Catalytic Oxidations with Hydrogen Peroxide in Fluorinated Alcohol Solvents

Catalytic Oxidations with Hydrogen Peroxide in Fluorinated Alcohol Solvents 

This review focuses on the use of hydrogen peroxide as the terminal oxidant. The virtues of hydrogen peroxide as a readily available and green carrier of active oxygen have been praised numerous times [4—6]. In the current context, it is particularly gratifying that hydrogen peroxide is activated toward electrophilic oxidations by 

1) Fluorinated alcohols are (mildly) acidic as exemplified by the pfC values of 12.4 for TFE and 9.3 for HFIP, fluorinated alcohols are significantly more acidic than their nonfluorinated counterparts [11]. 

2) Fluorinated alcohols are strong hydrogen bond donors the specific a-values for TFE and HFIP are 1.51 and 1.96, respectively [12aj. The remarkable strength of H-bond donation by, for example, HFIP is nicely illustrated by the existence of its 1 1-complex with THE, which can be distilled without decomposition [12bj. See Section 4.2.1 for a more detailed look at the hydrogen bond donor features 

4) Fluorinated alcohols are highly polar solvents With regard to the Reichard- 

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In fluorinated alcohol solvents, nonstrained ketones such as cyclohexanone (1) undergo oxidation to lactones in the presence of hydrogen peroxide and catalytic amounts of Brpnsted acids (Berkessel and An-dreae 2001 Berkessel et al. 2002). Unlike the classical Baeyer-Villiger reaction, ketone oxidation with H2O2 in e.g. HFIP proceeds via a spiro-bisperoxide 2 intermediate (Scheme 1). In contrast to other solvents, the acid-catalyzed rearrangement of the spiro-bisperoxide 2 to two equivalents of the product lactone 3 proceeds rapidly and cleanly in HFIP. Preliminary calculations indicate active participation of the fluorinated alcohol solvent in the rate-determining step also in this case. 

Fluorinated alcohols, TFE and HFIP, have often been used as catalytic solvents for epoxidation reactions under mild conditions [34]. The merit of these fluorinated alcohols is, surely, their resistant nature toward oxidation. Another merit of their use in oxidation reactions has often been said to be their catalytic activities via protonation of an active oxygen species, such as H2O2. Experimental and computational studies indicate that the protonated hydrogen peroxide, H3C>2+, which is generated by the action of H2O2 with a strong acid, is a very powerful oxidant. In contrast, weak acids such as TFE appear to... 

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