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.
次级和初级醇(1)中的羟基被四
氟硼酸根衍生的
氟原子取代,通过电
化学形成烷氧基
三苯基膦四
氟硼酸盐(2),然后进行热分解来实现。该过程相当简单,包括:(1)在无隔膜电池中对1、Ph3P和Ph3PH·
BF4的
CH2Cl2混合物进行恒电流电解;(2)将电解后在真空下蒸发溶剂得到的残渣的
四氢呋喃或
二氧六环溶液回流。当2中的离去基团的构型适合取代或消除过程,且2生成烯烃的立体
化学被禁止时,环状次级醇如3β-羟基甾体和
2-金刚烷醇可转化为相应的
氟化物,产率令人满意。
甾醇和4-苯基-1-
环己醇的
氟化反应进行完全反转,表明四
氟硼酸根阴离子中的
氟原子通过SN2机制从2中膦部分的对面攻击,而不是SN1机制。通过本方法,无环次级和初级醇的
氟化反应在合理产率下进行,尽管后者的反应需要更强烈条件,如在
二氧六环中回流。