Eight 2-substituted and nine 3-substituted 1,6-methano[ lO]annulenes have been synthesized, and their magnetic circular dichroism has been measured. The B terms of the Lb transition depend on the ?r-electron effect of the substituent in a way which reveals the ordering of the four frontier molecular orbitals: a,, az, b,, and bz in the order of increasing energy. This result implies the presence of a strong transannular interaction between the bridgehead positions 1 and 6 on the bridged [ lolannulene ring.

Abstract:The synthesis and the UV-visible absorption, polarized fluorescence, and MCD spectra of six U-shaped "hairpin" polyenes (1-6) are reported. Qualitative arguments and results of T-electron calculations permit the identification of four excited singlet states and their assignment to mixtures of singly and doubly excited configurations. The hairpin polyenes represent a link between the all-trans polyenes on the one hand and the annulenes and acenes on the other hand: they possess the topology of the former and a geometry near that of the latter. The understanding of their electronic states offers a unified view of low-energy transitions in polyenes and aromatics, which accounts for their differences in a simple and intuitive manner.

Das aus Triphenylphosphoniumbromid und Na‐bis(trimethylsilyl)amid hergestellte Ylid (II) reagiert mit dem Cycloheptatrien (I) zum Divinylderivat (III).

The idea of transforming a classical arene, by “bridging,” into a [4n+2]annulene has been further extended to the phenanthrenes. The spectroscopic data and structural parameters for the title compound 1 reveal it to be a borderline case, with respect to its π‐electron structure, for an aromatic molecule.

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