A novel tetrathiafulvalene building block

Article abstract of DOI:10.1055/s-1999-3473

Efficient synthesis of a novel tetrathiafulvalene building block, 2,3- bis(2-cyanoethylthio)-6,7-bis(thiocyanatomethyl)tetrathiafulvalene (7) useful for stepwise and asymmetrical bis-functionalization is reported.

Full text of DOI:10.1055/s-1999-3473

PAPER
803
A Novel Tetrathiafulvalene Building Block
Jan Oskar Jeppesen,a Kazuo Takimiya,a Niels Thorup,b Jan Becher*a
aDepartment of Chemistry, University of Odense, Campusvej 55, DK-5230 Odense M, Denmark
Fax + 45(66)158780; E-mail: jbe@chem.ou.dk
bDepartment of Chemistry, Technical University of Denmark, Building 207, DK-2800 Lyngby, Denmark
Received 19 August 1998; revised 30 November 1998
Abstract: Efficient synthesis of a novel tetrathiafulvalene building
block, 2,3-bis(2-cyanoethylthio)-6,7-bis(thiocyanatomethyl)tetra-
thiafulvalene (7) useful for stepwise and asymmetrical bis-function-
alization is reported.
Key words: tetrathiafulvalene building block, orthogonal sets of
protection groups, stepwise and asymmetrical bis-functionalization,
thiocyanatomethyl, bromomethyl
Among sulfur-containing heterocycles the tetrathiaful-
valenes have been intensively studied during the past two
decades.1 The tetrathiafulvalene moiety has been exten-
sively modified in the search of new donor molecules suit-
able for the formation of low dimensional organic metals.2
Figure 1 Symmetry properties of substituted tetrathiafulvalenes
Furthermore, there has been an increasing interest in in-
corporating tetrathiafulvalenes into macrocyclic and su-
pramolecular structures.3,4 From this point of view the
selective functionalization of tetrathiafulvalenes is a prob-
lem, because tetrathiafulvalene has a D2h symmetry with
four identical potential attachment sites (Figure 1, a).
possible to functionalize the tetrathiafulvalene moiety re-
gioselectively in the 6- and 7-position (see Figure 1 d) in
one synthetic step, followed by a functionalization of the
tetrathiafulvalene moiety in the 2- and 3-position in the
next step.
Versatile tetrathiafulvalene building blocks have been de-
veloped in recent years, and many problems concerning
the selective functionalization of tetrathiafulvalene have
been solved.5–8 Tetrakis(2-cyanoethylthio)tetrathiaful-
valene5 is a useful tetrafunctionalized tetrathiafulvalene
building block of type a (Figure 1). The four cyanoethyl
thiolate protecting groups are readily deprotected when
treated with 4.5 equivalents of cesium hydroxide monohy-
drate, and the resulting thiolates can subsequently be re-
alkylated. In addition it has been shown that selective and
stepwise deprotection of the cyanoethyl thiolate protect-
ing groups is possible, by treatment with one equivalent of
cesium hydroxide monohydrate.4,9 When tetrakis(2-cya-
noethylthio)tetrathiafulvalene is treated with 2.0 equiva-
lents of cesium hydroxide monohydrate, symmetrical/
pseudo-symmetrical deprotection takes place exclusively
to produce a cis/trans mixture of type b/c bis-thiolates, i.e.
no asymmetrical bis-deprotection (type d) is formed.4,9
We report here the synthesis of a versatile tetrafunctional-
ized tetrathiafulvalene of type d, having two thiolate
groups in the 2- and 3-positions protected by the conven-
tional cyanoethyl group,5,6,9 whereas the tetrathiaful-
valene is functionalized through two thiocyanato-
methyl groups in the 6- and 7-positions. The two thio-
cyanatomethyl groups can be deprotected with sodium
borohydride in a mixture of THF and ethanol to generate
the reactive 2,3-bis(2-cyanoethylthio)tetrathiafulvalene-
6,7-bis(methylthiolate) (7b) (Figure 2), without inter-
fering with the cyanoethyl protected thiolate groups.
NC
NC
S
S
S
S
S
S
SCN
SCN
NaBH (excess)
4
7a
7b
Although the discovery of the cyanoethyl thiolate protect-
ing group and the methodology for selective deprotection/
realkylation have revolutionized the possibilities for in-
corporation of tetrathiafulvalene into macrocyclic and su-
pramolecular structures, no preformed tetrafunctionalized
tetrathiafulvalene building block of type d has so far been
presented. A perfect type d building block should possess
two orthogonal sets of protection groups.10 Each set
should contain two identical protection groups, hence it is
NC
NC
S
S
-
S
S
S
S
S
-
S
Figure 2 Reductive deprotection of thiocyanate protection groups
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804
J. O. Jeppesen et al.
PAPER
Scheme 1
The key starting material in our synthesis is the 4,5- tetrabromide and triphenyl phosphine yielding 6 as orange
bis(hydroxymethyl)-1,3-dithiole-2-thione 2, which was needles in good yield. It is important that the starting di-
obtained by modification of the procedure reported by alcohol 5 is pure and the solvent is absolutely dry, other-
Fox and coworkers.11 The reported procedure for reduc- wise byproducts are formed. Compound 6 is a reactive
tion of 4,5-bis(methoxycarbonyl)-1,3-dithiole-2-thione12 bis-electrophile, due to the pseudo-benzylic position of
(1) with sodium borohydride in the presence of lithium the two bromine atoms, which makes 6 unstable
chloride was not suitable for scale-up in our hands. How- under ambient conditions. Elemental analyses and
ever, by using sodium borohydride in the presence of lith- spectral data for compound 6 were all consistent
ium bromide in a mixture of THF and ethanol at –10 °C, with the assigned structure. The high reactivity of 6 is
the reduction of 1 proceeded smoothly in 85% yield to seen in the reaction with potassium thiocyanate, which
give the desired dialcohol 2 in multigram quantities.
converts 6 quantitatively to 2,3-bis(2-cyanoethylthio)-
6,7-bis(thio-cyanatomethyl)-tetrathiafulvalene (7) in
< 5 minutes. Although bis(thiocyanatomethyl)aromatic/
heteroaromatic compounds are useful for the preparation
of the corresponding thiolates,15 no analog in the tetrathi-
afulvalene series has, to our knowledge, been described.
Next the two hydroxy groups in 4,5-bis(hydroxymethyl)-
1,3-dithiole-2-thione (2) was protected as the tert-butyl-
diphenylsilyl ethers13 using tert-butyldiphenylchlorosi-
lane in DMF in the presence of imidazole, affording 3 in
almost quantitative yield.
To study the potential of 7 as a type d building block, the
following regioselective deprotection and realkylation re-
actions were carried out. Treatment of 7 with excess sodi-
um borohydride in THF/ethanol afforded the reactive 2,3-
bis(2-cyanoethylthio)tetrathiafulvalene-6,7-bis(methylth-
iolate) (7b) (Figure 2) which was trapped in situ with dif-
ferent electrophiles to give S-alkylated tetrathiafulvalenes
8a–c in 76–80% yields. Subsequently the remaining two
cyanoethyl thiolate protecting groups in 8a–c were depro-
tected using 2.2 equivalents of cesium hydroxide mono-
hydrate followed by addition of iodomethane affording
The bis-protected dialcohol 3 was then cross-coupled with
an excess (2.8 equiv) of 4,5-bis(2-cyanoethylthio)-1,3-
dithiol-2-one5 in neat triethyl phosphite to give the asym-
metric tetrathiafulvalene 4 in 41% yield. Deprotection of
the two tert-butyldiphenylsilyl protecting groups with tet-
rabutylammonium fluoride (TBAF) in THF afforded 2,3-
bis(2-cyanoethylthio)-6,7-bis(hydroxymethyl)tetrathia-
fulvalene (5) in 85% yield. Conversion of dialcohol 5 to
the useful 2,3-bis(2-cyanoethylthio)-6,7-bis(bromometh-
yl)tetrathiafulvalene (6) posed a challenging problem.14
The bromination was carried out in THF using carbon
Synthesis 1999, No. 5, 803–810 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
A Novel Tetrathiafulvalene Building Block
805
the corresponding 2,3-bis(methylthio)tetrathiafulvalenes
9a–c in 77–82% yields after column chromatography.16
Furthermore, treatment of 7 with 2 equivalents of sodium
borohydride and quenching the reaction mixture with sat-
urated ammonium chloride after 15 minutes, followed by
column chromatography gave 2,3-bis(2-cyanoethylthio)-
6,7-bis(2,3-dithiabutane-1,4-diyl)tetrathiafulvalene (8d)
in 61% yield. This type of tetrathiafulvalene derivatives
are of considerable current interrest due to its peripheral
S-position isomerism with ethylenedithiotetrathiaful-
valene.17–19
Figure 4 Stereo drawing of the molecular structure of 9c
NC
NC
S
S
NaBH (2 eq.)
4
THF-EtOH, rt, 15 min
S
S
S
S
S
S
7
61%
8d
In conclusion, we have developed an effective synthesis
of 2,3-bis(2-cyanoethylthio)-6,7-bis(thiocyanatomethyl)-
tetrathiafulvalene (7) possessing two orthogonal sets of
thiolate protecting groups and demonstrated that it fulfills
the requirement for an asymmetrical tetrafunctionalized
tetrathiafulvalene of type d. Thus 7 is the first example of
a type d tetrathiafulvalene building block.
NaBH (excess)
NC
NC
S
S
4
S
S
S
S
SCH
3
THF-EtOH, CH I, rt, 75 min
3
91%
SCH
3
8a
Scheme 2
All reactions involving tetrathiafulvalenes were carried out under
an atmosphere of dry N2. THF was distilled from Na/benzophenone
immediately prior to use, MeOH and EtOH was distilled from Mg.
DMF was allowed to stand over molecular sieves (4 Å) for at least
3 d before use, while acetone was dried over CaSO4 (Drierite). LiBr
was dried overnight at approx. 200 °C in vacuo. All reagents were
standard grade and used as received unless otherwise stated. Ana-
lytical TLC was performed on Merck DC-Alufolien Kieselgel 60
F254 0.2 mm thickness precoated TLC plates, while column chro-
matography was performed using Merck Kieselgel 60 (0.040–
0.063 mm, 230–400 mesh ASTM). Melting points were determined
on a Büchi melting point apparatus and are uncorrected. Microanal-
yses were performed at the Microanalytical Laboratory, University
of Copenhagen, or the Atlantic Microlab, inc., Atlanta, Georgia. 1H
NMR spectra were recorded on a Gemini-300BB instrument at 300
MHz using the deuterated solvent as lock and TMS or the residual
solvent as internal standard. 13C NMR spectra were recorded at
75 MHz using Broad Band Decoupling. Electron Impact Ionisation
Mass Spectrometry (EIMS) was performed on a Varian MAT 311A
instrument and Plasma Desorption Mass Spectrometry (PDMS)
was performed on a Bio-Ion 20K time of flight instrument from Ap-
plied Biosystem on the basis of 5.000.000 fission events. IR spectra
were recorded on a Perkin-Elmer 580 spectrophotometer. Cyclic
Voltammetry (CV) was carried out on an Autolab/PGSTAT 10 in-
strument using CH2Cl2 as the solvent employing Bu4NPF6 (0.10 M)
as supporting electrolyte, with a sweep rate of 100 mV/s. Counter
and working electrodes were made of Pt, and the reference electrode
was Ag/AgCl.
The disufide 8d could be further reduced with excess so-
dium borohydride to give 7b which was trapped in situ
with iodomethane affording 8a in 91% yield. These re-
sults strongly indicate that the disulfide 8d is an interme-
diate in the generation of the bis-thiolate 7b.20
Single crystals of 9c were obtained by careful recrystalli-
zation from chloroform/methanol, and its structure was
determined by X-ray crystallography. As shown in Fig-
ures 3 and 4, the central tetrathiafulvalene framework of
9c is essentially planar. The asymmetric unit is half a mol-
ecule as a two-fold rotation axis passes through the central
C1–C2 bond.
4,5-Bis(hydroxymethyl)-1,3-dithiole-2-thione (2)
To a well stirred solution of 112 (15.0 g, 59.9 mmol) and dry LiBr
(10.4 g, 120 mmol) in anhyd THF (120 mL) and anhyd EtOH
21
(60 mL) at –15 to –10 °C was slowly added powdered NaBH4
(4.80 g, 127 mmol) in portions over 20 min. An exothermic reac-
tion took place and temperature should be kept under –10 °C for
further 30 min. Then H2O (600 mL, 0 °C) was added and then dil
HCl (4 N) was carefully added until the evolution of H2 gas ceased.
The mixture was extracted with EtOAc (8 î 80 mL), and the extract
was dried (MgSO4). Evaporation of the solvent gave an yellow oily
Figure 3 Molecular structure of 9c. Displacement ellipsoids are
drawn at the 50% level
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806
J. O. Jeppesen et al.
PAPER
residue which was redissolved in CH2Cl2/EtOAc (2:1, 50 mL) and
filtered through a short column (6 cm silica gel, 10 cm Ø). The col-
umn was first eluted with CH2Cl2/EtOAc (2:1, approx. 1 L). The
first yellow fraction contained trace amount of the starting material
and the mono alcohol. Next the desired dialcohol 2 (Rf 0.2, CH2Cl2/
EtOAc, 1:1) was eluted with CH2Cl2/EtOAc (1:1) until the solution
was colourless (approx. 2 L). The second fraction was concentrated
in vacuo to give yellow crystals of 2; yield: 9.93 g (85%). The prod-
uct was sufficiently pure for further reactions, but can be recrystal-
lized from EtOAc/petroleum ether (bp 60–80 °C) to give pale
yellow crystals; mp 88–89 °C (Lit11 mp 87–88 °C).
13C NMR (CDCl3/TMS): d = 18.72, 19.04, 26.50, 31.14, 59.44,
99.97, 102.21, 117.53, 127.85, 128.19, 129.99, 130.35, 132.49,
135.54.
MS(EI): m/z (%) = 910 (M+, 2.5), 199 (100).
IR (KBr): n = 3069, 3046, 3014, 2953, 2929, 2250 (CN), 1471,
1427, 1263, 1113 cm–1.
Anal. C46H50N2O2S6Si2 (911.4): calcd C 60.62, H 5.53, N 3.07;
found C 60.45, H 5.51, N 3.01.
CV (CH2Cl2, vs. Ag/AgCl): E = 0.54 V, 0.96 V.
½
2,3-Bis(2-cyanoethylthio)-6,7-bis(hydroxymethyl)tetrathiaful-
valene (5)
4,5-Bis(tert-butyldiphenylsiloxymethyl)-1,3-dithiole-2-thione
(3)
Compound 4 (2.90 g, 3.18 mmol) was dissolved in anhyd THF
(50 mL) under N2 in a 250 mL one-necked round-bottomed flask
equipped with a magnetic stirrer and a septum. 4 N HCl (1.65 mL,
6.60 mmol) was added by means of a syringe to the orange red so-
lution in one portion. TBAF (9.20 mL of a 1.0 M solution in THF,
9.20 mmol) was added dropwise to the mixture using a syringe
(10 min). After stirring for an additional 1.5 h the colour darkened
and the solvent was evaporated in vacuo at 25 °C. The resulting red
oil was dissolved in EtOAc (300 mL), washed with acid free H2O
(4 î 150 mL), and dried (Na2SO4).23 Concentration in vacuo
(T < 30 °C) gave an orange solid, which was recrystallized from
propan-2-ol (85 mL) to give 5 as thin orange needles; yield: 1.18 g
(85%); mp 139.5–140 °C.
1H NMR (DMSO-d6/TMS): d = 2.85 (t, 4 H, J = 6.7 Hz), 3.12 (t, 4
H, J = 6.7 Hz), 4.23 (d, 4 H, J = 5.2 Hz), 5.50 (t, 2 H, J = 5.2 Hz).
13C NMR (DMSO-d6/TMS): d = 18.22, 30.85, 56.62, 102.95,
115.25, 118.99, 127.77, 131.72.
MS(EI): m/z (%) = 434 (M+,1.1), 172 (21), 100 (27), 54 (100), 45
(43).
IR (KBr): n = 3435 (OH), 2923, 2254 cm–1 (CN).
To a solution of 2 (3.20 g, 16.5 mmol) in anhyd DMF (200 mL) was
first added tert-butyldiphenylchlorosilane (13.5 mL, 14.3 g,
52.0 mmol) in one portion followed by imidazole (24.0 g,
353 mmol). The yellow solution was stirred overnight at r.t., where-
upon the yellow orange solution was diluted with CH2Cl2 (400 mL).
The combined organic phase were washed with 4 N HCl
(3 î 300 mL), H2O (2 î 300 mL), dried (MgSO4) and the solvent
was evaporated. The resulting yellow residue was redissolved in
CH2Cl2 (15 mL) and subjected to column chromatography
(600 mL silica gel, 6 cm Ø, eluent: CH2Cl2). The broad yellow band
(Rf 0.8) was collected and evaporation of the solvent in vacuo af-
forded initially 3 as an yellow oil. Leaving the product in vacuum
overnight afforded 3 as an analytically pure yellow solid; yield:
10.7 g (97%); mp 104–105.5 °C.
1H NMR (CDCl3/TMS): d = 1.00 (s, 18 H) , 4.27 (s, 4 H),
7.34 (m, 12 H), 7.55 (m, 8 H).
13C NMR (CDCl3/TMS): d = 18.99, 26.45, 59.36, 128.00, 130.21,
132.06, 135.49, 140.40, 213.18.
MS(EI): m/z (%) = 670 (M+, 9), 237 (75), 199 (45), 197 (55), 161
(85), 135 (100).
Anal. C14H14N2O2S6 (434.6): calcd C 38.69, H 3.25, N 6.45; found
C 38.83, H 3.20, N 6.32.
IR (KBr): n = 3071, 3050, 3015, 2958, 2931, 1727, 1428, 1263,
1114, 1071 cm–1.
CV (CH2Cl2, vs. Ag/AgCl): E = 0.51 V, 0.85 V.
Anal. C37H42O2S3Si2 (671.1): calcd C 66.22, H 6.31; found C 65.85,
H 6.58.
½
2,3-Bis(2-cyanoethylthio)-6,7-bis(bromomethyl)tetrathiaful-
valene (6)
2,3-Bis(2-cyanoethylthio)-6,7-bis(tert-butyldiphenylsiloxymeth-
yl)tetrathiafulvalene (4)
Compound 5 (0.49 g, 1.13 mmol) was dissolved in anhyd THF
(15 mL) in a 100 mL one-necked round-bottomed flask equipped
with a magnetic stirrer and a N2 inlet. The orange solution was de-
gassed (N2, 5 min) and cooled (0 °C) before CBr4
(1.12 g, 3.38 mmol) was added. Then Ph3P (0.89 g, 3.39 mmol)
was added in one portion and the orange solution was stirred for 2 h
at 0 °C, during this time a white precipitate was formed. The sol-
vent was evaporated in vacuo at 25 °C and the orange oily residue
was redissolved in CH2Cl2/(EtOAc/cyclohexane, 1:2) (10 mL, 7:3)
and purified by column chromatography (200 mL silica gel,
4 cm Ø, eluent: EtOAc/cyclohexane, 1:2). The orange band (Rf 0.3)
was collected and concentrated (T < 30 °C) to give 6 as analytical-
ly pure orange needles;24 yield: 0.49 g (77%). Recrystallization
from CH2Cl2/petroleum ether (bp 60–80 °C) gave 6 as orange red
plates; mp 106–106.5 °C (dec).
Compound 3 (6.00 g, 8.94 mmol) and 4,5-bis(2-cyanoethylthio)-
1,3-dithiol-2-one5 (7.20 g, 24.9 mmol) were suspended in distilled
P(OEt)3 (50 mL) under N2 in a 250 mL one-necked round-bottomed
flask equipped with a magnetic stirrer and a reflux condenser, and
the suspension was heated on an oil bath at 135 °C, causing disso-
lution within 5 min, and leaving a red reaction mixture. After
10 min a orange precipitate of tetrakis(2-cyanoethylthio)tetrathi-
afulvalene was formed. The orange red suspension was stirred for 2
h, cooled to r.t. Addition of MeOH (50 mL) yielded an orange sol-
id,22 which was filtered and washed with MeOH (3 î 20 mL). The
combined filtrate was concentrated in vacuo and the resulting red
oil was purified by column chromatography (700 mL silica gel,
6 cm Ø, eluent: CH2Cl2/cyclohexane, 9:1). The broad red band
(Rf 0.4) was collected and concentrated to give crude 4 as a red oil
(3.69 g), which was redissolved in CH2Cl2 (10 mL) and
rechromatographed (500 mL silica gel, 6 cm Ø, eluent: CH2Cl2/cy-
clohexane, 9:1). The broad red band was collected and the solvent
evaporated to give a red semisolid, which was repeatedly dissolved
in CH2Cl2 (3 î 100 mL) and concentrated to give analytically pure
4 as a red semisolid; yield: 3.37 g (41%).
1H NMR (CDCl3/TMS): d = 2.75 (t, 4 H, J = 7.1 Hz), 3.09 (t, 4 H,
J = 7.1 Hz), 4.21 (s, 4 H).
PDMS: m/z (%) =
561.8 (M+ + 4,69); calcd25 for C14H1281Br2N2S6, 561.8.
559.8 (M+ + 2,93); calcd25 for C14H1279Br81BrN2S6, 559.8.
557.8 (M+, 38); calcd25 for C14H1279Br2N2S6, 557.8.
400.0 (M+ – 2 Br, 100); calcd25 for C14H12N2S6, 399.9.
1H NMR (CDCl3/TMS): d = 1.00 (s, 18 H), 2.75 (t, 4 H, J = 7.1 Hz),
3.10 (t, 4 H, J = 7.1 Hz), 4.10 (s, 4 H), 7.35 (m, 12 H),
7.56 (m, 8 H).
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PAPER
A Novel Tetrathiafulvalene Building Block
807
IR (KBr): n = 2954, 2927, 2251 (CN), 1636, 1590, 1417, 1205 cm–1.
tracted with CH2Cl2 (2 î 60 mL). The combined orange red extracts
were washed with H2O (2 î 75 mL) and dried (MgSO4). The sol-
vent was evaporated in vacuo and the resulting orange solid purified
by column chromatography (150 mL silica gel, 3 cm Ø, eluent:
CH2Cl2/cyclohexane, 9:1). The orange band (Rf 0.25) was collected
and concentrated to give 8a as a pure orange powder; yield: 0.097 g
(91%). Recrystallization from CH2Cl2/petroleum ether (bp 60–80
°C) gave 8a as fine orange needles. The analytical data of 8a ob-
tained by both Methods A and B were identical.
1H NMR (CDCl3/TMS): d = 2.15 (s, 6 H), 2.74 (t, 4 H, J = 7.2 Hz),
3.09 (t, 4 H, J = 7.2 Hz), 3.46 (s, 4 H).
13C NMR (CDCl3/TMS): d = 15.32, 18.77, 30.99, 31.16, 104.18,
115.62, 117.51, 128.15, 129.77.
Anal. C14H12Br2N2S6 (560.4): calcd C 30.00, H 2.16, Br 28.52, N
5.00, S 34.32; found C 30.16, H 2.15, Br 28.57, N 4.92, S 34.26.
CV (CH2Cl2, vs. Ag/AgCl): E = 0.70 V, 1.09 V.
½
2,3-Bis(2-cyanoethylthio)-6,7-bis(thiocyanatomethyl)tetrathia-
fulvalene (7)
Compound 6 (0.49 g, 0.87 mmol) was dissolved in acetone (25 mL)
in a 250 mL one-necked round-bottomed flask equipped with a
magnetic stirrer and a N2 inlet. KSCN (1.76 g, 18.1 mmol) was add-
ed in one portion to the orange solution. After stirring for 30 min at
r.t., the solvent was removed in vacuo. The orange solid residue was
dissolved in CH2Cl2 (250 mL), washed with H2O (4 î 150 mL) and
dried (MgSO4). Evaporation of the solvent gave 7 as an orange sol-
id; yield: 0.44 g (quant); mp 120–122 °C.
1H NMR (DMSO-d6/TMS): d = 2.89 (t, 4 H, J = 6.8 Hz), 3.16 (t,
4 H, J = 6.8 Hz), 4.46 (s, 4 H).
13C NMR (DMSO-d6/TMS): d = 18.18, 30.47, 30.95, 107.45,
110.26, 112,54, 119.01, 128.02, 130.57.
MS(EI): m/z (%) = 494 (M+, 48), 400 (45), 54 (83), 47 (58), 45
(100) .
IR (KBr): n = 2952, 2918, 2249 (CN), 1631, 1598, 1412 cm–1.
Anal. C16H18N2S8 (494.8): calcd C 38.84, H 3.67, N 5.66, S 51.83;
found C 38.78, H 3.69, N 5.56, S 51.77.
CV (CH2Cl2, vs. Ag/AgCl): E = 0.58 V, 0.99 V.
½
PDMS: m/z (%) =
516.6 (M+, 71); calcd25 for C16H12N4S8, 516.8.
400.5 (M+ – 2 SCN, 100); calcd25 for C14H12N2S6, 400.6.
IR (KBr): n = 2928, 2250 (CN), 2153 (SCN), 1417 cm–1.
2,3-Bis(2-cyanoethylthio)-6,7-bis(benzylthiomethyl)tetrathia-
fulvalene (8b)
Compound 7 (0.19 g, 0.37 mmol) was dissolved in a mixture of an-
hyd THF (10 mL) and anhyd EtOH (5 mL) in a 100 mL one-necked
round-bottomed flask equipped with a magnetic stirrer and a N2 in-
let and cooled (0 °C) before benzyl bromide (0.10 mL, 0.14 g,
0.84 mmol) was added. Then powdered NaBH4 (0.056 g,
1.48 mmol) was added in one portion at 0 °C. The solution was al-
lowed to come to r.t. and stirred for 60 min. The mixture was then
poured into ice containing sat. aq NH4Cl solution (50 mL), and ex-
tracted with CH2Cl2 (150 mL). The orange red extract was washed
with H2O (2 î 100 mL) and dried (MgSO4). The solvent was evap-
orated in vacuo and the resulting orange solid purified by column
chromatography (150 mL silica gel, 3 cm Ø, eluent: CH2Cl2). The
orange band (Rf 0.25) was collected and concentrated to give 8b as
an analytically pure orange powder; yield: 0.19 g (80%). Recrystal-
lization from CH2Cl2/petroleum ether (bp 60–80 °C) gave 8b as fine
orange needles; mp 112.5–113 °C.
Anal. C16H12N4S8 (516.8): calcd C 37.19, H 2.34, N 10.84; found C
37.56 H 2.38, N 10.45.
CV (CH2Cl2, vs. Ag/AgCl): E = 0.74 V, 1.04 V.
½
2,3-Bis(2-cyanoethylthio)-6,7-bis(methylthiomethyl)tetrathia-
fulvalene (8a)
Method A: Compound 7 (0.29 g, 0.56 mmol) was dissolved in a
mixture of anhyd THF (20 mL) and anhyd EtOH (10 mL) in a
100 mL one-necked round-bottomed flask equipped with a magnet-
ic stirrer and a N2 inlet and cooled (0 °C) before MeI (1.0 mL,
2.28 g, 16 mmol) was added. Then powdered NaBH4 (0.10 g,
2.64 mmol) was added in one portion at 0 °C, causing gas evolution.
The mixture was stirred at 0 °C for further 10 min, causing the ini-
tially orange red reaction mixture to become orange. The solution
was allowed to come to r.t. and stirred for 20 min followed by addi-
tion of powdered NaBH4 (0.10 g, 2.64 mmol) (gas evolution),
whereupon the orange reaction mixture was stirred for 2 h at r.t. and
then purged with N2 for 20 min (evaporating the excess of MeI).
The mixture was then poured into ice containing aq sat. NH4Cl so-
lution (70 mL), and extracted with CH2Cl2 (2 î 125 mL). The com-
bined orange red extracts were washed with H2O (2 î 125 mL) and
dried (MgSO4). The solvent was evaporated in vacuo and the result-
ing orange solid purified by column chromatography (200 mL silica
gel, 4 cm Ø, eluent: CH2Cl2/cyclohexane, 9:1). The orange band
(Rf 0.25) was collected and concentrated to give 8a as an analytical-
ly pure orange powder; yield: 0.21 g (76%). Recrystallization from
CH2Cl2/petroleum ether (bp 60–80 °C) gave 8a as fine orange nee-
dles; mp 118–119 °C.
1H NMR (CDCl3/TMS): d = 2.75 (t, 4 H, J = 7.1 Hz), 3.09 (t, 4 H,
J = 7.1 Hz), 3.09 (s, 4 H), 3.70 (s, 4 H), 7.28 (m, 10 H).
13C NMR (CDCl3/TMS): d = 18.78, 28.18, 31.16, 36.13, 104.08,
115.62, 117.50, 127.44, 128.15, 128.71, 128.95, 129.56, 137.02.
MS(EI): m/z (%) = 646 (M+, 0.11), 91 (100).
IR (KBr): n = 3059, 3026, 2917, 2250 (CN), 1631, 1601, 1493,
1452, 1413 cm–1.
Anal. C28H26N2S8 (647.0): calcd C 51.98, H 4.05, N 4.33, S 39.64;
found C 52.02, H 4.06, N 4.38, S 39.53.
CV (CH2Cl2, vs. Ag/AgCl): E = 0.55 V, 0.94 V.
½
2,3-Bis(2-cyanoethylthio)-6,7-bis(ethylthiomethyl)tetrathiaful-
valene (8c)
Method B: Compound 8d (0.10 g, 0.22 mmol) was dissolved in a
mixture of anhyd THF (10 mL) and anhyd EtOH (5 mL) in a
100 mL one-necked round-bottomed flask equipped with a magnet-
ic stirrer and a N2 inlet and cooled (0 °C) before MeI (0.4 mL,
0.91 g, 6.4 mmol) was added. Then powdered NaBH4 (0.033 g,
0.87 mmol) was added in one portion at 0 °C, causing gas evolution,
whereupon the reaction mixture was allowed to come to r.t. and
stirred for 30 min followed by addition of powdered NaBH4
(0.033 g, 0.87 mmol) (gas evolution). The orange reaction mixture
was stirred for further 45 min at r.t. and then purged with N2 for
20 min (evaporating the excess of MeI). The mixture was then
poured into ice containing sat. aq NH4Cl solution (50 mL), and ex-
Compound 7 (0.20 g, 0.39 mmol) was dissolved in a mixture of an-
hyd THF (10 mL) and anhyd EtOH (5 mL) in a 100 mL one-necked
round-bottomed flask equipped with a magnetic stirrer and a N2 in-
let and cooled (0 °C) before EtI (0.07 mL, 0.14 g, 0.88 mmol) was
added. Then powdered NaBH4 (0.059 g, 1.56 mmol) was added in
one portion at 0 °C. The solution was allowed to come to r.t. and
stirred for 45 min. The mixture was then poured into ice containing
sat. aq NH4Cl solution (50 mL), and extracted with CH2Cl2
(150 mL). The orange red extract was washed with H2O
(3 î 100 mL) and dried (MgSO4). The solvent was evaporated in
vacuo and the resulting orange solid purified by column chromatog-
Synthesis 1999, No. 5, 803–810 ISSN 0039-7881 © Thieme Stuttgart · New York

808
J. O. Jeppesen et al.
PAPER
raphy (100 mL silica gel, 3 cm Ø, eluent: CH2Cl2). The orange band
(Rf 0.2) was collected and concentrated to give 8c as an analytically
pure orange powder; yield: 0.16 g (79%). Recrystallization from
CH2Cl2/petroleum ether (bp 60–80 °C) gave 8c as fine orange
needles; mp 103.5–104 °C.
1H NMR (CDCl3/TMS): d = 1.29 (t, 6 H, J = 7.4 Hz), 2.59 (q, 4 H,
J = 7.4 Hz), 2.74 (t, 4 H, J = 7.0 Hz), 3.08 (t, 4 H, J = 7.0 Hz), 3.49
(s, 4 H).
1H NMR (CDCl3/TMS): d = 2.14 (s, 6 H), 2.42 (s, 6 H), 3.45 (s, 4
H).
13C NMR (CDCl3/TMS): d = 15.22, 19.03, 30.94, 107.17, 111.98,
127.51, 129.57.
MS(EI): m/z (%) = 416 (100, M+), 322 (92), 172 (29).
IR (KBr): n = 2952, 2918, 1630, 1599, 1421 cm–1.
Anal. C12H16S8 (416.7): calcd C 34.59, H 3.87, S 61.54; found C
34.49, H 3.87, S 61.58.
13C NMR (CDCl3/TMS): d = 14.28, 18.76, 26.00, 28.87, 31.15,
103.93, 115.80, 117.49, 128.12, 129.72.
CV (CH2Cl2, vs. Ag/AgCl): E = 0.47 V, 0.88 V.
½
MS(EI): m/z (%) = 522 (M+, 31), 400 (29), 172 (30), 87 (47), 76
(62), 54 (77), 45 (97), 32 (100).
2,3-Bis(methylthio)-6,7-bis(benzylthiomethyl)tetrathiaful-
valene (9b)
IR (KBr): n = 2965, 2924, 2868, 2251 (CN), 1631, 1598, 1422, 1376
cm–1.
Compound 8b (0.15 g, 0.23 mmol) was dissolved in anhyd DMF
(20 mL) in a 100 mL one-necked round-bottomed flask equipped
with a magnetic stirrer and purged with N2 (20 min). A solution of
CsOH•H2O (0.086 g, 0.51 mmol) in anhyd MeOH (5 mL) was add-
ed over a period of 5 min. The mixture was stirred for 20 min., caus-
ing the initially orange solution to become redish brown. MeI
(0.40 mL, 0.91 g, 6.4 mmol) was added in one portion, causing a
momentary colour change to orange. The mixture was stirred for
20 min and then purged with N2 (20 min). The solvent was removed
in vacuo and the resulting orange residue dissolved in CH2Cl2
(150 mL) and washed with H2O (4 î 100 mL). After drying
(MgSO4) and evaporation of the solvent, the resulting orange prod-
uct was purified by column chromatography (150 mL silica gel,
3 cm Ø, eluent: CH2Cl2/cyclohexane, 1:1). The orange band
(Rf 0.3) was collected and concentrated to give 9b as an orange oil;
yield: 0.10 g (78%).
1H NMR (CDCl3/TMS): d = 2.43 (s, 6 H), 3.08 (s, 4 H), 3.69 (s, 4
H), 7.28 (m, 10 H).
13C NMR (CDCl3/TMS): d = 19.08, 28.22, 36.07, 107.20, 112.16,
127.37, 127.60, 128.67, 128.96, 129.44, 137.14.
MS(EI): m/z (%) = 568 (M+, 14), 354 (18), 322 (17), 91 (100).
IR (KBr): n = 3059, 3026, 2917, 1600, 1493, 1453, 1420 cm–1.
Anal. C18H22N2S8 (522.9): calcd C 41.35, H 4.24, N 5.36, S 49.05;
found C 41.34, H 4.13, N 5.27, S 49.09.
CV (CH2Cl2, vs. Ag/AgCl): E = 0.54 V, 0.95 V.
½
2,3-Bis(2-cyanoethylthio)-6,7-bis(2,3-dithiabutane-1,4-diyl)-
tetrathiafulvalene (8d)
Compound 7 (0.24 g, 0.46 mmol) was dissolved in a mixture of an-
hyd THF (10 mL) and anhyd EtOH (5 mL) in a 100 mL one-necked
round-bottomed flask equipped with a magnetic stirrer and a N2 in-
let. Powdered NaBH4 (0.035 g, 0.092 mmol) was added in one por-
tion. The solution was stirred for 15 min whereupon addition of sat.
aq NH4Cl solution (50 mL) yielded an orange solid, which was fil-
tered and washed with H2O (3 î 10 mL). The orange solid was
dried in vacuo and purified by column chromatography (100 mL
silica gel, 3 cm Ø, eluent: CH2Cl2). The orange band (Rf 0.3) was
collected and concentrated to give 8d as analytically pure red or-
ange needles; yield: 0.13 g (61%). Recrystallization from CH2Cl2/
petroleum ether (bp 60–80 °C) gave 8d as fine orange needles;
mp 177–178 °C.
1H NMR (DMSO-d6/TMS): d = 2.87 (t, 4 H, J = 6.8 Hz), 3.15 (t,
4 H, J = 6.8 Hz), 3.72 (s, 4 H).
Anal. C24H24S8 (568.9): calcd C 50.67, H 4.25, S 45.08; found C
51.07, H 4.39, S 44.59.
13C NMR (DMSO-d6/TMS): d = 18.16, 29.75, 30.87, 105.92,
109.60, 118.97, 123.24, 127.89.
MS(EI): m/z (%) = 464 (5, M+), 430 (9), 400 (13), 54 (100).
CV (CH2Cl2, vs. Ag/AgCl): E = 0.44 V, 0.84 V.
½
IR (KBr): n = 2927, 2252 (CN), 1635, 1416, 1396 cm–1.
2,3-Bis(methylthio)-6,7-bis(ethylthiomethyl)tetrathiafulvalene
(9c)
Anal. C14H12N2S8 (464.7): calcd C 36.18, H 2.60, N 6.03, S 55.19;
found C 36.23, H 2.48, N 5.99, S 55.08.
Compound 9b (0.11 g, 0.21 mmol) was dissolved in anhyd DMF
(20 mL) in a 100 mL one-necked round-bottomed flask equipped
with a magnetic stirrer and purged with N2 (20 min). A solution of
CsOH•H2O (0.082 g, 0.49 mmol) in anhyd MeOH (5 mL) was add-
ed over a period of 5 min. The mixture was stirred for 20 min, caus-
ing the initially orange solution to become redish brown. MeI
(0.40 mL, 0.91 g, 6.4 mmol) was added in one portion, causing a
momentary colour change to orange. The mixture was stirred for
20 min and then purged with N2 (20 min). The solvent was removed
in vacuo and the resulting orange residue dissolved in CH2Cl2
(150 mL) and washed with H2O (4 î 100 mL). After drying
(MgSO4) and evaporation of the solvent, the resulting orange prod-
uct was purified by column chromatography (100 mL silica gel,
3 cm Ø, eluent: CH2Cl2/cyclohexane, 1:1). The orange band
(Rf 0.5) was collected and concentrated to give 9c as an orange
powder; yield: 0.072 g (77%). Recrystallization from CHCl3/
MeOH gave 9c as red needles; mp 103.5–104.5 °C.
CV (CH2Cl2, vs. Ag/AgCl): E = 0.62 V, 1.02 V.
½
2,3-Bis(methylthio)-6,7-bis(methylthiomethyl)tetrathiaful-
valene (9a)
Compound 8a (0.16 g, 0.32 mmol) was dissolved in anhyd DMF
(25 mL) in a 100 mL one-necked round-bottomed flask equipped
with a magnetic stirrer and purged with N2 (20 min.). A solution of
CsOH•H2O (0.12 g, 0.71 mmol) in anhyd MeOH (5 mL) was added
over a period of 5 min. The mixture was stirred for 20 min, causing
the initially orange solution to become redish brown. MeI (0.60 mL,
1.37 g, 9.6 mmol) was added in one portion, causing a momentary
colour change to orange. The mixture was stirred for 20 min and
then purged with N2 (20 min.). The solvent was removed in vacuo
and the resulting orange residue dissolved in CH2Cl2 (150 mL) and
washed with H2O (4î100 mL). After drying (MgSO4) and evapora-
tion of the solvent, the resulting orange product was purified by col-
umn chromatography (150 mL silica gel, 3 cm Ø, eluent: CH2Cl2/
cyclohexane, 1:1). The orange band (Rf 0.4) was collected and con-
centrated to give 9a as an analytically pure yellow orange powder;
yield: 0.11 g (82%). Recrystallization from CH2Cl2/petroleum ether
(bp 60–80 °C) gave 9a as yellow needles; mp 112–112.5 °C
1H NMR (CDCl3/TMS): d = 1.28 (t, 6 H, J = 7.4 Hz), 2.42 (s, 6 H),
2.58 (q, 4 H, J = 7.4 Hz), 3.48 (s, 4 H).
13C NMR (CDCl3/TMS): d = 14.28, 19.05, 25.92, 28.88, 106.99,
112.23, 127.56, 129.57.
MS(EI): m/z (%) = 444 (M+, 100), 398 (37), 322 (95), 276 (39).
Synthesis 1999, No. 5, 803–810 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
A Novel Tetrathiafulvalene Building Block
809
IR (KBr): n = 2959, 2920, 2867, 1630, 1599, 1452, 1423, 1375
coupling conditions, see:
cm–1.
Marshallsay, G. J; Bryce, M. R.; Cooke, G.; Jørgensen, T.;
Becher, J.; Reynolds, C. D.; Wood, S. Tetrahedron 1993, 49,
6849.
Marshallsay, G. J, Hansen, T. K.; Moore, A. J.; Bryce, M. R.;
Becher, J. Synthesis 1995, 926.
Anal. C14H20S8 (444.8): calcd C 37.81, H 4.53, S 57.66; found C
37.60, H 4.42, S 57.41.
CV (CH2Cl2, vs. Ag/AgCl): E = 0.44 V, 0.85 V.
½
A tetrathiafulvalene bearing a secondary alcohol functionality
has been produced by a trialkyl phosphite coupling in
reasonable yield, see:
Ozturk, T.; Rice, C.R.; Wallis, J.D. J. Mater Chem 1995, 5,
1553.
Crystallographic Data for Compound 9c
C14H20S8, M = 444.8, Crystal size 0.30 î 0.16 î 0.10 mm, hexago-
nal, a = 11.7402(2) Å, b = 11.7402(2) Å, c = 12.661(3) Å, a = 90°,
b = 90°, g = 120°, V = 1511.3(5) Å3, space group P62, Z = 3,
Dc = 1.466 g cm–3, F(000) = 696, Graphite monochromated MoKa
radiation, l = 0.71073 Å, m = 0.879 mm–1, T = 120 K. The intensi-
ties of 16224 reflections were measured on a Siemens SMART
CCD diffractometer to qmax = 26.35°, and were merged to 2065
unique reflections (Rint = 0.026). Structure solution, refinement and
analysis of the structure, and production of crystallographic illustra-
tions were carried out using the programs SHELXS97,26
SHELXL97,27 PLATON28 and SHELXTL.29 The refinement us-
ing 103 parameters converged at R = 0.0288 (for Fo > 4s(Fo)).
Atomic coordinates and further crystallographic details have been
deposited with the Cambridge Crystallographic Data Centre, Uni-
versity Chemical Laboratory, Lensfield Road, Cambridge CB2
1EW, England.
(14) 6,7-Bis(bromomethyl)-2,3-bis(methylthio)tetrathiafulvalene
is to our knowledge the only example of a tetrathiafulvalene
bearing two vicinal bromomethyl groups, so far described in
the literature. 6,7-Bis(bromomethyl)-2,3-bis(methylthio)-
tetrathiafulvalene was prepared by treatment of the correspon-
ding 6,7-bis(hydroxymethyl)-2,3-bis(methylthio)tetrathia-
fulvalene with phosphorus tribromide and have been used as a
diene precursor in a [4+2] Diels–Alder cycloaddition with C60,
see:
Boulle, C.; Rabreau, J. M.; Hudhomme, P.; Cariou, M.;
Jubault, M.; Gorgues, A.; Orduna, J.; Garin, J. Tetrahedron
Letters 1997, 38, 3909.
(15) Richter, R.; Ulrich, H. The Chemistry of Cyanates and Their
Thio Derivatives, Patai, S., Ed.; John Wiley & Sons: New
York, 1977; Vol. 2, Chapter 17.
Acknowledgement
Breitenbach, J.; Boosfeld, J.; Vögtle, F. Comprehensive
Supramolecular Chemistry, Atwood, J. L.; Davies, J. E. D.;
MacNicol, D. D., Vögtle, F.: Lehn, J.-L., Eds.; Pergamon:
1996, Vol. 2, Chapter 2.
We thank University of Odense for a Ph.D. scholarship to J.O.J. and
the Danish Research Academy for a post doctoral fellowship to
K.T., as well as helpful discussions with Prof. P. Hudhomme, Uni-
versité d'Angers.
(16) It was not possible to deprotect the cyanoethyl thiolate
protecting groups in presence of the thiocyanatomethyl
groups, using the standard deprotection technique (i.e. first
treatment of 7 with 2.2 equivalents of CsOH•H2O followed by
addition of MeI) nor by addition of 2.2 equivalents of
CsOH•H2O to a mixture of 7 and MeI.
(17) Hudhomme, P.; Blanchard, P.; Sallé, M.; Moustarder, S. L.;
Riou, A.; Jubault, M.; Gorgues, A.; Duguay, G. Angew. Chem.
Int. Ed. Engl. 1997, 36, 878.
(18) Durand, C.; Hudhomme, P.; Duguay, G.; Jubault, M.;
Gorgues, A. J. Chem. Soc., Chem. Commun. 1998, 361.
(19) Attempt to deprotect the remaining two cyanoethyl thiolate
protecting groups in 8d followed by addition of iodomethane
failed, probably due to the unstability of the disulfide moiety.
(20) It was not possible to isolate the bis-thiol
References
(1) For reviews, see:
Schukat, G.; Richter, A. M.; Fanghänel, E. Sulfur Reports
1987, 7, 155.
Schukat, G.; Fanghänel, E. ibid 1993, 14, 245.
(2) Bryce, M. R. Chem. Soc. Rev. 1991, 20, 355.
(3) Jørgensen, T.; Hansen, T. K.; Becher, J. Chem. Soc. Rev.
1994, 23, 41.
(4) Becher, J.; Li, Z.-T.; Blanchard, P.; Svenstrup, N.; Lau, J.;
Nielsen, M. B.; Leriche, P. Pure & Appl. Chem. 1997, 69, 465.
(5) Svenstrup; N.; Rasmussen, K. M.; Hansen, T. K.; Becher, J.
Synthesis 1994, 809.
(6) Simonsen, K. B.; Svenstrup, N.; Lau, J.; Simonsen, O.; Mørk,
P.; Kristensen, G. J.; Becher, J. Synthesis 1995, 407.
(7) Garin, J.; Orduna, J.; Uriel, J.; Moore, A. J.; Bryce, M. R;
Wegener, S.; Yufit, D. S.; Howard, J. A. K. Synthesis 1994,
489.
[i.e (RS)2TTF(CH2SH)2] by addition of excess NaBH4 to 7 in
THF/EtOH (2:1) followed by addition of deoxygenated water
and pH adjustment (pH = 7).
(21) Fresh NaBH4 should be used.
(22) The orange solid exclusively contains tetrakis(2-cyanoeth-
ylthio)tetrathiafulvalene (TLC and PDMS).
(8) Garin, J. Adv. Heterocycl. Chem. 1995, 62, 249.
(9) Simonsen, K. B.; Becher, J. Synlett 1997, 1211.
(10) For definition of orthogonal sets, see:
Barany, G.; Merrifield, R. B. J. Am Chem. Soc. 1977, 99,
7363.
Barany, G.; Albericio, F. J. ibid 1985, 107, 4936.
(11) Fox, A. F.; Pan, H. J. Org. Chem. 1994, 59, 6519.
(12) 4,5-Bis(methoxycarbonyl)-1,3-dithiole-2-thione (1) is
available in a one-step reaction between ethylene
trithiocarbonate and dimethyl acetylendicarboxylate, see:
Easton, D. B. J.; Leaver, D. J. Chem. Soc., Chem. Commun.
1965, 585.
O’Connor, B. R.; Jones, F. N. J. Org. Chem. 1970, 35, 219.
(13) Previous results have shown that an primary alcohol
functionality is unable to survive the standard trialkyl
phosphite coupling, whereas the tert-butyldiphenylsilyl
alcohol protecting group is able to withstand the standard
(23) If the organic phase darkens under aquous work up, the
following procedure was applied. The organic phase was
concentrated to approximately 50 mL and filtered through a
short column (4 cm silica gel, 10 cm Ø). The column was first
eluted with CH2Cl2 (300 mL). The first fraction contained a
colourless impurity (TLC, Rf 0.8, eluent EtOAc). Next the
desired dialcohol 5 (Rf 0.4, eluent EtOAc) was eluted with
EtOAc until the solution was colourless (ca 2 L). This fraction
contains trace of the above colourless impurity. Evaporation
of the solvent in vacuo (T < 30 °C) gave a orange solid.
Recrystallization from propan-2-ol (85 mL) gave the product
5 as thin orange needles with a slight decrease in the yield
(81%).
(24) Compound 6 is unstable and darkens after a few days when
exposed to air, and should be converted to 7 as rapidly as
possible. 6 reacted fast with H2O. 1H NMR spectroscopy of 6
Synthesis 1999, No. 5, 803–810 ISSN 0039-7881 © Thieme Stuttgart · New York

810
J. O. Jeppesen et al.
PAPER
(29) Sheldrick, G. M. SHELXTL. Structure Determination
in DMSO-d6 showed that the H2O signal (which normally
appears in spectra recorded in commercial DMSO-d6)
disappeared very fast.
Programs. Version 5.10., 1997, Bruker (formerly Siemens)
Analytical X-Ray Instruments Inc., Madison, Wisconsin,
USA.
(25) The calculated values are based on the exact mass for
compound 6 and for the molecular mass for compound 7.
(26) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(27) Sheldrick, G. M. SHELXL97. Program for the Refinement of
Crystal Structures, 1997, University of Göttingen, Germany.
(28) Spek, A .L. Acta Crystallogr. 1990, A46, C-34.
Article Identifier:
1437-210X,E;1999,0,05,0803,0810,ftx,en;P04698SS.pdf
Synthesis 1999, No. 5, 803–810 ISSN 0039-7881 © Thieme Stuttgart · New York