Supersilyl group

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). 

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. 

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. 

In this connection, we asked ourselves if disilyne with overloaded substituents has a linear structure analogous to acetylenes and if such an unsaturated system could be chemically demonstrated as a reaction intermediate or could even be isolated in substance. As overcrowded substituents we chose supersilyl groups, and as the method of preparation thermal salt elimination. 

It seems very probable that supersilyl sodium reacts with the mentioned bis(supersilyl)tetrabrom-disilane by bromine/sodium exchange. The sodium containing disilane so formed should have a conformation with the overcrowded supersilyl groups in /ra i-positions (Scheme 16). 

Therefore, NaBr elimination must produce a disilene with /rara-configured supersilyl groups, which cannot have any tendency for dimerization because of its overloaded supersilyl substituents. Its... 

Tetrasilatetrahedrane is accessible if the appropriate substituent is selected. As found in the case of the octasilacubane (/frr-BuMe2SiSi)s (15), silyl substituents decrease the strain in the polyhedrane (14). Thus, the synthesis of tetrasilatetrahedrane has been accomplished by use of the supersilyl group (/ert-BusSi) (10). 

Keywords Supersilyl / Group 1 metals / Group 12 metals / Group 13 metals... 

As shown in Eq. 4, Bu3SiBBr2 can be prepared by reaction of (fBu3Si)2Zn with BBrs in heptane at room temperature. When pyridine is added to the reaction mixture, colorless needles of iBusSiBBrs py precipitate at -20°C. Crystal structure determination shows a boron atom tetracoordinated by one supersilyl group, two bromine atoms and the nitrogen atom of pyridine (Fig. 5). 

According to crystal structure analyses of l,2-dibromo-l,2-disupersilyldisilane (3a) (Fig. 1) and tetrabromo-l,2-disupersilyldisilane (5) (Fig. 2) both compounds exhibit a tra/jj-relationship of the two bulky supersilyl groups. The bromine atoms of 3a are in cw-relationship. 

Besides yellow crystals of 21, red crystals were also obtained from R 2HSi-SiHBr-SiHBr-SiHR 2 and NaR which — after an X-ray structure analysis — wctc found to contain cyclotetrasilene molecules 11a (X = H). In fact, the latter product may be formed from the reactants after elimination of two molecules of HBr set off by NaR via the disilyne 24 as an intermediate. The 24 should then transform with simultaneous migration of two supersilyl groups into a tetrasilabutadiene 25, which thereafter transforms by an electrocycUc conrotatory process into the cyclotetrasilene 11a (X = H). Certainly, the formation of the latt product does not prove the intermediacy of a disilyne but nevertheless it is an indication of that. 

Supersilyllithium (A) may be readily converted to supersilyl halides (B and C below), which are useful for introducing supersilyl groups into organic molecules. Supersilyllithium is also a key building block in the synthesis of branched organosilanes (e.g., D) and of even bulkier silyllithiums (e.g., E). All these processes are depicted below. 

Scheme 2.12 Synthesis of (+)-EBC-23 by using one SAE as the chirality source (DiPT, diisopropyl tartrate PMB, p-methoxybenzyl DMSO, dimethylsulfoxide Si, supersilyl group (see text) LiHMDS, lithium hexamethyidisilazide and DDQ, 5,6-dichloro-2,3-dicyano-p-benzoquinone).

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