Supersilylation

Supersilyl substituents also stabilize negative charges extremely well In the radical anion of 1,4-di(tris(trimethylsilyl)silyl)benzene - as proven by ESR/ENDOR coupling constants [5a,c] augmented by HMO estimates for the "blind" centers (2 O) [5a,c] - more than half of the it spin population p is located in its Si(SiR3)3 groups. 

Summarizing, both the steric and electronic effects of supersilyl substituents, which stabilize radical cations as donors (1) and radical anions as acceptors (2), are recommended for further investigation. 

Since then, several different alkali metal silanide systems have been reported. One of them, the hypersilyl [Si(SiMe3)3] ligand, was first prepared by Gilman and Smith.172, 173 Another ligand includes supersilyl [SPBuJ-prepared in 1975.174-176 Several other variations of the ligand systems are known with the recent addition of [SkB Ph]-, for which several alkali metal derivatives have been prepared and characterized.177,178... 

It is instructive to compare the geometry of the supersilyl with the hypersilyl ligands. The geometry at the supersilyl ligands is clearly less pyramidal with G-Si-G angles frequently in the range of 106-107°. In contrast,... 

Figure 25 Synthesis and structures of supersilyl-substituted gallanyls 396 and 400, gallanide 398 and tetrahedrane 401.

The chemistry of supersilyl substituents has been reviewed Wiberg, N. Coord. Chem. Rev. 1997, 163, 217. 

The highly strained tetrasilatetrahedrane structure can only be stabilized using extremely bulky substituents like the tri-i-butylsilyl (= supersilyl) group. Thus the only tetrasilatetrahedrane derivative known so far has been synthesized by coupling (t-Bu)3SiSiBr2SiBr2Si(Bu-t)3 with two equivalents of (i-Bu SiNa92 (equation 24). 

The [2+2] cycloaddition of the Si=Si double bond of disilenes across a hetero double bond belongs to the most typical reactions for the preparation of disiletanes. Reaction of the supersilyl stabilized disilene 90 with PhHC=0 and Ph2C=S gave oxa- and thiadisiletanes 91 and 92, respectively (Scheme 15). The use of heterocumulenes 0=C=0 and 0=C=S in a similar cycloaddition reaction yielded oxa- and thiadisiletanes 44 and 31. The isolated disiletanes are colorless and oxygen, water, and thermostable compounds <2002CEJ2730>. 

Supersilyl substituents are also well-suited to stabilize negative charges leading to Gui-nesss Book of Records results ... 

The synthetic potential of silicon substituents in organic and organometallic chemistry has by far not been fully exploited, which is evidenced by the numerous contributions in this chapter. This is examplified for the description of the silyl group as a substituent and as a functional group in carbene and carbenoid chemistry, for the function of the trimethylsilyl substituent in the synthesis of low-valent compounds containing elements ofgroup 15 andfor the influence of a supersilyl ligand to a phosphorous center. 

Reaction of Supersilyl Sodium with White Phosphorus... 

Reaction of Supersilyl Disilane with White Phosphorus and Phosphorus Trichloride... 

In fact, an isolation of compounds - such as the substituted tetrasilatetrahedrane shown in Scheme 1 -is in many cases not possible without introducing overloaded groups like the rBu3Si group, that is the supersilyl group [2]. In addition, compounds which possess underloaded as well as overloaded silicon atoms may become isolable but retain - like the silaneimine shown in Scheme 1 - their chemical potency... 

Thermolysis of this in diethyl ether leads to a silaethene dimer, shown on the right-hand side in the first row of Scheme 6 as the only diastereomer. From the X-ray structure determination [14], both supersilyl groups are located on the same side of the four-membered ring. This fact speaks against the formation of the dimer by [2+2] cycloaddition. 

The formation of n-complexes of silenes, as well as of germenes or stannenes is of central importance to the reactivity of the unsaturated systems. I will only offer one example out of many to illustrate the facts. As we found out, our stable silaneimine (Bu2Si=NSirBu3 takes up carbon monoxide under formation of supersilyl isocyanide as well as cyclosiloxanes (Scheme 7). 

Obviously, the silaneimine like many transition metal complexes behaves like a Lewis acid even with regard to CO. Certainly, the CO adduct is unstable and transforms into the reaction products. Possibly the products with three-membered rings, shown on Scheme 7, are reaction intermediates. Supersilyl isocyanide could be prepared on this way for the first time because the reaction of cyanide with supersilyl halides exclusively gives supersilyl cyanide [21]. 

Altogether, the reaction with CO demonstrates impressively the overloaded-underloaded principle, mentioned at the begiiming. The imsaturated silicon atom is, on the one hand, coordinatively underloaded. This, at first, makes the reaction with CO possible (catalytic processes at the centre of transition metal complexes). On the other hand, the overloaded supersilyl group shields the unsaturated silicon atom and therefore lays open the reactivity of the latter atom against CO. In normal cases, other reactions wiiich will be operative never give the adduct formation with CO a chance. 

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