REARRANGEMENT OF THE RADICAL-ANIONS OF 1,6-BRIDGED[10]ANNULENES TO DERIVATIVES OF 5H-BENZOCYCLOHEPTENE AND BENZOTROPYLIUM

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 1documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} relative to 12documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} is rationalized by the assumption that the first and rate determining step in the case of 1documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} is the valence isomerization to the radical anion of tricyclo[4.4.1.01,6]undeca‐2,4,7,9‐tetraene (1a). In the reducing medium used in such reactions (potassium in 1,2‐dimethoxyethane), the final paramagnetic product of 1documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} is not 5H‐benzocycloheptene (15), but the benzotropylium radical dianion ( (Formula Presented.) ). This product ( (Formula Presented.) ) is also obtained from the radical anions of the halogen‐substituted 1,6‐methano[10]annulenes, 2 to 5, in the same medium. The temperatures required for the conversion of 2documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} and 3documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} into (Formula Presented.) lie above 293 and 243 K, respectively, whereas the short‐lived species 4documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} and 5documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} undergo such a rearrangement already at 163 K. The stability of the four halogen‐substituted radical anions thus decreases in the sequence 2documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} > 3documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} > 4documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} ≈ 5documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document}. Replacement of 2documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} and 3documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} by 2‐D4documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} and 3‐D4documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document}, respectively, leads to 1,4,5,8‐tetradeuteriobenzotropylium radical dianion ( (Formula Presented.) ). Experimental evidence and theoretical arguments indicate that the rearrangements in question are initiated by a loss of one (3documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} and 5documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document}) or two (2documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document} and 4documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document}) halogen atoms. Such a reaction step must involve the intermediacy of the radical 19 · (see below) which rapidly isomerizes to the benzotropylium radical 16:. Support for the transient existence of 19. is provided by the thermolysis of 1,6‐methano [10]annulene‐11‐t‐butylperoxyester (6) which yields 16. in a temperature dependent equilibrium with a mixture of its dimers (162). (Formula Presented.) In the hitherto unreported ESR. spectra of 2documentclass{article}pagestyle{empty}begin{document}$ 1^{ominus atop dot{}} $end{document}. and 3documentclass{article}pagestyle{empty}begin{document}$ 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. Copyright © 1979 Verlag GmbH & Co. KGaA, Weinheim