Nitrogen-Silicon Coordination

Selected Bond Lengths and Angles for the Neutral Pentacoordinate Complexes 

To determine with certainty the nature of the exchange process, a second stereogenic unit, a chiral carbon center, was introduced 

29Si and 15N NMR Data and Barriers for Ligand Exchange in 4a-4h14 

A discussion on the correlation between crystal structure and coupling constants appears in Section V. 

Coupling Constants (Hz) Through Three, Two, and One Bonds Across the N Si COORDINATIVE BOND IN 5, OBTAINED FROM 13C (AT 243 K), 15N, AND 15N H SPECTRA (at 300 K), Respectively16 

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III. PENTACOORDINATE NEUTRAL SILICON COMPLEXES A. Chelates with Nitrogen-Silicon Coordination... 

The silicon atom in Wiberg s silenes is a Lewis acidic center and can be coordinated with donors. These donors may be halide ions [like, e.g., in 104 LiX(T2-C-4)831, ethers or nitrogen bases84. Coordination with the donor stabilizes the silenes and, when the basicity suffices (LbOcBr, THF, NMe3, F ), the adducts serve as stock compounds from which the silenes may be liberated (equation 32). 92 decomposes at —100 °C, but its trimethylamine adduct is stable at room temperature85. Noticeable amounts of 97 are available from 97-LiBr already at — 78 °C, whereas from 97 LiF the temperature has to be raised to +30 °C. Structures of such silene adducts will be discussed in Section I.C.l. The silene adduct 124-TIIF has been discussed in Section I.A.2. 

Remarkably facile thermal generation of a silylene has recently been demonstrated for the pentacoordinate alkoxydisilane 92416. Intramolecular N—>-Si coordination in 92 was shown by X-ray crystallography. This disilane underwent thermolysis at 110°C in toluene, or 90 °C in DMF, producing the silylene 93, which was trapped with 2,3-dimethylbutadiene and diphenylacetylene (Scheme 26). The 2 1 adduct with diphenylacetylene was shown to have one nitrogen intramolecularly coordinated to silicon, even though the silicon atom lacks any electronegative substituents. 

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). 

Since both the and Si coordinative shifts appear to reflect the same phenomenon, i.e., the strength of nitrogen to silicon coordination, it is not surprising to find that they are linearly... 

More illustrative is the structure of the fluorosilane which exhibits the same tricapped tetrahedron geometry. The three N-Si interactions take place in the frontal position towards the Si-F bond instead of the rear coordination observed in penta- and hexacoordinated structures. Furthermore, the benzylamino groups have the possibility not to be coordinated at silicon since these groups have the capability of free rotation around the carbon-nitrogen bond. 

Regioselectivities that are usually high to excellent have been reported in novel palladium-catalyzed Heck arylation reactions with a variety of allylic substrates. The //-stabilizing effect of silicon enhanced regiocontrol in the internal arylation of allyl-trimethylsilane (Eq. 11.5) [18], and coordination between palladium and nitrogen... 

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