Nitroso compounds carbazoles

Tertiary and aromatic nitroso compounds are not readily accessible consequently not many reductions have been tried. Nitrosobenzene was converted to azobenzene by lithium aluminum hydride (yield 69%) [592], and o-nitrosobiphenyl to carbazole, probably via a hydroxylamino intermediate, by treatment with triphenylphosphine or triethyl phosphite (yields 69% and 76%, respectively) [298]. Nitrosothymol was transformed to amino-thymol with ammonium sulfide (yield 73-80%) [245], and a-nitroso-/J-naphthol to a-amino-/J-naphthol with sodium hydrosulfite (yield 66-74%) [255]. 

Similarly, aromatic nitroso compounds are deoxygenated. The nitrenes are intercepted to yield azoxyarenes (73). The internal trapping of the nitrene or the dipolar precursor (74) leads to heterocyclic products. Thus, 2-nitroso-biphenyl affords carbazole (75) in good yield. Spirodienyl intermediates are in-... 

In 1962, Prof. John I. G. Cadogan at University of St. Andrews observed that aromatic C-nitroso compounds were readily deoxygenated by triethyl phosphite. Carbazoles were prepared from 2-nitrosdb ary s in this fashion albeit in low yields. Later on that year, he published an article titled Reduction of Nitro Compounds by Triethyl Phosphite A New Cyclization Reaction in the journal Proceedings of the Chemical Society In the paper, Cadogan described that 2- /7robiaryIs were also readily deoxygenated by... 

It has been considered that a coordinated nitrene may give the same reaction and work in our group has shown that this is indeed possible in at least one case [260] (see also Chapter 5). However, it should be also noted that we now know that most reactions of nitro- and nitroso compounds with a metal complex occur through an intermediate electron transfer to the organic compound vide supra). Thus the formation of the radical anion of o-nitro- and/or o-nitrosobiphenyl should be considered as probable during the reaction. The reactivity of such radicals is virtually unknown and it cannot be excluded that these species, and not a nitrene intermediate, are responsible for the carbazole formation. The validity of carbazole formation as an indication for nitrene intermediates has been questioned very early [261]. It was shown that other reactions (including oxidation of 2-amino-biphenyl) can afford carbazole. Thus the results of this test should be taken with caution. 

Indazoles. A soln. of anthranilic acid in acetone added dropwise to a refluxing mixture of benzoyldiazomethane and isoamyl nitrite in methylene chloride 3-benzoylindazole. Y 88%. F. e. s. W. Ried and M. Schon, Ang. Gh. 76, 98 (1964) carbazoles from nitroso compounds cf. G. W. Steinhoff and M. G. Henry,... 

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