Disilanes nucleophilic

Disproportionation is also an important method of forming polysilanes. The method is used on an industrial scale. It is important that disilanes are much easier to disproportionate than monosilanes. The reason seems to be the strong nucleophilic reactivity of the SiSi bond. It has been shown that different substituents exhibit different reactivities and this reactivity sequence is the one that we have found in equilibrium reactions. Investigations of several substituents show a series with a decreasing exchange rate valid for equilibrium and disproportionation reactions [16]. 

Cleavage of the Si—Si bond of a disilane with nucleophiles such as organolithium compounds or alkali metal alkoxides or hydrides (equation 1) ... 

The first step of this conversion is assumed to be the formation of the silyl anion, which undergoes a subsequent nucleophilic attack on the starting material283. The resulting disilane may be isolated, when stoichiometric amounts of metal are used. Flowever, in contrast to peralkylated disilanes, disilanes which bear at least one aryl substituent at each silicon are susceptible to further reduction. Accordingly, the Si—Si bond of the fully or partially arylated disilane is easily cleaved under the reaction conditions by slow electron transfer from excess metal, eventually transforming both silyl units of the disilane into the desired metalated silane. 

As expected for a nucleophilic silicon(II) compound, 82 does not react with triethylsilane, but with the electrophilic trichlorosilane to form the corresponding unsymmetrical disilane (equation 69)189. Other polar and non-polar substrates react in a similar fashion to give the corresponding oxidative addition products (equations 69188... 

These silylenoid species have been made by lithium-tin exchange from a Si—Sn precursor96, or by reduction of chlorosilanes with lithium l-(dimethylamino)naphthalenide (LDMAN) at —78°C (equations 42 and 43)97. The resulting silyllithium species react as nucleophiles with Mc SiCI to give the corresponding disilanes (equation 44). An... 

The (chloromethyl)disilane readily forms a Grignard reagent (XXV), which undergoes many reactions in the normal way to produce certain carbon-functional disilanes that can not be prepared by nucleophilic substitution (25,104, 111). 

A practicable reaction mechanism is the nucleophilic attack by the silyl anion on the Si—Si bond, which leads to the symmetrical disilanes ... 

Perbenzoic acid reacts with organic disilanes by oxidation to the corresponding disiloxanes488) in a second-order reaction. A substitution on the phenyl group of the phenylpentamethyl-disilanes increases the nucleophilic reaction rate of the Si-Si oxidation and increases pw— d conjugation from the phenyl-group to the silicon. 

In the original paper [2], it was proposed that the results shown arise from a halophilic attack of the silylene nucleophile on the halogen atom of the reactants. On the other hand, Su has studied the Lewis type reaction of several silylenes with density functional theory [3, 4]. His basic idea is that the reaction may proceed in three steps. In the first, the halocarbon is expected to form a weak complex with the silylene. In the next, the complex is transferred into the insertion product. In the last step, the attack of a second silylene molecule leads to the formation of the final disilane product. 

Comparable results were obtained when 3 reacted with di- and trichlorosilanes (Eq. 5). The p-values (1 57 and 1.04, respectively) of these reactions, which allow the convenient synthesis of disilanes 13, show once more that the nucleophilic character of 4 is of importance for the rate... 

The stability of the Si-N bonds in 1 towards nucleophilic attack subsequently allows the introduction of further functional groups, which leads to the formation of multifunctional disilane derivatives simultaneously bearing three different kinds of substituents. Thus, for instance, 1 can react with /-butyllithium or the transition metal anion [Fe(CO)2Cp] without any detectable scission of the Si-N linkages ... 

Nucleophiles for the substitution can be generated in situ. For example, a synthesis of allylic carbamates and carbonates is conducted using amines and alcohols under CO2. Since disilanes are cleaved in the presence of Pd(0) complexes, their use in the synthesis of allylsilanes is feasible. 

Metallodisilanes LnM-SiX2-SiX3 (X = H, Cl) represent interesting model compounds for the study of the transition metal effect , because these species contain both a transition metal substituted- as well as a metal-ffee Si-X unit in the same molecule. EtsN-assisted hydrolysis of the ferrio-pentachlorodisilane Cp(OC)2Fe-Si2Cl5 yields exclusively the ferriodichloro-trihydroxy-disilane 10, demonstrating the reduced susceptibility to nucleophilic attack at the silicon in the a-position to the transition metal atom. ... 

Mizuno and his co-workers reported the photosilylation of electron-deficient alkenes by use of hexamethyldisilane and unsymmetrical disilanes. The photoreaction of l,l-dicyano-2-phenylethene and disilanes in acetonitrile in the presence of phenanthrene affords sily-lated dicyanoethanes in good yields. A key intermediate in this process is the silyl radical, generated by solvent (nucleophile)-assisted cleavage of the radical cation of disilanes. Silyl radical attack at the radical... 

The Pd-catalyzed cross-coupling reactions of metal nucleophiles with carbon electrophiles are of considerable value for the regio- and stereocontrolled synthesis of functionalized organometalhc compounds, in particular, silanes, stannanes, and boranes, which are important reagents for Pd-catalyzed carbon-carbon cross-coupling as shown in Sects. in.2.2-in.2.4. Symmetrical bimetallic compounds such as disilanes, distannanes, and diborons are usually used as metal nucleophiles. The present metallation is applicable to aryl, benzyl, vinyl, acyl, and aUyl (Sect. V.2.3.3) electrophiles. 

In the area of nucleophilic cleavage of the M—M bond there is less definitive data. Certainly, the silicon-silicon bond is cleaved more readily than the carbon-carbon bond, but digermanes, ditins, and dileads are cleaved under conditions similar to those required for disilanes. 

The disilane molecule is thought to be coordinated at the Lewis base by donor-acceptor interaction creating a pentacoordinated Si atom. Both the stability and the reactivity rate of this intermediate adduct depend on the steric factors and on the nucleophilicity of the donor. Dominantly the nucleophilicity determines the catalyst activity in the disproportionation reaction. 

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