text: Electrophilic additions of allylsilanes generally occur via an anti S2 process. Allylsilanes react through a conformation in which the smallest substituent on the carbon attached to silicon is essentially eclipsing the double bond. The silyl moiety forces electrophilic attack on the face opposite the silyl group for steric and electronic reasons, although the effects are not large. This model predicts that when the double bond is 1,2-disubstituted, the Z isomer should exhibit greater selectivity than the E' isomer, and this has been observed
  Vinylsilane additions proceed with retention of double bond configuration, and follow a similar principle. After addition of the electrophile to the top or bottom face of the double bond, the silyl moiety rotates to become parallel to the adjacent empty 2p orbital. The principle of least motion provides that the electrophile moves into a position close to that formerly occupied by the silyl group. Thus, the configuration of the double bond is retained after loss of the silyl group.