Student Summer Scholars


Resonance and the Carbon - Silicon Double Bond



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The allyl anion is a primary carbanion attached to a carbon-carbon double bond. It has fundamental significance, because it is the smallest organic molecule that displays resonance between two possible Lewis structures which gives the molecule added stability. The allyl anion is also of practical interest because of its similarity to the intermediate in anionic butadiene polymerization, which is important in the production of rubber. One measure of the extra stability due to resonance for the allyl anion is its rotational barrier, which has been found to be 10.7 kcal/mol for allyllithium.

The silaallyl anion is what is formed if one replaces a carbon atom of the allyl anion with a silicon atom. Like the allyl anion, the silaallyl anion has both practical and theoretical significance. Practically, the silaallyl anion could be useful as a ligand on transition metals or as an intermediate in the formation of new polymers. Once the silaallyl anion has been synthesized the rotational barrier can be measured in a manner similar to that for the allyl anion, which will be useful for determining the significance of the resonance structure of the silaallyl anion.

The steps that lead to the silaallyl anion have been optimized and a convenient method for modifying the substituents at silicon has been developed. Previously chlorodiphenyl(vinyl)silane was reacted with lithium which should form the anion through a radical pathway, but the diphenyl system proved to not be sterically hindered enough to inhibit polymerization so larger protecting groups needed to be explored. We are investigating the use of the chlorodimesitylvinyl silane as one route to the silaallyl anion. A second route we have studied involved synthesizing 1,1,1,3,3,3-hexamethyl-2-vinyltrisilane and then studying its reaction with tert-butyllithium. We observed a color change which is indicative of the formation of the silallyl anion, but because prior steps lead to products that are difficult to isolate, we did not have enough anion to fully characterize it. Currently the 1,3-di-tertbutyl-1,1,3,3-tetramethyl-2-vinyltrisilane system is being synthesized which should solve the issues involved in the synthesis of 1,1,1,3,3,3-hexamethyl-2-vinyltrisilane and give us more of the product anion to study.