Reactions with fluoride ion

All the rearranged products derived from (12) and (15) have been rationalized as arising by proton loss or reaction with fluoride ion of the respective homoallylic C-19 cations. The structures of the cations derived from (15) are represented by structures (20) to (24)." ... 

Electronegatively substituted acetylenes, such as dimethyl acetylenedicar-boxylate, do not react under normal conditions but will add the elements of hydrogen fluoride by reaction with fluoride ion (e g, CsF or tetraalkylammonium dihydrogen trifluoride) and a proton source under phase-transfer conditions [49, 50] (equation 8)... 

A carbon labelling study has elucidated the rearrangement mechanism for formation of chalcone (97) which accompanies formation of (91) by the expected vicinyl elimination of trimethylsilyl and benzotriazolyl groups from 2-benzotriazolyl-2-aryl-3-ketopropylsilanes, on reaction with fluoride ion in DMF. ° Thus, it has been possible to distinguish between the two alternative mechanisms depicted in Scheme 11 (via intermediates (93) or (95), respectively, by determining the fate of the labelled quaternary carbon of substrate (89). The results are consistent with the formation of a cyclopropane intermediate (95) which subsequently ring opens, with relief of strain, to form delocalized carbanion (96), from which the chalcone (97) is obtained (labelled... 

The use of /i-ketocstcrs and malonic ester enolates has largely been supplanted by the development of the newer procedures based on selective enolate formation that permit direct alkylation of ketone and ester enolates and avoid the hydrolysis and decarboxylation of ketoesters intermediates. Most enolate alkylations are carried out by deprotonating the ketone under conditions that are appropriate for kinetic or thermodynamic control. Enolates can also be prepared from silyl enol ethers and by reduction of enones (see Section 1.3). Alkylation also can be carried out using silyl enol ethers by reaction with fluoride ion.31 Tetraalkylammonium fluoride salts in anhydrous solvents are normally the... 

In dilute aqueous solutions, the boric acid molecule remains undissociated B(0H)3 but in concentrated solutions, B(OH)4 and complex polyborate anions, such as B303(0H)4 are formed. Reactions with fluoride ion produce fluoroborates, BF(OH)3, BF2(OH)2, BFsCOH), BF4, and B3O3F63 in stepwise sequence. It forms similar polyions with amides and borates. 

Reaction with fluoride ion forms the adduct oxofluoro ion, [0s04F2]2. For example, osmium tetroxide reacts with sodium fluoride to form an oxofluoro salt of sodium ... 

The nucleophile for which increased reactivity is most critical is the fluoride ion [3,4], Water molecules bind tightly to this ion, and their presence dramatically reduces its effective nucleophilicity. A variety of fluoride ion sources have been used in an effort to improve product yields in deoxyfluoro sugar synthesis [26,34]. The yields of substitution and elimination products generated from reactions with fluoride ion from several sources are listed in Table 2 [26]. Currently, the most attractive source of fluoride ion is tris(dimethylanuno)-5ulfur (trimetbylsilyl)difluoride (TASF), which is soluble in a variety of oiganic solvents and produces an anhydrous fluoride ion [33]. 

The striking nucleofugacity of the trimethylammomum leaving group compared with the mtro group in the reaction with fluoride ion is illustrated by the relative reactivities of p-nitrophenyltnmethylammonium perchlorate and p dini-trobenzene [73] (Table 4)... 

Reactions with fluoride ion With the exception of CF3OH (see the next section), fluorinated alcohols of the type RpCF20H are not known [124] but complexes of K, Rb, Cs, Ag or (C2H5)4N+ fluorides with hexafluoroacetone have been isolated [125, 126], following from the earlier isolation of some similar complexes with carbonyl fluoride [127]. These complexes have been reasonably formulated as fluorinated alkox-ides (Figure 8.27), but the use of these salts in synthesis is often difficult because the complexes may also act as fluoride-ion donors [128]. 

Solvation of the ions is certainly a factor in these reactions, with fluoride ion being much more strongly solvated than the other anions. However, the trend is also related to changes in the degree of interaction between the halides and the silver ions. The interactions can be expressed in terms of hard and soft acids and bases (HSAB), in which... 

Reactive ion etching (RIE) and deep reactive ion etching (DRIE) are common examples of plasma etching. In this incarnation, the substrate to be etched is placed on a powered electrode in a plasma chamber. Process gases are admitted into the chamber and a plasma is struck. Because the substrate is directly in the ion flux of the plasma, the ions impinge on the surface and may participate in chemistry. For example, RIE of polysilicon may use SFs as the reactive gas, and etches the Si by a reaction with fluoride ions fo form fhe volatile product SiF4, as illustrated by Eqs. (4) and (5). ... 

Fluorination of aromatics by XeF2 in the presence of HF is thought, contrary to earlier belief, not to involve the aromatic cation radical. However, a cation radical is thought to accept fluoride ion in reactions of XcF2 with easily oxidized aromatics (anisole) in the absence of HF (Anand etal., 1975). In this connection zinc tetraphenylporphyrin has been oxidized to its cation radical with xenon difluoride (Forman et al., 1971) although fluorine hyperfine splitting was found in the esr spectrum, reaction with fluoride ion did not occur. 

---