Reactivity of ketyl free radicals. Part 1.—Acid dissociation of aromatic ketyls and pinacols

Abstract

Acid-base equilibria of aromatic ketyl radicals and of their dimers (pinacols) were studied by flash photolysis of ketones and by electrochemical methods. pK values of the ketyls were found : fluorenone, 9.5 ; xanthone, 9.8 ; benzophenone, 9.15 ; 4-chlorobenzophenone, 9.2. The pK of the corresponding pinacols are 13.7, 13.5, 12.6 and 13-6, respectively. The difference in acidity between free radicals and their dimers is due to different hybridization of the C atom bonded to the OH group. Some new data are reported on the normal red-ox potentials of the ketone-ketyl anion systems, on the absorption spectra and dimerization rate constants of the ketyls. 918

Electroreduction of lichexanthone

“…12). According to these observations, the ether bond present in compound 1H was broken in compound 1H 3 , and a new hydroxyl group was formed at position 10a, corroborating the structure proposed.…”

“…Day and Biggers [2] found that in alkaline solutions xanthone exhibits one polarographic reduction wave. Kalinowski et al [3] studied the polarographic behavior of fluorenone, xanthone, benzophenone, and 4-chlorobenzophenone, observing that aromatic ketones were reduced in two one-electron steps, the first of them behaving as a reversible one-electron (plus eventually one-proton) transfer. The authors comment on the greater stability of the xanthone ketyl radical and report that all those molecules form dimmers (pinacols).…”

Evidence for Charge Delocalization in Diazafluorene Ligands Supporting Low‐Valent [Cp*Rh] Complexes**

“…In the case of dafone, the benzophenone-like core could undergo formation of radical species. [44] Similarly, the doubly benzylic methylene group in daf is likely to be sufficiently acidic to undergo deprotonation by nascent Rh(I) when it is formed. Two lines of evidence support this reactivity: first, the cyclic voltammogram of 2-NCMe features a first reduction wave that is significantly more reversible than the second (see Figure 2, middle panel and SI, Figure S42); second, chemical reduction experiments suggest that the Rh(I) form of 2-NCMe is sufficiently basic to deprotonate the starting Rh(III) form (See SI, pp.…”

Quenching Mechanisms and Diffusional Pathways in Micellar Systems Unravelled by Time‐Resolved Magnetic‐Field Effects

“…First, AC À has an absorption spectrum that is practically identical to that of 3 A* apart from a significant reduction of the overall absorbance. [6] Second, a protonation equilibrium (pK a = 9.8) [17] interconverts the products of quenching by electron transfer (the radical anion AC À ) and hydrogen abstraction (the ketyl radical HAC). To avoid protonation of AC À we, therefore, worked at pH 12.5 (adjusted with NaOH).…”