Study on the preparation and properties of polycyclic compounds bearing extremely bent benzene rings.

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1.Preparation of |1.1|paracyclophane and theoretical study on its structure and energetics. The preparation of |1.1|paracyclophane (2) via photochemical isomerization of bis (Dewar benzene) compound (1) was accomplished for the first time on the substituted derivative (R=CO_2Me) last year. To explore the chemistry of extremely strained cyclophanes further, unsubstituted 1a was prepared and 2a was successfully generated from 1a via its photochemical isomerization. According to theoretical calculations the interatomic distance between the opposing bridgehead carbons is exceptionally short at 2.3-2.4A and, thus, strong transannular interactions between the two aromatic rings in 2 is suggested. The calculations also suggest that removal of two electrons from 2a leads to electronically stabilized dication 2a^<2+> in which electronic charge is extensively delocalized and the interatomic distance mentioned above further diminishes to 2.05A,an unusually short interatomic distance for non-bonded carbon atoms. Investigation on the kinetic stabilization of 2 and the generation of 2^<2+> is now underway.Study on photochemical reactivities of Dewar benzenes 1.4-bridged with functionalized four-carbon-chain. Interested in a possibility that bridged Dewar benzene-|4|paracyclophane system may function as a photochemically driven molccular switching device. Dewar benzene derivatives bearing electron-withdrawing groups on the side chain were prepared and their photochemical behaviors were studied. Compounds 3b and 4a underwent photochemically reversible transformation into 5b and 6a, respectively, which exhibited characteristic UV/V is absorption spectra extending to 500nm. The photochemical reactivities of 3a and 4b were strikingly different from those of 3b and 4a, and their isomerization into the |4|paracyclophane structures could not be detected.