butylbenzyloxy)benzoate (STP) was
obtained by the ester formation reaction between propargyl
bromide and L3 in the presence of tetrabutylammonium
fluoride in dry THF.
To synthesize the [2]rotaxane, the azide-terminated [2]-
pseudorotaxane precursors CuPRT and NiPRT were separately
reacted with alkyne-functionalized stopper STP by the Cu(I)-
catalyzed click reaction. Our attempts to synthesize the
[2]rotaxane from CuPRT either under click reaction conditions
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of sodium ascorbate and Cu(II) or by the direct use of Cu(I)
with Na2CO3 in an inert atmosphere were unsuccessful. ESI-
MS studies of the above reaction mixtures indicated the
formation of the axle terminated with the stopper without any
interlocked product (Figure 25S in the Supporting Informa-
tion), which indicates complete dethreading of the axle
fragment from CuPRT under the reaction conditions followed
by azide−alkyne cycloaddition reaction between the dethreaded
axle fragment and the alkyne stoppers. In fact, no trace of
Cu(II)-templated rotaxane could be observed by ESI-MS. In
the presence of sodium ascorbate, the Cu(II) center in CuPRT
could easily be reduced to Cu(I), and the overall coordination
environment of the threaded adduct (3+2 format) might
disfavor Cu(I) templation, which would result in dethreading of
the azide axle from the reduced CuPRT. To avoid such a
reduction process during stoppering, we further attempted the
click reaction in an inert atmosphere using [Cu(CH3CN)4]-
(PF6) catalyst in the presence of Na2CO3. Unfortunately, the
reaction also failed under these reaction conditions. However,
such difficulties were overcome when the Ni(II)-templated
pseudorotaxane NiPRT was used as the precursor. The reaction
of azide-terminated NiPRT with the alkyne-terminated ester-
functionalized stopper STP in the presence of a catalytic
amount of Cu(CH3CN)4PF6 followed by in situ demetalation
afforded the metal-free [2]rotaxane (RTX) (Scheme 2B). RTX
was purified by column chromatography (SiO2, eluting with 3%
CH3OH/DCM) in 45% yield. The free axle with stopper
(AXL) was also isolated in 30% yield during the purification
process by column chromatography. Eventually, the click
reaction in the final step of the [2]rotaxane synthesis produces
a 1,2,3-triazole unit in the axle backbone, a versatile
functionality for anion recognition via polarized C−H hydro-
gen-bonding interactions. Furthermore, the N-donor sites of
the triazole unit in RTX could potentially be used for metal ion
coordination.
Hg and Hi of the wheel in RTX show upfield shifts with respect
to their non-interlocked counterparts. Such chemical shifts
indicate that these protons in RTX are located in the shielded
region composed of aromatic moieties of the wheel and axle.
This indicates that the aromatic units of the axle and wheel
components are positioned face-on in case of the interlocked
RTX system. Downfield perturbations of proton Hc and the
amide protons He of the macrocycle and the amide proton H4
of the axle indicate hydrogen-bonding interactions between the
macrocycle and the axle in RTX. Additionally, splitting of the
resonances corresponding to the methylene protons Hk and Hl
of the macrocycle indicate the reduced symmetry of these
protons in the interlocked structure of RTX.
The DOSY NMR spectrum of RTX provides further
evidence for the formation of the interlocked structure. Figure
4 depicts the DOSY spectrum of RTX in CDCl3 at 298 K. It is
evident from the spectrum that all of the peaks correlated to the
signals in the chemical shift dimensions are in a horizontal line.
Thus, all of proton signals due to the wheel BATAMC and the
axle AXL of the [2]rotaxane display the same diffusion
coefficient (D = 3.3 × 10−10 m2 s−1), indicating that they are
parts of the same species.
1
Two-dimensional H−1H rotating-frame Overhauser effect
spectroscopy (ROESY) provides further evidence of the
interlocked nature of RTX (Figure 20S in the Supporting
1
Information). The H−1H ROESY spectrum displays several
through-space cross-coupling interactions between the two
components of the [2]rotaxane. Important interactions include
those between Hg and H1 and those between Hc and H3, which
indicate that the bipyridine moiety of the axle is spatially close
to the wheel cavity, further confirming the interlocked
architecture of RTX in solution (Figure 5).
The stability of the interlocked structure and the location of
the wheel on the axle component were also investigated in the
protonated state of RTX by 1D and 2D NMR studies. Since
RTX contains three amine groups in the wheel and bipyridine/
triazole groups in the axle, a strong acid such as TFA can easily
protonate the system. Figure 6 shows the 1H NMR spectrum of
RTX in the presence of TFA (10 equiv) in CDCl3. Comparison
Formation of the [2]rotaxane was confirmed by ESI-MS and
1D and 2D NMR spectroscopy (COSY, DOSY, and ROESY)
experiments. The ESI-MS spectrum of RTX shows prominent
peaks at m/z 2106.22 and 1053.12 that correspond to [RTX +
H]+ and [RTX + 2H]2+, respectively, and the similarity between
the calculated and experimental isotope distribution patterns is
quite obvious (Figure 2).
1
of the H NMR spectra of protonated and unprotonated RTX
reveals protonation of RTX in which the three amine sites of
the wheel and the bipyridine and triazole units of the axle are
protonated (Figure 6). The resonances of protons Hi, Hk, and
Hl of the macrocycle, H1, H2, and H3 of the bipyridine unit, and
H7 of the triazole moiety are observed to be shifted downfield
in the case of protonated RTX. Furthermore, comparison of the
1H NMR spectra of protonated RTX, protonated BATAMC,
and protonated AXL and an 1H−1H NOESY study of
protonated RTX indicate that the wheel resides on the
bipyridinium unit of the axle (Figure 24S in the Supporting
Information and Figure 7). The analysis of the 1H−1H NOESY
spectrum of protonated RTX, which shows several clear cross-
peaks between protons related to the protonated axle and
protonated wheel (Hg and H2; Hg and H3; Hh and H3; and Hc
and H2), firmly proves the host−guest interaction (Figure 7).
The important interactions between Hg and H2, between Hg
and H3, and between Hh and H3 confirm that the electron-rich
phenylene units of the protonated wheel and the electron-poor
bipyridinium moiety of the protonated axle are in close
proximity and that strong aromatic π−π interactions are
present. These interactions might favor the wheel to reside on
the bipyridium moiety of the axle in protonated and
unprotonated RTX.
Figure 2. ESI-MS (positive-ion mode) spectrum of RTX. The inset
shows the similarity of the calculated (dotted) and experimental
(bold) isotope distribution patterns of [RTX + 2H]2+.
1
Comparative H NMR spectra of the wheel BATAMC, the
axle with stopper AXL, and the [2]rotaxane RTX in CDCl3 are
shown in Figure 3. In the spectrum of RTX, the triazole C−H
proton H7 is shifted downfield relative to that of AXL (Δδ = 0.2
ppm), indicating its hydrogen-bonding interaction. The
bipyridine protons H1, H2, and H3 of the axle and protons
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1
Figure 3. Partial stacked H NMR spectra (300 MHz) in CDCl3 at 298 K of (bottom) the macrocycle BATAMC, (middle) the [2]rotaxane RTX,
and (top) the stopper-terminated free axle AXL (top). The labels correspond to those shown in the inset pictures of BATAMC and AXL.
Figure 4. Partial DOSY spectrum in CDCl3 of RTX with a diffusion coefficient value (D) showing a single supramolecular aggregate.
The kinetically inert nature of the interlocked compound
RTX was verified via slippage experiments. No evidence of
dethreading was observed, as judged by variable-temperature
CONCLUSION
■
An azide-terminated 2,2′-bipyridine-based bis(amide)bis(azide)
axle has been established as a versatile thread for the tris-
chelating macrocycle wheel templated by Cu(II) and Ni(II).
The azide axle fragment allows easy coordination with the
template metal center in the cavity of the tris-chelating wheel in
an orthogonal orientation, forming metallopseudorotaxanes
with azide groups exposed outside the cavity of the wheel. This
indeed makes these pseudorotaxanes suitable precursors for
synthesis of other interlocked molecules. A metal-free [2]-
rotaxane enriched with a number of functional groups such as
2,2′-bipyridine, amide, ester, and triazole functionalities was
synthesized from the Ni(II)-templated [2]pseudorotaxane
upon coupling with a newly developed alkyne-terminated
stopper unit via the click reaction. The mechanically
interlocked bis(amide)tris(amine) wheel was found be located
at the center of the axle in both the free and protonated states
1
(VT) H NMR spectroscopy, even when RTX (4 mM) was
heated for several hours at 353 K in DMSO-d6. The
temperature of RTX solution was raised from 298 to 353 K,
and the spectra are shown in Figure 8. The VT-NMR analysis
reveals that there was no significant change in the protons of
the [2]rotaxane. However, the amide protons (−NH)
corresponding to the wheel (yellow) and axle (green) were
slightly shifted upfield by 0.16 and 0.23 ppm, respectively, at
353 K. These upfield shifts at elevated temperature could be
due to breakage of hydrogen bonds between amide −NH
protons and solvent molecules. Thus, the VT-NMR study
indicates that the stopper is sufficiently bulky to prevent
dethreading of the 30-membered wheel in the [2]rotaxane
architecture even at an elevated temperature.
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1
Figure 5. Chemical structure of RTX with intercomponent through-space H−1H interactions.
1
Figure 6. Comparison of the H NMR spectra of (top) protonated and (bottom) unprotonated RTX in CDCl3 at 298 K.
the axle of the [2]rotaxane even at an elevated temperature. We
are presently exploring various embedded functionalities in
such rotaxane molecules.
EXPERIMENTAL SECTION
■
General Details. The reagents were obtained from commercial
suppliers and used as received without further purification, unless
otherwise indicated. 2,2′-Bipyridine-5,5′-diacid chloride,77 2-azido-1-
ethylamine,78 the macrocycle BATAMC,71 and L179 were synthesized
using previously reported procedures. Peak assignments in the 1H
NMR spectra were confirmed using COSY spectra of selected
compounds. DEPT-135 spectra were used to interpret the 13C NMR
spectra. The special interactions between the protons of the wheel and
those of the axle were interpreted by COSY and ROESY NMR
spectroscopy in CDCl3.
X-ray Crystallography. Crystals of CuPRT suitable for single-
crystal X-ray diffraction studies were obtained upon slow evaporation
of a methanol solution at room temperature. A single crystal was
selected from the mother liquor, immersed in Paratone oil, mounted
on the tip of a glass fiber, and cemented using epoxy resin. Intensity
data for the CuPRT crystal were collected using Mo Kα radiation (λ =
0.7107 Å) at 298 K. The data integration and reduction were done
using the SAINT80 software provided with the software package for
the SMART APEX II system. An empirical absorption correction was
applied to the collected reflections using SADABS,81 and the structure
was solved by direct methods using SHELXTL82 and refined on F2 by
the full-matrix least-squares technique using the SHELXL-9783
program package. Graphics were generated using MERCURY 2.384
1
Figure 7. H−1H NOESY NMR spectrum of protonated [2]rotaxane
RTX in CDCl3 at 298 K. Assignable intercomponent through-space
interactions are highlighted.
of the [2]rotaxane. Moreover, the newly developed stopper is
able to prevent dethreading of the 30-membered wheel from
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Figure 8. Variable-temperature H NMR spectra of RTX in DMSO-d6 (300 MHz).
and PLATON,85 and the non-hydrogen atoms were refined
anisotropically until convergence.
(86%). ESI-MS+ m/z: calcd for C37H50O4K [M + K]+ 597.89, found
597.27. H NMR (CDCl3, 300 MHz): δ 1.36 (s, 36H, −C(CH3)3),
1
Synthesis of BAzBABPy. To a solution of 2-azido-1-ethylamine
(0.172 g, 2.0 mmol) and NEt3 (0.348 mL, 2.5 mmol) in dry THF (25
mL) was added 2,2′-bipyridine-5,5′-diacid chloride (281 mg, 1 mmol)
dissolved in dry THF (10 mL) dropwise using a pressure-equalizing
funnel. The mixture was allowed to stir in a nitrogen atmosphere for
0.5 h at 0 °C and then for 3 h at room temperature. An off-white
precipitate then developed and was filtered. The precipitate was
washed with NaHCO3 solution (50 mL) and water (100 mL) and
dried in vacuum to obtain BAzBABPy as an off-white solid in 84%
yield (0.320 g). ESI-MS+ m/z: calcd for C16H17N10O2 [M + H]+
381.37, found 381.16. 1H NMR (300 MHz, DMSO-d6): δ 3.35 (s, 4H,
−CH2), 8.39 (d, 2H, J = 9 Hz, Ar-H), 8.54 (d, 2H, J = 6 Hz, Ar-H),
9.03 (b, 2H, −NH), 9.13 (s, 2H, Ar-H). 13C NMR (75.47 MHz,
DMSO-d6): δ 39.1, 49.7, 120.6, 130.0, 136.3, 148.4, 156.4, 165.1. Anal.
Calcd for C16H16N10O2: C, 50.52; H, 4.24; N, 36.82. Found: C, 50.34;
H, 4.19; N, 36.93.
Synthesis of L2. In a 100 mL round-bottom flask, L1 (0.910 g, 5
mmol) was dissolved in 50 mL of dry acetonitrile. K2CO3 (2 g, 15
mmol) and 3,5-di-tert-butylbenzyl bromide (2.8 g, 10 mmol) were
added to that solution at room temperature. The reaction mixture was
allowed to stir at 80 °C in a nitrogen atmosphere for 24 h. Then the
solvent was evaporated under reduced pressure, and the solid was
dissolved in dichloromethane. This solution was extracted with a
solution of brine. After evaporation of the solvent, a liquid compound
was obtained that was became solid upon the addition of a small
portion of methanol (5 mL). The resulting off-white solid was purified
by silica gel column chromatography, eluting with 2% ethyl acetate/
petroleum ether, to afford the targeted disubstituted product L2 in
60% yield (1.75 g). ESI-MS+: m/z 609.38 [M + 23]+. 1H NMR
(CDCl3, 300 MHz): δ 1.367 (s, 36H, −C(CH3)3), 1.43 (t, 4H, J = 4
Hz, −CH3), 4.41 (q, 2H, J = 6.9 Hz, −CH2), 5.07 (s, 1H, Ar−CH2),
6.892 (t, 1H, J = 2.4 Hz, Ar-H), 7.31 (br, 4H, Ar-H), 7.36 (d, 2H, J =
2.4 Hz, Ar-H), 7.442 (t, 2H, J = 2.8 Hz, Ar-H). 13C NMR (CDCl3,
75.47 MHz): δ 14.5, 31.6, 30.0, 61.3, 71.4, 107.3, 108.5, 122.4, 122.5,
132.5, 135.6, 151.3, 160.1, 166.6. Anal. Calcd for C39H54O4: C, 79.82;
H, 9.27. Found: C, 80.02; H, 9.41.
5.07 (s, 4H, Ar−CH2), 6.93 (t, 1H, J = 3 Hz, Ar-H), 7.31 (br, 4H, Ar-
H), 7.42−7.44 (m, 4H, Ar-H). 13C NMR (CDCl3, 75.47 MHz): δ
32.5, 36.0, 72.3, 109.4, 110.0, 123.4, 123.5, 132.3, 136.4, 152.2, 161.2,
172.8. Anal. Calcd for C37H50O4: C, 79.53; H, 9.02. Found: C, 79.71;
H, 9.16.
Synthesis of the Stopper STP. To a solution of L3 (1.1 g, 2
mmol) in dry THF (5 mL) was added 1.2 equiv of TBAF (627.5 mg,
2.4 mmol) (1 M in THF), and the mixture was stirred at 30 °C in a
nitrogen atmosphere. To this solution was added propargyl bromide
(0.196 mL, 2.2 mmol). After the addition, the reaction mixture was
allowed to stir at room temperature in a nitrogen atmosphere for
another 6 h. The solution was evaporated, and the residue was
dissolved in CH2Cl2. The resulting solution was washed three times
with saturated NaHCO3 solution followed by saturated NaCl solution.
The off-white solid was isolated and dried over vacuum to yield the
desired product STP. Yield: 950 mg (80%). ESI-MS+ m/z: calcd for
C40H52O4Na [M + Na]+ 619.38, found 619.48. 1H NMR (CDCl3, 300
MHz): δ 1.33 (s, 36 H, −C(CH3)3), 2.51 (t, 1H, J = 3 Hz, −CH), 4.92
(d, 2H, J = 3 Hz, −CH2), 5.04 (s, 4H, Ar−CH2), 6.88 (t, 1H, J = 3 Hz,
Ar-H), 7.28 (d, 4H, J = 2 Hz, Ar-H), 7.36 (d, 2H, J = 2 Hz, Ar-H), 7.41
(t, 2H, J = 2 Hz, Ar-H). 13C NMR (CDCl3, 75.47 MHz): δ 31.6, 35.0,
52.7, 71.4, 75.2, 77.8, 107.9, 108.7, 122.4, 122.5, 131.3, 135.5, 151.3,
160.2, 166.8. Anal. Calcd for C40H52O4: C, 80.50; H, 8.78. Found: C,
80.24; H, 8.70.
Synthesis of [2]Pseudorotaxanes CuPRT and NiPRT. A
solution of M(ClO4)2·6H2O (M = Cu, Ni) (0.1 mmol) in CH3OH
(5 mL) was added to a solution of BATAMC (53.1 mg, 0.1 mmol) in
1:1 CH3OH/CH2Cl2 (5 mL) at room temperature. To this solution
was added the diazide axle BAzBABPy (38 mg, 0.1 mmol). The
resultant mixture was stirred for 4 h, and the solvent was evaporated.
The solid was washed repeatedly with CH2Cl2 and dried in vacuo to
give the pure products CuPRT and NiPRT in yields of 87 and 85 mg,
respectively (70−75%). The complexes were characterized by ESI-MS
and elemental analysis. Data for CuPRT: ESI-MS+ m/z: calcd for
−
C46H53ClCuN15O10 [CuPRT + ClO4 ]+ 1073.32, found 1072.97. Anal.
Calcd for C46H53Cl2CuN15O14: C, 47.04; H, 4.55; N, 17.89. Found: C,
Synthesis of L3. The reaction mixture of L2 (587 mg, 1 mmol)
and LiOH monohydrate (126 mg, 3 mmol) in 1:2 water/THF (3 mL)
was stirred at 30 °C for 24 h. The THF layer was evaporated, and
dilute HCl was added to the aqueous part. A white precipitate of L3
was formed. The solution was filtered, and the white precipitate was
isolated by repeated washing with water. This white solid L3 was dried
and used in the next step without further purification. Yield: 0.48 g
47.10; H, 4.62; N, 17.95. Data for NiPRT: ESI-MS+ m/z: calcd for
−
C46H53ClNiN15O10 [NiPRT + ClO4 ]+ 1068.31, found 1068.08. Anal.
Calcd for C46H53Cl2N15NiO14: C, 47.24; H, 4.57; N, 17.96. Found: C,
47.18; H, 4.62; N, 17.88. Single crystals of CuPRT suitable for X-ray
analysis were obtained by slow evaporation of a methanol solution at
room temperature.
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The Journal of Organic Chemistry
Article
Synthesis of the [2]Rotaxane RTX Using Cu(CH3CN)4PF6
inside the Glovebox. NiPRT (0.23 g, 0.2 mmol) and STP (0.23
g, 0.4 mmol) were suspended in 4 mL of 1:1 CH2Cl2/CH3CN
solution. Sodium carbonate (0.10 mg, 0.10 mmol) was added as a
solid, followed by the addition of Cu(CH3CN)4PF6 (112 mg, 0.03
mmol) in the glovebox. The reaction mixture was stirred overnight at
room temperature and then evaporated. A 10 mL saturated solution of
Na2EDTA in water was added to the crude residue, and the mixture
was stirred for 6 h. The solution was poured into 25 mL of CHCl3 and
washed with water. The organic layer was evaporated under reduced
pressure to obtain an off-white solid. The resulting solid was purified
by silica column chromatography, eluting with 3% CH3OH/CH2Cl2,
to afford the targeted [2]rotaxane RTX in 45% yield (170 mg) along
with the free axle AXL (95 mg, 30%).
ACKNOWLEDGMENTS
■
P.G. gratefully acknowledges the Science and Engineering
Research Board (SERB), New Delhi (Project SR/S1/IC-39/
2012) for financial support. S. Saha acknowledges the Indian
Association for the Cultivation of Science (IACS), Kolkata for a
research fellowship. S. Santra and B.A. acknowledge the
Council of Scientific & Industrial Research (CSIR), New
Delhi for senior research fellowships. X-ray crystallography of
CuPRT was performed at the DST-funded National Single
Crystal X-ray Facility at the Department of Inorganic
Chemistry at IACS.
REFERENCES
Synthesis of RTX Using CuSO4·5H2O and Sodium Ascorbate.
NiPRT (0.23 g, 0.2 mmol) and STP (0.23 g, 0.4 mmol) were
suspended in 4 mL of 1:1 THF/H2O solution. Then an aqueous
solution of sodium ascorbate (50 mg) and CuSO4·H2O (50 mg) was
added to the reaction mixture under a nitrogen atmosphere. The
reaction mixture was stirred overnight at room temperature and then
evaporated. A 10 mL saturated solution of Na2EDTA in water was
added to the crude residue, and the mixture was stirred for 6 h. The
solution was poured into 25 mL of CHCl3 and washed with water. The
organic layer was evaporated under reduced pressure to obtain an off-
white solid. The resulting solid was purified by silica column
chromatography, eluting with 3% CH3OH/CH2Cl2, to afford RTX
in 40% yield (150 mg) along with AXL.
■
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(d, 4H, J = 9 Hz, Ar-Hg), 6.86 (t, 2H, J = 3 Hz, Ar-H10), 7.20−7.41 (m,
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2
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ASSOCIATED CONTENT
■
S
* Supporting Information
Compound characterization data and crystallographic data
(CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: icpg@iacs.res.in.
Notes
The authors declare no competing financial interest.
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