ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The 1,6-methano[11]annulenyl (1·), 1,6:8, 14-propane-1,3-diylidene[15]annulenyl (2·), benzotropyl (3·) and 2,3-naphthotropyl (4·) radicals have been characterized by their ESR. spectra. The corresponding radical dianions, , , and , have also been studied both by ESR. and ENDOR. spectroscopy. Assignment of the coupling constants aHμ to protons in the individual positions μ of these radicals and radical dianions is to a large extent based on investigations of specifically deuteriated derivatives. The radicals 1·, 2·, 3· and 4· exist in temperature-dependent equilibria with (1)2, (2)2, (3)2 and (4)2, respectively, where (1)2 to (4)2 denote mixtures of dimers of 1· to 4·. The dissociation enthalpies, ΔH°, of (1)2 (102 kJ/mol) and (2)2 (88 kJ/mol) are considerably smaller than those of (3)2 and (4)2 which do not significantly differ from the ΔH° value of bitropyl (139 ± 6 kJ/mol). This finding indicates that the gain in π-electron delocalization energies, Δ(DE)π, upon dissociation markedly increases on going from bitropyl, (3)2 and (4)2 to (1)2 and (2)2, and thus points to an additional ‘resonance stabilization’ of 1· and 2·, as compared with 3· and 4·. A more pronounced π-spin localization on the 7-membered ring is observed in 3·, 4·, and relative to the corresponding species, 1·, 2·, and . It can be interpreted in terms of simple π-perimeter models without explicitly invoking substantial homoconjugative interactions between the bridged centres in 1·, 2·, and . However, the shortcomings of these crude models do not allow one to make a clear-cut statement about the contributions of the homotropyl structures to the π-systems of these paramagnetic species. The radical dianions and exhibit considerable hyperfine splittings from one 23Na or 39K nucleus of the counter-ion, whereas for and such splittings stem from two equivalent alkali metal nuclei. This finding is readily rationalized by different geometries of the bridged annulenyls and their benzo- and naphthotropyl analogues. Hyperfine data are also given for the radical anions of 7 H-benzocycloheptene, (3-H)\documentclass{article}\pagestyle{empty}\begin{document}$2^{\ominus \atop \dot{}}$\end{document}, and 6 H-(2,3-naphtho)cycloheptene, (4-H)\documentclass{article}\pagestyle{empty}\begin{document}$2^{\ominus \atop \dot{}}$\end{document}, as well as for the radical dianion of 1,6:8,14-bismethano[15]annulenyl,5\documentclass{article}\pagestyle{empty}\begin{document}$2^{\ominus \atop \dot{}}$\end{document}.
Additional Material:
7 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19810640838
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