Group 14 systems silicon-nitrogen

The activity of these particular groupings probably arises in part from the distance between the nitrogen atom and the hydrocarbon branch (at the silicon atom). A similar structure-activity relationship has also been noted in carbon systems (60). However, the silicon-nitrogen system may assume a cyclic conformation in which the unshared electrons of the nitrogen are coordinated with the d orbitals of the silicon atom (14). The potential for such a structural feature does not exist in most carbon systems. Silicon-to-nitrogen coordination is an important feature of the silatranes (61, 62), although physical evidence for such coordination in open-chain silylalkylamines is lacking (52). 

Because of the wide spectrum of activity shown by amines, definitive conclusions concerning structure-activity relationships cannot be made at this time. However, studies using single test systems reveal that the most active silylated amines contain the silicon atom in a y position relative to the nitrogen, as shown in partial structures 12 and 13 (55, 56). Some examples of compounds containing these groupings are found in Tables I and II. The silylated benzhydryl ethers (Section II,F) and sila-tranes (Section III,C) also contain this type of grouping. 

The molybdenum-silicon and titanium-nitrogen systems also belong to this group. For both, it was shown that U decreases when metal (Mo, Ti) particle size increases (curves 6 and 7, Fig. 48). 

A metallacycle in which uranium, silicon and nitrogen atoms connect to the methylene group has been shown to methylenate a variety of aldehydes and ketones, including some carbonyl compounds which do not react well in the Wittig process (equation 34). This material exemplifies the complexity and sophistication that is being used to develop reagent and catalyst systems. 

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). 

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