Silicon—carbon bonds lithium metal

The reaction of heterocyclic lithium derivatives with organic halides to form a C-C bond has been discussed in Section 3.3.3.8.2. This cannot, however, be extended to aryl, alkenyl or heteroaryl halides in which the halogen is attached to an sp2 carbon. Such cross-coupling can be successfully achieved by nickel or palladium-catalyzed reaction of the unsaturated organohalide with a suitable heterocyclic metal derivative. The metal is usually zinc, magnesium, boron or tin occasionally lithium, mercury, copper, and silicon derivatives of thiophene have also found application in such reactions. In addition to this type, the Pd-catalyzed reaction of halogenated heterocycles with suitable alkenes and alkynes, usually referred to as the Heck reaction, is also discussed in this section. 

The hypercarbon atom environment in this compound, with one silicon atom, two hydrogen atoms, and two copper atoms in the carbon coordination sphere, with a Cu-C-Cu bond angle of 74°, is consistent with the formation of three 2c-2e bonds to the silicon and hydrogen atoms, and a 3c-2e bond to the metal atoms. Hiis open cyclic structure, which may be contrasted with the more compact tetrahedral structures of typical tetrameric lithium alkyls, suggests that the metal atoms are sp hybridized, unable to make use of as many AOs as lithium atoms can. 

Alkoxysilane termination forms new bonds between the silicon of the alkoxysilane and the carbon polymer atoms in the polymer end group. Wear resistance is improved by about 50% and tensile strength is increased by some 30%. Further developments aim to maximize the effect by reacting an organolithium compound with a polymer in the presence of an alkali metal alkoxide, producing a polymer in which the main chain has been metallated with a plurality of lithium atoms, and then attaching a plurality of alkoxysilane molecules to the chain. 

The stereochemically controlled addition of organometallic species of copper, tin, silicon, palladium, zirconium, and boron to acetylenes has been investigated as a route to di-, tri-, and tetra-substituted olefins. The carbon-metal bond thus formed is cleaved in a stereoselective manner either directly, or indirectly, via the corresponding vinyl-lithium reagents with a wide variety of electrophiles. In three... 

The same situation is met in R-M-N ternary nitrides in which the nature of the M element determines the dominating type of bond involved in the material. This is illustrated by the fact that with lithium (or barium) as a cationic element, the R-M-N corresponding nitride is essentially ionic in character, whereas with silicon, more covalent nitrido-silicates are formed. In addition, metallic nitrided alloys exist, with nitrogen located as an interstitial element in octahedral voids of the metal atom lattice. The presence of insertion nitrogen (as well as carbon) in such compounds is sometimes necessary for their existence, and can strongly modify the physical properties. 

Transition metal-catalyzed silicon-based cross-coupling reaction has emerged as a versatile carbon-carbon bond-forming process with high stereocontrol and excellent functional group tolerance [35], For example, (a-benzoyloxy)alkenylsilanes 105, prepared as a pure -isomer by 0-acylation of a lithium enolate derived from the corresponding acylsilane, reacts with carboxylic acid anhydrides in the presence of [RhCl(CO)2]2, giving rise to a-acyloxy ketones 106, which are then converted into 1,2-diketones by acidic workup (Scheme 5.27) [36]. 

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