Dehydrocoupling, transition-metal catalyse

The formation of individual cycloborazanes can be achieved, in some cases, by transition metal-catalysed dehydrocoupling reactions. By using this strategy, secondary amine-boranes are converted to the four-membered ring (H2BNMe2)2 under mild conditions, whereas primary amine-boranes produce borazines. The cyclic pentamer is obtained as the exclusive product from ammonia-borane adduct H3B NH3 employing an iridium(fll) catalyst [see eqn (2.13) in Section 2.3)]. 

Because of these limitations, considerable effort has been focused on the development of new synthetic routes to polysilanes. The early transition metal-catalysed dehydrocoupling process discovered in 1985 [eqn (10.8)] is potentially very attractive however, the molecular weights of the polysilanes formed to date are generally fairly low (Mn<8000)." The catalysts used for these coupling reactions are usually titanocene or zirconocene derivatives." " " ... 

Among a variety of new preparative methods for creating Ae Si-Si bond such as transition metal catalysed dehydrocoupling of hydrosilanes [12-14a], ring opening polymerization [11,12], polymerization of masked disilenes [14b], the most practice one is still the Wurtz type condensation of organomonochloro- or dichlorosilanes which requires a stoichiometric amount of an alkali metal and high reaction temperatures in the cases of Na and K [3-9, 12] (Equations 1 and 2). Lower temperature variants have yet recently been reported [14c]. 

In recent years, dehydrocoupling reactions catalysed by early transition metal complexes have become an increasingly important method for generating catenated species of the p-block elements. In addition to producing cyclic oligomers, this approach is used to prepare linear oligomers and polymers such as polysilanes and polystannanes of the type H(MR2) H (M = Si, Sn) (see Section 10.1.4). ... 

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