Silylmetallic reagent

Solid silylmetallic reagents can be isolated. Evaporation of liq NHj siffords solid reagents, such as Et3GeK pure, solid MejSiLi is prepared by vacuum sublimation and (MejSOjSiLi S THF (tetrahydrofuran) may be recrystallized from pentane and Li(SiPh2)4Li 2 THF from cyclohexane. Crystalline 1 1 adducts, e.g., EtjSiLi-L, are formed by addition of hexamethylphosphoramide (HMPA), tetramethylethylenediamine (TMED), or 1,2-dimethoxyethane (DME) to Et3SiLi in hexane. Solid McjSiLi DME is also known . Isolation of KSiHj, free of solvent and KH is also possible . 

Reaction of hydrosilanes with alkali metals is seldom employed for generating silylmetallic reagents, except for HjSiM (M = Na, K, Pb, Cs). The silylmetallics of K, Rb and Cs may be prepared and isolated from 1,2-dimethoxyethane (DME) in 7-28 days The reaction of SiH4 and potassium in hexamethylphosphoramide (HMPA) (a carcinogen) gives a high yield of KSiHj, and Na and SiH. react in DME. 

Reaction of hydrosilanes with alkali metals is seldom employed for generating silylmetallic reagents, except for HjSiM (M = Na, K, Pb, The silylmetallics of K,... 

The synthesis of disilanes via silylmetallic reagents has been the subject of several recent reviews (4, 5, 14). Since the silylmetallic reagents are prepared by the reaction of metals with triorganohalosilanes ( direct preparation) or hexaorganodisilanes, the reagents are chiefly used for the synthesis of unsymmetrical rather than symmetrical disilanes. 

The preparation of a variety of higher polysilanes using silylmetallic reagents has been described (4, 5, 14). More recently, silylmetallics have been successfully employed in the synthesis of branched polysilanes. Treatment of silicochloroform with either triphenyl- or dimethylphenyl-silyllithium gives the corresponding tris derivatives. 

SYNTHETIC REACTIONS USING BROOK REARRANGEMENTS TRIGGERED BY ADDITION OF SILYLMETALLIC REAGENTS... 

Apart from problems of silicon-carbon bond cleavage, a second difficulty limiting the utility of this synthesis is that of obtaining the appropriate silylcarbinol. Whereas a-hydroxysilanes can be isolated from the reaction of silylmetallic reagents with aliphatic aldehydes 33,34), the initial adducts from reaction with aromatic aldehydes such as benzaldehyde rearrange to the isomeric silyl ethers too rapidly for any silylcarbinol to be isolated 33, 35). The mechanism of this reaction, summarized below, has been investigated in detail 30). 

An alternative route to the hydrosilylation product of monosubstituted alkynes (29) is that which involves silylmetallation of the alkynes (equation 16). A cuprate reagent (34) prepared from CuCN and 2 mol of Me2PhSiLi adds in a syn manner to the bond with the Me2PhSi group at the terminal carbon. Quenching with water affords a product of type (31). 

Two moles of silylmetallic form per mole of disilane, and the reagent obtained is salt free, but the disilane (or polysilane) must be prepared from a halosilane, which is the commercially available reagent. For a direct route from chlorosilane to silylmetallic through the disilane, which is not isolated, see 5.5.4.3. 

Organopolysilanes. Cleavage of Si—Si bonds in organopolysilanes also occurs but is not always discriminating, and mixtures of silylmetallics are formed. The best results are obtained with phenyl or silyl substituents. Polysilanes with Me substituents give the most reaetive disilanes species, but mixtures of metallic reagents are obtained. 

Reagent indicated is added and % yield corresponds to coupling product when chlorosilanes or Me PO are added, to isolated silane when H O is added and to active silylmetallic from double titration directly Biphenyl added,... 

This section deals with preparative methods of various organosilicon reagents in conventional ways using highly reactive organic/silyl nucleophile/electrophile in addition to modem protocols such as catalytic silylmetalation, cross-coupling, C-H silylation, and olefin metathesis. This kind of methods will be more employed in laboratories and industry in the next decade. 

Silylzinc reagents are also employed for the silylmetalation reaction of alkynes and alkenes. Bis(dimethylphenylsilyl)zinc adds across these unsaturated substrates in the presence of a copper(I) catalyst (Scheme 3-34). The addition reaction proceeds in a stereospecific manner always cis adducts are produced. 

---