Fluoroformate anion

Fluoroform (CHF3) efficiently trifluoromethylates aromatic aldehydes to the corresponding alcohols when deprotonated by potassium DMSylate in DMF. This is surprising, as species such as KCF3 have carbenoid character, and tend to be unstable. However, the reaction fails for solvents such as THF, and appears to depend on a highly specific role for DMF. It is proposed that CFs is trapped in situ by the solvent to form the gem-amino alcoholate (70), which acts as a stable, masked form of the anion, which then attacks the aldehyde, regenerating DMF. 

Furthermore, the repulsion between the electron pairs of fluorine atoms is responsible for the pyramidal structure of the carbanion derived from fluoroform. The inversion barrier of the anion is 100 kcal/mol, while that of CH3 is only 2 kcal/ mol. As the acidity of fluoroform is lO" times higher than that of methane, the role of the pyramidal form in stabihzing the carbanion CF3 is essential. 

There were not many new developments in the use of crown ethers for fluorination during the last decade, since the basicity of the fluoride anion is also enhanced along with the nucleophilicity. What is more, cheaper substitutes such as open-chain polyethers frequently led to similar results. However, there are still some reactions which are difficult to accomplish without these complexing agents and the synthesis of /-butyl fluoroformates... 

One of the first representative reaction with hard nucieophiies we deveioped was the reaction of 1-chloroai-kyl carbonates with fiuorides anion. This reaction proceeds through A1 attack mechanism, which is in accord to the H5AB theory, thus converting 1-chioroalkyi carbonates to fluoroformates in good yields (Ref. 91). 

Treatment of enolizable aldehydes with fluoroformates and KF in DMSO (55-100°C for 15-24 h) afforded 1-alkenyl carbonates in 72-92 % yield. According to Olofson s studies, the activated fluoride anion acting as a base deproto-nates the aldehyde to yield an enolate which reacts rapidly with the fluoroformate to give the desired vinylic carbonate as shown in scheme 99 (Ref. 131). Excess KF neutralises the HF which is liberated in the reaction as KHF2. 

Study carefully the pKgS for the haloform series, CHX3— they may not do what you think they should Chloroform is much more acidic than fluoroform even though fluorine is more electronegative (likewise with bromoform and chloroform). The anion CFamust be slightly destabilized because of some backdonation of electrons. The anion from chloroform and bromoform may also be stabilized by some interaction with the d orbitals (there aren t any on fluorine). The conjugate base anion of bromoform is relatively stable— you will meet this again in the bromoform/iodoform reaction (Chapter 21). 

Probably the most efficient way to generate CFf from the view point of atom economy is deprotonation of inexpensive CHFj with a strong base [59]. Unfortunately, this route poses two problems. First, the low boiling point of fluoroform (—82.2 °C) creates - at least on the laboratory scale - the practical problem of handling a gas. The second problem is the need to trap and stabilize the trifluoromethyl anion immediately after its generation, to suppress fragmentation. This second complication, in particular, impeded the apparently straightforward preparative... 

Electroreduction of aryl halides generates aryl anion, which also acts as EGB. For example, the electroreduction of iodobenzene (Ph-I) gives phenyl anion, which deprotonates fluoroform. The trifluoromethyl anion derived from the fluoroform reacts with aldehydes to give the trifluoromethylated alcohols as coupling products (Eqs. 18 and 19) [13]. 

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