Formation of [2]- and [3]Rotaxanes through Bridging under Kinetic and Thermodynamic Control

Article abstract of DOI:10.1021/acs.orglett.7b03615

An efficient synthesis of a doubly stranded [3]rotaxane has been developed through bridging of a pseudo[3]rotaxane featuring two axle components. Reversible azine formation was effective as the bridging reaction. Kinetic and thermodynamic conditions provided the [2]- and [3]rotaxanes, respectively.

Full text of DOI:10.1021/acs.orglett.7b03615

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Formation of [2]- and [3]Rotaxanes through Bridging under Kinetic
and Thermodynamic Control
†
†
§
†
§
†
‡
Takaaki Fujino, Hirotake Naitoh, Shinobu Miyagawa, Masaki Kimura, Tsuneomi Kawasaki,
§
,†
Kazuyuki Yoshida, Hajime Inoue, Hiroaki Takagawa, and Yuji Tokunaga*
†
Department of Materials Science and Engineering, Faculty of Engineering, University of Fukui, Bunkyo, Fukui 910-8507, Japan
Department of Applied Chemistry, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan
Forensic Science Laboratory, Fukui Prefectural Police Headquarters, Ohte, Fukui 910-8515, Japan
‡
§
*
S Supporting Information
ABSTRACT: An efficient synthesis of a doubly stranded [3]-
rotaxane has been developed through bridging of a pseudo[3]-
rotaxane featuring two axle components. Reversible azine formation
was effective as the bridging reaction. Kinetic and thermodynamic
conditions provided the [2]- and [3]rotaxanes, respectively.
any types of higher-order rotaxanes have been synthe-
capping of which has given the corresponding stable [n]-
rotaxanes in which the ring encircles multiple axles. In a doubly
stranded pseudo[3]rotaxane intermediate of such a system, if
intramolecular bridging of the macrocycle could proceed while
the two axles are positioned within it, a [3]rotaxane comprising
two [2]rotaxane units would be synthesized directly through a
process involving the production of two macrocyclic rings and
M
sized in recent years and investigated for their unique
1
2
structures and properties. Main-chain [n]rotaxanes, den-
3
1k,4
drimer-shaped rotaxanes, [c2]daisy chain rotaxanes,
and
5
,6
others are now well-established forms of these interlocked
molecules. The syntheses of these higher-order rotaxanes are
generally based on the accumulation of individual rotaxanes
synthesized from a ring and an axle; thus, the formation of each
rotaxane unit and monolithic rotaxane species should be high-
yielding.
15,16
ring shrinking
(Figure 1). In this Letter, we describe such a
Skillful molecular design for the stabilization of higher-order
(
pseudo)rotaxanes and/or the use of dynamic covalent
7
chemistry has led to the preparation of a molecular elevator,
a rotaxane capsule, a hetero[7]rotaxane, and a [20]-
rotaxane
interlocked molecules synthesized using ingenious means.
Templated syntheses using catalytically active metals have
also been developed:
8
9
1
0,11
all eye-catching examples of rotaxane-based
Figure 1. Bridging strategy for the synthesis of a [3]rotaxane.
12,13
continuous reactions of a dumbbell-
like axle have been performed inside large macrocyclic cavities
using macrocyclic metal catalysts, and a rotaxane possessing a
single macrocycle and multiple axles has been synthesized.
Conversely, sequential cyclization reactions performed around a
dumbbell-like axle have afforded a corresponding rotaxane
consisting of a single axle and multiple macrocycles. These
methods can produce heterorotaxanes possessing different
numbers of axles threaded through a macrocycle or different
numbers of macrocycles encircling an axle component.
novel method for the synthesis of [2]- and [3]rotaxanes through
intramolecular bridging of a pseudo[3]rotaxane formed from a
large crown ether and two dumbbell-like secondary ammonium
ions. In addition, we have examined the mechanism of the
process under kinetic and thermodynamic conditions.
We prepared two large crown ethers for this study: the 38- and
17
4
4-membered macrocycles 1a and 1b, respectively, which
afforded the 24- and 27-membered bis-crown ethers 2a and
Other methods using large macrocycles have also been
1
4
reported: macrocycles that recognize more than two axles have
Received: November 21, 2017
been used to form the corresponding pseudo[n]rotaxanes, end-
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03615
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
b, respectively, after bridging through azine formation from the
Letter
2
Intramolecular azine formation proceeded smoothly at room
temperature; the signal of the formyl H atoms at 10.39 ppm
disappeared, and a signal for the azine H atoms appeared at 8.85
ppm. 2a was detected in 87% NMR yield after 1.5 h at room
temperature. Bis-crown ether 2b was constructed using the same
procedure (Figure S1).
two aldehyde moieties and hydrazine (Figure 2). We suspected
Next, we examined the formation of pseudo[2]rotaxanes from
bis-crown ethers 2 and dibenzylammonium salts 3. In view of the
ring sizes of these bis-crown ethers and the bulk of the terminal
units of the dibenzylammonium ions, we used bis-[24]crown-8
(
2a)/phenyl group (3-H) and bis-[27]crown-9 (2b)/3,5-
dimethylphenyl group (3-Me) as matching pairs for NMR
titration experiments because association and dissociation of the
resulting pseudorotaxanes would be slow on the NMR time
19
scale.
We performed an NMR titration to monitor the pseudor-
otaxanes formed from bis-crown ether 2a (5 mM) and variable
concentrations of dibenzylammonium salt 3-H (0.8−12 equiv).
After the addition of 0.8 equiv of 3-H, a new set of signals
1
representing the pseudo[2]rotaxane 4a-H was evident in the H
Figure 2. Structures of the mono- and bis-crown ethers and the
dumbbell-like dibenzylammonium salts.
NMR spectrum (Figure 3c). The signals for the NH and
benzylic protons appeared at 8.28 and 4.50 ppm, respectively,
while the signals for the aliphatic protons of the crown ether
appeared at 3.34−3.39 and 3.46−3.50 ppm; such signals are
characteristic of crown ether/dibenzylammonium ion-type
rotaxanes. Moreover, two separate signals were present at 6.53
and 6.64 ppm for the aromatic protons at the meta positions of
the formimino groups of the crown ether, as would be expected
for the asymmetric 4a-H (Figure 4). After the addition of excess
that the reversible nature of this reaction would provide
thermodynamically stable [n]rotaxanes effectively; because
of its low basicity, we did not expect N H to deprotonate the
secondary dialkylammonium ions in the dumbbell-like axle
components. The details of the synthesis are described in the
Supporting Information.
Initially, we examined the preparation of bis-crown ether 2a
from mono-crown ether 1a in the presence of N H in 1:1
1
8
2
4
2
4
CDCl /CD CN (Figure 3). A 1:1 mixture (5 mM) of 1a and
3
3
1
hydrazine was monitored by H NMR spectroscopy (Figure 3b).
Figure 4. Association constants for the formation of pseudo[2]- and
pseudo[3]rotaxanes from bis-crown ethers 2 and dibenzylammonium
salts 3.
3
-H (Figure 3d), another new set of signals appeared
representing the pseudo[3]rotaxane 5a-H. Signals for the NH
7.91 ppm), benzylic (4.32 ppm), and ethylene glycol (2.99 and
.17 ppm) protons were observed at typical chemical shifts, and
(
3
only one signal was present for the aromatic protons at the meta
positions of the formimino groups in the crown ether, consistent
with a symmetric pseudo[3]rotaxane. ESI-MS spectra featured
signals at m/z 874 and 1218, corresponding to 4a-H and 5a-H,
−
+
respectively, that had lost the PF6 counterion ([4a-H − PF ] ,
6
+
calcd m/z 874.4; [5a-H − PF ] , calcd m/z 1217.5) (Figure
6
1
Figure 3. 1H NMR titrations using mono-crown ether 1a and
dibenzylammonium salt 3H. Shown are H NMR [600 MHz, 1:1
S14). Similarly, we used H NMR spectroscopy and mass
1
spectrometry (Figure S15) to confirm that the pseudo[2]- and
[
3]rotaxanes 4b-Me and 5b-Me formed from 1b and 3-Me in
the presence of N H .
CDCl /CD CN, 25 °C] spectra (a) of 1a, (b) 1.5 h after mixing of 1a
3
3
(
5 mM) with N H (1 equiv), and (c, d) the sample in (b) after the
2
4
2
4
addition of (c) 0.8 and (d) 12 equiv of 3-H. Labels: #, bis-crown ether
2a; [2], pseudo[2]rotaxane 4a-H; [3], pseudo[3]rotaxane 5a-H.
Integration of signals representing each species allowed us to
calculate association constants (Figure 4) for the pseudo[2]-
B
DOI: 10.1021/acs.orglett.7b03615
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
and pseudo[3]rotaxanes formed from 2a and 3-H (K = 1100 ±
1
3
9
30; K = 41 ± 5) and from 2b and 3-Me (K = 1400 ± 210; K =
2 1 2
6 ± 12). These values imply that negative allosteric effects
operated in the formation of the pseudo[3]rotaxanes, with the
negative effect for the smaller crown ether 2a being stronger than
that for the larger crown ether 2b. It is likely that electrostatic
repulsion of the two ammonium ions was a factor in these
pseudo[3]rotaxanes. The K value for 2b and 3-Me is larger than
1
that of the usual [27]crown-9 and the corresponding
ammonium salt, so it is likely that the azine moiety effectively
acts as a hydrogen-bond acceptor in pseudo[2]rotaxane 4b-Me.
For the synthesis of [3]rotaxanes, we performed NMR
titrations to monitor the products formed from the pairs 1a/3-
Me and 1b/bis(3,5-di-tert-butylbenzyl)ammonium hexafluor-
ophosphate (3-tBu); we suspected that these terminal groups
would be sufficiently large to inhibit decomplexation through
unthreading. After mixing of 1a and an excess of 3-Me in the
presence of N H , however, we observed no signals for the
Figure 6. Cartoon representation of the formation of [2]rotaxane 4b-
tBu and [3]rotaxane 5b-tBu.
2
4
1
[3]rotaxane 5a-Me in the H NMR spectrum; only those for 4a-
Me were evident, even after reaction for 194 h (Figure S2). We
suspect that the 3,5-dimethylphenyl groups in the pseudo[3]-
rotaxane provided too much steric bulk for the N H unit to
well as the amount of water and the pH of the reaction mixture
depends on the concentrations and/or ratio of the ammonium
2
4
salt and N H , we could not conclude the details of these
enter the cavity of 1a to react with the formyl groups. In contrast,
we detected small signals for the [3]rotaxane 5b-tBu as well as
signals for the [2]rotaxane 4b-tBu after mixing of 1b (5 mM)
and 3-tBu (25 mM) in the presence of N H (5 mM); the
2
4
2
1
processes. Finally, we isolated 5b-tBu in 33% yield after
reacting 2b with 10 equiv of 3-tBu and then separating the
products chromatographically. The spectroscopic analysis
supports the [3]rotaxane structure.
2
4
intensity of the signals for 5b-tBu increased as the reaction
progressed at ambient temperature (Figures S3 and 5). After 310
h, 4b-tBu and 5b-tBu were present in NMR yields of 27 and
In summary, we have constructed (pseudo)[2]- and [3]-
rotaxanes from large crown ethers and dumbbell-like dibenzy-
lammonium ions, with azine formation as a means of
intramolecular bridging. Negative allosteric effects were evident
in the assembly of the pseudo[3]rotaxanes, with the value of K2
for the larger crown ether being greater than that for the small
crown ether. The steric bulk of the end groups of the dumbbell-
like ammonium salts affected the formation of the correspond-
ing [3]rotaxanes. In the case of the 44-membered crown ether
7
1%, respectively.
1
b, the [2]- and [3]rotaxanes were formed predominantly under
kinetic and thermodynamic conditions, respectively.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03615.
Figure 5. Distribution of mono-crown ether 1b, bis-crown ether 2b,
2]rotaxane 4b-tBu, and [3]rotaxane 5b-tBu after mixing of 1b (5 mM)
and 3-tBu (25 mM) in 1:1 CD CN/CDCl in the presence of N H (5
Experimental details and NMR spectra (PDF)
[
3
3
2
4
AUTHOR INFORMATION
mM).
■
Corresponding Author
*E-mail: tokunaga@u-fukui.ac.jp.
ORCID
The spectra reveal that the second association was much
slower than the first, presumably because of electrostatic
repulsion of the ammonium ions and the steric bulk of the
Tsuneomi Kawasaki: 0000-0002-6332-2096
Yuji Tokunaga: 0000-0002-4183-7320
Notes
3
,5-di-tert-butylphenyl groups (Figure 6). Accordingly, the
formation of [2]rotaxane 4b-tBu is kinetically favorable, and
the content of [3]rotaxane 5b-tBu increases under thermody-
namic control because the 3,5-di-tert-butylphenyl groups are too
The authors declare no competing financial interest.
20
bulky to thread through the 27-membered crown ether units.
ACKNOWLEDGMENTS
This study was supported by JSPS KAKENHI (JP16K05691).
Moreover, it seems that the azine moiety contributes to relief of
the electronic repulsion in the [3]rotaxane through its ability to
function as a hydrogen-bond acceptor. We carried out the
■
[
3]rotaxane formation reaction in the presence of various
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DOI: 10.1021/acs.orglett.7b03615
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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20) We observed no formation of a pseudorotaxane from
dibenzo[27]crown-9 and 3-tBu (see Figure S4).
21) To find the intermediate, we performed an NMR titration using
b and 3-tBu in the absence of N H . However, the pseudo[3]rotaxane
(
4
58, 314. (b) Rotaxane consisting of two axles and four macrocyclic
components: Collin, J.-P.; Durola, F.; Frey, J.; Heitz, V.; Reviriego, F.;
(
Sauvage, J.-P.; Trolez, Y.; Rissanen, K. J. Am. Chem. Soc. 2010, 132,
6
840. (c) Pseudorotaxane based on a metal cage: Clever, G. H.;
(
Shionoya, M. Chem. - Eur. J. 2010, 16, 11792. (d) Multi-rotaxane
arrays: Yamada, Y.; Okada, M.; Tanaka, K. Chem. Commun. 2013, 49,
1
2
4
derived from 1b and 3-tBu could not be detected.
1
1053. (e) Rotaxane consisting of two axles and six macrocyclic
D
DOI: 10.1021/acs.orglett.7b03615
Org. Lett. XXXX, XXX, XXX−XXX