Silylenes isomerization

Conlin148 also studied the pyrolysis of 1-methyl-1-silacyclobutane in the presence of excess butadiene at various temperatures where the decomposition followed first-order kinetics and where the silene isomerized to the isomeric silylene prior to reacting with the butadiene. The value for the preexponential factor A for the silene-to-silylene isomerization was found to be 9.6 0.2 s-1 and the Ewl for the isomerization was 30.4 kcal mol-1 with A// = 28.9 0.7 kcal mol-1 and AS = -18.5 0.9 cal mol-1 deg. More recently, the photochemical ring opening of l,l-dimethyl-2-phenylcyclobut-3-ene and its recyclization was studied. The Eact for cycli-zation was 9.4 kcal mol-1.113... 

Arrhenius parameters for the reversible interconversion of 1-methylsilene (MeHSi=CH2, 2a) and dimethylsilylene (3) via 1,2-H shift, an example of the well-known silene-to-silylene isomerization (equation 5), have been reported by two groups24,25. The earlier values of Davidson and Scampton [ a = 166 kJmol-1 and log(A/s 1) 13.5 in both... 

In a similar fashion, the irradiation of 7 in hexane at room temperature in the presence of 2,3-dimethylbutadiene produced the adducts expected from (trimethylsilyl)-trimethyldisilene, 852. No silylene adduct 11 was detected, indicating that no disilene-to-silylene isomerization took place under these conditions (equation 7). However, such an isomerization was observed at 300°C52 (Section II.B.l.b). 

A silene-to-silylene isomerization by a 1,2-shift of a trimethylsilyl group from silicon to carbon was originally proposed in order to account for the formation of 1,1,3-trimethyl-1,3-disilacyclobutane during the pyrolysis of allylpentamethyldisilane (equation 93)214. Since silenes are planar and since the rc-component of the C=Si double bond is quite strong ( 40 kcal mol ) 19, m, it is not easy for the molecule to align the migrating SiH or SiSi bond with the carbon p-orbital that forms the new bond. 

Evidence for additional silene-to-silylene isomerizations on simple systems is now available. Thus the pyrolysis of a silabicyclo[2.2.2]octadiene precursor for 1-chlorosil-ene229 produces a mixture of matrix-isolated 1-chlorosilene and chloromethylsilylene. In this instance the thermal rearrangement of the silene to the silylene is probably facilitated by the increased exothermicity of the reaction, since halogenated silylenes are particularly stable (equation 95). 

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