Self-Initiated Autoxidation of a Sterically Crowded Cycloheptatriene Derivative via Norcaradienyloxyl Radicals

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An extremely crowded cycloheptatriene derivative, 1,3,5-tri- tert -butyl-7-(9-phenyl-9-fluorenyl)-1,3,5-cycloheptatriene ( 1 ), underwent autoxidation at 25 °C in cyclohexane to give 4- tert -butyl-2-pivaloylphenol ( 3 ) (33%), 5- tert -butyl-2-pivaloylphenol ( 4 ) (11%), 1,3,5-tri- tert -butylbenzene ( 5 ) (5%), tert -butyl 9-phenyl-9-fluorenyl peroxide ( 6 ) (44%), bis(9-phenyl-9-fluorenyl) peroxide ( 7 ) (7%), 1,1‘,3,3‘,5,5‘-hexa- tert -butyl-7,7‘-bicycloheptatriene ( 8 ) (2%), and carbon monoxide (4%). ESR studies showed that 1 dissociates into 1,3,5-tri- tert -butyltropyl radical ( 11 ) and 9-phenylfluorenyl radical ( 12 ) at 25−85 °C. The enthalpy and entropy of dissociation were determined to be 23.3 kcal/mol and 22.0 cal/mol·K, respectively. The formation of 3 − 5 can be explained by a mechanism involving attack of molecular oxygen to 11 and subsequent valence tautomerism of cycloheptatrienyloxyl radicals to norcaradienyloxyl radicals.