Fluorination with high valency

Introduction of Fluorine with High-Valency Oxidizing... 

Krypton difluoride is a fluorinating agent some 50 kJ/mol more powerful than fluorine itself. It forms adducts, salts of FKr+, with high valency fluorides such as AsF5 and SbF5, these react explosively with organic compounds. 

Fluorofullerenes have been prepared by reaction with halogen fluorides, by direct fluorination with F2 or by fluorination with noble gas fluorides [7,8,14], The reaction with high valence metal fluorides is the most versatile route for the synthesis of CsoFnH. 

Takubo, S. Sekiya, A. Monofluorination of fluorinated oxolanes with high-valency metal fluorides. J. Fluorine Chem. 1998, 87, 105-109. 

Replacement of Hydrogen. Three methods of substitution of a hydrogen atom by fluorine are (/) reaction of a G—H bond with elemental fluorine (direct fluorination, (2) reaction of a G—H bond with a high valence state metal fluoride like Agp2 or GoF, and (J) electrochemical fluorination in which the reaction occurs at the anode of a cell containing a source of fluoride, usually HF. 

High-valency metal fluoride fluorination of pyridine [82JFC(21)171], quinoline [82JFC(21)413], and 2-methylfurans [91 JFC(51)179] has been reported. With 2-methylfuran a complex mixture of stereoisomers of partially fluorinated oxolans was obtained. These can be dehydrofluorinated to fluorooxolens and no furans have been observed. Conformation and structural group were found to influence the direction and readiness toward dehydrofluorination [91 JFC(52) 165]. 

The essence of this theory on the mechanism of ECF is that reaction occurs via inorganic fluorinating agents generated at the anode these include nascent fluorine, molecular fluorine, or its loose complexes with nickel fluorides, and simple or complex forms of high valence nickel fluorides [62,65,169,178,179]. However, until the precise nature and structure of the active nickel anode surface is characterised the position remains a matter for conjecture. 

Work with pre-formed high-valency nickel fluorides, i.e. NiFs and Nip4, has demonstrated the very high level of reactivity of such compounds [105-107]. These are the only reagents described so far that will bring about complete fluorination at room temperature or below (Figure 2.19), and therefore mirror ECF procedures. 

An interesting rationalization of the mechanistic pathways involved in the duorination of aromatic compounds by high-valency metal fluorides has been provided. The main features of the mechanism [outlined for the conversion of fluorobenzene into /)-difluorobenzene by cobalt(in) fluoride in Scheme 2] are oxidation of the aromatic system to a radical-cation (1) and its subsequent reaction with a fluorine atom from the metal fluoride to give the type of Wheland intermediate [or benzenium ion (c/. p. 362)] (2) which would be formed in a conventional electrophilic process. Such a scheme implies that the relative stabilities of Wheland intermediates might be used to rationalize orientation in this type of reaction in much the same way as for more traditional processes, and some examples of such applications have been provided. Alternative routes from the radical-cation (1) are discussed and possible... 

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