Synthesis of [2]rotaxanes by tritylative endcapping of in situ formed pseudorotaxanes having thiol or hydroxyl functionality on the axle termini

Article abstract of DOI:10.1016/S0040-4020(02)00749-4

Tritylative endcapping of an in situ formed pseudorotaxane consisting of dibenzo-24-crown-8 and an axle having a thiol group at the end by treatment with trityl hexafluorophosphate at room temperature gave the corresponding sulfide-type [2]rotaxane in hig

Full text of DOI:10.1016/S0040-4020(02)00749-4

Tetrahedron 58 (2002) 6609–6613
Synthesis of [2]rotaxanes by tritylative endcapping of in situ
formed pseudorotaxanes having thiol or hydroxyl functionality on
the axle termini
*
Yoshio Furusho, G. Abraham Rajkumar, Tomoya Oku and Toshikazu Takata
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai,
Osaka 599-8531, Japan
Received 15 May 2002; accepted 27 June 2002
Abstract—Tritylative endcapping of an in situ formed pseudorotaxane consisting of dibenzo-24-crown-8 and an axle having a thiol group at
the end by treatment with trityl hexafluorophosphate at room temperature gave the corresponding sulfide-type [2]rotaxane in high yields.
Treatment of the pseudorotaxane having a hydroxyl group at the axle terminal with trityl hexafluorophophate followed by addition of a base
such as triethylamine afforded the corresponding ether-type [2]rotaxane in good yields. q 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
metathesis,7f–h thiol–disulfide interchange reaction,7i and
the reaction of halides with phosphines7j have been
employed for the rotaxane synthesis.
Rotaxanes1 are molecules that consist of dumbbell-like
units threaded into wheel-like ones. They have been
regarded as motifs for nano devices, such as motors,
sensors, switches, amplifier, and actuators at the nanoscale
level, based on their interlocked structures. One very
essential feature of these molecules is that their mechani-
cally interlocked units have a high degree of freedom in
relative motion of the components. Based on this aspect, one
could easily expect that polymers having rotaxane units,
such as polyrotaxanes,2 would exhibit unusual viscoelastic
properties, such as a very large loss modulus, low activation
energy for viscous flow, and rapid stress relaxation.3,4
However, the synthesis of rotaxane in high yield is still a
challenging task and it often relies on how exactly the
endcapping is achieved. Prompted by the fact that trityl
(thio)ether formation reaction can be rapidly and smoothly
achieved under neutral or acidic conditions,8 we explored
the possibility of utilizing this transformation as an
endcapping reaction.9 In this paper, we report a new
protocol for the synthesis of [2]rotaxanes by tritylative
endcapping of the thiol or hydroxyl functionality attached to
the axle units.
Rotaxanes can be synthesized efficiently using template-
directed protocols5 that utilize attractive interactions, such
as the coordination of heterocyclic ligands to a metal cation,
a p–p stacking interaction, or a hydrogen-bonding
interaction. A rotaxane recognition motif, which consists
of sec-ammonium salt centers in the rod sections of the
dumbbell components encircled by crown ethers, is a well-
established one.6 Synthesis of rotaxanes using threading-
endcapping methodology is attractive and it is emerging as a
convenient method in recent years. However, the synthesis
should be carried out under neutral or acidic conditions to
keep the hydrogen-bonding interaction intact during end-
capping of pseudorotaxanes.7 [2þ3]Cycloaddition
reaction,7a oxidative coupling of thiols,7b conjugate addition
of thiols,7c acylation of amines7d or alcohols,7e imine
2. Results and discussion
A sec-ammonium salt having a thiol group at the end (1)
was prepared as illustrated in Scheme 1. Treatment of 3,5-
di-t-butylbenzoic acid10 (5) with thionyl chloride followed
by the reaction with 2-aminoethanthiol afforded amide (6).
Reduction of 6 with LiAlH4 yielded the corresponding
amine, which was converted to 1 by treatment with
hydrochloric acid followed by counteranion exchange
with ammonium hexafluorophosphate. The analytical data
and the spectral data of 1 were consistent with the structure.
Tritylation of the pseudorotaxane (1·DB24C8) formed in
situ from 1 and dibenzo-24-crown-8 (DB24C8) was
examined by using trityl hexafluorophosphate (Ph3CPF6)
in dichloromethane at room temperature for 24 h (Scheme 1,
Table 1). Use of three equivalents of DB24C8 gave the
[2]rotaxane (2) in 98% yield (entry 1). A small excess of
Keywords: [2]rotaxane; tritylation; pseudorotaxane; thiol; alcohol;
endcapping.
*
Corresponding author. Tel.: þ81-72-254-9290; fax: þ81-72-254-9910;
e-mail: takata@chem.osakafu-u.ac.jp.
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00749-4

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Y. Furusho et al. / Tetrahedron 58 (2002) 6609–6613
Table 2. Solvent effect on the synthesis of sulfide-type [2]rotaxane (2) by
tritylation of the thiol group at the end of 1 in the presence of DB24C8
a
Entry
Solvent
DN
1a
Yield (%)b
1
2
3
4
5
6
Toluene
CH2Cl2
CH3NO2
CHCl3
CH3CN
0.1
1.0
2.7
4.0
14.1
14.8
2.2
8.9
35.9
4.8
35.9
2.2
80
93 (91)
98
88
82 (77)
89
1.4-Dioxane
1¼0.10 mmol, DB24C8¼0.12 mmol, Ph3CPF6¼0.15 mmol, solvent¼
0.60 mL.
Ref. 11.
a
b
Determined by 1H NMR using naphthalene as an internal standard.
Figures in parenthesis denote the isolated yields.
examined with respect to several aprotic solvents, and the
results are summarized in Table 2. The reaction between the
thiol group and trityl cation was completed in all cases.
Hence, the association constants of 1 and DB24C8 would be
mainly reflected in the yields of 2. The association constants
between ammonium salts and crown ethers generally
increase as solvent polarity decreases, because the hydro-
gen-bonding interaction working between them is the main
driving force for the complexation. Although solvent
polarity has been evaluated by various parameters,11 the
most suitable parameter for pseudorotaxane formation has
not been established. Stoddart et al. explained the
association constants of pseudorotaxane formation in
terms of donor numbers (DN) of solvent: since sec-
ammonium salts form pseudorotaxanes with crown ethers
mainly by the hydrogen-bonding interaction, a solvent
having a higher DN prevents the complexation, and vice
versa.12 We have recently reported that dielectric constants
(1 ) of solvent also affect the association constants of
pseudorotaxane formation together with DN: since the ion–
dipole interaction plays an important role for the complexa-
tion between sec-ammonium salts and crown ethers, a
solvent having a high 1 decreases the association constant.3g
However, use of toluene having a lower DN and an 1 than
those of dichloromethane resulted in a lowered yield (80%,
entry 1). The highest yield was obtained by use of
nitromethane (98%, entry 3). Chloroform and acetonitrile
afforded 2 in lower yields than dichloromethane (entries 4
and 5). 1,4-Dioxane, which has a DN as high as that of
acetonitrile, gave 2 in 89% yield (entry 6). Thus, the effects
of solvent on the yield of the tritylative endcapping are
obscure at best. However, the higher yield of 2 (.80%) in
all solvents that were employed is worth mentioning.
Scheme 1. Reagents: (a) (i) SOCl2, (ii) 2-aminoethanethiol; (b) (i)
LiA1H4/THF, (ii) HCl/MeOH, (iii) NH4PF6aq; (c) PPh3CX.
DB24C8 was enough to obtain 2 in more than 90% yield
(93%, entry 2). Combination of trityl chloride and silver
hexafluorophosphate, which generates trityl hexafluoro-
phosphate in situ, also worked to give 2 in 80% yield
(entry 3). When the reaction was conducted at 258C, the
yield decreased to 70% due to retarded reaction rate
between the thiol group and trityl cation (entry 4).
Prolonging the reaction time to 72 h hardly affected the
yield (entry 5).
Solvent effects on the synthesis of 2 by tritylation were
Table 1. Synthesis of sulfide-type [2]rotaxane (2) by tritylation of the thiol
group at the axle end of 1 in the presence of DB24C8
Synthesis of rotaxane by tritylation of the hydroxyl group at
the end was also examined (Table 3). Tritylation of the
hydroxyl group of the axle3g (3) was carried out in the
presence of DB24C8 at room temperature for 24 h and
afforded the [2]rotaxane (4) in 22% yield (entry 1). The low
yield is attributed to the much slower reaction rate of the
hydroxyl group than that of the thiol group, being consistent
with the difference in nucleophilicity between them. In fact,
3 was recovered from the reaction mixture. Addition of
triethylamine as a base to trap the hexafluorophosphoric
acid formed in the reaction mixture enhanced the tritylation
Entry DB24C8 (mmol)
X
Temperature Time (h) Yield (%)a
1
2
3
4
5
0.30
0.12
0.12
0.12
0.12
PF26 rt
24
24
24
24
72
98 (95)
93 (91)
– (80)
70
PF26 rt
CI2b rt
PF26 258C
PF26 258C
74
1¼0.10 mmol, Ph3CPF6¼0.15 mmol, CH2Cl2¼0.60 mL.
a
Determined by 1H NMR using naphthalene as an internal standard.
Figures in parenthesis denote the isolated yields.
An equimolar amount (0.15 mmol) of AgPF6 was used to generate
Ph3CPF6 in situ.
b

Y. Furusho et al. / Tetrahedron 58 (2002) 6609–6613
6611
Table 3. Synthesis of ether-type [2]rotaxane (4) by tritylation of the
hydroxyl group at the axle end of 3 in the presence of DB24C8
1H NMR were performed on JEOL JNM-GX-270 and JNM-
L-400 spectrometers in CDCl3 with tetramethylsilane as an
internal reference. FAB-MS measurements were performed
on a Finnigan TSQ-70 instrument. For preparative HPLC, a
JAICO LC-908 system using columns JAIGEL 1 (B
20 mm£600 mm) and JAIGEL 2 (B 20 mm£600 mm)
was used.
4.1.1. Synthesis of amide (6). A solution of 3,5-di-t-
butylbenzoic acid10 (5, 5.56 g, 23.7 mmol) in SOCl2
(13 mL) was stirred overnight at 508C. Then, the mixture
was concentrated in vacuo. The remaining SOCl2 was
removed as an azeotropic mixture with benzene to give the
corresponding acid chloride. To a solution of 2-amino-
ethanethiol (2.26 g, 29.3 mmol) and Et3N (4.2 mL,
30 mmol) in CH2Cl2 (115 mL) was slowly added a solution
of the acid chloride in Et2O (15 mL). The mixture was
stirred for 1 h at room temperature, washed with 1 M HCl
(20 mL£3) and successively with brine, anhydrous MgSO4,
and evaporated to dryness to give amide (6) as a white solid
(6.6 g, 94%). 6 was used in the next step without further
purification. Mp 180–1838C; 1H NMR (CDCl3, 270 MHz) d
7.60–7.58 (m, 3H, ArH ), 6.55 (br s, 1H, CONH ), 3.65 (dt,
J1¼J2¼6.3 Hz, 2H, CH2S), 2.81 (dt, J1¼8.6 Hz, J2¼6.3 Hz,
2H, CH2N), 1.42 (t, J¼8.6 Hz, 1H, SH ), 1.35 (s, 18H, t-Bu );
IR (KBr) 1633 cm21 (nCvO).
Entry
Base
None
X
Yield (%)a
1
2
3
4
5
PF26
PF26
PF26
PF26
Clb
22
72
74
67
74
Et3N
Pyridine
DMAP
Et3N
3¼0.10 mmol, DB24C8¼0.12 mmol, Ph3CX¼0.15 mmol, base¼
0.15 mmol. CH2Cl2¼0.60 mL.
a
4.1.2. Synthesis of axle (1). To a suspension of LiAlH4
(80%, 1.30 g, 27.4 mmol) in THF (10 mL) was added a
mixture of 1 (1.45 g, 4.94 mmol) and THF (20 mL) under
Ar atmosphere. The mixture was refluxed for 20 h. A
saturated aqueous solution of Na2SO4 was added to the
reaction mixture to form a white precipitate, which was
removed by suction filtration. The filtrate was poured into a
mixture of conc. HCl (2 mL) and MeOH (50 mL). The
resulting solution was evaporated to dryness. The residue
was dissolved in MeOH (15 mL). To this solution was
added a solution of NH4PF6 (1.93 g, 11.8 mmol) in H2O
(10 mL) to form a white precipitate. Water was added until
no further precipitate was formed. The precipitate was
collected by suction filtration, washed with water, dried in
vacuo, and recrystallized from H2O/EtOH to afford axle (1)
Isolated yields.
An equimolar amount of AgPF6 was used to generate Ph3CPF6 in situ.
b
and resulted in a high yield of 4 (72%, entry 2). It should be
noted here that triethylamine must be added finally, because
the addition of triethylamine prior to trityl hexafluorophos-
phate leads to decomplexation of the pseudorotaxane. Use
of other bases such as pyridine and DMAP gave 4 in similar
yields (entries 3 and 4). The combination of trityl chloride
and silver hexafluorophosphate also afforded 4 in 74% in the
presence of triethylamine (entry 5).
3. Conclusion
1
as a white solid (1.0 g, 7.2%). Mp 180–1828C; H NMR
Tritylative endcapping of the in situ formed pseudorotaxane
having a thiol moiety at the axle end was carried out simply
by treating a solution of the pseudorotaxane with tritylhexa-
fluorophophate at room temperature to give the correspond-
ing sulfide-type [2]rotaxane in high yields. Tritylation of the
pseudorotaxane having a hydroxyl group at the axle
terminus was achieved by treatment with trityl hexafluoro-
phophate followed by addition of a base such as triethyl-
amine to afford the corresponding ether-type [2]rotaxane in
good yields. This protocol has been demonstrated to be
effective for the synthesis of [2]rotaxanes, since it is simple,
high-yielding, and tolerant to various solvents.
(CD3CN, 270 MHz) d 7.55 (t, J¼1.6 Hz, 1H, ArH ), 7.32 (d,
J¼1.6 Hz, 2H, ArH ), 4.17 (s, 2H, ArCH2), 3.16 (t,
J¼7.3 Hz, 2H, CH2N), 2.80 (t, J¼7.3 Hz, 2H, CH2S), 1.33
(s, 18H, t-Bu) (the signals of the SH and NH2 were not
observed probably due to fast exchange process); Found: C,
47.92; H, 7.00; N, 3.20%. Calcd for C17H30F6NPS: C,
47.99; H, 7.11; N, 3.29%.
4.1.3. Representative procedure for the preparation of
sulfide-type [2]rotaxane (2). To a solution of thiol (1)
(43 mg, 0.10 mmol) and DB24C8 (54 mg, 0.12 mmol) in
dichloromethane (0.60 mL) was added trityl hexafluoro-
phosphate (59 mg, 0.15 mmol). The reaction mixture was
stirred at room temperature for 24 h and then partitioned
between CHCl3 (3 mL) and H2O (3 mL). The organic layer
was washed with H2O (3 mL£1), dried over anhydrous
MgSO4, and evaporated to dryness. The residue was purified
with preparative HPLC to afford the sulfide-type
4. Experimental
4.1. 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.
1
[2]rotaxane (2) in 91% yield. Mp 76.2–77.08C; H NMR
(400 MHz, CDCl3) d 7.33 (br t, 1H, axle-ArH ), 7.25–7.13

6612
Y. Furusho et al. / Tetrahedron 58 (2002) 6609–6613
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1996; pp 191–262. (e) Harada, A. Acta Polym. 1998, 49,
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(m, 19H, axle-ArH and NH2), 6.92–6.79 (m, 8H, wheel-
ArH ), 4.49 (m, 2H, ArCH2NH2), 4.15–3.30 (m, 26H,
crown-CH2 and ArCH2NH2CH2), 2.21 (t, J¼7.8 Hz, SCH2),
1.16 (s, 18H, t-C4H9); FAB-MS (matrix: mNBA)
[M2PF6þH]þ 970.5; Found: C, 63.89; H, 7.01; N,
1.32%. Calcd for C60H76F6NO8PS·(H2O)0.5: C, 64.04; H,
6.90; N, 1.24%.
4.1.4. Representative procedure for the preparation of
ether-type [2]rotaxane (4). To a solution of alcohol3g (3)
(42 mg, 0.10 mmol) and DB24C8 (54 mg, 0.12 mmol) in
dichloromethane (0.60 mL) was added trityl hexafluoro-
phosphate (59 mg, 0.15 mmol). Then Et3N (21 mL,
0.15 mmol) was added dropwise to the reaction mixture.
The mixture was stirred at room temperature for 24 h and
then partitioned between CHCl3 (3 mL) and H2O (3 mL).
The organic layer was washed with H2O (3 mL£1), dried
over anhydrous MgSO4, and evaporated to dryness. The
residue was purified with preparative HPLC to afford the
ether-type [2]rotaxane (2) in 72% yield. Mp 165–1668C; 1H
NMR (400 MHz, CDCl3) d 7.4–7.2 (m, 20H, axle-ArH and
NH2), 6.95–6.85 (m, 8H, wheel-ArH ), 4.66 (m, 2H,
ArCH2NH2), 4.30–3.30 (m, 28H, axle-CH2 and wheel-
CH2), 2.97 (t, J¼5.8 Hz, 2H, ArCH2NH2CH2), 1.75 (m, 2H,
CH2CH2CH2), 1.18 (s, 18H, t-C4H9); FAB-MS (matrix:
mNBA) [M2PF6þH]þ 968.5; Found: C, 65.18; H, 6.91; N,
1.26%. Calcd for C61H78F6NO9P·(H2O)0.5: C, 65.23; H,
7.09; N, 1.25%.
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We are grateful to Professor F. Sanda of Kyoto University
for the elemental analyses. This work was financially
supported by Grant-in-Aid for Scientific Research on
Priority Areas (A), (No. 413/14045262) from the Ministry
of Education, Science, Sports, Culture, and Technology.
G. A. R. thanks JSPS Postdoctoral Fellowships.
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