Dynamic covalent approach to [2]- and [3]rotaxanes by utilizing a reversible thiol-disulfide interchange reaction

Article abstract of DOI:10.1002/chem.200204644

A dynamic covalent approach to disulfide-containing [2]- and [3]rotaxanes is described. Symmetrical dumbbell-shaped compounds with two secondary ammonium centers and a central located disulfide bond were synthesized as components of rotaxanes. The rotaxan

Full text of DOI:10.1002/chem.200204644

FULL PAPER
Dynamic Covalent Approach to [2]- and [3]Rotaxanes by Utilizing a
Reversible Thiol Disulfide Interchange Reaction
Yoshio Furusho, Tomoya Oku, Toshihide Hasegawa, Akiyoshi Tsuboi, Nobuhiro Kihara,
and Toshikazu Takata*[a]
Abstract: A dynamic covalent ap-
proach to disulfide-containing [2]- and
[3]rotaxanes is described. Symmetrical
dumbbell-shaped compounds with two
secondary ammonium centers and a
central located disulfide bond were
synthesized as components of rotaxanes.
The rotaxanes were synthesized from
the dumbbell-shaped compounds and
dibenzo-[24]crown-8 (DB24C8) with
catalysis by benzenethiol. The yields of
isolated rotaxanes reached about 90%
under optimized conditions. A kinetic
study on the reaction forming [2]rotax-
ane 2a and [3]rotaxane 3a suggested a
plausible reaction mechanism compris-
ing several steps, including 1) initiation,
2) [2]rotaxane formation, and 3) [3]ro-
taxane formation. The whole reaction
was found to be reversible in the pres-
ence of thiols, and thermodynamic con-
trol over product distribution was thus
possible by varying the temperature,
solvent, initial ratio of substrates, and
concentration. The steric bulk of the
end-capping groups had almost no in-
fluence on rotaxane yields, but the
structure of the thiol was crucial for
reaction rates. Amines and phosphines
were also effective as catalysts. The
structural characterization of the rotax-
anes included an X-ray crystallographic
study on [3]rotaxane 3a.
Keywords: crown compounds
¥
dynamic covalent bonds ¥ rotaxanes
¥ sulfur ¥ supramolecular chemistry
Dynamic covalent chemistry has proved be a powerful way
to synthesize mechanically or topologically interlocked com-
pounds such as catenanes and rotaxanes.[3] To the best of our
knowledge, Harrison reported in 1972 the first use of trityl
ether bonds as dynamic covalent bonds to synthesize rotax-
anes by a statistical approach.[4] Similarly, Schill et al. used
trityl thioethers for the synthesis of interlocked molecules.[5]
Sanders et al. employed ring-closing metathesis in 1998 to
synthesize p-electron-poor/p-electron-rich[2] catenanes, and
this was the first template-directed approach to interlocked
molecules using dynamic covalent bonds under thermody-
namically controlled conditions.[6] Leigh et al. also utilized
Introduction
The synthesis of organic molecules has traditionally been
achieved by kinetically controlled reactions that form
™strong∫ covalent bonds. In this kind of synthesis, reagents
and conditions must be chosen very carefully to avoid
unfavorable bond formations. On the other hand, supra-
molecular chemistry, which utilizes noncovalent bonding
interactions to form supramolecules under thermodynami-
cally controlled conditions, has advanced so far that it is now
possible to create various kinds of complex molecular
assemblies.[1] However, in most cases, the noncovalent bond-
ing interactions are so weak that the supramolecular assem-
blies disassemble into their components upon any change in
conditions. Recently, chemists have opened up a new field of
synthetic chemistry that exploits ™dynamic∫ covalent bonds
which can be formed and broken reversibly under thermody-
namic control.[2]
olefin metathesis in the synthesis of ™magic rings∫ or benzylic
[7]
amide[2] catenanes having a C C bond in each ring. Addi-
ˆ
tionally, Stoddart et al. demonstrated that imine bonds are
very useful as dynamic covalent bonds for the construction of
rotaxanes and catenanes.[8]
One of the most intriguing dynamic covalent bonds exists in
nature: the thiol disulfide interchange reaction is widely
acknowledged to be important in stabilizing protein struc-
tures. Extensive mechanistic studies on thiols and disulfides
by Whitesides et al. showed that i) disulfide exchange takes
place efficiently under mild conditions in the presence of a
catalytic amount of thiol, and ii) disulfides are stable toward
many different functional groups.[9] Still et al. demonstrated
that the composition of an equilibrium system of three
[a] Prof. Dr. T. Takata, Dr. Y. Furusho, T. Oku,
T. Hasegawa, A. Tsuboi, Dr. N. Kihara
Department of Applied Chemistry
Graduate School of Engineering
Osaka Prefecture University
1-1 Gakuen-cho, Sakai, Osaka 599-8531 (Japan)
Fax : (‡81)72-254-9910
E-mail: takata@chem.osakafu-u.ac.jp
Chem. Eur. J. 2003, 9, 2895 2903
DOI: 10.1002/chem.200204644
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T. Takata et al.
FULL PAPER
different disulfides can indeed be influenced by host guest
interactions.[10] Tam-Chang et al. demonstrated an ingenious
use of disulfide bonds to make a capsule molecule.[11] Sanders
et al. took advantage of the thiol disulfide interchange
reaction to make dynamic combinatorial libraries of macro-
cyclic disulfides.[12]
Whereas interlocked molecules such as rotaxanes[13] and
catenanes[14] possessing disulfide linkages have long been
known, we first utilized the reversible nature of disulfide
linkages to synthesize a rotaxane in 2000.[15] We synthesized
the dumbbell-shaped molecule 1a having two secondary
ammonium salt centers and a central disulfide linkage
(Scheme 1). Although dibenzo-[24]crown-8 (DB24C8) is
known to bind secondary dialkylammonium ions in its
macroring, no complexation was observed for a solution of
1a and DB24C8 in CD3CN because the 3,5-di-tert-butylphenyl
groups were too large to invade the cavity of the DB24C8
macroring, even when the solution was heated to 1008C. On
addition of a catalytic amount of benzenethiol, however,
formation of [2]rotaxane 2a and [3]rotaxane 3a was observed,
and the mixture reached equilibrium in 30 d. Thus, this small
amount of benzenethiol acted as the ™key∫ to ™unlock∫ the
disulfide bond, and thereby allowed formation of the rotax-
anes. Here we describe the dynamic covalent approach to
rotaxanes having disulfide linkages in detail and the structures
of the rotaxanes, highlighting the reversible nature of disulfide
linkages.
Results and Discussion
Synthesis of axles: Dumbbell-shaped axles 1 with two
secondary ammonium centers and a central disulfide bond
were synthesized according to Scheme 2. Benzoic acid
derivatives 4 having bulky substituents were treated with
thionyl chloride to form acid chlorides, which were treated
with 2-aminoethanethiol to yield amides 5. Compounds 5
were reduced with LiAlH4 to the corresponding amines 6 with
a central disulfide bond. Conversion of 6 to ammonium salts 1
by treatment with hydrochloric acid was followed by anion
exchange with ammonium hex-
afluorophosphate. The analyti-
cal and spectral data of 1a
c
were consistent with the struc-
tures shown.
Synthesis of [2]- and [3]rotax-
anes: A degassed solution of 1a
(0.12 molLÀ1) and DB24C8
(0.24 molLÀ1)in CH3CN was
sealed in a Pyrex tube and
heated at 1008C for 12 h. No
formation of rotaxanes was de-
1
tected by H NMR spectrosco-
py. This is because the dialkyl
disulfide linkage is sufficiently
stable, at least up to 1008C, and
because the 3,5-di-tert-butyl-
phenyl end groups are far too
large to pass through the
DB24C8 macroring. Photoirra-
diation of the mixture with a Hg
lamp at room temperature for
12 h yielded neither the corre-
sponding [2]rotaxane nor the
[3]rotaxane.
A
catalytic
amount
(0.010 molLÀ1) of benzenethiol
was added to a solution of 1a
(0.10 molLÀ1) and DB24C8
(0.15 molLÀ1)
in
CDCl3/
CD3CN (7:3). The mixture was
heated at 508C for 148 h and
then allowed to stand at 208C
for 240 h. The progress of the
reaction was monitored by
1H NMR spectroscopy, and the
yields of [2]rotaxane 2a and
[3]rotaxane 3a determined by
Scheme 1. Synthesis of [2]rotaxane 2a and [3]rotaxane 3a.
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[2]- and [3]-Rotaxanes
2895 2903
reactions within a reasonable timescale. This is advantageous
in ensuring facile workup.
Structures of [2]- and [3]rotaxanes: T he1H NMR spectro-
scopic data of the rotaxanes revealed significant chemical
shifts relative to the separate axle and ring components. The
1H NMR spectra of 2a, 3a, and a mixture of 1a and DB24C8
recorded at 208C are shown in Figures 1 and 2. The signal of
the tBu protons of 3a shifted to higher field (Dd ꢀÀ 0.1 ppm)
relative to 1a. The upfield shift is accounted for by the
shielding effect of the benzene rings of the DB24C8 compo-
nent. The signals of the benzylic protons of 3a shifted to lower
field relative to 1a (Dd ꢀ‡ 0.4 ppm). Such a downfield shift of
benzylic protons with neighboring secondary ammonium
groups has been observed in most (pseudo)rotaxanes consist-
ing of crown ethers and secondary ammonium salts.[16] This is
attributed to the CH ¥¥¥ O hydrogen-bonding interaction
between the benzylic protons and the oxygen atoms of the
crown ether.
Scheme 2. Synthesis of [2]rotaxane 2a
a) i) SOCl2; ii) 2-aminoethanethiol, Et3N/CH2Cl2. b) LiAlH4/THF.
c and [3]rotaxane 3a c.
c) i) HCl/MeOH;
d) cat. PhSH.
ii) NH4PF6
aq.
1H NMR spectroscopy were 64
and 30%, respectively (see
above). Rotaxanes 2a and 3a
were isolated by preparative
HPLC in 52 and 34% yield,
respectively. Their structures
were characterized by 1H NMR
and IR spectroscopy and FAB
mass spectrometry. The yields
of 2a and 3a reached about
90% on optimizing the reaction
conditions (see below). It is
noteworthy that no special ma-
nipulation to remove the cata-
lyst is necessary: 2a and 3a
were isolated simply by evapo-
ration of the solvents and puri-
fication of the residue by HPLC
in almost the same yields as
1
determined by H NMR spec-
troscopy. Dynamic covalent
synthesis using olefin metathe-
sis requires quenching of the
catalysts to isolate the products.
When imine linkages are em-
ployed as dynamic covalent
bonds, it is necessary to convert
the imine bonds to amine bonds
for product isolation, since
imine metathesis is a fast and
reversible process at room tem-
perature. The thiol disulfide
interchange reaction is slow
enough to isolate rotaxanes in
the presence of a thiol catalyst,
but fast enough to complete the
Figure 1. 1H NMR spectra (CDCl3, 270 MHz) of a) [3]rotaxane 3a and b) [2]rotaxane 2a. The rings in the
structural formula denote DB24C8.
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T. Takata et al.
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benzenethiol was added to a
solution of 1a (0.10 molLÀ1)
and DB24C8 (0.20 molLÀ1) in
CD3CN. The mixture was heat-
ed at 508C, and the progress of
the reaction was monitored by
1H NMR spectroscpy. The time
courses of the yields of 2a and
3a are shown in Figure 4. Soon
after initiation, 2a was formed,
and the yield of 2a gradually
increased. The yield of 2a
reached its maximum value
(69%) after 4 h, and then even-
tually decreased. The signals of
3a appeared after about 4 h,
and the yield of 3a increased
with a concomitant decrease in
the yield of 2a. The system
reached equilibrium after about
36 h, where the yields of 2a and
3a were 29 and 65%, respec-
tively.
Figure 2. 1H NMR spectrum (CDCl3/CD3CN (7/3), 270 MHz) of a mixture of axle 1a and DB24C8.
The 1H NMR spectrum of [2]rotaxane 2a showed temper-
ature-dependent behavior. The signals of most protons of the
dumbbell component were split into two peaks at room
temperature, because the shuttling behavior of 2a is slower
than the 1H NMR timescale. The signal of the tBu protons of
the half of the dumbbell unit encircled DB24C8 shifted to
higher field, while those of the benzylic protons shifted to
lower field, as in the case of 3a. The signals of the other half of
the dumbbell unit, which is not encircled by DB24C8, did not
shift compared to those of the axle 1a. The signals specific to 2a
and 3a, such as those of tBu and benzylic protons, were used to
monitor the the synthesis of the rotaxanes, as described below.
X-ray crystal structure analysis of 3a revealed an inter-
locked structure (Figure 3a) which resembles that of the
disulfide-containing [3]rotaxane of Busch et al.[13] Each
DB24C8 macroring adopts a folded U-shaped conformation,
‡
with the NH2 inserted approximately axially through its
center. The two DB24C8 macrorings are folded together to
form a ™tennis ball∫-like assembly.[17] Complex stabilization is
‡
À
À
achieved by a combination of N H ¥¥¥ O and C H ¥¥¥ O
hydrogen bonds. The disulfide group is surrounded by the four
catechol rings of DB24C8. Inspection of the solid-state
structure reveals no interactions between the disulfide moiety
and the DB24C8 macrorings. The structural parameters of the
disulfide moiety (Figure 3b) are reminiscent of those of
dimethyl disulfide, as determined by gas-phase electron
Figure 3. X-ray crystallographic structure of [3]rotaxane 3a. a) ORT EP
plot of 3a. Thermal ellipsoids are scaled to the 50% probability level.
b) Schematic diagram of the structure around the disulfide linkage of 3a.
diffraction:[18] the S S and C S bond lengths of 3a are
2.035(1) and 1.818(4)/1.812(4) ä, respectively, while those of
dimethyl disulfide are 2.022(3) and 1.806(2) ä, respectively;
the S-S-C bond angles of 3a are 104.5(1) and 102.9(1)8, values
similar to those of dimethyl disulfide (104.1(3)8); the disulfide
moiety of 3a adopts a gauche conformation with a C-S-S-C
dihedral angle of 82.2(5)8, which is also very close to that of
dimethyl disulfide (83.9(9)8).
À
À
These results suggest a possible reaction mechanism
(Scheme 3). The reaction is initiated by the nucleophilic
attack of benzenethiol at the disulfide linkage, which results in
thiol A and benzenethiol-terminated disulfide B [Eq. (1)].
Then DB24C8 rapidly threads onto A and B to form
pseudorotaxanes A ¥ DB24C8 and B ¥ DB24C8 [Eqs. (2) and
(3)]. The nucleophilic displacement of the phenylthio group in
A ¥ DB24C8 by B gives [2]rotaxane 2 and benzenethiol
Reaction mechanism and reversible cleavage/recombination
of the disulfide linkage: A catalytic amount (0.010 molLÀ1) of
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[2]- and [3]-Rotaxanes
2895 2903
Figure 4. Time course of the yields of [2]rotaxane 2a and [3]rotaxane 3a.
[1a] ˆ 0.10 molLÀ1
,
[DB24C8] ˆ 0.20 molLÀ1
,
[PhSH] ˆ 0.010 molLÀ1 in
CD3CN.
equilibrium is affected by solvent polarity, especially by donor
numbers (DN).[16, 19, 20] We recently reported that dielectric
constants also influence the association constants of pseudor-
otaxane formation in addition to DN.[21] The yields of the
rotaxanes are expected to increase when solvents with lower
DN are used. However, the use of CDCl3, which has a lower
DN (4.0) than CD3CN (DN 14.1) as co-solvent did not change
the yields of the rotaxanes (Table 1, entries 3 and 4). A
Table 1. Effect of solvent on the formation of [2]rotaxane 2a and
[3]rotaxane 3a from 1a and DB24C8 (cat. 0.1 molLÀ1 PhSH).
Concentration/molLÀ1
Yield/%[a]
T/8C 2a
Entry
Solvent
1a
DB24C8
0.20
3a
1
2
3
4
5
6
7
CD3CN
0.10
50
20
50
20
50
50
50
29
15
24
15
37
17
65
81
64
77
60
83
0
CDCl3/CD3CN 7:3
0.10
0.20
CDCl3/CD3OD 1:1
CD3NO2
DMF
0.10
0.10
0.10
0.20
0.20
0.30
< 10
[a] Determined by 1H NMR spectroscopy at equilibrium (error ca. 5%).
mixture of CDCl3 and CD3OD with a higher DN than CD3CN
shifted the equilibrium to the [2]rotaxane side (entry 5).
[D3]Nitromethane (DN 2.7) gave the highest yield of [3]ro-
taxane 3a (83%, even at 508C, entry 6). The use of DMF (DN
26.6) resulted in a dramatic decrease in yield (entry 7). Thus,
the solvent effects on the yields of the rotaxanes could, to a
certain extent, be accounted for by donor numbers in this
system.
The effects of the ratio of DB24C8 to 1a were examined
(Table 2). When equimolar amounts of DB24C8 and 1a were
employed, 2a was obtained preferentially at 508C (entry 1).
Increasing the DB24C8/1a ratio dramatically shifted the
equilibrium to 3a (entries 2 4). As described above, con-
Scheme 3. Possible reaction mechanism.
[Eq. (4)]. Pseudorotaxane B ¥ DB24C8 nucleophilically at-
tacks 1 or A to afford 2 [Eqs. (5) and (6)]. [3]Rotaxane 3 is
formed by reaction of B ¥ DB24C8 and 2, or of B ¥ DB24C8
and A ¥ DB24C8 [Eqs. (7) and (8)].
The reaction mixture was then cooled and kept at room
temperature (Figure 4). After 80 h (240 h after initiation), the
system reached another equilibrium state, in which the yields
of 2a and 3a were 15 and 81%, respectively. Since the main
driving force for rotaxane formation is the exothermic
hydrogen-bonding interaction between the secondary ammo-
nium group and DB24C8, the equilibrium shifted to the
[3]rotaxane side on lowering the reaction temperature.
Raising the temperature again to 508C returned the equili-
brium to the original state. These results demonstrate that the
whole process is reversible and that the yields of the rotaxanes
can be changed under thermodynamic control.
Table 2. Effect of the initial ratio of DB24C8 to1a ([1a] ˆ 0.1 molLÀ1, cat.
0.01 molLÀ1 PhSH, CDCl3/CD3CN, 508C).
Yield/%[a]
Entry
[DB24C8]/molLÀ1
2a
3a
1
2
3
4
5
0.10
0.15
0.20
0.30
0.40
65
69
29
12
10
10
26
65
88
82
Thermodynamic control: Since the hydrogen-bonding inter-
action is the main driving force for rotaxane formation, the
[a] Determined by 1H NMR spectroscpy at equilibrium (error ca. 5%).
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T. Takata et al.
FULL PAPER
ducting the reaction at a DB24C8/1a ratio of 2 yielded
preferentially 3a (entry 3). Using three equivalents of
DB24C8 gave the highest yields of 3a (entry 4). However,
the use of four equivalents of DB24C8 resulted in a lower
yield of 3a (entry 5). The decrease in the yield of 3a when an
excess of DB24C8 is used is attributed to the increased
polarity of the reaction mixture caused by DB24C8 itself,
which diminishes the association constant for pseudorotaxane
formation, as reported previously.[21]
Increasing the concentrations of 1a (0.710 molLÀ1) and
DB24C8 (1.42 molLÀ1) in CDCl3/CD3CN (1:1) gave 3a in
89% yield at 208C, while the reaction carried out at initial
concentrations of [1a] ˆ 0.10 molLÀ1 and [DB24C8] ˆ
0.20 molLÀ1 afforded 3a in 77% yield at 208C (Table 3). In
Table 3. Effect of concentration on the formation of [2]rotaxane 2a and
[3]rotaxane 3a (cat. 0.01 molLÀ1 PhSH).
Concentration/molLÀ1
Yield/%[a]
Entry Solvent
1a
DB24C8
T/8C 2a
3a
1
2
3
CDCl3/CD3CN 7:3 0.10
0.20
50
20
20
24
15
10
64
77
89
CDCl3/CD3CN 1:1 0.710
1.42
[a] Determined by 1H NMR spectroscopy at equilibrium (error ca. 5%).
this case, the advantage of the high initial concentrations
exceeded the drawback of the increased polarity of the
reaction system. Thus, the product distribution of this reaction
can easily be controlled by changing the initial concentrations.
Figure 5. Time course of the yields of [2]rotaxane 2a and [3]rotaxane 3a
Effect of end-capping groups: Three axles 1a c were
employed for the synthesis of rotaxanes to examine the steric
effects of end-capping groups on the yield (Table 4). The axles
with a) PhSH, b) 7a, and c) tBuSH as initiator. [1a] ˆ 0.10 molLÀ1
,
[DB24C8] ˆ 0.20 molLÀ1, [initiator] ˆ 0.010 molLÀ1 in CDCl3/CD3CN 7:3
at 508C.
Table 4. Effect of end caps on the yields of [2]rotaxanes2 and [3]rotaxanes
3
([1a] ˆ 0.1 molLÀ1
,
[DB24C8] ˆ 0.2 molLÀ1
,
cat. 0.01 molLÀ1 PhSH,
rate than that obtained with PhSH. This decrease in reaction
rate is attributed to the lack of contributions from Equa-
tions (4), (6), and (8) in Scheme 3. This slow rate makes it
easier to isolate [2]rotaxanes during the period in which their
yields peak. In a separate experiment under the same
conditions, 2a was indeed isolated in 89% yield 40 h after
initiation. When tBuSH was employed as an initiator, the
reaction rate was much slower than those obtained with PhSH
and 7a. The system did not reach equilibrium, even after
300 h. This is attributable to the lack of contributions from
Equations (4), (6), and (8) in Scheme 3, as well as to the low
nucleophilicity of tBuSH because of the steric hindrance of
the tert-butyl group. Note that 2a could be isolated in 89%
yield under ™kinetic∫ conditions.
CD3CN).
Axle
T/8C
Yield/%[a]
2
3
1a
1b
1c
50
20
50
20
50
20
29
15
19
11
22
11
65
81
63
81
61
79
[a] Determined by 1H NMR spectroscopy at equilibrium (error ca. 5%).
having less hindered endcapping groups (1b and 1c) afforded
the corresponding [2]rotaxanes 2b and 2c and [3]rotaxanes
3b and 3c in almost the same yields. Thus, the steric effect of
end-capping groups appears relatively small from the view-
point of rotaxane yield.
Initiators other than thiols: Disulfide linkages can be cleaved
by various nucleophiles other than thiols.[23] To confirm the
efficiency of thiol catalysts, we examined 4-nitrophenol,
diethylamine, and hexamethylphosphoric triamide (HMPT)
as initiators (Table 5). 4-Nitrophenol did not react with 1a at
508C, even after 60 h (entry 2). Diethylamine catalyzed the
rotaxane synthesis, and the yields of 2a and 3a were 25 and
4%, respectively, at 508C after 67 h (entry 3), although the
Effect of thiols: To evaluate the effect of the thiol catalyst,
thiol 7a[22] and tBuSH were used as initiators. The use of 7a
caused a retarded reaction rate (Figure 5). The yield of 2a,
evaluated by 1H NMR spectroscopy, reached a maximum
value of 95% 33 h after initiation, indicative of a much slower
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[2]- and [3]-Rotaxanes
2895 2903
Table 5. Effect of catalysts on the yields of [2]rotaxane 2a and [3]rotaxane
remaining SOCl2 was removed as an azeotropic mixture with benzene to
give the corresponding acid chloride. A solution of the acid chloride in
Et2O (15 mL) was slowly added to a solution of 2-aminoethanethiol (2.26 g,
29.3 mmol) and Et3N (4.2 mL, 30 mmol) in CH2Cl2 (115 mL). The mixture
was stirred for 1 h at room temperature, washed with 1m HCl (20 mL Â 3),
and treated successively with brine, anhydrous MgSO4, and evaporated to
dryness to give amide 5 as a white solid.
3a ([1a] ˆ 0.1 molLÀ1
CD3CN, 208C).
,
[DB24C8] ˆ 0.2 molLÀ1
,
[cat.] ˆ 0.01 molLÀ1
,
Yield/%[b]
Catalyst
t/h[a]
2
3
PhSH
4-Nitrophenol
Et2NH
55
60
67
25
0
25
61
0
4
Compound 5a: quantitative; m.p. 188 1918C; 1H NMR (270 MHz, CDCl3,
238C): d ˆ 7.59 (m, 3H, ArH), 6.55 (s, 1H, NH), 3.64 (q, J ˆ 6.5 Hz, 2H,
CH), 2.80 (dt, J ˆ 8.4, 6.5 Hz, 2H, CH), 1.42 (t, 1H, J ˆ 8.4 Hz, SH), 1.35 (s,
HMPT54
60
24
À
À
ˆ
[a] All reactions did not reach equilibrium. [b] Determined by 1H NMR
spectroscopy (error ca. 5%).
18H, tBu); IR (KBr): nÄ ˆ 3249 (N H), 2570 (S H), 1633 (C O), 1595
À1
ˆ
ˆ
(C C), 1543 cm (C C).
1
Compound 5b: quantitative; H NMR (CDCl3, 270 MHz, 238C): d ˆ 7.74
(d, J ˆ 4.1 Hz, 2H, ArH), 7.70 (d, J ˆ 4.1 Hz, 2H, ArH), 6.59 (s, 1H, NH),
3.64 (q, J ˆ 6.2 Hz, 2H, CH2), 2.79 (dt, J ˆ 8.4, 6.2 Hz, 2H, CH2), 1.40 (t, J ˆ
À
À
8.4 Hz, 1H, SH), 1.34 (s, 9H, tBu); IR (KBr): nÄ ˆ 3246 (N H), 2570 (S H),
reaction rate was as slow as that with tBuSH. HMPTalso
reacted with 1a to catalyze the formation of 2a and 3a
(entry 4).[24] Thus, amines and phosphines are potential
catalysts for rotaxane synthesis utilizing the reversible nature
of disulfide linkages.
À1
ˆ
ˆ
1
ˆ
1633 (C O), 1595 (C C), 1545 cm (C C).
Compound 5c: 94%; H NMR (CDCl3, 270 MHz, 238C): d ˆ 7.38 (s, 2H,
ArH), 7.12 (s, 1H, ArH), 6.70 (s, 1H, NH), 3.62 (q, J ˆ 6.2 Hz, 2H, CH2),
2.74 (dt, J ˆ 8.6, 6.2 Hz, 2H, CH2), 2.34 (s, 6H, CH3), 1.41 (t, J ˆ 8.6 Hz, 1H,
À
À
ˆ
ˆ
SH); IR (KBr): nÄ ˆ 3311 (N H), 2561 (S H), 1639 (C O), 1601 (C C),
1539 cmÀ1 (C C).
ˆ
Representative procedure for the synthesis of amine 6: Compound 5a was
added to a suspension of LiAlH4 (80%, 1.32 g, 27.8 mmol) in THF (40 mL).
The mixture was heated under reflux for 17 h. Saturated aqueous sodium
sulfate solution was then added carefully to the mixture to form a white
precipitate, which was removed by suction filtration. The filtrate was
evaporated to dryness. The residue was subjected to column chromatog-
raphy on silica gel (chloroform) to afford the amine 6a as a colorless oil.
Conclusion
Symmetrical dumbbell-shaped compounds 1 with two ammo-
nium centers and a central disulfide bond were designed with
the aim of investigating the utility of disulfide linkages as
dynamic covalent bonds for rotaxane synthesis. Compound 1
and DB24C8 reacted with catalysis by benzenethiol to afford
[2]- and [3]rotaxanes in good yields. Kinetic studies on the
reaction suggested a mechanism involving several steps such
as initiation, [2]rotaxane formation, and [3]rotaxane forma-
tion. Since the whole reaction is fully reversible, product
distributions were controllable under thermodynamic condi-
tions by changing temperature, solvent, initial ratios of
substrates, and concentrations. The yield of [3]rotaxane 3a
reached up to 89%. On the other hand, [2]rotaxane 2a was
isolated in 89% yield by changing the structure of the thiol
catalyst to decelerate the reaction rate. Thus, disulfide link-
ages were shown to be effective as dynamic covalent bonds for
the construction of rotaxanes. We believe that more compli-
cated supramolecules such as catenanes and knots can be
synthesized by means of thiol disulfide interchange reac-
tions. since these proceed well under mild conditions and can
tolerate many functional groups.[25]
Compound 6a: 88%; oil; 1H NMR (400 MHz, CDCl3, 238C): d ˆ 7.32 (t,
J ˆ 1.6 Hz, 2H, ArH), 7.15 (d, J ˆ 1.6 Hz, 4H, ArH), 3.80 (s, 4H, CH2), 2.98
(t, J ˆ 6.0 Hz, 4H, CH2), 2.85 (t, J ˆ 6.0 Hz, CH2), 1.33 (s, 36H, tBu); IR
À1
À
ˆ
ˆ
ˆ
(KBr): nÄ ˆ 3604 (N H), 1599 (C C), 1476 (C C), 1458 cm (C C).
Compound 6b: quantitative; 1H NMR(CDCl3, 270 MHz, 238C): d ˆ 7.43
7.20 (m, 8H, ArH), 3.76 (s, 4H, CH2), 2.93 (t, J ˆ 5.9 Hz, 4H, CH2), 2.81 (t,
À
J ˆ 5.9 Hz, 4H, CH2), 1.31 (s, 18H, tBu); IR (KBr): nÄ ˆ 3288 (N H), 1511
À1
ˆ
ˆ
(C C), 1458 cm (C C).
Compound 6c: 99%; 1H NMR (CDCl3, 270 MHz, 238C): d ˆ 6.85 (m, 6H,
ArH), 3.72 (s, 2H, CH2), 2.81 (t, J ˆ 5.9 Hz, 4H, CH2), 2.68 (t, J ˆ 5.9 Hz,
À
ˆ
4H, CH2), 2.31 (s, 12H, CH3); IR (KBr): nÄ ˆ 3306 (N H), 1606 (C C),
1457 cmÀ1 (C C).
ˆ
Representative procedure for the synthesis of dumbbell 1: A solution of 6a
(3.00 g, 5.13 mmol) in methanol/conc. HCl (30/7 mL) was stirred for
45 min. Water (50 mL) was added to the solution to form a white
precipitate, which was collected by suction filtration. The white solid was
washed with water, dried under reduced pressure, and reprecipitated from
MeOH/water to give the corresponding hydrochloric acid salt of 6a as a
white solid. The hydrochloric acid salt (1.34 g, 2.04 mmol) was dissolved in
MeOH (15 mL). A solution of ammonium hexafluorophosphate (1.93 g,
11.8 mmol) in water (10 mL) was slowly added to the methanolic solution
to form a white precipitate. Water was added to the resulting solution until
no further precipitation occurred. The white solid was collected by suction
filtration, washed with water, dried under reduced pressure, and recrystal-
lized from water/ethanol to afforded the dumbbell 1a.
Experimental Section
Compound 1a: 95%; needlelike crystals; m.p. (decomp): 225 2308C;
1H NMR (270 MHz, CD3CN, 238C): d ˆ 7.55 (t, J ˆ 1.6 Hz, 2H, ArH), 7.31
(d, J ˆ 1.6 Hz, 4H, ArH), 4.38 (brs, 4H, NH2), 4.18 (s, 4H, CH2), 3.34 (t, J ˆ
General: Melting points were measured on a Yanagimoto micro melting-
point apparatus and were uncorrected. IR spectra were recorded on a
JASCO FT-IR model 230 spectrometer.1H NMR spectra were recorded on
JEOL JNM-GX-270 and JNM-L-400 spectrometers in CDCl3 with
tetramethylsilane as an internal reference. FABMS measurements were
performed on a Finnigan TSQ-70 instrument. For preparative HPLC, a
JAICO LC-908 system using columns JAIGEL 1 (20 mmÆ Â600 mm) and
JAIGEL 2 (20 mmÆ Â 600 mm) was employed. X-ray diffraction was
carried out on a Rigaku R-AXIS RAPID imaging plate system. All
reagents used were commercially available, except for 4a, which was
prepared according to the literature procedure.[21]
6.8 Hz, 4H, CH2), 2.92 (t, J ˆ 6.8 Hz, 4H, CH2), 1.33 (s, 36H, tBu); IR
À1
À
ˆ
À
À
(KBr): nÄ ˆ 3237 (N H), 1604 (C C), 839 (P F), 557 cm (P F); FABMS
‡
(matrix: mNBA): m/z: 558.7 [M À 2PF6] ; elemental analysis (%) calcd for
C34H58F12N2P2S2 ¥ H2O (848.90): C 47.11, H 6.98, N 3.23; found: C 47.39, H
6.73, N 3.26.
Compound 1b: 86%; white crystals; m.p. (decomp): 232 2358C; 1H NMR
(270 MHz, CD3CN, 238C): d ˆ 7.52 (d, J ˆ 8.1 Hz, 4H, ArH), 7.40 (d, J ˆ
8.1 Hz, 4H, ArH), 5.97 (brs, 4H, NH2), 4.20 (s, 4H, CH2), 3.36 (t, J ˆ
Representative procedure for the synthesis of amide 5: A solution of 3,5-di-
tert-butylbenzoic acid[21] (4a, 5.56 g, 23.7 mmol) in SOCl2 (13 mL) was
stirred overnight at 508C. Then the mixture was concentrated in vacuo. The
6.9 Hz, 4H, CH2), 2.93 (t, J ˆ 6.9 Hz, 4H, CH2), 1.32 (s, 18H, tBu); IR
À1
À
ˆ
À
À
‡
(KBr): nÄ ˆ 3253 (N H), 1586 (C C), 839 (P F), 557 cm (P F); FABMS
‡
(matrix: mNBA): m/z: 591.0 [M À PF6] , 445.0 [M À 2PF6] ; elemental
Chem. Eur. J. 2003, 9, 2895 2903
www.chemeurj.org
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2901

T. Takata et al.
FULL PAPER
analysis (%) calcd for C26H42F12N2P2S2 (736.69): C 42.39, H 5.75, N 3.80;
found: C 42.66, H 5.61, N 3.78.
X-ray crystallographic analysis of [3]rotaxane (3a): Single crystals suitable
for X-ray analysis were grown by recrystallization from MeOH. Crystal
data: C82H122F12N2O16P2S2 ¥ 2MeOH, M ˆ 1810.0, triclinic, a ˆ 161215(6),
Compound 1c: 46%; white crystals; m.p. (decomp): 215 2178C; 1H NMR
(270 MHz, [D6]DMSO, 238C): d ˆ 8.80 (brs, 4H, NH2), 7.08 (s, 6H, ArH),
4.11 (s, 4H, CH2), 3.23 (t, J ˆ 6.8 Hz, 4H, CH2), 2.96 (t, J ˆ 6.8 Hz, 4H,
b ˆ 17.1050(6), c ˆ 17.3503(6) ä, a ˆ 106.506(1), b ˆ 95.903(2), g ˆ
90.168(2)8, Vˆ 4560.4(3) ä3, space group P1, Z ˆ 2, 1 ˆ 1.318 gcmÀ3
,
≈
m(MoKa) ˆ 0.183mmÀ1
,
F(000) ˆ 1808. 20398 independent reflections
À
ˆ
CH2), 2.30 (s, 12H, CH3); IR (KBr): nÄ ˆ 3247 (N H), 1611 (C C), 1467
À1
(2q < 558) were collected with a Rigaku R-AXIS-RAPID imaging plate
system, equipped with a rotating anode generator, at 103 K using w scans
and graphite-monochromatized MoKa radiation. Refinement was carried
out for reflections with F2 > 3s(F2). The structure was solved by a direct
method (SIR97). wR, GOF, and R were based on F, which was set to zero
for negative values. The threshold expression of F2 > 2s(F2) was used only
for calculating the R factor. wR, GOF, and R were 0.058, 0.090, and 1.000,
respectively. Calculations were carried out with the CrystalStructure
package (version 2.0). CCDC-196932 contains the supplementary crystal-
lographic data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crys-
tallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax:
(‡44)1223-336-033; or deposit@ccdc.cam.uk).
ˆ
À
À
(C C), 839 (P F), 557 cm (P F); FABMS (matrix: mNBA): m/z: 534.6
‡
‡
[M À PF6] , 391.6 [M À 2PF6‡H] ; elemental analysis (%) calcd for
C22H34N2S2 ¥ 1.9 PF6 ¥ 0.1H2PO4 (680.58): C 39.10, H 5.10, N 4.15; found: C
39.21, H 4.79, N 4.26.
Representative procedure for the synthesis of [2]rotaxane 2 and [3]rotax-
ane 3: A catalytic amount of benzenethiol (1.0 mL, 0.01 mmol) was added to
a solution of 1a (85 g, 0.10 mmol) and DB24C8 (69 g, 0.15 mmol) in
CD3CN/CDCl3 (0.3/0.7 mL). The solution was sealed in an NMR tube (Æ
5 mm), which was heated at 508C for 6 d and cooled and kept at room
temperature for 10 d. The progress of the reaction was monitored by
1H NMR spectroscopy. The mixture was evaporated to dryness. The residue
was subjected to preparative HPLC to afford 2a and 3a.
1
Compound 2a: H NMR (270 MHz, CDCl3, 238C): d ˆ 7.46 (t, J ˆ 1.6 Hz,
1H, ArH), 7.40 (d, J ˆ 1.6 Hz, 2H, ArH), 7.39 (brs, 2H, NH2 complexed
with DB24C8), 7.37 (t, J ˆ 1.6 Hz, 1H, ArH), 7.30 (d, J ˆ 1.6 Hz, 2H, ArH),
6.92 (s, 8H, ArH), 4.67 (m, 4H, CH2), 4.35 4.05 (m, 10H, CH2), 3.92 3.25
(m, 20H, CH2), 2.70 (m, 4H, CH2), 1.33 (s, 18H, tBu), 1.21 (s, 18H, tBu), the
Acknowledgements
signal of NH2 uncomplexed with DB24C8 was not observed; IR (KBr): nÄ ˆ
We thank Dr. Kunihisa Sugimoto, Rigaku Corporation, for help with the
X-ray experiment. We are grateful to Prof. F. Sanda of Kyoto University for
the elemental analyses. This work was financially supported by Grant-in-
Aid for Research on Basic Area (B-2) No. 12440181 and Scientific
Research on Priority Areas (A), No. 413/14045262 from the Ministry of
Education, Science, Sports and Culture of the Japanese Government.
À1
À
ˆ
ˆ
À
À
3155 (N H), 1597 (C C), 1506 (C C), 845 (P F), 558 cm (P F); FABMS
‡
(matrix: mNBA): m/z: 1006 [M À 2PF6] ; elemental analysis (%) calcd for
C58H90F12N2O8P2S2 (1297.40): C 53.69, H 6.99, N 2.16, S 4.94; found: C 53.17,
H 6.97, N 2.45, S 4.83.
Compound 2b: 1H NMR (270 MHz, CDCl3, 238C): d ˆ 7.47 (d, J ˆ 8.6 Hz,
2H, ArH), 7.38(d, J ˆ 8.6 Hz, 2H, ArH), 7.33 (brs, 2H, NH2 complexed
with DB24C8), 7.28(d, J ˆ 8.6H, 2H, ArH), 7.22 (d, J ˆ 8.6 Hz, 2H, ArH),
6.88 (m, 8H, ArH), 4.7 (brs, 2H, NH2 uncomplexed with DB24C8), 4.59
(m, 2H, CH2), 4.32 3.97 (m, 10H, CH2), 3.95 3.40 (m, 18H, CH2), 3.13 (t,
[1] J.-M. Lehn, Supramolecular Chemistry, VCH, Weinheim, 1995.
[2] S. J. Rowan, S. J. Cantrill, G. R. L. Cousins, J. K. M. Sanders, J. F.
Stoddart, Angew. Chem. 2002, 114, 938 993; Angew. Chem. Int. Ed.
2002, 41, 898 952.
[3] a) D. B. Amabilino, J. F. Stoddart, Chem. Rev. 1995, 95, 2725 2828;
b) R. J‰ger, F. Vˆgtle, Angew. Chem. 1997, 109, 966 980; Angew.
Chem. Int. Ed. Engl. 1997, 36, 930 944; c) G. A. Breault, C. A.
Hunter, P. C. Meyers, Tetrahedron 1999, 55, 5265 5293; d) Moleuclar
Catenanes, Rotaxanes and Knots (Eds.: J.-P. Sauvage, C. O. Dietrich-
Buchecker), Wiley-VCH, Weinheim, 1999; e) T. J. Hubin, D. H.
Busch, Coord. Chem. Rev. 2000, 200 202, 5 52.
J ˆ 7.0 Hz, 2H, CH2), 2.81 (m, 4H, CH2), 1.28 (s, 9H, tBu), 1.25 (s, 9H, tBu);
À1
À
ˆ
ˆ
À
IR (KBr): nÄ ˆ 3135 (N H), 1595 (C C), 1506 (C C), 843 (P F), 558 cm
‡
À
(P F); FABMS (matrix: mNBA): m/z: 894 [M À 2PF6] ; elemental
analysis (%) calcd for C50H74N2O8S2 ¥ 2H2PO4 ¥ (1185.19): C 55.13, H 7.22,
N 2.57; found: C 54.97, H 7.00, N 2.37.
Compound 2c: m.p. 68 708C; 1H NMR (270 MHz, CDCl3, 238C): d ˆ 7.28
(brs, 2H, NH2 complexed with DB24C8), 7.15 (s, 2H, ArH), 6.98 (s, 1H,
ArH), 6.95 6.80 (m, 11H, ArH), 6.10 (brs, 2H, NH2 uncomplexed with
DB24C8), 4.52 (m, 2H, CH2), 4.30 4.00 (m, 10H, CH2), 3.95 3.40 (m,
18H, CH2), 3.23 (t, J ˆ 7.3 Hz, 2H, CH2), 2.79 (m, 4H, CH2), 2.29 (s, 6H,
[4] a) I. T. Harrison, J. Chem. Soc. Chem. Commun. 1972, 231 232;
b) I. T. Harrison, J. Chem. Soc. Perkin Trans. 1 1974, 301 304.
[5] G. Schill, W. Beckmann, N. Schweickert, H. Fritz, Chem. Ber. 1986,
2674 2655.
À
ˆ
CH3), 2.15 (s, 6H, CH3); IR (KBr): nÄ ˆ 3135 (N H), 1595 (C C), 1506
À1
ˆ
À
À
(C C), 843 (P F), 558 cm (P F); FABMS (matrix: mNBA): m/z: 837
‡
[M À 2PF6] ; elemental analysis (%) calcd for C46H66N2O8S2 ¥ PF6 ¥ H2PO4
(1129.09): C 51.10, H 6.34 N 2.59; found: C 51.14, H 6.57, N 2.43.
[6] a) D. G. Hamilton, N. Feeder, S. J. Teat, J. K. M. Sanders, New J.
Chem. 1998, 1019 1021; b) D. G. Hamilton, J. E. Davies, L. Prodi,
J. K. M. Sanders, Chem. Eur. J. 1998, 4, 608 620; c) D. G. Hamilton,
M. Montali, L. Prodi, M. Fontani, P. Zanello, J. K. M. Sanders, Chem.
Eur. J. 2000, 6, 608 617.
1
Compound 3a: m.p. 207 2088C; H NMR (270 MHz, CDCl3, 238C): d ˆ
7.35 (s, 2H, ArH), 7.34 (brs, 4H, NH2), 7.28 (s, 4H, ArH), 6.89 (s, 16H,
ArH), 4.57 (m, 4H, CH2), 4.30 3.95 (m, 8H, CH2), 3.85 3.30 (m, 20H,
CH2), 2.18 (t, J ˆ 6.2 Hz, 4H), 1.18 (s, 36H, tBu); IR (KBr): nÄ ˆ 3164
À1
À
ˆ
ˆ
À
À
(N H), 1596 (C C), 1506 (C C), 843 (P F), 558 cm (P F); FABMS
[7] T. J. Kidd, D. A. Leigh, A. J. Wilson, J. Am. Chem. Soc. 1999, 121,
1599 1600.
‡
(matrix: mNBA): m/z: 1454 [M À 2PF6] ; elemental analysis (%) calcd for
C82H122F12N2O16P2S2 (1745.91): C 56.41, H 7.04, N 1.60; found: C 56.14, H
7.16, N 1.44.
[8] a) S. J. Cantrill, S. J. Rowan, J. F. Stoddart, Org. Lett. 1999, 1, 1363
1366; b) S. J. Rowan, J. F. Stoddart, Org. Lett. 1999, 1, 1913 1916;
c) P. T. Glink, A. I. Oliva, J. F. Stoddart, A. J. P. White, D. J. Williams,
Angew. Chem. 2001, 113, 1922 1927; Angew. Chem. Int. Ed. Engl.
2001, 40, 1870 1875.
[9] W. J. Lee, G. M. Whitesides, J. Org. Chem. 1993, 58, 642 647.
[10] H. Hioki, W. C. Still, J. Org. Chem. 1998, 63, 904 905.
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904 905.
[12] S. Otto, R. L. E. Furlan, J. K. M. Sanders, J. Am. Chem. Soc. 2000, 122,
12063 12064.
[13] A. G. Kolchinski, N. W. Alcock, R. A. Roesner, D. H. Busch, Chem.
Commun. 1998, 1437 1438.
1
Compound 3b: m.p. 186 1888C; H NMR (270 MHz, CDCl3, 238C): d ˆ
7.28 (brs, 4H, NH2), 7.24 (d, J ˆ 8.6 Hz, 4H, ArH), 7.18 (d, J ˆ 8.6 Hz, 4H,
ArH), 6.83 (s, 8H, ArH), 4.55 (m, 4H, ArH), 4.25 3.95 (m, 16H, CH2),
3.90 3.45 (m, 36H, CH2), 2.45 (t, J ˆ 6.8 Hz, 4H), 1.23 (s, 18H, tBu); IR
À1
À
ˆ
ˆ
À
(KBr): nÄ ˆ 3178 (N H), 1593 (C C), 1506 (C C), 845 (P F), 557cm
‡
À
(P F); FABMS (matrix: mNBA): m/z: 1488 [M À PF6] ; elemental analysis
(%) calcd for C74H106F12N2O16P2S2 (1633.70): C 54.40, H 6.54, N 1.74; found:
C 54.39, H 6.64, N 1.74.
Compound 3c: m.p. 223 À 2248C; 1H NMR (270 MHz, [D6]DMSO, 238C):
d ˆ 7.17 (brs, 4H, NH2), 7.00 6.85 (m, 22H, ArH), 4.41 (m, 4H, CH2),
4.20 3.90 (m, 16H, CH2), 3.80 3.40 (m, 36H, CH2), 2.27 (t, J ˆ 6.8 Hz,
À
ˆ
4H), 2.13 (s, 12H, CH3); IR (KBr): nÄ ˆ 3154 (N H), 1594 (C C), 840
[14] L. Raehm, C. Hamann, J.-M. Kern, J.-P. Sauvage, Org. Lett. 2000, 2,
1991 1994.
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2000, 18 19.
À1
‡
À
À
(P F), 557 cm (P F); FABMS (matrix: mNBA): m/z: 1432 [M À PF6] ;
elemental analysis (%) calcd for C70H98F12N2O16P2S2 ¥ H2O (1577.59): C
52.69, H 6.32, N 1.76, S 4.02; found: C 52.70, H 6.21, N 1.94, S 4.26.
2902
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2003, 9, 2895 2903

[2]- and [3]-Rotaxanes
2895 2903
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2297 2307.
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[24] HMPTreacts with 1a to form the corresponding thiophosphonium salt
and thiol 7a. T. Oku, Y. Furusho, T. Takata, unpublished results.
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2003, 41, 119 123.
Received: December 5, 2002 [F4644]
Chem. Eur. J. 2003, 9, 2895 2903
www.chemeurj.org
¹ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2903