Ketyls Kinetic acidity

Ketyls, 218 Kinetic acidity, 280 Kinetic control, 42, 151 addition to 0=0, 235 addition to dienes, 195 Diels-Alder, 350... 

Figure 7.30 (a) Microdensitometer records of a photographic plate showing the absorption of the triplet state of anthracene-9-carboxylic acid. Horizontal axis, wavelength in nm vertical axis, absorbance, (b) Oscilloscope trace of the absorption of the ketyl radical of benzophenone in ethanol, (c) Second-order plot of the decay kinetics against time t... 

In a detailed kinetic study, Flowers showed that the rate of ketone reduction is directly related to the pof the alcohol proton source used in the reaction and that the proton source must be sufficiently acidic to protonate the ketyl radical anion intermediate 3.11 Interestingly, when H20 is used as a proton... 

To study the mechanism by which HCIO4 catalyzes electron transfer, the kinetic order in acid was measured by performing a series of Stem-Volmer luminescence quenching experiments at various concentrations of HCIO4. These experiments revealed a linear relationship between k t and [HCIO4] (Fig. 42) showing electron transfer from [Ru (bpy)3] " is first order in acid. In this work, the authors proposed a stepwise ET/PT mechanism in which protonation of the ketyl radical anion provides additional thermodynamic driving force which causes an increase in et- This work predated the wide-spread acceptance of concerted proton-coupled electron transfer as an elementary step, however these seminal observations provided the conceptual framework for PCET to be applied further in contemporary synthetic chemistry. 

---