P-Carbocation

Vogel, P. Carbocation Chemistry, Elsevier Amsterdam, Holland, 1985. 

Another approach uses silicon s ability to stabilize P-carbocations (P-effect). In addition, the TMS group behaves like a proton and is removed after nucleophilic activation ... 

Recall that maximum stabilization of a p-carbocation only occurs if the vacant p orbital and the carbon-silicon bond can be in the same plane. 

Chlorobutanoic acid is dehydrohalogenated to 2-butenoic acid and HCl is added. H adds to give a P-carbocation which bonds to Cl to form the -chloroacid. 

Sulfur-stabilized P-carbocations (three-membered rings)... 

Three-membered cyclic sulfonium ions, representing P carbocations, are often encountered in participation reactions. We have seen this already in the way mustard gas works, but almost any arrangement of a sulfide with a leaving group on the P carbon atom leads to participation and the formation of a three-membered ring. The product is formed by migration of the PhS group from one carbon atom to another (Chapter 37). 

Protonation of the hydroxyl and trimethylsilyloxy groups of A and D, respectively, generates carbocations B and E. In B, attack of the methyl-substituted triple bond leads to formation of the preferred linear vinyl cation. Its trapping by the formate ion furnishes the substituted bicyclo[2.2.2]octene C. On the other hand, attack of the silicon-substituted triple bond in E occurs to form the silicon-stabilized p-carbocation, which, on trapping by formate ion, yields the substituted bicyclo[3.2.2]nonene F. 

Vinylsilanes react with a variety of electrophiles to give substitution or addition products. In substitution reactions, the silyl group is replaced by the electrophile because formation of the carbon-carbon rc-bond is faster than capture of the incipient P-carbocation by the nucleophile. 

However we rationalized that using silicon as a migrating group could result in a unique stabilization of the energy surface of olefin isomerization. This rationalization was based on silicon s well known ability to stabilize both a-carbanions and p-carbocations. Thus a hypothetical "dual-stabilized" zwitterion would be produced by a 90° twist of a vinyl silane, and a following 1,2-shift of Silicon would produce a singlet carbene possessed of the same hyperconjugative stabilization as in a P-silyl cation (Eq. 5). 

In summary, it appears that bromination usually involves a complex that collapses to an ion pair intermediate. The ionization generates charge separation and is assisted by solvent, acids, or a second molecule of bromine. The cation can be a P-carbocation, as in the case of styrenes, or a bromonium ion. Reactions that proceed through bromonium ions are stereospecific anti additions. Reactions that proceed through open carbocations can be syn selective or nonstereospecific. 

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