Trimethylsilyl substituents, 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. 

On the basis of complete neglect of differential overlap (CNDO/2) calculations which include silicon p and d orbitals in the basis set, the effect of the trimethylsilyl substituent is due to electron withdrawal by the interaction of both the silicon p and d orbitals with the aromatic ir-system (85). In as much as the silicon p orbitals are primarily involved in the cr and cr orbitals of the trimethylsilyl group, interaction of the silicon p orbitals with the ir-system amounts to a hyperconjugative, v — cr, electron withdrawl (85). 

Block et al.194 examined the effects of trimethylsilyl substitution on the first vertical ionization potentials by photoelectron and Penning ionization electron spectroscopy studies of a range of cyclic and noncyclic sulfides and ethers. It was shown that substitution of oxirane 218 with a trimethylsilyl substituent as in 219 lowered the ionization potential by 0.90 eV (20.8 kcal/mol), while similar substitution of dimethyl ether 220 in 221 lowered the ionization potential by 0.64 eV (14.8 kcal/mol). By comparison, the effects of silyl substitution on sulfur lone-pair ionization potentials was found to be smaller thus the ionization potential of dimethyl sulfide 222 is lowered by 0.37 eV upon trimethylsilyl substitution in 223, and the trimethylsilyl-substituted thiirane 225 is lowered by 0.59 eV relative to thiirane 224. The raising of the energy of the sulfur lone-pair electrons in the thiirane 225 is also apparent from its UV spectrum, where there is a bathochromic shift in the absorption maximum compared to the parent 224. 

The o(m, p)-PBTMS-PPV series shows very similar absorption profiles with those of PMEH-PPV polymers. The maximum absorption wavelengths of o(m)-PBTMS-PPV are blue-shifted for the same reason as that of o(ra)-PMEH-PPV. But PBTMS-PPV polymers show about 20-50 nm blue-shifted absorption maxima compared to those of PMEH-PPV series, because the trimethylsilyl substituent has little electron-donating property. The PL spectra show very drastic changes in emission color because of the substituents and kink effects. Figure 19 shows PL emission profiles of the polymers mentioned above. 

In the case of the phosphaalkenes—in analogy to the phosphaalkynes—kinetic stabilization of the localized double bond by bulky substituents such as, tert-butyl-, trimethylsilyl-, adamantyl-, or 2,4,6-tri-ferr-butylphenyl groups has again proved useful (Scheme 3). In addition, however, electronic effects also have a significant influence on the stability thus, it has been shown that a sufficient overlap of the p orbitals in such double bond systems can be realized by reducing the polarity of the P—C bond—for example, by the presence of electron-withdrawing substituents at the phosphorus atom. 

Chatgilialoglu and Curran synthesized a variety of allyl tris(trimethylsilyl)silanes bearing substituents at the 2-position (Scheme 26) [70], These allylsilanes underwent reaction with alkyl halides when heated with a radical initiator to give very good yields of allylated products. The reactions were relatively sensitive to electronic effects electrophilic radicals reacted well only with electron-rich allyl silanes and vice versa. One potential drawback of this methodology is that the reactions reported were all carried out at 80 °C or above, suggesting that relatively high temperatures are necessary for efficient reaction. 

It is worth mentioning that under the conditions applied the silanediols 15d,g could not be obtained the final products of the acid hydrolysis of 9d,g were the 1-hydroxyalkylsilanols 14d,g. Similarly, 9d and 9g were reluctant to take part in a Cl/OSiMes exchange and the formation of lld,g and their resultant products. We suppose that the tert-butyl group in 9d prevents the molecule from adopting a suitable conformation for the replacement of the respective substituents, and the isomerisation of 9g fails as the result of the electronic effect of the dichlorophenyl substituent, destabilizing a necessary carbenium ion transition state. Interestingly, also in case of the related equally substituted 1-hydroxyalkyl tris(trimethylsilyl)silanes 1 (R = H, = tert-butyl R = H, R = 2,6-dichlorophenyl, respectively) no acid-induced isomerization could be performed. 

H, C and Si NMR spectral analyses carried out earlia for bis(trimethylsilyl)ethyle-nes and some types of vinylsUanes indicate an interaction between the vacant d-orbitals on the silicon atom and the r-electron system of the vinyl group. In an effort to broaden these notions and in continuation of previous studies concerned with the influence of electronic effects of silicon and vinyl substituents on the chemical shifts as well as the coupling constants, Lukevics and coworkers determined the H, C, 2 Si and 0 NMR spectra for the following chlorinated silylethylenes (31-34) and 1,2-disilylethylenes (35) ... 

The values of = —0.04 and Op = —0.07 for the trimethylsilyl substituent compare with values of —0.07 and —0.17 for the methyl substituent and values of —0.10 and —0.20 for the t-butyl substituent Hence compared with alkyl groups the MesSi group has a modest electronic effect in both the meta and para positions. The and Op constants suggest that McsSi will be a weakly activating substituent, but will not have a significant directing effect in aromatic substitutions. 

Incidentally, oxidation data of the pyrrole monomers show an interesting increase in oxidation potentials when containing heavier substituents (Table 25). However, the ionization potential of N -methylpyrrole (7.95 V) is smaller than that of pyrrole (8.21 V). The accepted linear relationship between ionization potential and oxidation potential210 would have it the other way round. Considering, however, that trimethylsilyl and trimethylgermyl groups are weak electron donors211, it is plausible that a nonelectronic effect is responsible for the observed trend and the potential shifts are associated with steric effects. 

Group IV substituents, especially the trimethylsilyl group, apparently enhance the electron affinity of aromatic systems. The effect is particularly noticeable in aniline derivatives. The strong electron-releasing effect of the amino group decreases the electron affinity of the aniline derivatives and hinders reduction to the radical anions. Nitroanilines may be reduced to radical anions (65). The only other aniline radical anions that have been reported bear silyl substituents either at nitrogen (62) or on the ring (83, 85, 86). 

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