Disilanes reductive

Reduction of the Si-Si bond of disilanes with alkali metals (Scheme 2.43, C). ... 

Combining two different hydrosilylation catalysts in sequence with chlorodimethylsilane has allowed the construction of complex dye assemblies and conducting polymers. In the example shown (Scheme 8), Karstedt s catalyst was chosen after a brief screen for the hydrosilylation of an aromatic diyne with chlorodimethylsilane. After reduction of the chlorosilane, an equimolar mixture of disilane B and diyne A was treated with a catalytic amount of Wilkinson s catalyst, resulting in the formation of polymer C.42 Hydrosilylation of alkynes has also been studied as a means of synthesizing oligo(phenylenevinylene) units with pendant alkoxysilanes to create curable, hole-transporting films.43,43 ... 

Allred et al. studied the electrochemical reduction of Me3SiCl in acetonitrile using a platinum cathode [82], They reported that the choice of the supporting electrolyte is important for the formation of the disilane. When Bu4NC104 was... 

Attempts have been made to prepare polysilanes containing the 8-dimethylaminonaphth-l-yl ligand.863 The coupling reaction of 8-dimethylaminonaphth-l-yl lithium with MeSiClj has given the dichlorosilane 909, whose mild reduction with Mg has surprisingly yielded the disilane 910 rather than the expected polysilane (Scheme 128). The formation of the disilane may be rationalized by the insertion of a transient silylene 904 into an Si-N bond of... 

The catalytic effect of tetra-n-butylammonium fluoride in the homogeneous reduction of heterocyclic A-oxides and nitroarenes by hexamethyldisilane in tetra-hydrofuran can occur with EXPLOSIVE violence, but can be controlled by the slow addition of the disilane to the A-oxide (or nitroarene) and tetra-n-butylammonium fluoride to yield the parent heterocycle (>70%) (or azobenzene 84%). In a similar manner, azoxybenzene is converted into azobenzene (95%), and 4-nitropyridine-l-oxide, is reduced to azoxypyridine-l,l -dioxide (78%), with minor amounts of azopyridine-1, l -dioxide and azopyridine-1-oxide [5,6]. 

Other silanes have been used as mediators in the intermolecular C—C bond formation. They can be used alone, as in the following example of the disilane (Ph2SiH)2 shown in Reaction (7.6), for the reductive addition of a bromide or a xanthate to phenyl vinyl sulfone [11,12]. 

P configuration as it proceeds with retention when SiHCl3, SiH2Ph2 and other hydrosilanes are used. UAIH4 or Si2Cl6 are also possible reductants, but inversion sometimes occurs, especially with disilanes (equation (1)). 

A silyl compound with two different alkyl substituents, (t-Bu)2MeSiK, was prepared by Ftirstner and Weidmann in low yield by the cleavage of the Si—Si bond of (t-Bu)2MeSi—SiMe(t-Bu)2 with CsK20. This reaction is quite exceptional, as usually the Si—Si bond of peralkylated disilanes is not susceptible to reductive cleavage by alkali metals. 

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. 

Reductive cleavage of disilanes by alkali metals is the most versatile method to prepare metalated arylsilanes, which carry, in addition to the aromatic group, also an aliphatic, benzylic or allylic substituent. Table 1 reports all the metalated arylsilanes which have been prepared by reductive cleavage of disilanes with alkali metals. 

The first metalated silole, 48, which was characterized unambiguously by means of NMR spectroscopy, has been obtained by Boudjouk and coworkers via reductive cleavage of the Si—Si bond of disilane 47 with lithium or sodium under ultrasonic activation (equation 54)110a. 

In contrast to these experimental and computational results, lithiated 1-silafluorenide 50, which was prepared by the reductive cleavage of the central Si—Si bond of disilane 49 with lithium under ultrasonic activation, provides some evidence for the existence of localized metalated siloles (equation 55)111. Thus, upon metalation of 49 to form 50, a highfield shift of the 29Si nucleus (AS = —47.9 ppm) is observed. In addition, the chemical shifts of the phenyl carbons indicate that there is no accumulation of tt electron density, which would be expected for a delocalized lithium silafluorenide111. 

The oxidation potential decreases in the order Si—Si Si—Ge>Ge—Ge>Si—Sn> Ge—Sn >Sn—Sn in accord with the ionization potential (7P) of the corresponding dimetal. Anodic generation of silicenium ions from disilanes was also reported. The reduction potentials of silicenium ions were determined by cyclic voltammetry of neutral precursor disilanes49. The reduction potential shifted to the negative direction as the center element changed from C to Ge as shown in equation 44. 

Cathodic reduction potentials of disilenes were determined by cyclic voltammetry56. As shown in Table 18, tetraaryldisilenes are reduced at less negative potentials than dialkyl-diaryl derivatives. This is in sharp contrast to the fact that anodic oxidation potentials are similar for both types of these disilanes (see Table 13). 

On the other hand, Hengge and colleagues first showed the possibility of the Si—Si coupling through cathodic reduction of various mono- and disilanes in DME using a platinum cathode and a mercury or lead anode without control of the applied potential75,76. In this case, the mercury and lead anodes work as a sacrificial anode . 

Corriu and colleagues also re-examined the cathodic reduction of halosilanes in anhydrous DME. They found that Ph3SiCl and Me3SiCl gave the corresponding disilanes... 

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... 

An extreme example is given by the reduction of hexachlorodisilane (10a). Whereas disilane, Si2H6, can be obtained in 87% yield when 15% excess of a solution of lithium aluminum hydride in ether is added to the chloro-disilane, the product is exclusively monosilane, SiH4, when a solution of the chlorodisilane is added to a solution of lithium aluminum hydride so that an excess of the latter is always present. 

Oxidation of the tris(pentamethyldisilanylmethyl)borane (XXVI) by alkaline hydrogen peroxide leads to the (hydroxymethyl)disilane, while reduction of the ethyl (pentamethyldisilanylmethyl)acetate (XXVII) with lithium aluminum hydride yields the (jS-hydroxyethyl)disilane. 

Under certain conditions, aroyl chlorides are converted to arylsilanes by the reaction with disilanes. The oxidative addition of aroyl chloride and decarbonylation are followed by transmetallation and reductive elimination to give aryl silanes. Neat trimellitic anhydride acid chloride (377) reacts with dichlorotetramethyldisilane (376) at 145 °C to generate 378, which affords 4-chlorodimethylsilylphthalic anhydride (379) by reductive elimination. Finally it was converted to 380 and used for polyimide formation [185], Biphenyltetracarboxylic anhydride 381 is obtained at a higher... 

As an example of carbometallation, the 1,4-carbosilylation product 218 is obtained by the reaction of dienes, disilanes and acid chlorides of aromatic and a,/i-unsaturatcd acids at 80 °C. The phenylpalladium 216 is formed by the oxidative addition of benzoyl chloride, followed by facile decarbonylation at 80 °C, and reacts with butadiene to generate the benzyl-7i-allylic complex 217. Then, transmetallation with the disilane and reductive elimination afford 4-silyl-2-butenylbenzene 218 [92], Regioselective carbomagnesation of isoprene with allylic magnesium bromide 219 catalysed by Cp2TiCl2 gives 220, which is useful for terpene synthesis [93,94],... 

Clearly, synthetic alternatives to the reductive elimination are missing. It is noteworthy that the application of photochemical methods, common for the synthesis of transient silylenes, have not been described for the generation of l,3,2(A2)-diazasilacyclopentenes and -pentanes. A potential alternative to the described reduction chemistry could be the formal disproportionation reaction of a l-halo-2-alkyl disilane with extrusion of a silylene as shown in Scheme 13. The scope and the limitation of this chemistry has however not been tested. 

Reductive coupling of ArCOCl.3 Aroyl halides substituted by an electron-withdrawing group couple to biphenyls when heated with a disilane such as 1 at 160° in the presence of a Pd catalyst. 

When a disilane is reacted with an isocyanide in the presence of a catalytic amount of tetrakis(triphenylphosphine)paUadium(0), an imine is formed. Subsequent reduction by... 

Therefore, reduction of steric hindrance allows the formation of oligomers higher than the disilane. 

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