Silicon substituent steric effects

An alternative SN2-Si mechanism may be considered, without a penta-coordinate intermediate. In this alternative, the silicon is rehybridized from spy to sp2. If bond order is conserved, then it is reasonable to ascribe a bond order of 0.5 to the Si—O bond of both the entering nucleophile and the leaving group [52], It has been shown that the bond order is related to the bond distance [45]. Even though there are five substituents in the vicinity of the silicon in an SN2-Si type mechanism, two of the substituents are significantly further away from the central atom (silicon) than the other three substituents. A looser transition state structure than that for an SN2 -Si or an SN2 -Si process results. The steric effects of the alkyl group bonded to silicon should, therefore, be considerably less in an SN2-Si type mechanism. 

The influence of both the steric and electronic properties of the silyl group on the rate of epoxidation have been examined experimentally [104], Two rate effects were considered. First, the overall rate of epoxidation of the silyl allylic alcohols was found to be one-fifth to one-sixth that of the similar carbon analogs. This rate difference was attributed to electronic differences between the silicon and carbon substituents. Second, the increase in k[el to 700 for silyl allylic alcohols compared with carbon analogs (e.g., 104 for entry 3, Table 6A.8) was attributed to the steric effect of the large trimethylsilyl group. As expected, when abulky (-butyl group was placed at C-3, k[e] increased to 300 [104],... 

Most of the organosilicon compounds contain bonds between the silicon and carbon atom. In the following paragraph the structural chemistry of the Si—C single bond is discussed, mostly in compounds with tetracoordinate silicon and tetracoordinate carbon atoms. The structural chemistry of the Si—C bond in compounds where the carbon coordination state is different, is also discussed. The Si—C bond is markedly polarized and the increase of the bond ionicity by attaching different substituents to either the silicon or the carbon atoms may affect its length. The electronic and steric effects are discussed later. 

As with vinylsilanes and alkynylsilanes, substitution is favoured over addition for allylsilanes. However, this can be affected by the steric and electronic effects of the silicon substituents. Mayr and Hagen have studied the reactivities of allylsilanes towards the p-methoxy substituted diphenylcarbocation (Scheme 7)136. 37 Relative rate data and observed products are summarized in Table 11 for allylsilanes of the structures 119-122, with various silicon substituents. 

Data listed in Table 2 include the substituent constants R1 of trialkylchlorosilanes and the relative rate constants fc(R1Me2SiCl)/A (Me3SiCl) for the reactions of the two chlorides with lithium silanolates and isopropylate (equation 39)57. The reaction rates of silanes are influenced almost exclusively by the steric effects of the alkyl groups attached to the silicon atom. The log(A rei) values of the compounds with various R1 groups give a satisfactory correlation with Taft s Es values151. Thus the steric hindrance of silyl groups follows the order listed in entry 4457 of Table 1. 

A quantitative scale for the structural effect of various silyl groups is established, as shown in entry 57 of Table 1, by the rates of solvolysis of 40 triorganosilyl chlorides in aqueous dioxane under neutral conditions69. The structural effect involves the steric effect and, in some examples, the electronic effect. Because little difference exists in the electronic effect among alkyl groups, their steric effect at silicon follows the order primary < secondary < tertiary substituents. 

Kinetic studies show that hydrolysis of 1-organyl- and 1-alkoxysilatranes in neutral aqueous solutions is a first-order reaction catalyzed by the formed tris(2-hydroxyalkyl)amine13 294. As a rule, electron release and steric effects of the substituent X hinder the reaction. However, the hydrolytic stability of 1-methylsilatrane is just below that of 1-chloromethylsilatrane294. Successive introduction of methyl groups into the 3, 7 and 10 sites of the silatrane skeleton13,294 and substitution with ethyl group on C-459 retard sharply the hydrolysis rate. It was proposed294 that nucleophilic attack at silicon by water proceeds via formation of the four-centered intermediate 57 (equation 56). 

It has been pointed out previously that silylation of ylides leads to stabilized products and that this is only one example of the very general phenomenon of carbanion stabilization through silicon (34, 61, 72). This effect was also found for arsenic ylides (34, 73), and is the basis for the preparation of other compounds of this series. The influence of silicon is by no means solely an electronic effect. In many cases, where alkylsilyl substituents are introduced, a steric effect may well dominate, which may reduce lattice energies for salts in transylidation reactions, preventing intermolecular contacts in decomposition processes, and rendering the formation of salt adducts unfavorable. This steric effect is reduced to a minimum, but not eliminated, if simple SiH3 groups are employed (61). Even then, however, a pronounced silicon effect is found, which must be based on electronic influences (49, 60, 61). 

The stabilities of silyl ethers are closely related to the electronic and steric effect of the substituents on the silicon atom and are generally proportional to the steric hindrance provided by the substituents. Moreover, electron-withdrawing substituents on the silicon atom increase the stability of the silyl groups toward acid but decrease their stability toward base. Consequently, their stability in acid follows the order TMSSelective deprotection among these groups can thus be achieved... 

In contrast, the rate constants for methanol addition to the series of silicon-substituted silenes 2a-i (Table 13) do not vary in a straightforward way with either inductive io ) or resonance (o ) substituent parameters associated with the R substituent. However, a multi-parameter fit of the data to equation 64, in which Es is the steric substituent parameter of Unger and Hansch122 and p, pr and ps are the related standard reaction constants describing the individual effects of inductive, resonance and steric effects on the rate (and are the variables in the analysis), led to an excellent least-squares fit of the data (r2 = 0.965). This afforded the coefficients pr = —3.6 1.2, pi = 3.1 1.0 and ps = 0.21 0.08, where the quoted errors represent the 95% confidence limits of the analysis. Figure 11 shows a plot of the data against the function obtained from the least-squares fit (equation 65). 

In all complexes of type Cp(OC)2Mn(H)SiR3 (and also PR3-substituted derivatives) the orientation of the silane relative to the Cp(OC)2Mn fragment is the same. If the Mn—H—Si triangle is taken as a reference plane, both carbonyl ligands are below this plane at about the same distance, one substituent at silicon (R1) is within this plane, and the other two substituents at silicon (R2) are above and below. The substituent R1 is the most electronegative one (in Fig. 2, the fluorine atom) how much this orientation of the silane is prone to steric effects has not yet been probed. 

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