Synthesis of linear oligo-TTFs and their [2]rotaxanes with cyclobis(paraquat-p-phenylene)

Article abstract of DOI:10.1039/b001868f

Two linear oligo-TTFs were synthesised employing a stepwise strategy involving two different thiolate protecting groups. These linear TTFs were incorporated into donor-acceptor rotaxanes with the cyclic acceptor, cyclobis(paraquat-p-phenylene). Moreover, a prototype rotaxane based on a bis(pyrrolo)-TTF was prepared and studied.

Full text of DOI:10.1039/b001868f

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J O U R N A L O F
Synthesis of linear oligo-TTFs and their [2]rotaxanes with
cyclobis(paraquat-p-phenylene)
Dorthe Damgaard,a Mogens Brùndsted Nielsen,a Jesper Lau,{a Kenneth Bendix Jensen,a
Roman Zubarev,b Eric Levillainb and Jan Becher*a
C H E M I S T R Y
aDepartment of Chemistry, University of Southern Denmark, Odense University, Campusvej
55, Odense M, Denmark DK-5230
bIMMO UMR 6501 du CNRS, 2 Boulevard de Lavoisier, Angers, France 49045
Received 8th March 2000, Accepted 10th July 2000
First published as an Advanced Article on the web 17th August 2000
Two linear oligo-TTFs were synthesised employing a stepwise strategy involving two different thiolate
protecting groups. These linear TTFs were incorporated into donor±acceptor rotaxanes with the cyclic
acceptor, cyclobis(paraquat-p-phenylene).{ Moreover, a prototype rotaxane based on a bis(pyrrolo)-TTF was
prepared and studied.
thiolate protecting groups (PG1 and PG2). The cyanoethyl
group has been commonly employed as a protecting group for
TTF-thiolates.6 Recently, the p-nitrophenylethyl group was
used successfully in combination with the cyanoethyl group for
a stepwise deprotection±alkylation (Scheme 2).7
Thus, it should be possible to stepwise deprotect a TTF
containing one cyanoethyl group and one p-nitrophenylethyl
group (Scheme 3). The key compounds 14 and 15 were
prepared by unsymmetrical phosphite-mediated cross cou-
plings.8 Different combinations of 1,3-dithiole-2-thiones and
1,3-dithiol-2-ones were tried in these couplings and the highest
yield of the TTF 14 is obtained when reacting the ketone 6 with
the thione 10.
The p-nitrophenylethyl monoprotected thione 10 can be
prepared in two ways (Scheme 4): (a) treatment of 167 with
1 equiv. of caesium hydroxide monohydrate followed by excess
methyl iodide in DMF; (b) treatment of 76 with 1 equiv. of
caesium hydroxide monohydrate followed by 2-(4-nitrophenyl)-
ethyl bromide in CH3CN.
Selective monoalkylation is now possible, thus treatment of
14 with 1 equiv. of caesium hydroxide monohydrate and
subsequently 1,2-bis(2-iodoethoxy)ethane in DMF (Scheme 5)
generated the iodide 17 in 77% yield. This compound was then
reacted with the bisthiolate generated by treating 186 with
2 equiv. caesium hydroxide monohydrate to afford the
tris(tetrathiafulvalene) 19 in 83% yield. The two p-nitrophe-
nylethyl protecting groups were then removed upon treatment
with 2 equiv. of caesium hydroxide, and the resulting bis-
thiolate was then reacted with 1-{2-[2-(2-iodoethoxy)ethoxy]-
ethoxy}-4-(triphenylmethyl)benzene1g 23a (Scheme 5) in
DMF, affording the dumbbell-shaped compound 20 in 82%
yield.
Introduction
Rotaxanes are a class of interlocked molecules containing a
dumbbell-shaped component (a rod and two bulky stoppers),
around which a cyclic entity is encircled. The bulky stoppers
prevent the cyclic entity from dethreading. The translational
isomerism of rotaxanes1 offers the possibility for employing
such interlocked molecules as molecular machines (Fig. 1).2
The reversible redox properties of tetrathiafulvalene (TTF)
make it a good candidate as an electron donor unit in donor±
acceptor based rotaxanes with the cyclic acceptor, cyclobis(-
paraquat-p-phenylene) 4 (Fig. 2).3 Thus a rotaxane, containing
two different TTFs, whose redox potentials can be ®nely tuned
by varying their substitution, could possibly be a molecular
switch.
The ®rst TTF-based rotaxane was reported by Stoddart and
coworkers1g and contained one central bis(2-oxypropylene-
dithio)-substituted TTF and two hydroquinone donor sites in
the dumbbell component. We have reported another rotaxane
based on a tetramercapto-substituted TTF (Fig. 3).1h
As an extension to this work, a rotaxane containing three
derivatised units of 2 has been prepared in order to study the
shuttling motions upon electrochemical oxidation. Moreover, a
rotaxane containing derivatives of the two different TTFs 2 and
3 is reported (Fig. 4). However, both these rotaxanes suffer
from the problem of cis±trans isomerism. As a solution to this
problem we describe the synthesis of the ®rst prototype
rotaxane based on the bispyrrolo-TTF 1, which is a strong p-
donor (Fig. 4). An association constant of 7900 M21 was
obtained for the pseudorotaxane complex between 1 and 4 in
acetone,4 while the association constant for the TTF?4 complex
is 2600 M21 in acetone.5
Next, a linear molecule containing two differently substi-
tuted tetrathiafulvalenes was prepared, the ®rst TTF unit
containing methylthio substituents in the 5(4),5'-positions and
the other methoxycarbonyl groups in the 5(4),5'-positions
(Scheme 6), from the diester 15, which was treated with one
equiv. of caesium hydroxide to afford the monothiolate that
was then reacted with the iodide 17, generating the bis(te-
trathiafulvalene) 21 in 85% yield. Deprotection of 21 by
treatment of the bisthiolate with 2 equiv. caesium hydroxide
monohydrate and addition of 2 equiv. of 23a1g gave the
dumbbell 22 in 65% yield. Both compounds 20 and 22 are
mixtures of cis±trans isomers.
Results and discussion
Preparation of linear oligo-tetrathiafulvalenes
In order to prepare linear molecules containing multiple TTFs,
the stepwise strategy outlined in Scheme 1 was employed. This
strategy relies on the availability of at least two orthogonal
{Present address: Novo Nordisk A/S, Novo Nordisk Alle, DK-2760
MaÊlùv, Denmark.
{The IUPAC name for cyclobis(paraquat-p-phenylene) is
1(1,4),2(4,1),6(1,4),7(4,1)-tetrapyridina-4,9(1,4)-dibenzenacyclodeca-
phane-11,21,61,71-tetraium.
Finally, a dumbbell based on the bis(pyrrolo)-TTF 1 was
DOI: 10.1039/b001868f
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Table 1 Peaks (m/z) assigned in the ESMS and MALDI MS spectra
27
28
ESMS
682
MALDI MS
ESMS
MALDI MS
[M24PF6]4z
[M24PF6]3z
[M24PF6]2z
[M24PF6]z
[M23PF6]4z
[M23PF6]3z
[M23PF6]2z
[M23PF6]z
[M22PF6]4z
[M22PF6]2z
[M22PF6]z
[M21PF6]z
[dumbbell]3z
[dumbbell]2z
[dumbbell]z
682
909
1364
2728
570
760
760
1139
2278
2728
718
958
958
808
1212
2873
754.5
1510
3018
3163
737
2873
2423
2568
1284
Fig. 1 Redox-controlled switch.
1105
2208
1758
Table 2 ESMS/MS data rotaxanes (m/z)
Parent
ion
Daughter
ions
[M24PF6]4z [C8H8]z [C28H24N4]2z [C18H16N2].z [dumbbell]2z
27 682
28 570
104
208
208
260
260
1105
879
Electrospray mass spectrometry (ESMS) and matrix assisted
laser desorption ionization (MALDI)
The rotaxanes were characterized using ES and MALDI mass
spectrometry. The data concerning 27 and 28 are listed in
Table 1. It can be seen that the compounds give peaks arising
from loss of PF62. Peaks resulting from fragmentation of the
rotaxane structures are also observed in the spectra. The
daughter ion spectra (ESMS/MS) of selected parent ions are
recollected in Table 2. Collisional activation of [M24PF6]4z
by argon results in fragmentation of the supramolecular
structure. The breakdown of the tetracationic cyclophane is
accompanied by one or two electron transfers from the
dumbbell to the cyclophane (Scheme 9). A similar fragmenta-
tion pattern was previously observed in related catenanes.9
Fig. 2 Donor properties of compounds 1±4
obtained in a yield of 81% by reacting bispyrrolo-TTF 1 with
sodium hydride and the bromide 23b (Scheme 7).
Preparation of rotaxanes
The rotaxane 27 was prepared in 20% yield by reacting the
dumbbell 20 with 1,1@-[1,4-phenylenebis(methylene)]bis(4,4'-
bipyridin-1-ium) bis(hexa¯uorophosphate) 26 and 1,4-bis(bro-
momethyl)benzene 25 in DMF and subjecting the mixture to
10 kbar for 4 d (Scheme 8). In a similar way the rotaxanes 28
and 29 were obtained, in yields of 35% and 26%, respectively
(Fig. 4). The UV-Vis of 27 and 28 revealed charge transfer
absorption bands at lmax 788 nm (e 496 M21 cm21) and lmax
748 nm (e 1150 M21 cm21), whereas 29 exhibited a charge
transfer band at lmax 807 nm (e 310 M21 cm21).
1H NMR spectroscopy
According to 1H NMR spectroscopy the multiple-TTF
rotaxanes 27 and 28 only contain one encircled unit of 4.
However, as a result of the cis±trans isomerism, their 1H NMR
spectra are complicated. Actually, 6 isomers of 27 and 4
isomers of 28 are possible. The spectrum of 27 is shown in
Fig. 5. The cyclophane bipyridinium a-protons resonate as one
Fig. 3 Mono-TTF based rotaxane.
2250
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Fig. 4 Structures 28a, 28b and 29
Scheme 1 Retrosynthetic strategy for preparing linearly connected oligo-TTFs.
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Scheme 4 Reagents and conditions: i, 1 equiv. CsOH?H2O±CH3OH,
DMF, room temp., 1 h; ii, CH3I (excess), room temp., 5 h, 42%; iii,
1 equiv. CsOH?H2O±CH3OH, CH3CN, room temp., 1 h; iv, 2-(4-
nitrophenyl)ethyl bromide, CH3CN, room temp. 1 h, 92%.
1
cyclic acceptor along the dumbbell on the H NMR timescale
(500 MHz).
Also for 28 two SCH3 multiplets were observed, positioned
at 2.31±2.45 and 2.55±2.64 ppm, respectively (Fig. 4). These
two sets of SCH3 protons were still present when cooling the
sample to 213 K in (CD3)2CO and can be assigned to
uncomplexed and complexed SCH3 groups, hence correspond-
ing to encirclement of either the diester-TTF or the tetra-
mercaptothio-TTF. The ratio between these two rotaxanes was
estimated to ca. 8 : 10 at room temperature in CD3CN
(500 MHz). The exchange was still slow on the 1H NMR
timescale (250 MHz) upon heating to 338 K. Thus, a kinetical
barrier prevents the conversion of rotaxane 28b into the
supposedly more stable 28a in which the tetramercapto-TTF
donor is encircled. The isomerically pure pyrrolo-TTF
rotaxane 29 gave a simple 1H NMR spectrum with sharp
signals for the cyclophane protons.
Scheme 2 Step-wise monodeprotection±realkylation protocol.
Conclusions
broad multiplet, whereas the b-protons are split into two broad
multiplets (Table 3). The dumbbell SCH3 and SCH2 protons
each resonate as two multiplets. One SCH3 multiplet was
shifted down®eld by 0.22 ppm relative to the other, which is
indicative of interactions with the cyclic acceptor.5 The
separated signals indicate slow shuttling motions of the
A strategy for preparing linear oligo-TTFs has been developed
using two different thiolate protection groups. From these,
rotaxanes were prepared and investigated, in both solution and
in the gas phase. In the gas phase we observe electron transfers
to the cyclic acceptor accompanied by its fragmentation.
Future rotaxane systems based on a pyrrolo-TTF10 and
another donor seem very promising owing to the strong
complexation between this TTF and the cyclic acceptor.
Moreover, such systems will be devoid of cis±trans isomerism
and hence more easy to characterise and study.
Experimental
General methods
All reactions were carried out under an atmosphere of dry N2.
CH3OH was distilled from Mg. DMF was allowed to stand
Ê
over molecular sieves (4 A) for at least 3 days before use. All
reagents were standard grade and used as received. Analytical
TLC was performed on Merck DC-Alufolien Kieselgel 60 F254
0.2 mm thickness. Column chromatography was carried out
using silica gel 60F (Merck, 9385, 230±400 mesh). Melting
points were determined on a BuÈchi melting point apparatus
and are uncorrected. 1H NMR spectra were recorded on a
Bruker AC250, a Varian 300 or a Varian 500 spectrometer; all
chemical shifts are referenced to Me4Si; J values are in Hz.
Electron impact (EI) and fast atom bombardment (FAB) mass
spectra were obtained on a Varian MAT 311 A instrument and
a Kratos MS 60 RF, respectively. Plasma desorption (PD) mass
spectra were carried out on a BioIon 10R time of ¯ight mass
spectrometer over 56105 ®ssions (252Cf). Electrospray (ES)
mass spectra were recorded using a Finnigan MAT TSQ 700
triple quadrupole mass spectrometer [a] and an IonSpec
Fourier Transform Mass Spectrometer [b]. The rotaxanes
were electrosprayed from acetonitrile solutions. ESMS/MS
experiments were performed on the TSQ using argon typically
at a pressure of 0.7 mTorr. The ion of interest was selected by
Scheme 3 Reagents and conditions: i, Hg(OAc)2, CHCl3±CH3CO2H,
room temp., 90 min, 87% (12), 70% (13); ii, P(OEt)3, PhMe, 120 ³C, 2 h.
2252
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Scheme 5 Reagents and conditions: i, 1 equiv. CsOH?H2O±CH3OH, DMF, room temp., 1 h; ii, (CH2OCH2CH2I)2, DMF, room temp., 5 h, 77%; iii,
2 equiv. CsOH?H2O±CH3OH, DMF, room temp., 1 h; iv, 2 equiv. 17, DMF, room temp., 5 h, 83%; v, 2 equiv. CsOH?H2O±CH3OH, DMF, room
temp., 1 h; vi, 2 equiv. 23a, DMF, room temp., 5 h, 82%.
the ®rst quadrupole, collisionally activated in the second
(actually an octapole), and the products analysed by the third
quadrupole. Matrix Assisted Laser Desorption Ionization
(MALDI) mass spectra were recorded using a Bruker Re¯ex
instrument based on 200 shots, the matrix was dihydroxyben-
zene±CH3OH 1 : 1. Electrochemical experiments were carried
out with Bu4NPF6 as supporting electrolyte (0.1 M). Counter
and working electrodes were made of Pt and the reference
electrode was Ag/AgCl . The solvent was CH3CN±CH2Cl2 9 : 1
unless otherwise indicated. All measurements were carried out
at room temperature. UV-VIS spectra were recorded on a
Shimadzu UV-160 instrument. Elemental analyses were
performed at the Microanalytical Laboratory, University of
Copenhagen and Atlantic Microlab, Inc., Georgia, USA.
solution of CsOH?H2O (0.36 g, 2.11 mmol) in methanol
(5 ml) was added dropwise via a syringe with stirring over
30 min. The solution was stirred for a further 30 min and CH3I
(1.43 g, 10.07 mmol) was added via a syringe and stirring was
continued for an additional 5 h, whereupon the reaction
mixture was concentrated in vacuo. CH2Cl2 (100 ml) was
added, and the organic solution washed with water, saturated
aqueous NaCl, and dried (MgSO4). The solvent was removed in
vacuo and the residue puri®ed by column chromatography
(silica gel, CH2Cl2±petroleum ether (bp 60±80 ³C) 1 : 1),
affording 10 (0.31 g, 0.85 mmol, 42%) as long yellow±orange
needles. Mp 93±94 ³C (from propan-2-ol).
Method 2. To
methylthio-1,3-dithiole-2-thione
CH3CN (50 ml), solution of CsOH?H2O (0.70 g,
a
solution of 4-(2-cyanoethylthio)-5-
7
(1.00 g, 3.77 mmol) in
5-Methylthio-4-[2-(4-nitrophenyl)ethylthio]-1,3-dithiole-2-thione
10
a
4.14 mmol) in methanol (10 ml) was added dropwise with
stirring over 30 min. The solution changed colour from yellow
to red and was stirred for 30 min. A solution of 2-(4-
Method 1. To
a solution of 4,5-bis[2-(4-nitropheny-
l)ethylthio]-1,3-dithiole-2-thione 166 (1.00 g, 2.01 mmol), a
Scheme 6 Reagents and conditions: i, 1 equiv. CsOH?H2O±CH3OH, DMF, room temp., 1 h; ii, 1 equiv. 17, DMF, room temp., 5 h, 85%; iii, 2 equiv.
CsOH?H2O±CH3OH, DMF, room temp., 1 h; iv, 2 equiv. 1-{2-[2-(2-iodoethoxy)ethoxy]ethoxy}-4-(triphenylmethyl)benzene), DMF, room temp.,
5 h, 65%.
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Scheme 7 Reagents and conditions: i, NaH (excess), DMF, 81%.
Scheme 8 Reagents and conditions: i, 20, 25 and 26, DMF, room. temp., 10 kbar, 4 d; ii, NH4PF6, 20%.
nitrophenyl)ethyl bromide (1.06 g, 4.61 mmol) in CH3CN
(20 ml) was added in one portion. After stirring for 1 h, the
reaction mixture was concentrated in vacuo. CH2Cl2 (75 ml)
was added, and the organic solution washed with water,
saturated aqueous NaCl, and dried (MgSO4). The solvent was
then removed in vacuo. Recrystallisation of the product from
propan-2-ol gave 10 (1.26 g, 3.49 mmol, 92%) as long yellow±
orange needles. Mp 93±94 ³C; dH (CDCl3) 2.52 (s, 3H; SCH3),
3.12 (m, 4H; SCH2CH2Ar), 7.40 (d, 2H, J 8.7; Ar 2,6-H), 8.19
(d, 2H, J 8.6; Ar 3,5-H); dC (CDCl3) 18.96, 35.69, 36.87, 123.93,
129.55, 130.40, 140.98, 146.44, 147.05, 210.66; MS (EI): m/z
(%): 361 (Mz, 100); Found: C, 39.84; H, 3.05; N, 3.76;
C12H11NO2S5 requires C, 39.87; H, 3.07; N, 3.87%.
washed with chloroform (3650 ml). The combined ®ltrate was
cautiously washed with aqueous NaHCO3 (56100 ml) and
water (26100 ml) and dried (MgSO4). The solvent was
removed in vacuo and the residue recrystallised from CH3OH
to give 12 (0.83 g, 2.41 mmol, 87%) as shining white needles.
Mp 80±81 ³C; dH (CDCl3) 2.48 (s, 3H; SCH3), 3.11 (m, 4H;
SCH2CH2Ar), 7.39 (d, 2H, J 8.7; Ar 2,6-H), 8.18 (d, 2H, J 8.6;
Ar 3,5-H); dC (CDCl3) 19.26, 35.81, 36.90, 122.01, 123.86,
129.50, 131.60, 146.58, 147.00, 188.94; MS (EI): m/z (%): 345
(Mz, 75); Found: C, 41.84; H, 3.11; N, 4.05; S, 37.24;
C12H11NO3S4 requires C, 41.72; H, 3.21; N, 4.05; S, 37.12%.
4'-(2-Cyanoethylthio)-5(4),5'-bis(methylthio)-4(5)-[2-(4-
nitrophenyl)ethylthio]tetrathiafulvalene 14, cis±trans-mixture
5-Methylthio-4-[2-(4-nitrophenyl)ethylthio]-1,3-dithiol-2-one 12
Method 1. 4-(2-Cyanoethylthio)-5-methylthio-1,3-dithiol-2-
one 6 (1.54 g, 6.18 mmol) and 5-methylthio-4-[2-(4-nitrophe-
nyl)ethylthio]-1,3-dithiole-2-thione 10 (2.90 g, 8.03 mmol) were
suspended in a mixture of P(OEt)3 (30 ml) and toluene (15 ml).
The mixture was heated under nitrogen to 120 ³C causing the
solid material to dissolve. The reaction mixture was heated for
To a solution of 5-methylthio-4-[2-(4-nitrophenyl)ethylthio]-
1,3-dithiole-2-thione 10 (1.00 g, 2.77 mmol) in a mixture of
CHCl3 (45 ml) and AcOH (15 ml) was added Hg(OAc) (2.38 g,
7.47 mmol). After stirring for 90 min, the suspension was
®ltered through a short layer of Celite. The Celite was carefully
Table 3 Selected 1H NMR chemical shifts of rotaxanes (cyclophane region)
Compound
Solvent
Ha
Hb
C6H4
NCH2
5
CD3CN
9.00
9.65
9.07
9.39
7.89
7.72
8.07
7.69
7.90
5.68
6.09
5.74
5.76
27
28
29
CD3COCD3
CD3CN
CD3SOCD3
8.67, 8.45
8.08, 7.83
7.94
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vacuo. The residue was subjected to column chromatography
(silica gel, (i) cyclohexane, (ii) CH2Cl2±cyclohexane 1 : 1)
affording 14 (0.28 g, 0.50 mmol, 33%) as a red solid. Mp
135±136 ³C.
4'-{2-[2-(2-Iodoethoxy)ethoxy]ethylthio}-5(4),5'-bis(methylthio)-
4(5)-[2-(4-nitrophenyl)ethylthio]tetrathiafulvalene 17, cis±trans-
mixture
To a solution of 4'-(2-cyanoethylthio)-5(4),5'-bis(methylthio)-
4(5)-[2-(4-nitrophenyl)ethylthio]tetrathiafulvalene 14 (0.80 g,
1.42 mmol) in DMF (100 ml), a solution of CsOH?H2O
(0.25 g, 1.49 mmol) in methanol (5 ml) was added dropwise
with stirring over 30 min. The solution was stirred for 30 min,
whereupon a solution of 1,2-bis(2-iodoethoxy)ethane (1.58 g,
4.26 mmol) in DMF (5 ml) was added in one portion. The
solution was stirred for 5 h, and the solvents were removed in
vacuo. The residue was extracted with CH2Cl2 (100 ml) and the
organic phase washed with water and saturated aqueous NaCl
solution. It was then dried over anhydrous MgSO4 and
concentrated in vacuo. The residue was subjected to column
chromatography (silica gel) with CH2CH2±petroleum ether (bp
60±80 ³C) (1 : 1) as the eluent. 17 (0.83 g, 1.10 mmol, 77%) was
obtained as a red solid. Mp 50±52 ³C; dH (CDCl3) 2.50 (br s,
6H; SCH3), 3.08 (m, 6H; SCH2 and SCH2CH2Ar), 3.27 (m, 2H;
ICH2), 3.75 (m, 8H; OCH2), 7.41 (d, 2H, J 8.6; Ar 2,6-H), 8.16
(d, 2H, J 8.4; Ar 3,5-H); MS (FAB): m/z: 751 (Mz); CV:
E11/2~0.52 V, E21/2~0.79 V; Found: C, 35.25; H, 3.19; N, 1.87;
C22H26INO4S8 requires C, 35.15; H, 3.49; N, 1.86%.
Tris(tetrathiafulvalene) 19, cis±trans-mixture
Scheme 9 Possible fragmentations explaining the observed daughter
ion peaks in ESMS/MS. The fragmentation of the cyclic acceptor is
accompanied by electron transfers from the TTF(s).
To
a
solution of 4(5),4'-bis(2-cyanoethylthio)-5(4),5'-bis
(methylthio)tetrathiafulvalene 1811 (0.21 g, 0.45 mmol) in
DMF (75 ml), a solution of CsOH?H2O (0.17 g, 0.99 mmol)
in methanol (10 ml) was added dropwise with stirring over
30 min. After stirring for 30 min, a solution of 4'-{2-[2-
(2-iodoethoxy)ethoxy]ethylthio}-5(4), 5'-bis(methylthio)-4(5)-
[2-(4-nitrophenyl)ethylthio]tetrathiafulvalene 17 in DMF was
added in one portion. The solution was stirred for 5 h, and the
reaction mixture was then concentrated in vacuo. CH2Cl2
(100 ml) was added, and the organic solution washed with
water, saturated aqueous NaCl, and dried (MgSO4). The
solvent was then removed and the residue puri®ed by column
chromatography (silica gel, (i) CH2Cl2, (ii) CH2Cl2±ethyl
acetate 19 : 1), affording 19 (0.60 g, 0.75 mmol, 83%) as a
dark red viscous oil. dH (CDCl3) 2.48 (br s, 18H; SCH3), 3.06
(m, 16H; SCH3 and SCH2CH2Ar), 3.64 (m, 16H; OCH2),
7.41 (d, 4H, J 8.5; Ar 2,6-H), 8.16 (d, 4H, J 8.8; Ar 3,5-H); MS
(FAB): m/z: 1606 (Mz); CV: E11/2~0.48 V, E21/2~0.80 V;
Found: C, 38.75; H, 3.51; N, 1.68; C52H58N2O8S24 requires C,
38.83; H, 3.63; N, 1.74%.
2 h, before it was cooled to room temperature. The solvents
were removed in vacuo. The residue was subjected to column
chromatography (silica gel) with CH2Cl2 as the eluent. 14
(1.51 g, 2.68 mmol, 43%) was obtained as a red solid. Mp 135±
137 ³C; dH (CDCl3) 2.45 (s, 3H; SCH3), 2.50 (s, 3H; SCH3), 2.75
(m, 2H; CH2CN), 3.07 (m, 6H; SCH2 and SCH2CH2Ar), 7.41
(d, 2H, J 8.6; Ar 2,6-H), 8.16 (d, 2H, J 8.6; Ar 3,5-H); MS (EI):
m/z (%): 562 (Mz, 100); CV: E11/2~0.53 V, E21/2~0.80 V;
Found: C, 40.48; H, 3.21; N, 4.84; C19H18N2O2S8 requires C,
40.55; H, 3.22; N, 4.98%.
Method 2. 4-(2-Cyanoethylthio)-5-methylthio-1,3-dithiole-
2-thione 7 (0.77 g, 2.91 mmol) and 5-methylthio-4-[2-(4-nitro-
phenyl)ethylthio]-1,3-dithiol-2-one 12 were suspended in a
mixture of P(OEt)3 (30 ml) and toluene (15 ml). The mixture
was heated under nitrogen to 120 ³C causing the solid material
to dissolve. The reaction mixture was heated for 2 h, before it
was cooled to room temperature. The solvents were removed in
Tris(tetrathiafulvalene) dumbbell 20, cis±trans-mixture
To a solution of 19 (0.26 g, 0.16 mmol) in DMF (50 ml) was
added dropwise via a syringe, a solution of CsOH?H2O
(0.059 g, 0.35 mmol) in methanol (5 ml) with stirring over
30 min. The solution was stirred for 30 min. A solution of 23a1g
(0.23 g, 0.40 mmol) in DMF (10 ml) was added in one portion,
and the reaction mixture was stirred for 5 h. The solvents were
removed in vacuo. The residue was extracted with CH2Cl2
(50 ml), and the organic phase washed with water and
saturated aqueous NaCl solution. It was then dried over
anhydrous MgSO4 and concentrated in vacuo. The residue was
puri®ed by column chromatography (silica gel, (i) CH2Cl2, (ii)
CH2Cl2±ethyl acetate 19 : 1), affording 20 (0.29 g, 0.13 mmol,
82%) as an orange oil. dH (CDCl3) 2.43 (br s, 18H; SCH3), 2.97
(m, 12H; SCH2), 3.68 (m, 28H; OCH2), 3.85 (m, 4H;
ArOCH2CH2), 4.11 (m, 4H; ArOCH2), 6.79 (d, 4H, J 9.0; Ar
3,5-H), 7.10 (d, 4H, J 8.9; Ar 2,6-H), 7.27 (m, 30H; C6H5);
Fig. 5 1H NMR (250 MHz) spectrum of 27 in (CD3)2CO.
J. Mater. Chem., 2000, 10, 2249±2258
2255

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MS (FAB): m/z: 2208 (Mz); CV: E11/2~0.48 V, E21/2~0.79 V;
Found: C, 52.68; H, 4.56; C98H104O10S24?H2O requires C,
52.80; H, 4.79%.
4-Methoxycarbonyl-5-[2-(4-nitrophenyl)ethylthio]-1,3-dithiol-2-
one 13
To
a solution of 4-methoxycarbonyl-5-[2-(4-nitrophenyl)
ethylthio]-1,3-dithiole-2-thione 11 (2.00 g, 5.36 mmol) in a
mixture of CHCl3 (75 ml) and AcOH (25 ml) was added
Hg(OAc) (4.61 g, 14.46 mmol). After stirring for 16 h the
suspension was ®ltered through a short layer of Celite. The
Celite was carefully washed with chloroform (100 ml). The
combined ®ltrate was cautiously washed with aqueous
NaHCO3 and water, and dried (MgSO4). The solvents were
removed in vacuo and the residue recrystallised from CH3OH
to give 13 (1.34 g, 3.75 mmol, 70%) as bright-yellow needles.
Mp 126±127 ³C; dH (CDCl3) 3.16 (m, 2H; ArCH2), 3.27 (m, 2H;
SCH2), 3.86 (s, 3H; CO2CH3), 7.41 (d, 2H, J 8.6; Ar 2,6-H),
8.20 (d, 2H, J 8.6; Ar 3,5-H); MS (EI): m/z (%): 357 (Mz,100);
Found: C, 43.67; H, 3.06; N, 3.92; S, 26.95; C13H11NO5S3
requires C, 43.69; H, 3.10; N, 3.92; S, 26.91%.
Tris(tetrathiafulvalene) rotaxane 27, cis±trans-mixture
A solution of 20 (0.271 g, 0.12 mmol), 26 (0.260 g, 0.37 mmol)
and 25 (0.110 g, 0.42 mmol) in DMF (12 ml) was transferred to
a high-pressure-reaction Te¯on tube, which was then com-
pressed (10 kbar) at room temperature for 4 d. The solvent was
then removed in vacuo to give a residue, which was subjected to
column chromatography on silica gel with ®rst acetone and
later acetone±NH4PF6 as eluent. Collection of the green
fraction afforded a green solid after evaporation of the solvent
in vacuo. CH3NO2 was added and the resulting solution washed
with water and cyclohexane. The solvent was then removed to
give 27 (0.081 g, 0.024 mmol, 20%) as a green solid. Mp 202±
205 ³C (decomp.); dH (CD3COCD3) 2.48 (m; SCH3), 2.70 (m;
SCH3), 2.48±2.70 (18H), 3.00 (m; SCH2), 3.35 (m; SCH2), 3.00±
3.35 (12H), 3.61±4.15 (m, 36H; OCH2), 6.09 (m, 8H; NCH2),
6.62 (br d, J 7.9; C6H5), 6.86 (br d, J 8.6; C6H5), 7.09 (br t, J 8.4;
C6H5), 7.34 (m; C6H5), 6.62±7.34 (38H), 8.07(m, 8H; C6H4),
8.45 (m; b-H), 8.67 (m; b-H), 8.45±8.67 (8H), 9.65 (m, 8H; b-
H); MS (ES): m/z: [a]: 682 [M24PF6]4z, 737 [20]3z, 958
4'-(2-Cyanoethylthio)-5(4),5'-bis(methoxycarbonyl)-4(5)-[2-(4-
nitrophenyl)ethylthio]tetrathiafulvalene 15, cis±trans-mixture
Method
thio]-1,3-dithiole-2-thione 11 (1.20 g, 3.21 mmol) and 4-(2-
cyanoethylthio)-5-methoxycarbonyl-1,3-dithiol-2-one
1. 4-Methoxycarbonyl-5-[2-(4-nitrophenyl)ethyl-
8
[M23PF6]3z
,
1105 [20]2z
,
1510 [M22PF6]2z
,
2728
(0.65 g, 2.47 mmol) were suspended in a mixture of freshly
distilled P(OEt)3 (30 ml) and toluene (15 ml) and heated to
120 ³C. The mixture was stirred for 2 h and then allowed to
cool to room temp. The product was precipitated with CH3OH
(50 ml), ®ltered off and washed with CH3OH. Column
chromatography (silica gel, CH2Cl2), afforded 15 (0.41 g,
0.69 mmol, 28%) as an orange±red solid. Mp 183±184 ³C; dH
(DMSO-d6) 2.99 (m, 2H; CH2CN), 3.17 (t, 2H, J 7.6;
SCH2CH2Ar), 3.39 (m, 4H; SCH3), 3.73 (s, 3H; CO2CH3),
7.59 (d, 2H, J 8.3; Ar 2,6-H), 8.18 (d, 2H, J 8.6; Ar 3,5-H); MS
(EI): m/z (%): 586 (Mz,100); CV: E11/2~0.65 V, E21/2~0.98 V;
Found: C, 43.26; H, 3.16; N, 4.65; S, 32.55; C21H18N2O6S6
requires C, 42.99; H, 3.09; N, 4.77; S, 32.78%.
[M24PF6]z, 2873 [M23PF6]z, 3018 [M22PF6]z, 3163
[M21PF6]z; [b]: 682 [M24PF6]4z, 718 [M23PF6]4z, 754.5
[M22PF6]4z, 958 [M23PF6]3z; MS (MALDI): m/z: 682
[M24PF6]4z, 909 [M24PF6]3z, 958 [M23PF6]3z, 1364 [M2
4PF6]2z, 2728 [M24PF6]z, 2873 [M23PF6]z; MS/MS (ES):
parent ion: m/z: 682, daughter ions: m/z: 104, 208, 260, 1105;
CV: E11/2~0.52 V, E21/2~0.81 V; UV-VIS: lmax~788 nm and
e~496 M21 cm21
.
4-Methoxycarbonyl-5-[2-(4-nitrophenyl)ethylthio]-1,3-dithiole-
2-thione 11
Method 1. A solution of caesium 5-methoxycarbonyl-1,3-
dithiole-2-thione-4-thiolate12 (2.00 g, 5.61 mmol) and 2-(4-
nitrophenyl)ethyl bromide (1.55 g, 6.74 mmol) in acetone was
re¯uxed for 2 h. The resulting solution was cooled to room
temp. and concentrated in vacuo. The crude product was
redissolved in CH2Cl2 (100 ml), washed with water, saturated
aqueous NaCl solution, and dried (MgSO4). The solvent was
then removed and the residue puri®ed by column chromato-
graphy (silica gel, (i) CH2Cl2±petroleum ether (bp 60±80 ³C)
1 : 1, (ii) CH2Cl2) affording 11 (1.40 g, 3.75 mmol, 66%) as
yellow needles. Mp 146±147 ³C (from propan-2-ol).
Method
2. 4-(2-Cyanoethylthio)-5-methoxycarbonyl-1,3-
dithiole-2-thione 9 (1.16 g, 4.20 mmol) and 4-methoxycarbo-
nyl-5-[2-(4-nitrophenyl)ethylthio]-1,3-dithiol-2-one 13 (1.00 g,
2.80 mmol) were suspended in a mixture of freshly distilled
P(OEt)3 (20 ml) and toluene (10 ml). The mixture was heated at
120 ³C for 2 h and then allowed to cool to room temp. The
product was precipitated with CH3OH (50 ml), ®ltered off and
washed with CH3OH. Column chromatography (silica gel,
CH2Cl2), afforded 15 (0.47 g, 0.80 mmol, 28%) as an orange±
red solid. Mp 183±184 ³C.
Bis(tetrathiafulvalene) 21, cis±trans-mixture
Method 2. To a solution of 5-methoxycarbonyl-4-mercapto-
1,3-dithiole-2-thione12 (10.00 g, 44.58 mmol) in DMF,
(400 ml). NaOEt (1.03 g, in EtOH, 44.58 mmol) was added
dropwise with stirring over 30 min. The solution was stirred for
30 min, whereupon a solution of 2-(4-nitrophenyl)ethyl bro-
mide (12.31 g, 5.35 mmol) in DMF was added. Stirring was
continued for an additional 5 h, and the reaction mixture was
then concentrated in vacuo. CH2Cl2 (400 ml) was added, and
the organic solution washed with water, saturated aqueous
NaCl solution, and dried (MgSO4). The solvent was then
removed and the residue puri®ed by column chromatography
(silica gel, CH2Cl2±petroleum ether (bp 60±80 ³C) 4 : 1),
affording 11 (4.08 g, 10.92 mmol, 20%) as a yellow solid. Mp
146±147 ³C (from propan-2-ol); dH (CDCl3) 3.17 (m, 2H;
ArCH2), 3.28 (m, 2H; SCH2), 3.86 (s, 3H; CO2CH3), 7.41 (d,
2H, J 8.8; Ar 2,6-H), 8.20 (d, 2H, J 8.7; Ar 3,5-H); dC (CDCl3)
35.12, 35.77, 52.94, 122.21, 124.00, 124.13, 129.51, 145.78,
147.27, 158.90, 207.43; MS (EI): m/z (%): 373 (Mz, 100);
Found: C, 41.77; H, 2.95; N, 3.79; S, 34.38; C13H1NO4S4
requires C, 41.81; H, 2.97; N, 3.75; S, 34.34%.
To a solution of 4'-(2-cyanoethylthio)-5(4),5'-bis(methoxycar-
bonyl)-4(5)-[2-(4-nitrophenyl)ethylthio]tetrathiafulvalene 15
(0.39 g, 0.66 mmol) in DMF (100 ml),
a solution of
CsOH?H2O (0.12 g, 0.70 mmol) in methanol (5 ml) was
added dropwise via a syringe with stirring over 30 min. After
stirring for 30 min, a solution of 4'-{2-[2-(2-iodoethoxy)ethoxy]-
ethylthio}-5(4), 5'-bis(methylthio)-4(5)-[2-(4-nitrophenyl)-
ethylthio]tetrathiafulvalene 17 (0.55 g, 0.73 mmol) in
DMF was added via a syringe. The solution was stirred
for 5 h, whereupon the solvent was removed in vacuo. The
residue was extracted with CH2Cl2 (100 ml), and the organic
phase washed with water and saturated aqueous NaCl
solution. The organic phase was dried over anhydrous
MgSO4 and concentrated in vacuo. The residue was subjected
to column chromatography (silica gel) with CH2Cl2 as the
eluent. 21 (0.66 g, 0.57 mmol, 85%) was obtained as an orange±
red glass. dH (CDCl3) 2.45 (br s, 6H; SCH3), 3.11 (m; CH2Ar),
3.30 (m; SCH2), 3.11±3.30 (12H), 3.78 (m, 14H; CO2CH3 and
OCH2), 7.42 (m, 4H; Ar 2,6-H), 8.20 (m, 4H; Ar 3,5-H); MS
2256
J. Mater. Chem., 2000, 10, 2249±2258

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(FAB): m/z: 1156 (Mz); Found: C, 41.69; H, 3.67; N, 2.43; S,
38.62; C40H40N2O10S14 requires C, 41.50; H, 3.48; N, 2.42; S,
38.77%.
deactivated with 2% H2O, CH2Cl2±cyclohexane 20 : 3). Collec-
tion of the yellow band gave 24 (0.59 g, 81%) as a pale yellow
solid. Mp 209±210 ³C (from CHCl3±CH3OH); dH (CDCl3) 2.15
(s, 12H; CH3), 3.53±3.65 (m, 12H), 3.75 (t, 4H, J 4.8), 3.88 (t,
4H), 4.06 (t, 4H, J 4.3), 6.80 (d, 4H, J 9.0; OC6H4 2,6-H), 7.14
(d, 4H, J 8.8; OC6H4 3,5-H), 7.16±7.30 (m, 30H; C6H5); MS
(PDMS): m/z: 1238.9 calcd. for C76H74N2O6S4 1239.7; CV
(CH3CN): E11/2~0.33 V, E21/2~0.74 V; Found: C, 68.23; H,
5.65; N, 2.06; C76H74N2O6S4 requires C, 68.05; H, 5.56; N,
2.06%.
Bis(tetrathiafulvalene) dumbbell 22, cis±trans-mixture
To a solution of 21 (0.64 g, 0.55 mmol) in DMF (75 ml) a
solution of CsOH?H2O (0.20 g, 1.19 mmol) in methanol (10 ml)
was added dropwise with stirring over 30 min. The solution was
stirred for 30 min. A solution of 23a1g (0.80 g, 1.38 mmol) in
DMF was added and the mixture was stirred for 5 h. The
reaction mixture was concentrated in vacuo. CH2Cl2 (100 ml)
was added, and the organic phase was washed with water,
saturated aqueous NaCl solution, and dried (MgSO4). The
solvent was then removed and the residue was chromato-
graphed on silica gel using CH2Cl2 as eluent, affording 22
(0.64 g, 0.36 mmol, 65%) as an orange±red oil. dH (CDCl3) 2.43
(br s, 6H; SCH3), 3.00 (m, 4H; SCH2), 3.24 (m, 4H; SCH2), 3.70
(m, 30H; OCH2 and CO2CH3), 4.10 (m, 4H; CH2OAr), 6.79 (d,
4H, J 8.8; Ar 2,6-H), 7.09 (d, 4H, J 8.9; Ar 2,6-H), 7.24 (m,
30H; C6H5); MS (FAB): m/z: 1758 (Mz); CV: E11/2~0.47 V,
E21/2~0.61 V, E31/2~0.83 V, E41/2~0.97 V; Found: C, 57.32;
H, 5.08; S, 24.73; C86H86O12S14?2.5 H2O requires C, 57.21; H,
5.08; S, 24.86%.
[2]Rotaxane 29
A solution of the dumbbell 24 (180 mg, 0.145 mmol), 26
(310 mg, 0.436 mmol) and 25 (127 mg, 0.480 mmol) in dry
degassed DMF (12 ml) was transferred to a high-pressure-
reaction Te¯on tube, which was compressed (10 kbar) at room
temperature for 3 d. The resulting green suspension was
concentrated in vacuo and the residue applied on a column
(SiO2) as a suspension in CH3CN and eluted with a mixture of
CH3OH±NH4Cl (2 M)±CH3NO2±CH3CN (14 : 4 : 2 : 5). The
broad green band was collected and the solvents were removed
in vacuo. The green residue was washed with H2O (50 ml) and
subsequently dissolved in CH3OH (25 ml). A concentrated
solution of NH4PF6 in CH3OH was added until precipitation
was complete. The precipitate was washed with H2O and dried
to give rotaxane 29 (88 mg, 26%). Mp (decomp.) over a wide
range; dH (DMSO-d6) 2.19 (s, 12H; CH3), 3.65±3.75 (m, 16H),
3.98 (t, 4H), 4.06 (t, 4H), 5.76 (s, 8H, ArCH2), 6.76 (d, 4H, J
9.1; OC6H4 2,6-H), 7.02 (d, 4H, J 8.9; OC6H4 3,5-H), 7.12±7.19
(m, 18H; C6H5 3,4,5-H), 7.26±7.31 (m, 12H; C6H5 2,6-H), 7.90
(s, 8H, C6H4), 7.94 (d, 8H, J 6.9; b-H), 9.39 (d, 8H, J 6.6; b-H);
dC (CDCl3) 11.91, 44.14, 63.26, 63.79, 66.87, 69.00, 69.86,
70.02, 70.07, 112.49, 113.49, 115.10, 118.91, 125.65, 126.04,
127.82, 130.51, 130.66, 131.70, 136.73, 138.72, 144.30, 145.39,
146.84, 156.30; CV (CH3CN): E11/2~0.65 V, E21/2~1.04 V;
Found: C, 56.72; H, 4.64; N, 3.78; C112H106F24N6O6P4-
S4?1H2O requires C, 57.04; H, 4.62; N, 3.56%.
Bis(tetrathiafulvalene) rotaxane 28, cis±trans-mixture
A solution of 22 (0.61 g, 0.35 mmol), 26 (0.73 g, 1.04 mmol)
and 25 (0.311 g, 1.14 mmol) in DMF (12 ml) was transferred to
a high-pressure-reaction Te¯on tube, which was then com-
pressed (10 kbar) at room temperature for 4 d. The solvent was
then removed in vacuo to give a residue, which was subjected to
column chromatography on silica gel with ®rst acetone and
later acetone±NH4PF6 as eluents. Collection of the green
fraction afforded a green solid after evaporation of the solvent
in vacuo. CH3NO2 was added and the solution washed with
water and cyclohexane, the solvent was then removed to give 28
(0.35 g, 0.123 mmol, 35%) as a green solid. Mp 162±165 ³C
(decomp.); dH (CD3CN) 2.45±2.31 (m; SCH3), 2.55±2.64 (m;
SCH3), 2.31±2.64 (6H), 2.90±3.22 (m, 8H; SCH2), 3.59±4.11 (m,
34H; OCH2 and CO2CH2), 5.74 (m, 8H; NCH2), 6.57 (m; Ar
2,6-H), 6.80 (d, J 8.8; Ar 2,6-H), 7.13 (m; Ar 3,5-H), 6.57±7.13
(8H), 7.30 (m, 30H; C6H5), 7.69 (m, 8H; C6H4), 7.83 (m; b-H),
8.08 (m; b-H), 7.83±8.08 (8H), 9.07 (m, 8H; b-H); dH
((CD3)2CO) 2.41±2.52 (m; SCH3), 2.65±2.79 (m; SCH3),
2.41±2.79 (6H), 3.23±3.55 (m, 8H; SCH2), 3.64±4.12 (m, 34H;
OCH2 and CO2CH2), 6.00±6.12 (m, 8H; NCH2), 6.45 (m; Ar
2,6-H), 6.84 (d, J 8.8; Ar 2,6-H), 7.13 (m; Ar 3,5-H), 6.45±7.13
(8H), 7.26 (m, 30H; C6H5), 8.05 (m, 8H; C6H4), 8.42 (m; b-H),
8.63 (m; b-H), 8.42±8.63 (8H), 9.61 (m, 8H; b-H); MS (ES): m/z:
References
1
For examples of rotaxanes based on either hydrogen bonding,
transition metal complexation, or donor±acceptor interactions,
see: (a) F. VoÈgtle, M. HaÈndel, S. Meier, S. Ottens-Hildebrandt,
F. Ott and T. Schmidt, Liebigs Ann. Chem., 1995, 739;
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[a]: 570 [M24PF6]4z, 760 [M24PF6]3z, 808 [M2-3PF6]3z
1212 [M23PF6]2z, 1284 [M22PF6]2z; [b]: 570 [M24PF6]4z
,
,
760 [M24PF6]3z, 808 [M23PF6]3z, 1212 [M23PF6]2z, 1284
[M22PF6]2z; MS (MALDI): m/z: 760 [M24PF6]3z, 1139
[M24PF6]2z, 2278 [M24PF6]z, 2423 [M23PF6]z, 2568
[M22PF6]z; MS/MS (ES): parent ion: m/z: 569.5, daughter
ions: m/z: 208, 260, 439.5, 879.2; CV: E11/2~0.53 V,
E21/2~0.63 V,
l
E31/2~0.77 V,
E41/2~0.96 V;
UV-VIS:
max~748 nm and e~1150 M21 cm21
.
2
3
For an account of molecular machines, see: V. Balzani, M.
GoÂmez-LoÂpez and J. F. Stoddart, Acc. Chem. Res. 1998 31, 405.
For a recent application of TTF-rotaxanes in molecular electro-
nics, see: C. P. Collier, E. W. Wong, M. Belohradsky, F. M.
Raymo, J. F. Stoddart, P. J. Kuekes, R. S. Williams and J. R.
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1994, 23, 41; (b) M. Adam and K. MuÈllen, Adv. Mater., 1994, 6,
439; (c) M. B. Nielsen, C. Lomholt and J. Becher, Chem. Soc. Rev.,
2000, 29, 153.
Bis(pyrrolo)-TTF dumbbell 24
Bis(2,5-dimethylpyrrolo)-fused tetrathiafulvalene
1
(0.20 g,
0.59 mmol) in DMF (50 ml) was cooled on an ice bath
before NaH (142 mg, 5.9 mmol) was added in small portions
over a period of 5 min. The orange reaction mixture was stirred
for 20 min before 23b13 (0.83 g, 1.56 mmol) was added in one
portion. The reaction mixture was stirred for 3 h before it was
poured on to brine±ice (400 ml). The yellow±brown precipitate
was ®ltered off and washed with H2O and CH3OH. The crude
product was subjected to column chromatography (basic Al2O3
4
5
J. Lau, M. B. Nielsen, N. Thorup, M. P. Cava and J. Becher, Eur.
J. Org. Chem., 1999, 3335.
(a) W. Devonport, M. A. Blower, M. R. Bryce and
L. M. Goldenberg, J. Org. Chem., 1997, 62, 885; (b) D. Philp,
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J. Chem. Soc., Chem. Commun., 1991, 1584; (c) P. R. Ashton,
V. Balzani, J. Becher, A. Credi, M. C. T. Fyfe, G. Mattersteig,
S. Menzer, M. B. Nielsen, F. M. Raymo, J. F. Stoddart, M. Venturi
and D. J. Williams, J. Am. Chem. Soc., 1999, 121, 3951.
(a) K. B. Simonsen, N. Svenstrup, J. Lau, O. Simonsen, P. Mùrk,
G. J. Kristensen and J. Becher, Synthesis, 1996, 407;
(b) K. B. Simonsen and J. Becher, Synlett, 1997, 11, 1121.
K. B. Simonsen, N. Svenstrup, J. Lau, N. Thorup and J. Becher,
Angew. Chem., Int. Ed., 1999, 38, 1417.
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33, 2685.
(a) M. B. Nielsen, Z.-T. Li and J. Becher, J. Mater. Chem., 1997, 7,
1175; (b) M. B. Nielsen, N. Thorup and J. Becher, J. Chem. Soc.,
Perkin Trans. 1, 1998, 1305.
9
6
10 J. O. Jeppesen, K. Takimiya, F. Jensen and J. Becher, Org. Lett.,
1999, 1, 1291.
11 J. Lau, P. Blanchard, A. Riou, M. Jubault, M. P. Cava and
J. Becher, J. Org. Chem., 1997, 62, 4936.
12 J. Lau, O. Simonsen and J. Becher, Synthesis, 1995, 521.
13 Z.-T. Li, P. C. Stein, N. Svenstrup, K. H. Lund and J. Becher,
Angew. Chem., Int. Ed. Engl., 1995, 34, 2524.
7
8
(a) C. Gemmel, J. D. Kilburn, H. Ueck and A. E. Underhill,
Tetrahedron Lett., 1992, 33, 3923; (b) P. Blanchard, M. SalleÂ,
2258
J. Mater. Chem., 2000, 10, 2249±2258