Bimolecular Formation of Radicals by Hydrogen Transfer, 10. On the Mechanism of Quinone Dehydrogenations

The mechanistic alternatives in the literature for quinone dehydrogenations of hydroaromatic to aromatic hydrocarbons are reviewed. In a kinetic study of the reaction of DDQ 16 with 9,10‐dihydroanthracene ( 13 ) or xanthene ( 17 ) small solvent effects and small rate differences between 13 and 17 (Table 1) but large kinetic isotope effects k H / k D were observed. In addition, intermediate 9‐hydroanthryl radical 20b and 9‐xanthyl radicals 19b were trapped by the addition of nitrosobenzene to the dehydrogenation reaction mixtures as shown by the ESR spectra of the spin adducts 22 and 23 . These results are better explained by an initial H atom transfer step (retrodisproportionation, RD) than by direct hydride transfer (DHT). Possible consequences for biochemical hydrogen transfer processes are addressed briefly.