Nitro ketones, reduction

When aliphatic nitro compounds are used instead of aldehydes or ketones, no reduction occurs, and the reaction is essentially a Knoevenagel reaction, though it is usually also called a Tollens reaction ... 

Nitroalkane anions are very stable and hence excellent at conjugate addition (chapter 22). Quite weak bases such as amines are enough to give the anion of 44 and hence the nitro-ketones 45. Reduction gives the amino-ketones 46 that cyclise to give imines, reduced under the reaction conditions to pyrrolidines 47. 

Although insects contain reductases that catalyze the reduction of xenobiotics, reduction is less common than oxidation. Three types of reduction reactions, i.e., nitro reduction, azo reduction, and aldehyde or ketone reduction, are known to occur in insects. 

When aliphatic nitro compounds are used instead of aldehydes or ketones, no reduction occurs, and the reaction has been referred to as a Tollens reaction (see 16-43). However, the classical condensation of an aliphatic nitro compound with an aldehyde or ketone is usually called the Henry reaction or the Kamlet reaction, and is essentially a nitro aldol reaction. A variety of conditions have been reported, including the use of a silica catalyst, Mg—A1 hydrotalcite, a tetraalkylam-monium hydroxide,proazaphosphatranes, " or an ionic liquid.A solvent free Henry reaction was reported in which a nitroalkane and an aldehyde were reacted on KOH powder. Potassium phosphate has been used with nitromethane and aryl aldehydes. The Henry reaction has been done using ZnEt2 and 20%... 

Reduction. An early report2 stated that aldehydes and a-hydroxy ketones are reduced by this reagent but that ketones, nitro compounds, carboxylic acids, and esters are not reduced. Borch and Durst,3 however, found that some ketones are reduced, but more slowly than with sodium borohydride. One advantage over the latter reagent is that lithium cyanohydridoborate is stable in acid up to pH 3 and hence can be used for reduction of groups sensitive to high pH (e.g., thiamine). 

The ej. can perform yet other reactions such as dechlorination of chlorinated aliphatic hydrocarbons. This can be either oxidative, in which case the products are ketones, or reductive, as with carbon tetrachloride which produces the highly toxic free radical CH2 (Salmon, Jones and Mackrodt, 1981). Moreover, the ej. carry at least two reducing enzymes a nitro-reductase and an azo-reductase, both of which produce primary amines. 

Potassium and sodium borohydride show greater selectivity in action than lithium aluminium hydride thus ketones or aldehydes may be reduced to alcohols whilst the cyano, nitro, amido and carbalkoxy groups remain unaffected. Furthermore, the reagent may be used in aqueous or aqueous-alcoholic solution. One simple application of its use will be described, viz., the reduction of m-nitrobenzaldehyde to m-nitrobenzyl alcohol ... 

The imides, primaiy and secondary nitro compounds, oximes and sulphon amides of Solubility Group III are weakly acidic nitrogen compounds they cannot be titrated satisfactorily with a standard alkaU nor do they exhibit the reactions characteristic of phenols. The neutral nitrogen compounds of Solubility Group VII include tertiary nitro compounds amides (simple and substituted) derivatives of aldehydes and ketones (hydrazones, semlcarb-azones, ete.) nitriles nitroso, azo, hydrazo and other Intermediate reduction products of aromatic nitro compounds. All the above nitrogen compounds, and also the sulphonamides of Solubility Group VII, respond, with few exceptions, to the same classification reactions (reduction and hydrolysis) and hence will be considered together. 

The N,]S -dialkyl-/)-PDAs are manufactured by reductively alkylating -PDA with ketones. Alternatively, these compounds can be prepared from the ketone and -lutroaruline with catalytic hydrogenation. The /V-alkyl-/V-aryl- -PDAs are made by reductively alkylating -nitro-, -nitroso-, or /)-aminodipheny1 amine with ketones. The AijAT-dialkyl- PDAs are made by condensing various anilines with hydroquinone in the presence of an acid catalyst (see Amines-aromatic,phenylenediamines). 

Analogous to DPNH (144-146), 1,4-dihydropyridines (147) act as reducing agents. For instance, the conversion of aromatic nitro compounds to amines (148) and reduction of enones to ketones (749) has been achieved. 

The nitro group of ct-nitro ketones is readily removed either by treatment v/ith BiuSnH or reduction v/ith LiAlHl of the cotrespondmg tosylhydrazones fEq 3 58 Details of denitration are discussed in Section 7 2, and some applications of this process are shovm in Schemes 35-37... 

The Michael addition of nitro compounds to ct,fi-unsatiirated ketones or esters followed by reduction of the nitro to amino group is useful for the preparation of various heterocycles This is presented in Chapter 10 (Symhesis ofHatarocyclas). 

The reduction of y-nitroketone acetals as in Eq. 6.50 v/ith ammonium formate in the presence of Pd/C gives the correspondmg amines in good yields. However, the reduction ofy-nitro ketones are reduced to cyclic nitrones fEq. 6.51. This reduction is far superior to the classical method using Zn/T4HlCl due to improved yield and simple workup. 

The Michael dclclidon of nitro ilkdnes to a,fi-unsdnirdted ketones gives Y-nitroketones, which re convetted into pyrroles by reduction of the nitro group with and PhSSPh fEq. 10.2. ... 

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