Nitrene rearrangements

Figure 6. MNDO heat of formation hypersurface for singlet vinyl nitrene rearrangements to 2H-azirine, ketene imine and the thermodynamically most favorable isomer, acetonitrile.

Reed and Lwowski (38) have done the photolysis of 1-azidonor-bornane (84) in MeOH and obtained two amines (85 and 86) in 54 and 24% yields, respectively. In the course of intramolecular nitrene rearrangements, an anti-Bredt s rule imine, 2-azabicyclo[3.2.1]oct-l-ene (87), can be regarded as a plausible intermediate from which the amine 85 is produced. On the other hand, a biradical 88 has been... 

Overall, then, the Curtius rearrangement converts an acid chloride to an amine with loss of a car- tk>n atom—very useful. Also useful is the related Hofmann rearrangement, which turns an amide ito an amine with loss of a carbon atom. This time we start with a primary amide and make a trene by treatment with base and bromine. Notice how close this nitrene-forming reaction is to the tarbene-forming reactions we talked about on p. 1072. The nitrene rearranges just as in the Curtius reaction, giving an isocyanate that can be hydrolysed to the amine. 

The photoreactions of azides can in most cases be rationalized in terms of the formation of intermediate nitrenes which then undergo rearrangement, insertion, or addition reactions. Nitrene rearrangement is presumably involved in the conversion of azidomethane (80) into methyleneimine (81) on irradiation... 

The photoreactions of azides can in most cases be rationalized in terms of the initial formation of nitrenes which then undergo rearrangement, insertion or addition reactions. Nitrene rearrangement is thought to be responsible for the conversion of the a-azido ethers (81) into the imino ethers (82), and the first silanediimine (83) to be described has been prepared in a similar fashion from the diazide (84) as shown in Scheme 4. Intramolecular nitrene addition to suitably aligned nitrogen-nitrogen double bonds has been employed in the synthesis of triaziridines. 

The D-hex-2-ulopyranosyl azides (327) lose nitrogen on photolysis and are converted to a mixture of two labile imidates. The intermediate nitrenes rearrange by either C2-C3 cleavage (major pathway) or C1-C2 cleavage (minor pathway) followed by carbon-nitrogen bond formation to yield the imidates (328) and (329) respectively, irrespective of the anomeric configuration of the azides studied. The E,E, E,Z- and Z,Z-isomers of 2,6-di(4 -azidobenzylidene)cyclohexanone do not interconvert on irradiation either in the crystalline state or when adsorbed on silica gel but decompose to yield nitrene-derived products. The main decomposition products of the azides of terephthalic and isophthalic acids are reported... 

Japanese chemists, on oxidation of l-amino-5-phenyl-l,2,3-triazolo[4,5-first example of the ring enlargement on oxidation of N-aminotriazoles. Probably, the reason for this involves the known instability of fivemembered hetarynes, which lower the activation energy for the )V-nitrene rearrangement relative to that for the fragmentation. 

AI mixed with the amides of isoxazole-5-carboxylic acids, the mixture being difficult to separate (75ACS(B)65). Such problems are typical in the preparation of many other amino-substituted heterocycles by these nitrene rearrangements (Scheme 73). 

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... 

A satisfactory reason for this selective nitrene-nitrene rearrangement is found in thermochemistry. Approximate heats of formation can be estimated as indicated in Table 14. The values are given in parentheses in [Eq. (36)]. The reason why isoquinolylnitrene 113) ring opens instead of expanding is simply that the former process is thermodjmamically less costly. The analogous ring opening in quinolyl-nitrene [Eq. (38)] is not observed. 

In 1983, Pople et al. recalculated geometries of 14 and 15, using the unrestricted SCF level of theory in conjunction with the 6-31G basis set. They used the MP4SDQ/6-31G" method for single-point energy calculations. The results of their calculations for the triplet nitrene rearrangement were very similar to those in previous paper. 

If a free singlet nitrene is an intermediate, produced upon decomposition of a vinylazide, then the activation energy of vinyl nitrene rearrangement to azirine must be very small. The situation is similar to the case of methylnitrene, produced upon photolysis of methyl azide (see Section 5.3). 

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