Silicon—nitrogen bonds double bonded

When the l-diazo-2-silyl moiety is incorporated into cycles then endocyclic silicon-carbon double bonds should be formed. Most interesting in this connection is the decomposition of l-diazo-2-sila-3,5-cyclohexadiene 331, because the initial Si=C product should be silabenzene 332170. The outcome of the nitrogen elimination depends on the conditions used. The products isolated are 333 (equation 81), 335 via bicyclic 334 (equation 82) and 338, formed via silafulvenes and 336 (equation 83). 

A body of data exists for two-bond silicon coupling to phosphorus with an intervening nitrogen. Very low values are observed if the nitrogen is sp -hybridized, but values of /(Si,N,P) between 8 and 42 Hz are typical for nitrogen with a double bond. 

This compound represents the first isolable base-free silanimine with an almost linear Si=N—C skeleton, and is of interest for the understanding of bonding nature of silicon—nitrogen multiple bonds. In addition, the Si=N double bond is synthetically useful for cyclic silicon compounds. 

The silicon-nitrogen bond distances in this molecule are 159.4 pm and 162.6 pm. These values are intermediate between those for a SiN double bond (156.8 pm)15 and those for a SiN single bond (174.8 pm).46... 

Photoswitching has allowed Kawashima et al. to control the configuration of the N-N double bond and the coordination number of potassium 18-crown-6 2-(phenylazo)phenyltetrafluorosilicates 836 and 837, which are accessible by the reaction of 2-(phenylazo)phenyltrifluorosilane 838 with KF/18-crown-6 (Scheme 116).830 In the (Z)-form, the silicon atom of 837 is pentacoordinated, whereas in the (E)-form, one nitrogen atom of the azo group of 836 intramoleculary coordinates to the silicon atom, thus extending its coordination sphere to six ligands. Almost... 

Another modification of the double silylation process reported by Tanaka and co-workers involves the use of a bis(hydrosilane) instead of a disilane as the reactant molecule.61 This reaction can be described as a dehydrogenative double silylation, in that two Si-H bonds are activated rather than an Si-Si bond. The system is best catalyzed by Pt(CH2=CH2)(PPh3)2 other Pt, Pd, Ru, and Rh complexes give only very low yields of the double-silylated products. Alkynes, alkenes, and dienes undergo reaction with the bis (hydrosilane) with a range of results. Silicon-oxygen bonds and silicon-nitrogen bonds can also be formed by this method and are discussed in the appropriate sections later. 

Whereas carbon is able to form stable (p-p) double and triple bonds with carbon itself, nitrogen and oxygen, leading to coordination numbers 2 and 3 at the carbon atom, silicon cannot build up such bonds (compare Table 2). Although the existence of monomeric species such as SiO and SiNH has been established under specific conditions, and (p-p)ff bonded intermediates have been postulated in a number of... 

As mentioned in this chapter, in recent years much progress has been made in the chemistry of silicon-chalcogen multiple bonds. For silicon-sulfur doubly-bonded compounds, we have now several isolated examples, both kinetically stabilized and thermodynamically stabilized. Furthermore, there have been reports of the synthesis and characterization of stable compounds with silicon-nitrogen double bonds (i.e. silanimines or iminosilanes) as well as their heavier group 15 element analogues such as phosphasilenes and arsasilenes. 

Electron donation from nitrogen to silicon is undoubtedly important in stabilizing all of these molecules. Partial double-bonding between N and Si will tend to occupy the vacant p-orbital on silicon, which is the usual site of electrophilic reactivity of silylenes. This stabilization is shown in resonance drawings in Scheme 16. Greater --donation from N to Si would be expected for 60 than for 59 or 61, since in 60 the nitrogens are more basic. The Si—N bond lengths are consistent with this model, as mentioned above. 

Even in the late 1970s one could find the statement that stable compounds with double bonds from silicon to carbon, oxygen, and nitrogen were not known (152). In the 1980s knowledge about such systems has developed dramatically. Theoretical calculations and studies of gas-phase processes, though still numerous, are slowly giving way to the relatively new area of stable unsaturated molecules and their reactions in solution (153-157). 

The class of phosphaalkenes with isolated P=C double bonds was first synthes ized by Becker.33 His synthetic strategy starting from trimethylsilylphosphines and acyl chlorides is still the most versatile (Protocol 3). The principle is based on the easily achievable, 1,3-silatropic migration of a silyl group bonded to phosphorus to a doubly bonded element such as nitrogen, oxygen, or sulfur. The process is favoured energetically by the construction of the P=C double bond with concomitant formation of a very stable silicon-element bond. 

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