Silylenes addition

The reversibility of most silylene addition reactions allows the cycloadduct of a silylene to 1,3-diene to be employed as a sUylene source. Extrusion of a silylene from l-silacyclopent-3-ene (3) has been achieved by thermolysis in the gas phase and also by photolysis in solution (Scheme 14.4). 

Silylene addition to dienes was first reported in 1968 (Scheme 17) (68JA3438). The l-silacyclopent-3-ene produced is again thought to result from a silirane intermediate, as a concerted 1,4-cycloaddition can be ruled out. Thermally generated silylene SiH2 does not add stereospecifically to trans,trans- and cis,trans-2,4-hexadiene but gives a mixture of the two silacyclopentenes (6 1 and 2 1 ratio) in each case (Scheme 18) (74JA6198). 

The photolysis of tris(trimethylsilyl)phenylsilane in the presence of a series of alkynes alforded the silacyclopropene through silylene addition to the triple bond. Those obtained from monosubstituted alkynes underwent photochemical isomerization to the disilanyl-alkyne through a 1,2-hydrogen shift (Scheme 48) (80JOM(190)117). Disubstituted alkynes form silirenes that can be isolated by preparative GLC. 

The first 1,2,4-thiadisiletane results from the reaction of carbon disulphide and the hindered silylene [2,4,6- (Me3Si)2CH 3C6H2]MesSi (TbtMesSi ) formed from the Z-disilene precursor. The mechanism is thought to involve a skeletal rearrangement of the 3,3 -spirobi(l,2-thiasilirane) intermediate formed by silylene addition to each carbon-sulphur double bond (equation 31)64. 

Extrusion of a silylene from a silirane or silirene is of course the inverse of silylene addition to alkenes or alkynes, respectively. The reversibility of most silylene reactions allows the inverse of addition to 1,3-dienes to also be employed as a silylene source. The first such reaction was reported by Chernyshev and coworkers, who found that transfer of SiCl2 units from l,l-dichlorosilacyclopent-3-enes was a unimolecular process and hence was likely to consist of silylene extrusion and readdition (equation 38)82. Dimethylsilylene extrusion has been found in the pyrolysis of silacyclopentenes and other products of... 

There is a dramatic kinetic stabilization by bulky substituents of the three-membered ring products from silylene addition to jr-bonds. Addition of t-Bu2Si to ethylene led to the first silirane with no substituents on its ring carbon atoms as a distillable liquid 164. Interestingly, l,l-di(terf-butyl)silirane does not undergo photochemical or thermal silylene extrusion, but instead polymerizes. A distillable silirane was also reported from addition of t-Bu2Si to 2-methylstyrene165. 

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. 

K. L. Bobbitt, Photochemical Generation of Germylenes and Silylenes Mechanism of Germylene and Silylene Addition to 1,3-Dienes, Doctoral Dissertation, Washington University, St. Louis, December 1990. 

The bonding in the silylene addition complexes was compared with that in their carbon analogues (i.e. such as H2C h3)294. Significant differences were found, particularly in the multiple bond character of the central atom. Many of the differences between silylenes and carbenes, particularly the higher barriers to hydrogen migrations in the silylene complexes, can be understood in terms of the reluctance of silicon to rehybridize294. 

In Table 7, the studies on silylene addition to c-bond systems are summarized. They are classified primarily according to the types of silylenes. The addition reactions of monomeric SiF2 formed by the high-temperature Si-SiF4 reactions are omitted in this table because they have already been summarized in Table 3. In the following sections, the addition reactions are discussed according to the type of molecules the silylenes are adding to, namely, olefins, dienes, alkynes, and aromatic compounds. 

Table 7. Summary of Silylene Addition Reactions to 7r-Bond Systems... 

The addition of SiMej and SiMeOMe to 2,3-dimethylbutadiene studied by Weyenberg and Atwell was actually the first reported silylene addition to a conjugated diene system. In the vapor phase, the yields of silacyclopentenes were generally high and over 60%. Experimental support for the mechanism of such addition to include an initial step of 1,2-addition followed by isomerization was recently provided by Ishikawa, Ohi, and Kumada, who studied the reaction as shown in equation (109) with SiMePh. They succeeded in intercepting the silacyclopropane intermediate with CH3OH and obtained 2,3-dimethyl-4-(methylphenylmethoxysilyl)-2-butene (9), as an identifiable product. 

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