Solid-state photocyclization of 2,4,6-triisopropyl-4′-(methoxycarbonyl)benzophenone. Evidence for a narrow reaction cavity and a photoenol diradical intermediate

The origin of the previously observed unusual photostability of 2,4,6-triisopropyl-4′-(methoxycarbonyl)-benzophenone (1-p-CO₂Me) in the solid state was investigated. 1-p-CO₂Me was found to photocyclize normally to produce the corresponding benzocyclobutenol 2-p-CO₂Me when its solid-state photolysis was carried out either (a) after thorough grinding, (b) after solid-solid mixing with 2,4,6-triisopropyl-4′-(ethoxycarbonyl)-benzophenone (1-p-CO₂Et), or (c) at elevated temperatures (an estimated energy barrier of 20 kcal/mol). Furthermore, when the photolysis was performed under more carefully deoxygenated conditions (closed argon atmosphere), formation of blue species that are persistent in the absence of oxygen was observed. On the basis of oxygen trapping and ESR experiments, the blue species are regarded as a mixture of a diradical intermediate DR and monoradicals derived thereof. The X-ray study of 1-p-CO₂Me had revealed that the distances between the carbonyl oxygen and the o-i-Pr methine hydrogens are within the critical limit for hydrogen abstraction to occur, but a small reaction cavity or the compact crystal packing around both of the o-i-Pr groups is interfering with the photocyclization. The present results are consistent with this X-ray crystal structure; i.e., the photochemical hydrogen abstraction of 1-p-CO₂Me to DR can take place, but DR reketonizes back to 1-p-CO₂Me under the usual photolysis conditions because there is a high topochemical barrier to cyclization leading to 2-p-CO₂Me.