Cycloaddition reactions of 2H-1-benzothietes and 1,3,5,7-tetrathio-s-indacene-2,6-dithiones

1 Introduction

2H-1-benzothietes represent very useful, versatile synthons for numerous syntheses of larger heterocyclic ring systems [1–6]. 2-Methylenecyclohexa-3,5-diene-1-thione, the open valence isomer 1′ of 2H-1-benzothiete (1), is a highly reactive 8π system, which adds to many double and triple bond systems in [8π + 2π] cycloaddition reactions (hetero-Diels–Alder reactions). Apart from CC, CN, CP double and triple bonds and CC, CN, CO, NN, NO, NS and PS double bonds, CS double bonds are interesting reaction partners [1–6]. The latter cycloaddition leads to 4H-1,3-benzodithiins [7, 8].

We report here on the reactions of 2H-1-benzothiete (1) and 2H,5H-benzo[1,2-b:4,5-b′]bisthiete (3) and 1,3,5,7-tetrathio-s-indacene-2,6-dithiones (2a–c) (Fig. 1). The combination of 1 and 2 can lead to spiro-compounds as 1:1- or 2:1-adducts. The bifunctional systems 2 and 3, on the other hand, could give 1:1-, 1:2-, 2:1-, 2:2-, 2:3- and higher adducts (AB oligomers).

Fig. 1:

Benzothietes 1 and 3 and C=S reaction partners 2.

2 Results and discussion

We prepared 2H-1-benzothiete (1) by flash vacuum pyrolysis (FVP) of 2-mercaptobenzyl alcohol [9, 10]. A large pyrolysis apparatus enables the preparation of several 100 g in a few hours. The 1,3,5,7-tetrathio-s-indacene-2,6-dithiones (benzo[1,2-d:4,5-d′]bis[1,3]dithiole-2,6-dithiones (2) were obtained according to literature protocols: 2a [11–13], 2b [14, 15], 2c [16, 17].

Heating of 1 to 110°C provokes the generation of the valence isomer 1′ with an opened four-membered ring. This 8π-system reacted with 2a; however, the virtual insolubility of 2a rendered the process unhelpful. Therefore, we studied this reaction in detail with the much better soluble alkoxy derivatives 2b and 2c. 1:1-Ratios of the starting compounds led to the spiro-compounds 4b (44%) and 4c (37%) and to diastereomeric pairs of the bis-spiro-compounds 5b/5b′ (32%) and 5c/5c′ (35%) (Scheme 1).

Scheme 1:

Reaction of 1 and 2b, c to monocycloadduct 4b, c and cis- and trans-bisadducts 5b, c/5b′, c′.

The [8π + 2π] cycloadditions were highly regioselective, so that 1,3-dithiin rings were formed. However, the twofold cycloadditions are not stereoselective. According to the ¹³C NMR spectra, the cis-products 5b, c and the trans-products 5b′, c′ were generated in a ratio of 1:1. Due to the symmetry of 2, all products are achiral. The pseudo-symmetry point groups of the products are indicated in Scheme 1 – in order to make the NMR characterization better understandable (Table 1).

The second reaction discussed here concerns 4,9-dithiatricyclo[6.2.0.0^3,6]deca-1,3(6),7-triene (2H-benzo[1,2-b:4,5-b′]bisthiete (3)) [18, 19], a highly reactive “bisdiene” system, which should be able to generate polymers/oligomers in the reaction with the bifunctional compounds 2 [20]. A diluted solution of 3 and 2b (ratio 1:1) reacted in boiling toluene to give a large amount of an insoluble solid and 15% of the 1:1 cycloadduct 6b (Scheme 2). When 2c was used, which has a higher solubility than 2b in toluene, and when the ratio of the two starting compounds 3 and 2c was 1:2, 36% of the 1:2 adduct 7c could be obtained. The major portion of the resulting products were again hardly soluble solids. The mass spectroscopic studies (FD spectra) plead for the formation of homo- and heterooligomers (Fig. 2).

Scheme 2:

Reaction of bisthiete 3 and the thiocarbonyl compounds 2b and 2c, respectively.

Fig. 2:

Oligomer series: 8 formed by oligomerization of 3; 9 formed by oligocondensation of 2c; 10 formed by oligocycloaddition of 3 and 2c.

The oligomers 8, derived from monomer 3, have been found earlier [19]. The generation of the series 9 (n = 2, m/z = 916; n = 3, m/z = 1342 and n = 4, m/z = 1768) was unexpected, because the condensation of 2 by elimination of S needs normally more drastic conditions [17, 21]. Members of the series 10 (AB oligomers) were detected up to m/z = 1802, which corresponds to a 2:3 adduct of 3 and 2b. The cycloadducts 8 and 9 have band structures; and 10 can be regarded as a twisted band.

The structure elucidation of 4b, c, 5b, c/5b′, c′, 6b and 7c was based on ¹H and ¹³C NMR measurements (Table 1) and on a crystal structure analysis of 4b. The most interesting ¹H NMR data concern the ring-CH₂ groups. Their δ values (in CDCl₃) are between 4.2 and 4.3 ppm for the four-membered rings of 1, 3 and 6b and between 3.85 and 3.95 ppm for the six-membered rings in 4b, 4c, 5b/5b′, 5c/5c′, 6b and 7c.

The most important ¹³C NMR data concern the C=S groups and the spiro-C atoms C(SR)₄. The sp²-C atoms of the thiocarbonyl functions have δ values of 211.1±0.1 ppm in 4b, 4c and 6b. The transformation into spiro-carbon atoms leads by a drastic up-field shift to δ values between 82.1 and 83.5 ppm for 4b, 4c, 5b/5b′, 5c/5c′, 6b and 7c.

A selection of X-ray data of 4b is compiled in Table 2. The 1,3-dithiole rings have an envelope conformation, whereas the 1,3-thirane rings adopt a twisted chair conformation.

Figure 3 depicts a Platon plot of 4b and Fig. 4 the unit cell of the crystals, which have 4b molecules in a herringbone arrangement with included toluene molecules.

Fig. 3:

A Platon plot of 4b.

Fig. 4:

Herring bone arrangement of the molecules and unit cell in crystalline 4b.

3 Conclusion

Benzothiete 1 and benzobisthiete 3 react in form of their 8π valence isomers with one or both CS double bonds of the tetrathia-s-indacenedithiones 2a–c. The resulting spiro-heterocycles 4 and 6 and the bis-spiro-heterocycles 5 and 7 were characterized by their ¹H and ¹³C NMR data and in the case of 4b by a crystal structure analysis. All observed [8π + 2π] cycloadditions are highly regioselective, so that 1,3-dithiin rings are generated, which bear two sulfur substituents on C–2. Accordingly, C–2 is a spiro-carbon atom with four sulfur groups. These compounds can be regarded as tetrathiocarbonic acid orthoesters. They are thermally stable and are cleaved by strong acids [22]. Moreover, hardly soluble oligomers were obtained, whose FD mass spectra suggest band structures.

4 Experimental section

NMR spectra were recorded on a Bruker AM 400 spectrometer operating at 400 MHz for ¹H and 100 MHz for ¹³C. FD MS (5 kV) and EI MS (70 eV) measurements were performed with a Finnigan MAT 95 spectrometer. Elemental analyses were determined in the microanalytical laboratory of the Chemistry Department of the University of Mainz.

The starting compounds were synthesized according to the following references: 1 [9, 10], 2a [11–13], 2b [14, 15], 2c [16, 17], 3 [19].