Nitrene chemistry

Nitrenes can be generated from many precursors such as azides, isocyanates, ylides, heterocycles, and nitro compounds.236,237 Amongst these, azides are the most convenient precursors since they are easily prepared and can be decomposed by heat, light or a suitable catalyst. Despite considerable endeavors, no one has yet provided a synthetically viable method to use azides as sources of nitrenes.237 The breakthrough of nitrene chemistry was the recognization of the value of A-arenesulfonyl iminoiodinanes (ArS02N=IPh) as nitrene precursors by Breslow and Mansuy. - They reported inter- and intramolecular C-H insertions by tosylimino phenyl-iodinane (TsN=IPh) in the presence of Mn(m) or Fe(m) porphyrins or [Rh2(OAc)4]. Subsequently, Muller... 

Snell, K.E., H. Ismaili, and M.S. Workentin, Photoactivated nitrene chemistry to prepare gold nanoparticle hybrids with carbonaceous materials. ChemPhysChem, 2012.13(13) p. 3185-3193. 

If metal carbene chemistry can be said to be mature, metal nitrene chemistry is in its infancy. Although the first report of a catalytic process used benzenesulfonyl azide,high temperatures were required, and no one has yet provided a synthetically viable method to use azides as sources of nitrenes. Instead, iminophenyliod-inanes (44), formed from the corresponding sulfonamide by oxidation with... 

Thus far, enantioselective intramolecular aziridination via metal nitrene intermediates has not been successful. Bromamine-T has recently been shown to be a viable source of nitrene for addition to alkenes in copper halide catalyzed reactions, " and so has iodosylbenzene (Phl=0) that forms 44 in situ. Conceptually, aziridination does not necessarily fall between cyclopropanation and epoxidation, as some have suggested. Instead, metal nitrene chemistry has unique problems and potential advantages associated with the electron pair at nitrogen that are yet to be fully overcome. 

Developing analogues to carbene metathesis with nitrene chemistry is just now being explored, but few examples offer significant promise. Transfer of a nitrene to... 

In contrast to considerations of 50 years ago, today carbene and nitrene chemistries are integral to synthetic design and applications. Always a unique methodology for the synthesis of cyclopropane and cyclopropene compounds, applications of carbene chemistry have been extended with notable success to insertion reactions, aromatic cycloaddition and substitution, and ylide generation and reactions. And metathesis is in the lexicon of everyone planning the synthesis of an organic compound. Intramolecular reactions now extend to ring sizes well beyond 20, and insertion reactions can be effectively and selectively implemented even for intermolecular processes. 

So what is left to be accomplished During the current decade one can expect further asymmetric applications and catalyst designs for metathesis reactions, a maturing of chiral catalyst development for cyclopropanation and insertion with increasing synthetic applications, and decreased reliance on traditional Fischer carbenes in synthesis. Major changes remain for ylide applications, especially those that can be enantioselective, in catalytic carbene chemistry, and advances in nitrene chemistry that are comparable to those achieved over the years in carbene chemistry are in their infancy. 

Nitrenium ions (or imidonium ions in the contemporaneous nomenclature) were described in a 1964 review of nitrene chemistry by Abramovitch and Davis. A later review by Lansbury in 1970 focused primarily on vinylidine nitrenium ions. Gassmann s ° 1970 review was particularly influential in that it described the application of detailed mechanistic methods to the question of the formation of nitrenium ions as discrete intermediates. McClelland" reviewed kinetic and lifetime properties of nitrenium ions, with a particular emphasis on those studied by laser flash photolysis (LFP). The role of singlet and triplet states in the reactions of nitrenium ions was reviewed in 1999. Photochemical routes to nitrenium ions were discussed in a 2000 review. Finally, a noteworthy review of arylnitrenium ion chemistry by Novak and Rajagopal " has recently appeared. 

Nitrene chemistry is discussed in Nitrenes, W. Lwowski, Ed., Wiley-Interscience, New York, 1970. 

Despite their limited reactivity aryl nitrenes have proved exceedingly useful as photogenerated reagents and this is documented in later chapters. Reviews on nitrene chemistry include those of Lwowski (1970), Moss and Jones (1978), Wentrup (1979), Iddon et al. (1979), Colman et al. (1981), and Scriven (1983). 

Following their investigations on nitrene, carbene, and oxo transfer reactions catalyzed by fluorinated silver tris(pyrazoyl)borate (see Chapter 6 on nitrene chemistry), Lovely et al. looked for a combined carbene transfer and [2,3]-sigmatropic rearrangement. On the basis of these mechanistic considerations, these authors showed that diazoacetates, indeed, reacted with allyl halides in the presence of this silver catalyst to give a-halo-y-unsaturated esters (Scheme 3.51).77... 

These reactive intermediates were proposed first for the Lossen rearrangements in 1891 by Tiemann b and for the photochemical decomposition of hydrazoic acid in 1928 by Beckman and Dickinson 2>. The chemistry in this field, which attracted little interest for a long time, was stimulated by the development of carbene chemistry. In the last fifteen years, beginning with a summary by Kirmse 3> in 1959, excellent reviews on nitrene chemistry appeared 4-12),... 

In contrast to the carbene chemistry, where addition to a double bond is a very common process, in nitrene chemistry the [1 + 2]-cycloaddition obviously occurs with carbethoxy- and cyanonitrene in an easy way only. There are three main reasons why [1 + 2]-cycloaddition of nitrenes to carbon-carbon double bonds is relatively rare ... 

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. 

Alkoxycarbonyl azides, which do not readily undergo the Curtius rearrangement, have a much richer triazoline and nitrene chemistry, the scope and theory of which are frilly discussed by Lwowski. Carb-ethoxynitrene may be generated by a-eliminations, c tiorudly assisted by ultrasound or PTC (Scheme 23). - ... 

For a more comprehensive treatment of nitrene chemistry in general, the reader is referred to Chapter 5 of this book, and to reviews that have appeared recently... 

Demonstrating the practical nature of nitrene chemistry in the context of heterocyclic synthesis... 

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