Nucleophilic substitution at silicon
المؤلف:
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص669-670
2025-07-03
525
You may wonder why trimethylsilyl chloride does not use the SN1 mechanism familiar from the analogous carbon compound t-butyl chloride. There is, in fact, nothing wrong with the Me3Si+ cation—it can be detected in mass spectra, for example. The reason is simply that the ‘SN2’ reaction at silicon is too good for SN1 to compete.
There are some important differences between the SN2 substitutions at Si and at C. Alkyl halides are soft electrophiles but silyl halides are hard electrophiles. Alkyl halides react only very slowly with fluoride ion but silyl halides react more rapidly with fluoride than with any other nucleophile. The best nucleophiles for saturated carbon are neutral or based on elements down the periodic table (S, Se, I) or both. The best nucleophiles for silicon are charged and based on highly electronegative atoms (chiefly F, Cl, and O). A familiar example is the reaction of enolates at carbon with alkyl halides but at oxygen with silyl chlorides.
When a Me3Si group is removed from an organic molecule with hydroxide ion, the product is not the silanol as you might expect but the silyl ether ‘hexamethyldisiloxane’.
The other side of the coin is that the SN2 reaction at carbon is not much affected by partial positive charge (δ+) on the carbon atom. The ‘SN2’ reaction at silicon is affected by the charge on silicon. The most electrophilic silicon compounds are the silyl trifl ates and it is estimated that they react some 108–109 times faster with oxygen nucleophiles than do silyl chlorides. Trimethylsilyl trifl ate is, in fact, an excellent Lewis acid and can be used to form acetals or silyl enol ethers from carbonyl compounds, and to react these two together in aldol-style reactions. In all three reactions the triflate attacks an oxygen atom. In the acetal formation, silylation occurs twice at the carbonyl oxygen atom and the final leaving group is hexamethyldisiloxane. where the carbonyl group is twice protonated and the leaving group is water.
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