Superoxide-Mediated Base-Catalyzed Autoxidation of Tetraphenylcyclopentadiene

The superoxide (KO₂/18-crown-6)-mediated oxidation of tetraphenylcyclopentadiene (1) in aprotic media generated a variety of products following aqueous workup, including 2-hydroxy-2,4,5-triphenylfuranone (3), 2,3,4,5-tetraphenylfuran (4), cis-dibenzoylstilbene (5), cis- and trans-1,2,4,5-tetraphenyl-2-pentene-1,5-dione (6 and 7), and the novel benzoylformic anhydride (8). Treatment of the acid fraction with diazomethane yielded methyl benzoate (9) as the major component. The mechanism of product formation involves a series of well-precedented, base-catalyzed autoxidative processes including deprotonation of 1 by O^●-, oxygenation, dehydration or cyclization of the resulting peroxy anion, and oxidative cleavage. Lactol 3, furan 4, and benzoic acid (the putative precursor to 9) are known oxidative cleavage products of tetracyclone (2), which was detected in trace amounts in the reaction mixture. Isomeric 2-ene-1,5-diones 6 and 7 each isomerize to give an equilibrium mixture of the two in a 8:5 ratio. Molecular-modeling studies confirm the greater stability of the cis-6 isomer over that of trans-7.