Dehalogenation, polysilanes

Although these polysilane by-products were not noticed at that time, they are now important main products. In the so-called "Yajima process" [10] (the usual reductive dehalogenating coupling of dimethyldichlorosilane with sodium) polysilanes were obtained and fibres could be formed from these which were then pyrolyzed to form silicon carbide fibres. 

Reductive dehalogenating coupling can be used for the preparation of other new and interesting compounds, and a brief consideration of the special field of polysilanes, the cyclosilanes, will show this. 

Reductive dehalogenation cannot be completely controlled, and mostly complicated mixtures were formed which are difficult to separate. Salt elimination opens the possibility of a reaction aimed at polysilane formation. Some examples are shown in Fig. 2. The key compounds are the alkali metal cyclosilanes, which we have isolated via the mercury compounds by the action of sodium/potassium alloy and used for the first time [13]. 

The partially chlorinated cyclosilanes are valuable starting materials for the synthesis of other derivatives like di- and polycyclic polysilanes (see Section V.A). The dehalogenative coupling with alkali metals, however, only gives low yields and many side products are formed. Salt elimination of cyclosilanyl halides and cyclosilanyl alkali metal compounds... 

Summary The formation of reactive intermediates via dehalogenation of chlorosilanes was investigated by using lithium powder and sonication. Whereas in the absence of a diene substrate mainly polysilanes are obtained, reactions with, e.g., dimethylbutadiene, yield the corresponding cycloaddition products, indicating silylenes and silaethenes as intermediates. 

Cyclogermanes. Cyclogermanes are prepared by methods similar to those used for polysilanes, for example, Wurtz dehalogenative coupling." Cyclogermanes (R2Ge) with n = 3, 4, and 5 can be isolated." ... 

The first polysilanes were synthesized more than 75 years ago by Kipping through Wurtz-type reductive dehalogenation of dihalodiorganosilanes. This procedure, which is alkali metal intensive, has in the last decade been better understood and has consequently acquired a degree of control that was not previously considered possible. It remains the easiest and most widely used route to a polysilane and it will be discussed later in this section after consideration has been given to three other methods of synthesis that have been researched in recent years. 

Metal-assisted dehalogenation reaction, analogous to the well-known Wurtz reaction has been very effectively employed for the preparation of polysilanes which contain catenated organosilicon units. Thus, the reaction of a variety of diorganodichlorosilanes with finely divided sodium in a... 

We have seen in the previous seetion that polysilanes, [RR Si] can be prepared by dehalogenation of RR SiCl2- These polymers can also be prepared by ROP of two different types of cyclie monomers. The tetrasilane [PhMeSi]4 is polymerized to [PhMeSiJn by using /iBuLi as the initiator (seeEq. 1.27) [40]. 

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