Synthesis of a new extended π-donor with 1,4-oxathiane annulation

Article abstract of DOI:10.1039/a603837i

4,5;4′,5′-Bis(1,4-oxathiane-2,3-diyldithio)tetrathiafulvalen e 7 (TOET) is synthesised in four steps from 1,4-oxathiane. Compound 7 shows two reversible redox couples at 0.56 and 0.90 V vs. SCE. Analysis of NMR spectra combined with energy minimisation calculations indicated that at least three isomers of TOET should be present in solution. TOET forms a charge-transfer complex with TCNQ which shows a conductivity of 3 x 10^-3 S cm^-1.

Full text of DOI:10.1039/a603837i

Synthesis of a new extended p-donor with 1,4-oxathiane annulation
¨
Jonas Hellberg,*a Karlis Balodis,a Mikael Moge,a Peter Koralla and J-U. von Schutzb
aOrganic Chemistry, Royal Institute of T echnology, S-100 44 Stockholm, Sweden
b3. Physikalisches Institut, Universitat Stuttgart, D-70550 Stuttgart, Germany
¨
4,5;4∞,5∞-Bis(1,4-oxathiane-2,3-diyldithio)tetrathiafulvalene 7 (TOET) is synthesised in four steps from 1,4-oxathiane. Compound 7
shows two reversible redox couples at 0.56 and 0.90 V vs. SCE. Analysis of NMR spectra combined with energy minimisation
calculations indicated that at least three isomers of TOET should be present in solution. TOET forms a charge-transfer complex
with TCNQ which shows a conductivity of 3×10−3 S cm−1.
Tetrathiafulvalene p-donors for molecular metals and super-
conductors have attracted considerable research interest. Most
successful cation radical systems are based on ET 1 as donor,
with various counter ions.1 Taking a simplified view, the p-
core is responsible for hosting the conduction process, whereas
the peripheral substituents act as guides for the docking of the
donor moieties between the anions. Changing these side groups
will therefore lead to new unforeseen packing modes in the
solid state. Although the initial high hopes for oxygen substi-
tution2 has not been fulfilled, a strong tendency to form
metallic compounds has been noted.3
diyldithio)-1,3-dithiole-2-thione 5 in 38% yield. Thione 5 was
converted to the corresponding carbonyl derivative 6 via
treatment with mercuric acetate in acetonitrile. Dimerization
to TOET 7 could be achieved by treating either the thione, or
the carbonyl, or a 151 mixture of the two, with trimethyl
phosphite in benzene at reflux temperatures. Best yields (ca.
40%) were achieved with either the carbonyl or the mixture,
while the thione alone gave slightly less product. The isolated
TOET showed a good elemental analysis, whereas the EI-MS
conditions produced clean retro Diels–Alder reactions and
therefore showed only the mass spectrum of 4. Proton NMR
spectra in CDCl or [2H ]DMSO could only give an indication
3
6
of the purity because of the insolubility of TOET. However,
in [2H ]pyridine at 353 K, a reasonable spectrum could be
5
recorded which showed a heavily coupled system with the
individual protons well separated (Fig. 1). At this temperature
the retro Diels–Alder reaction was too fast to allow a 13C
NMR spectrum to be recorded, i.e. after 6 h ca. 50% had
reverted to starting material. Also apparent from the spectrum
was the presence of at least two stereoisomers, since one of
the bridgehead methine protons was doubled.
Newly aroused interest in the 1,4-dioxane fused-ET (BDDT-
TTF) 2,4 together with the availability of 2,3-dihydro-1,4-
oxathiine 4 in our group, prompted us to synthesise the
analogous 4,5;4∞,5∞-bis(1,4-oxathiane-2,3-diyldithio)tetrathia-
fulvalene (ThiOxane-ET or TOET), which we report here.
The synthesis sequence is depicted in Scheme 1.
Commercially available 1,4-oxathiane 3 was treated with sulfu-
ryl chloride in carbon tetrachloride and refluxed for 3 days to
yield 2,3-dihydro-1,4-oxathiine 4 in reasonable yield after distil-
lation. Compound 4 is less stable than the corresponding dioxa
derivative, but can be stored in a freezer for several months if
pure; impurities seem to initiate polymerisation. Reaction of 4
with 1,3-dithiole-2,4,5-trithione oligomer was achieved in
refluxing dioxane to yield the cis-fused 4,5-(1,4-oxathiane-2,3-
At this point we found it necessary to try to estimate the
isomeric purity of the TOET. If the cycloaddition step is
assumed to give a product with the vicinal hydrogens between
the oxathiane and dithiine ring arranged in a cis-fashion, the
number of theoretically possible stereoisomers is reduced from
sixteen to eight. Among these eight there are two pairs of
identical molecules, which means that the final number of
stereoisomers are six, namely the two non-chiral diastereomers
anti-trans A and syn-cis B, and the two pairs of enantiomers,
syn-trans C/D and anti-cis E/F, respectively. Rotation around
the central double bond has been shown to occur even in
Scheme 1 Reagents and conditions: i, SO Cl , CCl , reflux, 1 week;
2
2
4
ii, dioxane, 1,3-dithiole-2,4,5-trithione oligomer, reflux, 18 h, 38%;
iii, Hg(OAC) , MeCN; iv, (MeO) P, benzene, reflux, 3.5 h
Fig. 1 500 MHz 1H NMR spectrum of TOET 7 in [2H ]pyridine
5
at 353 K
2
3
J. Mater. Chem., 1997, 7(1), 31–34
31

weakly protic solutions in related systems, which should make
A equilibrate with B, as well as C with E and D with F,
respectively. Our nomenclature for the stereoisomers, and
representations of these, are given in Fig. 2 and 3.
A simulation of the coupling pattern present in the 1H NMR
spectrum of the carbonyl compound 6, using coupling con-
stants calculated from the global minimum conformation
derived with the MACROMODEL program, was in good
agreement with that obtained from the experimental spectrum
(Fig. 4). The conformation of the global minimum is shown in
Fig. 5. The second lowest conformation turned out to be
16 kJ mol−1 higher in energy (12 kJ mol−1 using AM1 calcu-
lations), indicating a highly populated global minimum.
Starting with this conformation we energy-minimized the
four different diastereomers of 7 (A, B, C/D, E/F) assumed to
be present. The four conformations so achieved were thus all
in firm energy-minima with no other conformations energeti-
cally available. The energy differences between the four stereo-
isomers were negligible, the largest being 0.4 kJ mol−1. This
energy difference is probably within the calculation error-limit,
and would lead to only a few percent difference in concen-
tration between two isomers in equilibrium. AM1 optimization
of the four isomers gave energy differences very close to zero.
The four energy-minimized diastereomers are shown in Fig. 6.
Noteworthy is the strong tendency for the sulfur atom in the
oxathiane ring to point outwards from the convex part of the
molecule, whereas the oxygen atom in the same ring is on the
concave side towards the planar p-system.
Fig. 4 (a) Experimental and (b) calculated 1H NMR spectra for 6
The experimental 1H NMR spectrum for TOET 7 could be
simulated, anticipating two stereoisomers with identical coup-
ling systems but slightly different shifts. Good agreement was
found for all protons using the coupling constants found for
6, with the two isomers present in approximately 253 ratio
(see Fig. 7 for the proton around d 3.95).
Since there do not seem to be any large differences in energy
between the stereoisomers, the isomeric outcome is probably
governed by solubility factors rather than thermodynamic
Fig. 5 Energy-minimized structure of carbonyl derivative
6
(MACROMODEL, MM3*). Experimental coupling constants:
H11–H12=12.1 Hz, H11–H21=2.3 Hz, H11–H22=11.9 Hz, H12–H21=
1.9 Hz, H12–H22=4.3 Hz, H21–H22=14 Hz, H3–H4=1.5 Hz.
stabilities. Our work-up procedure relies on the crystallisation
of TOET from the cooled reaction mixture, so there is probably
enrichment of the less soluble stereoisomers at this stage. As
it has been shown that proton-assisted rotation–isomerization
around the central double bond is prevented by the addition
of pyridine, we can anticipate that this isomerization reaction
is not present in our spectrum of TOET in pyridine.
Fig. 2 Definitions of different stereoisomers of TOET 7. (a) ‘S–S’ cis-
trans iomers formed through proton-assisted rotation around central
double bond. (b) ‘H–H’ syn-anti isomers formed during phosphite-
assisted dimerization of 5+6. (c) Only cis ‘H–H’ isomers are formed
in the cycloaddition reaction to form 5.
So, without any definitive proof, is it then possible to say
something about the stereoisomeric distribution in this system?
Fig. 3 Different possible stereoisomers of TOET 7
32
J. Mater. Chem., 1997, 7(1), 31–34

Well, if one believes that it is the less soluble isomer that is
crystallising, we get some guidance from the fact that Kini
et al.4b were able to isolate a cation radical salt of an anti
isomer of the analogous dioxaderivative BDDT-TTF. If the
same situation is valid in our case, we can assume that the
recorded spectrum is showing a mixture of isomers A and E/F.
Cyclic voltammetry of TOET in benzonitrile showed two
reversible redox couples at E =0.56 and 0.90 V. These values
1/2
are ca. 50 mV higher than ET 1 measured under the same
conditions. Electrocrystallization of the new donor in benzo-
nitrile in the presence of the anions NO or PF gave microcrys-
3
6
talline red–brown powders of poor quality. We could also
isolate a 151 charge transfer complex with TCNQ as a
microcrystalline powder, with
a
rather low conductivity
(3×10−3 S cm−1 measured on a compressed pellet).
In conclusion, we have synthesised the new extended donor
TOET in a short synthesis, and showed that there are at least
two stereoisomers present in the crude reaction product. We
are currently investigating alternative conditions for the elec-
trocrystallization experiments in order to facilitate monoisom-
eric salts, but the difficulty of isolating pure isomers coupled
with the inherent ease of isomerisation in these systems will
probably prevent any extensive use of TOET as a donor for
organic metals.
Experimental
NMR spectra were recorded on a Bruker AM 400 (400 MHz)
or Bruker DMX 500 (500 MHz) spectrometer; J values are
given in Hz. Mass spectra were recorded on a Finnigan SSQ
7000. THF was distilled from sodium–benzophenone, all other
solvents were either distilled or HPLC grade. Cyclic voltamme-
try was carried out at 25 °C in benzonitrile at 100 mV s−1
with Bu NPF (0.15 ) as electrolyte and measured vs. a SCE
Fig. 6 The four different energy-minimized diastereomers of TOET 7
(AM1 calculated)
4
6
reference electrode. Electrocrystallizations were performed at
ambient temperature, with a platinum wire anode of about
1 cm2. Currents were typically 1.0 mA cm−2 and the experiment
was run for 1–2 weeks.
Molecular mechanics modelling was carried out using the
MACROMODEL program (ver. 4.5) using a Silicon Graphics
Indigo 2 workstation. The following parameters were used in
the conformational search on 7: force field, MM3*; minimiz-
ation algorithm, truncated Newton-Raphson; derivative con-
˚
vergent criteria, 0.05 kJ A−1 mol−1; permissible window above
lowest energy conformation, 50 kJ mol−1; number of Monte
Carlo steps, 1000. Vicinal proton coupling constants (3J) were
calculated within the MACROMODEL program (using the
method of Haasnoot et al.5). No Boltzmann averaging was
performed since the energy difference of 16.2 kJ mol−1 between
the global minimum and the second lowest conformer will
result in an almost exclusive population of the former. Semi-
empirical calculations were performed using the AM1 parame-
trization as implemented in SPARTAN SGI ver. 4.0.3 GL,
using global minima derived from MACROMODEL.
2,3-Dihydro-1,4-oxathiine 4
1,4-Oxathiane 3 (143,9 g, 1.38 mol) dissolved in carbon tetra-
chloride (1 l) was brought to reflux. Sulfuryl chloride (1.05
equiv. 196 g) dissolved in carbon tetrachloride (500 ml) was
added dropwise to the refluxing solution over ca. 90 min. The
solution was then refluxed for 3 days. The carbon tetrachloride
was then evaporated under reduced pressure and the dark
residue distilled under water aspirator vacuum. Pure com-
pound 4 was collected at 70 °C (lit.,6 54 °C, 20 mmHg) in yields
up to 73%. Yields varied considerably since polymerisation
frequently occurred, especially at temperatures above 100°C.
Attempts to inhibit this side reaction previous to distillation
by various drying and/or neutralizing procedures seemed to
have little or no effect on the yield. 1H NMR (400 MHz,
Fig. 7 (a) Experimental and (b) calculated 1H NMR spectra for the
proton at d 3.95 in TOET 7
J. Mater. Chem., 1997, 7(1), 31–34
33

CDCl ) d 6.57 (d, J 6.5, 1H), 5.04 (d, J 6.5, 1H), 4.30 (2H, m),
4,5;4∞,5∞-Bis(1,4-oxathiane-2,3-diyldithio)tetrathiafulvalene 7
3
3.00 (2H, m); 13C NMR (126 MHz, CDCl ) d 140.2, 93.3, 65.3,
3
Freshly distilled trimethyl phosphite (6 ml, 51 mmol) was
added to a stirred suspension of thione 5 (0.62 g, 2.08 mmol)
and carbonyl 6 (0.57 g, 2.02 mmol) in benzene (35 ml), and
the mixture was refluxed under nitrogen during 3.5 h. After
cooling to room temperature, fine crystals were filtered off and
washed with EtOH, acetone and diethyl ether. Attempts to
recrystallize the product led only to reversal of the Diels–Alder
reaction. Comparable yields could be achieved when the same
procedure was used with only one of the reactants 5 or 6; 1H
25.4; m/z 102 (100%, M+), 74, (84, M+−C H ), 45 (67,
2
4
M+−C H O).
3
5
4,5-(1,4-Oxathiane-2,3-diyldithio)-1,3-dithiole-2-thione 5
A suspension of 2,3-dihydro-1,4-oxathiine 4 (4.2 g, 40.7 mmol)
and 1,3-dithiole-2,4,5-trithione oligomer7 in 1,4-dioxane
(75 ml) was refluxed for 18 h. The hot mixture was filtered and
the residue was washed with hot toluene. The solvent was
evaporated under reduced pressure and the resulting dark
brown sticky residue was dissolved in toluene and filtered
through a short column of silica gel to remove unreacted
oligomer. Evaporation of the toluene gave pure 5 (by NMR)
(4.6 g 38%) as yellow–brown crystals. An analytically pure
sample could be obtained by chromatography with a 251
hexane–CH Cl gradient; mp 142–143 °C; 1H NMR (400 MHz,
NMR (500 MHz, [2H ]Pyridine) Isomer 1: d 5.72 (d, J 1.80,
5
1H), 4.86 (d, J 1.8, 1H), 4.32 (m, J 12.10, J 3.20, J 3.20, 1H),
1
2
3
3.96 (m, J 12.10, J 10.50, J 2.30, 1H), 3.20 (m, J 14.00, J
2
1
2
3
1
10.50, J 3.20, 1H), 2.38 (m, J 14.00, J 3.20, J 2.30, 1H).
3
1
2
3
Isomer 2: d 5.71, 4.80, 4.31, 3.97, 3.23, 2.34; m/z 102 (100%),
74, (84) (Calc. for C H O S : C, 31.55; H, 2.27. Found: C,
14 12 2 10
31.67; H, 2.33%).
2
2
CDCl ) d 5.47 (d, J 1.50, 1H), 4.66 (d, J 1.5, 1H), 4.46 (m, J
3
1
12.10, J 3.20, J 3.20, 1H), 4.07 (m, J 12.10, J 10.50, J 2.30,
2
3
1
1
2
3
References
1H), 3.31 (m, J 14.00, J 10.50, J 3.20, 1H), 2.41 (m, J 14.00,
2
3
1
J 3.20, J 2.30, 1H); 13C NMR (126 MHz, CDCl ) d 208.8,
1
2
See for instance, J. M. Williams, J. R. Ferraro, R. J. Thorn,
K. D. Carlson, U. Geiser, H. H. Wang, A. M. Kini and M-
H. Whangbo, Organic Superconductors, Prentice Hall, 1992.
T. Suzuki, H. Yamochi, G. Srdanov, K. Hinkelmann and F. Wudl,
J. Am. Chem. Soc., 1989, 111, 3108; F. Wudl, H. Yamochi, T. Suzuki,
H. Isotalo, C. Fite, H. Kasmai, K. Liou and G. Srdanov, J. Am.
Chem. Soc., 1990, 112, 2461.
2
3
3
124.6, 117.9, 78.0, 71.6, 44.3, 23.8; m/z 298 (17%, M+), 102
(100, M+−C S ) (Calc. for C H OS : C, 28.16; H, 2.03. Found:
3 5
7
6
6
C, 28.28; H, 2.09%).
4,5-(1,4-Oxathiane-2,3-diyldithio)-1,3-dithiol-2-one 6
3
4
H. Yamochi, S. Horiuchi and G. Saito, Synth. Met., 1993, 55–57,
2096.
Thione 5 (1.28 g, 4.3 mmol) was suspended in acetonitrile
(130 ml) and mercuric acetate (1.60 g, 5.0 mmol) was added.
The resulting mixture was refluxed for 3 h and then additional
mercuric acetate (0.8 g, 2.5 mmol) was added and the mixture
was stirred for an additional 10 min. The dark precipitate was
filtered off and washed with CH Cl , and the combined filtrates
(a) A. I. Kotov, C. Faulmann, P. Cassoux and E. Yagubskii, J. Org.
Chem., 1994, 59, 2626; (b) A. M. Kini, U. Geiser, H-H. Wang,
K. R. Lykke, J. M. Williams and C. F. Campana, J. Mater. Chem.,
1995, 5, 1647.
5
6
C. A. G. Haasnoot, F. A. A. M. de Leeuw and C. Altona,
T etrahedron, 1980, 36, 2783.
2
2
were evaporated to give crude 6. Chromatography on silica
gel with a 154 hexane–CH Cl gradient gave pure 6 as yellow
Compound 4 was first synthesised by W. E. Parham, I. Gordon and
J. D. Swalen, J. Am. Chem. Soc., 1952, 74, 1824. Other approaches
are, A. H. Haubein, J. Am. Chem. Soc., 1959, 81, 144; N. de
Wolf, P. W. Henniger and E. Havinga, Recl. Trav. Chim. Pays-Bas,
1967, 86, 1227; C. Berglund and S-O. Lawesson, Ark. Kemi, 1963,
20, 225.
2
2
crystals (0.8 g, 66%). An analytically pure specimen could be
achieved via recrystallization from EtOH; mp 155–157 °C; 1H
NMR (400 MHz, CDCl ) d 5.47 (d, J 1.70, 1H), 4.63 (d, J 1.7,
3
1H), 4.47 (m, J 12.10, J 3.20, J 3.20, 1H), 4.09 (m, J 12.10,
7
O. Ya. Neilands, Ya. Ya. Katsens and Ya. N. Kreitsberga, Zh. Org.
Khim., 1989, 25, 658; J. Becher and N. Svenstrup, Synthesis, 1995,
215.
1
2
3
1
1
J 10.50, J 2.30, 1H), 3.32 (m, J 14.00, J 10.50, J 3.20, 1H),
2
3
2
3
2.41 (m, J 14.00, J 3.20, J , 2.30, 1H); 13C NMR (126 MHz,
1
2
3
CDCl ) d 198.4, 114.6, 108.5, 79.2, 71.3, 45.1, 23.9; m/z 282
3
(10%, M+), 102 (100, M+−C S ).
Paper 6/03837I; Received 3rd June, 1996
3 5
34
J. Mater. Chem., 1997, 7(1), 31–34