Nitro compounds mechanism

Reduction of Nitro Compounds. The mechanism for catalytic hydrogenation of nitro compounds has been the subject of many iavestigations and there is much evidence that this reaction proceeds through several iatermediate species. The most widely accepted mechanism for the hydrogenation of nitro compounds was proposed by Haber ia 1898 (2) (see Fig. 1). 

Fig. 1. Mechanism for the hydrogenation of nitro compounds. FunctionaHties are labeled.

The reaction of alkyl nitro compounds with acetyl chloride in the presence of an alkenic compound produced a 2-isoxazoline. The mechanism is believed to proceed via a nitrile oxide and is illustrated in Scheme 112 (B-79MI41613). 

A mechanism of this type permits substitution of certain aromatic and ahphatic nitro compounds by a variety of nucleophiles. These reactions were discovered as the result of efforts to explain the mechanistic basis for high-yield carbon alkylation of the 2-nitropropane anion by p-nitrobenzyl chloride. p-Nitrobenzyl bromide and iodide and benzyl halides that do not contain a nitro substituent give mainly the unstable oxygen alkylation product with this ambident anion ... 

Recently, nitration of organolithiums and Gngnards v/iih has been developed for the preparation of certain kmds of nitro compounds fEqs. 2.14 and 2.15. The success of this process depends on the reaction conthdons (low remperanirei and the stnicnire of substrates. For example, 3-nitrothiophene can be obtained in 70% overall yield from 3-bromothiophene this is far superior to the older method. 3-Nitroveratrole cannot be prepared usefully by classical electrophilic nitradonof veratrole, but it can now be prepared by direct orr/io-lithiadon followed The mechanism is believed to proceed by dirutrogen rathcal, followed by the rathcal s combinadon. 

Aromatic nitro-compounds have also seen use as inhibitors in polymerization and as additives in radical reactions. The reactions of these compounds with radicals are very complex and may involve nitroso-compounds and nitroxide intermediates.20" 206 In this case, up to four moles of radicals may be consumed per mole of nitro-compound. The overall mechanism in the case of nitrobenzene has been written as shown in Scheme 5.18. The alkoxyamiuc 40 can be isolated in... 

Sodium nitrite can be used to form nitro compounds with primary or secondary alkyl bromides or iodides, though the method is of limited scope. Silver nitrite gives nitro compounds only when RX is a primary bromide or iodide. Nitrite esters are an important side product in all these cases (10-33) and become the major product (by an SnI mechanism) when secondary or tertiary halides are treated with silver nitrite. 

CHsSNa in an aprotic solvent or by BusSnH. Both reactions have free-radical mechanisms.Tertiary nitro compounds can be reduced to RH by NaHTe. The compound Bu3SnH also reduces isocyanides (RNC, prepared from RNH2 by formylation followed by 17-33) to a reaction that can also be... 

So do anhydrides and many compounds that enolize easily (e.g., malonic ester and aliphatic nitro compounds). The mechanism is usually regarded as proceeding through the enol as in 12-4. If chlorosulfuric acid (CISO2OH) is used as a catalyst, carboxylic acids can be ot-iodinated, as well as chlorinated or brominated. N-Bromosuccinimide in a mixture of sulfuric acid-trifluoroacetic acid can mono-brominate simple carboxylic acids. ... 

Mercuration of aromatic compounds can be accomplished with mercuric salts, most often Hg(OAc)2 ° to give ArHgOAc. This is ordinary electrophilic aromatic substitution and takes place by the arenium ion mechanism (p. 675). ° Aromatic compounds can also be converted to arylthallium bis(trifluoroacetates), ArTl(OOCCF3)2, by treatment with thallium(III) trifluoroacetate in trifluoroace-tic acid. ° These arylthallium compounds can be converted to phenols, aryl iodides or fluorides (12-28), aryl cyanides (12-31), aryl nitro compounds, or aryl esters (12-30). The mechanism of thallation appears to be complex, with electrophilic and electron-transfer mechanisms both taking place. 

These radical anions have been detected by ESR. This mechanism is consistent with the following result when nitrosobenzene and phenylhydroxylamine are coupled, and N labeling show that the two nitrogens and the two oxygens become equivalent. Unsymmetrical azoxy compounds can be prepared by combination of a nitroso compound with an N,N-dibromoamine. Symmetrical and unsymmetrical azo and azoxy compounds are produced when aromatic nitro compounds react with aryliminodimagnesium reagents, ArN(MgBr>2. ... 

The nitro group of aromatic nitro compounds has been removed with sodium borohydride. This reaction involves an addition-elimination mechanism. 

Nitro compounds can be formed in good yields by treatment of diazonium salts with sodium nitrite in the presence of cuprous ion. The reaction occurs only in neutral or alkaline solution. This is not usually called the Sandmeyer reaction, although, like 14-25, it was discovered by Sandmeyer. The BFJ ion is often used as the negative ion to avoid competition from the chloride ion. The mechanism is probably like that... 

With some alkenes, the initial p-halo nitroso compound is oxidized by the NOCl to a P-halo nitro compound. Many functional groups can be present without interference (e.g., COOH, COOR, CN, OR). The mechanism in most cases is probably simple electrophilic addition, and the addition is usually anti, though syn addition has been reported in some cases. Markovnikov s rule is followed, the positive NO going to the carbon that has more hydrogens. 

Primary or secondary aliphatic nitro compounds can be hydrolyzed, respectively, to aldehydes or ketones, by treatment of their conjugate bases with sulfuric acid. This is called the Nef reaction Tertiary aliphatic nitro compounds do not give the reaction because they cannot be converted to their conjugate bases. Like 16-2, this reaction involves hydrolysis of a C=N double bond. A possible mechanism is" ... 

When primary nitro compounds are treated with sulfuric acid without previous conversion to the conjugate bases, they give carboxylic acids. Hydroxamic acids are intermediates and can be isolated, so that this is also a method for preparing them. Both the Nef reaction and the hydroxamic acid process involve the aci form the difference in products arises from higher acidity, for example, a difference in sulfuric acid concentration from 2 to 15.5 M changes the product from the aldehyde to the hydroxamic acid. The mechanism of the hydroxamic acid reaction is not known with certainty, but if higher acidity is required, it may be that the protonated aci form of the nitro compound is further protonated. 

Azoxy compounds can be obtained from nitro compounds with certain reducing agents, notably sodium arsenite, sodium ethoxide, NaTeH, NaBH4—PhTeTePh, and glucose. The most probable mechanism with most reagents is that one molecule of nitro compound is reduced to a nitroso compound and another to a hydroxylamine 119-42), and these combine (12-51). The combination step is rapid compared to the reduction process. Nitroso compounds can be reduced to azoxy compounds with triethyl phosphite or triphenylphosphine or with an alkaline aqueous solution of an alcohol. ... 

The structures of the oxime complex Co(NO)(dmg)2 (84) and its diphenylglyoxime analog exhibit strongly bent Co—NO groups, which are easily oxidized to the corresponding nitro compound.353 A mechanistic study of NO transfer from Co(NO)(dmg)2 to hemoglobin established that the reaction involves NO association with the protein subsequent to dissociation of NO from the Co complex.354 This mechanism is also consistent with the observation of nitrato complexes in reactions... 

Unprotonated hydroxylamines are not reducible by electron transfer (10) as already mentioned and the electrohydrogenation of nitro compounds at Raney metal electrodes in neutral and basic aqueous alcoholic media gives the corresponding amines as shown by the numerous examples illustrated above. Therefore, in these media, hydroxylamines must be reduced to the amine by an ECH mechanism (eq. [1] followed by eqs. [18] and [19]). 

Therefore, such alkyl radical acceptors as quinones, nitroxyl radicals, and nitro compounds retard the oxidation of PP according to the following cyclic mechanism of chain termination ... 

In addition to nitric oxide, superoxide, and peroxynitrite, NO synthases are able to generate secondary free radicals because similar to cytochrome P-450 reductase, the reductase domain can transfer an electron from the heme to a xenobiotic. Thus it has been found [158,159] that neuronal NO synthase NOS I catalyzed the formation of CH3CH(OH) radical from ethanol. It was suggested that the perferryl complex of NOS I is responsible for the formation of such secondary radicals. Miller [160] also demonstrated that 1,3-dinitrobenzene mediated the formation of superoxide by nNOS. It was proposed that the enhancement of superoxide production in the presence of 1,3-dinitrobenzene converted nNOS into peroxynitrite-produced synthase and may be a mechanism of neurotoxicity of certain nitro compounds. 

In many cases, the yields of these products are high. However, the use of /V-silylated triazoles as nucleophiles or the use of cyclic nitroso acetals (475) substituted at the C-3 atom leads to a noticeable decrease in the yield of the oximes. Therefore, steric hindrance in nitroso acetals and a decrease in nucleophilicity of A-centered nucleophiles result in an increase in the contribution of side reactions. It should be emphasized that C -nucleophiles, such as anions of nitro compounds, are not involved in coupling reactions with cyclic nitroso acetals (475). However, the products, which formally correspond to the C,C-coupling mechanism, can be prepared by the nucleophilic substitution of chlorine in compound (476 d) by a Sa/2 mechanism (Scheme 3.254, product (483c), the yield was 79%). 

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