Silylene relative reaction rates

Another type of reaction allowed us to study the philicity of silylene 4. As we have shown earlier, 4 adds smoothly to a variety of alkynes giving way to the silacyclopropene framework (Eq. 4) [10], Varying the / ara-substituents of diphenylacetylene offers us the possibility to tune the electron density of the triple bond, and we now studied the rates of the reaction of 3 with diphenylacetylenes lOa-c. The absolute reaction rates of these three first-order reactions are identical (Ai = 6.3 0.2 10 4s-l) this result is in accordance with a mechanism, in which the formation of silylene 4 from cyclotrisilane 3 is the rate determining step [6], However, the relative reaction rates of the addition of 4 to the triple bond of lOa-c, which were determined by competition experiments, turned out to differ appreciably from each other. Electron withdrawing substituents favor the addition of 4 to the alkyne, whereas electron donating substituents, such as a methyl group, slow down the reaction rate. As shown in Fig. 5, the relative reactions rates correlate well with the [Pg.62]

In this model the reaction rate decreases with increasing temperature because dissociation of the intermediate complex to the reactants has a higher barrier (i.e. via TSi), than does the rearrangement of the complex (i.e. via TS2) to the product of the silylene reaction. Thus, as the temperature increases, product formation is disfavored relative to regeneration of the silylene from the intermediate complex. 

The top panel of Fig. 17.2 (Ts = 800 K) reveals that there is very little decomposition of the silane in the gas phase, which is a result of the relatively low temperature. As a result the net growth rates should be expected to be quite low, since the silane sticking coefficient is so low. At a surface temperature of Ts = 1300 K, however, the decomposition of silane to silylene in the gas-phase boundary layer is nearly complete. The relatively high silylene concentrations should lead to high growth-rates. The peak in the silylene profile at about 1.5 mm above the surface results from the competition between production by the homogeneous decomposition reaction and consumption at the surface by heterogeneous reaction. 

Photochemically generated silylene species may react either with themselves to form poly silane polymers, or with a trapping agent added to intercept the species. The fate of the silylene and the structure of reaction products depend on the relative rates of these two processes, which may proceed simultaneously. 

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