Reactivity of nitrenes

The various Nb and Ta nitrenes were converted to oxo derivatives by an excess of carboxylic compounds.291,296 This seems characteristic of the nucleophilic reactivity of nitrenes, but water is sometimes required (equation 25). 

Amines attached to positions 4, 5, 6 or 7 of benzo[6]thiophene behave as normal aromatic amines. The reactions of nitrenes attached to the benzene ring of benzo[6]thiophene are discussed in Section 3.14.3.5 alongside the reactivity of nitrenes attached to the thiophene ring, in order to obtain a unified picture. 

Apart from investigations of the mechanism of the Clemmensen reaction ensuing from the formation of 3, work on the reactivity of nitrenes (Section IV,A,4), as well as studies of photochemical rearrangements of acridine Af-oxides (Section IV,B), have led to 4-azaazulenes. 

The reactivity of nitrenes is similar to that of carbenes. They readily add to double bonds to give the corresponding three-membered heterocycles, aziridines. Insertion reactions are also common. 

Photolysis of azides again leads to loss of N2, but in this case what is left behind is a ni-trene (Eq. 16.75). Photolysis of azides is the best route to nitrenes. Because of the high reactivity of nitrenes, aryl azides have been popular reagents for photoaffinity labeling (see the... 

The types of reaction open to a nitrene are broadly similar to those undergone by carbenes, but the reactivity of nitrenes is considerably lower. Nitrenes are less indiscriminate than carbenes in their reactions with primary, secondary, and tertiary carbon-hydrogen bonds, for instance, and nitrenes are also somewhat electrophilic, preferring an 0-H over a C-H bond. By far the most frequently used nitrene source is an aryl azide (see Table IV). The derived aryl nitrenes are much less reactive than a-keto, a-sulfonyl, or a-phosphoryl nitrenes, but the use of these acyl nitrenes is ruled out by the high chemical reactivity of the precursor species acyl azides, sulfonyl azides, and phosphoryl azides. 

The very rich reactivity of nitrenes arising through their generation in the coordination sphere of metals such as copper and rhodium has been described. Difimctionalizations... 

Signihcant changes in the reactivity of nitrenes may also occur if the azides are substituted with strong 7i-donors or K- or o-acceptors (such as the dimethylamino or nitro substituents, fluorine atoms). 

Time-resolved spectroscopic techniques are important and effective tools for mechanistic photochemical studies. The most widely used of these tools, time-resolved UV-VIS absorption spectroscopy, has been applied to a variety of problems since its introduction by Norrish and Porter almost 60 years ago. Although a great deal of information about the reactivity of organic photochemical intermediates (e.g., excited states, radicals, carbenes, and nitrenes) in solution at ambient temperatures has been amassed with this technique, only limited structural information can be extracted from... 

Only limited success was achieved in determining the relative reactivity of primary, secondary, and tertiary carbon-hydrogen bonds to sulphonyl nitrenes 8>. Insertion of p-toluenesulphonyl nitrene into 2-methylbutane gave a mixture of products which could not be completely resolved. The ratio of (primary) (secondary + tertiary) = [38 + 39 40 + 41] was 1.53, compared to a ratio of 5.6 for carbethoxynitrene58>, indicating the lowered selectivity of the sulphonyl nitrene relative to the carbethoxynitrene, as might be expected from the possible resonance stabilization of the latter species. 

Metal-oxenoid (oxo metal) species and metal-nitrenoid (imino metal) species are isoelectronic and show similar reactivity both species can add to olefins and be inserted into C—H bonds. Naturally, the study of nitrene transfer reactions began with metalloporphyrins, which were originally used as the catalysts for oxene transfer reactions. 

Nitrenes, the nitrogen analogs of carbenes, are too reactive for isolation under ordinary conditions. The ground state of NH, and of most nitrenes, is a triplet. Aryl nitrenes have been trapped at 77 K,186 the more reactive alkyl nitrenes in matrices at 4 K.187... 

These indices have been used to study the reactivity for a series of chlorobenzenes and a good correlation is observed, for example, between W and toxicity of chlorobenzene [41]. For a detail discussion of this concept and its applications, we refer the readers to a recent review [41,42]. For studying intramolecular reactivity, these philicity indices and local softness contain the same information as obtained from the Fukui functions, because they simply scale the Fukui functions. In some cases the relative electrophilicity and relative nucleophilicity may be used although they provide similar trends as s(r) and co(r) in most cases [43]. In the same vein, the spin-donicity and spin-philicity, which refer to the philicity of open-shell systems [44], could also be utilized to unravel the reactivity of high-spin species, such as the carbenes, nitrenes, and phosphinidenes [45]. 

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. 

The great majority of matrix isolation studies of carbenes and nitrenes have employed their formal adducts with molecular nitrogen, that is, diazo compounds or diazirines in the case of carbenes, azides in the case of nitrenes, as precursors for their in situ generation. Usually, these compounds will readily release N2 on irradiation with a low-pressure mercury lamp (254 nm), and this fragment has the advantage that it will usually not react with or perturb the targeted reactive intermediate (see Scheme 17.2). 

CHi generated by photolysis of CH2N2 in the liquid phase is indiscriminate—totally non-selective—in its reactivity (p. 199). CH, generated in other ways and other carbenes are less reactive and insert in the order tertiary > secondary > primary.232 Halocarbenes insert much less readily, though a number of instances have been reported.233 Nevertheless, even for less reactive carbenes, the insertion reaction has seldom been used for synthetic purposes.234 The carbenes can be generated in any of the ways mentioned in Chapter 5 (p. 198). For the similar insertion of nitrenes, see 2-12. 

Information on the chemical reactivity of these nitrenes is limited the NR groups are tightly bound to the pentavalent metal and not readily displaced. 

The photochemistry of aryl azides is quite complex, suggesting that the nitrene 14 may not be the only reactive intermediate and that insertion reactions may not be the only route to form photoconjugates.Although aryl nitrenes are much less susceptible to rearrangements than acyl nitrenes, they may still occur and lead to the formation of reactive intermediates such as azepines, which may go on to react with nucleophiles.[911 141 Addition of nitrenes to double bonds will generate azirines, while dimerization will produce azobenzenesJ11 Aryl azides are stable to most of the procedures used in the course of peptide synthesis except for reduction reactions. Non-photochemical reduction of aryl azides to the primary amines by thiols has been reported by Staros et al.[15]... 

---