Silanes alkenyl, reactions with electrophiles

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

As with the silanes, the most useful synthetic procedures involve electrophilic attack on alkenyl and allylic stannanes. The stannanes are considerably more reactive than the corresponding silanes because there is more anionic character on carbon in the C-Sn bond and it is a weaker bond.156 The most useful reactions in terms of syntheses involve the Lewis acid-catalyzed addition of allylic stannanes to aldehydes.157 The reaction occurs with allylic transposition. 

Several new syntheses of vinylsilanes have been described. Tris(trimethyl-silyl)aluminium undergoes 5yn-addition to alkynes alternatively the same -isomers can be obtained by photochemical isomerisation of Z-1-alkenyl-silanes. Other methods described involve treatment of the lithium salts of hydrazones with trimethylsilyl chloride, Wurtz-type coupling with vinyl bromides, and reaction of acetylenes with a silyl-copper reagent followed by an electrophile. Using the hydrazone method, a route has been devised for 1,2-carbonyl transposition within ketones (Scheme 17). ... 

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