Rearrangement of the Radical Anions of 1,6‐Bridged[10]Annulenes to Derivatives of 5H‐Benzocycloheptene and Benzotropylium
Authors: Gerson Fabian , Huber Walter , Müllen Klaus 
Published: December 2004

The rearrangement products obtained upon reduction of 1,6‐methano[10]‐annulene (1) and its 11‐halogen derivatives have been studied by ESR. and, in part, by ENDOR. spectroscopy. These derivatives comprise 11,11‐difluoro‐ (2), 11‐fluoro‐ (3), 11,11‐dichloro‐ (4) and 11‐bromo‐1,6‐methano[10]annulene (5), as well as the 2,5,7,10‐tetradeuteriated compounds 2‐D4 and 3‐D4. The studies of the secondary products in question have been initiated by the finding that the radical anion of 11,11‐dimethyltricyclo[4.4.1.01,6]undeca‐2,4,7,9‐tetraene (12), i.e., the prevailing valence isomer of 11,11‐dimethyl‐1,6‐methano[10]annulene, undergoes above 163 K a rearrangement to the radical anion of 5,5‐dimethylbenzocycloheptene (14). A rearrangement of this kind also occurs for the radical anion of the parent compound 1, albeit only above 323 K. The lower reactivity of 1\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} relative to 12\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} is rationalized by the assumption that the first and rate determining step in the case of 1\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} is the valence isomerization to the radical anion of tricyclo[4.4.1.01,6]undeca‐2,4,7,9‐tetraene (1a). In the reducing medium used in such reactions (potassium in 1,2‐dimethoxyethane), the final paramagnetic product of 1\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} is not 5H‐benzocycloheptene (15), but the benzotropylium radical dianion (
equation image
). This product (
equation image
) is also obtained from the radical anions of the halogen‐substituted 1,6‐methano[10]annulenes, 2 to 5, in the same medium. The temperatures required for the conversion of 2\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} and 3\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} into
equation image
lie above 293 and 243 K, respectively, whereas the short‐lived species 4\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} and 5\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} undergo such a rearrangement already at 163 K. The stability of the four halogen‐substituted radical anions thus decreases in the sequence 2\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} > 3\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} > 4\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} ≈ 5\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document}. Replacement of 2\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} and 3\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} by 2‐D4\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} and 3‐D4\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document}, respectively, leads to 1,4,5,8‐tetradeuteriobenzotropylium radical dianion (
equation image
). Experimental evidence and theoretical arguments indicate that the rearrangements in question are initiated by a loss of one (3\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} and 5\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document}) or two (2\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} and 4\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document}) halogen atoms. Such a reaction step must involve the intermediacy of the radical 19 · (see below) which rapidly isomerizes to the benzotropylium radical 16:. Support for the transient existence of 19. is provided by the thermolysis of 1,6‐methano [10]annulene‐11‐t‐butylperoxyester (6) which yields 16. in a temperature dependent equilibrium with a mixture of its dimers (162).magnified image

In the hitherto unreported ESR. spectra of 2\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document}. and 3\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document}, the coupling constants of the ring protons differ considerably from the analogous values for the radical anions of other 1,6‐bridged [10]annulenes. These differences strongly suggest that the fluoro‐substitution substantially affects the character of the singly occupied orbital.