A metal-free "threading-followed-by-shrinking" protocol for rotaxane synthesis

Article abstract of DOI:10.1002/anie.201101524

Shrinking ring size: Synthesis of a [2]rotaxane has employed photoextrusion of SO₂ from the arylmethyl sulfone motif of a [2]pseudorotaxane. A dumbbell-shaped guest molecule allows a shrinking reaction to decrease the number of atoms in the rin

Full text of DOI:10.1002/anie.201101524

DOI: 10.1002/anie.201101524
Rotaxanes
A Metal-Free “Threading-Followed-by-Shrinking” Protocol for
Rotaxane Synthesis**
Sheng-Yao Hsueh, Jia-Ling Ko, Chien-Chen Lai, Yi-Hung Liu, Shie-Ming Peng, and
Sheng-Hsien Chiu*
The potential applicability of interlocked molecular switches
or actuators in mesoscale molecular electronic devices has
driven the development of new synthetic protocols for
preparing these functional molecules.[1] Recently, in addition
to the classical “threading-followed-by-stoppering”,[2] slip-
page,[3] and clipping[4] approaches, a new “threading-followed-
by-shrinking” protocol was described for [2]rotaxane syn-
thesis. It employed a Pd2+ ion to chelate the salophen moiety
of the macrocyclic unit in the corresponding [2]pseudorotax-
ane, thereby “shrinking” the size of the free space in the
complexed macrocycle and interlocking the components in
the form of a [2]rotaxane.[5] A more intuitive approach to
“shrinking” would be decreasing the number of atoms in the
skeleton of the macrocyclic component (Figure 1). Several
challenges must be resolved to realize such a metal-free ring-
shrinking protocol. 1) The rate of shrinking of the macrocycle
in its complexed state should not be much slower than that in
its free state. Therefore, the change in the structure of the
shrinking motif in the macrocyclic unit should not disrupt
most of the stabilizing interactions in the host–guest complex.
One way to ensure such behavior would be for the non-
shrinking portion of the macrocycle to be the primary source
of stabilizing interactions toward the threadlike component.
2) A mild chemical reaction would be preferred for the
shrinking of the ring skeleton of the macrocycle, so that the
relatively weakly associated components of the [2]pseudo-
rotaxane would not dissociate significantly during the trans-
formation. 3) The sizes of the stopper of the dumbbell-shaped
component and the cavity of the macrocycle must be finely
tuned so that they can associate freely prior to performing the
shrinking process, but not afterward. Herein, we report one
such metal-free “threading-followed-by-shrinking” synthesis
of a [2]rotaxane, mediated through photoextrusion[6] of SO2
from the arylmethyl sulfone motif of the crown ether-like
macrocyclic unit of a [2]pseudorotaxane featuring a dumb-
bell-shaped cycloheptyl-terminated dialkylammonium ion.
Macrocycles featuring oligo(ethylene glycol) units in their
ring structures can form complexes with dialkylammonium
ions with little assistance from the other components of their
molecular structures.[7] Thus, we synthesized the macrocycle 1
(Scheme 1) to demonstrate the concept of metal-free shrink-
ing; we expected its penta(ethylene glycol) unit to recognize
dialkylammonium ions and its arylmethyl sulfone unit to
extrude SO2 under photochemical conditions, thereby
decreasing the number of atoms in the ring skeleton. We
chose photochemical extrusion as the crucial step in the
synthesis because we suspected that this mild reaction would
not disrupt the weakly associated components of the [2]pseu-
dorotaxane in solution.
Figure 1. Cartoon representation of the concept of the metal-free
“threading-followed-by-shrinking” protocol.
[*] S.-Y. Hsueh,[+] J.-L. Ko,[+] Y.-H. Liu, Prof. S.-M. Peng, Prof. S.-H. Chiu
Department of Chemistry, National Taiwan University
No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan 10617 (R.O.C.)
Fax: (+886)2-33661677
E-mail: shchiu@ntu.edu.tw
Homepage: http://www.ch.ntu.edu.tw/faculty.db/en/faculty_perso-
nal_01.php?id=42
Prof. C.-C. Lai
Institute of Molecular Biology, National Chung Hsing University
and Graduate Institute of Chinese Medical Science
China Medical University, Taichung, Taiwan (R.O.C.)
[+] These authors contributed equally to this work.
[**] We thank the National Science Council (Taiwan) for financial
support (NSC-98-2113M-002-004-MY3).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101524.
Scheme 1. Photoextrusion of SO2 from macrocycle 1 and the struc-
tures of dumbbell-shaped guest dialkylammonium salts.
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To confirm that shrinking of the ring skeleton of macro-
macrocycle 1. The cyclohexyl termini of the threadlike salt
5-H·PF6 passed through the cavity of macrocycle 1 smoothly
(similar to the behavior of 3-H·PF6), whereas the correspond-
ing threading of the cycloheptyl-terminated salt 6-H·PF6 had
relatively slow kinetics, and required 1 h to reach equilibrium
at room temperature, which suggests appropriate size com-
plementarity for our proposed [2]rotaxane synthesis.
cycle 1 through extrusion of SO2 was possible, we irradiated a
degassed solution of 1 in C6H6/CH2Cl2/CH3CN (5:5:1) at
273 K for 90 min; gratifyingly, we obtained the macrocycle 2
in 65% yield after column chromatography (Scheme 1). Next,
it was necessary to select a suitable dumbbell-shaped guest
with which to perform the shrinking reaction for [2]rotaxane
synthesis. The macrocycles 1 and 2 are 25- and 24-membered-
ring systems, respectively. Based on the knowledge that
isopropylphenyl and cyclohexyl units are slippage stoppers[8]
and that p-tert-butylphenyl and cycloheptyl units are true
stoppers for dibenzo[24]crown-8 (DB24C8),[9] we synthesized
a series of symmetric dialkylammonium salts (3–6-H·PF6;
Scheme 1) featuring various terminal groups to test whether
they were capable of passing through the cavity of macrocycle
1 but not through that of macrocycle 2.
Next, we tested whether the “shrunk” macrocycle 2 could
form [2]pseudorotaxane-like complexes with any of these
four threadlike salts in CHCl3/CH3CN (9:1). 3-H·PF6 rapidly
formed a complex with macrocycle 2 at ambient temperature
with a Ka value of 470mÀ1 [12]
After heating their respective
.
1
mixtures at 323 K for 2 h, H NMR spectra revealed that the
cyclohexyl groups of 5-H·PF6 were slippage stoppers for this
macrocycle. In contrast, we observed no additional signals
corresponding to [2]pseudorotaxanes in the spectra of the
solutions of macrocycle 2 and the threadlike salts 4-H·PF6 and
6-H·PF6, which suggests that p-tert-butylphenyl and cyclo-
heptyl units are both true stoppers for the macrocycle 2. Thus,
6-H·PF6 appeared to be an ideal choice for use as a
component in the shrinking reaction: its cycloheptyl units
could penetrate through the cavity of macrocycle 1 at ambient
temperature to form the corresponding [2]pseudorotaxane
[1ꢀ6-H][PF6] (Scheme 2), but the smaller macrocyclic unit 2
obtained after shrinking would be unlikely to pass over these
stopper units.
The 1H NMR spectrum of an equimolar mixture (5 mm) of
macrocycle 1 and the threadlike salt 3-H·PF6 in CDCl3/
CD3CN (9:1) displayed three sets of signals: those of the two
free components and their 1:1 complex. For the complex, the
downfield shift of the signal of the methylene protons
+
adjacent to the NH2 center and the upfield shifts of the
signals of the OCH2CH2 protons of the macrocycle, relative to
the respective signals of the free species, are similar to those
observed for [2]pseudorotaxanes prepared from DB24C8 and
dibenzylammonium ions,[10] which suggests that the complex
[1·3-H]+ was likely to have a [2]pseudorotaxane geometry
(Figure 2). Using the single-point method, we determined the
Scheme 2. The cycloheptyl units of 6-H·PF6 penetrate the cavity of
macrocycle 1 to form the corresponding [2]pseudorotaxane [1ꢀ6-H]-
[PF6]. A shrinking reaction is performed by irradiating a solution of 1
and salt 6-H·PF6 to obtain the [2]rotaxane 7-H·PF6.
Figure 2. Partial 1H NMR spectra (400 MHz, CDCl3/CD3CN (9:1),
298 K) of a) macrocycle 1, b) an equimolar mixture of 1 and threadlike
salt 3-H·PF6 (5 mm), and c) 3-H·PF6. The descriptors (C) and (UC)
refer to the signals of the components’ complexed and uncomplexed
states, respectively.
association constant (Ka) for the complex formed between
We performed the shrinking reaction by irradiating
(254 nm) a C6H6/CH2Cl2/CH3CN (5:5:1) solution of macro-
cycle 1 (28.4 mm) and the threadlike salt 6-H·PF6 (70.9 mm) at
273 K for 90 min, and obtained the [2]rotaxane 7-H·PF6 in
28% yield after column chromatography (Scheme 2).[13] The
1H NMR spectrum of this [2]rotaxane revealed significant
downfield and upfield shifts of the ethylene glycol protons
macrocycle 1 and the threadlike salt 3-H·PF6 to be 300mÀ1 [11]
.
Under similar conditions (namely, standing at room temper-
ature for several hours), we observed no signals for such a
1
complex in the H NMR spectrum of a mixture of 4-H·PF6
and macrocycle 1. However, after heating the solution at
323 K for 2 h we detected a new set of signals representing a
1:1 complex, which suggests that the p-tert-butylphenyl
termini of this threadlike salt were slippage stoppers for the
+
and the methylene protons adjacent to the NH2 center,
respectively, relative to those of their free components, which
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+
À
suggests that [ N H···O] hydrogen bonding remained the
most dominant noncovalent interaction in the system, with
possible shielding of the methylene protons adjacent to the
+
NH2 center by the macrocycleꢀs aromatic motifs (see Fig-
ure 3a–c). The spectrum of an equimolar mixture of salt
Figure 4. Ball-and-stick representation of the solid-state structure of
the [2]rotaxane 7-H+.
solution of 7-H·PF6. The solid-state structure in Figure 4
reveals[16] the expected [2]rotaxane geometry, in which the
rodlike portion of the dumbbell-shaped component has
penetrated through the cavity of the macrocyclic component,
Figure 3. Partial 1H NMR spectra (400 MHz, CDCl3/CD3CN (9:1),
298 K) of a) threadlike salt 6-H·PF6, b) macrocycle 2, c) [2]rotaxane
7-H·PF6, d) an equimolar mixture of 2, 6-H·PF6, and 7-H·PF6 (10 mm),
and e) the solution in (d) after heating at 323 K for 16 h.
+
with the CH2NH2 units hydrogen bonded to the oxygen
atoms of the macrocyclic unit.
In summary, we have developed a “threading-followed-
by-shrinking” protocol for rotaxane synthesis under metal-
free conditions, by employing photoextrusion to decrease the
number of atoms in the ring skeleton of the macrocyclic
component. This methodology adds to the repertoire of
reactions available to the synthetic chemist for the construc-
tion of complicated interlocked structures.
6-H·PF6, macrocycle 2, and the [2]rotaxane 7-H·PF6 in CDCl3/
CD3CN (9:1) revealed (Figure 3d) three sets of signals, of
equal abundance, for the three species, which suggests that
exchange of the components did not occur under these
conditions. Heating this solution at 323 K for 16 h provided a
spectrum similar to that measured prior to heating, thereby
confirming that the [2]rotaxane had not assembled through
slipping of threadlike salt 6-H·PF6 through macrocycle 2.
Although the free radical intermediate in this photo-
extrusion reaction might have irreversibly combined to form
Experimental Section
General method for the photoextrusion process: The photoextrusion
reaction was performed using a Rayonet RPR-200 photoreactor
containing 16 35 W, 254 nm lamps. The reaction mixture was placed in
a quartz apparatus equipped with an internal cold finger.
À
the corresponding C C bond within the solvent cage rapidly
enough to retain the stereochemistry of the benzyl radicals,[14]
it remained necessary for us to dismiss the possibility that the
[2]rotaxane 7-H·PF6 was formed through clipping of the
uncomplexed free radical intermediate about the recognition
element in the threadlike salt. Thus, we irradiated (254 nm) a
solution of threadlike salt 4-H·PF6 (70.9 mm) and macrocycle
1 (28.4 mm) in C6H6/CH2Cl2/CH3CN (5:5:1) for 90 min at
7-H·PF6: Macrocycle 1 (60.0 mg, 0.125 mmol) and threadlike salt
6-H·PF6 (120 mg, 0.312 mmol) were dissolved in a mixture of CH2Cl2,
CH3CN, and benzene (2.0:0.4:2.0 mL) and left at room temperature
for 1 h before being irradiated under N2 for 1.5 h. The organic
solvents were evaporated under reduced pressure and the residue was
purified by column chromatography (SiO2; CH2Cl2/CH3CN, 49:1) to
afford the [2]rotaxane 7-H·PF6 as a white solid (28 mg, 28%). M.p.
176–1788C; 1H NMR (400 MHz, CDCl3): d = 1.00–1.08 (m, 4H),
1.38–1.71 (m, 22H), 2.58–2.63 (m, 4H), 3.00 (s, 4H), 3.65–3.71 (m,
12H), 3.80–3.81 (m, 4H), 4.05–4.06 (m, 4H), 6.40 (s, 2H), 6.66 (d, J =
8 Hz, 2H), 6.83 (d, J = 8 Hz, 2H), 7.18 ppm (t, J = 8 Hz, 2H);
13C NMR (100 MHz, CDCl3): d = 26.0, 28.4, 31.8, 35.4, 36.7, 55.5, 67.8,
71.1, 71.2, 71.3, 71.7, 110.0, 116.5, 122.4, 129.6, 142.9, 157.5 ppm;
1
273 K. The H NMR spectrum of the crude reaction product
featured no signals for the corresponding [2]rotaxane, thus
suggesting that the formation of a [2]pseudorotaxane prior to
shrinking was an essential aspect of our rotaxane synthesis.
Presumably, the lifetimes of the open-chain radical inter-
mediates were not long enough for any potential formation of
7-H·PF6 through clipping. Thus, this [2]rotaxane was obtained
almost exclusively through a process of threading of the
rodlike component through the macrocycle and subsequent
shrinkage of the macrocyclic component—a methodology
that is conceptually different from those reported previously
for synthesizing rotaxanes.[15]
+
HRMS (ESI): m/z [7-H]+ C40H64NO6 calcd: 654.4733; found:
654.4711.
Received: March 2, 2011
Published online: June 6, 2011
We grew single crystals suitable for X-ray crystallography
through liquid diffusion of toluene into a THF/CH2Cl2
Keywords: host–guest systems · macrocycles · photoextrusion ·
rotaxanes · synthetic methods
.
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Communications
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[16] Crystal data for [7-H·PF6]: [C40H64O6N][PF6]; Mr = 799.89;
ꢀ
triclinic; space group P1; a = 18.8647(4); b = 19.8552(4); c =
21.9993(3) ꢂ; V= 8190.8(3) ꢂ3; 1calcd = 1.297 gcmÀ3; m(MoKa) =
1.221 mmÀ1; T= 150(2) K; colorless needle; 29854 independent
measured reflections; F2 refinement; R1 = 0.1165; wR2 = 0.3506.
CCDC 812242 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
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