SOLID-STATE PHOTOCYCLIZATION OF ,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 ,4,6-t riisopropyl-4'-(methoxycarbonyl)benzophenone (1-p-CO2Me) in the solid state was investigated. 1-p-CO2Me was found to photocyclize normally t o 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'-(ethoxycarbony l)benzophenone (1-p-CO2Et), or (c) at elevated temperatures (an estima ted energy barrier of 20 kcal/mol). Furthermore, when the photolysis w as performed under more carefully deoxygenated conditions (closed argo n atmosphere), formation of blue species that are persistent in the ab sence 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 hyd rogen 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-r ay 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 u nder the usual photolysis conditions because there is a high topochemi cal barrier to cyclization leading to 2-p-CO2Me.