Nitrene-carbene rearrangements

The formation of fluorene from 2-biphenylcarbene is completely analogous to the carbazole formation described above. Contrary to earlier beliefs, the facile gas-phase rearrangement of diphenylcarbene to fluorene (Scheme 51) is not a direct process (path a),208 Other carbocyclic analogs of this reaction have been reported,10,329 and it has allowed the preparation of a number of heterocyclic compounds in high yields as well as a deeper understanding of the carbene-nitrene rearrangement.210 232... 

Flash thermolysis of 3-aryl[l,2,3]triazolo[l,5-a]pyridines (277) under mild conditions (380-500°C, 10-3 torr) affords carbazoles (279, 280) in nearly quantitative yields.232 The triazoles exist as the valence tautomeric diazo compounds 278 in the gas phase.189 The substitution patterns in the products (279 and 280) demonstrate that the reactions take place exclusively by a carbene-nitrene rearrangement in which the pyridylcarbenes insert into the 2,3-bond in pyridine (Scheme 52). 

Quinolylcarbene and 1-isoquinolylcarbene undergo carbene-nitrene rearrangement to 1-naphthylnitrene and 2-naphthylnitrene, respectively.10,202 Both 2- and 4-pyrimidylcarbenes and 2-pyrazinylcarbene isomerize to 3-and 4-pyridylnitrenes, respectively, which then contract to cyanopyrroles.10... 

Diazopyrazole (368) undergoes gas-phase thermal extrusion of two molecules of nitrogen and formation of the azirine 369 the same product is formed from the azide 370. This indicates the occurrence of the carbene-nitrene rearrangement 371 -> 372 (Scheme 72)432 Carbenes like 371 can be trapped in solution.432... 

Overall, then, the Curtius rearrangement converts an acid chloride to an amine with loss of a carbon atom— very useful. Also useful is the related Hofmann rearrangement, which turns an amide into an amine with loss of a carbon atom. This time we start with a primary amide and make a nitrene by treatment with base and bromine. Notice how close this nitrene-forming reaction is to the carbene-forming reactions we talked about on p. 000. The nitrene rearranges just as in the Curtius reaction, giving an isocyanate that can be hydrolysed to the amine. 

The reactions grouped in this sedion are all characterized by skeletal rearrangement. Some are generally considered to be concerted, and thus discrete electron-defident intermediates (carbonium ion, carbene, nitrene, etc.) are bypassed. Reactions in which migration is to carbon or nitrogen are the most important these are considered separately in the sections that follow. 

Tomioka, H., Ichikawa, N., and Komatsu, K., Photochemistry of (2-nitrophenyl)diazomethane studied by a matrix isolation technique (nitrophenyl)carbene to (carboxylphenyl)nitrene rearrangement by successive reduction of nitro group with carbenic center, /. Am. Chem. Soc., 114, 8045, 1992. 

The rearrangements that we have considered to date all have one feature in common the migration of an alkyl or aryl group, with its electron pair, to a carbon atom which, whether it be a carbocation or not, is electron-deficient. Another atom that can similarly become electron-deficient is nitrogen in, for example, R2N or RN (a nitrene, cf. carbenes above), and it might be expected that alkyl or aryl migration to such centres would take place, just as it did to R3C and R2C this is indeed found to be the case. 

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