Structural characteristics, spectroscopic properties, and unique reactions of a helmeted dialkylsilylene, 2,2,5,5-tetrakis(trimethylsilyl)silacyclopentane- 1,1-diyl, that was synthesized as the first isolable dialkylsilylene and its application to the synthesis of novel silicon unsaturated compounds are surveyed. The silylene was found to be the least electronically perturbed among known stable silylenes by 29SiNMR and UV-vis spectroscopies. The silylene reacts not only with well-known trapping reagents for transient silylenes, such as alcohols, triethylsilane, and 2,3-dimethylbutadiene, but also with less reactive haloalkanes and halosilanes to give the corresponding adducts. One-electron reduction of the silylene using alkali metals affords the corresponding radical anion as a relatively persistent species at low temperatures in solution. Irradiation of the silylene using visible light generates a singlet excited state with a lifetime of 80.5 ns, which reacts with various aromatic compounds and alkenes to give the corresponding silepins and siliranes in a highly regio- and stereoselective manner. The silylene is utilized for the synthesis of unique silicon unsaturated compounds, such as Si=X doubly bonded compounds (X = S, Se, Te, C=NR, etc.), a trisilaallene, a 1,3-disilagermaallene, and silylene transition-metal complexes.
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