Iminosilanes

After decades of unsuccessful attempted syntheses, Gusel nikov and Flowers in 1967 reported the first compelling evidence for the existence of silenes, compounds containing a double bond between silicon and carbon. This initiated a renewed interest in the synthesis and behavior of stable silenes,8 disilenes,b iminosilanes,c phosphasilenesd and their heavier homologs. 

The first silaethenes and disilenes were synthesized at the beginning of the 1980s, and the first iminosilanes were reported in the mid-1980s. The difference in the electronegativity between Si (1.8) and N (3.0) gives the iminosilanes an ylidic nature, which makes them susceptible to oligomerization and dimerization. 

However, monomers can be stabilized by the presence of bulky substituents. In this review (see also reviews by Walter and Klingebiel and by Raabe and Michl), we examine the methods of preparing iminosilanes, together with their structural features and some of their chemistry, that have been reported since 1985. 

Under the same reaction conditions, other hindered azidosilanes such as Mes2Si(N3)SiPh2-/-Bu or R2Si(N3)SiMes2-/-Bu (R = Me, /-Pr) form only the iminosilanes and/or the alcohol addition products.1... 

The first two unhindered iminosilanes were identified in two different ways7 ... 

Retro-ene reaction of an N-allylsilanamine under flash vacuum thermolysis (FVT) conditions. The iminosilane has been identified not only by reaction with the trapping agent /-butyl alcohol, but also directly by coupling the oven with a high-resolution mass spectrometer.7... 

Dehydrochlorination of a 1-chlorosilanamine by vacuum gas solid reaction (VGSR). The presence of the iminosilane was confirmed by direct analysis of the gaseous flow.7... 

The formation of the cyclodisilazane ring system can be explained by dimerization of the unstable iminosilane in the cold trap.7... 

In 1985/86 the first iminosilanes that are kinetically stable at room temperature were synthesized. Two groups demonstrated two different processes of preparation. 

Wiberg et al. prepared an TV-silyl substituted iminosilane by means of an N2/NaCl and -LiCl elimination. None of the intermediate products was isolated. The iminosilane was formed in the reaction of azido-di-f-butylchlorosilane and tri-t-butylsilylsodium in dibutyl ether at -78°C.13-16... 

The second preparation for a free iminosilane was found by the Klinge-biel group. It is based on a fluorine/chlorine exchange of lithiated fluoro-silylamines with subsequent thermal LiCl elimination.17... 

When the (Me2HC)2SiClNLi(THF)2C6H2(CMe3)3 is warmed to >80°C (vacuum, 0.01 mbar), LiCl is eliminated and the iminosilane sublimes, forming orange crystals, that melt into a red liquid. The downfield shift... 

The third preparation of a stable iminosilane is the photochemical reaction of di-t-butylsilylene with tri-f-butylsilylazide.22... 

A fourth preparation for a stable iminosilane as the THF adduct is the reaction of a silylene with trityl azide.23... 

Many iminosilane adducts with very different donors have been isolated. What are the characteristics of these adducts ... 

The THF adduct of the fully /-butyl-coordinated iminosilane shows a remarkable upheld shift of the unsaturated silicon atom (8 29Si = 1.08 ppm) compared to the free iminosilane described in the previous section.14-16,24... 

In the l3C-NMR spectrum of the iminosilane, the C20 signal of the THF molecule appears at 73.78 ppm—a downfield shift of nearly 6 ppm compared with that for free THF. This reflects the strong Lewis acid character of three-coordinated silicon in the iminosilane. 

The lithium derivative of the Ph-substituted aminofluorosilane crystallizes from THF as a monomeric LiF adduct of an iminosilane, (THF)3LiF (CMe3)2Si=NSiPh(CMe3)3.2I... 

The lithium derivative of the aminosilane (Me3C)2SiFNHSiMe(CMe3)2 forms a dimeric LiF adduct of the iminosilane [(Me3C)2SiFLiNSi-Me(CMe3)2]2 associated via Li—F contacts. 

In 1988 Wiberg et al. presented reactions of afree, stable iminosilane.14,27 Since that time the number of products has rapidly increased.28 Although the double bond is sterically shielded, the reactivity of iminosilanes is immense. 

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