Carbocations sulphur-stabilized

Electrophilic substitution of vinylsilanes may be combined with the fazarov cyclization to build five-membered rings, as depicted in Figures i5.11 and Si5.12 (LA = Lewis acid). Note that the reaction depicted in igure Si5.12 is anomalous because the vinylsilane is attacked by the lectrophile at the end of the double bond remote from silicon. This is ecause the sulphur lone pairs stabilize a carbocation more effectively than le silicon P effect. 

According to this mechanism, the reaction occurs through formation of the semithioketal and the carbocation intermediates. Moreover, at the same proton concentrations the concentration of (II) is higher than that of the protonated hydroxyacetone. This is an effect of the smaller electronegativity value of sulphur, which enables the higher stabilization of the carbocation by Ji-electrons. An increase of the rate of the limiting step causes an increase of catalysts activity and their selectivity. 

---