Rotaxanes with chiral stoppers and photoresponsive central unit

Article abstract of DOI:10.1055/s-1999-3466

Two new chiral and photoisomerizable rotaxanes (9m, 9p) have been prepared, beating the longest non-polymeric axle known so far: the axles (6m, 6p) are polyether chains made of alternating triethyleneglycol/hydroquinone units, bearing a central meta- or p

Full text of DOI:10.1055/s-1999-3466

PAPER
849
Rotaxanes with Chiral Stoppers and Photoresponsive Central Unit
Christopher Kauffmann,a Walter M. Müller,a Fritz Vögtle,*a Sarah Weinman,b Sarah Abramson,b Benzion Fuchs*b
aKekulé-Institut für Organische Chemie und Biochemie, Universität Bonn, Gerhard-Domagk-Str. 1, D-53121 Bonn, Germany
Fax +49(228)735662; E-mail: voegtle@uni-bonn.de
bTel-Aviv University, School of Chemistry, Ramat-Aviv, 69978 Tel-Aviv, Israel
E-mail: bfuchs@post.tau.ac.il
Received 15 October 1998; revised 17 December 1998
and CD spectroscopy.9 The isolated main fraction of the
Abstract: Two new chiral and photoisomerizable rotaxanes (9m,
9p) have been prepared, bearing the longest non-polymeric axle
known so far: the axles (6m, 6p) are polyether chains made of alter-
nating triethyleneglycol/hydroquinone units, bearing a central me-
sugar-stoppered axles clearly exhibits b-glucosidic con-
1
nections as could be confirmed by H NMR measure-
ments. This implication can directly be derived from the
ta- or para-azobenzoyl moiety and terminal tetraacetylglucoside value of the H-1/H-2 coupling constants of the terminal
sugar groups (3JH-1,H-2 = 7.9 and 8.0 Hz), which are in good
stopper groups, while the wheel is the cyclobis(p-xylylene)bis(4,4´-
bipyridinium) salt. Temperature dependent 1H NMR measurements
agreement with axially positioned hydrogen atoms at the
reveal that the translational process of the wheel across the azo-
C-1 and C-2 positions within a glucose unit.10 Smaller
benzene unit in the E-configuration is favoured for the para-rotax-
coupling constants, which would be exhibited by an a-
ane 9p compared to the meta isomer 9m. The azobenzene core of
glucosidic connection (H-1 in an equatorial, H-2 in axial
position) could not be detected.
the free axles (6m, 6p) and rotaxanes (9m, 9p) undergo photochem-
ical E/Z-isomerisation: from (E) to (Z) by irradiation at low wave-
length and back from (Z) to (E) at high wavelength. Complete (Z) to
(E) conversion was achieved by thermal isomerization.
The rotaxanes described present a world record regarding
the length of a non-polymeric axle moiety. We chose
these long axles to examine if the translational movement
of the wheel along the axle can be systematically influ-
enced compared to similar rotaxanes with a shorter
axle7,11, which could be detected by NMR measurements.
At ambient temperature the electron-deficient wheel com-
ponent, which prefers the proximity of the electron-rich
hydroquinone system, is in a translational movement
mainly along one side of the azobenzene unit, that is,
without slipping over. Since this movement is already
slow enough at r.t. on the NMR time scale, the signals for
the hydroquinone protons of the ”unoccupied side” show
up at d = 6.7–6.9 ppm, whereas the signals for the hydro-
quinone protons of the ”occupied” side are merged in the
base line at higher magnetic field as already described ac-
curately in the literature for similar rotaxanes.7,11
Key words: rotaxane, azobenzene isomerization, molecular recog-
nition, chirality, supramolecular chemistry
Azobenzene derivatives have been used to construct pho-
toswitchable devices for many years.1–3 The thermody-
namically stable (E)-isomer can usually be photo-
chemically converted to the (Z)-isomer,4 which is convert-
ed back to the (E)-isomer by light excitation and thermally
in the dark.5
For the synthesis of the title rotaxanes 9,6 we started with
the hydroquinone derived ethyleneglycol 17 and a-D-ace-
tobromoglucose 28 to yield the chiral mono-stoppered
half-axle 3 and, as a byproduct, the doubly tetraacetylglu-
coside stoppered short chain 4.7 By use of HgBr2/Hg(CN)2
in MeCN instead of Ag2O in CH2Cl2/t-BuOMe in this re-
action step, formation of the chiral di-stoppered axle 4 is
favoured, as we recently reported.7 By reaction of the tet-
raacetyl-b-D-glucoside stoppered glycol 3 with the meta-
or para-azobenzoic acid dichlorides (5m, 5p), the chiral
axles 6m and 6p were obtained, respectively, in 75%
yield. The cyclobis(p-xylylene)bis(4,4´-bipyridinium)
wheel of the rotaxanes was cyclized using the clipping
method worked out by Stoddart et al.,4b,c by reaction of the
In DMSO-d6 at 45 °C, the former broaden significantly to
likewise merge in the baseline for both rotaxanes. At
65 °C, in the case of the para-rotaxane 9p, a distinct coa-
lescence signal at d = 6.15 ppm can be detected, while for
the meta-rotaxane 9m, the corresponding signals are still
extremely broadened. At 85 °C, however, no difference
between the shape of the coalescence signals of both com-
pounds can be observed anymore. Thus, our NMR studies
indicate that the shuttle process of the tetracationic cyclo-
phane across the azobenzene unit is slightly disfavoured
for the meta-rotaxane compared to the para-substituted
compound. Low temperature 1H NMR studies down to –
60 °C in acetone-d6 lead to a complicated signal splitting
for both the wheel and the axle, so that a coalescence tem-
perature for the translational process of the cyclophane
between two neighbouring hydroquinone units cannot be
determined.
1,1´-p-xylylenebis(4,4´-bipyridinium)
bis(hexafluoro-
phosphate) salt 7 with 1,4-bis(bromomethyl)benzene (8)
in presence of the axles 6p or 6m, to yield the chiral meta-
or the para-rotaxanes 9p or 9m, respectively. They were
isolated mainly in the (E)-configuration, but depending on
light exposure, they may contain small amounts of the (Z)-
form.
The axles 6m, 6p and the rotaxanes 9m, 9p were charac-
terized by FAB and MALDI mass spectrometry, by NMR
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Rotaxanes with Chiral Stoppers and Photoresponsive Central Unit
851
In attempts to prepare rotaxanes bearing the isomeric mobility, respectively, one of the original aims of this
wheel of cyclo(p-xylylene/m-xylylene)bis-4,4´-bipyridin- study, yet could not be determined unequivocally, as pro-
ium salt under analogous conditions, both the free wheel longed irradiation seems to lead to byproducts that addi-
and the free axle components were isolated but no rotax- tionally complicate the interpretation of UV, as well as
ane could be detected. This may be due to the smaller NMR spectra compared to our earlier investigations.2,14
wheel perimeter as well as to an unfavourable torsional
disposition of the aromatic rings.
Mps: Tottoli-type melting point apparatus SMP-20 (Büchi), and are
uncorrected. TLC: TLC plastic sheets with silica gel (SiO2) 60 F254
The (E)-meta-axle ((E)-6m) underwent (E) Æ (Z) photo-
isomerization on irradiation at 338 nm. This is manifested (Merck 5748). Detections: UV or development with iodine vapor.
Column chromatography: preparative medium pressure chromato-
by a shift to lower wavelength and an increase in intensity
graphy system (MPLC) by Büchi. CHN analyses: Mikroanalytis-
of the > 400 nm absorption maximum.
ches Labor Pascher, Remagen, Germany. Optical rotations were re-
Quantum yield was measured (on concentrated solutions)
and found to be f = 0.1. The photostationary state exhib-
ited a ca. 95% (Z)-6m content (measured by both, UV and
NMR methods). Four isosbestic points at 232, 257, 269
and 375 nm indicate ”pure” (E) Æ (Z) isomerization.
corded at r.t. with a Perkin-Elmer model 241 automatic polarimeter.
1H and 13C NMR: Bruker AC-200 Cryospec (200 MHz and 50.3
MHz), AC-360-WB (360 MHz and 90.5 MHz), WM 250 (250 MHz
and 62.9 MHz), AM 400 (400 MHz and 100.6 MHz) and DRX 500
(500 MHz, for the temperature dependent measurements) spectrom-
eters. MS: MS-30 and MS-50 A.E.I. instruments. FAB-MS spectra
were obtained on a Concept 1H, Cratos, Manchester, UK in NBA
(m-nitrobenzyl alcohol) as matrix. MALDI-TOF spectra were re-
corded with a Micromass TOF Spec E (Micromass, Manchester,
UK); the matrices used were 9-nitroanthracene (9-NA) and 2,5-di-
hydroxybenzoic acid (2.5-DHB). UV spectra were measured on a
UVIKON-931 spectrophotometer.
Irradiation of the (Z)-axle (Z)-6m in the above (Z)-en-
riched product at 446 nm gave a (E)/(Z) mixture of 3/1 (by
NMR), and f E/f Z = 2.3 at 446 nm. Finally, the thermal
(Z)-6m Æ (E)-6m isomerization was measured by NMR
spectroscopy in toluene-d8 with a half-life time of 60 min
at 95°C and 270 min at 60°C.
Photochemical experiments were performed on a JASCO CRM-FA
Spectro-irradiator, equipped with a photon-counter. For quantum
yields, the latter was periodically calibrated by chemical actinome-
try (using the Aberchrome 540 actinometer); at 338 nm the output
was 2.5*10–8 Einstein/s. The usual bandwidth for 1 cm cells was
±13 nm; when needed, this range was reduced by using narrower
windows. Irradiations were carried out, for both preparative and an-
alytical purposes, at 338 nm. All irradiations were performed on
deoxygenated (Ar or N2) MeCN solutions in quartz (< 300 nm) or
pyrex (300–500 nm) UV cells or NMR tubes. The products of the
preparative runs at 338 nm were used for thermokinetic studies in
NMR tubes held at constant temperatures, kept by reflux of various
solvents (e.g., CHCl3 for 60 °C). Both photochemical and thermo-
The meta-rotaxane (E)-9m was irradiated at 338 nm for
photoisomerization to the (Z)-form. Three isobestic points
were observed at 256, 270, 378 nm. The quantum yield is
similar as for (E)-6m, f = 0.06.
The thermal (Z)-9m Æ (E)-9m process takes place at
room temperature with a half-life time of ca. 200 h.
The (E)-para-axle (E)-6p was irradiated in the 311–365
nm range, which caused (E) Æ (Z) photoisomerization. At
the most effective 345±6 nm irradiation, a photostationary
state with a ca. 72% (Z)-6p-content was reached. The
quantum yield in this process was determined to give f =
0.02 with a 20% uncertainty margin. The photoisomeriza-
tion is again indicated by the occurrence of four isobestic
points at 240, 283, 383 and 490 nm.12
1
chemical experiments were monitored using UV and/or H NMR
spectroscopy.
b-D-Glucoside 3
Compound 17 (17.95 g, 30 mmol) was dissolved in anhyd CH2Cl2
(100 mL) and diluted with anhyd t-BuOMe (200 mL), and 2,3,4,6-
tetra-O-acetyl-a-D-glucopyranosyl bromide 28a,b (13.57 g, 33
mmol) was added. After the reaction flask had been wrapped by alu-
minium foil freshly precipitated Ag2O (6.95 g, 30 mmol) was added
to the mixture. It was stirred overnight at r.t. The progress of the re-
action was monitored by TLC. The solution was filtered through
Celite. The solvent was evaporated. The remaining residue was dis-
solved in CH2Cl2 and separated by MPLC using Lichroprep Si 60
(15–25 mm) with CH2Cl2/MeOH (9.8:0.2) as eluent. Yield: 11.2 g
(40%).
The (Z)-para-axle (Z)-6p in the above (Z)-enriched pho-
tochemical product was irradiated at 446 nm, which
caused nearly total reversal to (E)-6p (the relative quan-
tum yield at this wavelength could be estimated: fZÆE
/
f
EÆZ ≥ 10). Complete (Z) Æ (E) reversal could, however,
be achieved by heating the sample. Indeed, this thermal
(Z) Æ (E) isomerization was readily measured (by NMR
at the d = 7.6 ppm signals) and exhibits, at 60 °C, a first-
order rate constant of k1 = 1.3 * 10–4 sec–1, i.e., a half-life
time of 88 min. This is in accord with the behaviour of
simply substituted azobenzenes.13
After recrystallization from THF, mp 59–60 °C. [a]2D1 = -2.7 (c =
0.9, in CHCl3).
1H NMR (400 MHz, CDCl3, 25 °C): d = 1.97–2.08 (m, 12H,
OCCH3), 2.56 (br s, 1H, OH), 3.57–4.14 (m, 36H, OCH2), 4.20–
4.27 (m, 1H, 5-H), 4.59 (d, 1H, 3JH,H = 8.1 Hz, 1-H), 4.97 (dd, 1H,
3JH,H = 8.1 Hz, 3JH,H = 9.6 Hz, 2-H), 5.06 (t, 1H, 3JH,H = 9.5 Hz, 4-H),
5.18 (t, 1H, 3JH,H = 9.5 Hz, 3-H), 6.82 (s, 8H, ar. H).
13C NMR (100.6 MHz, CDCl3, 25 °C): d = 20.61, 20.65, 20.69,
20.77 (OCCH3), 61.76, 61.93, 67.99, 68.03, 69.13, 69.85, 69.89,
70.34, 70.35, 70.74, 70.78, 70.79, 70.83, 71.26, 71.75, 72.52, 72.83
(CH2, glucose-CH), 100.85, 115.51, 115.57, 115.59 (ar. CH),
According to space filling models, the (E) Æ (Z) isomer-
ization generates a stopper-like barrier for the wheel, thus
the isomerization process was supposed to be associated
with a reduced translational distance for the wheel along
the axle. Hence, the mechanical frequency of this transla-
tional movement should about double at a given tempera-
ture, since the effective length for this motion was cut in
half. This ”mechanical frequency doubling” in going from
the (E) to the (Z)-isomer, a switching of conformational
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852
C. Kauffmann et al.
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152.98, 153.04, 153.10, 153.15 (Cq), 169.45, 169.46, 170.32,
170.74 (C=O).
anhyd MeCN (160 mL). After 15 min 1,4-bis(bromomethyl)ben-
zene (8) (0.35 g, 1.325 mmol) and silver hexafluorophosphate (0.74
g, 2.88 mmol) were added. The mixture was stirred for 2d under the
exclusion of light. After filtration the solvent was evaporated. The
residue was dissolved in nitromethane (60 mL), and a solution of
Et4NCl in nitromethane (8 mL, 20%) was added. The water-soluble
precipitate was separated by MPLC using Lichroprep Si 60 (15–25
mm) with MeOH/1 M NH4Cl/ nitromethane (7:2:1). Yield: 640 mg
(16%). [a]2D1 = -1.7 (c = 1.55, in acetone).
C44H64O21: calc. C 56.8, H 6.94; found C 57.1, H 7.25.
FAB-MS: m/z = 928.4 (M+).
As a byproduct, the b-D-bisglucoside 47 was isolated. Yield: 2.2 g
(5%).
[a]2D1 = 5.29 (c = 1.6, in CHCl3).
FAB-MS: m/z = 1258.5 (M+).
1H NMR (400 MHz, acetone-d6, 25 °C): d = 1.83–2.15 (m, 24H,
OCCH3), 3.57–4.15 (m, 72H, OCH2), 4.19–4.29 (m, 2H, 5-H), 4.53
(br s, 4H, CH2OAc), 4.77 (br s, 2H, 1-H), 4.90 (dd, 2H, 3JH,H = 7.9
Hz, 3JH,H = 9.6 Hz, 2-H), 5.01 (t, 2H, 3JH,H = 9.6 Hz, 4-H), 5.21 (br
s, 2H, 3-H), 5.99 (s, 8H, NCH2), 6.79 (s, 4H, ar. H), 6.85 (s, 4H, ar.
meta-Axle 6m
At r.t. 3 (7.83 g, 8.43 mmol) and m-azobenzoic acid dichloride 5m
(1.29 g, 4.2 mmol) were dissolved in THF (40 mL). After the solu-
tion had been cooled to +4°C, a solution of pyridine (0.67 g, 8.48
mmol) and a few crystals of 4-dimethylaminopyridine in THF (10
mL) were added dropwise under stirring. After 12 h the precipitate
was filtered off and the solvent was evaporated. The residue was
separated using Lichroprep Si 60 (15–25 mm) with CH2Cl2/MeOH
(9.75:0.25) as eluent, to give 6m as a reddish oil. Yield: 6.78 g
(77%). [a]2D1 = -16.5 (c = 1.8, in CHCl3).
3
H), 7.76 (t, 2H, JH,H = 7.5 Hz, ar. H), 7.98–8.59 (m, 22H, ar. H),
9.33 (br s, 8H, bipy-H).
13C NMR (100.6 MHz, acetone-d6, 25 °C): d = 20.59, 20.65, 20.72
(OCCH3), 55.45, 62.81, 65.39, 65.78, 68.81, 69.67, 69.89, 70.47,
70.86, 71.09, 71.31, 71.41, 71.82 (CH2), 69.44, 72.16, 72.37, 73.50
(glucose-CH), 101.42, 116.25, 116.30, 123.88, 126.90, 130.92,
131.61, 145.89 (ar. CH), 124.31, 132.61, 133.09, 137.80, 147.65
(Cq), 153.09, 154.09, 166.20, 169.90, 170.13, 170.85 (C=O).
1H NMR (400 MHz, CDCl3, 25 °C): d = 1.95–2.15 (m, 24H,
OCCH3), 3.58–4.15 (m, 72H, OCH2), 4.19–4.29 (m, 2H, 5-H),
4.48–4.56 (m, 4H, CH2OAc), 4.59 (d, 2H, 3JH,H = 8.0 Hz, 1-H), 4.98
(dd, 2H, 3JH,H = 8.1 Hz, JH,H = 9.6 Hz, 2-H), 5.06 (t, 2H, 3JH,H = 9.6
Hz, 4-H), 5.19 (t, 2H, 3JH,H = 9.5 Hz, 3-H), 6.79 (s, 8H, ar. H), 6.82
(s, 8H, ar. H), 7.58 (t, 2H, 3JH,H = 7.9 Hz, ar. H), 8.10 (dt, 2H, 3JH,H
MALDI -TOF (2,5-DHB): m/z = 2757.2 (M+ - 4 PF6; + Na+).
C138H164N6O44P4F24 ◊ 3 H2O (3244.73): calc. C 51.08, H 5.28; found
C 50.74, H 5.29.
After the evaporation of the nitromethane mother liquor, the unre-
acted axle 6m was recovered by CHCl3 extraction.
= 8.1 Hz, 4JH,H = 1.2 Hz, ar. H), 8.17 (dt, 2H, 3JH,H = 7.9 Hz, 4JH,H
1.5 Hz, ar. H), 8.60 (t, 2H, 4JH,H = 1.7 Hz, ar. H).
=
13C NMR (100.6 MHz, CDCl3, 25 °C): d = 20.62, 20.66, 20.69,
20.78 (OCCH3), 61.91, 64.45, 67.96, 69.14, 69.19, 69.88, 70.34,
70.73, 70.75, 70.81 (CH2), 68.35,, 71.23, 71.73, 72.81 (glucose-
CH), 100.84, 115.47, 115.53, 124.57, 126.87, 129.31, 132.24 (ar.
CH), 131.35, 152.31, 153.02 (Cq), 153.04. 153.07, 165.90, 169.42,
169.45, 170.30, 170.71 (C=O).
para-Rotaxane 9p
As described for 9m the rotaxane 9p is formed from 6p (3.66 g, 1.75
mmol), 7 (0.62 g, 0.875 mmol), 8 (0.24 g, 0.9 mmol), silver hexaflu-
orophosphate (0.51 g, 2 mmol) in MeCN (130 mL). Yield: 120 mg
(4%). [a]2D1 = -4.1 (c = 1.25, in acetone).
1H NMR (400 MHz, acetone-d6, 25 °C): d = 1.80–2.15 (m, 24H,
OCCH3), 3.55–4.15 (m, 72H, OCH2), 4.18–4.30 (m, 2H, 5-H), 4.52
(br s, 4H, CH2OAc), 4.77 (br s, 2H, 1-H), 4.90 (dd, 2H, 3JH,H = 8.1
Hz, 3JH,H = 9.8 Hz, 2-H), 5.02 (t, 2H, 3JH,H = 9.4 Hz, 4-H), 5.20 (br
s, 2H, 3-H), 6.00 (s, 8H, NCH2), 6.83 (s, 4H, ar. H), 6.86 (s, 4H, ar.
H), 7.95–8.35 (m, 24H, ar. H), 9.36 (d, 8H, 3JH,H = 5.8 Hz, bipy-H).
13C NMR (100.6 MHz, acetone-d6, 25°C): d = 20.58, 20.64, 20.71
(OCCH3), 55.47, 62.80, 65.39, 65.79, 68.81, 69.88, 70.48, 70.87,
71.08, 71.41 (CH2), 69.44, 72.16, 72.37, 73.50 (glucose-CH),
101.41, 116.29, 116.31, 123.87, 126.92, 130.89, 131.61, 145.91 (ar.
CH), 128.11, 137.82, 147.68 (Cq), 154.10, 166.15, 170.13 (C=O).
C138H164N6O44P4F24◊4H2O (3262.75): calc. C 50.80, H 5.31; found
C 50.94, H 5.31.
FAB-MS: m/z = 2091.6 (M+).
para-Axle 6p
At r.t. 3 (6.35 g, 6.84 mmol) and p-azobenzoic acid dichloride 5p
(1.05 g, 3.42 mmol) were dissolved in THF (40 mL). After the so-
lution had been cooled to +4°C, a solution of pyridine (0.54 g, 6.84
mmol) and a few crystals of 4-dimethylaminopyridine in THF (10
mL) were added dropwise under stirring. After 24 h the precipitate
was filtered off, and the solvent was evaporated. The residue was
separated using Lichroprep Si 60 (15–25 mm) with CH2Cl2/MeOH
(9.75:0.25) as eluent, to give 6p as a reddish oil. Yield: 5.4 g (75%).
[a]2D1 = -6.3 (c = 1.4, in CHCl3).
1H NMR (400 MHz, CDCl3, 25 °C): d = 1.85–2.10 (m, 24H,
OCCH3), 3.50–4.06 (m, 72H, OCH2), 4.15–4.25 (m, 2H, 5-H),
4.41–4.51 (m, 4H, CH2OAc), 4.55 (d, 2H, 3JH,H = 7.9 Hz, 1-H), 4.92
(dd, 2H, 3JH,H = 8.0 Hz, 3JH,H = 9.5 Hz, 2-H), 5.01 (t, 2H, 3JH,H = 9.4
Hz, 4-H), 5.15 (t, 2H, 3JH,H = 9.4 Hz, 3-H), 6.78 (s, 8H, ar. H), 6.80
(s, 8H, ar. H), 7.90 (d, 4H, 3JH,H = 8.5 Hz, ar. H), 8.15 (d, 4H, 3JH,H
= 8.5 Hz, ar. H).
MALDI-TOF (2,5-DHB): m/z = 2756.9 (M+ -4 PF6; + Na+).
After the evaporation of the nitromethane mother liquor, the unre-
acted axle 6p was recovered by CHCl3 extraction.
Acknowledgment
13C NMR (100.6 MHz, CDCl3, 25°C): d = 20.53, 20.60, 20.66
(OCCH3), 61.85, 64.39, 67.93, 69.03, 69.09, 69.78, 70.25, 70.71
(CH2), 68.31, 71.19, 71.66, 72.75 (glucose-CH), 100.74, 115.47,
115.48, 115.50, 115.52, 122.87, 130.70 (ar. CH), 132.32, 152.96,
153.01 (Cq), 154.80, 165.73, 169.34, 170.14 (C=O).
Support for this work was provided by the Israel Science Foundati-
on and by the Fonds der Chemischen Industrie.
References
FAB-MS: m/z = 2091.7 (M+).
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ridin-4-ylpyridinium-1-yl)methyl]phenyl}methyl)pyridinium di-
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(9) We also measured the circular dichroism (CD) of 6m, 6p and
9m, 9p. Unfortunately, as in some previous cases,7 these
compounds only show small dichroitic absorptions.
Tetrabenzoylglucose stoppers reported by us15 should be more
suitable, as they offer more efficient chromophores near the
stereocentres.
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Article Identifier:
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Synthesis 1999, No. 5, 849–853 ISSN 0039-7881 © Thieme Stuttgart · New York