Fluoride anion cleavage

Firstly, the possibility of generating an a-aminomethyl anion equivalent under mild and neutral conditions and in the presence of the electrophilic receptor via the fluoride ion cleavage of the Si-C bond was investigated by Patel and Joule,11 who observed the addition product of the anion to the starting material. Katritzsky and Sengupta12 then found that fluoride-mediated desilylation could be carried out in the presence of an aldehyde, allowing the facile synthesis of aminoethanols. 

Formation of a C-C bond (C-Si to C-C) a. Alkylation via fluoride anion assisted cleavage of the C-Si bond... 

If the cleavage of siloxycyclopropanes with fluoride anions could be performed under strictly anhydrous conditions, the ester enolate generated by ring opening should be interceptable by electrophiles different from protons. This process could be realized... 

Elaboration of 3.119 (prepared as in Fig. 3.45) to 3.121 (steps a,b. Fig. 3.46) was performed in standard experimental conditions. Final cleavage of 3.121 to 3.107 (steps c,d) was better performed using a recently developed traceless residue protocol (32) which converts fluoride anions to volatile silyl fluorides which are evaporated from the solution together with pyridine. The product 3.107 was obtained in a good 37% overall yield, calculated for the entire 14-steps SP protocol starting with the loading of 3.112 onto PS-DES resin. 

In the case of the tricyclic epoxides la-c the outcome of the reaction depends on the ring size. The formation of 2 and 3 was explained by the following mechanism. Cleavage of the epoxide ring with boron trifluoride diethyl ether complex gives the zwitterionic intermediate 4. Subsequent transfer of fluoride anion to the cationic center C2 leads to fluoroborate 5 (path a), which is hydrolyzed to yield 2. Spiro ketone 3 is obtained by a 1,2-alkyl shift in 4 (path b). 

A metal-free trimethylsilyl anion is formed from hexamethyldisilane by cleavage with TBAF in HMPA in equilibrium concentration, as revealed by H and F NMR analysis of the reaction mixture. Treatment of trisilane with TBAF provides Me3SiMe2Si /NBu4 and Me3SiF the fluoride anion attacks at the terminal silicon of trisilane [Eq. (21)] (59). Synthetic application of this species is described in Section VII. Other metal-free silyl anions are described in Section V. 

Besides the Fmoc (Section 2.1.1.1.1.3) and the substituted sulfonylethyl groups (Section 2.1.1.1.1.4) that are cleaved by (3-elimination under basic conditions, the A -2-(trimethylsi-lyl)ethoxycarbonyl (Teoc, 43)P 1 and 2-phenyl-2-(trimethylsilyl)ethoxycarbonyl (44)P 1 derivatives (Scheme 21) are readily removed by the action of the fluoride anion, the elimination products of the Grob-type reaction being TMSF, ethene or styrene, and a carbamate which releases carbon dioxide upon aqueous workup. The nature of the cleavage byproducts allows for easy isolation of the free amino compound. 

Numerous methods are now available in the literature for the deprotection of TBS ethers under a variety of conditions. One of the most effective ways for the cleavage of silyl ethers is based on the exploitation of the high affinity of silicon towards fluoride ions. Thus, a number of reagents involving one form of fluoride or another, such as tetrabutylammonium fluoride [292], BF3-Et20 [301], hydrofluoric acid [302], fluorosilicic acid [303], ammonium fluoride [304], silicon fluoride [305], lithium tetrafluoroborate [306], and chlorotrimethylsilane/potassium fluoride dehydrate [307] have been developed for the deprotection of TBDMS ethers. Among these, TBAF is most frequently used but the strong basicity of the fluoride anion makes it inappropriate for base sensitive functionalities. 

The reactions of bases with borylstannylalkanes generally lead to cleavage of the tin moietyand lithium thiophenoxide is especially specific. It is of interest that the reaction of fluoride anion with di-mesitylboryl(trimethylsilyl)alkanes also gives a-boryl carbanions, thus obviating the need to use organo-metallics. ... 

Hydrolysis of the esters (107) and (108) involves concomitant cleavage of both C-O and P-O bonds, the relative contributions of such fragmentations depending on experimental conditions. The reaction at carbon is favoured by an increa.se in temperature and is accelerated (60 times) by thiosulphate. Reaction at the P-O bond is favoured relative to that at the C-O bond by the addition of an organic. solvent (acetone) to the aqueous medium. Only P-O cleavage occurs in THF, in which the process is accelerated by fluoride anion (100 times). ... 

The reaction of 1 -trimethylsilylnaphthalene with pivaldehyde in the presence of 20 mol% Bu-P4 base proceeded smoothly at room temperature to give the alcohol in 91% yield. Other phosphazene bases with weaker basicities, such as Bu-P2 base and BEMP, showed no catalytic activity. As one of the conventional strong organic bases, DBU was found to be inactive. Caesium fluoride (CsF) was then examined as a fluoride anion donor, but no carbon-silicon bond cleavage was observed. Reactions with other aldehydes have been examined that with benzaldehyde was found to proceed somewhat slowly at room temperature. Other aryl aldehydes with electron-donating groups were also employed as electrophiles and the reactions proceeded smoothly at room temperature [57] (Table 5.5). 

The fluoride anion produces the cleavage of the silyl ether. 

Competitive reactions of FCN, F2C=NF, and F3CN=CF2 over CsF or KF establish the reactivity order towards the fluoride anion CF2=NF>CF3N = CF2>FCN. The reaction of N2F4 with RC=CR in inert solvents gives RFC(NF2)C(F)=NF via an intermediate RC(NF2) = C(NF2)R. Under solvolysis conditions, reaction can involve S yl cleavage of the N—F bond. Slow exchange of fluorine between CIF5 and CsF in anhydrous HF occurs via the formation of [CIF ]". 

An example of cleavage ol the sulfur-oxygen bond in trifluoromethane-sulfonic ester has been reported Tnfluororaethyl triflate reacts with neutral or anionic nucleophiles with elimination of carbonyl difluoride and formation of trifluoromethanesulfonyl fluoride [57] (equation 32) The mechanism of this reaction involves elimination of fluoride ion, which is a chain carrier in the substitution of fluorine for the trifluoromethoxy group... 

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