Cyclic polysilanes, photolysis

The photolysis of cyclic polysilanes results in ring contraction with concomitant extrusion of a silylene fragment. Although the formation of two reactive intermediates potentially complicates mechanisms for product formation, it has provided a useful method for the synthesis of both unstable and stable disilenes... 

Other workers have reported that prolonged sonolysis of organolithiums derived from organic halides [94], chlorosilanes and chlorostannanes [95] results in Wurtz-type coupling. Yields are moderate and the reactions are of little synthetic interest. However, coupling of dichlorosilanes and stannanes produces a novel route to the cyclic polysilanes (9) and (10) (Scheme 37). The product obtained is determined by the steric bulk of the alkyl groups and only low levels of contamination by other silanes is observed [95]. Silylene intermediates did not appear to be involved. However, Boudjouk et al. [101] later reported formation of the tetramesityl silylene (11) which had previously been made by photolysis of (Mes)2Si(TMS)2 [102] (Scheme 38). 

The chemistry of linear and cyclic polysilanes and stannanes is summarized, and the generation of Si—C double bonded intermediates by photolysis of... 

Scheme 38 Photolysis of ladder polysilane and trapping products of extruded cyclic disilene (substituents /-Pr, except as noted)308...

Photochemical behavior of monosilanes has been investigated by mercury-sensitized photolysis (15-19), flash photolysis (20, 21), vacuum ultraviolet photolysis (22-27), and matrix photolysis (28-30). The first examples of the photolysis of permethylated polysilanes were published in 1970 (14). All of the cyclic and acyclic permethylpolysilanes with the exception of hexamethyldisilane readily undergo photolysis on irradiation with ultraviolet light to give shorter chain compounds with the concurrent generation of the divalent silicon intermediate, dimethylsilylene (8). 

M-M bonds in polysilanes and polygermanes as well as silylgermanes are readily cleaved by photolysis to generate reactive silylenes and germylenes which are trapped with butadienes affording the cyclic products 155 and 156 <20000M3232, 2002JOM(649)25>. In the case of 156, the replacement of At by less bulky substituents leads to small yields and the cycloaddition is accompanied by the formation of many side products. 

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