Silyl groups, electronic effects

The ability to promote /S elimination and the electron-donor capacity of the /3-metalloid substituents can be exploited in a very useful way in synthetic chemistry. Vinylstannanes and vinylsilanes react readily with electrophiles. The resulting intermediates then undergo elimination of the stannyl or silyl substituent, so that the net effect is replacement of the stannyl or silyl group by the electrophile. An example is the replacement of a trimethylsilyl substituent by an acetyl group by reaction with acetyl chloride. 

Silyl ethers are among the most frequently used protective groups for the alcohol function. This stems largely from the fact that their reactivity (both formation and cleavage) can be modulated by a suitable choice of substituents on the silicon atom. Both steric and electronic effects are the basic controlling elements that regulate the ease of cleavage in multiply functionalized substrates. In plan-... 

Figure 1 shows the electron attachment energies (AE) and ionization potentials (IP) of silyl substituted 7t-systems and related compounds [4], AE can be correlated with the energy level of the LUMO (lowest unoccupied molecular orbital) and IP can be correlated with the energy level of the HOMO (highest occupied molecular orbital). For a-substituted 7t-systems, the introduction of a silyl group produces a decrease in the tc -(LUMO) level. This effect is attributed to the interaction between a low-lying silicon-based unoccupied orbital such as the empty d orbital of silicon and the it orbital (d -p interaction) as shown in Fig. 2. Recent investigations on these systems, however, indicate that d orbitals on silicon are not necessarily required for interpreting this effect a-effects of SiR3 can also be explained by the interaction between Si-R a orbitals and the 7r-system.

The effects of silyl groups on the chemical behavior of the anion radicals generated by cathodic reduction is also noteworthy. It is well known that silyl groups stabilize a negative charge at the a position. Therefore, it seems to be reasonable to consider that the anion radicals of re-systems are stabilized by a-silyl substitution. The interaction of the half-filled re orbital of the anion radical with the empty low-lying orbital of the silicon (such as dx-pK interaction) results in partial electron donation from the re-system to the silicon atom which eventually stabilizes the anion radical. 

It has been known for some time that the basicities of a heteroatom decrease upon a-silyl substitution [12], For example, alkyl silyl ethers (R3Si-0-R ) are less basic than dialkly ethers. Silylamines are weak bases compared to alkylam-ines. This electron-withdrawing effect of silyl groups has been explained in terms of the interaction between low lying vacant orbitals such as 3d orbitals of silicon or a orbitals with the nonbonding p orbitals (lone pairs) of the heteroatom (Fig. 4). This interaction decreases the HOMO level which in turn lowers the basicity of the heteroatom. Such effect may also cause the increase of the oxidation potentials, but little study has been reported on the electrochemical properties of this type of compounds. 

Although little information has been available for the effect of a-silyl substitution on the electrochemical properties of heteroatom compounds, extensive studies have been carried out on the effect of /f-silyl substitution [10,13]. For the -substituted heteroatom compounds (substitution at the a carbon), the introduction of a silyl group results in a significant decrease of the oxidation potentials, although the magnitude depends upon the nature of the heteroatom. This effect is explained in terms of the interaction between the C Si a orbital and the nonbonding p orbital of the heteroatom (Fig. 5). This interaction raises the HOMO level which in turn favours the electron transfer. 

Acylsilanes are a class of compounds in which a silyl group is directly bound to the carbonyl carbon, and they have received considerable research interest from the point of view of both physical organic and synthetic organic chemistry [15]. Acylsilanes have a structure quite similar to the structure of a-silyl-substituted ethers a silyl group is attached to the carbon adjacent to the oxygen atom, although the nature of the C-O bond is different. Therefore, one can expect /1-silicon effects in the electron-transfer reactions of acylsilanes. 

We know that reduction potentials of arylsilanes are less negative than those of the corresponding aromatic compounds without silyl substituents (Sect. 2.2.1). The effect of silyl groups to facilitate the electron transfer to the neigbouring aromatic group is explained in terms of p interaction. For example, half wave reduction potentials of naphthylsilanes are less negative than that of... 

Electron-poor allylic alcohols, studied by Adam and coworkers, were found to react r/treo-selectively, regardless of whether the hydroxy functionality is free or masked by alkyl or silyl groups (Scheme 35). This diastereoselectivity, for which the nature of the solvent had no significant influence, was attributed to stereoelectronic effects rather than intramolecular hydrogen bonding or steric strain. 

Conjugate addition of 1 to a,(3-unsaturated esters, ketones, or aldehydes followed by reaction with an alkyl iodide (or bromide) proceeds by stereoselective attack anti to the silyl group, apparently because of electronic effects.2 Protonation of the (3-silyl enolate of 5 proceeds in the same sense to give the opposite diaster-eomer (Chart I). 

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],... 

It was concluded that for halogen and methyl substituted silyl groups, the magnitude of the /J-effect is directly related to the electron-withdrawing ability of the ligands. 

Different silyl groups in substrates exert steric influence on organic reactions in various degrees. Evidence for this exists in the following 29 types of important organic reactions, which can be controlled by bulky silyl groups. Their steric effect often dominates the outcome of those reactions. Nevertheless, the electronic or the stereoelectronic effect may simultaneously also play a minor role. 

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. 

Triorganosilyl groups can control stereochemistry in organic reactions through a steric, an electronic or a stereoelectronic effect. More than one of these effects may exist simultaneously in some chemical processes. The trend listed in Section II could provide chemists with a clear guideline, yet definitive order, to choose a silyl group with appropriate size in control of reactions based on the steric effect. 

As in other benzylic-type carbocations, the chemical shift of the para carbon is a probe to the electronic demand of the carbocation centre and it can be used to evaluate the effect of a /J-silyl group on the positive charge. 

It should be noted that in the case of geranyltrimethylsilane, the carbon-carbon double bond of the allylsilane moiety is oxidized selectively to give two regioisomeric products (equation ll)13. This is due to the electron-donating effect of the /i-silyl group. Since a methoxylated silane was formed in the anodic oxidation of an allylsilane as shown in equation 12, the reaction mechanism can be illustrated as in equation 1313. 

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