10.3.3.2 Allyl Silanes

Allylsilanes can be readily prepared from allyl halide via Grignard reaction (Scheme 13).

Scheme 13

Allyl silanes are more reactive compared to vinyl silanes (Scheme 14). This is because vinyl silanes have C-Si bonds orthogonal to the p orbitals of the alkene, in contrast, allyl silanes have C-Si bonds that can be parallel to the p orbitals of the double bond so that interaction can be possible. However, both react with electrophiles at the ipso atom occupied by silicon. In both cases a b -silylcation is an intermediate.

For example,

Scheme 14

In optically active compounds, one enantiomer of the allyl silane gives one enantiomer of the product (Scheme 15). The stereogenic centre next to silicon disappears and new one appears.

Scheme 15

Allyl silanes also attack carbonyl compounds in the presence of Lewis acid which activates the carbonyl group (Scheme 16).

Scheme 16