Abstract
The radical anions of 6a‐thiathiophthenes ([1,2]dithiolo[1,5‐b] [1,2]dithioles), I(R), convert into those of 4__H__‐thiapyran‐4‐thiones, III(R), via cis‐trans isomerization. The reaction is slowed down when the size of the substituent R in the 2,5‐positions of 6a‐thiathiophthene increases, and it is prevented by the introduction of a 3,4‐polymethylene bridge. The primary and the secondary radical anions, I(R)\documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document} and III(R)\documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document}, respectively, exhibit very similar hyperfine splitting patterns. E.g., in the case of the unsubstituted 6a‐thiathiophthene, I(H), and 4__H__‐thiapyran‐4‐thione, III(H), the proton coupling constants are a~H2,5~=6.72 and a~H3,4~=1.73 Gauss for I(H)\documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document}, and a~H2,6~=6.35 and a~H3,5~=2.07 Gauss for III(H)\documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document}. In contrast to I(H)\documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document}, cis‐trans isomerization could not thus far be proved to occur with its 1,6‐dioxa‐analogue, IV(H)\documentclass{article}\pagestyle{empty}\begin{document}$ ^{\ominus \atop \dot{}} $\end{document}, since no ESR. spectrum of the radical anion of 4__H__‐pyran‐4‐thione, V(H), was detected upon reduction of IV(H).