Carbanions silyl enol ether formation

Having defined the types of commonly used carbon nucleophiles and carbon electrophiles, it would seem that if you react any of the carbon nucleophiles (electron donors) with any of the carbon electrophiles (electron acceptors), then a carbon-carbon bond should be formed. While this is theoretically true, it is unworkable from a practical point of view. If, for example, a carbanion nucleophile was reacted with a cationic electrophile, it is unlikely that the desired carbon-carbon bond formation would be detected, even after the smoke cleared. Or if a silyl enol ether nucleophile was reacted with an a, /f-unsaturated ester, no reaction could be observed to take place in any reasonable time frame. 

Variable experimental conditions are suitable for these arylations, from low to reflux temperatures in t-butanol or DMF the substrate is first converted into a carbanionic form in situ by a strong base. Several ordinary ketones were phenylated under conditions favouring the formation of potassium or lithium enolates and also through their silyl enol ethers. A general method has been developed for the synthesis of a-phenylketones and a,a-diphenylketones from their silyl enol ethers and diphenyliodonium fluoride, as shown in Table 8.3. 

The reaction of acylsilanes with a-sulfinyl carbanions proceeds through the initial formation of an a-silyl alkoxide intermediates (5), followed by the cationotropic migration of silyl group to oxyanion and the elimination of sulfenate ion, to afford the corresponding silyl enol ethers (Scheme 3). ... 

Addition of lithiated trimethylsilyldiazomethane to aldehydes provided adduct 103 which was shown through in situ IR measurements and quenching experiments to be in equilibrium with the 1,3-Brook-rearranged carbanion 104. Addition of methanol or methyl iodide provided 105 and 106, respectively. Stereocontrolled hydride shift upon formation of the rhodium-stabilized carbenoid provided (2)-selective formation of silyl enol ethers 107 and 108. ... 

Methods for the introduction of an a-alkylthio-group into an aliphatic ketone have been established, involving either treatment of the silyl enol ether derived from the ketone with MeLi and a sulphenyl chloride, or a-carbanion formation from the ketone with the Li salt of a secondary amine, followed by treatment with a disulphide or a sulphenyl halide. ... 

Among common carbon-carbon bond formation reactions involving carbanionic species, the nucleophilic substitution of alkyl halides with active methylene compounds in the presence of a base, e. g., malonic and acetoacetic ester syntheses, is one of the most well documented important methods in organic synthesis. Ketone enolates and protected ones such as vinyl silyl ethers are also versatile nucleophiles for the reaction with various electrophiles including alkyl halides. On the other hand, for the reaction of aryl halides with such nucleophiles to proceed, photostimulation or addition of transition metal catalysts or promoters is usually required, unless the halides are activated by strong electron-withdrawing substituents [7]. Of the metal species, palladium has proved to be especially useful, while copper may also be used in some reactions [81. Thus, aryl halides can react with a variety of substrates having acidic C-H bonds under palladium catalysis. 

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