Lithium carbon-silicon bonds

Aldimines can be trifluoromethylated at the imine carbon using Me3SiCF3 in dimethyl formamide at —20 °C, using a lithium carboxylate as catalyst.71 It is proposed that the carbon-silicon bond of the reagent is activated via formation of a lithium silicate bearing carboxylate and DMF ligands on silicon. A similar process has been used for diastereoselective addition to sulftnylimims.12... 

The silyl halides can now" be prepared in high purity and high yield by the facile hydrogen halide cleavage of the carbon-silicon bond in arylsilanes. " No catalyst is required, and the use of the hazardous intermediate reagent, silane, is avoided. Bromosilane is prepared by the reaction of hydrogen bromide and phenylsilane. The latter is obtained by lithium hydro-aluminate reduction of the commercially available phenyltri-chlorosilane. lodosilane can be prepared in a similar fashion however, mixtures of iodosilane and benzene are difficult to separate. The preferred alternative procedure described below utilizes an isomeric mixture of 2-, 3-, and 4-chlorophenylsilanes as the intermediate. This intermediate is obtained by the chlorination of phenyltrichlorosilane, and is then reduced to the hydride. 

After considerable experimentation, a similar hydrosilylation protocol was used as a key step for the syntheses of jatrophatrione and citlalitrione by Paquette and co-workers.32 Following the stereoselective reduction of a tricyclic ketone with lithium aluminum hydride to provide alochol 28, silylation and platinum catalyzed hydrosilylation were effected to produce 29. Finally, the carbon-silicon bond was successfully cleaved to generate diol 30 in an impressive 93% yield. 

Formation of Bis(trimethylsilyl)methyllithium. The carbon-silicon bond of tris(trimethylsilyl)methane can be cleaved by lithium methoxide in HMPA to formbis(trimethylsilyl)methyl-lithium, which subsequently reacts with carbonyl compounds to form alkenes (eq 4). ... 

Propargylic trimethylsilanes, now readily accessible by reaction of lithium acetylides with (trimethylsilyl)methyl halides or triflate, are emerging as useful synthetic intermediates. The general pattern of reactivity involves attack by an electrophile with cleavage of the carbon-silicon bond. For example, treatment of propargylic trimethylsilanes with TFA or bromine leads to allenes or 3-bromoallenes respectively, " and reactions with acetals furnish 4-alkoxy-allenes" (e.g. Scheme 101). 

Similarly, when silene 2a is generated in presence of excess tris(trimethylsilyl)silyllithium, the lithium silanide is added across the silicon-carbon double bond to give an organolithiiun intermediate, which undergoes a rearrangement, a l,3-Si,C-trimethylsilyl migration, resulting in formation of a lithium silanide, which is trapped with chlorotrimethylsilane to yield the polysilane 7. The H-silane 8 is obtained as the protonation product after usual hydrolytic work up (Eq. 4-5). 

Recently, we were able to show that lithium organyls are also able to interact with CO [6] in a newly developed LXe cell constructed from one piece of single crystal silicon [7]. In a first step carbon monoxide is complexed by back-bonding to BuLi (n(CO) 2047 cm ) and inserts in a second step at higher temperature into the lithium-carbon bond (n(CO) 1635 cm ). Further... 

Most of the known 1,3-disilacyclobutanes were synthesized by [2+2] cycloaddition of the corresponding silicon-carbon double bonds or the "Kriner Reaction" of chloro(chloromethyl)silanes with magnesium or lithium [3], The 1,3-disilacyclobutanes prepared by these reactions are normally symmetrically substituted (Scheme 1). 

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... 

Silicon, unlike carbon, does notiorm a very large number of hydrides. A series of covalently bonded volatile hydrides called silanes analogous to the alkane hydrocarbons is known, with the general formula Si H2 + 2- I uf less than ten members of the series have so far been prepared. Mono- and disilanes are more readily prepared by the reaction of the corresponding silicon chloride with lithium aluminium hydride in ether ... 

Because carbon bonds so readily with itself, there are many hydrocarbons (see Chapter 18). Silicon forms a much smaller number of compounds with hydrogen, called the silanes. The simplest silane is silane itself, SiH4, the analog of methane. Silane is formed by the action of lithium aluminum hydride on silicon halides in ether ... 

The first stable silaallene, 56, was synthesized in 1993 " " by the intramolecular attack of an organolithium reagent at the /f-carbon of a fluoroalkynylsilane (Scheme 16). Addition of two equivalents of r-butyllithium in toluene at O C to compound 54 gave intermediate 55. The a-lithiofluorosilane then eliminated lithium fluoride at room temperature to form the 1-silaallene 56, which was so sterically hindered that it did not react with ethanol even at reflux temperatures. 1-Silaallene 56 was the first, and so far the only, multiply bonded silicon species to be unreactive toward air and water. The X-ray crystal structure and NMR spectra of 56 is discussed in Sect. IVA. 

The same principles that are valid for the surface of crystalline substances hold for the surface of amorphous solids. Crystals can be of the purely ionic type, e.g., NaF, or of the purely covalent type, e.g., diamond. Most substances, however, are somewhere in between these extremes [even in lithium fluoride, a slight tendency towards bond formation between cations and anions has been shown by precise determinations of the electron density distribution (/)]. Mostly, amorphous solids are found with predominantly covalent bonds. As with liquids, there is usually some close-range ordering of the atoms similar to the ordering in the corresponding crystalline structures. Obviously, this is caused by the tendency of the atoms to retain their normal electron configuration, such as the sp hybridization of silicon in silica. Here, too, transitions from crystalline to amorphous do occur. The microcrystalline forms of carbon which are structurally descended from graphite are an example. 

Organic groups are bound to the silica surface after grinding silica in organic liquids (277). A more controlled substitution of surface silanol groups was reported by Wartmann and Deuel (194). Silica gel which had been treated with thionyl chloride was allowed to react with phenyl lithium. Silicon-phenyl bonds could be detected by infrared spectroscopy. The phenyl content of Aerosil treated in this way as estimated from carbon analysis corresponded to 85% of the silanol groups (188). However, it is not certain whether the reaction... 

Organic groups were detected on the surface after grinding of silica in the presence of organic solvents. Silicon-carbon bonds are also formed by nucleophilic attack on the siloxane bonds with lithium organic compounds. This reaction is analogous to the dissolution of silica with alkali hydroxides. 

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. 

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