Fluoride-ion-induced reactions

In the preceding sections we outlined reactions of fluoroalkenes with many types of nucleophilic species, but reactions involving initial attack on fluoroalkenes by fluoride ion have been reserved for a separate discussion here. 

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Intramolecular cyclization of tethered phenyl ketones (58 X = Br, SiMe3) show contrasting stereochemical outcomes for indium catalysis of the alkyl bromides and fluoride ion-induced reaction of the allylsilanes. The reactions thus allow complementarity in product diastereoselectivity, and the difference appears to be related to an... 

The inverse relationship that exists between fluoride ion-induced reactions of fluorinated alkenes [3] and proton-induced reactions of alkenes maybe adduced from the processes described in the following sections. 

A particularly useful reaction of this type involves the direct formation of hexakis(trifluoromethyl)cyclopentadiene (71) (Scheme 31), or the corresponding cyclopentadienide (72), from the diene (38) by a fluoride ion induced reaction with pentafluoropropene [67-69]. Recent work [54] has shown that very active sources of fluoride ion can be generated by direct reaction of amines, especially TDAE (43), with perfluorinated alkenes or perfluorinated aromatic compounds and these essentially solventless systems promote both oligomerisations (see above) and polyfluoroalkylations. The absence of solvent makes recovery of product very easy, e.g. in high-yielding formation of (73), (74) or (75) (Scheme 32). 

Fluoride-ion-induced reactions A similar polymer to that in Figure 7.87 is obtained upon anionic polymerisation of hexafluoro-2-butyne initiated by fluoride ion in a solvent [311-313] (Figure 7.89). This is a clear example of an anionic polymerisation of an unsaturated fluorocarbon, although the growing anion can be trapped by a sufficiently reactive system [291, 314], such as pentafluoropyridine [315] (Figure 7.90). There is little difference between the ultraviolet spectra of 7.90A and 7.90B, confirming that conjugation in the polyene system is inhibited by steric effects. 

An effective route to a number of bis(perfluoroalkyl)- and bis(perfluorocycloalkyl)-mercurials involves fluoride-ion-induced reactions of fluoroalkenes [31] this follows an earlier method involving addition of mercury(ll) fluoride to fluoroalkenes, e.g. using anhydrous hydrogen fluoride as solvent [62] (Figure 10.18). 

The fluoropyridazines have been intensively investigated by Musgrave and co-workers. Earlier work or nucleophilic substitutions in perfluoro-pyridazines has been reviewed. Tetrafluoropyridazine reacts with hexa-fluoropropene to give products formed from both kinetic and thermodynamic control of the polyfluoroalkylation, i.e., the fluoride ion-induced reaction between a fluoroolefin and an activated polyfluoroaromatic compound. The reaction with tetrafluoroethylene is kinetically controlled. Products arising only from thermodynamic control are formed, however, in the reaction with octafluoroisobutene. 

The structure (13) of the trimer formed in the fluoride-ion-induced reaction of perfluorocyclobutene has been assigned on the basis of its C(ls) core electron spectrum. 

Scheme 49 Kinetic vs thermodynamic control in fluoride-ion-induced reactions...

Similarly, Itexafluoroprapylene undergoes fluoride ion induced homotelo-merization to give a series of dimers and trimers These telomerizations can be induced by other nucleophiles, such as amines Indeed, the selectivity of the pi oce-,s can be changed significantly by varying reagents and reaction conditions [25, 26] (equations 19 and 20)... 

Mixtures of anhydrous hydrogen fluoride and tetrahydrofuran are successfully used as fluonnating agents to convert 1,1,2-trifluoro-l-alken-3-ols, easily prepared from bromotrifluoroethene via lithiation followed by the reaction with aldehydes or ketones, to 1,1, l,2-tetrafluoro-2-alkenes The yields are optimal with a 5 1 ratio of hydrogen fluoride to tetrahydrofuran The fluonnation reaction involves a fluoride ion-induced rearrangement (SN2 mechanism) of allylic alcohols [65] (equation 40)... 

ESCA has been used to determine the molecular structure of the fluoride ion-induced tnmerization product of perfluorocyclobutene [74] and the products of the sodium borohydnde reduction of perfluoroindene [75] ESCA is also used to analyze and optimize gas-phase reactions, such as the bromination of trifluoro-methane to produce bromotrifluoromethane, a valuable fire suppression agent [76] The ionization energies for several hundred fluorine-containing compounds are summarized in a recent review [77]... 

The fluoride ion-induced reductive / -elimination makes it possible to generate highly strained olefins [Eq. (31)]. /J-Silyl A3-iodane 40 generates five-mem-bered cumulene with remarkable reactivity at room temperature and affords Diels-Alder adduct 40a (7 % yield) by the reaction with benzene [63]. 

Various oligomers of fluorinated alkenes and cycloalkenes have been prepared by fluoride ion induced oligomerisation of various monomers (Sect. 5.3), and the chemistry of these systems provides some unique reactions. The oligomers of special interest here may be described as of types (95) or (96) (Scheme 59), i. e. systems with either four (95) or three (96) perfluoro-alkyl or -cycloalkyl groups attached to the double bond, whereas systems with two perfluoroalkyl groups attached, i. e. (97) and (98), have a chemistry more similar to fluorinated alkenes that may be derived from other sources. 

The reaction of the decenyliodonium tetrafluoroborate with tetrabutylammonium fluoride has also been studied but follows a different course84. Thus, the relatively basic fluoride ion induces the a-elimination of iodobenzene and affords products derived from an intermediate alkylidenecarbene (equation 205). 

An efficient procedure for the nucleophilic displacement of the A, A -dimethylamino group of 1-triisopropylsilyl-gramines via the fluoride ion-induced elimination-addition reaction has been devised. 1-Triisopropylsilylgramine methiodide 1371 reacted smoothly with a variety of nucleophiles in the presence of tetrabutylammonium fluoride (TBAF) to give 3-substituted indoles 1372 (Scheme 262) <1995TL5929>. 

Many useful, higher-molecular-weight fluoroalkenes can be conveniently prepared by fluoride-ion-induced oligomerisation reactions of smaller fluoroalkenes such as tetrafluoroethene and hexafluoropropene, and these methods are discussed in Section C. 

In general it is very difficult to make any distinction between a concerted process and the involvement of a short-lived carbanion. For clarity a number of alkene rearrangements in the following text are written as two-step processes, but it should be emphasised that they could involve concerted mechanisms. Products arising from substitution with rearrangement are frequently encountered in reactions of cyclic fluoroalkenes and in fluoride-ion-induced rearrangements (Subsection 6, below). 

In addition to the generation of phosphonium ylides from phosphonium salts by deprotonation with bases in some instances ylides may result from pyrolysis of phosphonium salts, especially silylated salts (equation 17). Similar fluoride ion induced desilylation (equation 18) of phosphonium salts proved to be a very useful alternative for the synthesis of ylides which are difficult to synthesize by the conventional salt method (as in the case of R, R = alkyl). - The most effective fluoride source is cesium fluoride and the reaction proceeds at room temperature. 

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