Silylenes dimerization

When silylenes are generated photochemically in hydrocarbon matrices in the presence of electron-pair donors, they may form Lewis acid-base complexes that act as intermediates in the silylene dimerization to disilenes.3233 In a typical example, Mes2Si(SiMe3)2 was photolyzed in 3-meth-ylpentane (3-MP) matrix containing 5% of 2-methyltetrahydrofuran. At 77 K, dimesitylsilylene (Amax 577 nm) was formed. When the matrix was... 

The first 1,2-disilacyclobutene (82) was prepared in 1973 by the gas phase reaction of dimethylsilylene and 2-butyne (73JOM(52)C21). It probably results through silylene insertion into the intermediate silacyclopropene (Section 1.20.3.4), but silylene dimerization followed by addition to the alkyne is also suggested (76JA7746), since (82) is formed in good yield if the disilene is generated directly (Scheme 127) (78JOM(162)C43). 

Silylene addition to 9,10-dimethylanthracene had previously been found to compete effectively with silylene dimerization (equation 71)167. Control experiments established that the silylene adduct did not undergo ring expansion to the disilene adduct. 

The addition of t-Bu2>Si to 1,4-diaza-l,3-butadienes competes with dimerization of the silylene only when the concentration of t-BinSi is low170. Subtle steric effects must also be responsible for the addition of /-BinSi to the W-cyclohexyl mono-imine of benzil, while only the silylene dimer undergoes addition under similar conditions in the presence of the IV-methyl mono-imine171. It may be that t-Bi Si and its dimer t-Bu2Si=SiBu-t2, both formed simultaneously upon photolysis of cyclo (t-Bu2Si)3, are in equilibrium, and the steric effect is upon the (2+4) cycloaddition of the disilene. 

Another route to stable disilenes is photolysis of hindered cyclotriarylsilanes (equation 76)185. In this reaction one disilene molecule is formed directly, and a second one is produced by eventual silylene dimerization. 

The lower heats of formation of silylenes compared with carbenes translate into somewhat lower, albeit similar, reactivity. There is a general lesson here that slowing down the reactions of carbene-like species makes possible processes like dimerization that are more or less impossible for carbenes. We have already seen that slowing down their rearrangements allows the observation of intermolecular reactions of alkylsilylenes not observed for alkylcarbenes. The dimerization of silylenes to disilenes, discovered in the gas phase [36], was employed to brilliant effect by West and coworkers in the condensed phase synthesis of isolable disilenes [37]. While these were not the first disilenes, silylene dimerization opened the way for the preparation of many new molecules, including digermenes from the dimerization of germylenes [38]. 

The CO complexes of silylenes with bulky substituents which were observed by West et al. [8] in organic matrices eliminated CO upon annealing of the matrix the remaining silylenes dimerized to the corresponding disilenes. If this reaction took place in the CH20Si system, it would lead to the longsought-after parent disilene molecule. 

Two singlet silylenes dimerize to give a ground-state disilene with a substantial barrier via the least-motion path and without a barrier via the non-least-motion path. On the other hand, two triplet silylenes dimerize to give adiabatically an excited state of disilene. 

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