Silylenes substituent effects

Table 4. Selected structural data of silylene complexes, substituent effects and pyramidaliza-tion at Si [35-38]...

Substituent effects on the transition energy of silylene have been investigated systematically by ab initio MO calculations and good agreement between the calculations and experiment has been attained175 176. 

The nature of substituent effects on the position of absorption maxima corresponding to n — p transition in polyatomic silylenes have been considered in detail in theoretical studies of Apeloig and coworkers126 128. Unfortunately, there is no similar study for germylenes, stannylenes or plumbylenes. However, the available experimental data show that the main conclusions obtained by Apeloig and coworkers are also applicable to polyatomic germylenes, stannylenes and plumbylenes. 

It is often difficult to deduce from the restricted experimental data the main trends in reactivity. Theoretical calculations could give the needed information to fill these gaps. In other words, they could be used to sometimes interpolate scarce experimental data, e.g. in studying substituent effects. To examine general trends in the series of Group 14 elements carbene analogs, data for silylenes and even for carbenes are presented in this section for some cases. 

Substituents effect dramatically also the bond dissociation energies of disilenes to the corresponding silylenes (Eq. 1), as is shown in Fig 12. which describes the correlation of the calculated dissociation energies for a series of substituted disilenes vs the sum of the singlet-triplet energy differences in the corresponding silylenes [20]. The theoretical background for the existence of such a correlation is discussed elsewhere [21]. 

Dienes and heterodienes differ in the regioselectivity of their cycloadditions. In case of dienes they are controlled by substituent effects, in case of heterodienes by the strongest primary Lewis acid-base interaction [silylene to lone-pair of heteroatom with O > N (Scheme 3)]. 

Two features are immediately obvious from the data in Table 32. The singlet and triplet states respond differently to substitution and the effects are mostly larger (especially in the singlet state) in the carbenes than in the silylenes. Let us examine first the substituent effects in the singlet state. Most substituents stabilize the singlet silylene, but the most effective stabilization is achieved with 7t-donors and it decreases with the decrease in the... 

The singlet carbenes behave similarly, except that the stabilization energies are usually much greater than for the corresponding silylenes, especially for rc-donors (Table 32). The smaller substituent effects in silylenes result mainly from the lower tendency of silicon to form 7r-bonds (Section V.A). The similar qualitative response of silylenes and carbenes to substituent effects makes it possible to use the extensive knowledge on substituted carbenes to predict qualitatively the effect of other substituents on singlet silylenes. 

Silicenium ions received some theoretical attention, but to a smaller extent than silylenes. Interest centered on their thermodynamic stabilities relative to the analogous carbenium ions and on substituent effects. These are important questions, because the low thermodynamic stability of silicenium ions was believed by many workers to be one of the major factors frustrating their experimental preparation258. 

The effect of substituents on the properties and structure of silylenes was analyzed in detail by Apeloig and Karni on the basis of ab initio calculations137. 

Silicon substituted silylenes attract the chemists interest for a special reason. According to ab initio calculations 85 137 176], substituents acting as cr-donors should induce a relatively large red-shift of the UV maximum or, in other words, the n(Si) —> 3p(Si) transition energy should be relatively small. Therefore these species are potential candidates for the long-sought triplet ground state silylenes, especially if this electronic effect is supported by a steric one. Nevertheless, reports on the matrix isolation of silicon substituted silylenes are comparatively scarce. 

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