Semibullvalenes photorearrangement

Liu has found that 2,3-histrifluoromethylbarrelene undergoes sensitized photorearrangement to three semibullvalenes<43) ... 

Cyclooctatetraene (COT) —> Semibullvalene (SB) Photorearrangement. Irradiation of COT yields semibullvalene [97], in spite of the fact that this photochemical reaction is forbidden by orbital conservation mles. The Longuet-Higgins loop for this system actually predicts that this should happen, although the reaction is phase preserving. (Fig. 42). This is another example of type C loop (Fig. 11). Only six of the eight electrons re-pair as COT transforms to SB. The reaction is made possible by the fact that COT valence isomerization, a phase-inverting reaction (four electron-pair Hiickel system), takes place simultaneously. One expects to produce in the reaction a COT isomer, that can be detected solely by proper substitution. 

Circularly polarised 190 nm radiation has been used to study the direct photoracemisation of the enantiomers of trans cyclo-octene (Inoue et al.), and new evidence has been reported for the 90° twisted intermediate on the SI surface of stilbene (Gano et al.). Intramolecular charge-transfer excitation of 4-dimethyl-amino-4 -cyanostilbene and 4-azetidinyl-4 -cyanostilbene is considered to involve at most an intermediate with a lifetime of less than 1 ps (lUchev et al.), and a novel synthetic route to 5,6-dihydro-4H-l,2-oxazines (26) from y,5-unsaturated oximes has been described by Armesto et al. The photorearrangement of the ammonium salts of the dibenzobarrelene (27) to the semibullvalene isomers has... 

The barrelene (72a) undergoes efficient photorearrangement to the semibullvalene (73a). The reaction is best carried out in cyclohexane with 254 nm radiation. The yield of product (73a) is 94yo and the isomeric barrelene (72b) is... 

Other interesting fused thiophene derivatives include the photorearrangement of thio-phenetriptocene to form semibullvalene (96) <93CL68i>. Photochemically induced intramolecular reaction of thiophenones with alkenes in methanol produces novel tricyclic thiolane derivatives such as (97) <88HCA502>. 

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