Silanes, dehydrocoupling

In summary, although there are some indications that the stereochemistry of silane dehydrocoupling may not be completely random, the effect is not dramatic and can be rationalized by any of the mechanisms discussed previously. In the case of radical coupling, steric interaction between the substituents on the end silicons would tend to favor syndiotactic enchainment. Coupling in the coordination sphere of the catalyst would involve interactions between catalyst substituents and substituents at the end of the silicon chain. 

The reactivity of silanes has been intensively investigated in connexion with hydrosi-lylation and silane dehydrocoupling reactions (see Sec. 22-3). 

In general the rate of silane dehydrocoupling catalyzed by late transition metal homogeneous complexes decreases in the order primary > secondary > tertiary. Other clear-cut relationships between the rate of dehydrocoupling to factors such as the metal catalyst precursor, its ligands, or the silane substituents are difficult to make from the limited data. This is shown in several examples. Brown-Wensley showed that the complex (Ph2P) RhCl is the best catalyst precursor for the conversion of the secondary silane Et2SiH2 to the... 

Silane Dehydrocoupling Reactions Catalyzed by the Late IVansition Metals... 

Scheme 26 (a) Calcium- and (b) magnesium-catalyzed amine-silane dehydrocoupling... 

Synthesis is typically by alkali metal-mediated coupling of dichlorosilanes or 1,2-dichlorodisilanes, although electrochemical coupling of chlorosilanes and dehydrocoupling of primary and secondary silanes also often lead to oligomeric (as opposed to high polymer) fractions. 

The Harrod group has investigated dehydrocoupling reactions of silanes with amines, hydrazines, and ammonia. In the presence of catalytic amounts of CuCl, hydrosilanes react with primary amines to give silazanes in high yields.180 For example, the reaction of phenylmethylsilane with benzylamine leads to a mixture of three products, with dependence on initial molar ratio of silane to amine [Eq. (72)]. Equimolar silane to amine ratio leads to formation of 1 as major product, whereas 2 is the main product if a 2 1... 

Polysilazanes can also be synthesized by catalytic dehydrocoupling (equation 4)28,29. The reaction works best with monosubstituted silanes (MeSiH3 is flammable) however, as with the ammonolysis reactions, molecular weights are normally less than 2000 Da. Hence, as-formed oligomers are not useful precursors. However, combinations of ammonolysis followed by catalytic dehydrocouphng provide access to useful precursors as discussed in the SiCN section. 

A. Linear-selective Dehydrocoupling of Hydrosilanes to Poly silanes. 144... 

Similarly, other disilanes such as bis(l-sila-3-butyl)benzene,57,58 2,5-disilaoct-7-ene, 2,5-disilahexane61 underwent dehydrocoupling in the presence of group 4 metallocene combination catalysts Cp2MCl2/Red-Al (M = Ti, Zr, Hf) and Cp2ZrCl2/2w-BuLi. Thus, types of substituent on silane and catalyst can make a difference in the coupling process. 

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