Reactions of 10-methyl-9,10-dihydroacridine with inorganic oxidants
Reactions of 10-methyl-9,10-dihydroacridine with various inorganic oxidants in 20% acetonitrile/80% water solvent mixture fall into two categories depending on the strength of the oxidant used. Rapid electron transfer to the strong oxidizing reagents, Ce IV and IrCl62-, produces dihydroacridine radical cations, AcrH2·+ ([lambda] max 650 nm). This is followed by the rate-determining loss of a proton and rapid oxidation of thus formed AcrH· by the second equivalent of Ce IV or IrCl62-. The protonation of AcrH· and deprotonation of AcrH2·+ exhibit significant kinetic isotope effects. The use of d2-acridine in H2O/CH3CN yields the secondary and solvent isotope effects separately. A large normal secondary isotope effect of 1.86 ± 0.17 for the protonation of the acridinium radical suggests a possibility of nuclear tunneling. The reaction of dihydroacridine with a mild oxidizing reagent, Fe3+, is slow and shows no kinetic isotope effect. The initial electron transfer from AcrH2 to Fe3+ is believed to be rate-determining;The oxidation of AcrH2 to AcrH+ by hydrogen chromate ions is a chain reaction that is strongly inhibited by oxygen. The initiation reaction between AcrH2 or AcrD2 and H2CrO4 forms AcrH2·+ and occurs by a 1e mechanism, kH = kD = 4.6x102 L mol-1s-1 (25°C). The Cr V produced along with AcrH· (from the acid ionization of AcrH2·+ are chain-carrying intermediates. The propagating reaction between AcrH2 and Cr V, k = 1 x 108 L mol-1s-1, is of key importance since it is a branching reaction that yields two chain carriers, AcrH· and CrO2+, by hydrogen atom abstraction. The same partners react competitively by hydride ion abstraction, to yield Cr3+ and AcrH+, k = 1.2 x 107 L mol-1 s-1, in the principal termination step. The reaction of CrO2+ and AcrH2, kH = 1.0 x 104 and kD = 4.8 x 103 L mol-1 s-1, proceeds by hydride ion transfer. The Cr2+ so produced could be trapped as CrOO2+ when O2 was present, thereupon terminating the chain. AcrH2 itself reacts with Cr2O72- + H+, k = 5.6 x 103 L2 mol-2s-1, but this step is not an initiating reaction. From that, two successive electron transfer steps are believed to occur, yielding CrIV + CrVI + AcrH+.