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

In an attempt to explore whether singlet molecular oxygen reactions could be used to obtain access to antiaromatic species, we carried out the Rose Bengal photosensitized oxygenation of (E)- and (Z)-1,1'-bi(benzocyclobutenylidene) (7-E and 7-Z). The photooxidation resulted in a facile E/Z isomerization accompanied by the sluggish uptake of oxygen and formation of benzocyclobutenone 8, spiroindanone 9, phthalide 10, and dione 11. We found no evidence, however, for the generation of the corresponding benzocyclobutadiene via a O-1(2)-ene reaction. On the contrary, the data suggests that the isomerization of the diaryl olefin can occur in the absence of sensitizer, while product formation is a free radical autoxidative process, presumably initiated by the interaction of excited dye with molecular oxygen (''type IIc''). The formation of 8-11 may be rationalized in terms of secondary rearrangements (unzipping and cyclization) of the initially formed polyperoxide or polyperoxy radical. The absence of singlet oxygen ene product may reflect the increased activation energy required to generate an antiaromatic product. Alternatively, the inability of the ring methylene hydrogens to attain a pseudoaxial conformation may actually be the controlling factor in inhibiting the ene process.