Ionic carbonyl compound reduction

In aprotic solvents, the carbanions, generated by reduction of carbon tetrachloride or ethyl trichloroacetate at mercury, can be trapped by reaction with an added carbonyl compound [74], This reaction has been developed as a useful step in synthesis. Cathodic reduction of a system containing a catalytic amount of carbon tetrachloride, excess chloroform and an aldehyde leads to an effective ionic chain reaction sustained by trichlormethyl carbanions as indicated in Scheme 4.4. A carbon-felt cathode is used with diraethylformamide as solvent [75]. Aldehydes react with cuiTent efficiency of 700 %, which indicates a short chain reaction. Ketones... 

Aromatic aldehydes and aromatic ketones also can be reduced to hydrocarbons in a completely different manner, namely via the so-called ionic hydrogenation followed by an ionic hydrogenolysis. This kind of reduction is possible only if it can proceed via resonance-stabilized cationic intermediates. This resonance stabilization is readily achieved in a benzylic position, and it is therefore advantageous to employ aromatic carbonyl compounds in this kind of reduction. The carboxonium ion A, formed... 

Although hydrosilanes reduce ketones, in trifluoroacetic acid, to the corresponding methylene compounds or dimeric ethers via ionic hydrogenation, the reduction of a-amino and a-oxy ketones and p-keto acid derivatives with hydrosilanes, particularly PhMe2SiH, under these conditions proceeded with high anti selectivity to the alcohols. No racemization was observed at the carbon a to the carbonyl group. Intramolecular hydrosilylation catalyzed by Lewis acids provided a highly stereoselective route to anti-1,3-diols from p-hydroxy ketones (Section 1.1.3. ). ... 

Simple electrochemical reductions of carbonyl compounds and organic halides have been achieved in ionic liquids. For example, the electrochemical reduction of benzoylformic acid in [emim][Br] (emim = l-ethyl-3-methylimidazolium) gave man-delic acid in 91% yield (Equation 12.2) [7]. 

Ionic routes to allylic (2) and benzylic (4) alcohols use reduction of carbonyl compounds (1) and (3) since these are readily available by condensation or Friedel-Crafts reaction. 

As a further example, a,j8-unsaturated carbonyl compounds possess two sites which can undergo reduction the carbonyl group and the adjacent C—C-double bond. While an abundant hterature is available on the selective reduction of the carbonyl group, often achieved with relative ease, the selective hydrogenation of the double bond suffers from the lack of simple methods using practical chemicals. Selective ionic hydrogenation of ,j8-unsaturated carbonyl compounds with cyclohexane was previously known to proceed only in superacidic conditions due to the necessity of dicationic, superelec-trophilic activation of the enones. H-form zeolites with acidity well below superacidity are however also able to induce the reduction of o j8-unsaturated carbonyl compounds with cyclohexane, in analogy, or sometimes even better than the parent , superacid mediated reactions. ... 

Cathodic reduction of carbonyl compounds like benzaldehyde and acetophenone was investigated in ionic liquids, and notably their dimerization proceeded predominantly [11, 12], For acetophenone, the corresponding pinacol was formed as a diastereomeric mixture, and the diastereoselectivity is greatly affected by iraiic liquids used as shown in Scheme 1 [11],... 

Titanium. Catalyses of hydrogenation of alkenes, alkynes, carbonyl-, and nitro-compounds have been described. The effect of the nature of the ligand L and of the alkene to be reduced on reactivity in catalytic hydrogenation by Ti(7r-C5H5)2L2 has been quantitatively studied. The dependence of rate constants on solvent for reduction of decene in the presence of Ti(7r-C5H5)Me+ is interpreted in terms of electrostatic interaction between the active ionic species and the solvent. There is also a thermochemical report relevant here, and that is of a determination of the heats of mixing of cyclohexene and of hex-l-ene with titanium tetrachloride. The heats of mixing are close to zero, which implies very small heats of complex formation between these alkenes and titanium. ... 

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