Aromatic compounds Clemmensen reduction

Like any aldehydes aromatic aldehydes undergo Clemmensen reduction [758, 778] and Wolff-Kizhner reduction [759, 774] and give the corresponding methyl compounds, generally in good yields. The same effect is accomplished by conversion of the aldehydes to p-toluenesulfonyl hydrazones followed by reduction with lithium aluminum hydride (p. 106). 

Some features of the Clemmensen reduction are discussed in Section 5.1.3, p. 476. Purely aromatic ketones generally do not give satisfactory results pinacols and resinous products often predominate. The reduction of ketonic compounds of high molecular weight and very slight solubility is facilitated by the addition of a solvent, such as ethanol, acetic acid or dioxane, which is miscible with aqueous hydrochloric acid. With some carbonyl compounds, notably keto acids, poor yields are obtained even in the presence of ethanol, etc., and the difficulty has been ascribed to the formation of insoluble polymolecular reduction products, which coat the surface of the zinc. The addition of a hydrocarbon solvent, such as toluene, is beneficial because it keeps most of the material out of contact with the zinc and the reduction occurs in the aqueous layer at such a high dilution that polymolecular reactions are largely inhibited (see Expt 6.123). 

The reagents and conditions for the Clemmensen reduction are similar to those used to reduce a nitro group to an amine. Aromatic substitution followed by reduction is a valuable process for making compounds with specific substitution patterns, such as in the following synthesis. 

Because of carbonium ion generation, aromatic aldehydes and ketones can usually be reduced more easily than the corresponding aliphatic compounds. However, a modified Clemmensen reduction is an effective method to reduce isolated aliphatic carbonyl groups directly to methylene groups, and typical examples are shown in equations (4)-(6)." ... 

Haworth phenanthrene synthesis. Acylation of aromatic compounds with aliphatic dibasic acid anhydrides to (i-aroylpropionic acids, reduction of the carbonyl group according to Clemmensen or Wolff-Kishner procedures, cyclization of the y-arylbutyric acid with 85% sulfuric acid, and conversion of the cyclic ketone to polycyclic hydroaromatic and subsequently to aromatic compounds. 

Fig. (15). The ketone (173) from the unsaturated alcohol (23) is converted to the tricyclic ketone (175) by standard organic reactions. It is made to react with acetone to obtain the compound (176). This on dehydration followed by aromatization yielded the compound (178), which on demethoxylation, oxidation and methylation produces the ketone (179) whose transformation to taxodione (180) has been accomplished by Japanese workers. The Clemmensen reduction of the ketone (179) followed by oxidation and demethoxylation produces sugiol (181).

When cyclic anhydrides are used as one component, the Friedel-Crafts acylation provides a means of adding a new ring to an aromatic compound. One illustration is shown here. Note that only the ketone is reduced in the Clemmensen reduction step. The carboxylic acid is unaffected. The same result can be achieved using the Wolff-Kishner reduction. 

Clemmensen-type reduction.1 Aromatic ketones can be reduced to the corresponding methylene compounds with ammonium formate on transfer hydrogenation in acetic acid catalyzed by 10% Pd/C. The reduction is usually complete in 10-30 minutes at 110°. Halo and nitro substituents can be reduced under these conditions, and a,p-unsaturated carbonyl groups are reduced to saturated carbonyl groups. 

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