Nitrosoarenes, deoxygenation

Nucleophilic displacement of the butoxy group in 2-butoxy-3//-azepine (1) by the use of excess secondary amine is preferred by some workers64 to the photolysis or thermolysis of aryl azides, or the deoxygenation of nitro- or nitrosoarenes in amine solution, as a preparative route to Ar,Ar-dialkyl-3//-azcpin-2-amines, e.g. 2,... 

Benzofurazans can be obtained by several methods (1) by dehydration of o-quinone dioxime (2) from o-substituted nitrosoarenes and (3) deoxygenation of benzofuroxans <1984CHEC(6)393, 1996CHEC-II(4)229>. For example, trihydroxyimino derivative 280 treated with sodium hypobromite, or when boiled in aqueous ammonia, affords the corresponding substituted tetrahydrobenzofurazan 281 (Equation 54) <2000CHE996>. 

Several mechanisms are possible for these cyclizations. Formation of the dioxide (4) from the unknown 2,2 -dinitrosobiphenyl would parallel the dimerization observed with acyclic nitrosoarenes, and all the other cycliza-tion steps shown in Scheme 1 could be simple condensations, or involve radical anions, or radicals. Where deoxygenating reagents are used, there is also the possibility of cyclization via nitrene intermediates. It has been reported that low yields of benzo[c]cinnolines result from reduction of 2,2 -dinitrobiphenyls with carbon monoxide in the presence of iron penta-carbonyF or by heating in triethyl phosphite. In polarographic reduction studies, Ross et al - claimed to have demonstrated the sequence ... 

Amino-3//-azepines are also obtained by deoxygenation of nitro- or nitrosoarenes with trivalent phosphorus compounds such as P(OR)3, CH3P(OR)2 or (C6H5)2POR in the presence of secondary amines. Again arylnitrenes 18 are intermediates [18]. 

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... 

At 120 °C and 1 atm, AH =18 Kcal mol and AS = -30 cal mol K have been calculated [112]. In nonpolar solvents such as decane the reaction does not proceed. The deoxygenation of the nitro group to nitroso is considered to be the rate determining step of the reaction, since the subsequent carbonylation of the nitroso compounds to give the isocyanate derivative is about ten times faster than the one of the nitro compound in the presence of [Rh(CO)2Cl]2 as catalyst [114, 115], a reaction which is accelerated by M0CI5. The mechanism of this reactions is discussed in more detail in Chapter 6 we mention here that side-on rhodium nitrosoarene complexes were proposed as intermediates. However, which really are the coordination modes of the nitrosoarene to rhodium(I) in this system is still a question to be solved. As a matter of fact, the known arylnitroso complexes of mononuclear rhodium(I) complexes have a nitrogen-o-bonded nitroso group [116], and some of them can catalyse the carbonylation of nitrosobenzene to PhNCO[117]. 

Complex (J) was crystallographically characterised in the case of Ar = 3,4-dichlorophenyl and shown to have the structure schematically reported in the scheme. Note that the CO2 molecule formed in the first deoxygenation of the nitro compound has not left the coordination sphere of the metal, but has remained included into a metallacyclic structure. Tliis is the earliest intermediate ever isolated in deoxygenation reactions of nitro compounds, though complexes having the same metallacyclic structure had been earlier obtained by interaction of a nitrosoarene complex with CO2 [106-108, 195]. 

---