Catalysts dehydrocoupling

A significant problem is the dehydrocoupling reaction, which proceeds only at low yields per pass and is accompanied by rapid deactivation of the catalyst. The metathesis step, although chemically feasible, requires that polar contaminants resulting from partial oxidation be removed so that they will not deactivate the metathesis catalyst. In addition, apparendy both cis- and /ra/ j -stilbenes are obtained consequendy, a means of converting the unreactive i j -stilbene to the more reactive trans isomer must also be provided, thus complicating the process. 

With diphosphanes recently Stephan et al. reported an intriguing Al and P based macrocyclic structure [37]. A zirconium based catalyst precursor first was employed in the catalytic dehydrocoupling of the primary bidentate phosphane to give the tetraphosphane 6, (Scheme 4). The function of 6 as a molecular building block has been demonstrated by its reaction with MMe3(M = Al, Ga). Although, the gallium derivative 7 has not been... 

For the amino-borane dehydrocoupling using [Rh(l,5-cod)(p-Cl)]2 as starting catalyst, an induction period and a sigmoid-shaped kinetic curve (plot of substrate conversion versus time) were also observed, consistent with metal-particle formation. But, for Ph2PH BH3... 

It seems likely that further improvements in this coordination polymerization technique will require design of more active catalysts. The catalyst systems currently available are very amenable to modifications that dramatically influence dehydrocoupling rates. Significantly, results so far indicate that there is no inherent limitation to molecular weight control via polymerizations of this type, and in principle it should be possible to identify conditions and catalysts that allow production of high polymers. 

Dehydrocoupling of polyhydrosiloxane 44 with thiol 45 proceeds smoothly in the presence of Wilkinson s catalyst to furnish polythioether 46 in high yield <00TL1127>. 

Harrod-type catalytic dehydrocoupling method using early transition metal catalysts (see COMC II (1995), chapter Organopolysilanes, p 99, and earlier in this review, Section 3.11.4.1.3. (i)).69,75 Si-H bonds are susceptible to free radical attack, and use of this was made in the free radical substitution of 38 to prepare a number of oxy-functionalized polysilanes, as shown in Scheme 25.185,186... 

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." " " ... 

Woo recently developed a rapid, highly linear-selective dehydrocoupling catalyst system of phenylsilane CpjMCL/Hydride (C p = C5H5 or C5Me5 M = Ti, Zr, Hf Hydride = Red-Al, Selectride, Super Hydride) combination catalysts [Eq. (2)].11,17... 

Woo s combinative catalyst system of CpjMCE/Hydride is different from the catalyst systems using Cp2MCl2/2 alkyllithiums of Corey, Tanaka, and Harrod. Real catalytic species in the dehydrocoupling of hydrosilanes could be a metallocene hydride based on a sigma-bond metathesis mechanism.12,1315 Inorganic hydrides effectively produce a metallocene hydride whereas alkyllithium can produce a metallocene hydride via... 

---