Synthesis of a molecular charm bracelet via click cyclization and olefin metathesis clipping

Article abstract of DOI:10.1021/ja9090337

We describe the synthesis of a polycatenated cyclic polymer, a structure that resembles a molecular charm bracelet. Ruthenium-catalyzed ring-opening metathesis polymerization of an aminocontaining cyclic olefin monomer in the presence of a chain transfer agent generated an α,ω-diazide functionalized polyamine. Cyclization of the resulting linear polyamine using pseudo-high-dilution coppercatalyzed click cyclization produced a cyclic polymer in 19% yield. The click reaction was then further employed to remove linear contaminants from the cyclic polymer using azide- and alkyne-functionalized scavenging resins, and the purified cyclic polymer product was characterized by gel permeation chromatography, ¹H NMR spectroscopy, and IR spectroscopy. Polymer hydrogenation and conversion to the corresponding polyammonium species enabled coordination and interlocking of diolefin polyether fragments around the cyclic polymer backbone using ruthenium-catalyzed ring-closing olefin metathesis to afford a molecular charm bracelet structure. This charm bracelet complex was characterized by ¹H NMR spectroscopy, and the catenated nature of the small rings was confirmed using two-dimensional diffusionordered NMR spectroscopy.

Full text of DOI:10.1021/ja9090337

Published on Web 02/16/2010
Synthesis of a Molecular Charm Bracelet via Click Cyclization
and Olefin Metathesis Clipping
Paul G. Clark,† Erin N. Guidry,† Wing Yan Chan,† Wayne E. Steinmetz,‡ and
Robert H. Grubbs*,†
The Arnold and Mabel Beckman Laboratory of Chemical Synthesis, DiVision of Chemistry and
Chemical Engineering, California Institute of Technology, Pasadena, California 91125, and
Chemistry Department, Pomona College, 645 North College AVenue, Claremont, California 91711
Received October 22, 2009; E-mail: rhg@caltech.edu
Abstract: We describe the synthesis of a polycatenated cyclic polymer, a structure that resembles a
molecular charm bracelet. Ruthenium-catalyzed ring-opening metathesis polymerization of an amino-
containing cyclic olefin monomer in the presence of a chain transfer agent generated an R,ω-diazide
functionalized polyamine. Cyclization of the resulting linear polyamine using pseudo-high-dilution copper-
catalyzed click cyclization produced a cyclic polymer in 19% yield. The click reaction was then further
employed to remove linear contaminants from the cyclic polymer using azide- and alkyne-functionalized
scavenging resins, and the purified cyclic polymer product was characterized by gel permeation
1
chromatography, H NMR spectroscopy, and IR spectroscopy. Polymer hydrogenation and conversion to
the corresponding polyammonium species enabled coordination and interlocking of diolefin polyether
fragments around the cyclic polymer backbone using ruthenium-catalyzed ring-closing olefin metathesis to
1
afford a molecular charm bracelet structure. This charm bracelet complex was characterized by H NMR
spectroscopy, and the catenated nature of the small rings was confirmed using two-dimensional diffusion-
ordered NMR spectroscopy.
Introduction
Novel polymer architectures have attracted significant interest
from scientists in an unceasing quest to tailor the structural
attributes of a material for specific applications. Cyclic polymers
(CPs) have garnered particular attention due to a number of
attractive traits, including desirable physical properties, increased
functional group density, and smaller hydrodynamic radii
relative to linear polymer analogues.1-7 Recent work by Fre´chet,
Szoka, and co-workers8 has also indicated that CPs have
potential biological applications, displaying better circulation
half-lives than their linear counterparts and thus making them
attractive candidates as possible drug carriers.
Figure 1. Macrocyclization of an R,ω-heterotelechelic polymer under high-
dilution conditions.
via high-dilution cyclization of dianionic linear polymers and
a two-site coupling agent. However, due to issues associated
with polymer purity using this technique, modern efforts have
focused on the development of a number of new routes to cyclic
architectures, including macrocyclization of R,ω-heterotelechelic
linear polymers (Figure 1)13-15 or olefin-terminated polymers,16,17
cyclization of electrostatically templated telechelic polymers,18-25
and ring-expansion reactions of cyclic species, including lac-
In addition to their intriguing properties, CPs also present a
unique challenge to synthetic chemists, as there are few
procedures that favor the formation of these “endless” polymers
in high purity.9-12 Traditionally, CPs have been synthesized
† California Institute of Technology.
‡ Chemistry Department, Pomona College.
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10.1021/ja9090337  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 3405–3412 3405

A R T I C L E S
Clark et al.
Figure 2. Ruthenium olefin metathesis catalysts 1 and 2.
Figure 3. Mechanically interlocked structures synthesized by olefin
metathesis. A rodlike structure, with bulky terminal “stopper” units, encircled
by a ring (A) is termed a [2]rotaxane, while two interlocked rings (B) are
designated a [2]catenane.
tides26 and olefin monomers.27-30 Macrocyclization reactions
of R,ω-heterotelechelic polymers often employ high-fidelity,
high-conversion “click” reactions,31-36 such as the Huisgen 1,3-
dipolar cycloaddition between an alkyne and an azide, but
necessitate high-dilution or pseudo-high-dilution conditions to
favor cycles rather than oligomers. In contrast, reactions such
as ring-expansion metathesis polymerization27-30 (REMP),
catalyzed by cyclic ruthenium alkylidene species, overcome this
limitation and enable rapid synthesis of multigram quantities
of pure cyclic polymer. In addition to the cyclic ruthenium
catalysts used in REMP, a number of other functional-group-
tolerant ruthenium alkylidene catalysts, for example, catalysts
1 and 2 (Figure 2), have seen broad application in polymer
synthesis.37-40
supramolecular chemistry45-47 (often involving hydrogen-
bonding interactions between crown ether-type species and
secondary ammonium ions) to template the formation of
preorganized structures and dynamic covalent chemistry48-54
to induce interlocking of the resulting complex, high-yielding
syntheses of rotaxanes55-58 and catenanes59-65 (Figure 3), as
well as a variety of other, more complex architectures,66-72 have
been realized. One particularly effective strategy to synthesize
mechanically interlocked molecules employs the dynamic,
ruthenium-catalyzed ring-closing metathesis (RCM) reaction,
whereby a diolefin polyether fragment is subjected to RCM
conditions and “clipped” around a disubstituted ammonium
ion.56,63,69
Another intriguing and challenging area of research is the
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any covalent bonds between them. Utilizing a combination of
Due to the complexity of mechanically linked structures,
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Synthesis of a Molecular Charm Bracelet
A R T I C L E S
complexes containing threaded moieties,86-91 and Harada has
reported the cyclization of a threaded, capped polyrotaxane.92
In complement to these reports, we wanted to explore a
“clipping” approach to such a polycatenated molecular charm
bracelet. We synthesized a CP architecture via a pseudo-high-
dilution, two-component click macrocyclization and subse-
quently converted this species to the polyammonium analogue.
We were then able to clip diolefin crown ether-type rings around
the ammonium sites of the polymer scaffold using RCM.
Confirmation of the interlocked nature of the charm bracelet
product was achieved via 2D-DOSY NMR.
Results and Discussion
Monomer Design and Synthesis. We believed the nine-
membered cyclic boc-amine compound 3 (Scheme 1)93 pos-
sessed sufficient ring strain energy to be a suitable metathesis
polymerization monomer and would afford the desired polyam-
monium polymer upon removal of the Boc-protecting group.
Synthesis of 3 was readily achieved in eight steps and 31%
overall yield. cis-1,5-Cyclooctadiene (4) was treated with m-3-
chloroperoxybenzoic acid to afford the monoepoxidized cy-
clooctene species 5. Reduction of 5 with lithium aluminum
hydride opened the epoxide to the corresponding alcohol 6,
which was subjected to Swern oxidation conditions, giving
ketone 7. Refluxing 7 in the presence of hydroxylamine
hydrochloride generated oxime 8, which was transiently con-
verted to the tosyl oxime intermediate prior to undergoing a
base-promoted Beckmann rearrangement that produced a re-
gioisomeric mixture of lactams 9a and 9b. By 1H NMR
spectroscopic analysis, isomer 9b appeared to dominate the
product mixture, which is not entirely surprising given that, in
8, the ratio of oxime anti to Ha versus syn to Ha was
approximately 6:1, respectively. The mixture of lactams was
reduced to the cyclic amine by lithium aluminum hydride, and
the amine was subsequently Boc-protected to yield the desired
unsaturated nine-membered cyclic Boc-amine monomer 3.
Interestingly, despite the mixture of starting materials, only the
nonsymmetric regioisomer of 3 was isolated.
Figure 4. Graphic representation of various polycatenane structures,
including main-chain polycatenanes (A and B), side-chain polycatenanes
(C and D), and the targeted cyclic interlocked charm bracelet polycatenane
(E).
spectroscopy73-79 (2D-DOSY) facilitates the analysis of distinct
complexes in solution on the basis of their unique diffusion rate.
Using this technique, small molecules with fast diffusion rates
that are intimately associated (either covalently or mechanically)
with a slowly diffusing macromolecule will diffuse at the speed
of the larger structure to which they are appended. The
application of this technique to linear polymers with threaded
rings has been demonstrated previously.80
The inclusion of interlocked moieties as a part of a larger
macromolecular structure43,81 has been shown to have an impact
on both the solution state and bulk properties82-85 of the
resulting material, and to this end, researchers have synthesized
a number of polyrotaxanes,82,83 as well as polycatenane species
(Figure 4A-D).72,84,85 The polycatenane charm bracelet struc-
ture (Figure 4E), however, has been particularly elusive.9-11,45,70
Recently, a few reports have emerged describing smaller cyclic
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A R T I C L E S
Clark et al.
Scheme 1. Synthesis of Nine-Membered Cyclic Boc-amine Monomer 3a
a Reagents and conditions: (a) m-CPBA, CHCl3, 0 °C to room temperature, 12 h; (b) LAH, THF, 0 °C to reflux, 4 h; (c) oxalyl chloride, DMSO, TEA,
-78 °C to room temperature, 4 h; (d) NH2OH·HCl, NaHCO3, MeOH, reflux, 4 h; (e) TsCl, Pyr, DCM, 0 °C to room temperature, 12 h; (f) K2CO3, H2O,
THF, room temperature, 12 h; (g) LAH, THF, reflux, 4 h; (h) Boc2O, TEA, DMAP, DCM, room temperature, 24 h.
Scheme 2. Synthesis of Linear Telechelic Dibromide Polymer L-11, Diazide Polymer L-12, and Cyclic Polymer C-13a
a Reagents and conditions: (a) catalyst 1, 1.0 M DCM, 43 °C, 1 day; (b) NaN3, DMF, 50 °C, 12 h.
(Scheme 2). The GPC traces for L-11 and L-12 overlapped
closely (Figure 5), and polymer end-group conversion from
1
bromide to azide was monitored by H NMR spectroscopy
(Figure 6B), which showed an upfield shift of the bromometh-
ylene protons (from 3.37 to 3.23 ppm) after substitution with
sodium azide. To verify that L-12 contained azide functional-
ities, we studied polymers L-11 and L-12 by FT-IR (Figure 7).
As expected, the spectrum of L-12 contained a strong band at
2096 cm-1, while no azide peak was observed in the spectrum
of L-11.
We effected the cyclization of the linear diazide polymer L-12
to the targeted CP C-13 via a slow-addition, pseudo-high-
dilution process13-15 (Scheme 2), whereby a solution of diazide
linear polymer L-12 and 1,4-diethynylbenzene (14) was added
Figure 5. GPC traces of linear bromide polymer L-11 (black), linear azide
polymer L-12 (red), doubly clicked linear polymer L-15 (green), and cyclic
by syringe pump over several days to a well-stirred DMF
reservoir containing Cu catalyst and N,N,N′,N′′,N′′-penta-
methyl-diethylenetriamine (PMDETA) ligand. Rather than at-
tempt to fractionally precipitate linear polymer impurities99 from
C-13, we instead utilized the high fidelity of the click reaction.
Since any linear polymer contaminants would likely contain
either an azide or an alkyne chain-end functionality, we
subjected the crude cyclization products to a more concentrated
click reaction in the presence of alkyne-functionalized polymer
beads and azide-functionalized Merrifield resin (Scheme 3).100
Filtration of the solid beads resulted in the concomitant removal
of all linear contaminants from solution and afforded a solution
of pure cylic polymer C-13.
The cyclic topology of C-13 was confirmed by 1H NMR end-
group analysis and shifting of GPC peak elution volume.
Evidence for the success of the click reaction could be readily
observed in the dramatic downfield shift of the azidomethylene
proton (Hd) signal from 3.23 to 4.37 ppm (Figure 6C), indicating
conversion of the azides to triazole functionalities. IR spectro-
scopy of C-13 (Figure 7) also showed that the strong azide band
in the spectrum of L-12 was no longer present. Inclusion of
the 1,4-diethynylbenzene unit within the backbone of C-13 was
polymer C-13 (blue).
suitably functionalized telechelic polymer would be structurally
analogous to early dianionic polymers used in the synthesis of
CPs.9,99
Given the successful application of the alkyne-azide click
reaction to the synthesis of CPs,13-15 we employed this
procedure to cyclize our linear polymer. Though our efforts to
use a diazide CTA during ROMP to monomer 3 were unsuc-
cessful, we readily accessed the dibromo telechelic polymer
L-11 upon treatment of a solution of 3 and dibromo CTA 10100
with catalyst 1 (Scheme 2). Gel permeation chromatography
(GPC, Figure 5) analysis of L-11 revealed that the polymer had
an Mn value of 4.1 kDa, correlating to a degree of polymerization
1
(DP) of 17, and a polydispersity index (PDI) of 1.49. By H
NMR end-group analysis (Figure 6A), the DP of L-11 was found
to be 19, in close agreement with the GPC results. Synthesis of
the desired diazide telechelic polymer L-12 was accomplished
by treating dibromotelechelic polymer L-11 with sodium azide
(99) Gan, Y.; Dong, D.; Hogen-Esch, T. E. Macromolecules 2002, 35,
6799.
(100) See the Supporting Information for full details.
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Synthesis of a Molecular Charm Bracelet
A R T I C L E S
1
Figure 6. Partial 600 MHz H NMR spectra at 25 °C in CDCl3 of polymer end-group resonances, showing conversion of telechelic bromide L-11 (A) to
diazide L-12 (B) to post-click cyclic species C-13 (C). After cyclization, the end-group signal shifts downfield to 4.37 ppm. Full spectra can be seen in the
Supporting Information.
Figure 8. Partial 600 MHz 1H NMR spectra at 25 °C in CDCl3 of the
clicked end-groups of symmetric cyclic polymer C-13 (A) and nonsymmetric
double-clicked linear polymer analogue L-15 (B). Integration values in red
were obtained by assigning the end group signal from Hg an integration
value of 4.
synthesized a control polymer using the original diazide polymer
Figure 7. FT-IR spectra for linear bromide polymer L-11 (black), linear
azide polymer L-12 (red), cyclic polymer C-13 (blue), and doubly clicked
linear polymer L-15 (green).
L-12. We again subjected L-12 to click conditions, but in the
presence of a significant excess of dialkyne 14 to give linear,
doubly clicked analogue L-15 (Scheme 4). The GPC trace for
L-15 (Figure 5) overlapped identically with both L-11 and L-12,
again with only a slight MW increase (Mn ) 4.4 kDa) due to
the addition of the clicked units. In contrast to the symmetric
singlet of the phenyl clicked end group of C-13, the signal from
Hh and Hi of the phenyl group of the clicked units of L-15
produced two doublets (Figure 8) with coincidental overlap of
the triazole signal from Hj with the downfield doublet. When
the signal from Hg was assigned a value of four protons, each
of the doublets from Hh and Hi integrated to four protons with
an additional two protons from the triazole, confirming the
clicking of two 1,4-diethynylbenzene units per polymer chain.
IR spectroscopy (Figure 7) also showed no remaining azide
resonance after the “click” reaction, though peaks at 3300 and
2200 cm-1 indicated the presence of remaining alkyne func-
tionality. The alkyne protons, too, could be observed in the 1H
NMR spectrum (3.10 ppm) as a sharp singlet extending above
the broad signal from Hc.100
1
readily apparent in the H NMR spectrum (Figure 8A), with
the phenyl protons of the linker (Hf) generating a sharp singlet
at 7.88 ppm that integrated to four protons upon assigning the
end-group signal from the R-triazole methylene proton Hg an
integration value of four (Scheme 2). Absence of splitting of
the aromatic signal indicated the symmetric nature of the clicked
unit and implied click coupling at both ends of the
1,4-diethynylbenzene. The triazole moiety also produced a broad
singlet at 7.78 ppm (Figure 8) that integrated to two protons.
GPC analysis of C-13 (Figure 5) showed the expected increase
in peak retention time characteristic of cyclic polymers,9 with
no appreciable change in molecular weight (MW) except for
the addition of the diethynylbenzene unit (Mn ) 4.4 kDa).101
1
Additionally, H NMR end group integration values remained
identical to the linear species L-11 and L-12. Though we
attempted to confirm the MW of the polymers by matrix-assisted
laser desorption ionization time-of-flight mass spectrometry
(MALDI TOF MS), the lability of the boc groups, as well as
the lability of the bromide and azide end groups of the linear
precursors, prevented accurate mass analysis due to complex
and intractable fragmentation patterns.
Polymer Functionalization. Once we had confirmed the cyclic
structure of C-13, we began the process of functionalizing
the polymer in preparation for the desired interlocking reac-
tion. The use of the ruthenium olefin metathesis catalyst 1 to
affect the final clipping RCM reaction necessitated hydrogena-
tion of all olefin residues within the cyclic polymer backbone,
To confirm that the observed increase in retention time upon
cyclization was not an artifact of the click reaction, we
Scheme 3. Click Removal of Linear Contaminants from Cyclic Polymer C-13 Using Azide- and Alkyne-Functionalized Beads
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A R T I C L E S
Clark et al.
Scheme 4. Synthesis of Doubly Clicked Linear Polymer L-15
a
Scheme 5. Synthesis of (A) Cyclic Polyammonium C-17-nH·nPF6 and (B) Linear Polyammonium L-18-nH·nPF6
a Reagents and conditions: (a) Wilkinson’s catalyst (Rh(PPh3)3Cl), 800 psi of H2, THF, 50 °C, 24 h; (b) TFA, DCM, room temperature, 4 h; (c) NH4PF6,
MeOH, room temperature, 12 h.
and this was achieved by treating C-13 with Wilkinson’s catalyst
under high-pressure hydrogenation conditions (Scheme 5A) to
give C-16. No remaining olefin residues at 5.40 ppm were
1
observed by H NMR spectroscopy, indicating saturation of
C-13.
To reveal the coordinating ammonium sites within the
backbone of the polymer, we treated C-16 with trifluoroacetic
acid (TFA), removing the Boc protecting group. The ammo-
nium-TFA adduct was then subjected to counterion exchange
with ammonium hexafluorophosphate to produce C-17-
nH·nPF6. Hexafluorophosphate counterions have been shown
to increase the binding constant between crown ether-type
species and ammonium ions and also enhance the solubility of
the charged complex in organic solvents.102-105 The H NMR
1
spectrum of C-17-nH·nPF6 contained a broad singlet (6.45 ppm)
corresponding to the ammonium residues, and the sharp signal
at 1.4 ppm corresponding to protons on the Boc-group was no
longer present. Additionally the R-ammonium methylene proton
1
Figure 9. Partial 600 MHz H NMR spectra at 25 °C in CD3CN of (A)
signal from Hc (Scheme 2) shifted upfield to 2.96 ppm
(originally 3.13 ppm), an effect of the conversion of the Boc-
amine to an ammonium salt. Broadening and shifting of the
triazole resonance (from 7.80 ppm to 8.14 ppm) suggested that
the acidic conditions of the deprotection may have resulted in
protonation of the triazole.
cyclic polyammonium C-17-nH2 ·nPF6 with 24-crown-8 ether (24C8)
showing less than 1.5% threading and (B) linear double-clicked poly-
ammonium L-18-nH2 ·nPF6 with 24C8 showing more than 28% threading.
(Note: approximately 0.5 equiv of 24C8 per ammonium; see the Supporting
Information for complete details.)
Unfortunately, none of the charged polymers were amenable
to GPC analysis, due to challenges associated with solubility.
As an alternate method for cyclic/linear topology confirmation,
we exploited the well-known hydrogen-bonding association
between 24-crown-8 ether (24C8) and dialkylammonium ions.
Upon encircling a dialkylammonium ion, the strongly coordinat-
ing 24C8 causes a downfield shift of the signal from the protons
on the carbons adjacent to that ammonium site. Correlating the
integration value of the original signal with the shifted signal
enables quantitation of the threading of the polymeric chains
by 24C8. When a solution of the “endless” C-17-nH·nPF6 was
mixed with 24C8 (0.5 equiv per ammonium site), barely
The linear doubly clicked analogue L-15 was subjected
to the same hydrogenation conditions (Scheme 5B) as C-13 to
give the saturated product. Deprotection and anion exchange
were also performed, generating the linear polyammonium
adduct L-18-nH·nPF6.
(101) All GPC MW measurements were obtained through multiangle laser
light scattering detection, giving the absolute molecular weight of
the polymer.
(102) Ashton, P. R.; Cantrill, S. J.; Preece, J. A.; Stoddart, J. F.; Wang,
Z.-H.; White, A. J. P.; Williams, D. J. Org. Lett. 1999, 1, 1917.
(103) Montalti, M. Chem. Commun. 1998, 1461.
(104) Doxsee, K. M. J. Org. Chem. 1989, 54, 4712.
(105) Jones, J. W.; Gibson, H. W. J. Am. Chem. Soc. 2003, 125, 7001.
1
detectable threading (<2%) was observed by H NMR spec-
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A R T I C L E S
Scheme 6. Screen Reaction To Determine Effect of Nitromethane
nH2 ·nPF6 and 19 in the 1:1 dichloromethane/nitromethane (v/
v) solvent mixture. Simple precipitation of the polymer into pure
dichloromethane and isolation of the insoluble material afforded
the desired interlocked charm bracelet C-22-nH·nPF6 (Scheme
7). By 1H NMR spectroscopic analysis, ∼15% of the ammonium
sites were clipped, indicating that each polymer chain contained
approximately two to three interlocked “charms”. We believe
there are several factors that resulted in this low clipping percent.
Because of the limited solubility of the polymer, the charged
ammonium residues were likely concentrated in a dense core
surrounded by the more soluble alkyl components, severely
limiting access of 19 and catalyst to the polymer binding sites
and preventing extensive clipping. Also, the large amount of
nitromethane solvent required to dissolve C-17-nH2 ·nPF6 would
significantly decrease the association constant governing coor-
dination of 19 to the ammonium sites, a phenomenon further
compounded by the inherently lower binding constant of
dialkylammonium species relative to their dibenzyl counter-
parts.45
Concentration on RCM Olefin Conversion
troscopy (Figure 9A; see the Supporting Information for full
details). However, the linear analogue L-18-nH·nPF6 engaged
in significant threading interactions upon addition of the same
amount of 24C8 (nearly 30% threading, Figure 9B). As
expected, introduction of additional 24C8 (total of 2.0 equiv
per ammonium site) resulted in increased threading for both
polymers, with coordination remaining low for C-17-nH·nPF6
(∼10%) while L-18-nH·nPF6 threaded to ∼45%. It is worth
noting that, due to the dominance of cyclic architectures in the
sample of C-17-nH·nPF6, the effective concentration of avail-
able 24C8 would be much higher per ammonium site for any
linear, threadable polymers, and could explain the proportionally
greater increase in threading upon introduction of additional
crown. This topologically based threading technique was
particularly valuable, as it enabled quantitative determination
that the presence of linear contaminants C-17-nH·nPF6 was very
low and unambiguously showed that the sample contained near
or above 90% cyclic polymer.
Molecular Charm Bracelet Synthesis and Analysis. To enable
“clipping” of CP C-17-nH·nPF6, we had to develop a suitable
solvent system that would both solubilize the highly charged
polymer and facilitate the olefin metathesis reaction. Because
the polymer displayed good solubility in nitromethane, we
explored the possibility of using this as our RCM solvent. To
test the efficacy of this system, we monitored the effect of
nitromethane concentration on the conversion of protons Hk and
Hm to Hn upon clipping of the diolefin crown ether-type fragment
19 around the template 20-H·PF6 to form the pseudorotaxane
21-H·PF6 (Scheme 6).100 We observed that an increased
nitromethane volume fraction (relative to DCM) led to decreased
olefin conversion. Consequently, we concluded that a 1:1
mixture (v/v) of dichloromethane to nitromethane would
maximize the solubility of the polymer while still enabling
reasonable olefin conversion.
Though efforts to neutralize C-22-nH·nPF6 were unsuccess-
ful, conclusive evidence for the interlocked nature of the
molecular charm bracelet was obtained using 1H and 2D-DOSY
NMR spectroscopy. Analysis of the proton signals of the
interlocked crown showed significant line broadening100 relative
to the free, noninterlocked ring-closed analogue, a phenomenon
that is commonly observed for interlocked complexes56,63,69 and
arises from an increase in the rotational correlation time of the
interlocked crown as a result of increased MW and volume (due
to interlocking of the small ring with the large polymer).
Additionally, a downfield shift in the primary -CH2- crown
proton signals from 3.57 ppm to 3.64 ppm was observed, a
phenomenon attributed to subtle electron-withdrawing effects
from hydrogen-bonding interactions occurring between the
ammonium protons and crown oxygens. Though one-dimen-
1
sional H NMR analysis suggested the interlocked nature of
C-22-nH·nPF6, 2D-DOSY NMR provided convincing evidence.
This technique enabled the direct detection of the diffusion rates
of ring-closed product 23 after RCM in the absence and presence
of polymer (Figure 10A and 10B, respectively). Typically, the
diffusion values obtained from 2D-DOSY NMR are reported
as log D, where the diffusion constant, D, has units of m2 s-1
.
Thus, a smaller (more negative) diffusion value implies a slower
diffusion rate. For example, the large macromolecular template
C-17-nH·nPF6 had a diffusion value of log D ) -8.83
(slow),100 while free crown 23 had a faster diffusion value of
Synthesis of the molecular charm bracelet polymer was
achieved via addition of catalyst 1 to a solution of C-17-
Scheme 7. Graphical Representation of the Synthesis of Molecular Charm Bracelet C-22-nH·nPF6
9
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A R T I C L E S
Clark et al.
signals at the same diffusion rate indicated an intimate associa-
tion between the two species, only possible if 23 was interlocked
around the cyclic polymer framework. To rule out the possibility
of coincidental overlap between crown and polymer diffusion
values observed in Figure 10B, a sample of C-22-nH·nPF6 was
spiked with noninterlocked 23. If the slow diffusion rate of
crown signals in the 2D-DOSY NMR of C-22-nH·nPF6 (Figure
10B) originated from hydrogen-bonding interactions between
the crown and polymer and not from interlocking of the crown
around the polymer backbone, we would expect to see the
introduced, noninterlocked crown diffuse at a similarly slow
rate as the interlocked crown in C-22-nH·nPF6. However, 2D-
DOSY NMR analysis of a physical mixture of C-22-nH·nPF6
and 23 (Figure 10C) revealed a distinct difference between
diffusion rates of the two types of crown (log D ) -8.80 and
-8.52, bound and free, respectively), confirming the interlocked
nature of the crown in the molecular charm bracelet C-22-
nH·nPF6.
Conclusions
We have described a clipping approach to a polycatenated
cyclic polymer, a structure that resembles a molecular charm
bracelet. We have shown that the use of ring-opening metathesis
polymerization of a Boc-amine macromer in the presence of a
chain transfer agent allows for the synthesis of a linear polymer
that could be subsequently functionalized and cyclized to the
corresponding cyclic analogue. This cyclic polymer was char-
acterized through a variety of techniques and subjected to further
functionalization reactions, affording a cyclic polyammonium
scaffold. Diolefin polyether fragments were coordinated and
clipped around the ammonium sites within the polymer back-
bone using ring-closing olefin metathesis, giving the molecular
charm bracelet. Confirmation of the interlocked nature of the
product was achieved via 1H NMR spectroscopy and two-
dimensional diffusion-ordered NMR spectroscopy.
Acknowledgment. We thank Professor Daniel J. O’Leary, Dr.
David VanderVelde, Dr. Andrew J. Boydston, Dr. John. B. Matson,
and Yan Xia for helpful discussions. The work was supported by
the Office of Naval Research through its MURI program and the
NSF (NSF CHE-0809418). W.Y.C. acknowledges the NSERC of
Canada for a postdoctoral fellowship.
Figure 10. 2D-DOSY 1H NMR spectra at 400 MHz and 25 °C in CD3CN
of free ring-closed crown 23 (A), purified molecular “charm bracelet” C-22-
nH·nPF6 (B), and a physical mixture of C-22-nH·nPF6 and 23 (C). The
units of the diffusion constant D are m2 s-1
.
Supporting Information Available: Figures, tables, and text
giving complete experimental procedures as well as full
characterization data. This material is available free of charge
via the Internet at http://pubs.acs.org.
log D ) -8.45 (Figure 10A), expected behavior for a small
molecule. However, when the crown signals from C-22-
nH·nPF6 were analyzed by 2D-DOSY NMR, a significant
reduction in crown diffusion speed (log D ) -8.85) was
observed (Figure 10B). The alignment of crown and polymer
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