Hydrosilane dehydrogenative coupling

Hydride species were also formed in the dehydrogenative coupling of hydrosilanes with DMF [45]. The catalytic system is applicable to tertiary silanes, which are known to be difficult to be converted into disiloxanes (Fig. 17). The catalytic reaction pathway involves the intermediacy of a hydrido(disilyl)iron complex... 

Among the various synthetic procedures for polysilanes is the Harrod-type dehydrogenative coupling of RSiH3 in the presence of Group 4 metallocenes (Reaction 8.1) [5,6]. One of the characteristics of the product obtained by this procedure is the presence of Si—H moieties, hence the name poly(hydrosilane)s. Since the bond dissociation enthalpy of Si—H is relatively weak when silyl groups are attached at the silicon atom (see Chapter 2), poly(hydrosilane)s are expected to exhibit rich radical-based chemistry. In the following sections, we have collected and discussed the available data in this area. 

The dehydrogenative coupling of hydrosilanes and hydrogermanes catalyzed by the early transition metal complexes is an attractive route for the synthesis of derivative polymers. " A major advantage of this... 

However, such a marked ligand effect of the ester moiety was not observed in the case of methyl acetoacetate (23.5% e.e.)153 or ethyl benzoylacetate (62.3-68.4% e.e.)207. This may be due to the fact that the silyl ether could be produced not only by the hydrosilylation, but also by in situ transfer hydrogenation of the silyl enol ether (a mixture of E and Z isomers), which is formed in the first step of the reaction by dehydrogenative coupling of the enol with the hydrosilane. 

Alkynylsilanes. A terminal alkyne and a hydrosilane undergo dehydrogenative coupling on solid bases. Magnesium oxide can be used to promote the reaction at room temperature. 

The chemistry of silylene complexes has become much more developed in the past 10 years as silylene complexes have been found to be important intermediates in reactions such as the dehydrogenative coupling of hydrosilanes, redistribution reactions, and the Direct Process for the production of simple chlorosilanes. For reviews of this work, see References 43, 44 and 294. Several complexes have now been prepared in which there is a silene or disilene ligand (R2C=SiR2 or R2Si=SiR2 species) but, since all these contain several Si-C bonds, they will not be discussed further here. 

The chemistry of group 4 silyl compounds is closely associated with the dehydrogenating coupling of hydrosilanes. This reaction was discovered by Harrod and coworkers who converted PhSiHa with a catalytic amount of Cp2TiMe2 to H(PhSiH) H [370]. Later Tilley and coworkers showed that if zirconocene and hafnocene complexes are used in the reaction a catalytic cycle is involved which consists of o-bond metathesis steps [371]. To enhance o-bond metathesis reactivity with Si-H and C-H bonds Cp2Hf(Me)Si(SiMe3)3 and related compounds were treated with B(C6F5)3 to obtain cationic Hf complexes [372]. 

Although for the catalytic transformations of organosilicon compounds only hydrosilylation is well known as industrially important process, in the last 20 years other reactions of silicon compounds catalyzed by transition metal complexes have been discovered and developed. They include double (bis)silylation of alkenes and alkynes, silylative coupling of alkenes and alkynes with vinylsi-lanes, dehydrocoupling of hydrosilanes, silylformylation and silylcarbonylation of unsaturated compounds, and dehydrogenative silylation of alkenes and alkynes with hydrosilanes. Only the latter, as related to hydrosilylation (and very often its side reaction), has been discussed here (13). 

Various transition metal catalysts of Pt, Pd, and Rh were not effective in this dehydrogenative cross-coupling condensation reaction, even though some of them had been reported as efficient catalysts for linear hydrosilanes and silanols in our previous paper [95,115]. 

Yb[N(SiMe3)2]2 THF)2 and l,3-bis 2,4,6-trimethylphenyl)imi-dazol-2-ylidene (IMes) were prepared by reported methods, respectively (3,4). The rare earth compound is highly active for the catalytic dehydrogenative cross-coupling of amines with hydrosilanes. 

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