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Various reactions for a stable dialkylsilylene, 2,2,5,5-tetrakis(trimethylsilyl)silacyclopentane-1,1-diyl (1), are summarized and their mechanisms are discussed. Silylene 1 isomerizes to the corresponding silaethene via the 1,2-trimethylsilyl migration. Reduction of 1 with alkali metals affords the corresponding radical anion 1 . − with a relatively small 29Si hfs constant (2.99 mT) and a large g-factor (g = 2.0077) compared with those for trivalent silyl radicals. Photo-excitation of 1 generates the corresponding singlet excited state (1 1 ∗ ) with the lifetime of 80.5 ns. The excited state reacts with C=C double bond compounds including benzene, naphthalene, and (E)- and (Z)-2-butenes. Although the thermal reactions of 1 with haloalkanes occur via radical mechanisms, the insertion into O–H, Si–H and Si–Cl bonds proceeds concertedly via the three-membered cyclic transition states. The reaction of 1 with H2SiCl2 gives the Si–Cl insertion product exclusively, while the quantitative insertion to Si–H bond occurs when Me2SiHCl is used as a substrate. The origin of the rather unusual Si–H/Si–Cl selectivity is elucidated using DFT calculations. Silylene 1 adds to C=C, C≡C, and C=O π bonds to afford the corresponding silacycles as stable compounds. The importance of the carbonyl silaylides during the reactions of silylenes with aldehydes and ketones is emphasized.

Tetra(t-butyldimethylsilyl)germasilene (8b) was synthesized using di(t-butyldimethylsilyl)hydridogermyllithium prepared by the germa-metallation of the corresponding dihydridogermane. Tetrasilylgermasilene 8b was obtained as single crystals by recrystallization from hexane at rt. The molecular structure of 8b is similar to those of the corresponding disilene 8a and digermene 8c but with an intermediate unsaturated bond length and twist angle between those of 8a and 8c. The reactions of 8b with methanol and 2,3-dimethylbutadiene gave the corresponding adducts. Equilibrium between 8b and the corresponding (trisilylsilyl)(silyl)germylene is suggested by the reaction of 8b with triethylsilane giving the germylene insertion product into the Si–H bond.

The reactions of isolable dialkylgermylene and -stannylene with a galvinoxyl radical were found to give rather unusual cyclic compounds in high yields via intermediate adduct radicals. The structures of the cyclic compounds were determined by X-ray crystallography. We propose a concerted cyclization mechanism in which the abstraction of a hydrogen atom from the tert-butyl group by the galvinoxyl radical is accompanied by the simultaneous attack of the germyl radical center to the tert-butyl carbon atom.

The reactions of a stable dialkylsilylene with chlorosilanes such as tetrachlorosilane, dimethyldichlorosilane, and dichlorosilane occurred smoothly at room temperature in hydrocarbon solvents to give the corresponding Si-Cl bond insertion products. In the reaction of the silylene with dichlorosilane, only the Si-Cl bond insertion product was obtained, while a similar reaction with dimethylchlorosilane gave only the Si-H insertion product, emphasizing the remarkable difference in the steric requirements between these two insertion reactions. No reaction took place during the treatment of the silylene with trimethylchlorosilane. The Si-Cl insertion reactions are expected to be applied in the synthesis of new organosilicon frameworks that cannot be obtained by conventional methods.

Syn and anti isomers of a novel saturated sila-macrobicyclic compound, hexasilabicyclo[12.8.8]triacontane, were synthesized via the functionalization of 1,1,10,10-tetraphenylhexasilacyclooctadecane derivative, which was obtained by the diimide hydrogenation of the corresponding hexasilacyclooctadecahexayne derivative. The structures of these macrocyclic compounds were determined by 1H, 13C, and29Si NMR and X-ray crystallography.

A new series of 2,5-bis(arylethenyl)siloles 1a-d was prepared by the reaction of the corresponding zirconacyclopentadienes with H2SiCl2 in CH2Cl2. Absorption maxima of siloles 1a-d were observed in the significantly longer wavelength region compared to those of distyryl-substituted benzenes and heteroles. The emission spectrum of distyrylsilole 1a showed a large Stokes shift compared with distyrylsilole 2.