Cation stabilisation

In acid solution, cobalt creates a Ji-stabilised cation at the site of the OH group that cyclises onto the allyl silane with the expected regioselectivity and excellent enantioselectivity. The nature of the cation stabilisation and the removal of the cobalt are discussed in the workbook. The structure of this group Co(dmgH)2py is explained in the workbook. 

Of course, if the intermediate cation, stabilised by the P-silyl cation effect, has a lifetime long enough to allow rotation about the C C bond, the stereochemical information is lost. Clear-cut examples with retention are often intramolecular and the most valuable are those that create an exocyclic alkene. A famous example is the cyclisation of the two acetals E- and Z-195 with SnCI4 as the Lewis acid to give reasonable yields of the oxepanes 196 with complete retention of configuration at the exocyclic alkene.47... 

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. 

Nucleophilic radicals carry cation-stabilising groups on the radical carbon, allowing electron density to be transferred from the radical to an electron-deficient heterocycle they react, therefore, only with electron-poor heterocycies and will not attack electron-rich systems examples of such radicals are CH20H, alkyl", and acyl". Substitution by such a radical can be represented in the following general way ... 

HX gives the 6 coordinate cation L2Si. Bis(8-dimethylaniinonaphthyl)sUane is deprotonated by I2 to give the 5 coordinate cation stabilised by p-t-BuLLi reacts... 

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