Dehydrocoupling reactions

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. 

The latter part of the chapter focusses on catalyzed dehydrocoupling reactions as a route to polysilazanes. 

Si-N bonds can also be formed by a dehydrocoupling reaction catalyzed by alkali (6-11) or transition metals (12) ... 

Transition metal catalyzed dehydrocoupling, reaction (27), offers an alternative to the alkali metal dehydrocyclizatlon process... 

We have also explored the use of the dehydrocoupling reaction for the synthesis of preceramlc polysilazanes starting from the precursor MeNH-[H2SlNMe]x-H. As described above, MeNH-[H2SlNMe]X H Is produced by aminolysis of H2SiCl2 with MeNH2 under conditions where x 18-20 (Mn - 1150). 

IV Facile Preparation of Organoboron Polymer Electrolytes via Dehydrocoupling Reaction of 9-Borabicyclo[3.3.1]nonane... 

Since the initial cross-linking reactions do not permit applications requiring a thermoplastic polymer, further studies have been devoted to the control of the dehydrocoupling reaction, in particular between the already established chains. For this purpose, Sneddon et al. demonstrated that the preparation of dialkylamine-modified... 

IV. FACILE PREPARATION OF ORGANOBORON POLYMER ELECTROLYTES VIA DEHYDROCOUPLING REACTION OF 9-BORABICYCLO[3.3.1JNONANE AND... 

More recent studies, particularly with slower hafnium complexes, have provided more detailed mechanistic insight As a step polymerization, the reaction is "nonideal" in that inequivalent reactivities for different Si-H functional groups in the system are observed. For exaniple, disilanes tend to be more reactive than monosilanes. Beyond disilane formation, the preferred dehydrocoupling reaction appears to involve addition of one silicon at a time to the growing chain, via M-S1H2R intermediates (n = 1 above). The Si-Si bond-forming reactions are also reversible. 

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). ... 

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)]. 

Processing the polymer 9.16 followed by pyrolysis provides an excellent method for preparing this valuable ceramic in shaped forms such as fibres and monoliths. Derivatisation of 9.16 by a thermal dehydrocoupling reaction with amines NHR2 to replace -H substituents on boron by -NR2 groups can be achieved.Examples of void-free BN fibres prepared from polyaminoborazylene precursors formed by... 

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... 

Similarly, Woo et al. carried out the combinative Si-Si/Si-O dehydrocoupling reactions of mixed hydrosilanes with alcohols (two different hydrosilanes were used in the same mole ratio 0.5 0.5 1.5 mole ratio) at 50 °C, catalyzed by Cp2MCl2/Red-Al (M = Ti, Zr) and Cp (M = Co, Ni), producing copoly(alkoxysilane)s in one-pot in high yield [Eq. (9)].37a... 

The most widely accepted mechanism for Group 4 metallocene promoted dehydrocoupling reactions is the a-bond metathesis mechanism proposed by Tilley as shown in Fig. 3 3h,3°,30,4°,96 reac on of the metallocene... 

The carbosilane monomer, H2MeSiCH2CH2SiH3, contains both a secondary and a primary silicon center. After about 48 h with Cp2TiMe2 or Cp2TiCl2/ BuLi at r.t. the dehydrocoupling reaction occurred at the primary silane center to give a polysilane with a substituent at each silicon center that contained a tether to the secondary silane center. The initial polymer had a Mw of approximately 1000. After another 72 h at r.t., the... 

---