Reactions of a stable dialkylsilylene and their mechanisms

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.