Silanimines

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. 

FIGURE 29. Optimized geometry of H2Si=NH (6-31G ). Bond lengths are in A, bond angles in deg. Reprinted with permission from J. Am. Chem. Soc.y 108,1775 (1986). Copyright (1986) American Chemical Society211. 

The model silylnitrene-silanimine isomerization (H3SiN- H2Si=NH) was stud- 

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Addition of water to the Si=N multiple bond in silanimine R2Si= NSiBul3 gives the expected R2(HO)Si-NH(SiBut3) (R = Me or Bul). Ste-ric factors presumably prevent the rapid cleavage of the Si-N single bonds that remain in the product (139,148). 

The thermolysis products of silaazetidines support the formulation of stepwise decomposition to alkene and Cl2Si=NR derivatives [3,4], The silene is not liberated from the silaazetidines when performing thermolysis reactions, but the Si-dichloro substituted silanimine (Cl2Si = N/Bu) is formed. This can be trapped by M SiOMe or Ph2C=NfBu, to give the addition products. 

The relationship between all these compounds can be seen from the formula diagram. The decisive intermediate is silanimine 20, which in a side reaction equilibrates with aminosilylene 21 but mainly forms isonitrile 23 by dehydrogenation of the SiH2 group. 

The stability within the series of H3NSi isomers increases in the order singlet-silylnitrene / trip let-silylnitrene / silanimine 20 / aminosilylene 21. Therefore it is no surprise that a silylnitrene cannot be observed. 

Metallo-silylamines and Metallo-silanimines 3.1 Bifunctional Silylamines... 

While r 2-coordination of silanimines has been realized in species such as Cp2Zr(r 2-SiMe2=Nt-Bu)(PMe3) [13], no -(silanimine) transition metal complexes are known so far [14 - 16]. Access to these Si=N systems is opened up by treatment of 19b,c with Me3P=CH2 at low temperature leading to elimination of hydrogen chloride and formation of the fert-butyl and mesityl-N-derivative 23a,b. These species are stable only for a short period in solution (two hours in toluene at -30°C), but can be... 

Scheme 1. Iron-substituted Silanimines. Synthesis and Reactions with Alcohols, Amins and Acetone.

An exception is the silaazetidine 516 prepared from 349 and iV-lhiorenylidincaniline, which decomposes slowly already at room temperature, yielding the head-to-head dimer 517 of the intermediate silanimine 518 and siloxyalkene 519 (equation 176)248. 

The synthesis of silanimines by salt elimination has been independently developed in the laboratories of Klingebiel304-307. An example of the synthesis of a stable donor adduct (681) is shown in equation 222 and equation 223 illustrates the general dimerization... 

Aminofluorosilanes 679 are formed in the reaction of difluorosilanes with lithiated amines. 679 can then be metalated by RLi (R = Me, Bu) to give the corresponding lithium salts (e.g. 680a). The structure of salts 680 depends on the basicity of the nitrogen atom and on the solvent. Representative structure types (680a-c) will be discussed in the subsection dedicated to structural features. The thermal elimination of LiF leads to silanimine dimers (e.g. 682 from 680c), if sterically possible (equation 223), or to rearrangements. 

The thus formed heterocycles 412a decompose or isomerize thermally the required reaction temperature depends on the substituents. The isomerization leads to diazomethane derivatives 413a, whereas the decomposition by [2 + 3] cycloreversion reaction gives bis(trimethylsilyl)diazomethane and short-lived silanimines 414a, which dimerize in most cases. The ratio isomerization/cycloreversion depends on R, the solvent and the temperature and more cycloreversion is observed at higher temperature. An unfavourable side reaction is the insertion of Me2Si=NR into Si—N bonds (formation of 415a and 686 see below). 

The formation of silanimines can further be proven by trapping reactions. One such reaction is the [2 + 3] cycloaddition with azides to give siladihydrotetrazoles 344a (equation 227). 

The yield of the [2 + 3] cycloadducts lies between 15 and 100%. The siladihydrotetra-zoles 344a are thermally quite stable and decompose in solution at temperatures higher than 130 °C in the reversal of their formation reaction to give silanimines and azides. The Si=N species dimerize to 687 or may be trapped by, e.g., acetone to give the ene product 688 (equation 228)310. 

The [2 + 2] cycloreversion of l-aza-2-silacyclobutanes (silaazetidines) is a possible mode of silanimine preparation (equation 229). 

The cyclization of allyl silyl amine 697 by hydrosilylation led to silaazetidine 698, which was subjected to flash vacuum thermolysis at 700-900°C at 10-4 hPa313. The silanimines 699 and 700 themselves were too reactive to be observed by high resolution mass spectrometry of the reaction mixture, but their cyclic dimers, the cyclodisilazane 701 and 702 and a trapping product with t-BuOH 703, were definitely confirmed... 

The photolysis of the three other azidosilanes 708-710 in 3-MP glass at 77 K produced no new UV bands, nor was a trapping product obtained in the presence of alcohols. In solution, however, the silanimines 711-713 were produced by photolysis and could be intercepted with alcohols (equation 237). 

The authors propose that N-silanimines result from a migration of a silyl group with simultaneous loss of N2 in the excited state of the azidosilane. This is in accordance with evidence published earlier by Kyba and Abramovich318,319 that the photochemical decomposition of alkyl azides does not proceed via a nitrene intermediate (see also Reference 320). 

An elegant study to probe the effects of jr-bonding on the properties of the Si=N bond has been carried out with bridgehead silanimines 714-716, in which the double bond is twisted321. 

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