ALBERT

Notes: The dianions of phenanthrene and 1,2,3,4-dibenzocyclooctatetraene have been prepared by metal reduction of the neutral compounds and characterized via the study of their 1H- and 13C-NMR. spectra. A description of the charge distribution can be achieved which is consistent with both MO-models and the spin density distribution of the corresponding radical anions. Thus, the dianion of 1,2,3,4-dibenzocyclooctatetraene appears as a π-bond delocalized species having its excess charge mainly localized in the eight membered ring.

Notes: [2+2]-Cycloadditions and -cycloreversions in radical anions. An ESR. spectroscopic study for 2,2′-disubstituted diphenyl derivativesThe radical anions derived from the polycyclic olefins 1, 2 and 3 are shown by ESR. spectroscopy to undergo [π2+π2]-cycloaddition reactions even at low temperatures. Similarly, facile cleavage by [σ2+σ2]-cycloreversion processes is observed for the radical anions of the corresponding cyclobutane species. This reactivity, which is in marked contrast with the thermal stability of the neutral parent compounds, is discussed taking into account the molecular geometry and the spin density distribution.

Notes: The Photochemistry of Conjugated γ,δ-Epoxy-ene-carbonyl Compounds of the Ionone Series: UV.-Irradiation of α,β-Unsaturated ε-Oxo-γ,δ-epoxy Compounds and Investigation of the Mechanism of the Isomerization of Epoxy-enones to FuranesOn 1n, π*-excitation (λ ≥ 347 nm; pentane) of the enonechromophore of 3, three different reactions are induced: (E/Z)-isomerization to give 13 (7%), isomerization by cleavage of the C(γ)-C(δ) bond to yield the bicyclic ether 14 (36%) and isomerization by cleavage of the C(γ)—O bond to give the cyclopentanones 15 (13%) and 16 (11%; s. Scheme 2). On 1π, π*-excitation (λ = 254 nm; acetonitrile) 13 (14%), 15 (6%), and 16 (6%) are formed, but no 14 is detected. In contrast, isomerization by cleavage of the C(δ)—O bond to give the cyclopentanone 17 (23%) is observed. The reaction 3→17 appears to be the consequence of an energy transfer from the excited enone chromophore to the cyclohexanone chromophore, which then undergoes β-cleavage. Irradiation of 4 with light of λ = 254 nm (pentane) yields the analogous products 20 (18%), 21 (9%), 22 (7%), and 24 (7%; s. Scheme 2). Selective 1n, π*-excitation (λ ≥ 280 nm) of the cyclohexanone chromophore of 4 induces isomerization by cleavage of the C(δ)—O bond to give the cyclopentanones 23 (9%) and 24 (44%). Triplet-sensitization of 4 by excited acetophenone induces (E/Z)-isomerization to provide 20 (12%) and isomerization by cleavage of the C(δ)—O bond to yield 21 (26%) and 22 (20%), but no isomerization via cleavage of the C(δ)—O bond.It has been shown, that the presence of the ε;-keto group facilitates C(γ)—C(δ) bond cleavage to give a bicyclic ether 14, but hinders the epoxy-en-carbonyl compounds 3 and 4 from undergoing cycloeliminations.The activation parameters of the valence isomerization 13 → 18, a thermal process, have been determined in polar and non-polar solvents by analysing the 1H-NMR. signal intensities. The rearrangement proceeds faster in polar solvents, where the entropy of activation is about -20 e.u. Opening of the epoxide ring and formation fo the furan ring are probably concerted.

Notes: On the Charge Distribution in Ionic π-SystemsThe charge distribution in the anthracene dianion and dication is investigated by means of 13C-NMR. spectroscopy and compared with the spin density distribution in the corresponding radical ions. It appears that the excess charge in the dianion is distributed in a highly non-uniform manner and that, quite surprisingly, the quaternary carbon atoms bear positive charges. These findings can be rationalized by means of MO-models.

Notes: ESR. and ENDOR. spectra are reported for the radical anion of the 2-methyl derivative (III) of 1,6-methano[10]annulene (I). Comparison of the hyperfine data for I·⊖ and III·⊖ with those for the radical anions of naphthalene (II) and its 1-methyl derivative (IV), respectively, confirms the suggestion that - despite the vastly different α-proton coupling constants - the π-spin distributions should be similar in the two series. This result provides further support for the view that the π-spin distribution, in particular, and the cyclic π-delocalisation, in general, are not seriously perturbed by moderate deviations of the π-perimeter from planarity.

Notes: 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{}} $\end{document} is not 5H-benzocycloheptene (15), but the benzotropylium radical dianion (). This product () is also obtained from the radical anions of the halogen-substituted 1,6-methano[10]annulenes, 2 to 5, in the same medium. The temperatures required for the conversion of 2\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} and 3\documentclass{article}\pagestyle{empty}\begin{document}$ 1^{\ominus \atop \dot{}} $\end{document} into lie above 293 and 243 K, respectively, whereas the short-lived species 4\documentclass{article}\pagestyle{empty}\begin{document}$ 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{}} $\end{document}, respectively, leads to 1,4,5,8-tetradeuteriobenzotropylium radical dianion (). Experimental evidence and theoretical arguments indicate that the rearrangements in question are initiated by a loss of one (3\documentclass{article}\pagestyle{empty}\begin{document}$ 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{}} $\end{document}) halogen atoms. Such a reaction step must involve the intermediacy of the radical 19 · (see below) which rapidly isomerizes to the benzotropylium radical 16:. Support for the transient existence of 19. is provided by the thermolysis of 1,6-methano [10]annulene-11-t-butylperoxyester (6) which yields 16. in a temperature dependent equilibrium with a mixture of its dimers (162).In the hitherto unreported ESR. spectra of 2\documentclass{article}\pagestyle{empty}\begin{document}$ 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.

Notes: Metal reduction of pyrene and of pyrene isomers under carefully controlled conditions yields the corresponding dianions. The properties of the neutral compounds and of the dianions are studied via 1H-NMR. spectroscopy. Whereas the former exhibit similar behaviour, the charge distribution within the dianionic species proves to be very different. As a consequence, some of the polycyclic dianions can be regarded essentially as (4n)π-perimeters, an approach which is satisfactorily rationalized by simple MO-models. Further evidence in favour of this bonding scheme comes from the ESR. spectroscopic measurement of the spin density distribution within the corresponding radical anions.

Notes: Metal reduction of the polycycle 2 yields a surprisingly stable dianion with intact molecular framework. This conclusion can be drawn from reoxidation experiments and from an analysis of the 1H-NMR. spectrum. The excess charge turns out to be distributed over both the π- and σ-fragments of the molecule. The relative stabilities of ionic species derived from 1, 2 and 3 are considered.