Control over the macrocyclisation pathway and product topology in a copper-templated catenane synthesis

Article abstract of DOI:10.1039/c9cc02263e

We report here that the product topology in a copper-templated catenane synthesis can be controlled by favouring a particular macrocyclisation pathway, offering an additional strategy for improving the efficiency of catenane formation. A linear [4]catenan

Full text of DOI:10.1039/c9cc02263e

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DOI: 10.1039/C9CC02263E
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Control over the macrocyclisation pathway and product topology
in a copper-templated catenane synthesis
Chi-Chung Yee,a Antony Wing Hung Nga and Ho Yu Au-Yeunga,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report here that the product topology in a copper-templated
Clearly, strategies for preorganisation and ring-closing are
catenane synthesis can be controlled by favouring a particular not sufficient for macrocycle interlocking and catenane
macrocyclisation pathway, offering an additional strategy for formation. A further understanding on how the macrocycles
improving the efficiency of catenane formation.
A linear are formed from the templated precursors is necessary for a
[4]catenane was obtained by non-covalently modifying a flexible predictable and reliable synthesis of molecular links. While
building block that favours the intra-ligand cyclisation.
structural modifications on the building blocks to optimise the
stability and orientation of the preorganised precursors to
improve the yields and topological selectivity is a viable
strategy,9-12 how the precursor intertwining and macrocycle
interlocking are affected by the subtle and dynamic
intermolecular interactions is not always obvious and clear.13
On the other hand, strategies to control the macrocyclisation
pathway could be a more straightforward and predictable
strategy to achieve selectivity in product topology. By a good
understanding of the structural features of the preorganised
precursors and a judicious design of the complementary
building blocks, some macrocyclisation pathways may be
(dis)favoured to control the product topology. Here, we report
the effect of covalent and non-covalent modification of the
building block on the topological outcome in a copper-
templated, imine-based catenane synthesis. By favouring the
intra-ligand macrocyclisation, mechanically interlocked Hopf
links and a linear [4]catenane were obtained in good yields.
Cu+-bis(phenanthroline) was chosen as a model template
because of its wide application in the synthesis of different
interlocked molecules.14 Due to the tetrahedral geometry of
Cu+, the two phenanthrolines are orthogonally arranged such
that the intra-ligand macrocyclisation will give a Hopf link (Fig.
1c, green pathway), whereas the inter-ligand macrocyclisation
will produce the topologically trivial molecular figure-eight
(Fig. 1c, blue pathway). If the intra- or inter-ligand
macrocyclisation can be promoted or suppressed by the use of
an appropriate building block, formation of the interlocked
rings could be controlled. X-ray crystallographic analysis of
[Cu(L1)2][PF6] showed the expected tetrahedral geometry.
Stacking of the benzaldehyde and phenanthroline units are
observed which may further stabilise the complex and direct
The unique mechanical, co-conformational and stereochemical
properties of molecular links have recently emerged to
different applications including molecular recognition,1
sensing,2 catalysis3 and constructing molecular machines4
which may not be achievable by their non-interlocked
counterparts. Developing strategies to the efficient formation
of interlocked macrocycles are therefore of central importance
in the field. Although some simple catenanes (e.g. Hopf link)
can now be considered to be straightforwardly accessible,5
there are also reports of unsuccessful macrocycle interlocking
despite of the use of some well-studied templates and ring-
closing reactions. For example, it has been shown that the
double macrocyclisation of two orthogonally arranged ligands
on a Fe(II),6 Co(III)7 or Pd(II)8 template gave no catenanes but
only trivial macrocycles. Two independent works that both
feature the ring-closing alkene metathesis of three pyridyl-2,6-
diamine ligands coordinated on a lanthanide template also
showed that a [3]catenane was obtained in one case,9 whereas
a mixture of a trefoil knot and a trivial macrocycle was resulted
in another.10 Few other examples that different topologies
were obtained from the same template and ring-closing
macrocyclisation have also been reported.11 In all these cases,
the subtle differences in the building block structures may
have led to a different stability and/or orientation of the
templated building blocks, which eventually resulted in
different macrocyclisation pathways that affected their
interlocking and the overall product topology.
a. Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong
Kong, P. R. China. Email: hoyuay@hku.hk
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
1
the benzaldehydes to an orthogonal orientation. H NMR of
[Cu(L1)2]+ showed upfield shifts of the benzaldehyde protons
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by ~0.6–1 ppm when compared with that of L1, suggesting the
stacking interactions and orthogonal orientation are
maintained in solutions (Fig. S33). The intra- and inter-ligand
aldehyde distances are ca. 8.2 Å and 13.3–15.8 Å respectively.
Ring-closing with a relatively short diamine could therefore
favour the intra-ligand cyclisation to give a Hopf link, whereas
a molecular figure-eight could be obtained by using a relatively
long diamine. Of note, it is more common in literatures that
only the yield of the (desired) interlocked molecules is
reported, and the formation of other topology, especially the
non-interlocked products, are rarely discussed. Detail analysis
on building block structure and topological selectivity in a
templated synthesis are therefore highly limited.15
View Article Online
DOI: 10.1039/C9CC02263E
Fig. 2. (a) Cu+-templated synthesis using L1 and DN3, and the
LC chromatogram of the product mixture. Also shown are the
ESI-MS and MS2 spectra of (b) F3 and (c) H3.
addition, δ of the phenanthroline protons in H3 are only
slightly upfield shifted by ~0.1 ppm when compared with that
of [Cu(L1)2]+, suggesting the Cu+-phenanthroline core remains
in a similar chemical environment. On the other hand,
significant shifts were observed for the phenanthroline
protons of F3, showing that the Cu+-phenanthroline core is
having a different conformation. In addition, NOE cross peaks
between HDN and HAr were observed in F3 but not in H3,
suggesting the aromatic units are in close proximity in the
figure-eight but not the Hopf link (Fig. S43). Both the 1H
spectra showed one set of signals that suggest a symmetrical
conformation for H3 and F3 on the NMR timescale.
Since the intra-ligand aldehyde distance is relatively
shorter than of the inter-ligand, the use of a shorter diamine
could favour the formation of the Hopf link. As expected, when
the number of ethylene glycol units in DN3 was decreased to 3
(DN2) and 2 (DN1), an increase in the yield of the Hopf link
from 28% to 43% to 66% at the expense of the figure-eight was
observed (Table 1). Ratio of the Hopf link to figure-eight
increases from 0.48 to 1.06 to 3.14, with only a small variation
in the yields of the [1+1] macrocycles. On the other hand, a
further increase in the number of ethylene glycol units to 5
(DN4) and 6 (DN5) did not change significantly the ratio of the
two products, and that the Hopf link to figure-eight ratio is
kept at around 0.5. The plateauing of the Hopf link to figure-
eight ratio could be due to the dynamic nature of the imine
which results in a statistical ratio of the intra- and inter-ligand
macrocyclisation. When the diamines are of a length such that
the two [2+2] products are energetically similar, a 2:1 ratio of
the inter- to intra-ligand macrocyclisation would be resulted
upon the formation of the first imine on the tetrahedral
template. In all cases, the [1+1] macrocycle was observed in
13–17% yield despite of the different length of the diamines. It
Fig. 1. (a) Chemical structures of the dialdehyde and diamine
building blocks; (b) X-ray structure of [Cu(L1)2]+; and (c)
possible products from different macrocyclisation pathways on
a tetrahedral template.
The copper complex [Cu(L1)2]+ was condensed with a series
of dioxynaphthalene-derived diamine of various lengths. The
yield and topology of all the condensation products were
analysed after reducing the dynamic imines to the
corresponding amines. LCMS studies of a reaction mixture
from [Cu(L1)2]+/DN3 showed three major products: a [1+1]
macrocycle M3 in 14% yield and two isomeric [2+2] species
(Fig. 2a). Topology of the two [2+2] products were
characterised by MS2, in which the major [2+2] product (F3,
58%) was suggested to be a trivial macrocycle because a
sequential fragmentation with fragments larger than that of
M3 was observed (Fig. 2b). On the other hand, the minor [2+2]
product (H3, 28%) was suggested to be a Hopf link due to its
direct fragmentation to the [1+1] species (Fig. 2c). Notably, F3
and H3 are structural isomers (different bond connectivity) of
different topology, and that H3 and a pair of M3 are
topological isomers that differ only in their topology.16
H3 and F3 were further characterised by 1H NMR. As a
figure-eight is chiral, Hd (ca. 4.4 ppm) in the Cu+-coordinated,
racemic F3 are diastereotopic and were observed as a pair of
doublet but remained as a singlet for the achiral H3 (Fig. 3). In
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Table 1. Distribution of products of diffDerOeI:n1t0.t1o0p39o/lCo9gCieCs02f2ro63mE
templated syntheses using [Cu(L1)2]+ and [Cu(L2)2]+. Note that
H13, F13 and M13 are respectively a [4]-, [3]- and [2]catenane.
[2+2]
[2+2]
[1+1]
intra:
inter
diamine
Hopf link
Figure-eight
Macrocycle
DN1*
DN2
H1b, 66%
H2, 43%
H3, 28%
H4, 28%
H5, 29%
H6, 82%
H7, 86%
H8, 84%
H9, 82%
H10, 82%
H11, 83%
H12b, 20%
H13, 77%
F1b, 21%
M1, 13%
M2, 17%
M3, 14%
M4, 16%
M5, 15%
M6, 18%
M7, 14%
M8, 16%
M9, 18%
M10, 18%
M11, 17%
M12, 11%
M13, 15%
3.14
1.06
0.48
0.50
0.52
/
F2, 40%
DN3
F3, 58%
DN4
F4, 56%
DN5
F5, 56%
Alk7
/
Alk8
/
/
1
Fig. 3. Partial H NMR (D2O, 500 MHz, 298 K) of (a) F3 and (b)
H3.
Alk9
/
/
Alk10
Alk11
Alk12
Hex*
/
/
is likely that these [1+1] macrocycles are formed from a non-
preorganised ligand rather than a “back-side” macrocyclisation
which is highly improbable (Fig. 1c, red). Based on these
observations, it appears that the observed selectivity may be
thermodynamic in origin, yet the need of imine reduction for
the analysis may present additional complication in
understanding the nature of the Hopf/figure-eight selectivity.
Of note, DN1 to DN5 differ only by the number of the flexible
ethylene glycol units with no obvious curvature, rigidity or
moiety for additional interactions. With the same template
and preorganised precursors being used, the difference in the
Hopf link to figure-eight ratio is probably due to the different
macrocyclisation pathway but not the stability or orientation
of the preorganised precursors.10,17
/
/
/
/
F12b, 68%
F13, 8%
0.29
8.56
Hex∙CB[6]
* Determined after demetallation because of peak overlapping.
adopting a relatively rigid conformation in solution (Fig. S74).
The comparable δ values for the aromatic protons of H13 to
that of H3, and the observed singlet of Hd (at ~4.8 ppm at 358
K) are also consistent with the linear [4]catenane topology. On
the other hand, inter-ligand macrocyclisation using Hex∙CB[6]
gave the [3]catenane F13 with a twisted central macrocycle in
a figure-eight conformation in only 8% yield, corresponding to
a switch in the product selectivity by about 30 times (Fig. 4).
Further demonstration of controlling the topological
selectivity by favouring the intra-ligand macrocyclisation
comes from another set of templated synthesis using
[Cu(L1)2]+ and ,-alkyldiamines. Because of the significantly
shorter length of the ,-alkyldiamines, the inter-ligand
macrocyclisation could be further suppressed to favour Hopf
link formation. Consistent with this proposal, LCMS analysis of
mixtures from the reaction of [Cu(L1)2]+ and Alkn (n = 7–12) all
showed the Hopf link as the only [2+2] product with no
molecular figure-eight, indicating that the intra-ligand
macrocyclisation is exclusively favoured (Table 1). The Hopf
1
The H spectrum of F13 consists of only broad signals that are
sharpened slightly at higher temperature, suggesting that F13
is conformationally more flexible than H13. Of note, linear
[n]catenanes are often obtained in low yields and are relatively
rare when compared to radial counterparts.5a,20 The present
efficient synthesis of H13 could be implicated in the synthesis
of other high-order catenanes that are to be realised.
In summary, control over the product topology in a
templated synthesis using either covalent or non-covalent
strategies is demonstrated, and a rare linear [4]catenane was
obtained in one simple step in 77% yield. In addition to
conventional optimisation of the non-obvious and subtle
stability/orientation of the preorganised precursors and
limiting possible macrocyclisation pathways, the present
results offer straightforward and alternative strategies for the
simple and effective control over the product topology in a
templated synthesis which is central to both the fundamental
chemistry and new applications of molecular links and knots.
This work is supported by the Croucher Foundation and a
grant from the Research Grants Council of the Hong Kong SAR,
China (HKU 27300014). C.-C. Y. and A. W. H. N. are recipients
of the Postgraduate Scholarship from The University of Hong
1
links H6 to H11 have been characterised by MS2, H and 13C
NMR and are consistent with their interlocked structures.
In addition to the use of diamine with a different covalent
structure, effects of non-covalent modification of the building
block were also investigated. Similar to DN3–DN5,
condensation of Hex with [Cu(L2)2]+ gave a mixture with the
non-interlocked F12b as the major product in 68% yield,18
showing that the inter-ligand macrocyclisation is preferred.
Interestingly, binding of CB[6] to the central hexanediamine
unit that converted Hex to a [2]pseudorotaxane resulted in a
change of the preferred macrocyclisation pathway to intra-
ligand,19 and the linear [4]catenane H13 was obtained in 77%
1
yield. A single set of temperature insensitive H resonances
was observed for H13, showing that the [4]catenane is
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Kong. We acknowledge UGC funding administered by The
University of Hong Kong for support of the Electrospray
Ionisation Quadrupole Time-of-Flight Mass Spectrometry
Facilities under the support for Interdisciplinary Research in
Chemical Science.
Conflict of interest
There are no conflicts to declare.
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DOI: 10.1039/C9CC02263E
Strategies to control building block intertwining and the efficient assembly of a linear [4]catenane
are presented.