Xanthenol, oxidation

The oxidation of benzhydrol and 9-fluorenol in basic solution again shows a difference in regard to mechanism that can be primarily attributed to a difference in acidity as carbon acids. In tert-butyl alcohol benzhydrol enters into an oxidation scheme as the mono (oxy) anion. The data strongly suggest a free radical chain. Under these conditions the more acidic fluorenol or xanthenol oxidizes via carbanions or dianions. These oxidations can be catalyzed to occur via a free radical chain process by one-electron acceptors, such as nitrobenzene, and a free radical chain process may well be involved in the absence of the catalyst. 

Oxidation of 9-Fluorenol and 9-Xanthenol in Basic Solution. Fluor-enol and xanthenol react with base and oxygen to give high yields of the ketones, or in the case of fluorenol in DMSO solution, the DMSO-fluore-none adduct. The stoichiometry of the oxidation (Table XI) varies with... 

The oxidations involving lithium ferf-butoxide reach completion (3 minutes for fluorenol, 12 minutes for xanthenol) much sooner than the corresponding reactions utilizing potassium terf-butoxide as base (35 minutes for fluorenol, 27 minutes for xanthenol). This behavior obviously involves Reaction 20 since the initial rates of oxidation were all approximately the same for the lithium- and potassium ferf-butoxide-catalyzed reactions. 

The oxidation of xanthenol and fluorenol showed a number of differences from the oxidation of benzhydrol. No induction period was observed (Figure 5). The rate was enhanced by ferric ion, and the stoichiometry was altered to 0.5 mole of oxygen per mole of fluorenol (Figure 5), apparently because of Reaction 25. 

Oxidation of xanthenol or fluorenol with deficient quantities of oxygen in tert-butyl alcohol produced large quantities of the ketyl, as did reaction of equal molar amounts of the ketone and alcohol in basic solution. In fact, the reaction of the pinacol of fluorenone with excess base in tert-butyl alcohol produced an essentially quantitative yield of the ketyl (19). 

Oxidation of fluorenol and xanthenol alcoholates in f-butanol and mixtures with pyridine and dimethylsulfoxide is accelerated by nitrobenzene [113]. The mechanism suggested is... 

Sahin, Nieger, and Brdse have reported also the oxidative coupling of various hexahydro-xanthenols. The application of an iron complex as oxidant converted the 2-hydroxy-substituted xanthenes 1056 (Scheme 13.26) to the 3,3- t S-coupled biaryl 1057 (565). It appears that these two papers ((564) and (674)) represent the only chemical studies published on the topic of xanthone or xanthene biaiyl-coupUng. 

Sahin H, Nieger M, Erase S (2009) Probing the Oxidation of Functionalized (Hexahydro) xanthenols. Eur J Org Chem 5576... 

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