Silene-silylene rearrangement

Tilley reported silene-silylene rearrangement in a cationic iridium complex (Eq. 13) [13b]. 

Rearrangement of a silene to a silylene (40) via migration of a Me3Si group has been suggested as a step in the gas-phase silylene to silylene rearrangement. Labeling experiments, however, have indicated an alternative mechanism (Scheme 14.23). ... 

Several examples in which a C-H bond of a silylene shifts to yield a silene are mentioned in Section III.B.l.b, which discusses the reverse process of thermal silene-to-silylene rearrangement (see in particular References 75 and 229). 

Although the silene-to-silylene rearrangement would presumably be approximately thermoneutral in most cases, it has so far been only observed for substituents with particularly high migratory aptitudes, H and SiMe3. 

Another report of a silene-to-silylene rearrangement involves a 1,2-shift of a trimethylsilyl group, converting l-(trimethylsilyl)- 1-methylsilene into methyl[(tri-methylsilyl)methyl]silylene (equation 96)207. 

The pyrolysis of a precursor to cyclopropyl(trimethylsilyl)silylene is most easily interpreted by assuming that the silylene undergoes a rearrangement to l-(trimethylsilyl)-I-silacyclobutene which in turn undergoes a silene-to-silylene rearrangement (equation... 

Another case in which a silylene-silene-silylene sequence is believed to occur is shown in equation 98227. Other examples of possible silylene-to-silene-to-silylene rearrangements are known225,226,277. 

We have mentioned in Section III.A.2.b the reverse of the silene-to-silylene rearrangement. In particular, cyclopropylsilylenes are prone to rearrange to 1-silacyclo-butenes207,223,224. 

Grobe15 has described the pyrolysis of 1 -methyl-1 -vinyl- and 1,1 -diviny 1-1-silacyclobutanes 166 which led to the formation of methylvinylsilene and divinylsilene, respectively. Under the experimental conditions used, it was suggested that the silenes rearrange to exo-methylene- 1-silacyclo-propanes 167 which extrude methylsilylene or vinylsilylene, respectively. In support of this proposal, when the reactions were carried out in the presence of 2,3-dimethylbutadiene, the anticipated silylenes were trapped as their respective l-silacyclopent-3-enes 168. 

A much explored pathway to simple silenes involves the thermolysis of silacyclobutanes at 400-700°C, the original Gusel nikov-Flowers (155) route. Such temperatures are not readily conducive to the isolation and study of reactive species such as silenes except under special conditions, and flash thermolysis, or low pressure thermolysis, coupled with use of liquid nitrogen or argon traps has frequently been employed if study of the physical properties is desired. Under these high temperature conditions rearrangements of simple silenes to the isomeric silylenes have been observed which can lead to complications in the interpretation of results (53,65). Occasionally phenyl-substituted silacyclobutanes have been photolyzed at 254 nm to yield silenes (113) as has dimethylsilacyclobutane in the vapor phase (147 nm) (162). 

Two indirect routes to silenes, one derived from silylenes and the other from silylcarbenes, are of some generality and importance. Silylenes (e.g., Me3Si—Si—<]) (53) have been derived from the thermolysis of either methoxy or chloro polysilyl compounds. Thermolysis resulted in the elimination of trimethylmethoxy- or trimethylchlorosilane and yielded the silylene, which, based on products of trapping, clearly had rearranged in part to the isomeric silene [Eq. (5)]. Alternatively the silylene Me2Si has... 

Barton has reported a wide variety of elegant studies in which various silenes or silylenes have been created, usually thermally, and their subsequent rearrangements investigated in terms of the observed products of trapping (51,53,65,145). It has been clearly established that interconversion between silenes and silylenes, especially where H atoms or Me3Si groups migrate, are facile processes. In some cases, radicals can be the precursors to silenes (65). 

Scheme 14.18). The silylene-silene rearrangement 27 28 is nearly thermoneutral, with the silene being slightly more stable. The photolysis of a-diazo compounds (30) is the only frequently used reaction path to silenes (31) via a carbene-silene... 

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). 

A photochemical approach via a silylene-to-silene rearrangement was followed by Fink and coworkers in their synthesis of silacyclobutadienes in a 3-methylpentane matrix at low temperatures161. Irradiation of the cyclopropenyltrisilane 294 gives the relatively stable cyclopropenylsilylene 295. 295 can be efficiently converted to silacyclobutadiene 296 by irradiation into the visible absorption band of the silylene (equation 72)161 162. 

Conlin and coworkers photolyzed vinyltris(trimethylsilyl)silane 188 in the presence of a variety of trapping reagents such as butadiene, substituted butadienes or silanes and observed products derived from intermediate silenes 189 (formed by rearrangement) or from silylenes 190 resulting from elimination of hexamethyldisilane93. In some cases complex mixtures of products which could have been derived from intermediate silyl radicals were also observed. The reaction products formed from the silene and the silylene in the presence of butadiene, 191 and 192 respectively, are shown in Scheme 32. 

The mechanisms proposed by Caspar and coworkers in their approach to the problem have a common point, the formation of the intermediate 9. This silylene then undergoes two prototropic rearrangements initial isomerization to a silene, the 2//-silole (11), followed by a second to give the 1H-silole (2)19 (equation 3). 

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