Alkyl fluorides reduction

Alkyl fluorides are for practical purposes resistant to reduction. In a few instances, where fluorine in fluoroalkanes was replaced by hydrogen, very energetic conditions were required and usually gave poor yields 500, SOI], For this reason it is not difficult to replace other halogens present in the molecule without affecting fluorine. 

Reductive removal of fluorine from alkyl fluorides requires a potent reducing agent and so is not normally encountered However, hydrogenolysis of an unactivated carbon-fluorine bond in, for example, 3 p-fluorocholestane has been efficiently accomplished in 88% yield with a solution of potassium and dicyclohexyl 18 crown-6 in toluene at 25 °C [/] Similarly, sodium naphthalene in tetrahydrofuran converts 6 fluorohexene-1 and 1-fluorohexane to hydrocarbons in 50% yield at 25 °C over a 7-h period [2]... 

Reduction of RX.1 Alkyl halides, even alkyl fluorides, are reduced in high yield by LiAlH4/CeCl3 (3 1) in refluxing DME or THF. The actual reagent may be a low-valent cerium compound, since direct hydride attack is not involved. Under the same conditions phosphine oxides are reduced to phosphines. 

Reductive cleavage of alkyl fluorides.1 Potassium or Na/K and dicyclohexyl-18-crown-6 in toluene is the most effective of known systems for reductive cleavage of unactivated C—F bonds. Addition of a proton source is not necessary and actually is deleterious. 

Alkyl fluorides.1 a-Fluoro sulfides, available by reaction of mercury(II) fluoride with thioacetals, are convertible into alkyl fluorides by reductive desulfurization with sodium in ethanol. 

Not within the scope of this section are the synthesis of a-fluoro sulfides from dithioace-tals, their reduction to alkyl fluorides, and the synthesis of glycosyl fluorides from (phenyl-sulfanyl)glycosides. ... 

ABSTRACT. Toluene radical anion, generated by dissolving potasssium metal in toluene by the assistance of dicyclohexano-18-crown-6, has been proved to be especially effective for reductive removal of fluorine atom from unactivated alkyl fluorides that resist common reduction conditions. Stereochemical and mechanistic aspects of the present method is discussed. In connection with the preparation of substrates the effect of dipolar aprotic solvents on the nucleophilic fluorination with potassium fluoride/dicyclohexano-18-crown-6 system was also examined, and sulfolane or N,N-dimethylformamide was shown to be a solvent of choice. 

Alkyl fluorides were chosen as the substrates because the carbon-fluorine bond is known to be very strong" and resists common reduction conditions . 

The results of the reduction of fluorocholestane (2) under various conditions (solvent, metal, crown ether, H" ) are shown in Table II. Alkyl fluorides (primary, secondary, tertiary) were reduced under standard conditions [K/DC-18-C-6 (2 mol eq.)/toluene at ambient temperature], and the results are shown in Table III. 

Additional support for an electron transfer-like mechanism comes from the observation that reactions with n-alkyl halides are incomplete, with ca- 40% of substrate unreacted, whereas reactions with alkyl iodides are nearly complete. This is consistent with the fact that the reducing power of n-type polyacetylene decreases as the carbanion concentration decreases, and that normal alkyl chlorides are the most difficult to reduce in the series employed in our experiments (see Table 1). (Note that the reduction potentials of polyacetylene and the alkyl halides cannot be compared directly as they were measured in different solvents.) Alkyl fluorides, as expected, do not react to a measurable extent. 

Methods reported this year for the reduction of alkyl halides to alkanes include the potassium-dicyclohexyl-18-crown-6 reduction of alkyl fluorides, sodium borohydride reduction of alkyl chlorides, bromides, and iodides (or sulphonate esters) under liquid-liquid phase-transfer conditions, and the selective reduction of tertiary alkyl, benzyl, and allyl halides with the borate (61). Continuing... 

Silyl enol ethers are other ketone or aldehyde enolate equivalents and react with allyl carbonate to give allyl ketones or aldehydes 13,300. The transme-tallation of the 7r-allylpalladium methoxide, formed from allyl alkyl carbonate, with the silyl enol ether 464 forms the palladium enolate 465, which undergoes reductive elimination to afford the allyl ketone or aldehyde 466. For this reaction, neither fluoride anion nor a Lewis acid is necessary for the activation of silyl enol ethers. The reaction also proceed.s with metallic Pd supported on silica by a special method[301j. The ketene silyl acetal 467 derived from esters or lactones also reacts with allyl carbonates, affording allylated esters or lactones by using dppe as a ligand[302]... 

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