Silazanes cyclic

Scheme 1. Reaction between cyclic silazane and a,

It was observed that ammonolysis of B(C2H,Si(R)H2)3 (Scheme 2, route A) requires basic catalysts such as n-butyl lithium. The reaction is performed in analogy to the potassium hydride-catalyzed cross-linking of cyclic silazanes described by Seyferth et al. [8]. Most probably, n-BuLi initially deprotonates the weak nucleophile ammonia with the formation of lithiiun amide and evaporation of n-butane. The stronger nucleophilic amide then replaces a silicon-bonded hydride, which subsequently deprotonates ammonia, leading to the evolution of molecular hydrogen. The silylamines that arise are not stable under the reaction conditions applied (refluxing solvent), and by fast condensation of ammonia the polymeric precursors form [6]. 

Hexamethylcyclotrisilazane and octamethylcyclotetra-silazane have been prepared by Brewer and Haber from the reaction of dry, gaseous ammonia with dimethyl-dichlorosilane in benzene. When the ammonolysis was carried out in liquid ammonia, the combined yield of silazanes was only slightly less, but the trimer was formed in greater proportion than the tetramer. Although the yield of cyclic silazanes was always high, it was difficult to reproduce the proportions of the individual compounds. The procedure given here is an adaptation of that of Brewer and Haber. 

Cyclic silazanes are the reaction products when diorganyldihalogenosilanes are employed with ammonia or primary amines, especially when the reaction takes place at a... 

The same reaction is exploited on a commercial scale to convert higher silazanes to cyclic silazanes, but the hydride intermediate is never isolated (115). 

The structures of several related naphtho-fused ring systems have been determined by X-ray crystallography. Thus, the cyclic silazane 112 (R = Ph) is planar at the nitrogen atom and all the rings lie almost coplanar, while the related compound 112 (R = Bu but with an anisyl group attached to one of the silicon atoms) has the silicon atoms out of the plane of the tricyclic system, and the ylide 113 has the heterocyclic ring in an envelope conformation and the ylidic carbon is pyramidal and not planar <1996CB495> (see Section 9.18.4.3.2). 

Polymers prepared by the above procedure have molecular weights up to 80,000. It is also possible to start with diphenoxysiloxane and catalyze the reaction with sodium or lithium metals. Reactions of cyclic silazanes with arylene disalanols yield polymers with molecular weights as high as 900,000 ... 

The reaction of the cyclic silazane with almost any type of SiOH group (Eq. 2) is complete within minutes, even at room temperature, due to the high reactivity of the cyclic disilazane. The products obtained are 100 % functionalized. Furthermore, no catalyst is needed and no unwanted byproducts are released. So these precursors are the best choice to prepare silicone-urea block copolymers. 

This idea was later extended to cyclic silazanes (equation 3) (36), The reactions proceeded readily at room temperature to give good yields of the desired silylperoxides. Reaction of one equivalent of hydroperoxide per Si-N bond in the silazane gave complete substitution of the cyclic silazane trimer. 

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