1.1.5.Fluorination of unsaturated compounds by hydrogen fluoride in the presence of electrophilic reagents.

Addition of a positive particle X⁺ (Cl⁺, Br⁺, I⁺) of a halogen in the presence of a fluoride-ion source to form a product of trans-addition of X-F elements to the double bond of the olefin is a selective reaction for a wide range of unsaturated compounds. The source of X⁺ cation is mainly such reagents as halogen-amides (N-bromosuccinamide, N-bromoacetamide etc.) or free halogen (I₂ or Br₂). The fluoride-ion source is usually anhydrous hydrogen fluoride or its complexes with bases.

Above there were mentioned examples of the use of anhydrous hydrogen fluoride and its complexes for implementation of the hydrofluorination process. The latter runs in two stages: at first hydrogen fluoride as an acid protonates an unsaturated organic compound or a substrate containing a heteroatom with unpaired electron couple to generate a cationoid particle. Further an attack of fluoride ion towards the carbon atom with the greatest positive charge takes place. This process scheme may be realized otherwise. At first due to interaction of electrophilic agents with the multiple bond or due to a different influence on the bond a carbanion is generated and hydrogen fluoride or its complex with a base then becomes the fluoride-ion source. Introduction of fluorine into the organic molecule takes place in this case also but a different functional group is involved. It should be noted that this approach was found very productive and was widely used. This review presents only examples illustrating opportunities of using hydrogen fluoride and its complexes with bases in such a way.

Generally the reaction runs according to the following scheme. At first the cationoid particle reacts with the multiple bond to form halogenoid cation which is subjected to the attack of the fluoride-ion to give, as a rule, a product of anti-addition . So, in case of interaction of N-chlorosuccinamide in a medium of anhydrous hydrogen fluoride with alkenes there is formed -chloro--fluoroalkane.

AgF, BF₃ etc. are often used in these processes as catalysts. Et₃N/3HF system is an universal source of fluoride-ion in reactions of halogenfluorination of unsaturated hydrocarbons [168]. Table 10 presents the examples of stereo- and regio-selective synthesis of organic compounds by chlorofluorination and bromo-fluorination of alkenes.

Table 10. Halogenfluorination of alkenes

A combination of Et₃N/3HF system and N-halosuccinimides was found an effective halofluorinating reagent of alkenes [169,170]. The reaction runs at room temperature in glassware. The anti-addition has a  stereospecific character.

It has been shown [171] that the fluorinating agent being a complex of trialkylamine/HF ( for example Me₃N/3HF in combination with N-bromosuccinimide (NBS)) in reactions with styrenes is more effective in comparison with Et₃N/3HF and gives bromofluorination products according to Markovnikov law with high regioselectivity.

In case of terminal aliphatic alkenes, hexene for example, the regioselectivity decreases a little at transition from Me₃N/ 3HF  (95:5) system to Et₃N/3HF (87:13) system.

The high regioselectivity takes place in bromofluorination of terminal olefins when Et3N 3HF complex is used in the presence of N-bromosuccinimide [172] The yield of the products is almost quantitative.

• 1-bromo-2-fluoro-3,3-dimethylbutane: 2-bromo-1-fluoro-3,3-dimethylbutane
• 1-bromo-2-fluoro-2,4,4-trimethylpentane: 2-bromo-1-fluoro-2,4,4-trimethylpentane

In case of cis- and trans-6-dodecene there are formed threo- and erythro-6-bromo-7-fluorododecenes respectively in a quantitative yield. (E)-6-bromo-7-fluoro-3,7-dimethyl-2-octen-1-ol was obtained in a similar way in 96% yield [172].