Catenation through a Combination of Radical Templation and Ring-Closing Metathesis

Article abstract of DOI:10.1021/jacs.5b10623

Synthesis of an electrochemically addressable [2]catenane has been achieved following formation by templation of a [2]pseudorotaxane employing radically enhanced molecular recognition between the bisradical dication obtained on reduction of the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), and the radical cation generated on reduction of a viologen disubstituted with p-xylylene units, both carrying tetraethylene glycol chains terminated by allyl groups. This inclusion complex was subjected to olefin ring-closing metathesis, which was observed to proceed under reduced conditions, to mechanically interlock the two components. Upon oxidation, Coulombic repulsion between the positively charged and mechanically interlocked components results in the adoption of a co-conformation where the newly formed alkene resides inside the cavity of the tetracationic cyclophane. ¹H NMR spectroscopic analysis of this hexacationic [2]catenane shows a dramatic upfield shift of the resonances associated with the olefinic and allylic protons as a result of them residing inside the tetracationic component. Further analysis shows high diastereoselectivity during catenation, as only a single (Z)-isomer is formed.

Full text of DOI:10.1021/jacs.5b10623

Communication
pubs.acs.org/JACS
Catenation through a Combination of Radical Templation and Ring-
Closing Metathesis
Ian C. Gibbs-Hall, Nicolaas A. Vermeulen, Edward J. Dale, James J. Henkelis, Anthea K. Blackburn,
Jonathan C. Barnes, and J. Fraser Stoddart*
Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States
*
S Supporting Information
exhibit radical−radical dimerization and have allowed for the
expedient syntheses of otherwise inaccessible MIMs. The stable
inclusion complex formed between a reduced viologen radical
cation and diradical dicationic state of the cyclophane has
ABSTRACT: Synthesis of an electrochemically address-
able [2]catenane has been achieved following formation by
templation of a [2]pseudorotaxane employing radically
enhanced molecular recognition between the bisradical
dication obtained on reduction of the tetracationic
cyclophane, cyclobis(paraquat-p-phenylene), and the rad-
ical cation generated on reduction of a viologen
disubstituted with p-xylylene units, both carrying tetra-
ethylene glycol chains terminated by allyl groups. This
inclusion complex was subjected to olefin ring-closing
metathesis, which was observed to proceed under reduced
conditions, to mechanically interlock the two components.
Upon oxidation, Coulombic repulsion between the
positively charged and mechanically interlocked compo-
nents results in the adoption of a co-conformation where
the newly formed alkene resides inside the cavity of the
11
allowed for the synthesis of (i) a series of rotaxane derivatives
devoid of stabilizing interactions in their oxidized states, (ii) a
12
homo[2]catenane harboring a stable organic radical, and (iii)
13
an artificial molecular pump capable of exhibiting an energy
ratchet mechanism where components are shifted away from
equilibrium toward higher local concentrations. While these
MIMs and their properties are novel, a wider variety of methods
for their synthesis would be welcome.
Scheme 1. Synthesis of the Hexacationic [2]Catenane 4·6PF6
1
tetracationic cyclophane. H NMR spectroscopic analysis
of this hexacationic [2]catenane shows a dramatic upfield
shift of the resonances associated with the olefinic and
allylic protons as a result of them residing inside the
tetracationic component. Further analysis shows high
diastereoselectivity during catenation, as only a single
(
Z)-isomer is formed.
1
lefin ring-closing metathesis (RCM) has been employed
O
to construct a diverse array of mechanically interlocked
2
molecules (MIMs), specifically catenanes. This reversible
reaction has proven to be effective in the synthesis of relatively
3
small ring compoundstypically five- to eight-membered ones.
The additional robustness of Grubbs’s ruthenium catalysts has
allowed RCM to be applied to much larger ring systems and so
feature in catenane synthesis, a situation in which macro-
cyclization proceeds in the presence of a wide range of
templation procedures under both neutral and oxidative
conditions. In these synthetic protocols, transition metal−ligand
4
5
6
8
coordination, donor−acceptor interactions, and both neutral
7
We report herein the synthesis and characterization of an
and charged hydrogen-bonding, as well as anion recognition,
6
+
electrochemically addressable [2]catenane, 4 (Scheme 1),
where Grubbs’s RCM is employed to install the mechanical bond
have served as sources of template direction, providing access to
catenated compounds.
Recently, we demonstrated that radically enhanced molecular
•+
2(•+)
9
following the radically driven inclusion of 1 by CBPQT
The existence of the mechanical bond in 4 was confirmed in the
solution phase by H NMR and UV-vis-NIR spectroscopies as
.
6
+
recognition offers a route toward positively charged MIMs in
which the individual molecular components, in their ground
states, naturally repel one another. Here, intermediate
bipyridinium radical cations present in viologens and cyclobis-
1
Received: October 10, 2015
10
4+
(
paraquat-p-phenylene) (CBPQT ) when they are reduced
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b10623
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Journal of the American Chemical Society
Communication
well as cyclic voltammetry (CV). Degradation analysis led to the
system, in keeping with the Beer−Lambert law. Conversely, plots
3
(•+)
identification of high diastereoselectivity during the carbon−
of the absorbance at 850 nm against the concentrations of 4
6
+
carbon double bond formation in 4 when compared to a
afford a linear fit, indicative of a single compound in solution (see
Figure S26).
control involving the formation of the non-catenated macrocycle.
The [2]catenane 4·6PF was obtained (Scheme 1) following a
Data obtained by electrochemical analysis of a 0.5 mM
solution of 4·6PF in Me CO using CV at various scan rates
6
sequence of reactions that involved (i) the allylation of
tetraethylene glycol to afford its mono-allyl ether 2, which was
then (ii) reacted (NaH/THF) with 1,4-bis(bromomethyl)-
benzene to give the benzylic bromide 3 which, (iii) on reaction
with 4,4′-bipyridine in MeCN (40 °C), followed by counterion
exchange (NH PF /H O), yielded the bis-allyl ether 1·2PF .
6
2
(Figure 2) allowed us to qualitatively distinguish the [2]catenane
4
6
2
6
After 1·2PF was dissolved in Me CO, it was treated with a large
6
2
excess of Zn dust under N in a glovebox in the presence of
2
CBPQT·4PF , producing a dark blue solution which indicates
6
the initial formation of cationic bipyridinium radicals, giving way
rapidly to a deep purple solution, indicative of the formation of a
14
trisradical complex. Following the addition of 20 equiv of
HOAc, the second-generation Hoveyda−Grubbs catalyst was
•
+
added to promote macrocyclization of 1 under radical
2
(•+)
templation conditions with CBPQT
to give the [2]catenane.
Following treatment of the crude reaction product with an excess
of HCl, it was subjected to reverse-phase HPLC with TFA in
H O and MeCN as eluents. Treatment of the hexachloride with
2
NH PF , after removal of the MeCN, produced 4·6PF in 30%
4
6
6
yield.
Figure 2. CV spectra of 0.5 mM (a) 4 and 0.5 mM (b) BV2+/
6+
CBPQT4 as PF6 salts in Me CO, 0.1 M TBAPF6 supporting
+
−
2
electrolyte, at increasing scan rates (mV/s). At low scan rates (100−
2
50 mV/s), a single oxidation peak near −100 mV is indicative of rapid
•
+
dissociation between the partially reduced benzyl viologen (BV ) and
CBPQT2
CBPQT2
(•+)
(•+)
subunits of (a) 4
2(•+)(2+)
and separate (b) BV and
•+
species before oxidation to their fully oxidized states. At
higher scan rates (500−1500 mV/s), a new oxidation peak at +50 mV is
observed, indicating the presence of the diradical inclusion complexes
associated with (a) 42
(•+)(2+)
and (b) BV ⊂CBPQT
•+
(•+)(2+)
during the
, there
2
(•+)(2+)
CV measurements. Because of the interlocked nature of 4
is a higher population of the radical metastable state.
from a 0.5 mM solution containing a mixture of the separate
4
+
2+
9
components, CBPQT and BV . Previous work has
2
(•+)
demonstrated that CBPQT
inclusion complexes with
•
+
partially reduced viologen guests such as BV are able to
undergo two 1e− oxidations at separate potentials from the
2
+
4+
proposed transient species BV ⊂CBPQT . Following the first
−
•+
1
e
reduction, the metastable intermediate complex BV ⊂
(
•+)(2+)
CBPQT
the CBPQT
is destabilized from the dicationic component of
host, and the complex dissociates rapidly to
(
•+)(2+)
4
+
2+
afford CBPQT and BV (see Figure S22a). The equilibration
•
+
(•+)(2+)
of the oxidized species from the BV ⊂CBPQT
is rapid
on the time scale of the experiment and can only be observed
using scan rates exceeding 1000 mV/s. Figure 2b shows the
emergence of a peak at +50 mV, of rising intensity with increasing
Figure 1. UV-vis spectra of 43
(•+)
(a) and BV ⊂CBPQT
•+
2(•+)
(b) in
−
•+
Me CO at increasing concentrations.
scan rate, corresponding to the two 1e oxidations of the BV
2
(
•+)(2+)
•+
(•+)(2+)
and CBPQT
components in BV ⊂CBPQT
to
2
+
4+
16
UV-vis spectroscopy was employed (Figure 1) in a comparison
their respective BV and CBPQT states. The analogous
peak at +50 mV can be observed (Figure 2a) in the 0.5 mM 4·
6PF sample but with greater intensity on account of a shift in the
4
+
of 4·6PF to an equimolar mixture of CBPQT and the guest,
6
2
+
N,N′-dibenzyl-4,4′-bipyridinium (BV ). Solutions of the
6
reduced 43
(•+)
(Figure 1a) and BV ⊂CBPQT
•+
2(•+)
(Figure 1b)
equilibrium to the BV ⊂CBPQT
•+
(•+)(2+)
co-conformation of the
and away from the corresponding BV
separated co-conformation (see Figure
2
(•+)(2+)
•+
were analyzed between 450 and 1400 nm. The emergence of a
components of 4
15
(•+)(2+)
peak at 850 nm associated with viologen radical dimerization
and CBPQT
S22b, middle entry).
was observed for both the [2]catenane and the corresponding
mixture of components. Plots of the absorbance at 850 nm for
1
Figure 3 presents the H NMR spectra recorded in CD CN for
CBPQT , 4 , and 5 in order that a comparison can be drawn
3
•
+
2(•+)
4+ 6+
2+
BV ⊂CBPQT
versus concentration (see Figure S24)
revealed an exponential trend, indicative of a two-component
between the resonances for the [2]catenane and those for its
B
DOI: 10.1021/jacs.5b10623
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Journal of the American Chemical Society
Communication
isomerthe (Z)-isomeris present in the mechanically
1
interlocked macrocycle. See the H NMR spectra in Figures 3b
and S13.
6
+
The diastereoselectivities of the RCMs in the synthesis of 4
2+
and the free macrocycle 5 were probed (Figure 4) by subjecting
the two compounds to the same degradation conditions.
Aqueous solutions of 4·6PF and 5·2PF were treated with an
6
6
excess of base (NaOD) to afford the diols (Z)-7 and (E/Z)-7,
respectively, as shown in Figure 4a,b, after hydrolytic removal of
1
the bipyridine units from their respective macrocycles. The H
NMR spectrum (Figure 4c) recorded in CD CN of the diol from
3
4
·6PF reveals the presence of only the (Z)-isomer, whereas the
6
1
H NMR spectrum (Figure 4d) of the diol from 5·2PF reveals
6
the presence of both the (Z)- and (E)-isomers in a ratio of 5:2.
In summary, we have described the synthesis of a hexacationic
[2]catenane, constructed through olefin ring-closing metathesis
following radical−radical templation under reducing conditions.
Solution-phase analysis through UV-vis-NIR spectroscopy and
cyclic voltammetry reveals that the radical−radical dimerization
observed within the [2]catenane is indicative of a single
molecular species. Coulombic repulsion between the tetra- and
dicationic rings of the fully oxidized [2]catenane forms a single
1
co-conformation, as observed by H NMR spectroscopy. Further
spectroscopic analyses show that the olefinic and allylic
resonances are shifted significantly upfield as a result of residing
within the tetracationic cavity of cyclobis(paraquat-p-phenylene)
on account of Coulombic repulsion experienced within the
molecule. High diastereoselectivity within the [2]catenane differs
from that of a free macrocycle synthesized through RCM under
1
Figure 3. H NMR spectra in CD CN at room temperature of (a)
3
CBPQT·4PF , (b) the [2]catenane 4·6PF , and (c) the non-catenated
macrocycle 5·2PF . The resonances associated with the alkene and
allylic protons (c, (Z/E)-H and (Z/E)-H ) of 5·2PF are shifted upfield
significantly (b, (Z)-H′ and (Z)-H ′) in 4·6PF . The accidentally
equivalent resonances for the constitutionally heterotopic protons (c, Hc
6
6
6
a
6
a
6
and H ) associated with the phenylene rings in the non-catenated
d
macrocycle 5·2PF become anisochronous (b, H ′ and H ′) when the
6
c
d
macrocycle is a component of the [2]catenane 4·6PF .
6
component rings. With the exception of resonances for the p-
xylylene ring, the chemical shift differences for the aromatic
2
+
6+
protons on going from 5 (Figure 3b) to 4 (Figure 3c) are not
all that dissimilar. While the resonances for the O-methylene
protons in the glycol loops in these two compounds experience
some small changes in chemical shifts, by far the most
pronounced differences are observed for the olefinic and
neighboring allylic protons. Significant upfield shifts are
registered in the case of the [2]catenane for the olefinic ((Z)-
H′) and allylic ((Z)-H ′) proton resonances at 2.75 and 2.53
a
ppm, compared with 5.63 ((Z)-H) and 3.83 ppm ((Z)-H ),
a
respectively, in the macrocycle. The (E)-isomer for the
macrocycle also has resonances at 5.50 and 3.93 ppm for the
respective olefinic and allylic protons. The dramatic upfield shifts
1
6+
for these resonances in the H NMR spectrum of 4 suggest the
olefinic and allylic portions of the mechanically interlocked
macrocycle are spending nearly all of their time in the cavity of
4+
the CBPQT ring.
1
1
The H− H NOESY NMR spectrum (see Figure S11) of the
2]catenane reveals through-space correlations between the
[
olefinic and allylic protons in the macrocyclic component with all
4
+
the protons (H , H , and H ) on the CBPQT ring
α1
β1
xyl
component. A noteworthy observation resides in the difference
between the stereochemistry of the alkene fragment in the
[
2]catenane and that associated with the free macrocycle 52+
prepared by RCM using the second-generation Hoveyda−
Figure 4. Degradation conditions of (a) 4·6PF and (b) 5·2PF , and the
6
6
2
+
1
Grubbs catalyst. In the case of 5 , a mixture of the (Z)- and (E)-
isomers associated with the alkene fragment was obtained in a 5:2
ratio, whereas, in the case of the [2]catenane, only a single
resulting H NMR spectra (c,d) in CD CN/D O at room temperature,
3
2
of the respective [2]catenane- and bipyridinium-based macrocycle after
exposure to an excess of NaOD.
C
DOI: 10.1021/jacs.5b10623
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Journal of the American Chemical Society
Communication
ambient conditions, as only the (Z)-isomer is present in the
mechanically interlocked molecule. This synthetic protocol
serves as a proof-of-concept demonstrating that olefin metathesis
is able to proceed in the presence of persistent organic radical
cations and can be utilized in the synthesis of increasingly
elaborate mechanically interlocked compounds.
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■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.5b10623.
(
8) (a) Sambrook, M. R.; Beer, P. D.; Wisner, J. A.; Paul, R. L.; Cowley,
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Allinson, E. S. H.; Lankshear, M. D.; Ng, K.-Y.; Cowley, A. R.; Serpell, C.
Experimental procedures and spectral data ( H and 13C
NMR, HRMS, CV) for all new compounds, as well as
NOESY and DOSY spectra of 4·6PF (PDF)
1
6
́
J.; Santos, S. M.; Costa, P. J.; Felix, V.; Beer, P. D. RSC Adv. 2011, 1, 995.
AUTHOR INFORMATION
(f) Evans, N. H.; Rahman, H.; Leontiev, A. V.; Greenham, N. D.;
Orlowski, G. A.; Zeng, Q.; Jacobs, R. M. J.; Serpell, C. J.; Kilah, N. L.;
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■
*
Corresponding Author
stoddart@northwestern.edu
Y.; Ruiz-Gonzal
.; Perez, E. M. Angew. Chem., Int. Ed. 2014, 53, 5394. (h) Mercurio, J.
M.; Caballero, A.; Cookson, J.; Beer, P. D. RSC Adv. 2015, 5, 9298.
9) Trabolsi, A.; Khashab, N.; Fahrenbach, A. C.; Friedman, D.; Colvin,
́
ez, L.; Torres-Pardo, A.; Casado, S.; Martín, N.; Rubio,
Notes
A
́
́
The authors declare no competing financial interest.
(
M. T.; Cotí, K. K.; Benítez, D.; Tkatchouk, E.; Olsen, J.-C.; Belowich, M.
E.; Carmielli, R.; Khatib, H. A.; Goddard, W. A., III; Wasielewski, M. R.;
Stoddart, J. F. Nat. Chem. 2010, 2, 42.
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Stoddart, J. F.; Williams, D. J. Angew. Chem., Int. Ed. Engl. 1988, 27, 1547.
ACKNOWLEDGMENTS
■
This research is part of the Joint Center of Excellence in
Integrated Nano-Systems (JCIN) at the King Abdulaziz City of
Science and Technology (KACST) and Northwestern Uni-
versity (NU). The authors thank KACST and NU for their
continued support of this research. I.C.G.-H. thanks the National
Defense Science and Engineering Graduate Fellowship
(b) Barnes, J. C.; Juríce
J. Org. Chem. 2013, 78, 11962.
11) (a) Li, H.; Fahrenbach, A. C.; Dey, S. K.; Basu, S.; Trabolsi, A.;
Zhu, Z.; Botros, Y. Y.; Stoddart, J. F. Angew. Chem., Int. Ed. 2010, 49,
260. (b) Li, H.; Zhu, Z.; Fahrenbach, A. C.; Savoie, B. M.; Ke, C.;
̌
k, M.; Vermeulen, N. A.; Dale, E. J.; Stoddart, J. F.
(
(
FA9550-11-C-0028) from the U.S. Department of Defense.
8
E.J.D. acknowledges the award of a Graduate Research
Fellowship from the National Science Foundation (NSF) and
a Ryan Fellowship from the NU International Institute for
Nanotechnology (IIN). A.K.B. thanks Fulbright New Zealand
for a Fulbright Graduate Award and the New Zealand Federation
of Graduate Women for a Postgraduate Fellowship Award.
Barnes, J. C.; Lei, J.; Zhao, Y.-L.; Lilley, L. M.; Marks, T. J.; Ratner, M. A.;
Stoddart, J. F. J. Am. Chem. Soc. 2013, 135, 456.
(
(
12) Barnes, J. C.; et al. Science 2013, 339, 429.
13) (a) Cheng, C.; McGonigal, P. R.; Liu, W.-G.; Li, H.; Vermeulen,
N. A.; Ke, C.; Frasconi, M.; Stern, C. L.; Goddard, W. A., III; Stoddart, J.
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Schneebeli, S. T.; Li, H.; Vermeulen, N. A.; Ke, C.; Stoddart, J. F. Nat.
Nanotechnol. 2015, 10, 547.
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(16) At relatively slow scan rates (<100 mV/s), the reduced inclusion
complex between the interacting species has ample time to dissociate
and oxidize to the non-interlocked components (see Figure S22). At
relatively rapid scan rates (>1500 mV/s), the trisradical inclusion
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