Reaction with Nitrosoarenes

Neiman found that 2-azido-2 -nitrobiphenyl cyclizes readily to benzo[c]cinnoline-iV-oxide on photolysis or thermolysis. The intermolecular counterpart of this reaction now has been found to be equally facile in some cases. Heating a- or jS-naphthyl azide and p-nitroso-iV, -dimethylaniline at reflux in bromobenzene produces the azoxy compounds 40 and 41 in virtually quantitative yield (Np = naphthyl). 

No movement of the nitroso oxygen was found and so only one azoxy 

Azide decomposition in these reactions is always strictly first order, indicating nitrene intermediacy and ruling out a 1,3-dipolar cycloaddition mechanism. The substituent effects and the total suppression of azo compound and aniline formation suggests spin trapping of triplet nitrene by nitroso compound.  

A comparative study on the reactivity of the methylene hydrogen atoms in sulfur-nitrogen heterocycles towards an electrophilic reagent as exemplified by their reactions with aldehydes was undertaken. It was found that the reactivity decreased in the order of 249a 35 502 503. Compound 504 is as unreactive as 503. This clearly demonstrates that the 

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The formation of the Wheland intermediate from the ion-radical pair as the critical reactive intermediate is common in both nitration and nitrosation processes. However, the contrasting reactivity trend in various nitrosation reactions with NO + (as well as the observation of substantial kinetic deuterium isotope effects) is ascribed to a rate-limiting deprotonation of the reversibly formed Wheland intermediate. In the case of aromatic nitration with NO, deprotonation is fast and occurs with no kinetic (deuterium) isotope effect. However, the nitrosoarenes (unlike their nitro counterparts) are excellent electron donors as judged by their low oxidation potentials as compared to parent arene.246 As a result, nitrosoarenes are also much better Bronsted bases249 than the corresponding nitro derivatives, and this marked distinction readily accounts for the large differentiation in the deprotonation rates of their respective conjugate acids (i.e., Wheland intermediates). 

TABLE 1. Comparison of characteristic properties of N—S conjugates emerging during die reaction of nitrosoarenes with alkanethiols1... 

Azo- and azoxyarenes have been repeatedly observed during reactions of nitrosoarenes with thiols5-7,11,29 33 35 36 38. The latter family presumably emerges from the interaction of the TV-hydroxy arylamine with unreacted nitrosoarene, a reaction proceeding even in neutral solutions2,6 58. The formation of azoarenes may be due to condensation of the end-product arylamine with still unreacted nitrosoarene59. 

The reaction of nitrosoarenes with alkanethiols may provide a new and simple synthetic route to iV-aryl-S-alkylsullinarnidcs which has not been mentioned hitherto62. Nitrosoarenes are frequently accessible by simple redox reactions of the commercially available arylamines or nitroarenes2,71. High yields of the desired sulfinamide may be achieved by adjusting stoichiometry, pH and solvent polarity. With aryl thiols, however, this method may not be applicable because of the very sluggish reaction (see Table 2). Whether such a synthetic route can be extended to alkylnitroso compounds remains to be established. 

Formation of several colored products during reaction of nitrosoarenes with thiols has been repeatedly observed12,26,68. Two different orange-colored conjugates were found during HPLC separation of mixtures of 4-nitrosophenetol and GSH. The UV spectra were indicative of a quinoid structure, and further studies revealed these adducts to be a monocyclic and a bicyclic conjugate. In both cases the reactive quinoid structure gives rise to formation of secondary, stable end products. 

Another example is the reaction of indoles with nitrosoarenes in the presence of acids. The redox potentials of the reactants cannot justify an outer-sphere ET process, thus the formation of the phenylaminoxyl detected for the reaction carried out in the ESR cavity, could be more likely justified by an inner-sphere ET mechanism95. In fact the reaction of quinoline N-oxide with primary alkyl Grignards for which an outer-sphere mechanism was earlier proposed, takes place through classical nucleophilic addition96. 

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