Isolable silylene, disilenes, trisilaallene, and related compounds

Our recent studies of synthesis, structure, and reactions of an isolable silylene, stable novel cyclic and conjugated disilenes, a trisilaallene are summarized. Due to the distinctive electronic and steric effects of trialkylsilyl substituents, tetrakis(trialkylsilyl)disilenes showed interesting structural features around Sidouble bond; length as m-dashSi bonds, electronic spectra, and reactions. The tetrasilyldisilenes were useful reagents for the synthesis of novel types of organosilicon compounds such as η2-disilene transition metal complexes and a 1,3-disilabicyclo[1.1.0]butane. Photochemical and thermal interconversion among Si4R6 isomers including a cyclotetrasilene, a silylcyclotrisilene, and a bicyclo[1.1.0]tetrasilane occured without apparent participation of the corresponding tetrasila-1,3-diene. The first spiropentasiladiene was thermally very stable and showed remarkable spiroconjugation between the two ring π systems. An isolable dialkylsilylene was found to be well-protected sterically from dimerization but least perturbed electronically. Using the silylene, a trisilaallene, the first stable compound with formal sp-hybridized silicon atom, was synthesized. In contrast to carbon allenes, the skeleton of the trisilaallene was significantly bent and remarkably fluxional.