Silenes polysilanes

Silenes derived from aromatic di- or polysilanes have been characterized in particular by the ene-type reactions they undergo when treated with isobutene or acetone. Recently, Leigh80 observed the first reported case of one of these silenes undergoing [2+2] cycloaddition (21% yield) with acetone. The ene product, the only product previously detected from the reaction of such silenes, was formed in 41% yield, as shown in Eq. (33). 

Another family of silenes, those derived from the photolysis of acyldi- or polysilanes at A > 360 nm, also show somewhat unusual behavior compared to simpler silenes. These silenes exhibit great stability which in some cases has allowed isolation of solid silenes, and which has allowed acquisition of much physical data relating to silicon-carbon double bonds as mentioned earlier. 

TABLE 3. Silenes synthesized by photolysis or thermolysis of acyl di- or polysilanes... 

Recently, several studies have been made of the photolysis of disilanes or polysilanes in the presence of an electron-deficient alkene using a photosensitizer (such as phenanthrene) and acetonitrile as solvent. These conditions result in the addition of silyl groups to one end of the alkene double bond and hydrogen to the other end (equation 18) and evidently involve the reaction of the radical anions of the electron-deficient silene with silyl radicals67 (see also Section VIII.A). 

But till now only methods involving an in situ formation of deprotonated 1 have been described. In this paper we present a procedure leading to pure, isolated 1-hydroxyalkyl-polysilanes 1. In presence of base fliey are easily converted into transient silenes, which were characterized by various dimerization and addition reactions. The availability of isolated 1 in the synthesis of silenes according to the Peterson concept offers the possibility of a free choice of the solvent and the base used to initiate the silanolate elimination. With respect to the significance of the reaction medium for the silene generation and its subsequent reactions, this is of particular importance. 

Similarly, when silene 2a is generated in presence of excess tris(trimethylsilyl)silyllithium, the lithium silanide is added across the silicon-carbon double bond to give an organolithiiun intermediate, which undergoes a rearrangement, a l,3-Si,C-trimethylsilyl migration, resulting in formation of a lithium silanide, which is trapped with chlorotrimethylsilane to yield the polysilane 7. The H-silane 8 is obtained as the protonation product after usual hydrolytic work up (Eq. 4-5). 

Several areas dealing with the photochemical processes in silicon containing compounds have also received renewed attention. Thus the photochemical reactivity of oligosilanes, polysilanes and silylenes has been the subject of a detailed review. Aspects of the mechanisms for the processes were highlighted. Other publications have been concerned with the photoreactions of silanes (cyclic and acyclic) disilanes, silenes, etc and the photoinduced electron-transfer reactions involving organosilicon compounds. ... 

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