Nitrogen-silicon double bonds

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. 

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). 

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. 

The amide ion 3 then either eliminates H" from silicon to give an intermediate with a silicon-nitrogen double bond, R2Si = NR, which then undergoes head-to-tail dimerization to form 2 or, alternatively, reacts with a molecule of R2Si(H)NHR to give an intermediate that undergoes cyclization to 2 with displacement of H". 

The silicon atom is surrounded tetrahedrally by two chlorine and two nitrogen atoms. The ring skeleton is nearly planar and the bond lengths of C(l)-C(2) agree with usual C=C-double bonds. All distances and angles correspond to values derived from analogous compounds [2, 4, 5],... 

Oxidation of alcohol, carbonyl and acid functions, hydroxylation of aliphatic carbon atoms, hydroxylation of alicyclic carbon atoms, oxidation of aromatic carbon atoms, oxidation of carbon-carbon double bonds, oxidation of nitrogen-containing functional groups, oxidation of silicon, phosphorus, arsenic and sulfur, oxidative N-dealkylation, oxidative O- and S-dealkylation, oxidative deamination, other oxidative reactions... 

Silanimines 64 containing double bonds between nitrogen and silicon have been studied much less extensively than C=Si or Si=Si double bond s. The recent successful synthesis145 of sterically protected substituted silanimines, e.g. 64, R1 =R2 = t-Bu, R3 = (t-Bu3)Si, has stimulated the theoretical interest in this group of compounds. 

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