Supramolecular polymerization of a ureidopyrimidinone-based [2]catenane prepared via ring-closing metathesis

Article abstract of DOI:10.1002/pola.28694

The synthesis of a Sauvage-type [2]catenane featuring a quadruple hydrogen bonding ureidopyrimidinone (UPy) motif in each ring was reported. Intermolecular dimerization of the UPy motifs induces the hydrogen-bond-driven supramolecular polymerization of the [2]catenane monomer, thereby creating a linear polymer consisting of both hydrogen bonding and mechanical bonds. As the rings in the UPy catenane are asymmetric, two stereoisomers can be formed upon catenation, that is, with the phenanthroline moieties oriented +90° or -90° with respect to each other. Based on the phenanthroline-Cu(I) and ring-closing metathesis (RCM) approach, we first devised a synthetic procedure for the synthesis of the UPy-based catenane. Here, phenanthroline was first functionalized with phenol moieties in a two-step approach with an overall yield of 46%. The resulting biphenol 3 was then alkylated in a statistical manner with a mixture of 4-bromobut-1-ene and t-Boc-protected bromide resulting in t-Boc-protected compound. The results show that protection of the UPy motifs is necessary for this reaction to reach completion. Analysis of the unprotected UPy catenane by ¹H NMR revealed the formation of UPy-UPy dimers and significant broadening of the signals, both in presence and absence of Cu(I).

Full text of DOI:10.1002/pola.28694

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Supramolecular Polymerization of a Ureidopyrimidinone-Based
[2]Catenane Prepared via Ring-Closing Metathesis
1,2
Abraham J.P. Teunissen,1,2* Jose Augusto Berrocal,
*
ꢀ
1,2
Christiaan H.W.A. Corbet,2 E.W. Meijer
1Institute for Complex Molecular Systems, Eindhoven University of Technology, 5600 MB Eindhoven, P.O. Box 513,
Eindhoven, The Netherlands
2Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, 5600 MB Eindhoven,
P.O. Box 513, Eindhoven, The Netherlands
Correspondence to: E. W. Meijer (E-mail: e.w.meijer@tue.nl)
Received 23 April 2017; accepted 6 June 2017; published online 00 Month 2017
DOI: 10.1002/pola.28694
KEYWORDS: polycatenane; supramolecular polymer; ureidopyrimidinone; ring-closing-metathesis; mechanical bond.
INTRODUCTION Since its first report in 1993,1 Ru-catalyzed
systems has expanded the field toward interlocked polymeric
systems, such as polycatenanes9 and polyrotaxanes.10 The
ring-closing metathesis (RCM) has established itself as one of
flexibility of the mechanical bonds in these polymers is
the most reliable and efficient reactions to synthesize (macro)-
expected to give rise to unusual rheological and mechanical
cyclic molecules. Many of the catalysts prepared in the Grubbs
properties. Many covalent strategies have been developed to
laboratory are commercially available and show high tolerance
synthesize such interlocked polymers,9,11 including the ring-
to a number of functional groups, thereby making the reaction
opening metathesis polymerization (ROMP) of monomeric
widely applicable.2 It is even more remarkable that after many
[2]catenanes featuring one or two double bonds in each
years of improvement in catalyst design and efficiency, the
ring.12 Polymers consisting of covalent linkages alternated
“simple” first generation Grubbs catalyst (G1, Fig. 1) is still a
benchmark Ru-complex for RCM and olefin metathesis in gen-
eral. The immense versatility of G1 in RCM was further
highlighted when the first 1,10-phenanthroline-based (Phen,
Fig. 1) catenanes3a and knots3b were prepared upon merging
with [2]catenanes,13 and covalent polymers functionalized
with [2]catenane side-groups, have been made with average
degree of polymerization (DPn) up to approximately 25
repeating units.9,11 However, the challenges associated with
creating polymers purely consisting of interlocked rings has
limited their length to 5 units for linear structures14 and 7
the ingenious chemistries of Bob Grubbs and Jean–Pierre
units for interlocked networks.15 It follows that obtaining
Sauvage. Although kinetically controlled RCM (ethylene evolu-
tion) is the most frequently applied strategy to prepare these
interlocked architectures, selected studies have also exploited
the reversible character of the process4 by using internal ole-
fins5a and metal templates.5b
high DPn polymers solely composed of interlocked rings
remains an exciting synthetic challenge.
We envisaged that a strategy based on a [2]catenane mono-
mer which polymerizes via hydrogen bonding represents an
alternative to arrive at polymeric systems featuring mechani-
cal bonds. Recently, the self-complementary quadruple
hydrogen bonding ureidopyrimidinone (UPy, Fig. 1) motif
has been used to obtain “dynamic” [2]catenanes16a and pH-
Mechanically interlocked molecules are prime components of
molecular machines and have found applications in molecu-
lar switches,6 coatings,7 and electronics.8 The RCM-assisted
introduction of mechanical bonds in defined molecular
We would like to congratulate Bob with his 75th birthday and thank him for his inspirational contributions to science.
*These authors contributed equally to this article.
This is an open access article under the terms of the Creative Commons Attribution NonCommercial License, which permits use,
distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial
purposes.
Additional Supporting Information may be found in the online version of this article.
C
V
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biphenol 3 was then alkylated in a statistical manner with a
mixture of 4-bromobut-1-ene and t-Boc-protected bromide 4,
resulting in t-Boc-protected compound 5. Subsequently, 5
was deprotected using HCl, and amine 9 was coupled to CDI
activated isocytosine 8 to afford the UPy-functionalized phenan-
throline 10. Cu(I) complex 11 was then prepared in quantita-
tive yields by stirring 10 and commercially available
Cu(CH3CN)4PF6 in 2:1 molar ratio. As last part of the synthesis,
we planned to perform the G1-catalyzed RCM of 11 and subse-
quently remove Cu(I) to yield the desired UPy catenane 1.
To avoid the formation of intermolecular bonds, this reaction
was performed at a relatively low concentration of 10 mM.
Unfortunately, all our attempts to close the rings of 11 using
G1 resulted in insoluble precipitates, very likely cross-linked
networks deriving from intermolecular reactions. Similar
results were obtained using the second generation Grubbs cata-
lyst, or first or second generation Hoveyda–Grubbs catalysts.
FIGURE 1 Molecular structure of the first generation Grubbs
catalyst G1,
a phenanthroline (Phen)-based [2]catenane, a
dimer of the self-complementary ureidopyrimidinone UPy, and
the envisioned UPy-functionalized catenane. At high concentra-
tions, the UPys are expected to dimerize intermolecularly,
resulting in a supramolecular polycatenane. This compound
can exist as two stereoisomers, which are expected to be
formed in equal amounts. For clarity, only one of the two
stereoisomers is depicted throughout this article.
The RCM of 11 was then attempted at a lower concentration
(1 mM) to prevent the formation of intermolecular contacts.
Interestingly, ring-closure of just one of the phenanthroline-
based ligands of 11 was consistently obtained in all cases
(both E and Z isomers were formed). As Ru-catalyzed metathe-
sis reactions have been successfully performed on similar cate-
nanes5b,12 and UPy motifs,20 it is unlikely that deactivation of
the catalyst took place. Instead, we hypothesized that the first
ring-closure event induced the formation of an unusually stable
intramolecular UPy–UPy contact, which preorganizes the
unreacted terminal double bonds in an unfavorable manner
(see Supporting Information for more information).
actuated [c2] daisy chain rotaxanes,16b showing its compati-
bility with mechanically interlocked architectures. Inspired
by these examples, we present the synthesis of a symmetri-
cal Sauvage-type [2]catenane featuring a UPy motif in each
ring (Fig. 1). Intermolecular dimerization of the UPy motifs
induces the hydrogen-bond-driven supramolecular polymeri-
zation of the [2]catenane monomer, thereby creating a linear
polymer consisting of both hydrogen bonding and mechani-
cal bonds. Our supramolecular approach eliminates the need
of a covalent polymerization of the catenane monomers,
which potentially reduces the synthetic efforts. As the rings
in the UPy catenane are asymmetric, two stereoisomers can
be formed upon catenation, that is, with the phenanthroline
moieties oriented 1908 or 2908 with respect to each
other.15,17a In the absence of a driving force that favors the
formation of either isomer, we expect both to be formed in
equal amounts. For simplicity, we will show only one of
these isomers throughout this article.
All our attempts to disrupt the hypothesized intramolecular
UPy–UPy contact by performing the RCM in the presence of
methanol, a well-known hydrogen bond disrupting agent, or
a large excess of competing unfunctionalized UPys (10 eq.)
consistently afforded the closure of only one ring. It is possi-
ble that the first RCM allowed the formation of an intramolec-
ular UPy–UPy contact with a relatively high effective molarity.
As a result, no intermolecular competitive strategy19 could
induce the second RCM event to yield 1, which forced us to
develop another synthetic approach.
We hypothesized that covalently protecting the UPy moieties
with a benzyl group would destabilize the intramolecular UPy–
UPy dimerization21 and afford full conversion in the RCM. To
verify our hypothesis, we synthesized benzyl protected ligand
12, which afforded catenane precursor 13 after Cu(I) complex-
ation (Scheme 2). Gratifyingly, RCM on 13 resulted in almost
quantitative closure of both rings. After Cu(I) removal, benzyl
protected UPy catenane 14 was obtained as a mixture of EE,
EZ, and ZZ isomers. Unfortunately, all our attempts to remove
the benzyl protecting groups via a number of hydrogenation
procedures were unsuccessful, most likely due to coordination
of 14 to the catalysts used (see Supporting Information for
details on the procedures attempted).
Before arriving at the successful synthesis of the catenane-
based supramolecular polymer, we investigated a number of
synthetic routes that for a variety of reasons did not lead to the
target structure. We discuss them here in some detail to show
some remarkable synthetic findings, including a high selectivity
in the RCM. The aim of this article is to present the synthetic
strategy and furnish a proof of concept, while future work will
focus on a more detailed study of the material properties.
Based on the phenanthroline-Cu(I) and RCM approach, we
first devised a synthetic procedure for the synthesis of the
UPy-based catenane (Scheme 1). Here, phenanthroline was
first functionalized with phenol moieties in a two-step
approach with an overall yield of 46%.18 The resulting
To circumvent the need of using a catalyst, we opted for the
UV-labile o-nitrobenzyl protecting group21b,22 in the UPys.
2
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SCHEME 1 Synthetic procedure of UPy catenane 1. While compound 11 was successfully synthesized, the subsequent ring-
closing-metathesis was unsuccessful in closing both rings of 11. Instead, RCM using 5 wt% first generation Grubbs catalyst at
10 mM in DCE afforded an insoluble network, while RCM at 1 mM consistently afforded a compound with only one ring closed in
near quantitative yields.
Compound 17 was synthesized with a similar route to 14
(Scheme 2). This molecule was irradiated with UV light and
purified by column chromatography and preparative GPC.
The obtained UPy catenane 1 was characterized with 1H-
NMR and mass spectrometry. The characteristic low field
resonances (10–13 ppm range) observed in the 1H-NMR
spectrum are indicative of UPy–UPy dimerization, thereby
suggesting the formation of a supramolecular polymer (Fig.
UPy catenane 1 were broadened compared to the protected
UPy catenane 17 (Fig. 2). In addition, the observed changes
upon Cu(I) complexation are in good agreement with struc-
turally similar catenanes reported in literature.23 More spe-
cifically, the signal at 6 ppm observed in the presence of
Cu(I) results from the ArAH protons adjacent to the ether
bonds when catenated.23 Further characterization of the
solution-phase behavior of metalated and demetalated 17
and 1 was attempted by two-dimensional diffusion-ordered
1
2). Consistently, all H-NMR signals observed for deprotected
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SCHEME 2 Successful synthesis of UPy catenane 1. UPy-functionalized phenanthroline 10 was protected with benzyl and o-nitro-
benzyl ethers.
spectroscopy (2D-DOSY) NMR, but the broadness and signal
overlap in the NMR spectrum of (de)metalated 1 did not
offer reliable results.
metalated and demetalated 1 were prepared in CHCl3 at 1 mM
and compared to similar solutions of metalated and demeta-
lated o-nitrobenzyl protected 17. Featureless correlation func-
tions were measured for protected catenane 17, both in the
absence and presence of Cu(I), which suggests that these com-
pounds do not self-assemble. This is consistent with the sharp
As an alternative, the supramolecular polymerization of 1 was
investigated with dynamic light scattering (DLS). Solutions of
FIGURE 2 1H-NMR spectra of o-nitrobenzyl protected UPy catenane 17 in CDCl3 and deprotected UPy catenane 1 in CD2Cl2. The
insets shows the changes in the aromatic region upon the insertion of Cu(I). The signals at 6 ppm observed upon Cu(I) coordina-
tion are indicative of catenation and result from the ArAH protons directly adjacent to the ether bonds connecting the aromatic
rings and aliphatic spacers. [Color figure can be viewed at wileyonlinelibrary.com]
4
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1H NMR spectra measured for both molecules. In stark contrast,
the 1 mM CHCl3 solutions of metalated 1 provided correlation
functions characterized by long decay times corresponding to
sizes in the 100 nm to 1 mm range (see Supporting Informa-
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tion). Hence, the consistency with the H NMR measurements
was also respected for metalated 1. Surprisingly, studies of
demetalated UPy catenane 1 did not reveal any large aggregates
in solution, in poor agreement with the broadened 1H-NMR sig-
nals observed. A plausible explanation may lie in the previously
mentioned intramolecular dimerization of the UPy moieties.
Such intramolecular contacts would severely limit the polymeri-
zation process to very short oligomers at the examined concen-
tration. Although further characterization is required to fully
understand the polymerization and material properties of 1,
the procedures reported herein pave the way to the synthesis
of larger quantities and more detailed analyses.
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In conclusions, we have reported on the synthesis of a
Sauvage-type [2]catenane featuring a quadruple hydrogen
bonding UPy motif in each ring and have shown that this motif
allows supramolecular polymerization of the catenane via
UPy–UPy dimerization. The paramount reaction in our
approach was the Grubbs catalyzed RCM, which proved excel-
lently suited to create the necessary interlocked structure of
the catenane monomer. Our results also show that protection
of the UPy motifs is necessary for this reaction to reach com-
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pletion. Analysis of the unprotected UPy catenane by H NMR
revealed the formation of UPy–UPy dimers and significant
broadening of the signals, both in presence and absence of
Cu(I). Such broadening is in line with DLS measurements per-
formed on the unprotected metalated UPy catenane, which
showed the presence of aggregates with sizes in the 100 nm to
1 mm range in solution. The absence of such aggregates
observed for the deprotected demetalated UPy catenane is
believed to result from the formation of intramolecular UPy
contacts. Such contacts will severely limit the polymerization
to very short oligomers, which might therefore not be detected
by DLS. Our approach represents an alternative method to form
polymers containing mechanically interlocked junctions, and
aids the development of functionalized catenanes in general.
Future work will focus on the characterization of the supramo-
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ACKNOWLEDGMENTS
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The authors would like to thank Ralf Bovee and Dr Xianwen
Lou for their help with the mass analysis. Mr Gijs M. ter Huurne
is acknowledged for DLS measurements. This work is financed
by the Dutch Organization for Scientific Research (Graduation
School and TOPPUNT Program) and the Dutch Ministry of
Education, Culture and Science (Gravity program 024.001.035).
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