Aromatic compounds triethylsilane

Because of Us high polarity and low nucleophilicity, a trifluoroacetic acid medium is usually used for the investigation of such carbocationic processes as solvolysis, protonation of alkenes, skeletal rearrangements, and hydride shifts [22-24] It also has been used for several synthetically useful reachons, such as electrophilic aromatic substitution [25], reductions [26, 27], and oxidations [28] Trifluoroacetic acid is a good medium for the nitration of aromatic compounds Nitration of benzene or toluene with sodium nitrate in trifluoroacetic acid is almost quantitative after 4 h at room temperature [25] Under these conditions, toluene gives the usual mixture of mononitrotoluenes in an o m p ratio of 61 6 2 6 35 8 A trifluoroacetic acid medium can be used for the reduction of acids, ketones, and alcohols with sodium borohydnde [26] or triethylsilane [27] Diary Iketones are smoothly reduced by sodium borohydnde in trifluoroacetic acid to diarylmethanes (equation 13)... 

Nitrilium salts (40 equation 10) can be prepared from nitriles and triethyloxonium fluoroborate. Early workers concluded that the nitrilium salts could be reduced to amines but that the reduction could not be stopped at the aldehyde (aldimine) stage.Later, it was demonstrated that reduction of the nitrilium salts with NaBH4 was rapid and did give the corresponding amine, but with triethylsilane yields of 60-90% of both aliphatic and aromatic aldehydes were obtained. Triethylsilane is a mild reagent which reduces carbocations but neutral compounds are generally not reduced and so overreduction is not a problem with this reagent. Formation of the nitrilium salts with triethyloxonium fluoroborate is slow but can be speeded up by the use of iron(III) chloride (Scheme 19). There were some notable exceptions to aldehyde formation with some aromatic nitriles. 

Alkoxynitroxyl radicals, isoelectronic with ketyls, are produced when aromatic nitro compounds trap other radicals [e.g., the nitroxyl 10 (R = tetrahydrofuran-2-yl)] when 2-nitrofuran is irradiated in THF a similar radical, 10 (R = EtjSi), is obtained when the solvent is triethylsilane. The latter radical exhibits separate rotamers in proportions of 7 3 at low temperatures, and at 60°C two well-resolved spectra were measured. They were not, however, assigned. ... 

Many aromatic ketones and aldehydes can be reduced to the corresponding methylene compound by reaction with triethylsilane in trifluoroacetic acid. The reaction fails for ketones, such as p-cyanoacetophenone, which have strong electron-attracting substituents. In this case, the trifluoroacetate ester of the corresponding alcohol is produced. Suggest a mechanism for these reactions. 

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