Dimerization of silylenes

The mechanism of substitution reactions at saturated silicon centers is well studied, regarding both kinetics and stereochemistry13,14. In contrast, addition reactions to unsaturated silicon centers, such as to disilenes and silenes, are relatively unexplored. The reason is clear suitable substrates for investigations of regio- and stereochemistry and reaction kinetics are not readily available due to inherent kinetic instability of disilenes and silenes. Kinetically stabilized disilenes and silenes are now available, but these are not always convenient for studying the precise mechanism of addition reactions. For example, stable disilenes are usually prepared by the dimerization of silylenes with bulky substituents. Therefore, it is extremely difficult to prepare unsymmetrically substituted disilenes necessary for regio- and/or stereochemical studies. 

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 key to the synthesis of stable disilenes, which are readily obtained by dimerization of silylenes, is protection of the silicon-silicon double bond by substituents. 1,2-Di-(1-adamantyl)dimesityldisilene (198), a disilene which is resistant... 

Finally, the dissociation of disilene to two silylenes has been studied and this reaction is discussed (as the dimerization of silylene in Section VII.A.3.a. 

Overall, for the addition of silylenes to alkynes to give 1,4-disilacyclo-hexadienes, the operation of mechanism b seems to be firmly established. The complete operation of mechanism a is probably in doubt, but the conversion of silacyclopropene to disilacyclobutene (patch c) is still possible. However, since the primary steps of both mechanisms, the addition of silylene to n bonds and the dimerization of silylenes, are both well established, the relative importance of the two mechanisms should be kinetically controlled by the concentration of the silylenes. At some lower concentration of the reacting silylenes, the silacyclopropene route may actually prevail. 

As stated before, there is little knowledge on the unsymmetrically substituted stable disilenes because stable disilenes are usually prepared by dimerization of silylenes, thus leading to symmetrical disilenes. Unsymmetrically substituted disilenes are produced mostly as transient species (see the preceding section), and it was found that ( )- and (Z)-l,2-dimethyl-l,2-diphenyldisilenes undergo the addition reaction with alcohols very rapidly (k2 = 10 -10 s ). The rates are only 1 to 2 orders of magnitude smaller... 

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

Monomeric base adducts of silylene complexes can be transformed into dimeric compounds at elevated temperatures with loss of the donor. This applies also to reactive donor-free compounds. 

The dimerization process of silylene Mes2Si to 1 was studied using matrices with varying viscosity. The dimerization rate was found to be dependent on the viscosity. Interestingly, dimerization took place even in the matrix at 77 K without annealing when a soft matrix (e.g., isopentane/3-methylpentane = 9/1) was used.13... 

Synthesis, Structures, Reactions, and Dimerizations of Stable Silylenes. 684... 

Pyrolysis of polysilanes played an important role in the discovery of silylene reactions. Through the pyrolysis of alkoxydisilanes in the presence of diphenylacety-lene, Atwell and Weynberg obtained a product regarded as a dimer of dimethylsilylene adduct. 1,2-Shift of a methoxy group in disilanes takes place under relatively mild conditions (Scheme 14.1). ... 

SYNTHESIS, STRUCTURES, REACTIONS, AND DIMERIZATIONS OF STABLE SILYLENES... 

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. 

The first step of the retro-reaction involves loss of silylene 79, which could be trapped with 1-pentyne to give the known silirene 81 (equation 125). In the absence of a trapping agent, 79 recondenses to 77, probably by first dimerizing to the disilene Ar2Si=SiAr2 followed by 2 +1 cycloaddition to give 77 (equation 126). From the principle of microscopic reversibility, the fact that silylene is formed in the retro-reaction leads to the conclusion that 79 must also be an intermediate in the cycloaddition reaction. 

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