5-chloro-3-methylthio-1,2,4-thiadiazol-2-ium chlorides as useful synthetic precursors to a variety of 6aλ4-thiapentalene systems

Article abstract of DOI:10.1002/hc.10106

Title salts 3 were easily obtained by treatment of formimidoyl isothiocyanates 1 with a twofold excess of methanesulfenyl chloride. They showed interesting chemical behavior toward several nitrogen and carbon nucleophiles. Substitution reactions with isot

Full text of DOI:10.1002/hc.10106

Heteroatom Chemistry
Volume 14, Number 1, 2003
5-Chloro-3-methylthio-1,2,4-thiadiazol-2-ium
Chlorides as Useful Synthetic Precursors
4
to a Variety of 6aλ -Thiapentalene Systems
Georges Morel,1 Evelyne Marchand,1 Sourisak Sinbandhit,2
and Loic Toupet3
1Synthe`se et Electrosynthe`se Organiques, UMR 6510, Universite´ de Rennes I, Campus de Beaulieu,
F-35042 Rennes Cedex, France
2Centre Re´gional de Mesures Physiques de l’Ouest, Universite´ de Rennes I, Campus de Beaulieu,
F-35042 Rennes Cedex, France
3Groupe Matie`re Condense´e et Mate´riaux, UMR 6626, Universite´ de Rennes I, Campus de Beaulieu,
F-35042 Rennes Cedex, France
Received 21 May 2002
additional support from X-ray analysis of substrate
ABSTRACT: Title salts 3 were easily obtained by
ꢁ
C
16a.
2003 Wiley Periodicals, Inc. Heteroatom Chem
treatment of formimidoyl isothiocyanates 1 with a
twofold excess of methanesulfenyl chloride. They
showed interesting chemical behavior toward several
nitrogen and carbon nucleophiles. Substitution reac-
tions with isothioureas and acetamide in the presence
of triethylamine gave the 1H, 6 H-6aꢀ4-thia-1,3,4,6-
tetraazapentalenes 7 and 6 H-6aꢀ4-thia-1-oxa-3,4,6-
triazapentalene 9, respectively. Addition of p-toluidine
furnished the 5-imino-thiadiazole derivatives 10,
which reacted further with diverse heterocumulenes
to yield the corresponding thiatriaza- and tetraaza-
pentalene species 11. The N,Nꢀ-bis(1,2,4-thiadiazol-5-
ylidene)diaminobenzenes 13 were also prepared and
reacted with phenyl isothiocyanate. Two stable rota-
tional isomers were separated for the 1,2-phenylene
product 14b. Other ꢁ-hypervalent sulfur compounds
16 were synthesized under similar conditions from
salts 3 and methyl cyanoacetate or dimethyl malonate.
The structural assignments were discussed on the ba-
sis of IR and NMR spectroscopic data and received
14:95–105, 2003; Published online in Wiley InterScience
(www.interscience.wiley.com). DOI 10.1002/hc.10106
INTRODUCTION
We have previously reported [1] the reaction of
sulfenyl thiocyanates with a series of (methylthio)-
and (arylthio)formimidoyl isothiocyanates 1 readily
forming the compounds 2, which may be designated
as 6H-1,6aꢀ4-dithia-3,4,6-triazapentalenes. This re-
sult was explained by a nucleophilic addition to the
C S unsaturation of the heterocumulene 1, with sub-
sequent cyclization and displacement of the sulfenyl
anion (Scheme 1).
Products 2 are members of an important class
of heterocycles which are now described as three-
centered hypervalent molecules with a 10 ꢁ-electron
system (10-S-3 sulfuranes in the case of a central sul-
fur atom) [2]. Study of such isolable aromatic species
has attracted much attention over the past decades
because of their unique binding behavior [3–5]. It
has been pointed out that their structure may be the
result of the existence of a “single-bond–no-bond”
Correspondence to: Georges Morel; e-mail: georges.morel@univ-
rennes1.fr.
c
ꢁ
2003 Wiley Periodicals, Inc.
95

96 Morel et al.
SCHEME 1
resonance between several canonical forms, repre-
sented by the three general formulas in Scheme 2.
Alternatively to this proposition, a rapid equilibrium
between the two valence tautomers a and c may be
postulated for which the bicyclic form b represents
only a transition state. However, there is at present no
evidence for individual existence of valence isomers
a and c and most of these adducts are satisfactory
represented by the fused structure b [6,7].
Process of Scheme 1 encouraged us to investigate
the utility of the imidoyl isothiocyanates 1 for the
preparation of 5-chloro-1,2,4-thiadiazolium species
in the presence of sulfenyl chlorides. It was expected
that the 5-chlorine atom of such salts could be dis-
placed by nucleophiles and we were particularly at-
tracted by the possibility of obtaining diverse poly-
heterapentalene systems under the action of suitable
reagents.
Herein report about a facile synthesis of the
5-chloro-1,2,4-thiadiazol-2-ium chlorides 3 and their
use in a novel, general, and effective approach to
new hypervalent sulfur compounds. In this paper,
we assume the replacement nomenclature [3c] and
numbering [8] corresponding to the valence formula
b whatever the real importance of this particular res-
onance structure.
of methanesulfenyl chloride, in carbon tetrachlo-
ride at room temperature. The pale-yellow solids 3,
which precipitated from the reaction medium, were
collected by filtration in 80% yield or better. Large
amounts of dimethyl disulfide were recovered in the
filtrate supporting the stepwise mechanism via 4-
chloro-4-(methyldithio)-1,3-diazabutadiene 4 as in-
termediate (Scheme 3).
Salts 3 are hitherto unknown variations of the
thiadiazolium structure. To the best of our knowl-
edge, systems of this type have been limited to 5-
aryl- and 5-(dialkylamino)-1,2,4-thiadiazolium salts
through the oxidative ring closure of N-thiono-
amidino compounds in the presence of perchloric
acid [4].
Thiadiazolium chlorides 3 exhibited good stabil-
ity in the solid state under a dry atmosphere. Salts
3a–c were gradually hydrolyzed on standing in air
moisture, and much faster in alcohol solution, to
give the isothiourea hydrochlorides 5. We suggest a
course via an unstable carbamic acid (Scheme 3).
The 2-methyl-3,5-diphenyl-1,2,4-thiadiazolium salts
were shown to be similarly cleaved by H2O with
elimination of sulfur [4]. The source of instabil-
ity of 3d,e lies rather in the nature of the N-
substituent as these salts spontaneously decomposed
in solution into a mixture of tert-butyl chloride,
isobutene, and 5-chloro-1,2,4-thiadiazoles 6. A sim-
ilar isobutene evolution dominates the chemistry of
the N-tert-butylimidazolium [9] and pyrrolium [10]
salts. Patented 5-chloro-1,2,4-thiadiazoles, including
3-(alkylthio) derivatives, were previously prepared
by the reaction of trichloromethylsulfenyl chloride
with amidino substrates [4,11].
RESULTS AND DISCUSSION
Preparation of 5-Chloro-1,2,4-thiadiazol-2-ium
Chlorides 3
Our strategy consisted in treating the easily accessi-
ble imidoyl isothiocyanates 1 with a twofold excess
SCHEME 2

Precursors to a Variety of 6aꢀ4-Thiapentalene Systems 97
SCHEME 3
may also be formulated as 6H-1-oxa-6aꢀ4-thia-3,4,6-
triazapentalene 9 (Eq. (2)). Structure 9 was assumed
on the basis of NMR and IR spectral properties.
Thus, the obtained adduct exhibits C2, C3a, and C5
resonances (Table 1) in consonance with those of
nearly identical polyheterapentalenes [16]. It does
not display the normal carbonyl vibration of N-
acetylimidates in the 1650–1660 cm−1 range [17] but
shows a weak band at 1635 cm−1 together with two
strong bands in the 1500–1530 cm−1 region. All at-
tempts to synthesize related dithiatriazapentalenes
by treatment of salt 3a with thioamides were unsuc-
cessful, the substrate 7b being essentially recovered
on account of a partial hydrolysis to the transient
isothiourea 5a.
The 5-imino-2,5-dihydro-1,2,4-thiadiazole hy-
drochlorides 10 were readily accessible by conden-
sation of salts 3a,d with p-toluidine. They were
converted into a variety of 2,3-dihydro-6aꢀ4-thia-
polyheterapentalenes 11 in nearly quantitative
yields, using addition of heterocumulenes (Eq. (3)).
Literature synthetic pathways to analogous 5-
iminothiadiazoles [4] mainly involved the oxida-
tive cyclization of N-imidoylthioureas [18,19]. Hy-
drochloride derivatives proved to be rather stable
heterocycles in contrast to their deprotonated coun-
terparts, which underwent a rapid ring-opening pro-
cess [18b]. Dipolar intermediates were implicated
in their reactions with carbon disulfide [18a] and
iso-, isothio-, and isoselenocyanates [5,13,18a], pro-
ceeding either on the C N or the C X unsaturation
of such heteroallenes RNCX. Evidence for an addi-
tion of 10a on the carbon–nitrogen double bond of
phenyl isothiocyanate was found in the ready alky-
lation of the 2-thioxo-thiatetraazapentalene 11b to
Reactions of 5-Chloro-1,2,4-thiadiazolium
Chlorides 3 with Nitrogen Nucleophiles
Thiadiazolium salts 3 were shown to preferentialy
react with nucleophiles in the 5- and not in the
3-position. Substitutions by compounds that con-
tain the NH2 functional group are summarized in
Scheme 4. Adducts were generally obtained in sat-
isfying yields after a short time at room tempera-
ture in dry THF containing the specified amount of
triethylamine.
Reactions with S-methyl isothioureas gener-
ated the 1H,6H-6aꢀ4-thia-1,3,4,6-tetraazapentalenes
7 (Eq. (1)), which were characterized by elemental
analysis, mass, and NMR spectroscopies with partic-
ular regard to the chemical shifts for the three endo-
cyclic carbons. The pertinent 13C NMR data are listed
in Table 1 and other spectral results are recorded
in the Experimental section. Interestingly, the 1H
and 13C NMR spectra of symmetrically substituted
product 7b show the equivalence of ring carbons
C2 and C5 and of substituents on the pairs of sites
N1/N6 and C2/C5 (cf. Experimental section). Num-
ber and pattern of signals also remained unchanged
1
for the H NMR spectrum of 7b down to −95◦C in
CD2Cl2. Such a total magnetic equivalence on the
NMR time scale is consistent with a C2v -symmetrical
structure b (Scheme 2) [3c,12,13]. Relatively few ex-
amples of similar thiatetraazapentalene derivatives,
without substituent on the N3 and N4 positions,
have been mentioned in the literature [14,15]. They
were principally prepared from 2,5-dihydro-5-imino-
1,2,4-thiadiazoles and imidoyl chlorides [14].
Salt 3a and acetamide were reacted to give the 5-
(acetylimino)-2,5-dihydro-1,2,4-thiadiazole 8, which

98 Morel et al.
SCHEME 4
provide the methiodide 12b (Scheme 5). Structures
11 were also supported by the 13C NMR data reported
in Table 1, in satisfying agreement with values for re-
lated polyheterapentalenes [13,18a].
In a similar manner, nucleophilic substitution
on salt 3a by 1,4- and 1,2-phenylenediamines in the
presence of stoichiometric amount of triethylamine
promoted the formation of the N,Nꢀ-bis[1,2,4-thia-
diazol-5-ylidene]diaminobenzene dihydrochlorides
13. These 1:2 adducts readily furnished the
N,Nꢀ-phenylene-bridged bis(thiatetraazapentalene)
derivatives 14 by reaction with phenyl isothio-
cyanate (Scheme 4, Eq. (4)).
Analysis of crude solid 14b revealed the existence
of two isomers in a ratio of about 50:50, which were
easily separated by a flash chromatography. 1H NMR
spectra of both isomers 14b(I) and 14b(II) show a
singlet for the 2 equiv. methylthio groups together
with a septuplet and two different doublets for the
CH and the diastereotopic methyls of the 2 equiv. iso-
propyl groups. In the same way, 13C NMR spectra of
both isomers (Table 1) display two resonances in the
22–23 ppm range attributed to the methyl carbons
of the isopropyl groups while only 12 signals are de-
tected for all the other carbons of the molecule. These
data were rationalized assuming the presence of two

Precursors to a Variety of 6aꢀ4-Thiapentalene Systems 99
TABLE 1 NMR Chemical Shifts and Multiplicities for the Main Carbon Atoms of 6aλ4-Thiapentalene Systemsa,b
C2
C5
C3a (s)
SCH3 (q, 1J = 143)
N1 C c
N6 C c
7a
7c
7d
151.8 (q, 3J = 4.9)
152.7 (q, 3J = 5)
151.6 (q, 3J = 5.1)
156.8 (m)
169.4
171.3
168.2
177.5
169.6
165.4
163.9
167.5
163.3
167.1
178.9
164.3
164.8
178.5
177.6
176.5
13.4, 14.0
13.8, 14.1
14.0, 14.8
14.1
13.9
14.0
13.7
14.9
15.1
15.4, 17.3
14.3
13.6
14.3
15.7
15.4
17.5
135.4 (m)
135.0 (m)
135.3 (m)
51.0 (dm)
144.5 (m)
55.4 (m)
50.4 (dm)
51.6 (dm)
50.9 (dm)
50.6 (dm)
57.3 (m)
57.1 (m)
52.7 (dm)
52.9 (dm)
50.6 (dm)
50.4 (dm)
52.1 (dm)
51.3 (dm)
61.4 (m)
160.4 (q, 3J = 4)
154.4 (q, 3J = 3.9)
9d
182.5 (q, 2 J = 6.5)
203.1 (s)
165.5 (m)
162.1 (m)
163.2 (m)
–
–
11a
11b
11c
11d
11e
172.5 (s)
136.2 (m)
53.4 (dm)
168.2 (d, 3J = 4.7)
161.6 (m)
201.9 (s)
159.9 (q, 3J = 4.4)
–
150.7 (s)
161.1 (q, 3J = 4.4)
132.7 (t, 3J = 9.5)
12b
14a
154.9 (q, 3J = 5.1)
167.8 (m)
167.4 (m)
138.1 (m)
136.3 (m)
172.5 (s)
14b(I)d
14b(II)d
16ad
171.5 (s)
171.2 (s)
161.6 (m)
163.4 (m)
168.5 (m)
139.8 (t, 3J = 8)
140.4 (t, 3J = 8)
170.7 (q, 3J = 4)
171.6 (q, 3J = 4)
170.7 (q, 3J = 3.7)
–
–
–
16bd
166.7 (m)
16dd
167.9 (q, 3J = 4.7)
aδ (ppm) and J (Hz) at 75.5 MHz in CDCl3 solutions (except for 14a: CDCl3/CF3CO2H).
bThe ring atoms are numbered in such a way that the R1 group is at the 6-position.
cCarbon of the substituent connected to the specified nitrogen atom.
dOther selected values for 9: δ 23.2 (q, J = 128 Hz, CH3 on the 2 position); for 14b(I): δ 22.8 and 23.3 (qm, J = 128 Hz, CH3 of the i Pr
1
1
1
1
group); for 14b(II): δ 22.9 and 23.0 (qm, J = 128 Hz); for 16a: δ 52.6 (q, J = 146 Hz), 70.4 (s, C3), 115.9 (s, CN); for 16b: δ 51.2, 52.7 (q,
1J = 146 Hz), 89.6 (s, C3), 164.8 (q, 3 J = 4 Hz, CO2Me); and for 16d: δ 52.6 (q, 1J = 147 Hz), 69.6 (s, C3), 116.1 (s, CN).
rotational isomers due to a severe steric hindrance
(atropisomers [20]). Steric interactions induce a high
energy barrier of one of the rotational processes
around the central C N bonds. Such an explanation
was confirmed by a slow isomerization of each pure
conformer 14b on standing in CDCl3 at room temper-
ature. We obtained the same thermodynamic mix-
ture of the above-mentioned atropisomers [21] and
the thiadiazole derivative 15b via a monoelimination
of phenyl isothiocyanate (Scheme 6). Evaporation
of the solution gave a residual solid that contained
only the two conformers 14b by a recombination
with PhNCS! A high temperature 1H NMR study
(C5D5N, 95◦C) did not show the coalescence of corre-
sponding signals but promoted the fast formation of
15b and decomposition by-products. Only one iso-
mer was observed for the para-phenylene counter-
part 14a, which does not offer the same restricted
rotation.
Reactions of 5-Chloro-1,2,4-thiadiazolium
Chlorides 3 with Active Methylene Anions
Under similar conditions, treatment of substrates
3a,d with triethylammonium salts of methyl cyano-
acetate and dimethyl malonate afforded compounds
which may be designated as 6H-1-oxa-6aꢀ4-thia-
4,6-diazapentalenes 16 (Scheme 7). In principle,
the methyl 2-(1,2,4-thiazol-5-ylidene)cyanoacetates
17a,d corresponding to monocyclic form a in
Scheme 2 would be a mixture of Z/E isomers but
only one stereoisomer was detected in the reaction
medium from 3a,d and cyanoacetic ester. Structures
16 were further supported by 13C NMR (Table 1) and
IR spectroscopies as well as X-ray crystallographic
determination of 16a.
SCHEME 5
SCHEME 6

100 Morel et al.
SCHEME 7
The prominent feature of IR spectra of 16a,d
approximately in a straight line with a 167.5(3)◦ bond
angle. Other important result is the contact distance
between the S and O atoms (2.395 A), which is much
is the appearance of a strong absorption band at
about 1625 cm−1. Such a carbonyl stretchching vi-
bration is markedly affected with regard to the
typical conjugated ester frequency in the 1655–
1670 cm−1 region as previously reported for ethyl
2-(1,3-dithiol-2-ylidene)cyanoacetates [22] and alkyl
2-(5H-1,2,3-dithiazol-5-ylidene)cyanoacetates [23].
In the same way, thermostable adduct 16b exhibits
spectral data and strong [M]+. in good agreement with
the assigned structure.
The single crystal X-ray analysis of 16a con-
firmed the expected coplanar structure of the heter-
apentalene framework, as depicted in Fig. 1 [24]. The
angle between the thiadiazole ring and the isoxathi-
ole reference plane was calculated to be 0.8(3)◦ and
the heteroatoms of the N S O arrangement are
˚
shorter than the sum of the van der Waals radii
˚
(3.30 A). The relatively short interatomic S O dis-
tance, together with the coplanarity of the system,
suggest the existence of a no-bond interaction by ꢂ-
˚
delocalization. The C3a S6a length (1.746 A) lies in
the range expected for thiapentalenes [3–5].
However, as suggested by one of the referees,
the crystal data may be also considered in terms
of the open-chain form 17a where conjugated het-
eroatoms are held together by fractional S+ and O−
charges (Scheme 7). Such monocyclic structure is
rather consistent with the distance observed between
˚
the thiadiazole S and N6 atoms (1.724 A), which
is nearly equal to the sum of the covalent radii of
˚
FIGURE 1 Solid-state molecular structure of 16a. Selected bond lengths (A): N6 C5 1.326 (8), C5 N4 1.331 (7), N4 C3a
1.344 (8), C3a S6a 1.746 (6), S6a N6 1.724 (5), C3a C3 1.394 (8), C3 C2 1.422 (8), C2 O1 1.228 (7), O1 S6a 2.395
(5). Selected bond angles (◦): N6 C5 N4 117.5 (5), C5 N4 C3a 109.4 (5), N4 C3a S6a 114.4 (4), C3a S6a N6 87.2
(3), S6a N6 C5 111.4 (4), S6a C3a C3 119.0 (4), C3a C3 C2 117.2 (5), C3 C2 O1 121.9 (6), N4 C3a C3 126.6 (5),
O1 S6a N6 167.5 (3); dihedral angle (N6 C5 N3 C3a S6a/O1 C2 C3 C3a S6a) 0.8 (3).

Precursors to a Variety of 6aꢀ4-Thiapentalene Systems 101
˚
(20 mmol). A yellowish material precipitated im-
mediately. The mixture was stirred at room tem-
perature for 1 h. The salts 3 were collected by fil-
tration, washed with anhydrous diethyl ether, dried
in vacuo, and used for next reactions without fur-
ther workup. They showed very low solubility in
apolar solvents and were hygroscopic. For exam-
ple, 3a was allowed to stand overnight in the solid
state under atmospheric moisture to give the isoth-
iourea hydrochloride 5a, which was recrystallized
from CH2Cl2/diethyl ether. NMR spectra of salt 3d
were unable to be recorded because of the rapid elim-
ination of tert-butyl chloride in CDCl3 solution. This
instability problem even prevented the salt 3e to be
isolated from the CCl4 medium. After removal of the
solvent, the 5-chloro-1,2,4-thiadiazoles were purified
by distillation under reduced pressure (6d) or recrys-
tallization from petroleum ether (6e).
sulfur and nitrogen (1.74 A). This molecular N S
dimension corresponds to a typical two-center two-
electron bond length and agrees favorably with that
of a structurally comparable 5-(benzoylimino)-2,5-
dihydro-1,2,4-thiadiazole [25]. In the same way, the
open structure 8 should be preferred over the bicyclic
9 (Scheme 4).
In conclusion, the reactivity of the readily
available 5-chloro-1,2,4-thiadiazol-2-ium salts 3 was
shown to be exclusively determined by the nucle-
ophilic substitution of the chlorine atom. Reagents
having two active hydrogen atoms were used for
the one-step synthesis of new heterocycles, whose
spectroscopic data and X-ray analysis suggest an
hybridization between the canonical structures out-
lined in Scheme 2, with a great contribution from b.
The formation and separation of two atropisomers
in the case of compound 14b also deserve to be em-
phasised.
5-Chloro-2-isopropyl-3-(methylthio)-1,2,4-thiadi-
azol-2-ium Chloride (3a). m.p. 184◦C dec. (95%
1
yield). H NMR δ 1.68 (d, 6H, J = 6.6 Hz), 2.95 (s,
EXPERIMENTAL
3H), 4.59 (m, 1H). 13C NMR δ 16.6 (q, 1 J = 145 Hz),
23.0 (qm, 1 J = 130 Hz), 55.7 (dm, 1 J = 143 Hz),
169.9 (m, C2), 179.3 (s, C5).
NMR spectra: Bruker AM 300 WB spectrometer
1
(300.1 MHz for H and 75.5 MHz for 13C) in CDCl3
solution (internal standard Me4Si) at room tempera-
ture unless otherwise indicated. HRMS: Centre
Re´gional de Mesures Physiques de l’Ouest; Varian
MAT 311 instrument, electron impact mode using
a potential of 70 eV, except for compounds 12–
15: MS/MS ZabSpec TOF Micromass spectrometer,
ionization mode positive LSIMS with Cs+, matrix
m NBA; only mass-spectral fragments with relative
intensities of 10% or more are reported. IR spec-
tra: Perkin-Elmer 1420 spectrophotometer, suspen-
sions in nujol. Elemental analyses: Analytical labora-
tory, CNRS. Solvents: Carbon tetrachloride was dried
over calcium chloride and distilled; tetrahydrofuran
(THF) and dichloromethane were freshly distilled
from NaH and P2O5, respectively.
5-Chloro-2-isopropyl-3-[(4-methylphenyl)thio]-1,
2,4-thiadiazol-2-ium Chloride (3b). m.p. 171◦C
(88% yield). 1H NMR δ 1.76 (d, 6H, J = 6.8 Hz), 2.45
(s, 3H), 4.76 (m, 1H), 7.30, 7.55 (2d, 4H, J = 7.8 Hz).
5-Chloro-2-(2,6-dimethylphenyl)-3-(methylthio)-
1,2,4-thiadiazol-2-ium Chloride (3c). m.p. 192–
196◦C dec. (80% yield). 1H NMR δ 2.16 (s, 6H), 2.87
(s, 3H), 7.25 (m, 3H).
2-tert-Butyl-5-chloro-3-(methylthio)-1,2,4-thiadi-
azol-2-ium Chloride (3d). m.p. 135◦C (96% yield).
MS m/z (rel. int.): 166 [M-t BuCl]+. (10), 131 (100),
105 (12), 99 (30).
Starting Materials: The imidoyl isothiocyanates 1
were conveniently prepared as previously described
[1] from the insertion reaction of isocyanides R1NC
into the S Cl bond of sulfenyl chlorides R2SCl fol-
lowed by the addition of ammonium thiocyanate in
CCl4 solution. The isopropyl isoselenocyanate [13b]
and S-methyl-N-phenylisothiourea hydroiodide [26]
were obtained according to known procedures.
N-Isopropyl(methylthio)formamidine Hydrochlo-
ride (5a). m.p. 154◦C (62% yield). IR 3160, 1643,
1583 cm−1. 1H NMR δ 1.32 (d, 6H, J = 6.4 Hz), 2.79
(s, 3H), 3.77 (m, 1H), 8.90 (l, 2H), 9.55 (d, NH, J = 7
Hz). Anal. calcd. for C5H13ClN2S: C, 35.61; H, 7.72;
N, 16.62; S, 18.99. Found: C, 35.72; H, 7.71; N, 16.70;
S, 18.54.
Preparation of 5-Chloro-1,2,4-thiadiazol-2-ium
Salts 3
5-Chloro-3-(methylthio)-1,2,4-thiadiazole (6d).
b.p. 50◦C/0.025 Torr (Bu¨chi Kugelrohr apparatus)
1
(65% yield). H NMR δ 2.70 (s). 13C NMR δ 14.7 (q,
Methanesulfenyl chloride (3.3 g, 40 mmol) was dis-
solved in CCl4 (10 ml) and added dropwise to a
CCl4 solution (30 ml) of imidoyl isothiocyanate 1
1 J = 142 Hz), 172.0 (q, 3 J = 5 Hz, C3), 173.2 (s, C5).
MS m/z (rel. int.): 166 [M]+. (80), 131 (20), 105 (50),

102 Morel et al.
22.4 (qm, 1 J = 127 Hz), 48.4 (dm, 1 J = 136 Hz),
160.8 (m, C2 and C5), 171 (s, C3a). MS calcd. for
C11H20N4S3 m/z: 304.0850 [M]+. , found 304.0851; m/z
(rel. int.): 304 (15), 289 (10), 257 (100), 215 (45), 173
(95).
95 (100), 90 (40), 85 (30), 73 (30). Anal. calcd. for
C3H3ClN2S2: C, 21.62; H, 1.80; Cl, 21.32; N, 16.81; S,
38.43. Found: C, 21.66; H, 1.76; Cl, 21.19; N, 16.65;
S, 38.35.
5-Chloro-3-[(2-nitrophenyl)thio]-1,2,4-thiadia-
zole (6e). m.p. 87◦C (55% yield). IR 1585, 1564,
1513 cm−1. 13C NMR δ 125.6, 127.5, 129.3, 133.4,
134.5, 149.2 (arom.C), 16+8.4 (s, C3), 174.3 (s, C5).
MS m/z(rel. int.): 273 [M]. (80), 227 (60), 209 (50),
192 (65), 174 (100). Anal. calcd. for C8H4ClN3O2S2:
C, 35.10; H, 1.46; Cl, 12.98; N, 15.35; S, 23.40. Found:
C, 35.09; H, 1.56; Cl, 12.98; N, 15.39; S, 23.67.
6-(2,6-Dimethylphenyl)-2,5-di(methylthio)-1-
4
phenyl - 6aꢀ - thia-1,3,4,6-tetraazapentalene
(7c).
m.p. 187◦C (59% yield). 1H NMR δ 2.10 (s, 6H), 2.15,
2.52 (2s, 6H), 6.98 (m, 3H), 7.45 (m, 5H). MS calcd.
for C19H20N4S3 m/z: 400.0850 [M]+. , found 400.0850;
m/z (rel. int.): 400 (16), 353 (100), 327 (10), 280 (14),
248 (27). Anal. calcd. C, 57.00; H, 5.00; N, 14.00; S,
24.00. Found: C, 56.92; H, 4.98; N, 14.05; S, 24.23.
Reactions of Thiadiazolium Salts 3 with
Nitrogen Nucleophiles
6-tert-Butyl-2,5-di(methylthio)-1-phenyl-6aꢀ4-thia-
1,3,4,6-tetraazapentalene (7d). m.p. 135◦C (65%
yield). H NMR δ 1.47 (s, 9H), 2.19, 2.52 (2s, 6H),
1
In a general procedure, salt 3 (5 mmol) was added to
a solution of amino compound and triethylamine in
THF (50 ml). Quantities of the amino nucleophile
were the following: 1.47 g, 5 mmol for S-methyl-
N-phenylisothiourea hydroiodide; 0.84 g, 5 mmol
for S-methyl-N-isopropylisothiourea hydrochloride
5a; 0.3 g, 5 mmol for acetamide; 0.54 g, 5 mmol
for p-toluidine; 0.27 g, 2.5 mmol for 1,4- and 1,2-
phenylenediamines. Three, 2, or 1 equiv. (0.5 g) of
triethylamine were used as specified in Scheme 4.
The suspension was stirred at room temperature for
2 h. A mixture of triethylammonium chloride and
5-iminothiadiazole hydrochlorides 10,13 was col-
lected by filtration and slightly washed with H2O
to remove the ammonium salt. In other cases, con-
centration of the filtrate after elimination of the tri-
ethylammonium chloride gave a residue that was
purified by recrystallization from methanol (7a,c,d)
or flash chromatography on silica gel using di-
ethyl ether/petroleum ether (2:1) as eluent (7b, 9).
Selected 13C NMR data for compounds 7a,c,d and
9: see Table 1.
7.40 (m, 5H). MS calcd. for C15H20N4S3 m/z: 352.0850
[M]+. , found 352.0851; m/z (rel. int.): 352 (2), 305
(38), 249 (100). Anal. calcd. C, 51.12; H, 5.68; N,
15.90; S, 27.27. Found: C, 51.06; H, 5.73; N, 15.99;
S, 27.01.
6-Isopropyl-2-methyl-5-(methylthio)-1-oxa-6aꢀ4-
thia-3,4,6-triazapentalene (9). m.p. 112◦C (pen-
tane/diethyl ether) (56% yield). IR 1635, 1530, 1502
cm−1. 1H NMR δ 1.52 (d, 6H, J = 6.6 Hz), 2.41, 2.80
(2s, 6H), 4.40 (m, 1H). MS calcd. for C8H13N3OS2
m/z: 231.0500 [M]+. , found 231.0496; m/z (rel. int.):
231 (60), 216 (100), 174 (40), 147 (60). Anal. calcd.
C, 41.56; H, 5.63; N, 18.18. Found: C, 41.31; H, 5.49;
N, 17.96.
2-Isopropyl-5-[(4-methylphenyl)imino]-3-(methyl-
thio)-2,5-dihydro-1,2,4-thiadiazole
Hydrochloride
(10a). m.p. 205–207◦C (CH2Cl2/diethyl ether) (70%
yield). 1H NMR δ 1.55 (d, 6H, J = 6.6 Hz), 2.35, 2.86
(2s, 6H), 4.43 (m, 1H), 7.18, 7.62 (2d, 4H, J = 8.3
Hz), 13.00 (l, NH). 13C NMR δ 16.3 (q, 1 J = 144
6-Isopropyl-2,5-di(methylthio)-1-phenyl-6aꢀ4-thia-
1,3,4,6-tetraazapentalene (7a). m.p. 107◦C (69%
yield). 1H NMR δ 1.31 (d, 6H, J = 6.6 Hz), 2.19, 2.54
(2s, 6H), 3.82 (m, 1H), 7.45 (m, 5H). MS calcd. for
C14H18N4S3 m/z: 338.0694 [M]+. , found 338.0694; m/z
(rel. int.): 338 (6), 291 (76), 265 (11), 249 (100). Anal.
calcd. C, 49.70; H, 5.32; N, 16.56; S, 28.40. Found:
C, 49.50; H, 5.33; N, 16.78; S, 28.69.
1
3
Hz), 21.0 (qt, J = 127 Hz, J = 4.3 Hz), 22.7 (qm,
1 J = 129 Hz), 53.2 (dm, J = 141 Hz), 120.3, 129.6,
1
135.1, 135.5 (arom. C), 169.3 (m, C3), 170.7 (s, C5+).
MS calcd. for C13H17N3S2 m/z: 279.0864 [M − HCl]. ,
found 279.0867; m/z (rel. int.): 279 (100), 237 (60),
190 (20), 164 (65). Anal. calcd. for C13H18ClN3S2: C,
49.45; H, 5.71; N, 13.31; S, 20.29. Found: C, 49.39;
H, 5.88; N, 13.35; S, 20.33.
1,6-Diisopropyl-2,5-di(methylthio)-6aꢀ4-thia-1,3,
4,6-tetraazapentalene (7b). Orange viscous oil (75%
2-tert-Butyl-5-[(4-methylphenyl)imino]-3-(methyl-
thio)-2,5-dihydro-1,2,4-thiadiazole
Hydrochloride
1
yield). H NMR δ 1.44 (d, 12H, J = 6.6 Hz), 2.70
(10d). m.p. 184–186◦C (CH2Cl2/diethyl ether) (76%
(s, 6H), 4.10 (m, 2H); at −95◦C (CD2Cl2) δ 1.40 (d),
yield). H NMR δ 1.77 (s, 9H), 2.32, 2.84 (2s, 6H),
1
1
2.59 (s), 4.02 (m). 13C NMR δ 12.6 (q, J = 143 Hz),
7.15, 7.60 (2d, 4H, J = 8.2 Hz). 13C NMR δ 18.1

Precursors to a Variety of 6aꢀ4-Thiapentalene Systems 103
(q, 1 J = 144 Hz), 21.0 (qt, 1 J = 127 Hz, 3 J = 4.3
solvents. It was washed with boiling acetone. The
procedure for the purification of 14b was the same
as described for products 11. The two isomers were
separated by a column chromatography on silica-gel,
using CH2Cl2/diethyl ether (10:1) as eluent or by a
selective dissolution in acetone [isomer (I) insoluble,
isomer (II) soluble].
1
Hz), 24.9 (qm, J = 129 Hz), 62.8 (m), 120.4, 129.6,
135.2, 135.3 (arom. C), 166.7 (q, 3 J = 4.7 Hz, C3),
169.2 (s, C5). MS calcd. for C14H19N3S2 m/z: 293.1020
[M − HCl]+. , found 293.1025; m/z (rel. int.): 293 (19),
237 (100), 190 (17).
N,Nꢀ-Bis[2-isopropyl-3-(methylthio)-2,5-dihydro-
1,2,4-thiadiazol-5-ylidene]-benzene-1,4-diamine Di-
hydrochloride (13a). m.p. 278–282◦C (CH2Cl2/
A CDCl3 solution of pure sample 14b(I) or
14b(II) was maintained at room temperature for 48
h to generate the two starting conformers 14b in
addition with phenyl isothiocyanate and monoelim-
ination product 15b [14b(I)/14b(II)/15b ≈ 18:42:40
1
diethyl ether) (76% yield). H NMR δ 1.57 (d, 12H,
J = 6.6 Hz), 2.90 (s, 6H), 4.46 (m, 2H), 7.80 (s, 4H).
13C NMR δ 16.3 (q, 1 J = 145 Hz), 22.8 (qm, 1 J = 129
Hz), 53.4 (dm, 1 J = 142 Hz), 121.0 (d, 1 J = 165 Hz),
135.0 (m), 169.3 (m, C3), 170.9 (s, C5). MS calcd. for
C18H25N6S4 m/z: 453.1024 [M − HCl − Cl]+, found
453.1013. Anal. calcd. for C18H26Cl2N6S4: C, 41.14;
H, 4.95; N, 16.00. Found: C, 41.09; H, 4.96; N, 15.92.
1
by H NMR estimation]. Compound 15b was only
identified in this mixture by NMR spectroscopies [1H
NMR δ 1.26, 1.31 (2d, 6H, J = 6.5 Hz), 1.42, 1.43 (2d,
6H, J = 6.6 Hz), 2.33, 2.70 (2s, 6H), 4.00, 4.25 (2m,
2H), 7.25–7.55 (m, 9H); selected 13C NMR values: δ
131.8 (l), 140.3 (t, 3 J = 8.5 Hz), 150.0 (l), 162.5 (m),
164.7 (s), 167.3 (s), 168.3 (m), 172.3 (s)].
N,Nꢀ-Bis[2-isopropyl-3-(methylthio)-2,5-dihydro-
1,2,4-thiadiazol-5-ylidene]-benzene-1,2-diamine Di-
hydrochloride (13b). m.p. 210–212◦C (CH2Cl2/
diethyl ether) (77% yield). IR 3360, 1583, 1550 cm−1.
1H NMR δ 1.54 (d, 12H, J = 6.6 Hz), 2.69 (s, 6H),
4.40 (m, 2H), 7.45 (m, 4H), 12.00 (l, NH). MS calcd.
for C18H25N6S4 m/z: 453.1024 [M − HCl − Cl]+, found
453.1008; calcd. for C18H25N6S3 m/z: 421.1303 [M
− HCl − Cl − S]+, found 421.1324. Anal. calcd. for
C18H26Cl2N6S4: C, 41.14; H, 4.95; N, 16.00. Found: C,
41.37; H, 4.89; N, 16.29.
2,3-Dihydro-6-isopropyl-3-(4-methylphenyl)-5-
(methylthio)-1,6aꢀ4-dithia-3,4,6-triazapentalene-2-thi-
one (11a). m.p. 136–138◦C (MeOH) (90% yield). IR
1530, 1040 cm−1. 1H NMR δ 1.44 (d, 6H, J = 6.5 Hz),
2.13, 2.42 (2s, 6H), 3.96 (m, 1H), 7.13, 7.32 (2d, 4H,
J = 8.3 Hz). Anal. calcd. for C14H17N3S4: C, 47.32;
H, 4.79; N, 11.83; S, 36.06. Found: C, 47.44; H, 4.86;
N, 11.72; S, 35.89.
2,3-Dihydro-6-isopropyl-3-(4-methylphenyl)-5-
(methylthio)-1-phenyl-6aꢀ4-thia-1,3,4,6-tetraazapen-
talene-2-thione (11b). m.p. 168–170◦C (MeOH)
Addition of Heterocumulenes to
5-(Arylimino)-2,5-Dihydro-1,2,4-Thiadiazole
Hydrochlorides 10, 13
1
(87% yield). H NMR δ 1.44 (d, 6H, J = 6.6 Hz),
2.37, 2.44 (2s, 6H), 4.05 (m, 1H), 7.25–7.55 (m, 9H).
MS calcd. for C20H22N4S3 m/z: 414.1007 [M]+. , found
414.0983; m/z (rel. int.): 414 (1), 279 (80), 237 (50),
135 (100). Anal. calcd. C, 57.97; H, 5.31; N, 13.53; S,
23.19. Found: C, 58.27; H, 5.47; N, 13.63; S, 23.21.
Salt 10a,d (2.5 mmol) was added to a solution of
triethylamine (0.3 g, 3 mmol) and carbon disulfide
(1 ml) or appropriate iso-, isothio-, or isoseleno-
cyanate (2.7 mmol) in anhydrous CH2Cl2 (35 ml).
The mixture was stirred at room temperature for 1 h
and concentrated in vacuo. Trituration of the residue
with methanol allowed the filtration of the polyheter-
apentalenes 11a–e. Compound 11b was quantita-
tively alkylated by iodomethane in CH2Cl2 for 3 h
at room temperature. After removal of the solvent,
the crude methiodide 12b was recrystallized from
CH2Cl2/diethyl ether.
In a similar way, the synthesis of the phenylene-
linked bis(thiatetraazapentalene) derivatives 14a,b
was performed from a mixture of dihydrochloride
13a,b (0.8 g, 1.5 mmol), phenyl isothiocyanate (0.46
g, 3.4 mmol), and triethylamine (0.4 g, 4 mmol) in
CH2Cl2 (40 ml). Compound 14a rapidly precipitated
(20 min) from the reaction medium as stable and
colorless solid, which is quite insoluble in all usual
2,3-Dihydro-1,6-diisopropyl-3-(4-methylphenyl)-
5-(methylthio)-6aꢀ4-thia-1,3,4,6-tetraazapentalene-2-
selone (11c). m.p. 176◦C (MeOH) (93% yield).1H
NMR δ 1.46, 1.51 (2d, 12H, J = 6.6 Hz), 2.30,
2.43 (2s, 6H), 4.00, 5.10 (2m, 2H), 7.25, 7.30 (2d,
4H, J = 8.6 Hz). MS calcd. for C17H24N4S2Se m/z:
428.0608 [M]+. , found 428.0593; m/z (rel. int.): 428
(20), 278 (100), 236 (45), 190 (10). Anal. calcd. C,
47.78; H, 5.62; N, 13.11; S, 14.99. Found: C, 47.53;
H, 5.61; N, 13.13; S, 14.97.
6-tert-Butyl-2,3-dihydro-3-(4-methylphenyl)-5-
(methylthio)-1,6aλ4-dithia-3,4,6-triazapentalene-2-
1
thione (11d). m.p. 151◦C (MeCN) (90% yield). H
NMR δ 1.57 (s, 9H), 2.10, 2.44 (2s, 6H), 7.12, 7.32

104 Morel et al.
(2d, 4H, J = 8.4 Hz). Anal. calcd. for C15H19N3S4: C,
48.78; H, 5.15; N, 11.38; S, 34.68. Found: C, 49.09;
H, 5.27; N, 11.61; S, 34.25.
3-Cyano-6-isopropyl-2-methoxy-5-(methylthio)-
1-oxa-6aꢀ4-thia-4,6-diazapentalene
(16a). m.p.
173◦C (MeOH) (59% yield). IR 2205, 1620 cm−1.
1H NMR δ 1.50 (d, 6H, J = 6.6 Hz), 2.82, 3.85 (2s,
6H), 4.40 (m, 1H). MS calcd. for C10H13N3O2S2 m/z:
271.0449 [M]+. , found 271.0446; m/z (rel. int.): 271
(63), 229 (100), 197 (71). Anal. calcd. C, 44.28; H,
4.79; N, 15.49; S, 23.61. Found: C, 44.34; H, 4.79; N,
15.30; S, 23.31.
6-tert-Butyl-2,3-dihydro-3-(4-methylphenyl)-5-
(methylthio)-6aꢀ4-thia-1,3,4,6-tetraazapentalene-2-
one (11e). m.p. 182◦C (MeOH) (87% yield). IR
1685, 1590, 1530 cm−1. 1H NMR δ 1.62 (s, 9H), 2.37,
2.40 (2s, 6H), 7.10–7.70 (m, 9H). Anal. calcd. for
C21H24N4OS2: C, 61.16; H, 5.82; N, 13.59; S, 15.53.
Found: C, 61.18; H, 5.91; N, 13.85; S, 15.78.
6-Isopropyl-2-methoxy-3-(methoxycarbonyl)-5-
(methylthio)-1- oxa- 6aꢀ4 -thia-4,6-diazapentalene
(16b). m.p. 136◦C (MeOH) (42% yield). IR 1685,
1585 cm−1. 1H NMR δ 1.52 (d, 6H, J = 6.6 Hz),
2.87, 3.86, 3.90 (3s, 9H), 4.35 (m,+1H). MS calcd. for
C11H16N2O4S2 m/z: 304.0551 [M]. , found 304.0524;
m/z (rel. int.): 304 (100), 273 (19), 262 (23), 231 (82),
230 (43), 204 (31). Anal. calcd. C, 43.42; H, 5.26; N,
9.21. Found: C, 43.61; H, 5.28; N, 9.13.
6-Isopropyl-3-(4-methylphenyl)-2,5-di(methyl-
thio)-1-phenyl-6aꢀ4-thia-1,3,4,6-tetraaza-1H,3H,6H-
pentalenium Iodide (12b). m.p. 173–175◦C. 1H NMR
δ 1.58 (d, 6H, J = 6.7 Hz), 2.06, 2.48, 2.51 (3s, 9H),
4.35 (m, 1H), 7.38–7.65 (m, 9H). MS calcd. for C21
H25N4S3 m/z: 429.1241 [M − I]+, found 429.1237.
1,4-Bis[2,3-dihydro-6-isopropyl-5-(methylthio)-1-
phenyl-6aꢀ4-thia-1,3,4,6-tetraaza-2-thioxopentalene-
3-yl]-benzene (14a). m.p. 226–228◦C (98% yield). 1H
NMR (CDCl3/CF3CO2H) δ 1.63 (d, 12H, J = 6.6 Hz),
2.41 (s, 6H), 4.36 (m, 2H), 7.15 (m, 4H), 7.35 (m,
6H), 7.99 (s, 4H). Anal. calcd. for C32H34N8S6: C,
53.19; H, 4.71; N, 15.51. Found: C, 53.44; H, 4.69; N,
15.20.
6-tert-Butyl-3-cyano-2-methoxy-5-(methylthio)-1-
oxa-6aꢀ4-thia-4,6-diazapentalene (16d). m.p. 240◦C
(MeOH/CH2Cl2) (58% yield). IR 2200, 1630, 1495
cm−1. 1H NMR δ 1.74 (s, 9H), 2.82, 3.85 (2s, 6H). MS
calcd. for C11H15N3O2S2 m/z: 285.0606 [M]+. , found
285.0609; m/z (rel. int.): 285 (12), 229 (74), 197 (22),
105 (17), 57 (100). Anal. calcd. C, 46.31; H, 5.26; N,
14.73; S, 22.45. Found: C, 46.43; H, 5.23; N, 14.72;
S, 22.20.
1,2-Bis[2,3-dihydro-6-isopropyl-5-(methylthio)-1-
phenyl-6aꢀ4-thia-1,3,4,6-tetraaza-2-thioxopentalene-
3-yl]-benzene (80% yield) 14b(I). m.p. 159–161◦C
(EtOH). 1H NMR δ 1.27, 1.42 (2d, 12H, J = 6.6 Hz),
2.37 (s, 6H), 3.96 (m, 2H), 7.20–7.45 (m, 10H), 7.62
(m, 2H), 7.95 (m, 2H). MS calcd. for C32H34N8S6 m/z:
722.1231 [M]+. , found 722.1233; m/z (rel. int.): 722
(80), 588 (50), 453 (100), 419 (20), 291 (30). Oily
crude 14b(II): 1H NMR δ 1.47, 1.48 (2d, 12H, J = 6.6
Hz), 2.50 (s, 6H), 4.05 (m, 2H), 7.20–7.40 (m, 10H),
7.66 (m, 2H), 7.85 (m, 2H). MS found 722.1221; the
same fragments as described for 14b(I).
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Precursors to a Variety of 6aꢀ4-Thiapentalene Systems 105
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