Silicon Cation stabilisation

The second Prins reaction goes through the oxonium ion 223 to give the final product. Again the nucleophile is an allyl silane 223 and the second intermediate is a cation stabilised by the silicon p-effect. The relative stereochemistry is decided in the second reaction and 220 has the favoured diequatorial conformation. 

Fig-1- The silicon (3-effect stabilisation of a (3-cation enhances the nudeophilicity of allyl-... 

Numerous investigations, especially by the groups of Lambert, Olah and Reed demonstrate that the verification of a silyl cation requires counteranions and solvents with a nucleophilicity as low as possible and bulky substituents at the silicon center. For thermodynamic stabilisation of silyl cations amino-, alkylthio-, or alkoxy- substituents NR2, SR or OR are recommended, using their ability for back bonding into the empty p orbital at silicon. 

Synthesis silylation, Wittig, and cycloaddition reactions The stabilisation of cations by silicon... 

Even if silicon chemistry is new to you, you should by now have a picture of stable compounds with C-Si bonds and selective reaction with fluoride. You are already familiar with silyl enol ethers as nucleophilic enolate equivalents and allyl silanes resemble these in many ways. The missing link is the i-silyl effect. A Si atom stabilises a cation in the p-posit ion by overlap of the populated and relatively high energy C-Si c-orbital with the empty p orbital of the cation. This overlap is already present in the preferred conformation 95a of the allyl silane 95 as an anti-bonding interaction 95b between the C-Si c-orbital and the n orbital of the double bond. The resultant molecular orbital (the new HOMO) 95c increases the nucleophilic reactivity of the carbon atom in the y-position. 

Another way that allenes can be used to provide an acyl anion equivalent is by the Lewis acid catalysed addition of propargyl silanes to electrophiles 109 followed by oxidative cleavage of the allene. The intermediate vinyl cation 110 is stabilised by the silicon (3-effect (chapter 12) and loss of the Me3Si group gives the allene 111. Oxidative cleavage of the sensitive allene reveals that it has acted as a reagent for the formyl anion. 

The first step 221, whether you call it a Prins reaction or not, gives a cation 222 stabilised by both silicon and tin (chapter 12). Nucleophilic attack on tin is preferred (tin is lower down the... 

As the transition alumina stabilisation is more effective when the added cation is larger or more charged, it is recommanded that silicon, lanthanum and zirconium are used. 

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