Allylic fluorines reactivity

The 1,3-dipolar cycloadditions offluonnatedallenes provide a rich and varied chemistry Allenes, such as 1,1-difluoroallene and fluoroallene, that have fluorine substitution on only one of their two cumulated double bonds are very reactive toward 1,3-dipoles Such activation derives from the electron attracting inductive and hyperconjugative effects of the allylic fluorine substituent(s) that give nse to a considerable lowering of the energy of the LUMO of the C(2)-C(3) n bond [27]... 

Allylic fluorines in the starting compounds are reactive enough to be displaced, in an SN2 reaction, by the methoxide anion. The allylic fluorine at the other end of the double bond is displaced by the oxygen of the... 

Vinyl and phenyl mfluoromethyl groups are reactive in the presence of aluminum chloride [10] Replacement of fluorine by chlorine often occurs Polyfluori-nated trifluoromethylbenzenes form reactive a,a-difluorobenzyl cations in antimony pentafluoride [11] 1 Phenylperfluoropropene cyclizes in aluminum chloride to afford 1,1,3-trichloro 2 fluoroindene [10] (equation 10) The reaction IS hypothesized to proceed via an allylic carbocation, whose fluoride atoms undergo halogen exchange... 

Another powerful approach to prepare a-amino acids bearing an aromatic or unsaturated side chain in /I (but also many other compounds) is based on the reactivity of 5-fluoro-4-trifluoromethyloxazole, a starting material easily accessible from hexafluoroacetone. The fluorine atom in the 5 position is easily displaced by an allylic or benzylic alcohol. Then, the obtained ethers spontaneously undergo a Claisen rearrangement to afford, after acidic hydrolysis, an a-trifluoromethyl amino acid... 

Unsaturated fluorinated compounds are fundamentally different from those of hydrocarbon chemistry. Whereas conventional alkenes are electron rich at the double bond, fluoroal-kenes suffer from a deficiency of electrons due to the negative inductive effect. Therefore, fluoroalkenes react smoothly in a very typical way with oxygen, sulfur, nitrogen and carbon nucleophiles.31 Usually, the reaction path of the addition or addition-elimination reaction goes through an intermediate carbanion. The reaction conditions decide whether the product is saturated or unsaturated and if vinylic or allylic substitution is required. Highly branched fluoroalkenes, obtained from the fluoride-initiated ionic oligomerization of tetrafluoroethene or hexafluoropropene, are different and more complex in their reactions and reactivities. 

Fluorinated carbocations play an important role as intermediates in electrophilic reactions of fluoroolefins and other unsaturated compounds. For example, F-allyl cation 1 was proposed as a reactive intermediate in reactions of HFP with fluoroolefins catalyzed by Lewis acids [7]. The difference in stability of the corresponding allylic cations was suggested as the explanation for regio-specific electrophilic conjugated addition to CF2=CC1CF=CF2 [11]. Allylic polyfluorinated carbocations were proposed as intermediates in the reactions of terminal allenes with HF [53] and BF3 [54], ring-opening reactions of cyclopropanes [55], Carbocations are also an important part of the classic mechanism of electrophilic addition to olefins (see Eq. 2). This section deals with the questions of existence and stability of poly- and perfluorinated carbocations. 

The first step in the hydrolysis in tert-butyl alcohol is probably nucleophilic displacement of fluorine in the reactive allylic position. Spontaneous elimination of hydrogen fluoride from the geminal fluorohydroxy compound yields perfluorocyclopent-2-en-l-one. 

Nucleophilic displacement of fluorine atoms in the reactive allylic position affords an enol of a fluorinated 1,3-diketone, 2H-pentafluoro-cyclopentane-l,3-dione (compound H) [57]. ... 

Perfluorophenyl)copper exhibits high reactivity towards organic substrates. and reacts with fluorinated or nonfluorinated aryl iodides (e.g., formation of 5), fluorinated vinyl iodides, al-kynyl bromides and iodides, allyl halides, alkanoyl halides, and iodomethane to afford the corresponding coupled products in good yields. (Perfluorophenyl)copper readily undergoes addition in a syn fashion to perfluorobut-2-yne to form a perfluorovinylcoppcr reagent 6, which can be quenched by electrophiles (e.g. formation of 7). Recall that (per-fluorophcnyl)coppcr reacts with (trifluoromethyl)copper to form (perfluorophenethyl)copper (Section 2.1.1.3.3.). 

These reactions are based on hydrogen fluoride and involve, essentially, a nucleophilic displacement of halogen (for convenience, in the sense intended throughout this book, this term usually excludes fluorine). However, only the most reactive halides such as allylic and benzylic ones can be fluorinated by anhydrous HF alone [9] (Figure 2.2). 

Several examples are known which demonstrate the greater reactivity of chlorine vs fluorine in unsaturated compounds in their behaviour towards phosphorus(III) esters. Thus, the interaction of 2,3-dichlorotetrafluoropropene and trimethyl phosphite proceeds through an allylic displacement, by either an ionic or a concerted mechanism (Scheme 1) to give the phosphonate 36 The 1,2-dichloroperfluorocycloalkenes 37 (/i = 2,3 or 4) also react with phosphite or phosphonite esters to give the phosphonates 38 or phosphinates... 

The zinc and cadmium reagents " " (R0)2P(0)CF2MBr have also proved useful as alkylation substrates for particularly reactive species, e.g. allylic, benzylic or propargylic halides with the last type, some rearrangement to allenic compounds is observed. Scheme 12 illustrates the use of such reagents in the synthesis of a-fluorinated alkylphosphonic acid derivatives. 

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