Fluoride-ion-initiated reactions

Reactions which partly compensate for the unsuitability of the perfluoroalkyl-lithium route involve addition of fluoride ion to an unsaturated site giving corresponding carb-anions (Cs or K derivatives) that may be used in synthesis (see Chapter 7, Section IIC, 

---

Mixed oligomers can also be produced by fluoride-ion-initiated reaction between two different fluoroalkenes [146] (Figure 7.45), in which initial fluoride-ion attack occurs on the most reactive fluoroalkene (section IIC, Subsection 2). 

Examples of fluoride-ion-initiated reactions involving perfluorinated epoxides are shown in Figure 8.47 [155-158]. Hexafluoropropene oxide (HFPO) and tetrafluoroethene oxide will polymerise under certain conditions in the presence of fluoride ion. The process involves an extending alkoxide and it is terminated by elimination of fluoride ion to give... 

An effective route to some sulphur(IV) compounds involves fluoride-ion-initiated reactions of a fluoroalkene with sulphur(IV) fluoride [249] (Figure 8.90). Alternatively, similar reactions with sulphuryl fluoride give perfluorodialkylsulphones or perfluoroalk-anesulphonyl fluorides [250] (Figure 8.91). 

Subsection 6). Mercurials have been obtained by fluoride-ion-initiated reactions of per-fluoroalkenes with mercury(II) chloride [31], and it is probable that the process could be extended considerably (Figure 10.6). 

With the background theoretical and experimental studies detailed in the previous sections, it is now possible to study structure and bonding in polymer systems which have proved intractable by more conventional techniques. As an interesting example of this, we may consider the identification of a polymer produced in substantial yield as a by-product in the fluoride ion initiated reaction of hexaf1uorobut-2-yne with fluorinated heterocyclic molecules. The reaction produces substantial amounts of an... 

Trimethylsilylation of enolizable carbonyl compounds and alcohols has also been accomplished by the fluoride ion promoted reaction with hexamethyldisilane and ethyl trimethylsilylacetate [48, 49], with high stereospecificity giving Z-enol ethers from ketones [50]. l-Trimethylsilyl-(l-trimethylsilyloxy)alkanes, produced from the reaction of aldehydes with hexamethyldisilane, undergo acid-catalysed hydrolysis during work up to yield the trimethylsilylcarbinols [51]. In the case of aryl aldehydes, the initially formed trimethylsiloxy carbanion produces the pinacol (Scheme 3.1). 

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. 

Displacement of perfluoroalkyl from silicon occurs, initiated by catalytic amounts of added fluoride ion, and reaction is especially effective with carbonyl sites as electrophiles. The process that has been established is outlined in Figure 10.43 [94]. 

The recently disclosed facile generation of benzyne 61 from 2-(trimethylsilyl)phenyltriflate 60, by fluoride ion-initiated elimination has generated renewed interest in this reactive species. For example, Cheng et al. have reported a palladinm-catalyzed three-component reaction involving benzyne, allyl chloride, and an allenylstannane to afford l-allyl-2-allenyl benzene 62 (Scheme 5.6.21). 

Acetylene equivalent. As a dienophile for the Diels-Alder reaction, it incorporates into the adducts two hetero substituents on trigonal carbons. After conjugate reduction with Li AIH4 the )8-silyl sulfone moiety is susceptible to fluoride ion-initiated fragmentation. 

Chambers R, Korn S, Sandford G (1994) Reactions involving fluoride ion. Part 37. Proton Sponge hydrofluoride as a fluoride ion donor. J Fluor Chem 69 103-108 Chambers R, Gray W, Korn S (1995) Reactions involving fluoride ion. Part 40. Amines as initiators of fluoride ion catalysed reactions. Tetrahedron 51 13167-13176... 

Fluoride ion produced from the nucleophilic addition-elimination reactions of fluoroolefins can cataly7e isomerizations and rearrangements The reaction of per fluoro-3-methyl-l-butene with dimethylamine gives as products 1-/V,/Vdimeth-ylamino-1,1,2,2,4,4,4-heptafluoro-3-trifluoromethylbutane, N,W-dimetliyl-2,2,4,4,4-pentafluoro 3 trifluoromethylbutyramide, and approximately 3% of an unidentified olefin [10] The butylamide results from hydrolysis of the observed tertiary amine, and thus they share a common intermediate, l-Al,A -dimethylamino-l,l 24 44-hexafluoro-3-trifluoromethyl-2-butene, the product from the initial addition-elimination reaction (equation 4) The expected product from simple addition was not found... 

Kinetic studies also provide other evidence for the SnI mechanism. One technique used F NMR to follow the solvolysis of trifluoroacetyl esters. If this mechanism operates essentially as shown on page 393, the rate should be the same for a given substrate under a given set of conditions, regardless of the identity of the nucleophile or its concentration. In one experiment that demonstrates this, benzhy-dryl chloride (Ph2CHCl) was treated in SO2 with the nucleophiles fluoride ion, pyridine, and triethylamine at several concentrations of each nucleophile. In each case, the initial rate of the reaction was approximately the same when corrections were made for the salt effect. The same type of behavior has been shown in a number of other cases, even when the reagents are as different in their nucleophilicities (see p. 438) as H2O and OH . 

The initiation reaction in the polymerization of vinyl ethers by BF3R20 (R20 = various dialkyl ethers and tetrahydrofuran) was shown by Eley to involve an alkyl ion from the dialkyl ether, which therefore acts as a (necessary) co-catalyst [35, 67]. This initiation by an alkyl ion from a BF3-ether complex means that the alkyl vinyl ethers are so much more basic than the mono-olefins, that they can abstract alkylium ions from the boron fluoride etherate. This difference in basicity is also illustrated by the observations that triethoxonium fluoroborate, Et30+BF4", will not polymerise isobutene [68] but polymerises w-butyl vinyl ether instantaneously [69]. It was also shown [67] that in an extremely dry system boron fluoride will not catalyse the polymerization of alkyl vinyl ethers in hydrocarbons thus, an earlier suggestion that an alkyl vinyl ether might act as its own co-catalyst [30] was shown to be invalid, at least under these conditions. 

Trifluoromethyl thioethers are produced in a fluoride-catalysed one-pot reaction of alkyl or aryl thiocyanates with trifluoromethyl silanes [37]. The reaction is initiated by fluoride ion displacement of the trifluoromethyl anion from the silane the thioether is formed from the thiocyanate by displacement by the trifluoromethyl anion of the cyanide ion, which then perpetuates the reaction. Trifluoromethyl selenoethers are obtained by an analogous route. In a similar manner, disulphides can be converted into trifluoromethyl thio- or selenoethers [38],... 

As previously reported, the radical addition of CF2Br2 on glycals (initiated by sodium dithionite) affords difluorobromomethylated compounds. These latter molecules are easily dehydrohalogenated in the presence of TBAF. Under such conditions, these difluorovinyl compounds can add a fluoride ion (from TBAF). The subsequent elimination of the acetate moiety yields trifluoromethyl unsaturated compounds. The double bond can then be reduced (Figure 6.37). The same kind of reaction occurs in the presence of DAST with gcm-difluor-omethylene compounds, which are obtained by addition of an ylide onto an ulose (Figure 6.37). 

---