Fluorination of Secondary and Primary Alcohols by Thermal Decomposition of Electrochemically Generated Alkoxy Triphenylphosphonium Tetrafluoroborates

Replacement of hydroxyl groups in secondary and primary alcohols (1) with a fluorine atom arising from tetrafluorobote anion has been performed by the electrochemical formation of alkoxy triphenylphosphonium tetrafluoroborarates (2) from 1, followed by their thermal decomposition. The procedure is quite simple, involving : (1) constant-current electrolysis of a mixture of 1, Ph3P, and Ph3PH·BF4 in CH2Cl2 in an undivided cell; (2) refluxing a tetrahydrofuran or dioxane solution of the residue afforded by evaporation of the solvent in vacuo after the electrolysis. Cyclic secondary alcohols such as 3β-hydroxy steroids and 2-adamantanol are transformed into the corresponding fluorides in satisfactory yields when the geometry of the leaving group in 2 is suitable for the substitution or an elimination process for 2 to give an alkene is stereochemically forbidden. The fluorination of steroidal alcohols and 4-phenyl-1-cyclohexanol proceeded with complete inversion, demonstrating that a fluorine atom from the tetrafluoroborate anion attacks from the side opposite to the phosphonium moiety in 2 via an SN2 mechanism rather than as SN1 mechanism. The fluorination of acyclic secondary and primary alcohols was performed by the present method in reasonable yields, although the reaction for the latter required more forcing conditions, such as refluxing in dioxane.