Disilane from silyl radical + silane

The much studied photochemistry of aryldisilanes carried out in earlier years has been reviewed51,52. Cleavage of the silicon-silicon bond of the disilyl moiety is always involved, but various other reactions have been observed depending on the structure of the disilane and the conditions employed. Thus cleavage to a pair of silyl radicals, path a of Scheme 15, is normally observed, and their subsequent disproportionation to a silene and silane, path b, is often observed. There is evidence that the formation of this latter pair of compounds may also occur by a concerted process directly from the photoex-cited aryldisilane (path c). Probably the most common photoreaction is a 1,3-silyl shift onto the aromatic ring to form a silatriene, 105, path d, which may proceed via radical recombination52. A very minor process, observed occasionally, is the extrusion of a silylene from the molecule (path e), as shown in Scheme 15. 

Similarly, the attempted synthesis of diadamantylsilylene by extrusion from a trisilane 267 did not give the wanted product ". The predominant photoreactions are silicon-silicon bond homolysis to give the radicals 268 and 269 (equation 67). Disproportionation of 268 and 269 results in the formation of silene 271 and silane 272. TTie silene is identified by isolation of its head-to-tail dimer 273. In the presence of scavenger reagents like 2,3-dimethylbuta-l,3-diene radical trapping products like 270 could be detected in low yields. Secondary photoprocesses involving the disilane 272 take place. Formation of silyl radicals 269 and 274 with subsequent disproportionation of the radicals explain the formation of diadamantylsilane 275. 

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