Nitrenes and Related Intermediates

The nitrogen analogs of carbenes are called nitrenes. As with carbenes, both singlet and triplet electronic states are possible. The triplet state is usually the ground state for simple structures, but either species can be involved in reactions. The most common 

V Scriven, ed. Azides and Nitrenes Reactivity and Utility, Academic Press, Orlando, Florida, 1984. 

CHAPTER 10 REACTIONS INVOLVING CARBOCATIONS, CARBENES, AND RADICALS AS REACTIVE INTERMEDIATES 

The types of azides that have been used for generation of nitrenes include alkyl/ aryl/ acyl/ and sulfonyl derivatives. 

The characteristic reactions of an alkyl nitrene is migration of one of the substituents to nitrogen, giving an imine  

The nitrogen analogs of carbenes are called nitrenes. As with carbenes, both singlet and triplet electronic states are possible. 

Reactions Involving Carbocations, Carbenes, and Radicals as Reactive Intermediates 

Intramolecular insertion and addition reactions are almost unknown for alkylnitrenes. In fact, it is not clear that the nitrenes are formed as discrete species. The migration may be concerted with elimination, as in the case of the Wolff rearrangement. 

Arylnitrenes also generally rearrange rather than undergo addition or insertion 

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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... 

Fusion of a benzene ring to another aromatic ring or rings changes the electronic structure of the modified nitrenes and related intermediates sufficiently enough to alter their chemistry, kinetics and thermodynamics. Thus photolysis of polynuclear aromatic azides in the presence of primary and secondary amines leads not to 3H-azepines, but to corresponding... 

Carbenes, Nitrenes, and Related Electron-Deficient Intermediates... 

The Cadogan reaction refers to the deoxygenation of o-nitrostyrenes 1 or o-nitrostilbenes with trialkyl phosphite or trialkylphosphine and subsequent cyclization of the resulting intermediate nitrene 2 to form indoles 3. The reductive cyclization protocol has also been exploited to prepare a variety of A -containing heterocyclic compounds including carbazoles, indazoles, benzimidazole, benzotriazoles, anthranils, phenazines, phenothiazines, quinolines, and related compounds. 

Nitrenes were first proposed as reactive intermediates in the Lossen rearrangement by Tiemann in 1891, and subsequently by Stieglitz (1896) to account for the mechanisms of the related Hofmann, Curtius, and Beckmann rearrangements. Apart from an extensive amount of work by Curtius on the thermal decomposition of aryl and sulphonyl azides, interest in nitrenes declined until the reemergence of carbene chemistry in the 1950s. Reviews on nitrene chemistry by Kirmse (1959), Horner (1963), and Abramovitch (1964) reflected this renewed interest in nitrenes and provided a stimulus for the immense research effort undertaken in the 1960s, which has been surveyed in many reviews and two books.Of particular note are two chapters by Abramovitch" describing all facets of nitrenes, and an excellent chapter devoted specifically to aryl- and hetarylnitrenes by P. A. S. Smith. The present chapter is intended to update these reviews and it is hoped that it will stimulate further work, particularly on the synthetic aspects of intermolecular arylnitrene reactions. 

More recently, the effect of substituents in the arenesulfonyl moiety on Cu(I)-catalyzed aziridinations of cyclohexene with a series of [(arenesulfonyl-imino)iodo]benzenes was evaluated (Scheme 65) [177]. Iminoiodanes possessing p-OMe,p-CF3, andp-N02 substituents gave higher yields of aziridines than the tosylimino analog. Product yields in these reactions are not simply related to relative rates of aziridination (p-MeO >p-Me >p-N02), and appear to reflect partitioning of the copper(III)-nitrene intermediates between aziridination of the C,C-double bond and reduction to the corresponding sulfonamides. 

Hie Hofmann, Losseir and Curtins rearrangements (Scheme 6.38) are also thought to occur via an acyl nitrene-type intermediate and involve the migration of an R group to form an isocyanate. In the related Schmidt... 

A related transformation to the previous carbene transfer reaction involves a nitrene ligand bonded to the metal center, in a metallonitrene intermediate in situ generated upon the appropriate selection of the catalyst and the nitrene precursor. As shown in Scheme 17, some transition metal complexes react with such a precursor to generate an unsaturated intermediate, generally electrophilic in nature, which might react with olefins or C—H bonds affording aziridines or amines in a catalytic manner. The most employed nitrene sources are hypervalent I(III) compounds such as PhI=NTs, chloramine-T or organic azides. 

When discussing the interconversion of reactive intermediates it is of interest to have some idea of their relative stabilities. For this purpose, we are making extensive use of thermochemical estimates. These estimates will not be precise of course, for only two of the most crucial heats of formation are known with reasonable accuracy those of CH2 and NH. Nevertheless, when comparing closely related carbenes and nitrenes, the errors committed in the estimates will be nearly identical for the different species, and the differences in heats of formation — that is, the relative stabilities—will be at least qualitatively correct. Entropy differences are usually disregarded in this context, since they would contribute only ca. 2—3 kcal/mol to the free energies, and thus are within the uncertainties in the heat of formation estimates. 

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