Cyclohexasilanes conformation

Summary The use of oligosilyl dianions greatly facilitates the synthesis of cyclohexasilanes with a specific substituent pattern. A number of such conqiounds differently substituted in 1,4-positions are compared according to their stractural data. While almost all cyclohexasilanes studied so far prefer a chair conformation, we find the unusual example of a twist conformation for l,l,4,4-tetrakis(trimefliylsilyl)-octamethyl-cyclohexasilane (2). 

The preparation and structural characterization of several cyclohexasilanes with different substituents in the 1,4-position [4] (Tables 1 and 2) provided us with the unique opportunity to study the influence of the substituents on the conformation of the cyclohexasilane ring. Almost all known cyclohexasilanes adopt a chair conformation. The crystal structure of the dibr< no compound 13 (Fig. 2 and Table 2) [5] provides a rather typical example. We decided to compare the flattening of the chair as a function of the substituents. This property can be described by two angles (a, P) between the actual seat plane and the planes spanned by the atoms above and below and the seat edges (Fig. 1 and Table 1). 

Fig. 1. The chair conformation of cyclohexasilane with selected angles (a, p).

As mentioned above, usually a chair conformation with the large substituents in equatorial position is typical for the 1,4-substituted cyclohexasilanes. l,l,4,4-tetrakis(trimethylsilyl)-... 

The di- and trisubstituted compounds 6 - 8 are obtained as statistical mixtures of cis/tram isomers. Isolation and purification of the least soluble isomer can be achieved by crystallization from 2-propanol. Single-crystal X-ray diffraction studies of 6 and 7 exhibit the cyclohexasilane ring in a slightly distorted chair conformation and the bulky -OSiMe2 u groups in equatorial sites in order to minimize non-bonding interactions (Fig. 1). 

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