Synthesis of Pentathiepanes and Isolation of the Conformers Based on High Inversion Barrier of the Pentathiepane Ring

Abstract

Acenaphtho[1,2‐a]acenaphthylene (1) was sulfurated with elemental sulfur to give a pentathiepane derivative (2). A dynamic NMR spectrum analysis revealed that the two naphthalene rings of 2 are chemically nonequivalent up to 100 °C. The pentathiepane ring of 2 was shown to adopt a chair conformation both in solution and crystals. In accordance with the NMR spectrum analysis, the sulfuration of 5‐phenylacenaphtho[1,2‐a]acenaphthylene (12) gave a pair of conformers (16a and 16b), which were isolated in pure form. The Friedel–Crafts acetylation of 2, followed by dithioacetalization with 1,2‐ethanedithiol, also gave a pair of isolable conformers (18a and 18b). These two pairs of conformers isomerized, as a result of the ring inversion, to each other in solution at room temperature in the first‐order kinetics. The activation parameters for this process, Ea, ΔH#, and ΔS#, were determined. Based on the experimental observations, a probable mechanism for the inversion process is proposed.

Synthesis, structure, and ring inversion of a trithiolane (6b,12b-epitrithioacenaphtho[1,2-a]acenaphthylene)

“…The hydrogen signals of the two naphthalene rings of 7 commenced broadening and coalescing with a rise in temperature in the 1 H NMR spectra (Figure 4). This is in contrast to the results with the pentathiepane 2, in which the hydrogen signals neither broaden nor coalesce up to 100ЊC [19]. The activation energy of the ring inversion of 7 was investigated by dynamic 13 C NMR experiments.…”

“…Taking this into account, the synthesis of 7 was planned by deoxygenation of 6b,12b-epitrithioacenaphtho[1,2-a]acenaphthylene 2-oxide 8, which might be derived from 2. The previous study showed that the reduction of 2 with LiEt 3 BH led to the formation of the 1,2-dithiolate 9 [18,19]. Therefore, treatment of 2 with LiEt 3 BH followed by suc-cessive addition of Me 2 SnCl 2 and SOCl 2 was expected to give the trithiolane 2-oxide 8.…”

“…1 H NMR data at ‫04מ‬ЊC, which were assigned based on 1 H-1 H COSY correlation and coupling constant values [22] are summarized in Figure 1. Slight differences in the chemical shift values are observed between H 1 (d 7.86) and H 7 (7.87) and between H 3 (d 7.82) and H 9 (7.74) because of the influence of the ring sulfur atoms [7,12,19]. However, we cannot determine whether the trithiolane ring lies over the H 1 or H 7 side.…”

“…In general, trithiolanes are thermodynamically more stable than the corresponding pentathiepanes, which occasionally decompose to trithiolanes and SCHEME 1 SCHEME 2 sulfur when dissolved in polar solvents or on attempted purification by silica-gel column chromatography [7] or distillation [9]. In contrast to these results, the pentathiepane 2 was produced as the sole product for the sulfuration of acenaphtho[1,2a]acenaphthylene 1 with elemental sulfur in dimethylformamide (DMF) at 135ЊC [18,19]. The trithiolane 7 was never formed.…”

“…To our knowledge, however, conformational studies of 1,2,3-trithiolanes and 1,2,3,4,5-pentathiepanes in solution have never been reported [1,14], though those of benzopentathiepins were reported in some detail [15][16][17]. Recently, we reported the preparation and the conformational study of 6b,12b-epipentathioacenaphtho[1,2a]acenaphthylene 2 and the related compounds 3-6, which possess a [5.3.3] propellane structure (Scheme 1) [18,19]. The C S symmetry of 2 made its conformational analysis easier.…”