Silicon cleavage

Alkynes are viable substrates for the tandem intramolecular silylformylation/allylsilyla-tion reaction. The 1,5-diastereoselechvity is reversed from that observed with alkene substrates, as the 1,5-anti product predominates (Scheme 5.24) [38]. Synthetic flexibility was demonstrated in that either of two carbon-silicon cleavage procedures may be employed. Protodesilylahon with tetra-n-butylammonium fluoride followed by acetylation (determi-... 

Exactly the same sort of mechanism accounts for the reactions of aryl silanes with electrophiles under Friedel-Crafts conditions. Instead of the usual rules governing ortho, meta, and para substitution using the directing effects of the substituents, there is just one rule the silyl group is replaced by the electrophile at the same atom on the ring—this is known as ipso substitution. Actually, this selectivity comes from the same principles as those used for ordinary aromatic substitution (Chapter 22) the electrophile reacts to produce the most stable cation—in this case (3 to silicon. Cleavage of the weakened C-Si bond by any nucleophile leads directly to the ipso product. 

Oxidative aryl-silicon cleavage [14a, 61a] occurred almost quantitatively on exposure to lead tetralds(trifluoroacetate) to produce dl-estrone (Scheme 13.39) [61b]. 

Silicon-carbon bonds are cleaved by the action of various reagents, and electronegative groups such as halogen and alkoxy are introduced into the silicon part. Therefore these reactions are utilized for introduction of functionality into silicon. Cleavage of the Ar-Si bonds is simple with electrophilic reagents. Halodesilation is important for the preparation of chlorosUanes (equation 37). 

This often enables Si—Ph groups to be used as shielding groups in organometallic synthesis [60]. As more electronegative substituents are introduced onto silicon, cleavage is greatly retarded, and is eventually completely prevented. 

Baciocchi, E., Del Giacco, T, EUsei, R, and loele, M., Chloranil-sensitized photolysis of benzyltri-methylsilanes. Solvent effect on the competition between carbon-hydrogen and carbon-silicon cleavage, /. Org. Chem., 60, 7974,1995. 

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