Cyclic disilanes, photolysis

Rearrangements of disilanes to a-silylsilenes are well established and are involved in the exchange of substituents between a silylene center and the adjacent silicon.Pulsed flash pyrolysis of acetylenic disilane (41) gave rise to the acetylenic silene (42), which subsequently rearranged to the cyclic silylene, 1-silacyclopropenylidene (43). Irradiation of the cyclic silylene resulted in the isomerization to the isomeric 42, which itself could be photochemically converted into the allenic silylene (44). Both 42 and 43 also were reported to isomerize on photolysis to the unusual (45), which was characterized spectroscopically (Scheme 14.24). 

Maier and coworkers have found that pulsed flash pyrolysis of an acetylenic disilane 323 gave rise to the acetylenic silylene 324, which subsequently rearranged to the cyclic silylene 1-silacyclopropenylidene 325162. Irradiation of this cyclic silylene resulted in its isomerization to the isomeric acetylenic silylene 324, which itself could be photo-chemically converted to the allenic silylene 326. As well, both 324 and 325 were said to isomerize on photolysis to the unusual silacycloalkyne 327, which was characterized... 

Photolysis of cyclic trisilanes like 13 takes place with substantial retention of stereochemistry in the resulting disilane (equation 27), a result which has been explained... 

All known isolable two-coordinated silylenes are cyclic monomers stabilized by bulky substituted amino groups at silicon and in most cases supplemented by a Huckel-type delocalized electron system. They were obtained by reduction of the corresponding dihalogenated derivates [1]. Attempts to synthesize analogous pyrido- ornaphtho-l,3,2A, -diaminosiloles by the same route gave small amounts of the desired pure product or they failed [Id, 2]. This prompted us to investigate (i) the photolysis of bis(trimethylsilyl)diaminosilanes and (ii) the thermolysis of recently reported disilanes [3] and of the trisilanes as alternative methods to prepare stabilized low-coordinated silicon compounds. 

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