Insertion into c-Bonds

Carbene, silylene a-bond Reaction product Calculation method Activation barrier, kcal/mol Refs. 

To date no experimental evidence is available regarding the insertion of silylene into the H2 molecule, however, according to the theoretical estimates in [75], the lower limit of the activation barrier amounts to 5 kcal/mol. Other theoretical estimates [70, 76] are consistent with this value (see Table 8.4). An analysis of structural alterations in the course of the reaction SiH2 H- H2 - SiH4 has revealed that they are practically identical to those found in the reaction 

The effect of substitution of hydrogen atoms in silylene on exothermicity and activation barriers of the reaction of insertion into the H—H bond has been studied on its fluoro- and difluoro-derivatives [71]. The activation barrier grows by about 20 and 30 kcal/mol upon introduction of, respectively, one and two atoms of fluorine. This result does not practically depend on the size of the basis set nor on whether the correlation energy is taken into account. Analysis of the structure of transition states has shown that they become more late with concurrent lessening of exothermicity of insertion reactions, which is consistent with the Hammond postulate. 

A very high (40 kcal/mol) activation barrier is predicted in the ab initio calculations [65] for the insertion reaction of cyclic carbene of cyclo-propenylidene into the H—H bond. This may be accounted for by the strong destablization of the electrophilic 7r -orbital of carbene and the obstacles associated with it to the development of reaction in the initial stage. 

Insertion into the C—H and Si—H bonds. The insertion of singlet methylene into the C—H bonds occurs, according to experimental [79] and theoretical [66, 72] data, either without or with very low (0.2 kcal/mol) activation barrier, whereas for silylene a considerable (27 kcal/mol) barrier is predicted in agreement with experimental observations [80, 81]. The transition states have angular three-center bond structure. 

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