New method of fluorination using potassium fluoride in ionic liquid: significantly enhanced reactivity of fluoride and improved selectivity.
We have found the new nucleophilic fluorination reaction of some halo- and mesylalkanes to the corresponding fluoroalkanes with KF in the presence of [bmim][BF4] under various reaction conditions. 2-(3-Methanesulfonyloxypropoxy)naphthalene (1) was used as a model compound to optimize this fluorination reaction. Whereas the fluorination of the mesylate 1 with KF in an organic solvent such as CH3CN at 100 degrees C occurred hardly even after 24 h, the same reaction in ionic liquids, [bmim][BF4], as a reaction solvent was completed within 1.5 h, affording the wanted product 2-(3-fluoropropoxy)naphthalene 2a (85%) together with the alkene byproduct 2c (10%). Very interestingly, however, the addition of water (5 equiv) completely eliminated the formation of the undesired alkene 2c and thus gave higher yield of 2a (92%, entry 2). The use of acetonitrile as a cosolvent did not affect the reactivity of the fluorination. The presence of a proper amount of cosolvent was rather desirable (94% yield of 2a). We performed fluorination reactions with other ionic liquids ([bmim][PF6], [bmim][SbF6], [bmim][OTf], and [bmim][N(Tf)2], and two other cosolvents, to find the optimal ionic liquid and cosolvent. Nine different compounds were examined, including the 10 g-synthesis of 2-(fluoromethyl)naphthalene in 93% of isolated yield.
Kim
Dong Wook
DW
Department of Chemistry, Inha University, 253 Yonghyundong Namgu, Inchon 402-751, Korea.
Song
Choong Eui
CE
Chi
Dae Yoon
DY
D002625:Chemistry, Organic / Q000379:methods; D005459:Fluorides / Q000737:chemistry*; D006845:Hydrocarbons, Fluorinated / Q000138:chemical synthesis*; D017680:Potassium Compounds / Q000737:chemistry*; D012996:Solutions
D006845:Hydrocarbons, Fluorinated; D017680:Potassium Compounds; D012996:Solutions; C033320:potassium fluoride; D005459:Fluorides