In principle, the alkene could react with the electrophile at either end but reaction next to silicon leads to the more stable cation β to silicon. In the vinyl silane, the C-Si bond is orthogonal to the p orbital of the π bond, but as the electrophile attacks, the silyl group starts to move upwards, increasing the interaction between the C-Si bond and the empty p orbital of the cation. Rotation is restricted and the C-Si bond stabilization is decreased, resulting in the quick removal of the silyl group to form the alkene which retains it’s geometry.
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