Nitroarenes, deoxygenated

Photolyses of 3-substituted phenyl azides 53 in hot diethylamine containing pyrene, a singlet sensitizer, furnish mixtures of the 4- and 6-substituted 3ff-azepines 54 and 55. The isomers, however, were not separated and the yields were based on quantitative GC analysis of the reaction mixtures.176 Of mechanistic significance is that the ratios of the isomeric azepines obtained compare favorably with those observed in the deoxygenation of 3-nitroarenes with trivalent phosphorus compounds. 

Diethyl methylphosphonite in refluxing dialkylamine is the favored method for the formation of 37/-azepines from nitroarenes,75,176,207 although tributylphosphane and tri-piperidinophosphane are recommended for the deoxygenation of nitrobenzene in piperidine.79 Deoxygenation of nitrobenzene in diethylamine furnishes Ar,Ar-diethyl-3/f-azepin-2-amine, and a range of 5-substituted 3//-azepines 97 have been prepared in a similar manner from 4-sub-stituted nitroarenes.79,176 Curiously, the corresponding 2-substituted nitroarenes, with the exception of 2-nitrotoluene, yield only tarry products. 

As expected, similar treatment of 3-nitroarenes furnishes mixtures of 4- and 6-substituted 3H-azepines, 54 and 55, respectively.176 Comparable yields of mixed azepines were also obtained by deoxygenation of 3-nitroarenes with alkylphophorous triamides, formed in situ from hexa-methylphosphorous triamide and excess of a secondary amine.66 In a few cases the 3//-azepines were separated by fractional crystallization of their oxalate salts66 but, in general, pure isomers were not isolated and the yields cited in the table were determined by HNMR spectroscopy. 

Attempts to prepare 2-butoxy-3//-azepines by heating nitro compounds with tris(di-ethylamino)phosphane in /m-butyl alcohol failed, as diethylamine, liberated by alcoholysis of the aminophosphane, reacts in preference with the alcohol (vide supra) to give the Ar,/V-di-ethyl-3//-azepin-2-amine in good yield.176 However, the deoxygenation of nitroarenes with tributylphosphane in the presence of primary and secondary alcohols furnishes 2-alkoxy-3//-azepines 98 in practicable yields.79... 

Alkoxy-3//-azepines 98 by Deoxygenation of Nitroarenes in Alcohol Solution Typical Procedures 79 Method A 2-Methoxy-3//-azepine (98, R = Me) ... 

Deoxygenation of the nitroarene in the presence of phenol or an enol (acetyl acetone) fails to yield the 3//-azepine. On the basis of these studies it is concluded that there is a correlation between azepine formation and the p/(a of the hydroxy compound and only those hydroxy compounds of pATa >12 yield 3//-azepines. The detrimental effect of acid conditions on 3H-azepine formation has been referred to earlier.214... 

The intermediacy of a discrete nitrene species in these deoxygenations has been the subject of some debate. Supporting evidence comes from studies on the deoxygenation of a series of mainly m-nitroarenes in the presence of diethylamine, which reveal that the... 

Codeposition experiments have shown that vapors the early transition metals Ti, V, and Cr deoxygenate desulfurate organic compounds. Epoxides yield alkenes with these metal atoms. The early transition metals are generally more efficient for these types of reactions. Aminoxides and DMSO undergo deoxygenation, while nitro compounds and nitroarenes yield coupled azo and azoxy products ... 

Deoxygenation of nitroarenes. Aromatic nitro compounds when heated... 

Other reactions. The deoxygenation of bis(tributyltin)oxide to afford hexabutylditin in EtOH, 2h, 83% yield) and the conversion of Af-alkenylpyridinium salts to ketones are accomplished using NaBH as reagent. Anilines are obtained by reduction of nitroarenes u ith NaBH -fNH l SO. ... 

The reductive dimerization of nitroarenes is very dependent on the substitution in aromatic ring and electron-withdrawing groups (p-OCOMe, p-C02Me, and P-NO2) suppress the reaction (Table 6.17). Reaction was tested with several other metals (tin, manganese, copper, aluminum, antimony, and lead), and metal activity correlates with oxidation-reduction potentials of these metals in the order A1 > Mn > Sn > Pb > H > Sb > Bi > Cu (Table 6.18). The pathway for the deoxygenative dimerization of nitroarenes on the activated bismuth surface proceeds via the stepwise reduction processes from nitroarene to nitrosoarene and W-arylhydroxylamine, followed by dehydrative coupling. 

The reaction of 1-nitronaphthalene and other bicyclic nitroarenes, for instance, 5-nitroquinoline, with dimethyl phosphite in methanol in the presence of sodium methoxide proceeds via formation of the corresponding o adducts with the phosphite anion, which are converted into substituted nitrosoarenes. Subsequent N-deoxygenation results in the formation of nitrenes that react further to give benzazepines and analogues (Scheme 20) [88]. 

It is generally accepted that the mechanism of this trialkyl phosphite deoxygenation of nitroarenes involves initial transformation to the nitrosoarene and trialkyl phosphate followed by a second deoxygenation to the corresponding nitrene. Support for this pathway is seen by the fact that noatching aryl azides afforded similar product ratios under thermolysis conditions [10-12], Indeed, prior to the work of Cadogan and Sundberg, several workers described indole... 

It is interesting to point out the similarity between the intermediate formed during the deoxygenation of nitroarenes by [Rh(C0)4] (see Chapter 6) and the one that should lead to the oxime in the copper-catalysed reaction (Scheme 1) ... 

Reis PM, Royo B (2009) Chemoselective hydrogenation of nitroarenes and deoxygenation of pyridine N-oxides with H2 catalyzed by M0O2CI2. Tetrahedron Lett 50 949-952... 

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