Silicon-carbon compounds silyl anions

The resulting dark green solution was converted into the expected 1-methyl-1-(trimethylsilyl)-l-silafluorene by reaction with an excess of MesSiCl. The Si NMR chemical shift for 128 (S = —22.09 ppm) is in the range of aryl-substituted silyllithium compounds, e.g. Ph2MeSi Li+, which have no delocalization of the negative charge on silicon to the phenyl substituents. Downfield shifts of the ring carbons are also consistent with a localized silyl anion. 

Reaction with ei,fi-Unsaturated Sulfoxides. The reaction of TMSI with a, -unsaturated sulfoxides in chloroform at ambient temperature is a mild, efficient, and general method for the preparation of carbonyl compounds (eq 63). The proposed reaction mechanism is shown in eq 63. Formation of a strong oxygen-silicon bond is followed by reduction of the sulfur function and oxidation of iodide to iodine, the latter precipitating in chloroform. The trimethylsiloxy anion attacks the unsaturated carbon linked to the sulfur function, which leaves the substrate, allowing the formation of the sUyl enol ether species. Finally, hydrolysis converts the silyl enol ether into the carbonyl compound. ... 

The Wittig reagent is made by alkylation of the simplest ylid with the same silicon reagent. Notice that the leaving group (iodide) is on the carbon next to silicon, not on the silicon itself. Anion formation occurs next to phosphorus because PhsF is much more anion-stabilizing than MesSi. The ylid reacts with carbonyl compounds such as cyclohexanone in the usual way to produce the allyl silane with no ambiguity over which end of the allyl system is silylated. 

In the Peterson reaction of an a-silyl carbanion bearing a phosphorus substituent on the anionic carbon atom, there are two possibilities for alkene formation, that is, the Peterson reaction to form phosphorus-substituted alkenes and the Wittig-Horner reaction and the Wadsworth-Emmons reaction to form silicon-substituted alkenes. Most of the reports on these competing reactions have been focused on the reactions of a-silyl phosphonates with carbonyl compounds. It is noteworthy that the alkenylphosphonates have been exclusively obtained in almost every case. That is, the Peterson reactions override the Wittig and Wadsworth-Emmons reactions. 

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