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Uncatalyzed Transfer Hydrogenation of Quinones and Related Systems: A Theoretical Mechanistic Study

“…Among the four solvents studied, the barriers for both steps lie within a range of 10 kJ mol -1 . This is consistent with the experimental observation that the rate of reaction is rather independent of the solvent 1a. The barrier for the first step generally decreases with increasing polarity of the solvent.…”

“…On the other hand, for the reactions in ether, acetone, and CH 3 CN, the ionic intermediates are substantially lower in energy than the radical intermediates. This is in apparent contradiction to experiment, in which spin-trapping with nitrosobenzene indicates the presence of radicals in the reaction of acridan with DDQ in acetonitrile. 1a, To explore for possible reasons behind the apparent discrepancy between our calculated results and the experimental observations, we investigated the reactions between the 9-monohydroacridine intermediates, both radical and cationic, with the spin-trapping agent nitrosobenzene. The relative free energies for the species involved in the transfer-hydrogenation reaction in the presence of PhNO in acetonitrile are shown in Figure , in which the donor acridan is denoted DH 2 while the acceptor DDQ is denoted A.…”

“…A spin-trapping experiment with nitrosobenzene yielded the EPR spectrum of the radical-adduct a shown in Figure . Similarly, in the reaction between acridan and DDQ in acetonitrile, with nitrosobenzene as the spin-trap, a well-resolved EPR spectrum was obtained, 1a, which is likely to be associated with a similar radical adduct (adduct b in Figure ). These results indicate the presence of radicals under the conditions employed in these two reactions, and thus argue in favor of radical over ionic mechanisms.…”

Pentacene Disproportionation during Sublimation for Field-Effect Transistors

“…Rüchardt's hydrogen transfer experiments using 9,10-dihydroanthracene as the radical initiator have led us to believe that small concentrations of DHP are acting similarly in our experiments. , We note that HLPC trace chromatograms showed the presence of small amounts of DHP in pentacene, and it is possible that this is an autocatalytic reaction, with DHP acting as the radical initiator with the loss of hydrogen at the 6 position at temperatures exceeding 320 °C (Figure ). This mechanism is consistent with Rüchardt's observations on the hydrogen transfer reduction of styrene. , We note that the crossover experiments, indicating scrambling of hydrogen and deuterium in the DHP products, are permissive of such a mechanism but not exclusive to it. Moreover, the temperature of the transformation is similar to that required under Rüchardt's conditions using dihydroanthracene as the hydrogen donor.…”

“…This mechanism is consistent with Ru ¨chardt's observations on the hydrogen transfer reduction of styrene. 26,27 We note that the crossover experiments, indicating scrambling of hydrogen and deuterium in the DHP products, are permissive of such a mechanism but not exclusive to it. Moreover, the temperature of the transformation is similar to that required under Ru ¨chardt's conditions using dihydroanthracene as the hydrogen donor.…”