On the mechanism of formation of 1,4,5,8-tetrathianaphthalene (TTN) from dimercaptoisotrithione (dmit) derivatives

Article abstract of DOI:10.1016/S0040-4039(00)00816-9

The mechanism of formation of TTN (an important precursor to tetrathiafulvalene (TTF)) from 4,5-bis(benzoylthio)-1,3-dithiole-2-thione is discussed. Contrary to previous reports, ethylenetetrathiolate is not an intermediate in this synthesis, whereas 4,5-vinylenedithio-1,3-dithiole-2- thione is likely to be. The latter is prepared by a new synthesis and converted into TTN in one step. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00816-9

Tetrahedron Letters 41 (2000) 5207±5210
On the mechanism of formation of
1,4,5,8-tetrathianaphthalene (TTN) from
dimercaptoisotrithione (dmit) derivativesy
Raquel Andreu, Javier Garõn,* Jesus Orduna and Jose M. Royo
Â
Departamento de Quõmica Organica, ICMA, Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain
Â
Received 14 April 2000; accepted 19 May 2000
Abstract
The mechanism of formation of TTN (an important precursor to tetrathiafulvalene (TTF)) from 4,5-
bis(benzoylthio)-1,3-dithiole-2-thione is discussed. Contrary to previous reports, ethylenetetrathiolate is
not an intermediate in this synthesis, whereas 4,5-vinylenedithio-1,3-dithiole-2-thione is likely to be. The
latter is prepared by a new synthesis and converted into TTN in one step. # 2000 Elsevier Science Ltd. All
rights reserved.
Keywords: Diels±Alder reactions; dithioles; sulfoxides; sulfur heterocycles.
1,4,5,8-Tetrathianaphthalene (TTN), ®rst prepared by Cava and co-workers,1 is a compound
of current interest since its isomerization in a basic medium is reported to a€ord tetra-
thiafulvalene (TTF) in very good yield.2 7 Most syntheses of TTN start from 4,5-bis(benzoyl-
thio)-1,3-dithiole-2-thione 1,3,5 8 although the use of thiapendione 2 has occasionally been
reported3,7 (Scheme 1). In order to explain the formation of TTN in these reactions, ethylenetetra-
thiolate 3 is supposed to be generated, presumably by attack of excess sodium ethoxide on the
initially formed disodium salt of dmit 4 (Scheme 2). Nevertheless, the existence of 3 has been
challenged recently,9,10 at least in reactions which make use of thiapendione as starting material.
Scheme 1.
* Corresponding author. Fax: +34-976-761194; e-mail: jgarin@posta.unizar.es
y
Dedicated to Professor Jose Barluenga on the occasion of his 60th anniversary.
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00816-9

5208
In this communication we report that, despite previous reports, ethylenetetrathiolate 3 is not an
intermediate in the synthesis of TTN from 1 and suggest an alternative mechanism for this reaction.
Treatment of 1 with six equivalents of NaOEt in re¯uxing EtOH (4 hours), followed by reaction
with an excess (20 equivalents) of benzoyl chloride did not give the expected tetrakis(benzoylthio)-
ethylene. Instead, compound 1 was obtained in 60% yield (Scheme 2).{ This observation points to
the fact that dianion 4, which is generated quantitatively in the basic medium after a few minutes
at room temperature,15 reacts stepwise. It is likely that 4,5-vinylenedithio-1,3-dithiole-2-thione (7)
is ®rst formed from 4 and 1,2-dichloroethylene by a sequence of elimination and addition steps, as
®rst demonstrated by Truce on his papers on nucleophilic substitutions of vinyl halides by thiols.16,17
The fact that compound 7 is obtained in low yield from 4 and 1,2-dibromoethylene18 must not be
taken as evidence supporting the intermediacy of 3 in the present case,6 since the reaction in Ref.
18 was carried out in the absence of NaOEt, what precludes the same kind of mechanism and
points to an alternative addition±elimination sequence.19,20
Scheme 2.
To test this hypothesis, we decided to prepare 7. Although the reported syntheses of this com-
pound rely on the ®nal elimination of EtOH from ethoxyethylenedithio-1,3-dithiole-2-thione,21,22
we found a new, experimentally simpler method, which makes use of phenyl vinyl sulfoxide, a
synthetic equivalent of acetylene in Diels±Alder cycloadditions.23 Thus, reaction of trithione 612
with phenyl vinyl sulfoxide in re¯uxing xylene a€orded 7 in one step in 50% yield (Scheme 3).
Scheme 3.
Finally, reaction of 7 with NaOEt (5 equivalents) and cis-1,2-dichloroethylene (1.5 equivalents)
led to TTN in 85% yield (Scheme 4). It is noteworthy that this new synthesis of TTN is similar to
that originally reported by Cava1 but the reactants are at the right oxidation level, thus elim-
inating the extra oxidation step.
The analogous reaction with AcCl at ^80ꢀC gave a complex mixture in which 4,5-bis(acetylthio)-1,3-dithiole-2-
{
thione11 and a compound of formula C6S8 (HRMS, M+ (m/z = 327.7764), calcd: 327.7766) were identi®ed. Although
.
the exact structure of the latter is not known, its formation can be related to reactions of dmit with very reactive
halides, giving rise to oxidation products.12 Moreover, two di€erent structures have been proposed for C6S8 isomers
formed from oxidation reactions of dmit derivatives.13,14

5209
Scheme 4.
These observations demonstrate that tetraanion 3 is not an intermediate in the synthesis of
TTN from 1, as is often stated. On the other hand, 7 can be formed under the reaction conditions
and rapidly cleaved by additional base to dianion 9,24 which explains that 7 cannot be isolated
from the reaction of 1 with 1,2-dichloroethylene in the presence of excess NaOEt. A possible
mechanism for the conversion of 1 to TTN which accounts for the experimental facts is depicted
in Scheme 5: the ethynylthio group in 8 results from addition of one thiolate group of 4 to
chloroacetylene followed by anti elimination; subsequent intramolecular addition of the remaining
thiolate a€ords 7 and eventually, by a similar sequence starting from 9, TTN. An alternative
mechanism involving intermediates 10 and 11 (dashed arrows in Scheme 5) can be ruled out for
the following reasons:
(i) Intramolecular cyclization of 8 is likely to proceed faster than intermolecular attack to
chloroacetylene to a€ord 10.
(ii) Even if 10 is formed, dithiole ring-opening would a€ord salt 11(Z) which can only
cyclize to TTF (not to TTN), contrary to observations.
(iii) Possible isomerization of 11(Z) to 11(E), which could give rise to TTN, is unlikely since
it is generally believed that 11(E) is the intermediate in the conversion of TTN to
TTF2,6 and it is well known that (Z)-1,2-bis(alkylthio)ethylene-1,2-dithiolates are con-
®gurationally stable.25
Scheme 5.
To summarize, we have demonstrated that ethylenetetrathiolate 3 is not formed by basic cleavage
of 1. On the other hand, compound 7 has been converted into TTN and seems a likely inter-
mediate in the synthesis of the latter from 1. A possible mechanism for the overall sequence
1!TTN which explains all experimental facts is proposed.

5210
Acknowledgements
Financial support from DGESIC (MAT1999-1009-C02-02) is gratefully acknowledged.
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