Halo aromatics, reduction

Electrochemically generated nickel is very selective for the reduction of aromatic nitro compounds into anilines, in which alkenyl, alkynyl, halo, cyano, formyl, and benzyloxy groups are not affected.84 Sodium sulfide has been used for the selective reduction of aromatic nitro group in the presence of aliphatic nitro groups (Eq. 6.44).85... 

Reduction of ArN02.4 Aromatic nitro compounds are reduced by SnCl2 2H20 in ethanol or ethyl acetate at 70° to amines in 90-99% yield. Under these conditions, carbonyl, cyano, halo, and benzyl groups are not reduced. 

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. 

The reductive photodehalogenation of aryl halides has been actively investigated in recent years. Special attention has been given to (poly)halobenzenes and (poly)halo-biphenyls. The reactions are of interest in view of their mechanisms, and because of the importance of chlorinated aromatic hydrocarbons as environmental pollutants and the possibility of their photoinduced degradation. The photochemistry of aryl halides and related compounds in general14 and the photochemistry of polyhaloarenes in particular18 have been reviewed. 

In contrast to the above intramolecular reactions, Yamamoto and coworkers have demonstrated that aromatic halo-ketones having an alkyl substituent at their a-position are transformed to cyclized products at the carbonyl carbon (Eq. 21) [60]. This suggests that a new mechanism, nucleophilic addition of the arylpalladium moiety in the key intermediate to the carbonyl group (Scheme 2), can occur. Addition of an alcohol such as 1-hexanol is essential for the reaction. A possible role of it might be to facilitate the reduction of Pd(II) to Pd(0). 

Reduction of NO. This Ti(ll) reagent reduces aromatic and aliphatic nitro compounds to amines in THF/r-butyl alcohol at 0° in yields of 85-95%. Halo, cyano, and ester groups are not reduced. 

Various groups on the aromatic nucleus including halo, hydroxyl, alkoxyl, and amino groups are stable during reduction of the carbonyl group by one or more of the above procedures. The Clem-mensen reduction of keto acids is treated in method 269. 

A synthesis of nitriles from the cyanohydrins of aromatic aldehydes via the reduction of the corresponding a-halo cyanides has been proposed. As an example, benzaldehyde cyanohydrin is converted by the action of thionyl chloride to phenylchloroacetonitrile (80%). This substance is reduced with zinc in acetic acid to phenylacetonitrile (70%). ... 

Unsymmetrical secondary amines are readily prepared in good yields by the catalytic reduction of Schiff bases at moderate temperatures in high-or low-pressure equipment. Many examples have been cited. The intermediate imines are prepared from primary amines and aldehydes—very seldom from ketones—and may be used without isolation (cf. method 431). For the preparation of aliphatic amines, e.g., ethyl-w-propylamine and n-butylisoamylamine, a prereduced platinum oxide catalyst is preferred with alcohol as the solvent. Schiff bases from the condensation of aromatic aldehydes with either aromatic or aliphatic amines are more readily prepared and are reduced over a nickel catalyst. In this manner, a large number of N-alkylbenzylamines having halo, hydroxyl, or methoxyl groups on the nucleus have been made. Reductions by means of sodium and alcohol and lithium aluminum hydride have also been described,... 

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