Using photoresponsive end-closing and end-opening reactions for the synthesis and disassembly of [2]rotaxanes: Implications for dynamic covalent chemistry

Article abstract of DOI:10.1021/jo8025143

We have synthesized two [2]rotaxanes, each possessing a (Z)-α-methylstilbene unit as one of its stoppers, in good yield through the photoisomerization of terminal (E)-α-methylstilbene units of dialkylammonium salts in the presence of the crown ether diben

Full text of DOI:10.1021/jo8025143

Using Photoresponsive End-Closing and End-Opening Reactions for
the Synthesis and Disassembly of [2]Rotaxanes: Implications for
Dynamic Covalent Chemistry
Yuji Tokunaga,*,† Koichiro Akasaka,† Nobuharu Hashimoto,† Shou Yamanaka,‡
Kenji Hisada,‡ Youji Shimomura,† and Suzuka Kakuchi†
Department of Materials Science and Engineering, Faculty of Engineering, UniVersity of Fukui,
Fukui, 910-8507, Japan, and Department of Fiber Amenity Engineering, Faculty of Engineering,
UniVersity of Fukui, Fukui, 910-8507, Japan
tokunaga@matse.u-fukui.ac.jp
ReceiVed NoVember 25, 2008
We have synthesized two [2]rotaxanes, each possessing a (Z)-R-methylstilbene unit as one of its stoppers,
in good yield through the photoisomerization of terminal (E)-R-methylstilbene units of dialkylammonium
salts in the presence of the crown ether dibenzo[24]crown-8 (DB24C8). The synthesis relies on the
formation of pseudorotaxane intermediates through hydrogen bond-guided self-assembly and subsequent
end-closing photoisomerization. An (E)-R-methylstilbene unit is not sufficiently bulky to prevent
dissociation of the DB24C8 unit, whereas a (Z)-R-methylstilbene unit acts as a true stopper. We also
synthesized these [2]rotaxanes from the (Z)-R-methylstilbene-terminated axle-like salts though thermo-
dynamic covalent chemistry by taking advantage of the reversibility of the photoisomerization. To dissociate
the components of the [2]rotaxanes, we performed the reverse end-opening process under UV irradiation
(i.e., Z-to-E isomerization of the R-methylstilbene termini) in a polar solvent. These rotaxanes are stable
at room temperature, but dissociate slowly to their two components at elevated temperatures.
Introduction
anes possess two or more stations that interact noncovalently
to varying degrees with the macrocyclic component; photoir-
radiation alters the strength of these interactions. Other photo-
responsive systems possessing rotaxane-like structures function
through the association or dissociation of pseudorotaxanes in
response to photoirradiation. Copper(I),3 anthracene,4,5 cyclo-
heptatriene,6 bipyridinium/hydroquinone systems,7 1,8-naph-
thalimide,8 and transition metal complexes9,10 have all been used
as key photoactive units in reversible switching systems that
Artificial molecular switches based on photoreactions rely
on differences in the chemical properties of organic and
organometallic compounds in response to irradiation.1 Several
rotaxanes function as switchable systems based on photocon-
trolled translational motion (molecular shuttling).2 Such rotax-
† Department of Materials Science and Engineering.
‡ Department of Fiber Amenity Engineering.
(1) Selected reviews on photoresponsive artificial molecular switches: (a)
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2000, 39, 3348–3391. (c) Feringa, B. L. In Molecular Switches; Wiley-VCH:
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2374 J. Org. Chem. 2009, 74, 2374–2379
10.1021/jo8025143 CCC: $40.75 2009 American Chemical Society
Published on Web 02/18/2009

Synthesis and Disassembly of [2]Rotaxanes
function through photoresponsive reactions and their reversal
under orthogonal conditions.
conditions: i.e., the Z (or E) isomer slowly isomerizes to the E
(or Z) isomer under the conditions that predominantly afford
the Z (or E) isomer. The E isomer possesses a planar conforma-
tion because of its extended π conjugation, whereas the two
benzene rings of the Z isomer are slightly twisted about the
olefinic bond because of the steric repulsion between the protons
at the 2 and 2′ positions. Accordingly, a stilbene unit is “thicker”
(less flat) after isomerization of the E isomer to the Z isomer.
Recently Leigh and co-workers reported a molecular information
ratchet using the rotaxane system, which consists of asymmetric
ammonium stations and the photoresponsive R-methylstilbene
“gate” in the axle and of a crown ether possessing the
photosensitizer part.12f
In this paper we report (1) the synthesis of two [2]rotaxanes
through the photoisomerization of terminal (E)-stilbene units
to their Z isomers in the presence of dibenzo[24]crown-8
(DB24C8) in an aprotic solvent (a so-called “end-closing”
process), (2) the synthesis of a rotaxane through UV irradiation
of a terminal (Z)-stilbene unit in the presence of DB24C8 under
thermodynamic control,17-21 and (3) the degradation of these
rotaxanes (a) through photoisomerization of the (Z)-stilbene unit
to its E isomer on the rotaxane axle (a so-called “end-opening”
process) and (b) under thermal conditions in a polar solvent
(Figure 1).22
Photochemical reactions that afford a change in the three-
dimensional structure of a molecule can be considered as the
signaling of a molecular switch.1 For example, the Z-to-E
isomerization of a carbon-carbon double bond in the reti-
nylidene chromophore in rhodopsin plays an important role in
the principal visual process of the eye. The activity of rhodopsins
is triggered by the photoinduced isomerization of the corre-
sponding chromophores. The conformational change that occurs
upon isomerization is usually referred to as the primary event
of the protein photocycle.11 Rotaxane switching systems have
been developed to take advantage of structural changes arising
from photoisomerization reactions; in these systems, the geo-
metrical changes of carbon-carbon12 or nitrogen-nitrogen12e,13,14
double bonds affect the molecular shuttling or association/
dissociation reactions between the ring-shaped and axle-like
components.15
Stilbene is one of the most widely investigated molecules in
organic photochemistry and photophysics.16 (E)-Stilbene isomer-
izes to its Z isomer upon exposure to UV light in the presence
of an approximate sensitizer. Upon irradiation in the presence
of an alternative sensitizer, the Z isomer reverts back to its E
isomer. Both reactions proceed reversibly under these irradiation
Results and Discussion
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Design Principle. For this study, we selected the well-
established self-assembly of dialkylammonium ions and
DB24C823 because of the strong hydrogen bonding between
the components and the adequate size of the DB24C8 cavity
with respect to a stilbene unit. Rotaxanes possessing stilbene
units on their axle moieties have been reported previously by
the Stoddart group, but these nonsubstituted (E)- and (Z)-stilbene
units were not sufficiently large to prevent disassembly of the
components.24 Computer simulation revealed that introduction
of a methyl group at the R position of stilbene increases its
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J. Org. Chem. Vol. 74, No. 6, 2009 2375

Tokunaga et al.
FIGURE 1. Outline of the end-closing synthesis of a [2]rotaxane, its end-opening degradation through photoisomerization, and thermal degradation.
SCHEME 1
FIGURE 2. Space-filling representations of the energy-minimized
structures of (a) (E)-R-methylstilbene, (b) (Z)-stilbene, and (c) (Z)-R-
methylstilbene.
bulk and causes a dramatic difference between the sizes of the
(E)- and (Z)-R-methylstilbenes (Figure 2). Because the isopro-
pylphenyl group permits slippage of DB24C8,25 we expected
this macrocycle to have little trouble passing over the linear
and flat (E)-R-methylstilbene terminus (Figure 2a), i.e., through
a mechanism involving (i) passage over the p-phenylene unit,
CDCl3/CD3CN (1:1) revealed the presence of (E)-4 through the
appearance of signals for the CH2N+CH2 protons at 4.67 and
4.77 ppm and for the aliphatic and aromatic protons of the
DB24C8 component at 3.59-3.63 and 6.83-6.93 ppm, respec-
tively. Such chemical shifts are typical for these types of
rotaxanes.23 In contrast, we observed no new signals in the 1H
NMR spectrum of a mixture of (Z)-3 and DB24C8 in CDCl3/
CD3CN, suggesting that the (Z)-R-methylstilbene unit is an
impassable stopper. Next, we examined the feasibility of forming
a rotaxane through photoisomerization of (E)-4. UV irradiation
of a CDCl3/CD3CN (1:1) mixture of (E)-3 (272 mM) and
DB24C8 (408 mM), in the presence of benzil16 (272 mM) as a
sensitizer, led to efficient E-to-Z isomerization of the stilbene
unit of the pseudorotaxane (Figure 1 end-closing process: (E)-3
+ DB24C8 f (Z)-4)), resulting in the formation of the
[2]rotaxane (Z)-4 in a 73% isolated yield (NMR yield: 87%).26,27
The (Z)-R-methylstilbene moiety was sufficiently bulky to
prevent unthreading at room temperature in polar solvents in
(ii) “slippage” over the isopropylene unit, and (iii) dissociation
over the terminal phenyl group. In contrast, molecular modeling
of the (Z)-R-methylstilbene terminus (Figure 2c) suggested that
a DB24C8 unit would dissociate through a different mechanism:
hovering over the p-phenylene unit (but not passing over it
completely), stretching to overcome the R-methyl group, and
then passing over the phenyl group. The need to pass over the
methyl and phenylene groups simultaneously would make the
(Z)-R-methylstilbene unit a much bulkier stopper unit for a
rotaxane possessing DB24C8 as its ring-shaped component.
Synthesis of Axle Components. We synthesized the key axle
components 3 using a previously reported procedure (Scheme
1).22 4-Carboxybenzaldehyde dimethyl acetal 1 was first me-
thylated and then subjected to a Wittig reaction; deprotection
of the acetal provided a separable mixture of the (E)- and (Z)-
stilbene derivatives 2 in 45% and 43% yields, respectively.
Condensation of these isomers with 4-tert-butylbenzylamine and
subsequent reduction of the imines produced the corresponding
secondary amines, which we converted into the dialkylammo-
nium salts (E)- and (Z)-3 through protonation and counterion
exchange.
1
the dark. We characterized the [2]rotaxane (Z)-4 using H and
13C NMR spectroscopy, FAB mass spectrometry, and IR
spectroscopy; the 1H NMR spectrum was fully assigned by using
1H-1H COSY and ROESY techniques. The shifts to higher field
for the signals of the crown ether and to lower field for those
of the benzylic protons of the axle support the presence of the
components in a rotaxane structure. In the FAB mass spectrum,
Rotaxane Synthesis. We found that the (E)-R-methylstilbene
terminus was sufficiently small to allow its passage through the
cavity of DB24C8. The solubility of (E)-3 in CDCl3/CD3CN
was poor; the addition of DB24C8 improved its solubility,
suggesting the immediate formation of pseudorotaxane (E)-4.
1
(26) The use of 9-fluorenone, duroquinone, and pyrene as photosensitizers
provided the [2]rotaxane (Z)-4 in 54% (NMR yield: 79%), 71% (79%), and 58%
(77%) yields, respectively.
A H NMR spectrum of a mixture of (E)-3 and DB24C8 in
(25) Ashton, P. R.; Baxter, I.; Fyfe, M. C. T.; Raymo, F. M.; Spencer, N.;
Stoddart, J. F.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1998, 120,
2297–2307.
(27) The geometry ratio of the R-methylstilbene was similar to Leigh’s results
in indirect photoisomerization reaction with benzil as a sensitizer (Z:E )
80:20).12f
2376 J. Org. Chem. Vol. 74, No. 6, 2009

Synthesis and Disassembly of [2]Rotaxanes
FIGURE 4. Plots of the concentrations of (E)-3, (Z)-3, (E)-4, and (Z)-4
over time during the irradiation of a CDCl3/CD3CN solution (1:1) of
(Z)-3 and DB24C8 in the presence of benzil at 0 °C. The photoreaction
was performed in an NMR tube under nitrogen with light from a high-
pressure mercury lamp.
FIGURE 3. Plots of the concentrations of (E)-3, (Z)-3, (E)-4, and (Z)-4
over time during the irradiation of a CDCl3/CD3CN (1:1) solution of
(E)-3 and DB24C8 in the presence of benzil at 0 °C. The photoreaction
was performed in an NMR tube under nitrogen with light from a high-
pressure mercury lamp.
the presence of an intense peak at m/z 818, corresponding to
the molecular weight of the [2]rotaxane in the absence of its
counterion, confirmed the structural assignment.
end-closing process: (Z)-3 + DB24C8 f (Z)-4): (i) (Z)-3
isomerized under UV irradiation to afford (E)-3; (ii) DB24C8
and (E)-3 self-assembled to produce the [2]pseudorotaxane (E)-4
through hydrogen bonding between the NH2+ unit and the crown
ether; and (iii) (Z)-4 was produced through isomerization of
(E)-4. As a result, the thermodynamically stable and photoi-
somerizationally predominant [2]rotaxane (Z)-4 was afforded
though a sequence of end-opening and end-closing processes.
The final compositions of the [2]rotaxane mixtures formed
after irradiation of the E and Z isomers in Figures 3 and 4 were
similar. The ratio of compounds (E)-3, (Z)-3, (E)-4, and (Z)-4
was 3:20:18:59 after irradiation of (E)-3 in the presence of
DB24C8 and benzil for 930 s; it was 6:24:12:58 after irradiation
of (Z)-3 in the presence of DB24C8 and benzil for 3600 s. Note
that the synthesis with the Z isomer required a longer reaction
time, despite the otherwise identical reaction conditions. This
finding suggests that the reaction rate for the isomerization of
(Z)-3 is slower than that of its E isomer. In addition, the longer
UV irradiation time results in decomposition of the axle-like
units, thereby decreasing the yield of the [2]rotaxane in this
case. The photoreactions of the E and Z isomers generated ca.
14 and 11 mM solutions, respectively, of the rotaxane (Z)-4
after irradiating moderate initial concentrations of the precursors
for 10 (Figure 3) and 30 min (Figure 4), respectively.
1
We used H NMR spectroscopy to monitor the progress of
the photochemical isomerization of (E)-3 leading to the [2]ro-
taxane.28 After irradiating a solution containing equimolar (23
mM) quantities of (E)-3, DB24C8, and benzil in CDCl3/CD3CN
(1:1) at 0 °C, we observed over time the appearance and
disappearance of resonances assigned to (Z)-4, (Z)-3, (E)-4, and
(E)-3. The decreases in intensity of the signals of the protons
of the E isomers were concomitant with the increases in the
intensities of the signals associated with the Z isomers during
the course of the reaction (Figure 3). On the basis of integration
of pertinent signals in these spectra, we determined the
association constant for the formation of the [2]pseudorotaxane
29
(E)-4 to be 900 ( 110 M-1
.
1
There was a significant difference between the H NMR
spectra of the crude products obtained at high (272 mM) and
moderate (23 mM) concentrations. At high concentration, we
identified only the signals of the substrates, expected products,
and sensitizer; in contrast, we observed signals for some
unidentified products when performing the reaction at moderate
concentration, where the percentage of products decreased after
12 min (Figure 3). It is likely that the ca. 20% of axle-like
species that existed in the uncomplexed form at moderate
concentration decomposed under UV irradiation.14a,30 Indeed,
long-term irradiation of a solution of the axle in the absence of
DB24C8 afforded complex mixtures.
Furthermore, we also prepared (Scheme 2) the dialkylam-
monium salts 5, obtained as a 2:1 mixture of the E and Z
regioisomers, possessing 3,5-dimethylphenyl groups as bulky
stoppers on one end; we used the same photoisomerization
reaction to convert these isomers into the corresponding
[2]rotaxane (Z)-6. Thus, irradiating a suspension of the dialky-
lammonium salts 5 in the presence of DB24C8 and benzil for
330 min gave the corresponding [2]rotaxane (Z)-6 in 71%
isolated yield (NMR yield: 76%).
Rotaxane Formation through Dynamic Covalent Chem-
istry. The salt (Z)-3 is only partially soluble in CDCl3/CD3CN
at high concentrations, even when DB24C8 is present, because
the species cannot form a [2]pseudorotaxane. We used 1H NMR
spectroscopy to monitor the progress (Figure 4) of the UV
irradiation of a CDCl3/CD3CN (1:1) solution of (Z)-3 (23 mM)
and DB24C8 (23 mM) in the presence of benzil (23 mM).28
Three equilibrating reactions occurred (Figure 1 process 1 and
Degradation of [2]Rotaxane through Photochemical
and Thermal Reactions. We examined the rate of the end-
opening reaction of the [2]rotaxane (Z)-4. Irradiation of a
solution of (Z)-4 (8.5 mM) and benzophenone (8.5 mM)sa
nonselective sensitizer16sin DMSO-d6, which disrupts hydrogen
bonding between the axle- and wheel-like components, led to
the production of (E)-3, (Z)-3, and DB24C8. Monitoring this
(28) See the Supporting Information.
(29) The association constant was measured at 25 °C with the following
equation: K ) [pseudorotaxane]/[ammonium ion + ammonium salt][DB24C8].
(30) Rotaxane-encapsulation can enhance the stability of the axle’s compo-
nents; for examples, see: (a) Parham, A. H.; Windisch, B.; Vo¨gtle, F. Eur. J.
Org. Chem. 1999, 1233–1238. (b) Buston, J. E. H.; Young, J. R.; Anderson,
H. L. Chem. Commun. 2000, 905–906. (c) Craig, M. R.; Hutchings, M. G.;
Claridge, T. D. W.; Anderson, H. L. Angew. Chem., Int. Ed. 2001, 40, 1071–
1074. (d) Ghosh, P.; Mermagen, O.; Schalley, C. A. Chem. Commun. 2002,
2628–2629. (e) Choi, S.; Park, S. H.; Ziganshina, A. Y.; Ko, Y. H.; Lee, J. W.;
Kim, K. Chem. Commun. 2003, 2176–2177. (f) Arunkumar, E.; Forbes, C. C.;
Noll, B. C.; Smith, B. D. J. Am. Chem. Soc. 2005, 127, 3288–3289. (g) Hsueh,
S.-Y.; Lai, C.-C.; Liu, Y.-H.; Wang, Y.; Peng, S.-M.; Chiu, S.-H. Org. Lett.
2007, 9, 4523–4526.
1
process with H NMR spectroscopy revealed that the photo-
degradation of (Z)-4 fitted a first-order curve; the half-life was
103 min ((Figure 1 end-opening process and process 1′: (Z)-4
f (E) and (Z)-3 + DB24C8, Figure 5).28
Next, we investigated the thermal degradation of the [2]ro-
taxane (Z)-6 (stopper unit: 3,5-dimethylphenyl group) (Figure
1 thermal degradation: (Z)-6 f (Z)-5 + DB24C8 and (Z)-4 f
J. Org. Chem. Vol. 74, No. 6, 2009 2377

Tokunaga et al.
SCHEME 2
(Z)-3 + DB24C8). We heated a solution of (Z)-6 and 3,5-di-
tert-butylphenol (a radical scavenger) in DMSO-d6 in the dark
at 80-110 °C.31 Plots of the concentration of rotaxane (Z)-6,
monitored with 1H NMR spectroscopy,28 gave first-order curves;
we obtained the values of kdeg from the slope of the straight
line of the plot of ln([(Z)-6t]/[(Z)-60] against t, where [(Z)-60]
and [(Z)-6t] correspond to the initial concentration of (Z)-6 and
the concentration of (Z)-6 at time t, respectively (Figure 6). On
the basis of transition state theory, we estimated the activation
enthalpy (∆Hq), entropy (∆Sq), and free energy (∆Gq) at 25 °C
to be 56.5 kJ/mol, -210 J/mol K, and 119 kJ/mol, respectively.32
We also obtained kinetic parameters of degradation of (Z)-4:
∆Hq ) 60.3 kJ/mol, ∆Sq ) -199 J/mol K, and ∆Gq ) 120
kJ/mol at 25 °C.28 These kinetic parameters are close in value
to those previously reported for the decomplexation of a rotaxane
consisting of a mono-meta-fused dibenzo[25]crown-8 unit and
a tert-butyl stopper in methanol (∆Hq ) 60.2 kJ/mol; ∆Sq )
-144 J/mol K; ∆Gq ) 103 kJ/mol).33 In our system, however,
entropy contributes more strongly to the stability of the transition
state than it did in the previous case. We suspect that several
factors are responsible for this phenomenon: (1) the R-meth-
ylstilbene stopper may be strongly restricted by DB24C8 in the
transition state; (2) differences of stoppers; and (3) differences
in solvation.
Conclusion
It is possible to synthesize [2]rotaxanes comprising DB24C8
and secondary dialkylammonium ion units through the end-closing
isomerization of (E)-R-methylstilbene termini to their Z isomer
counterparts. The end-closing process, which includes [2]pseu-
dorotaxane formation and photoisomerization reactions, forms
[2]rotaxanes predominantly in aprotic, nonpolar solvents in the
presence of an appropriate sensitizer. We also succeeded in
constructing these [2]rotaxanes from the dialkylammonium ions
possessing (Z)-R-methylstilbene termini, taking advantage of the
reversible Z-to-E isomerization reactions. This process consisted
of three reversible reactions: isomerization of the (Z)-R-methyl-
stilbene unit of the dialkylammonium ion, [2]pseudorotaxane
formation, and isomerization of the (E)-R-methylstilbene unit in
the [2]pseudorotaxane. UV irradiation of the (Z)-R-methylstilbene
under appropriate conditions shifts the equilibria to favor the
formation of the [2]rotaxane. The photoinduced end-opening
reaction fits a first-order curve to afford the axle- and wheel-like
components when the [2]rotaxane is dissolved in a polar solvent
in the presence of an appropriate sensitizer. We also investigated
the thermal stability of [2]rotaxanes, monitoring the slow disas-
sociation of DB24C8 from the NH2+ center at high temperature
to afford the kinetic parameters.
FIGURE 5. Plots of the concentrations of (E)-3, (Z)-3, and (Z)-4 over
time during the photodegradation of the [2]rotaxane (Z)-4. The reactions
were performed in an NMR tube under nitrogen with light from a high-
pressure mercury lamp.
Experimental Section
(E)- and (Z)-4-(1-Methyl-2-phenylvinyl)benzaldehyde [(E)-
and (Z)-2]. Butyllithium (1.59 M in hexane; 8.17 mL, 13.0 mmol)
was added to a stirred suspension of benzylphosphonium chloride
(5.56 g, 14.3 mmol) in THF (55 mL) at 0 °C and then the resulting
mixture was stirred at 0 °C for 30 min. At this point, a solution of
4-acetylbenzaldehyde dimethylacetal (1.26 g, 6.49 mmol) in THF
(13 mL) was added dropwise at 0 °C. After being stirred at room
temperature overnight, the reaction mixture was quenched with 10%
HCl, the organic solvent was evaporated off, and the residue was
extracted with EtOAc. The extracts were washed with saturated
brine and the residue obtained upon workup was chromatographed
(100:1) to afford the (E)-2 (0.648 g, 45%) as colorless needles after
recrystallization (hexane). Mp 77-78 °C. IR νmax (KBr) 2976, 2939,
1
2895, 1701, 1599 cm-1. H NMR (300 MHz, CDCl3) δ 10.03 (s,
1H), 7.91-7.87 (m, 2H), 7.71-7.66 (m, 2H), 7.43-7.36 (m, 4H),
7.32-7.25 (m, 1H), 6.97 (br s, 1H), 2.32 (br s, 3H). 13C NMR
(125 MHz, CDCl3) δ 191.4, 149.6, 137.4, 136.0, 134.9, 130.0,
129.6, 129.0, 128.1, 126.8, 126.2, 17.0. Anal. Calcd for C16H14O:
C; 86.45, H; 6.35. Found: C; 86.57, H; 6.50. MS (FAB) m/z for
C16H14O [M + H]+ calcd 223, found 223. Further elution (SiO2;
FIGURE 6. (a) Plots of ln([(Z)-6t]/[(Z)-60] over time at 80 °C (O), 86
°C (∆), 92 °C (3), 96 °C (]), 100 °C (0), and 110 °C (×). (b) Plots
of ln k versus 1/T.
2378 J. Org. Chem. Vol. 74, No. 6, 2009

Synthesis and Disassembly of [2]Rotaxanes
hexane/EtOAc, 20:1) yielded (Z)-2 (0.615 g, 43%) as a powder.
Mp 38-40 °C. IR νmax (KBr) cm-1 2976, 2939, 2895, 1732, 1522.
1H NMR (300 MHz, CDCl3) δ 9.98 (s, 1H), 7.81-7.77 (m, 2H),
7.37-7.33 (m, 2H), 7.14-7.08 (m, 3H), 6.95-6.90 (m, 2H), 6.58
(br s, 1H), 2.23 (d, J ) 1.5 Hz, 3H). 13C NMR (125 MHz, CDCl3)
δ 191.7, 148.7, 137.2, 136.9, 134.9, 129.8, 129.0, 128.9, 128.2,
127.9, 126.5, 26.3. Anal. Calcd for C16H14O: C; 86.45, H; 6.35.
Found: C; 86.34, H; 6.60. MS (FAB) m/z for C16H14O [M + H]+
calcd 223, found 223.
hexane and toluene [1:1 (v/v), 8 mL] were added and then the mixture
was stirred for 1 day at room temperature. The solid was filtered off
and then purified through chromatography (SiO2; hexane/EtOAc, 1:2)
to yield crude (Z)-4, which was washed with toluene and hexane to
afford (Z)-4 (95.1 mg, 73%) as a solid. Mp 123-125 °C. IR νmax (KBr)
3161, 2934, 2876, 1506, 1456, 1254, 1124, 1055, 840, 557 cm-1. 1H
NMR (500 MHz, CDCl3) δ 7.75-7.40 (br s, 2H), 7.30-7.20 (m, 2H),
7.20-7.10 (m, 4H), 7.10-7.02 (m, 5H), 6.95-6.83 (m, 6H), 6.80-6.73
(m, 4H), 6.50 (br s, 1H), 4.65-4.56 (m, 2H), 4.53-4.42 (m, 2H),
4.18-4.02 (m, 8H), 3.83-3.68 (m, 8H), 3.55-3.37 (m, 8H), 2.50 (d,
J ) 1.5 Hz, 3H), 1.23 (s, 9H). 13C NMR (125 MHz, CDCl3) δ 152.5,
147.5, 143.0, 137.7, 137.6, 130.5, 129.3, 129.1, 129.0, 128.7, 128.3,
127.9, 127.3, 126.3, 125.5, 121.8, 112.7, 70.6, 70.2, 68.2, 52.4, 52.2,
34.6, 31.2, 26.8. Anal. Calcd for C51H64F6NO8P·0.5H2O: C, 62.95;
H, 6.73; N, 1.44. Found: C, 62.85; H, 6.57; N, 1.59. MS (FAB) m/z
for C51H64NO8+ [M - PF6]+ calcd 818, found 818.
(E)-N-(4-tert-Butylbenzyl)-4-(1-methyl-2-phenylvinyl)benzyl-
ammonium Hexafluorophosphate [(E)-3]. A suspension of the
aldehyde (E)-2 (0.300 g, 1.35 mmol), 4-tert-butylbenzylammo-
nium chloride (0.268 g, 1.35 mmol), potassium carbonate (0.934
g, 6.76 mmol), and magnesium sulfate (3.25 g, 27.0 mmol) in
THF was stirred at room temperature for 4 h. The reaction
mixture was filtered and the filtrate concentrated. The residue
(0.464 g) obtained upon workup was taken up into THF/ethanol
[1:1 (v/v), 4 mL] and cooled to 0 °C; sodium borohydride (76.6
mg, 2.03 mmol) was added and the mixture was stirred at room
temperature for 16 h. The resulting mixture was treated with
10% HCl and then the solvent was evaporated off to give a
residue; 10% NaOH was added and the mixture extracted with
chloroform. The extracts were washed with saturated brine, dried,
and evaporated to give a residue; 10% HCl was added at 0 °C
and then the water was evaporated. The residue was washed
with diisopropyl ether to provide the hydrochloride salt (0.430
g). Saturated aqueous ammonium hexafluorophosphate (1 mL) was
added to a suspension of the hydrochloride salt (0.350 g) in water/
acetone [1:1 (v/v), 30 mL] at 0 °C and then the mixture was stirred
for 1 h at 0 °C. After evaporation of the solvent, the solid was filtered
off and washed with water and benzene to afford (E)-3 [0.361 g, 64%
from (E)-2]. Mp >230 °C. IR νmax (KBr) 3251, 2961, 2787, 1600,
1411, 837, 559 cm-1. 1H NMR (500 MHz, DMSO-d6) δ 9.40-9.04
(br, 2H), 7.75-7.71 (m, 2H), 7.59-7.51 (m, 4H), 7.51-7.44 (m, 6H),
7.38-7.32 (m, 1H), 7.04-7.01 (m, 1H), 4.31-4.17 (m, 4H), 2.33-2.29
(m, 3H), 1.35 (s, 9H). 13C NMR (125 MHz, DMSO-d6) δ 151.7, 143.7,
137.5, 135.9, 130.6, 130.0, 129.7, 129.0, 128.9, 128.2, 127.8, 126.7,
126.0, 125.5, 49.77, 49.76, 34.4, 31.0, 17.0. HR-MS (FAB) m/z for
C27H32N+ [M - PF6]+ calcd 370.2535, found 370.2533.
(E)- and (Z)-N-(3,5-Dimethylbenzyl)-4-(1-methyl-2-phenylvi-
nyl)benzylammonium Hexafluorophosphate [(E)- and (Z)-5].
Imine formation from a mixture of aldehydes (E:Z ) 2:1; 1.11 g, 5.00
mmol), reduction, and salt formation were performed as described above,
yielding (E)- and (Z)-5 (1.50 g, 3.08 mmol, 65%, three steps; E:Z ) 2:1)
as a powder. Mp 173-175 °C. IR νmax (KBr) 3250, 3229, 2949, 2920,
1
1611, 842, 559 cm-1. H NMR (300 MHz, CDCl3:CD3CN, 1:1) δ
7.66-7.61 (m, 4/3H), 7.53-7.23 (m, 6H), 7.14-7.00 (m, 4H), 6.94-6.88
(m, 4/3H), 6.53-6.54 (br s, 1/3H), 4.24-4.08 (m, 4H), 2.34 (s, 6H), 2.28
(d, J ) 1.5 Hz, 2H), 2.21 (d, J ) 1.5 Hz, 2H). 13C NMR (125 MHz,
CDCl3) δ 143.7, 142.4, 137.81, 137.78, 137.7, 137.5, 137.0, 135.9, 131.7,
131.6, 130.7, 130.5, 130.30, 130.28, 130.2, 130.1, 129.1, 128.6, 128.3,
128.2, 128.0, 127.8, 127.6, 126.8, 126.7, 126.3, 126.0, 50.14, 50.06, 49.9,
49.8, 26.7, 20.81, 20.79, 17.1. Anal. Calcd for C25H28F6NP: C, 61.60; H,
5.79; N, 2.87. Found: C, 61.64; H, 5.87; N, 2.73. MS (FAB) m/z for
C25H28N+ [M - PF6]+ calcd 342, found 342.
[2]-{(Dibenzo[24]crown-8)-[(Z)-N-(3,5-dimethylbenzyl)-4-(1-
methyl-2-phenylvinyl)benzylammonium]}rotaxane Hexafluoro-
phosphate [(Z)-6]. The isomerization of the ammonium salts (Z)-
and (E)-5 (E:Z ) 2:1; 65.8 mg, 0.135 mmol, 270 mM) in the
presence of DB24C8 (90.8 mg, 0.203 mmol, 405 mM) and benzil
(28.4 mg, 0.135 mmol, 270 mM) was performed as described above
to give (Z)-6 (44.4 mg, 71%) as a powder. Mp 138.5-140.0 °C.
IR νmax (KBr) 3293, 3145, 2923, 2853, 1505, 1256, 1122, 841, 558
(Z)-N-(4-tert-Butylbenzyl)-4-(1-methyl-2-phenylvinyl)benzyl-
ammonium Hexafluorophosphate [(Z)-3]. Imine formation of
(Z)-2 (0.153 g, 0.689 mmol) followed by reduction and salt
formation was performed as described above to give (Z)-3 (0.171
g, 48%) as a powder. Mp >200 °C. IR νmax (KBr) 3250, 2964,
cm-1 1H NMR (500 MHz, CDCl3) δ 7.68-7.48 (br, 2H),
.
7.32-7.27 (m, 2H), 7.13-7.09 (m, 2H), 7.08-7.03 (m, 4H),
6.96-6.86 (m, 5H), 6.84-6.74 (m, 7H), 6.52-6.49 (m, 1H),
4.64-4.57 (m, 2H), 4.42-4.35 (m, 2H), 4.17-4.03 (m, 8H),
3.82-3.70 (m, 8H), 3.53-3.35 (m, 8H), 2.16 (d, J ) 1.2 Hz, 3H),
2.12 (s, 6H). 13C NMR (125 MHz, CDCl3) δ 147.5, 143.0, 138.3,
137.63, 137.57, 131.1, 130.7, 130.4, 129.4, 129.0, 128.7, 127.8,
127.3, 126.8, 126.3, 121.8, 112.7, 70.6, 70.1, 68.3, 52.8, 52.2, 26.7,
21.1. Anal. Calcd for C49H60F6NO8P: C, 62.88; H, 6.46; N, 1.50.
1
2789, 1585, 840, 559 cm-1. H NMR (500 MHz, DMSO-d6) δ
9.35-9.04 (br, 2H), 7.56-7.44 (m, 6H), 7.34-7.27 (m, 2H),
7.21-7.12 (m, 3H), 7.04-6.97 (m, 2H), 6.63 (br s, 1H), 4.27-4.16
(m, 4H), 2.23 (br s, 3H), 1.35 (s, 9H). 13C NMR (125 MHz, DMSO-
d6) δ 151.7, 142.4, 137.7, 137.0, 130.5, 130.2, 129.7, 128.9, 128.6,
128.2, 127.9, 126.7, 126.3, 125.4, 49.9, 49.8, 34.4, 31.0, 26.6. Anal.
Calcd for C27H32F6NP: C; 62.91, H; 6.26, N; 2.72. Found: C; 62.64,
H; 6.20, N; 2.75. MS (FAB) m/z for C27H32N+ [M - PF6]+ calcd
370, found 370.
+
Found: C, 62.62; H, 6.45; N, 1.36. MS (FAB) m/z for C49H60NO8
[M - PF6]+ calcd 790, found 790.
General Procedure for the Decomplexation of Rotaxane
(Z)-6. A solution of the [2]rotaxane (Z)-6 (4 mM) and 2,6-di-tert-
butylphenol (4.8 mM) in DMSO-d6 was heated at 80-110 °C. 1H
NMR spectra were recorded at various time intervals.
[2]-{[(Z)-N-(4-tert-Butylbenzyl)-4-(1-methyl-2-phenylvinyl)-
benzylammonium]-(dibenzo[24]crown-8)}rotaxane Hexafluo-
rophosphate [(Z)-4]. A solution of the ammonium salt (E)-3 (70.0
mg, 0.136 mmol), DB24C8 (91.3 mg, 0.204 mmol), and benzil (28.6
mg, 0.136 mmol) in CDCl3/CD3CN [1:1 (v/v), 0.5 mL] was irradiated
at 0 °C for 1.5 h with light from a 250-W high-pressure mercury lamp
passed through a Pyrex filter. Evaporation of the solvent gave a residue;
Acknowledgment. We thank Professor K. Isa (Fukui Uni-
versity) and Dr Hoshi (Tohoku University) for performing
spectroscopic measurements.
1
Supporting Information Available: H NMR data on the
(31) A Z to E isomerization reaction proceeded in the absence of the radical
scavenger.
progress of the formation of rotaxane ((Z)-4) and degradation
of rotaxanes ((Z)-4 and (Z)-6) and 1H and 13C NMR spectra of
(E)-2, (Z)-2, (E)-3, (Z)-3, (Z)-4, (E)-5, (Z)-5, and (Z)-6. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(32) The transition state theory is described by using the following equation:
kdeg ) (kT/h) exp(-∆Gq/RT), where R, h, and k correspond to the gas, Planck,
and Boltzmann constants, respectively, kdeg is the rate constant of the thermal
degradation reaction, and ∆Gq is the activation free energy, given by the activation
enthalpy (∆Hq) and activation entropy (∆Sq) as ∆Gq ) ∆Hq- T∆Sq.
(33) Chiu, S.-H.; Rowan, S. J.; Cantrill, S. J.; Glink, P. T.; Garrell, R. L.;
Stoddart, J. F. Org. Lett. 2000, 2, 3631–3634.
JO8025143
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