Carbonium ions silicon-stabilized

The reactivity of the organosilanes65 and organostannanes66 towards electrophiles is dependent on the characteristics of the organic ligands. Typically, the alkylsilanes and alkylstannanes are unreactive, which is a consequence of the weakly polarized carbon-silicon and carbon-tin cr-bonds (C8-—Ms+). However, allylsilanes67 and allylstannanes are highly reactive to electrophiles because of extensive ct-tt (C—Si or C—Sn) conjugation in the ally metals and the 0-carbonium ion stabilization effect of the metal center. Consequently, electrophiles add exclusively with allylic transposition. 

The failure of difluoramine to appear among the final products is not particularly surprising. In the presence of nitric acid and/or nitrogen oxides, it might easily be oxidized and may well constitute the source of the silicon tetrafluoride. The formation of a carbonium ion from trityl-difluoramine would be favored by resonance stabilization. In the tert-butyl case, on the other hand, this driving force is not present and formation of the ion would be expected to occur less readily. In addition, both the tert-butyl carbonium ion and the difluorammonium ion from which it is derived would be more subject to a variety of side reactions than the corresponding trityl species. 

The stabilization of a 3-carbonium ion is also involved in the reaction of an allylsilane with an electrophile (Eq. 14.16). An advantage of silicon over other metals in this context is that it does not undergo 1,3 shifts, and so the point of attachment of the electrophile can be reliably predicted (Eq. 14.17). - This 3 stabilization of the carbonium ion also has stereochemical implications Eq. 14.18 shows how the stereochemistry of a vinylsilicon reagent can be retained on protonation. A TMS group on carbon has been described as a... 

Vinyl—silicon bonds are also rather easily cleaved by electrophiles. It appears that the R3Si group is able to stabilize //-carbonium ions. Allylsilanes are prone to attack even by fairly weak electrophiles. Acetals, aldehydes and ketones react in the presence of Lewis acids to form unsaturated alcohols. The reaction is accompanied by an allylic transposition ... 

The ability of silicon to stabilize jS-carbonium ions results in regioselective Baeyer-Villiger oxidations in ketones such as (33) leading to acids (34) after removal of the silane function.The full potential of this observation has yet to be delineated. 

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