Synthesis and photosensitized oxygenation of cyclopropylidenecyclobutenes

Cyclopropylidenecyclobutenes and -cyclobutanes were conveniently prepared using the Petasis titanocene approach. The cyclobutenes were unreactive to singlet oxygen, reacting sluggishly via a photoinitiated free radical autooxidative epoxidation process, to yield the corresponding spiroketones. By contrast, cyclopropylidenecyclobutanes react rapidly with 1O2, via an ‘ene’ process, initially generating a cyclopropyl hydroperoxide, which proceeds to products via Hock cleavage. The inertness of cyclopropylidenecyclobutenes to a 1O2 ‘ene’ reaction mode may be attributed to the fact that it would require the formation of the relatively high energy cyclobutadiene moiety.