Bromofluorination of alkenes

Nucleophilic displacement using [ F] fluoride works well in aUphatic systems where reactive haUdes or sulfonates esters can undergo substitution at unhindered sites. In order to introduce a F fluorine atom in a secondary or tertiary position, a two steps strategy was developed. It involves a F-bromofluorination of alkenes, followed by reductive debromination (n-BujSnH, AIBN). [ F]BrF is usually generated in situ from [ F]potassium fluoride and l,3-dibromo-5,5-dimethylhydantoin (DBH) in sulfuric acid. This methodology was successfully applied to label steroids at the 11 and 6a positions [245] (Scheme 60) and to prepare [ F]fluorocyclohexanes [246]. 

Iodofluorination and bromofluorination of alkenes are also effected in hydrogen fluoride systems by using bromine or iodine with an equivalent amount of silver(I) nitrate. In some cases, iodofluorination can also be achieved by iodine without adding the silver(I) salt, as illustrated in the following procedure. 

Selected examples are given in Table 9 for bromofluorinations of alkenes using A-bromosuccin-imide in combination with 70% hydrogen fluoride/pyridine (Method A) and hydrogen fluo-ride/polyvinylpyridine (Method B), respectively. Table 10 shows examples of the selective monoaddition of in situ produced halofluorides (70 % hydrogen fluoride/pyridine and A-iodo-. A-bromo- or /V-ehloro-succinimide) to symmetrically substituted alkynes. 

Bromofluorination of Alkenes with the DBH/PVPHF (64/36) System l-Bromo-2-fluorocyclohexane Typical Procedure (Method B) 41... 

Stable dialkyl ether poly(hydrogen fluoride) complexes have been shown to be convenient and effective fluorinating agents.553 Various open-chain and cyclic alkenes undergo hydrofluorination with dimethyl ether-5 HF (DMEPHF) atroom temperature to furnish the corresponding fluoro derivatives in high yields (73-94%) with excellent selectivities. The fluorination of secondary and tertiary alcohols exhibit similar features. Bromofluorination of alkenes can also carried out with DMEPHF in combination with /V-bromosuccinimide. The homologous diethyl ether and dipropyl ether complexes are also suitable for fluorinations. 

Fluorinatioa.1 Alkyl bromides are converted to alkyl fluorides by reaction with this reagent in CFI3CN (35-92% yield). Bromofluorination of alkenes can be effected with AgF/CaF2 and l,3-dibromo-5,5-dimethylhydantoin (DBH). Iodo-... 

BROMOFLUORINATION OF ALKENES 1-BROMO-2-FLUORO-2-PHENYLPROPANE (Benzene, (2-bromo-1-fluoro-1 -methylethyl))... 

BROMOFLUORINATION OF ALKENES WITH THE REAGENT COMBINATION N-BROMOSUCCINIMIDE/TRIETHYLAMINE TRISHYDROFLUORIDE... 

BROMOFLUORINATION OF ALKENES 1-BROMO-2-FLUORO-2-PHENYL-PROPANE... 

Addition of NBS to an alkene in the presence of aqueous sodium azide affords fair yields of the corresponding /3-bromoazides, which can be converted by lithium aluminum hydride reduction to aziridines. Intermolecular reactions of alkenes with NBS and weaker nucleophiles can be achieved if conducted under anhydrous conditions to avoid the facile bromohydration reaction. In this manner, bromofluorination of alkenes has been extensively... 

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