Silicon-oxygen bond, cleavage

Alkyl silyl ethers are cleaved by a variety of reagents Whether the silicon-oxygen or the carbon-oxygen bond is cleaved depends on the nature of the reagent used Treatment of alkoxysilanes with electrophilic reagents like antimony tri-fluonde, 40% hydrofluonc acid, or a boron tnfluonde-ether complex results in the cleavage of the silicon-oxygen bond to form mono-, di-, and tnfluorosiloxanes or silanes [19, 20, 21) (equations 18-20)... 

CLEAVAGE OF SILICON-CARBON AND SILICON-OXYGEN BONDS... 

Cleavage of Silicon-Carbon and Silicon-Oxygen Bonds... 

The complete process for synthesizing such species using this approach would entail the acquisition of an appropriate natural silicate or the preparation of an appropriate synthetic silicate and then the conversion of this silicate into the alkyl silicate or organosiloxane by suitable substitution reactions. In terms of bond cleavage, this process could entail no destruction and reformation of framework silicon-oxygen bonds, and, in terms of oxidation number, it would entail no reduction and reoxidation of the silicon. 

This process contrasts with the elemental-silicon processes sometimes used for alkyl silicates (8) and the elemental-silicon processes generally used for oligomeric and polymeric organosi-loxanes ( ,7) Since the silicon in these processes is obtained from quartz, these processes entail, in terms of bond cleavage, the destruction of four silicon-oxygen bonds per silicon and the subsequent reformation of the required number of such bonds. In terms of oxidation number, they entail the reduction of the silicon from four to zero and then its reoxidation back to four, Figures 2 and 3. 

It has been suggested317 that the position of ring cleavage in such cyclic, (tetraisopropyldisiloxane-l,3-diyl) derivatives on partial hydrolysis could possibly be rationalized in terms of the acidity of the carbon-bound hydroxyl group that is formed on hydrolysis of a silicon-oxygen bond. 

Basic hydrolysis of secondary alkyl-substituted silicon and germanium peroxides results in oxygen-oxygen bond cleavage. 

PCC can used for the oxidation of silyl-protected hydroquinones to quinones, except where there are electron-withdrawing substituents on the aromatic ring. Interestingly, there was no evidence for cleavage of the silicon-oxygen bond as the first step, which might be expected under the acidic conditions of a PCC oxidation. 

Introducing the Triazole Rina. The final step of the synthesis was displacement of the carbon-bound chlorine with triazole salts. Once again, silicon made life easy for us, since it activates such hindered systems toward displacement. The corresponding all-carbon compounds react very sluggishly with triazole salts. Luckily, silicon-carbon bond cleavage is not observed, provided water or other oxygen nucleophiles are excluded. The displacement reaction is illustrated in Equation 5 for DPX-H6573. 

The intervention of Berry pseudorotation processes was invoked to account for the intramolecular ligand permutation. The authors noted, however, that an isomerization by silicon-oxygen bond breaking cannot be strictly excluded. Indeed, it represents a plausible mechanism since a high activation energy is required (within the expected range for a bond cleavage in these anionic alkoxy-silanes). To eliminate this possibility Farnham and Harlow prepared the spiro-siliconate 196 (eq. [108]). Compound 196, which contains an apical fluorine... 

Fig. 27. Cleavage of a silicon-oxygen bond with NaNH2-...

In addition to their use for the preparation of specific Uthium enolates, silyl enol ethers are also excellent substrates for actrf-catalysed alkylation. In the presence of a Lewis acid (e.g. TICU, SnCU, BF3 OEt2> they react readily with tertiary alkyl halides to give the alkylated product in high yield. This procedure thus complements the more-common base-catalysed alkylation of enolates which fails with tertiary haUdes. It is supposed that the Lewis acid promotes ionization of the electrophile, RX, to form the cation R+, which is trapped by the silyl enol ether to give the addition product with cleavage of the silicon-oxygen bond. 

As with the analogous diazomethane case, the reaction can be interpreted as an intramolecular nucleophilic attack on silicon by the electron-rich oxygen of the original carbonyl group, leading to silicon-carbon bond cleavage, formation of the alkene double bond, and loss of triphenyl-phosphine. 

As can be easily appreciated from the table of bond dissociation energies, reactions involving formation of silicon-oxygen bonds from silyl iodides, bromides, and chlorides are exothermic. Of equal significance, reactions that result in formation of the silicon-fluorine bond (the strongest single bond known) enable cleavage of virtually all other bonds to silicon. 

Seven-membered ring trani-alkenes have been accessed diastereoselectively from dienes and aldehydes by the silver-catalysed transfer of silylene (Scheme 13). The trans geometry of the double bond has been assigned from cleavage of the silicon-oxygen bond (path (a)). The high strain of these cyclic trawi-alkenes can... 

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