Amines, dehydrocoupling

Recently, Manners and co-workers have found another type of reaction, the inorganic process corresponding to Rh-catalysed dehydrocoupling of borane adducts, which is homogeneous or heterogeneous catalysed depending on the substrate, in particular the nature of the Lewis base, phosphine or amine respectively [15]. 

Scheme 2.5 Formation of B-N ring systems by dehydrocoupling of amine-boranes.

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 coupling of a trialkylsilane and an amine with loss of H2, catalyzed by palladium on carbon, was first reported by Sommer and Citron in 1967.178 More recent work by Laine and Blum has involved the application of catalytic dehydrocoupling of compounds containing Si-H and N-H bonds to form aligo- and polysilazanes. These polymers, with silicon-nitrogen bonds in the backbone, are useful precursors to silicon nitride. In the presence of Ru3(CO)i2, silicon-nitrogen bonds are cleaved and reformed... 

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... 

A CuCl/tertiary amine catalyst was also found to be highly selective for the dehydrocoupling of some arylacetylenes with primary and secondary silanes.160 With primary silanes the Si-H bonds react in a stepwise sequence with the acetylene, and by appropriate choice of conditions the mono-, di-, or triethynyl silane can be the major product. 1,3-Diethynylbenzene couples with primary and secondary silanes to give polymers of moderate molecular weights of the general structure 11. 

Liu and Harrod ° reported on dehydrocoupling of ammonia and silanes catalyzed by dimethyltitanocene. Tertiary silanes, for example, Ph2SiMeH, were transformed into disilazanes. Yet no polysilazanes could be obtained when reacting PhSiH3 under similar conditions. Homodehydrocoupling (which proceeds with Si-Si linking compare Scheme 18.3) effectively competed with amination reactions, indicating that the products obtained by ammonolysis of PhSiHj are polyaminosilanes [SiPh(NH2)] rather than polysilazanes [SiPhH-NH] . 

Summary Precursor-derived quaternary Si-B-C-N ceramics frequently possess an enhanced thermal stability compared to SiC, SisN4 or Si-C-N ceramics. The stability of the materials towards crystallization and/or decomposition is directly coimected to the molecular structure and the elemental composition of the polymeric precursors. This paper highlights recent investigations on the synthesis of boron-modified polysilazanes and polysilylcarbodiimides. Hydroboration of polyvinylsilazanes and dehydrocoupling reactions of boron-modified silanes with ammonia or amines as well as cyanamide are described. It is shown that simple organosilicon chemistry provides a means to efficiently optimize ceramic yields and tune elemental composition as well as thermal properties of the polymer-derived ceramics. 

An alternative access to boron-modified polysilazanes is a dehydrocoupling of ammonia or alkyl amines with tris(hydridosilylethyl)boranes, B(C2H4Si(R)H2)3 (R = H, CH3 C2H4 = CH2CH2, CHCH3 Scheme 2) [6]. This avoids the formation of solid by-products and allows for the synthesis... 

Transition metal-catalyzed approaches to boron-nitrogen compounds are potentially of significant importance. Very interestingly. Manners et ah have reported the catalytic dehydrocoupling of primary or secondary amine-borane adducts... 

Scheme 11.13 Catalytic dehydrocoupling of primary amine-borane adducts.

Scheme 11.14 Catalytic dehydrocoupling of secondary amine-borane adducts.

One remarkable application of the Re(I) complexes of type 16-19 is the catalytic dehydrogenation of amine boranes [54], In recent years, the research on amine borane R R NH-BHs (R R = alkyl, H) became an important topic based on their capability of chemical hydrogen storage [55-57], In this respect transition metal-catalyzed dehydrocoupling reactions of amine borane offer great potential for control over both extent and rate of hydrogen release [58, 59]. 

Jiang Y, Blacque O, Fox T, Freeh CM, Berke H (2009) Development of rhenium catalysts for amine borane dehydrocoupling and transfer hydrogenation of olefins. Oiganometallics 28 5493-5504... 

Jaska CA, Temple K, Lough AJ, Manners I (2003) Transition metal-catalyzed formation of boron-nitrogen bonds catalytic dehydrocoupling of amine-borane adducts to form aminoboranes and borazines. J Am Chem Soc 125 9424—9434... 

Clark TJ, Russell CA, Manners I (2006) Homogeneous, titanocene-catalyzed dehydrocoupling of amine-borane adducts. J Am Chem Soc 128 9582-9583... 

A general article has discussed catalytic dehydrocoupling as a general method for the formation of element-element bonds. The stereoselective a-alkylation of aictmoxycarbene metallacycles has been lepculed. The enantioselective zirconocene mediated synthesis of allyl amines has been observed. ... 

Scheme 25 Magnesium-catalyzed amine-boiane dehydrocoupling...

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