Aromatics chemical reduction

Electrolytic reductions generally caimot compete economically with chemical reductions of nitro compounds to amines, but they have been appHed in some specific reactions, such as the preparation of aminophenols (qv) from aromatic nitro compounds. For example, in the presence of sulfuric acid, cathodic reduction of aromatic nitro compounds with a free para-position leads to -aminophenol [123-30-8] hy rearrangement of the intermediate N-phenyl-hydroxylamine [100-65-2] (61). 

Dehydration converting the imidazolone ring to imidazolinone seems to be sensitive to the aromatic nature of residue 66 [61, 62]. This step is thought to lead to the formation of an enolate intermediate, which can be trapped by reverse anaerobic chemical reduction of the mature chromophore using dithionite and other reducing agents [63]. 

On electrochemical or chemical reduction, aromatic phosphaallene derivatives yield anion-radicals. These species have two equivalent phosphorus nuclei. The unpaired electron oscillates between the two phosphorus atoms (Sidorenkova et al. 1998, Alberti et al. 1999) ArP=C( )-P( ) Ar o ArP( )-C(-)=PAr. 

Chemical reduction of aromatic aldehydes to alcohols was accomplished with lithium aluminum hydride [5i], alane [770], lithium borohydride [750], sodium borohydride [757], sodium trimethoxyborohydride [99], tetrabutylam-monium borohydride [777], tetrabutylammonium cyanoborohydride [757], B-3-pinanyl-9-borabicyclo[3.3.1]nonane [709], tributylstannane [756], diphenylstan-nane [114], sodium dithionite [262], isopropyl alcohol [755], formaldehyde (crossed Cannizzaro reaction) [i7i] and others. 

Reduction of aromatic ketones to hydrocarbons occurs very easily as the carbonyl group is directly attached to an aromatic ring. In these cases reduction produces benzylic-type alcohols which are readily hydrogenolyzed to hydrocarbons. This happens during catalytic hydrogenations as well as in chemical reductions. 

For ESR studies, cation radicals of aromatic molecules have most generally been formed by dissolution of the parent compound in concentrated sulfuric acid.19 Neither this nor any of the several new chemical methods of generating these species in solution26-35 provides a particularly suitable medium for subsequent energetic chemical reduction. 

Since proton addition is known (3, 4) to be a rate-determining step in the chemical reduction of aromatic and olefinic double bonds in the alkali-amine system, the effect of adding a proton donor (ferf-butyl alcohol) on current efficiency in the electrochemical reduction of 1-decene was investigated. 

The catalytic reduction of carboxylic acid chlorides by the Rosenmund procedure may be used for the preparation of aliphatic aldehydes but its application is mainly for the synthesis of aromatic aldehydes (e.g. Expt 6.120). Alternative procedures for the chemical reduction of acid chlorides include reduction with... 

A convenient and efficient synthetic route to a new class of macrocyclic aryl ether ether sulfide oligomers was reported. The process is shown in Fig. 28. This new class of cyclic oligomers is prepared in excellent yield by quantitative chemical reduction of macrocyclic aryl ether ether sulfoxide oligomers with oxalyl chloride and tetrabutylammonium iodide. The cyclic sulfoxide oligomeric precursors are prepared in high yields by an aromatic nucleophilic substitution reaction from bis(4-fluorophenyl) sulfoxide with potassium salts of bisphenols under high-dilution conditions [99]. 

Aromatic sym-disubstituted hydrazines are obtained by reduction of azo compounds, which in turn are intermediates in properly controlled reductions of nitro compounds. The over-all reduction can be accomplished with zinc dust and alkali or electrolytically. For example, hydrazobenzene, the simplest member, is made by both procedures. Chemical reduction is carried out on o-nitrobromobenzene to form 2,2 -dibromohydrazobenzene (57%), the halo groups remaining intact. Many examples of the electrolytic procedure have been cited the yields vary from 50% to 95%. To a limited extent, a magnesium-magnesium iodide system has been employed as a reducting agent for the azobenzenes. ... 

Reduction of heterocyclic compounds parallels reduction of aromatic compounds with the added factor that fission of the ring may occur. In most instances, catalytic hydrogenation is preferred to chemical reduction, purer products and more consistent results being obtained. 

Since aliphatic aldehydes and ketones are not hydrogenated over palladium, this reaction provides a means of selectively removing an aromatic carbonyl group in the presence of an aliphatic aldehyde or ketone (Eqn. 18.10).32 The palladium catalyzed hydrogenolysis of aryl aldehydes and ketones is preferable to any of the chemical reduction procedures such as the Wolff-Kishner or Clemmenson reactions for the removal of an aryl carbonyl group. 

Research activities on heterogeneously-catalyzed stereoselective reductions have generally increased during the past years[l]. Among various functionalities the stereoselective aromatic ring reduction is of importance in the synthesis of fine chemicals such as pharmaceuticals, herbicides, fragrances and liquid crystals. Particularly, substituted phenol derivatives offer the possibility to control the product selectivity via reduction of the prochiral intermediate cyclohexanone [2,3]. 

Aromatic hydroxylamines are generally prepared by chemical reduction or selective hydrogenation of aromatic nitrocompounds by using metal catalysts promoted with dimethylsulfoxide. However, such methods of synthesis are characterized by difficult products purification and low yields /1,2/. The low cost production of arylhydroxylamines can be of great practical interest because these compounds can undergo rearrangement to yield a variety of important chemicals 111. 

The aerobic biodegradation of N-heterocylic aromatic compounds frequently involves a reductive step (Section 6.3.1.3), but purely chemical reduction may take place under highly anaerobic conditions and has, for example, been encountered with the substituted l,2,4-triazolo[l,5fl]pyrimidine Flu-metsulam (Wolt et al. 1992) (Figure 4.23). 

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