Conformational isomers of 1,2,5,6-tetrathiocins and the photoisomerization of a 1,2,5,6-tetrathiocin into a 1,2,3,6-tetrathiocin: X- ray structures of (C6X4S2)2 (X = F, Cl) and C6f4SSSC<sup

Article abstract of DOI:10.1139/v98-106

The 1,2,5,6-tetrathiocins (C₆X₄S₂)₂ (3a, X = F; 3b, X =Cl) are obtained in high yields by the oxidation of the dithiols 1,2-C₆X₄(SH)₂ (X = F, Cl) with I₂ or SO₂

Full text of DOI:10.1139/v98-106

1093
Conformational isomers of 1,2,5,6-tetrathiocins
and the photoisomerization of a
1,2,5,6-tetrathiocin into a 1,2,3,6-tetrathiocin:
X-ray structures of (C6X4S2)2 (X = F, Cl)
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and
C6F4 SSSC6F4 S
Tristram Chivers, Masood Parvez, Ignacio Vargas-Baca, and Gabriele Schatte
Abstract: The 1,2,5,6-tetrathiocins (C6X4S2)2 (3a, X = F; 3b, X = Cl) are obtained in high yields by the oxidation of
the dithiols 1,2-C6X4(SH)2 (X = F, Cl) with I2 or SO2Cl2, respectively. In the solid state 3a has the C2h (chair)
conformation and crystallizes in two different phases: α-(C6F4S2)2, monoclinic, P21/a, a = 9.351(2), b = 6.465(2), and
c = 11.546(2) Å, β = 95.60(1)°, V = 694.6(2) Å3, Z = 2; and β-(C6F4S2)2, monoclinic, P21/c, a = 4.825(2), b =
11.302(2), and c = 12.453(2) Å, β = 91.45(3)°, V = 678.8(3) Å3, Z = 2. By contrast, 3b displays a D2 (twist-boat)
structure and crystallizes in the C2/c space group with a = 15.243(3), b = 8.703(2), and c = 27.010(14) Å, β =
92.81(4)°, V = 3578(1) Å3, and Z = 8. The derivative 3a exists as an equilibrium mixture of two conformational
isomers in toluene solution. The VT 19F NMR data afford the thermodynamic parameters ∆H° = 9.9 ± 0.4 kJ mol–1
and ∆S° = 14 ± 1 J K–1 mol–1. Density functional theory calculations for 3a indicate that the D2 conformation is lower
in energy than the C2h isomer by 4.6 kJ mol–1. The photolysis of 3a in benzene promotes a transannular sulfur
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migration to give the 1,2,3,6-tetrathiocin, C6F4SSSC6F4S (6), which exists in a chair conformation with respect to
antipodal sulfur atoms. Crystal structure of 6: orthorhombic, space group Pnma, a = 8.652(6), b = 19.084(4), and c =
8.301(6) Å, V = 1370.6(14) Å3, and Z = 4. The isomers 3a and 6 were also characterized by EI mass spectrometry, 19F
NMR, FTIR, and Raman spectroscopies.
Key words: tetrathiocins, conformational isomers, photoisomerization.
Résumé : Les oxydations des dithiols 1,2-C6X4(SH)2 (X = F, Cl) par I2 ou SO2Cl2 conduisent aux 1,2,5,6-tétrathiocines
(C6X4S2)2 (3a, X = F; 3b, X = Cl) avec des rendements élevés. À l’état solide, le composé 3a adopte la conformation
C2h (chaise) et cristallise dans deux phases différentes: α-(C6F4S2)2, monoclinique, P21/a, a = 9,351(2), b = 6,465(2) et
c = 11,546(2) Å, β = 95,60(1)°, V = 694,6(2) Å3, Z = 2 et β-(C6F4S2)2, monoclinique, P21/c, a = 4,825(2), b =
11,302(2) et c = 12,453(2) Å, β = 91,45(3)°, V = 678,8(3) Å3, Z = 2. Par opposition, le composé 3b existe sous la
forme D2 (bateau-croisé) et cristallise dans le groupe d’espace C2/c avec a = 15,243(3), b = 8,703(2) et c = 27,010(14)
Å, β = 92,81(4)°, V = 3578(1) Å3, Z = 8. En solution dans le toluène, le dérivé 3a existe sous la forme d’un mélange
à l’équilibre de deux isomères conformationnels. Des données de RMN du 19F à TV permettent de déterminer les
paramètres thermodynamiques suivants: ∆H0 = 9,9 ± 0,4 kJ mol−1 et ∆S0 = 14 ± 1 J K−1 mol−1. Des calculs de densité
fonctionnelle sur le composé 3a indiquent que l’énergie de la conformation D2 est 4,6 kJ mol−1 plus faible que celle de
l’isomère C2h. La photolyse du composé 3a dans le benzène provoque une migration transannulaire du soufre
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conduisant à la formation de la 1,2,3,6-tétrathiocine, C6F4SSSC6F4S (6), qui existe dans une conformation chaise par
rapport aux atomes de soufre antipodaux. Structure cristalline du composé 6: orthorhombique, groupe d’espace Pnma, a
= 8,652(6), b = 19,084(4) et c = 8,301(6) Å, V = 1370,6(14) Å3, Z = 4. Les isomères 3a et 6 ont aussi été caractérisés
par spectrométrie de masse sous IE, par RMN du 19F et par spectroscopies FTIR et Raman.
Mots clés : tétrathiocines, isomères conformationnels, photoisomérisation.
[Traduit par la Rédaction] Chivers et al. 1101
system, the exocyclic electron pairs are nonbonding for -S-
but bonding for -CH2-. Consequently, the effect that the re-
A sulfur atom (-S-) and a -CH2- group can be viewed as
placement of some of the -CH2- groups by -S- linkages has
isoelectronic with the important difference that, in a ring
Received February 2, 1998.
T. Chivers,1 M. Parvez, I. Vargas-Baca, and G. Schatte. Department of Chemistry, The University of Calgary, 2500 University
Drive NW, Calgary, AB T2N 1N4, Canada.
1Author to whom correspondence may be addressed. Telephone: (403) 220-5741. Fax: (403) 289-9488. E-mail: chivers@ucalgary.ca
Can. J. Chem. 76: 1093–1101 (1998)
© 1998 NRC Canada

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Can. J. Chem. Vol. 76, 1998
Scheme 1.
Reagents and general procedures
All solvents were dried by treatment with the appropriate
drying agent and freshly distilled before use.
1,2,3,4-Tetrafluorobenzene, C6Cl6, n-BuLi (2.5 M solution
in hexanes), Na2S·9H2O, and I2 were commercial reagents
used as received. The SO2Cl2 was distilled under an N2 at-
mosphere before use. Sulfur was recrystallized from CS2.
Tetrafluorobenzene-1,2-dithiol, C6F4-1,2-(SH)2 (bp 50°C, 5
on the structures and conformational barriers of organic ring
systems is of interest. This substitution generally facilitates
the observation of interconversion by increasing the magni-
tude of the inversion barriers (1). Since the number of de-
grees of freedom and the complexity of the system increase
with the number of atoms in the ring, the majority of
conformational investigations has been concerned with
six-membered heterocycles (2).
1
× 10–2 Torr (1 Torr = 133.3 Pa); H NMR (CDCl3), δ: 3.82;
19F NMR (CDCl3): –131.7 (m) and –158.5 (m)) was ob-
tained in 70% yield by the literature method (9). The
C6Cl4-1,2-(SH)2 was prepared by a modification (vide infra)
of the literature method (10). The (C6F4S)2 was also synthe-
sized by a known procedure (11).
Instrumentation
The study of larger rings is simplified when some atoms
are constrained to remain in a plane, e.g., 1,2,5,6-tetrathiocin
1
The H NMR spectra were obtained on a Bruker ACE
1.2
Although
200 MHz spectrometer, and chemical shifts are reported in
ppm relative to Me4Si. The 19F NMR spectra were recorded
on a Bruker AMX 300 spectrometer; chemical shifts are re-
ported relative to CFCl3 and were measured with an external
standard of C6F6 (δ –162.9 ppm). Electron impact mass
spectra were obtained by using a VG Micromass spectrome-
ter (VG 7070) set at 70 eV. The FTIR spectra were recorded
as Nujol mulls (KBr plates) on a Mattson 4030 FTIR spec-
trometer in the range 4000–300 cm–1 (resolution: 4 cm–1;
number of scans: 128). Raman spectra were recorded using a
Jarrell–Ash model 25-100 monochromator interfaced to a
microcomputer. A Coherent Radiation Model 540 argon la-
ser fitted with an Innova 70 plasma tube was used as the ex-
citing source (emission wavelength: 514.5 nm; laser power:
100 mW; laser current: 30 A). Scattered radiation was col-
lected at 90° to the incident light. The Raman spectra were
obtained from neat samples sealed in melting point tubes
and measured in the range 2500–100 cm–1 at room tempera-
ture. The frequencies are believed accurate to ±2 cm–1. Ele-
3,4,7,8-tetrakis(trifluoromethyl)-1,2,5,6-tetrathiocin has been
known since 1960 (3), it has not been structurally character-
ized. The parent ring system 1 was synthesized for the first
time in 1996 (4). As in the case of cycloocta-1,5-diene (5),
1,2,5,6-tetrathiocin (1) should exhibit several conformers.
Recent ab initio molecular orbital calculations predict that a
(Z,Z) twist form of 1 (approximately D2 symmetry) repre-
sents a global minimum (6) and that structure has been
found in the solid state by an X-ray crystallographic analysis
(Scheme 1) (4). By contrast the twist-boat form (C2 symme-
try) was calculated to be the global minimum for
cycloocta-1,5-diene (5). The calculations also predict that
the chair (Z,Z) conformation of
1 (C2h symmetry)
(Scheme 1) is only 22.2 kJ mol–1 higher in energy than the
D2 isomer (6), and consequently, this alternative geometry
should be readily accessible. Indeed the chair conformation
of the C4S4 ring has been established for C6S10 (2a) (7) and
the related derivative (2b) (8).
mental
analyses
were
performed
by
Canadian
In this article we describe the synthesis and X-ray struc-
tures of the octahalogenated derivatives 3a and 3b. The
fluoro derivative 3a exists as an equilibrium mixture of two
conformational isomers in solution. The thermodynamic pa-
rameters for this interconversion process, obtained from
variable temperature 19F NMR data, are compared with the
values calculated by density functional theory (DFT) calcu-
lations. In view of the photochemical sensitivity of the par-
ent system 1 (4), the photolysis of 3a was also investigated.
Microanalytical Service Ltd., Vancouver, B.C., and by
Galbraith Laboratories Inc., Knoxville, Tennessee.
Preparation of C6Cl4-1,2-(SH)2
In a typical preparation C6Cl6 (32.7 g, 0.11 mol), Fe pow-
der (6.2 g, 0.11 mol) and Na2S·9H2O (54 g, 0.22 mol) were
heated at reflux in DMF (1 L) for 48 h. Caution: a
wide-necked condenser and vigorous reflux are necessary to
prevent sublimed C6Cl6 from plugging the condenser and
2 1,2,5,6-Tetrathiacycloocta-3,7-diene.
© 1998 NRC Canada

Chivers et al.
1095
causing a pressure build-up. The mixture was allowed to
cool to 23°C and then 2 L of NaOH (0.75 M) were added
with vigorous stirring. The mixture was allowed to settle
(48 h) and then centrifuged to separate the solid, which was
dissolved in 300 mL of MeOH and refluxed for 3 h with
ZnO (8 g) and NaOH (150 mL, 0.75 M). The hot slurry was
filtered, and the solution was acidified to a neutral pH with
H2SO4. A pale-yellow precipitate was filtered and dried with
an air stream in the filtration funnel. This product was ex-
tained as a white solid by vacuum sublimation at 90°C and
10–3 Torr; mp 116–117°C. Anal. calcd. for C6F4S2: C 33.96,
S 30.23; found: C 33.53, S 32.10.3 EIMS (m/z %): 424, 30
+
+
(M+), 392, 26 (C12F8S3 ), 360, 40 (C12F8S2 ), 328, 100
+
+
(C12F8S+), 244, 20 (C6F4S3 ), 212, 100 (C6F4S2 ), 180, 37
(C6F4S+), 168, 40 (C5F4S+); 19F NMR (CDCl3, δ): –117.3
(m), –122.9 (m), –147.5 (m), –147.8 (m). IR (Nujol, cm–1):
1600 (s), 1487 (vs), 1305 (s), 1237 (s), 1163 (s), 1115 (s),
1042 (vs), 970 (m), 951 (m, sh), 866 (s), 817 (s), 771 (m),
724 (s), 491 (m), 446 (m).
tracted (Soxhlet apparatus) with Et2O to remove
a
dark-green insoluble material. The solvent was evaporated
from the solution to leave 11.3 g of a yellow solid. This
crude product was shown by EIMS to consist of a mixture of
C6Cl5SH and C6Cl4-1,2-(SH)2 (yield ~36%). Recry-
stallization from toluene and subsequent sublimation
(140°C, 5 × 10–2 torr) produced C6Cl4-1,2-(SH)2 containing
only traces of C6Cl5SH.
All measurements were made on a Rigaku AFC6S
diffractometer using graphite monochromated Mo-Kα radia-
tion. Crystallographic data for α-3a, β-3a, 3b, and 6 are
given in Table 1.4
α-3a
Preparation of (C6F4S2)2 (3a)
A yellow hexagonal-plate crystal of α-(C6F4S2)2 was
mounted on a glass fibre. Cell constants and an orientation
matrix for data collection, obtained from a least-squares re-
finement using the setting angles of 17 carefully centred re-
flections in the range 30.00 < 2θ < 50.00°, corresponded to a
primitive monoclinic cell. The data were corrected for Lo-
rentz, polarization, and empirical absorption effects (12).
The structure was solved by direct methods (13) and ex-
panded using Fourier techniques (14). All non-hydrogen at-
oms were refined anisotropically. All calculations were
performed using the teXsan crystallographic software pack-
age (15).
A solution of C6F4-1,2-(SH)2 (1.66 g, 7.70 mmol) in
methanol (50 mL) was treated with a solution of I2 (1.97 g,
7.70 mmol) in methanol (50 mL).
A pale-yellow,
microcrystalline product was formed. The crude product was
filtered, rinsed with cold methanol, and recrystallized from
Et2O to give (C6F4S2)2 (3a) (1.42 g, 3.32 mmol, 86%); mp
193°C. Anal. calcd. for C6F4S2: C 33.96, F 35.82, S 30.23;
found: C 33.81, F 33.85, S 30.55.3
EIMS (m/z, %): 424, 40 (M+), 328, 15 (C12F8S+), 212,
+
100 (C6F4S2 ), 168, 38 (C5F4S+). 19F NMR (CDCl3, δ): two
sets of resonances at –130.1 (m), –154.4 (m) and –122.1
(m), –147.4 (m) with approximate relative intensities 1:4. IR
(Nujol, cm–1): 1594 m, 1307 s, 1237 s, 1038 s, 973 m, 872 s,
818 s, 724 s, 483 w, 414 s.
β-3a
A yellow prismatic crystal of β-(C6F4S2)2 was mounted on
a glass fibre. Cell constants and an orientation matrix for
data collection, obtained from a least-squares refinement us-
ing the setting angles of 13 carefully centred reflections in
the range 40.00 < 2θ < 50.00°, corresponded to a primitive
monoclinic cell. The structure was solved by direct methods
(16) and expanded using Fourier techniques (14). The rest of
the calculations involving data processing and structure re-
finement followed the same procedures as described for
α-3a.
Preparation of (C6Cl4S2)2 (3b)
A solution of SO2Cl2 (0.67 g, 5.0 mmol) in benzene
(10 mL) was added to a solution of C6Cl4-1,2-(SH)2 (1.33 g,
4.75 mmol) in benzene (50 mL). After 1 h the solvent was
decanted by cannula to leave a yellow solid, which was
rinsed with pentane (2 × 10 mL) to give (C6Cl4S2)2 (3b)
(1.18 g, 2.12 mmol, 90%); mp 290°C (dec.). Anal. calcd. for
C6Cl4S2: C 25.92, Cl 51.01, S 23.07; found: C 26.30, Cl
48.88, S 23.27.3 EIMS (m/z): 556 (M+). IR (Nujol, cm–1):
1377 s, 1335 m, 1303 s, 1095 m, 869 m.
3b
An orange prismatic crystal of (C6Cl4S2)2 was mounted
on a glass fibre. Cell constants and an orientation matrix for
data collection, obtained from a least-squares refinement us-
ing the setting angles of 25 carefully centred reflections in
the range 15.00 < 2θ < 30.00°, corresponded to a C-centred
monoclinic cell. Based on the systematic absences hkl, h + k
≠ 2n and h0l, l ≠ 2n, packing considerations, a statistical
analysis of intensity distribution, and the successful solution
and refinement of the structure, the space group was deter-
Photoisomerization of 3a
A solution of 3a (0.60 g, 1.41 mmol) in benzene (50 mL)
in a Pyrex vessel at 23°C was irradiated with a Hg lamp.
The progress of the reaction was monitored by 19F NMR
spectroscopy. After 7 h, solvent was removed under vacuum
and the pale-yellow product was recrystallized from
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CH2Cl2–hexane to give C6F4SSSC6F4S (6) (0.35 g) con-
taminated with 3a (19F NMR). A pure sample of 6 was ob-
3 Consistently low analyses for halogens were obtained for both 3a and 3b by two different commercial analysts. Low analyses for fluorine
were also obtained for 6.
4 Crystal data, tables of atomic coordinates and displacement parameters, complete listings of bond lengths and angles, and other refinement
data may be purchased from: The Depository of Unpublished Data, Document Delivery, CISTI, National Research Council of Canada, Ot-
tawa, Canada, K1A 0S2. Crystal data, atomic coordinates, and bond lengths and angles have also been deposited with the Cambridge Crys-
tallographic Data Centre and can be obtained on request from: The Director, Cambridge Crystallographic Data Centre, University Chemical
Laboratory, 12 Union Road, Cambridge, CB2 1EZ, U.K.
© 1998 NRC Canada

1096
Can. J. Chem. Vol. 76, 1998
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Table 1. Crystallographic data for (C6X4S2)2 (X = F, Cl) and C6F4SSSC6F4S.
α-3a
β-3a
3b
6
Dimensions (mm)
Formula
fw
Space group
a (Å)
b (Å)
c (Å)
β (°)
V (Å3)
0.60 × 0.50 × 0.15
α-C12F8S4
424.36
P21/a
9.351(2)
6.465(2)
11.546(2)
95.60(1)
694.6(2)
2
0.50 × 0.27 × 0.24
β-C12F8S4
424.36
P21/c
4.825(2)
11.302(2)
12.453(2)
91.45(3)
678.8(3)
2
0.30 × 0.27 × 0.10
C12Cl8S4
556.00
C2/c
15.243(3)
8.703(2)
27.010(14)
92.81(4)
3578(1)
8
0.40 × 0.30 × 0.20
C12F8S4
424.36
Pnma
8.652(6)
19.084(4)
8.301(6)
—
1370.6(14)
4
Z
T (°C)
λ (Å)
ρcalcd (g cm–3)
µ (cm–1)
–103.0
0.71069
2.029
–123.0
0.71069
2.076
–103.0
0.71069
2.064
–103.0
0.71069
2.056
7.42
7.89
17.17
7.81
Reflections: Collected
Unique
1440
1356
1427
1272
3557
3413
1443
1443
Observed
1022
912
1666
1252
Criteria
I > 3.0σ(I)
0.803–1.00
F
0.033,a 0.033b
0.005
I > 3.0σ(I)
0.972–1.00
F
0.025,a 0.024b
0.010
I > 3.0σ(I)
0.786–1.000
F
0.047,a 0.045b
0.008
I > 2.0σ(I)
0.918–1.000
F2
0.032,a 0.077c
—
Transmission factors
Coefficients used
R, Rw
p-Factor
Goodness of fit, S
Max shift/error
Max and min peaks
In final diff. map
3.84
0.00
0.37
–0.25
1.88
0.00
0.26
–0.25
1.82
0.17
0.65
–0.52
1.22
0.00
0.37
–0.33
a
R = Σ||Fo| – |Fc||/Σ|Fo|.
b
2
2
2
2
2
Rw = [Σw(|Fo| – |Fc|)2/ΣwFo ]1/2, where w = 1/σ2(Fo) = 4Fo /σ2(Fo ) and σ2(Fo ) = {S2(C+R2B) + (pFo )2}/Lp2, S = scan rate, C = total
integrated peak count, R = ratio of scan time to background counting time, B = total background count, Lp = Lorentz-polarization factor, p
= p-factor.
c
2
2
Rw = [Σw(|Fo| – |Fc|)2/ΣwFo ]1/2, where w = 1/[σ2(Fo)2 + (0.037P)2 + 0.610P], P = (Fo + 2Fc2)/3.
mined to be C2/c (no. 15). The data processing, structure so-
lution, and refinement followed the same procedures as
described for α-3a.
Synthesis and X-ray structures of dibenzotetrathiocins
3a and 3b
The tetrathiocins (C6F4S2)2 (3a) and (C6Cl4S2)2 (3b) were
prepared by the oxidation of the corresponding dithiols with
I2 or SO2Cl2, respectively, in >85% yields. Compounds 3a
and 3b are air stable, pale-yellow solids with high thermal
stability. They are also stable in polar solvents, e.g.,
acetonitrile or chloroform. For comparison, the parent
tetrathiocin (1), which is obtained in only 14% yield by the
oxidation of cis-disodium ethene-1,2-dithiolate, is converted
to the tetramer (-CHϭCH-S-S-)4 in acetonitrile at room tem-
perature (4).
Single crystals of 3a were grown from a diethyl ether so-
lution. Two different crystalline phases, which could be sep-
arated manually, were apparent. The most abundant (α-3a)
consists of hexagonal plates, while the other phase (β-3a)
forms rectangular prisms. X-ray diffraction analysis reveals
that the chair conformer (C2h) of the 1,2,5,6-tetrathiocin ring
pertains in both types of crystals (see Fig. 1). The difference
between the α and β phases is caused by distinctive packing
arrangements (see Figs. 2 and 3). A chair conformation for
the 1,2,5,6-tetrathiocin ring has also been observed for 2a
(7) and 2b (8). By contrast, crystals of 3b, obtained by slow
evaporation of a very dilute THF solution, provided an
6
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A colourless, plate-like crystal of C6F4SSSC6F4S was
mounted on a glass fibre. Cell constants and an orientation
matrix for data collection, obtained from a least-squares re-
finement using the setting angles of 25 carefully centred re-
flections in the range 18.21 < 2θ < 21.43°, corresponded to a
primitive orthorhombic cell. Based on the systematic ab-
sences of 0kl, k + 1 = 2n + 1 and hk0, h = 2n + 1, packing
considerations, a statistical analysis of intensity distribution,
and the successful solution and refinement of the structure,
the space group was determined to be Pnma (no. 62). The
structure was solved by direct methods (16) and expanded
using Fourier techniques (14). The structure was refined
with the aid of SHELX93 (17). Other details of the structure
refinement were similar to those described for β-3a.
Details of the methods used for the DFT calculations can
be found in ref. 18.
© 1998 NRC Canada

Chivers et al.
1097
Fig. 3. Packing diagram for the β crystalline phase of (C6F4S2)2
Fig. 1. The ORTEP diagram for (C6F4S2)2 (3a).
(3a).
Fig. 2. Packing diagram for the α crystalline phase of (C6F4S2)2
(3a).
respectively. The C-S-S-C torsion angles are ca. 111° for 3a
and 115.3(4)° and 118.2(4)° for 3b. The major disparity re-
sulting from the different ring conformations of 3a and 3b is
in the S-S-C-C torsion angles, which are 79.9° for 3a and
42.3-52.2° for 3b.
Conformational isomers of 3a
The 19F NMR spectrum of 3a in toluene solution at 300 K
displays four resonances. These can be separated into two
pairs, on the basis of their relative intensities. The less in-
tense pair of AA′XX′ multiplets is centred at δ –130.7 and
–154.7 ppm, while the more intense pair appears at δ –123.3
and –148.3 ppm. These observations suggest that 3a exists
as a mixture of the C2h and D2 conformers in solution (see
Table 4). The interconversion of these two conformational
isomers in toluene solution was investigated by variable
temperature 19F NMR spectroscopy. The relative intensity of
the two pairs of resonances changes reversibly within the
temperature range 300–380 K, as a result of the equilibrium
between the two conformers. However, no significant line
broadening or chemical-shift changes were observed, indi-
cating that the fluxional process is too slow to be observed
on the NMR time scale. The equilibrium constants at differ-
ent temperatures were obtained from the intensity ratios for
the two pairs of resonance (see Table 5) and a plot of ln K
vs. T–1 (see Fig. 5) was used to determine ∆H° and ∆S° for
this process. The experimental value of ∆H° is 9.9 ± kJ
mol−1. The ∆S° value of 14 ± 1 J K–1 can be attributed pri-
marily to changes in solvation.
X-ray structure corresponding to the known twist-boat con-
formation (D2) of the tetrathiocin ring 1 (4)) (see Fig. 4).
Relevant molecular dimensions for 3a and 3b are summa-
rized in Tables 2 and 3. The mean bond distances and bond
angles in the two dibenzotetrathiocins are very similar
(|d(C—C)| = 1.40 Å, d|(C—S)| = 1.77 Å, d|(S—S)| = 2.05 Å,
|angle CCS| = 124.3°, |angle CSS| = 103.8°). For compari-
son, the corresponding values for the parent compound 1 are
1.33 Å, 1.75 Å, 2.06 Å, 129.0°, and 104.8°(4). The
benzo-dithiolato units in 3a and 3b are nearly planar, with
S-C-C-S and S-C-C-C torsion angles close to 0° and 180°,
In view of the apparent inversion of the order of stability
of the C2h and D2 conformers for 3a compared to that calcu-
lated for 1 (6), the energies of these two conformers were
determined by density functional theory calculations. The
structure of 3a was fully optimised for both C2h and D2 sym-
© 1998 NRC Canada

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Can. J. Chem. Vol. 76, 1998
Fig. 4. The ORTEP diagram for (C6Cl4S2)2 (3b).
Table 2. Selected bond distances (Å), bond angles (°), and torsion angles (°) for (C6F4S2)2 (3a).a
α
β
S(1)—S(2)*
S(1)—C(1)
C(1)—C(2)
S(2)—C(2)
2.057(1)
1.771(3)
1.403(4)
1.771(3)
2.064(1)
1.771(3)
1.409(4)
1.772(3)
102.73(9)
123.1(2)
102.97(9)
123.6(2)
79.9(2)
S(2)*-S(1)-C(1)
S(1)-C(1)-C(2)
S(1)*-S(2)-C(2)
S(2)-C(2)-C(1)
S(1)*-S(2)-C(2)-C(1)
S(1)-C(1)-C(2)-C(3)
C(1)-S(1)-S*(2)-C*(2)
S(1)-C(1)-C(2)-S(2)
S(2)-C(2)-C(1)-C(6)
a
103.0(1)
123.1(2)
102.8(1)
123.5(3)
83.1 (3)
–177.9(3)
111.4(2)
–2.1(4)
176.1(2)
–111.3(1)
–1.8(3)
174.7(3)
–179.6(2)
Starred atoms are related to the unstarred atoms by symmetry operation: x, 1/2 – y, z.
Table 3. Selected bond distances (Å), bond angles (°), and torsion angles (°) for (C6Cl4S2)2 (3b).
S(1)—S(4)
S(1)—C(1)
S(3)—C(7)
C(1)—C(6)
2.033(3)
1.782(8)
1.799(8)
1.40(1)
S(2)—S(3)
S(2)—C(6)
S(4)—C(12)
C(7)—C(12)
2.042(3)
1.783(9)
1.772(8)
1.40(1)
S(1)-C(1)-C(6)
125.2(7)
S(2)-C(6)-C(1)
124.8(6)
S(3)-C(7)-C(12)
S(4)-S(1)-C(1)
123.5(7)
104.7(3)
104.5(3)
–48.7(9)
–42.3(8)
3(1)
–178.1(7)
171.9(7)
115.3(4)
S(4)-C(12)-S(7)
S(3)-S(2)-C(6)
S(2)-S(3)-C(7)
S(3)-S(2)-C(6)-C(1)
S(1)-S(4)-C(12)-C(7)
S(3)-C(7)-C(12)-S(4)
S(2)-C(6)-C(1)-C(2)
S(4)-C(12)-C(7)-C(8)
C(6)-S(2)-S(3)-C(7)
127.3(6)
102.7(3)
104.4(3)
–52.2(8)
–42.4(9)
–7(1)
–178.8(7)
171.9(7)
118.2(4)
S(1)-S(4)-C(12)
S(4)-S(1)-C(1)-C(6)
S(2)-S(3)-C(7)-C(12)
S(1)-C(1)-C(6)-S(2)
S(1)-C(1)-C(6)-C(5)
S(3)-C(7)-C(12)-C(11)
C(1)-S(1)-S(4)-C(12)
metries. The molecular dimensions were in agreement with
the experimental values for 3a and 3b to within 0.01 Å for
bond distances, 1° for bond angles, and 5° for torsional an-
gles. The calculations indicate that the D2 conformer should
be the most stable in this case, but the difference of total en-
ergies of the two conformers is only 4.6 kJ mol–1, signifi-
cantly less than the value of 22.2 kJ mol–1 estimated for 1
(6).
Photolysis of octafluorodibenzo-1,2,5,6-tetrathiocin (3a)
The parent ring system 1 is photochemically sensitive
even towards daylight (4). The photolysis of 1 in benzene
solution in the presence of excess 2,3-dimethylbutadine
© 1998 NRC Canada

Chivers et al.
1099
Table 4. The 19F NMR chemical shifts (in ppm) for (C6F4S)2 and (C6F4S2)2 isomers.
F1:
F2:
F3:
F4:
–134
–155
–155
–134
11
–130.1
–154.4
–154.4
–130.1
–122.1
–147.4
–147.4
–122.1
–117.3
–147.5
–147.8
–122.9
Reference:
This work
This work
This work
Table 5. Measured conformation
equilibrium constant for (C6F4S2)2 (3a)
Fig. 5. Temperature dependence of the conformational
equilibrium constant for (C6F4S2)2 (3a).
at different temperatures.
T (K)
K
300
310
320
330
340
350
360
370
380
9.99
8.55
8.16
6.55
6.38
5.51
4.98
4.65
4.42
produced the bicyclic product 4 in 62% yield, suggesting the
intermediate formation of the thioaldehyde 5.
In view of these observations it was of interest to deter-
mine the influence of the rigid benzo substituents in 3a on
the outcome of the photochemical transformation. A solution
of 3a in benzene in a Pyrex vessel was irradiated with a me-
dium-pressure mercury lamp through a Pyrex filter. The re-
action was monitored by 19F NMR, which revealed the
formation of an intermediate with resonances at δ –121.8,
–123.3, –146.7, and –147.1. The final product was identified
as octafluorodibenzo-1,2,3,6-tetrathiocin (6) by elemental
analysis, EIMS, 19F NMR, IR, and Raman spectra, and by an
X-ray structure (vide infra).
increasingly high abundances of the ions [C12F8S3]+ and
[C12F8S2]+, suggesting that loss of sulfur (from the S3 chain)
is the dominant fragmentation process, which leads to the
parent ion [C12F8S+]. By contrast, the parent ion for 3a is the
+
monomeric fragment C6F4S2 , indicating that cleavage of
both disulfide bonds occurs predominantly in this case.
Raman spectroscopy is a diagnostic technique for the
identification of SS vibrations, which are normally observed
in the 450–550 cm–1 region (19). In the present work, the
photoisomerized product 6 should be readily distinguished
by the presence of both asymmetric and symmetric stretch-
ing vibrations for the trisulfide (S3) unit, whereas the precur-
sor 3a will exhibit only νs(SS). Consistently, the Raman
spectrum of 6 shows bands at 492 and 469 cm–1. On the
basis of its stronger intensity, the former is attributed to νs(SSS)
(cf. HSSSH (20)). The Raman spectrum of 3a exhibits only
one band in the 450–550 cm–1 region at 497 cm−1, which is
assigned to νs(SS) (cf. 489 cm–1 for C6F5SSC6F5 (21)).
The photoisomerized product 6 is readily distinguished
from the isomers of 3a by the 19F NMR spectrum, which ex-
hibits an AMXY pattern (Table 4). The multiplets attributed
to the inequivalent ortho-fluorine atoms have significantly
different chemical shifts whereas those assigned to the other
pair of inequivalent fluorine atoms differ by only 0.3 ppm.
The mass spectra of 3a and 6 show major differences. Both
+
compounds exhibit the molecular ion C12F8S4 with similar
intensities. However, the mass spectrum of 6 also exhibits
[1]
© 1998 NRC Canada

1100
Can. J. Chem. Vol. 76, 1998
Table 6. Selected bond distances (Å), bond angles
|
|
−
−
Fig. 6. The ORTEP diagram for C6F4SSSC6F4S (6).
|
|
−
−
(°), and torsion angles (°) for C6F4SSSC6F4S (6).a
S(1)—C(1)
S(2)—C(2)
S(2)—S(3)
1.799(2)
1.782(2)
2.047(1)
S(1)-C(1)-C(2)
C(1)-C(2)-S(2)
C(2)-S(2)-S(3)
121.7(1)
123.5(1)
103.1(1)
S(2)-S(3)-S(2)*
C(1)-S(1)-C(1)*
C(2)-S(2)-S(3)-S(2)*
C(1)*-S(1)-C(1)-C(2)
C(1)*-S(1)-C(1)-C(6)
S(1)-C(1)-C(2)-C(3)
S(1)-C(1)-C(2)-S(2)
S(3)-S(2)-C(2)-C(3)
S(3)-S(2)-C(2)-C(1)
S(2)-C(2)-C(3)-C(4)
S(1)-C(1)-C(6)-S(5)
a
107.2(1)
102.8 (1)
–93.78(10)
–94.94(13)
85.01(14)
179.95(15)
–6.0(2)
–103.79(11)
81.97(14)
–174.5(2)
–179.95(14)
The photochemical transformation of 3a into 6 (eq. [1])
formally involves a transannular migration of a sulfur atom.
In view of the similarity of the 19F NMR spectra of 6 and
the intermediate observed in this process (vide supra), it is
suggested that this intermediate is a conformational isomer
of 6. A similar isomerization of a C4S4 ring has been re-
ported for C6S10 (2a) but, in that example, the transforma-
tion from a 1,2,5,6- to a 1,2,3,6-tetrathiocin involves a
Hg2+-promoted hydrolysis to give C6S8O2 (22a). A mecha-
nism involving a thiosulfoxide intermediate has been pro-
posed for the sulfur transfer process (22b). Although organic
Starred atoms are related to the unstarred atoms by
symmetry operation: x, 1/2 – y, z.
93.8(1)° for the torsion angle C-S-S-S in 6 is significantly
smaller than the corresponding values for the torsion angles
involving a central disulfide linkage in 3a or 3b, which are
in the range 111–118°. We tentatively suggest that the inter-
mediate detected by 19F NMR in the photochemical transfor-
mation of 3a into 6 is the corresponding boat conformer of
6.
The 1,2,5,6-tetrathiocins (C6F4S2)2 (3a) and (C6Cl4S2)2
(3b) adopt chair (C2h) and twist-boat (D2) conformations of
the C4S4 ring, respectively. The difference in the energies of
the two conformers for 3a is very small (<5 kJ mol–1), and
thiosulfoxides R2SS have not characterized, the SS double
bond is known to be stabilized by electronegative groups (23).
|
|
−
−
For example, the cyclic thiosulfite
ROS(= S)OR (R =
cyclohexylidene) is stable at 20°C (24). In the case of the
transformation represented by eq. [1], the proposed interme-
this derivative undergoes
a
slow conformational
isomerization in solution. The photolysis of 3a promotes a
reversible, transannular sulfur migration to give the
1,2,3,6-isomer (6).
diate
7
may be stabilized by the electronegative
perfluoroaryl groups attached to sulfur. Finally, we note that
6 is unstable in solution in polar solvents and slowly reverts
to 3a.
X-ray structure of octafluorodibenzo-1,2,3,6-tetrathiocin
(6)
We thank NSERC (Canada) for financial support, Dr. T.
Shimizu for a preprint of ref. 6, and Prof. T.B. Rauchfuss for
helpful correspondence and a copy of the Ph.D. thesis of
C.P. Galloway in which the possible rearrangement pathway
for the transformation of C6S10 into C6S8O2 is presented. We
also thank Professor C. Langford for the use of photochemi-
cal equipment and Professor R. Kydd for his assistance with
the measurement of Raman spectra.
The structure of 6 was established by X-ray crystallogra-
phy. An ORTEP diagram is represented in Fig. 6, and rele-
vant structural parameters are given in Table 6. The S—S
bond length of 2.047(1) Å is, as expected, a typical sin-
gle-bond value. The bond angle S-S-S = 107.2(1)° is close
to the mean value of 107.9° reported for cyclo-octasulfur
(21). However, the mean bond distance, d|(C—S)| = 1.79 Å,
and mean bond angles, |angle CCS| = 122.6°, |angle CSS| =
103.1°, show small, but significant, differences from the cor-
responding values for 3a and 3b, possibly owing to slightly
greater ring strain in 6. The small bond angle
C(1)-S(1)-S(1)* of 102.81(1)° is perhaps indicative of some
ring strain. The C4S4 ring in 6 adopts a chair conformation
with respect to the sulfur atoms S(1) and S(3). The value of
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