Cyclic silylenes rearrangement

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). 

Nefedov, Michl and coworkers reported the successful spectroscopic characterization of the l-silacyclopenta-2,4-diene 287 and its photochemical transformation into the isomeric species 288-290 (equation 71)159. Caspar and coworkers160 had suggested that silole 287 might be formed by a rearrangement of the cyclic silylene 288 via the 1-silacyclopenta-1,3-diene 289. This proposal was confirmed in an elegant UV-visible, IR-matrix isolation... 

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... 

Similar cyclic and sterically protected silylene has been prepared recently (equation 88). The silylene must be stabilized to some extent by a-n conjngation instead of n-7T conjugation for other stable cyclic silylenes shown in Figure 9. Indeed the silylene undergoes facile rearrangement (eqnation 88). 

Since, as could be shown by trapping experiments with 2,3-dimethylbutadiene, the primary products upon trimethylsilane extrusion are always the expected open-chain silylenes, the formation of the cyclic compounds must arise from a rearrangement reaction. Whether really a one-step process with a transition state of type 155 occurs as indicated in equation 43, or whether the reaction pathway from 154 to 156 involves intermediates,... 

Both silene isomers 278 and 279 are ideal precursors for the generation of silylene 284, since their interconversion to 284 is spontaneous (in the case of 278) or can be easily induced by irradiation (in the case of 279). There are numerous well-established methods to prepare transient silylenes 279. Three important examples are shown in equation 69, namely the photolytic genCTation from a trisilane 280, thermolytic or photolytic decomposition of cyclic silanes 281 " " and degradation of diazidosilanes 282 56 -pjjg photolysis of the diazido silane 282 is an especially clean reaction which has been used in several spectroscopic studies. The photolysis of or-diazo compounds 283 is the only frequently used reaction path to sUenes 284 via a carbene-silene rearrangement. ... 

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