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Rearrangement of the Radical Anions of 1,6‐Bridged[10]Annulenes to Derivatives of 5 H ‐Benzocycloheptene and Benzotropylium
Author(s) -
Gerson Fabian,
Huber Walter,
Müllen Klaus
Publication year - 1979
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19790620709
Subject(s) - annulene , chemistry , isomerization , valence (chemistry) , ion , stereochemistry , medicinal chemistry , organic chemistry , catalysis
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 ‐D 4 and 3 ‐D 4 . The studies of the secondary products in question have been initiated by the finding that the radical anion of 11,11‐dimethyltricyclo[4.4.1.0 1,6 ]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{}} $\end{document} is the valence isomerization to the radical anion of tricyclo[4.4.1.0 1,6 ]undeca‐2,4,7,9‐tetraene ( 1a ). In the reducing medium used in such reactions (potassium in 1,2‐dimethoxyethane), the final paramagnetic product of 1 \documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} is not 5 H ‐benzocycloheptene ( 15 ), but the benzotropylium radical dianion (). This product () is also obtained from the radical anions of the halogen‐substituted 1,6‐methano[10]annulenes, 2 to 5 , in the same medium. The temperatures required for the conversion of 2 \documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} and 3 \documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} intolie above 293 and 243 K, respectively, whereas the short‐lived species 4 \documentclass{article}\pagestyle{empty}\begin{document}$ 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{}} $\end{document} by 2 ‐D 4 \documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} and 3 ‐D 4 \documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} , respectively, leads to 1,4,5,8‐tetradeuteriobenzotropylium radical dianion (). Experimental evidence and theoretical arguments indicate that the rearrangements in question are initiated by a loss of one ( 3 \documentclass{article}\pagestyle{empty}\begin{document}$ 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{}} $\end{document} ) halogen atoms. Such a reaction step must involve the intermediacy of the radical 19 · (see below) which rapidly isomerizes to the benzotropylium radical 16 :. Support for the transient existence of 19 . is provided by the thermolysis of 1,6‐methano [10]annulene‐11‐t‐butylperoxyester (6) which yields 16 . in a temperature dependent equilibrium with a mixture of its dimers ( 16 2 ).In the hitherto unreported ESR. spectra of 2 \documentclass{article}\pagestyle{empty}\begin{document}$ 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.