PHOTOSENSITIZED OXIDATION OF 1,1'-BI(BENZOCYCLOBUTENYLIDENE)

In an attempt to explore whether singlet molecular oxygen reactions co uld be used to obtain access to antiaromatic species, we carried out t he Rose Bengal photosensitized oxygenation of (E)- and (Z)-1,1'-bi(ben zocyclobutenylidene) (7-E and 7-Z). The photooxidation resulted in a f acile E/Z isomerization accompanied by the sluggish uptake of oxygen a nd formation of benzocyclobutenone 8, spiroindanone 9, phthalide 10, a nd dione 11. We found no evidence, however, for the generation of the corresponding benzocyclobutadiene via a O-1(2)-ene reaction. On the co ntrary, the data suggests that the isomerization of the diaryl olefin can occur in the absence of sensitizer, while product formation is a f ree radical autoxidative process, presumably initiated by the interact ion of excited dye with molecular oxygen (''type IIc''). The formation of 8-11 may be rationalized in terms of secondary rearrangements (unz ipping and cyclization) of the initially formed polyperoxide or polype roxy radical. The absence of singlet oxygen ene product may reflect th e increased activation energy required to generate an antiaromatic pro duct. Alternatively, the inability of the ring methylene hydrogens to attain a pseudoaxial conformation may actually be the controlling fact or in inhibiting the ene process.