Sequential deprotonation-dehydrogenation of dihydrogroups

The direct transfer of an electron to 02 is an unlikely process because of the extreme instability of the O species (Chapter 2). As a result, most of the reported electron-transfer oxidations by O2 - actually represent an initial proton abstraction to give substrate anion and HOO-, which disproportionates (or reacts with 02 ) to give O2 and HOOH (or HOO ). One or more of these species oxidizes the substrate anion. 

Superoxide ion is an effective hydrogen-atom oxidant for substrates with coupled hetero-atom (O or N) dihydrogroups such as 3,5-di-f-butylcatechol (DTBCH2), ascorbic acid (H2ASC), 1,2-disubstituted hydrazines, dihydrophenazine (H2Phen), and dihydrolumiflavin (H2FI).56-58 Table 7-3 summarizes the stoichiometric and kinetic data for the oxidation of these compounds by O2 -. 

0 DTBCH2, 3,5-di-fert-butylcatechol H2ASC, ascorbic acid H2Phen, dihydrophenazine H2FI, dihydrolumiflavin. 

The uv-vislble absorption spectra for the anion radical products were compared with those for the products from controlled potential electrolytic reduction of 3,5-di-tert-butyl-o-benzoquinone (Xmaxz 340 and 380 nm), dehydroascorblc acid (Xmax/ 360 nm), phenazine, lumiflavin (Xmax, 420 nm), and azobenzene (Xmax 410 nm). 

Pseudo-first-order rate constants, fci (normalized to unit substrate concentration IS]), were determined from current-collection-efficiency data (02 decay rates) with a glassy-carbon-glassy-carbon ring-disk electrode that was rotated at 900 rpm. (O2 was produced at the disk electrode from dissolved O2, which reacted with 2 mM substrate, and the unreacted O2 was determined by its oxidation at the ring electrode.) 

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