Nitrenes rearrangement reactions

Reaction of metal nitrosyls with azide ion proceeds with formation of N2 and N20 (56). This can be viewed as the result of a nitrene transfer reaction in analogy with the Curtius rearrangement (62) and its organome-tallic counterpart (63). 

We will also consider a number of rearrangement reactions that probably do not involve carbene or nitrene intermediates but give overall transformations that correspond to those characteristic of a carbene or nitrene. 

Both the singlet energy and electron transfer methods suffer from competing isocyanate formation. Energy and electron transfer in the triplet state avoids this rearrangement reaction which can result in higher yields of cycloaddition products. However, in both sensitization modes, side reactions of the triplet nitrene such as H-abstraction reactions have to be taken into account. 

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. 

Although this chapter is primarily concerned with the functionalization of unactivated spi C—bonds by nitrene insertion, other relevant aspects of nitrene chemistry are included. Thus the brief section on ni> trene reactivity highlights the rearrangement reactions which often compete with C—H insertion, and the final section covers insertion into sp C—H bonds, since many of these reactions have found wide use in recent years in the synthesis of natural products. 

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... 

Eor a treatise on azides, which includes discussion of rearrangement reactions, see Scriven, E.F.V. Azides and Nitrenes, Academic Press, NY, 1984. For a review of rearrangements of alkyl and aryl azides, see Stevens, T.S. Watts, W.E. Selected Molecular Rearrangements, Van Nostrand-Reinhold, Princeton, NJ, 1973, pp. 45-52. For reviews of the formation of nitrenes from alkyl and aryl azides, see, in Lwowski, W. Nitrenes, Wiley, NY, 1970, the chapters by Lewis, E.D. Saunders, Jr., W.H. pp. 47-97, 47-78 and by Smith, P.A.S. pp. 99-162. 

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 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). 

As pointed out, the dihydrazozene would undergo a rapid benzidine rearrangement reaction to benzidine. When the nucleophile is water, which is also a good nucleophile, the product would be p-aminophenol (PAP), which is produced from the hydrolysis reaction of the nitrene cation according to the reaction. 

A related nitrene insertion reaction which quite plausibly might have been expected to lead to a benzo-3,2-thiazocine was shown in fact to result in formation of a thiazepine as the result of an unusual rearrangement upon flash pyrolysis (Scheme 14) <84JOC3ii4>. 

The reactions to be described in this chapter have in common the formal involvement of even-electron intermediates having unfilled orbitals of low energy. The most familiar of these intermediates are carbonium ions. Sections 8.4 and 8.5 contain examples of fragmentation and rearrangement reactions of carbonium ions that are of synthetic value the discussion of these reactions supplements the mechanistically oriented discussion presented in Part A, Chapter 5. Also important are the neutral divalent carbon and monovalent nitrogen species, carbenes and nitrenes, respectively. 

Structure and Reactivity of Carbenes Generation of Carbenes Addition Reactions Insertion Reactions Rearrangement Reactions Related Reactions Nitrenes and Related Intermediates Rearrangements to Electron-Deficient Nitrogen... 

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