Negative hyperconjugation in silicon

The continued presence of second-row type stability in certain organosilicon compounds is known as the silicon α and β effects, after the corresponding locants.

In 1946, Leo Sommer and Frank C. Whitmore reported that radically chlorinating liquid ethyltrichlorosilane gave an isomeric mixture with exhibited unexpected reactivity in aqueous base.

[1] In 1994, Yong and coworkers compared the free-energy effects of α- and β-Si(CH3)3 moieties on C–H homo- and heterolysis.

[3] The silicon α and β effects arise because 3rd period heteroatoms can stabilize adjacent carbanions charges via (negative) hyperconjugation.

In the α effect, reactions that develop negative charge adjacent to the silicon, such as metalations, exhibit accelerated rates.

More generally, (i.e. even for "naked" carbanions) the Si σ* orbitals help stabilize the electrons on the α carbon.

[2] Generally, this effect has been rationalized as an intramolecular donor-acceptor interaction between the lone pair of the organofunctional group (such as NR2, OC(O)R, N(H)COOMe) and the silicon atom.

Sommer, Leo H.; Dorfman, Edwin; Goldberg, Gershon M.; and Whitmore, Frank C. "The reactivity with alkali of chlorine-carbon bonds alpha, beta and gamma to silicon."