Intermediacy of cyclobutylidene in photochemical methylenecyclopropane rearrangement

Methylenecyclopropane 1 undergoes photochemical rearrangement to 2. Intervention of cyclobutylidene 3 explains not only the rearrangement but also newly obtained products such as cyclobutene 4 and cyclobutylidenecyclobutane 6. Experiments designed to generate cyclobutylidene 3 independently have provided some support for the intermediacy of 3.