Cyclic silylenes matrix isolation

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... 

Other recent applications of the diazide photolysis route include the matrix isolation of some interesting cyclic silylenes (equations 23 and 24)54,55. Solution photolysis of diazides can give other reactive species in addition to silylenes. 

Among the cyclic silylenes 4-7, SiC2 (4) [2] and 1-silacyclopropenylidene (5) [3] have already been investigated by matrix spectroscopy. This series is now completed by silylene 6, the formal hydrogenation product of 5. We also report on the isolation of 7, which can be derived from 5 by formal substitution of one CH unit by a nitrogen atom. 

Three unusual and interesting examples of silylene formation from photolyses are shown in Scheme 54. In the first, Fink and coworkers photolyzed the trisilane 294 at 254 nm and produced the relatively stable silylene 295. This on further photolysis gave rise to the sterically crowded silacyclobutadiene 296 which was trapped with several reagents. In the second example Michl and coworkers photolyzed the matrix-isolated bis-azide 297 to form the cyclic silylene 298, and this on further photolysis at selected wavelengths, using matrices and low temperatures, isomerized to the silacyclopentadienes 299 and 300 and finally to the l-sUa-2,4-cyclopentadiene 301. Finally, Sakurai and coworkers were able to convert the trisUane 302 to the cyclic divinylsilylene 303. 

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