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A convenient synthesis of 5-methylthieno[2,3-d]-1,3-dithiole-2-thione and its tetrathiafulvalene (TTF)

E.V.K. Suresh Kumar,* J.D. Singh and H.B. Singh

Department of Chemistry, Indian Institute of Technology, Powai, Bombay 400 076, India


The synthesis of multi-sulfur p-donors and their charge transfer salts is of considerable current interest. pi-donors with thiophene-fused TTFs have shown some interesting redox properties . The synthetic routes to the thiophene-fused TTFs involve formation of the precursor thiones by thermolysis of the corresponding 1,2,3-thiadiazoles and the subsequent trapping of the formed 1,3-dipoles by carbon disulfide.

We describe here a convenient route to the synthesis of one such thione. This involves a thio-Claisen rearrangement of 4,5-bis(prop-2-ynylthio)-1,3-dithiole-2-thione (13) to give the novel thione, 5-methylthieno[2,3-d]-1,3-dithiole-2-thione (18) and subsequent coupling of the thione to give [TTF](19) in trace amounts.


Tetrathiafulvalene (TTF) based donors have yielded several organic superconductors. The S...S intermolecular interactions and C-H...anion interactions play a key role in determining the solid state properties of the superconductors [1]. Rovira et al. [2] and Engler et al. [3] have synthesised several isomeric tetrathiafulvalene based pi-donors with one peripheral sulfur atom and their radical cations. Some of the radical cations have shown metallic behaviour at low temperatures [2,4].

The synthetic routes for these type of donors involve formation of the precursor thiones by thermolysis of the corresponding 1,2,3-thiadiazoles and subsequent trapping of the formed 1,3-dipoles by carbon disulfide. The starting material, tosylhydrazone of tetrahydrothiophen-3-one (1) when treated with thionylchloride gave a mixture of 1,2,3-thiadiazole isomers 2, 3 and 4 [2]. Thermal decomposition of thiadiazoles 2 and 4 in the presence of carbon disulfide resulted in the formation of 1,3,dithiole-2-thiones 5, 6 and 7, 8 respectively (Scheme 1). It is interesting to note that the aromatic thiones 6 and 8 were obtained in very low yields.

Another method reported [3] for the synthesis of thiophene-fused thiones involves an oxo ester 9 which was cyclized to the iminium salt 10. Treating this iminium salt 10 with hydrogen sulfide gives the precursor thione 5 which can be converted to 6 by sulfur dehydrogenation at 180 oC. The other method reported by the same authors involve an oxoxanthate ester 11 which when cyclized with P4S10 in refluxing decalin afforded the precursor thione 5 as the major product along with smaller amounts of 6 (Scheme2).

We now report a more convenient route to the synthesis of one such thione using a different synthetic methodology.

Results and discussion

The precursor thione 5-methylthieno[2,3-d]1,3-dithiole-2-thione 18 was synthesised by treating 4,5-bis(prop-2-ynylthio)-1,3-dithiole-2-thione 13 with triphenylphosphine in refluxing benzene/toluene for 9 h and chromatographing the resulting reaction mixture on silica gel using light petroleum (bp 40-60 oC) as eluent. Initial fractions collected yielded unreacted triphenylphosphine, next three fractions of 200 ml each yielded 18 along with traces of its [TTF] (19) as identified by mass spectrometry . The final fractions contained triphenylphosphine sulfide (PPh3S) as identified by NMR and elemental analysis.(Scheme 3)

Analytical data for 18

Mp: 155 oC, 1H NMR (CDCl3) : 2.57 (s,3H, CH3), 6.75 (s,1H, CH3), 13C NMR (CDCl3) 214.57 (C=S), 137.98 (C=C), 146.11 (C-CH3), 118.13(CH), , 16.05 (CH3), Mass : 205 (M+, 100%), 344 (25%).

The thione 13 was synthesised by treating prop-2-ynyl bromide with Zn (DMIT)2- salt 12 in acetone at 0 oC for 1 h and eluting the reaction mixture on silica gel using light petroleum/ethyl acetate (95:5) as eluent. However, small amounts of monocyclised product were also obtained.

A probable mechanism for the formation of 18 involving an allenic intermediate 14 is proposed and is shown in Scheme 4. It is interesting to note that thione 18 was not formed on refluxing the thione 13 in benzene-toluene without using triphenyl phosphine, thus confirming the crucial role played by triphenyl phosphine in abstracting the sulfur atom from the intermediate14. The thione 18 then appears to undergo coupling reaction with PPh3 yielding trace amounts of the isomeric mixtures of [TTF] 19 (as identified by mass spectrometry). Recently, some reactions were reported where a single sulfur atom was abstracted from thiones by phosphorous reagents such as P(OEt)3 yielding rearranged thiones or TTFs [5].

More recently Zard et al. have reported an unexpected, non-radical rearrangement while studying the behaviour of S-prop-2-ynyl xanthates 20 [6]. They found that these substances undergo upon heating a thermal sigmatropic rearrangement to give the corresponding 5-allenyl xanthates 21 which appear to be in equilibrium with a bine 22 (Scheme 5).

In view of this result, we cannot rule out the initial formation of the intermediate 15 which subsequently undergoes thio-Claisen rearrangement and later sulfur abstraction by PPh3 and cyclization to give the thione 18 (Scheme 6).


  1. H. Yamochi, T. Komatsu, G. Saito, T. Mori, M. Kusunoki and K. Sakaguchi, J. Am. Chem. Soc., 1993, 115, 11319 and references cited therein.
  2. C. Rovira, J. Veciana, N. Santalo, J. Tarres, J. Cirujeda, E. Molins, J. Llorca and E. Espinosa, J. Org. Chem., 1994, 59, 3307.
  3. E.M. Engler, V.V. Patel, J.R. Andersen, R.R. Schumaker and A.A Fukushima, J. Am. Chem. Soc., 1978, 100, 3769.
  4. N. Santalo, J. Tarres, E. Espinosa, J. Llorca, E. Molins, J. Veciana, C. Rovira, M. Mays, S.Yang, D. O. Cowan, C. Garrigou-Lagrange, J. Amiell, P. Delhaes, Synth. Met., 1993, 55-57, 2050.
  5. (a) N. Robertson, A.E. Underhill, M.B. Hurtshouse, D.E. Hibbs and K.M. Abdul Malik, Tetrahedron Lett., 1995, 36, 7297; (b) X. Yang, T.B. Rauchfuss, S. Wilson J. Chem. Soc. J. Chem. Soc., Chem. Commun., 1990, 34.
  6. M.A Poelert and S. Z. Zard, Synlett, 1995, 325.