Nitrenes, Lossen rearrangement intermediates

Compounds containing neutral, monovalent nitrogen atoms are known as nitrenes. The parent structure, NH, is also called imidogen. Because most stable compounds of neutral nitrogen have a valence of 3, it is no surprise that nitrenes typically are very short lived, reactive intermediates. A short history of nitrenes has been presented by Lwowski who points out that they were first proposed by Tiemann in 1891 as transient intermediates in the Lossen rearrangement. 

Treatment of amides with bromine in alkaline medium promotes the Hofmann rearrangement, which may or may not involve a free nitrene intermediate." The oxidation of primary amides with lead tetraacetate and the resulting Lossen rearrangement also produces isocyanates, with the possible intervention of an acylni-... 

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),... 

However, hydroxamic acids, such as 474, react with carbodiimides to produce a nitrene intermediate 475, which undergoes the Lossen rearrangement to give an isocyanate 476." " The latter reacts with the starting hydroxamic acid to give the adduct 477. 

Nitrenes are also obtained as reaction intermediates in Hofmann, Schmidt and Lossen rearrangements. 

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. 

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