Nitrenes stable compounds

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 term nitrene is reminiscent of carbene. In fact, the two types of intermediates have more than a superficial resemblance. Carbenes and nitrenes have two bonds fewer than most stable compounds of carbon and nitrogen, respectively. As we will see, many of the properties of nitrenes are best appreciated on comparison with carbenes. 

In general, the reactive nitrenes can be generated by thermolytic or photolytic elimination of stable compounds from suitable precursors such as heterocycles, ylides, and azides. The most commonly used method for the generation of nitrenes is the thermolysis or photolysis of the corresponding... 

Few examples of azirine synthesis via enaminones are found in the literature, as azirines tend to undergo further reactions to give more stable compounds. One important method is the vinyl azide-olefin cycloaddition where the azides can be considered as derivatives of enaminones. The chemistry of this unique reaction has been reviewed (75AGE775). A recent example is included in Scheme 17, in which intramolecular cycloadditions of vinyl azides 53 furnish azirines 54 (87CB2003). In a similar reaction, the aziridinoindoies 56 are synthesized from 55 by insertion of the azide-derived nitrene into the olefinic bond (89H2029). 

From N-oxides of aromatic bases oxaziridines were obtained only at very low temperatures, but oxaziridines were often postulated as intermediates in the photoconversion of such N-oxides (Section 5.08.3.1.2). Isolation of the more stable photoisomers of nitrones also causes some problems due to their thermal and photochemical instability leading to acid amides, e.g. (69TL2281), or, by fragmentation, to carbonyl compounds and products of stabilization of nitrenes, e.g. from (260) (69ZN(B)477). 

Phosphinidenes (R-P) differ from other low-coordinate organophosphorus compounds, such as phosphaalkynes (R-C=P), phosphaalkenes (R2C=PR), and phosphaaromatics, in that the phosphorus atom carries only a single a-bonded substituent [7-9]. They relate to carbenes, nitrenes, and silylenes and likewise can exist as singlet and triplet species. The advances that led to stable carbenes [10, 11] and silylenes [12] stimulated an exploration of the chemistry of phosphinidenes. 

V-Aminopyrroles, easily prepared from the reaction of azoalkenes with enamines and /3-dicarbonyl compounds, have been shown to react with electron deficient alkynes to afford substituted benzenes (79TL2969). While the N-methoxycarbonylaminopyrrole (208) reacted with DM AD under rather vigorous conditions to afford (211) in only 13% yield, the N-unsubstituted aminopyrrole (209) prepared from (208) by NaCN treatment reacted with DMAD in CHC13 solution at room temperature to give (211) in 50% isolated yield. The formation of the aromatic system probably occurs by extrusion of the heteroatom bridge from (210) to afford a relatively stable nitrene (212 Scheme 45). 

Another general approach is the use of photoaffinity labels.9-11 A compound that is stable in the absence of light but that is activated by photolysis is reversibly bound to an enzyme and photolyzed. The usual reagents are diazo compounds that when photolyzed give highly reactive carbenes, or azides that give highly reactive nitrenes ... 

For sometime, phosphinidines, monovalent organophosphorus species with the general formulation (R-P), have been postulated as reactive intermediates in the thermal and photochemical decomposition of several types of stable organophosphorus compounds. While not isolated as stable molecules, these monovalent species can be inferred on the basis of the isolation of products whose structures can be derived rationally from the postulation of such species, as well as on the mass spectra of the transient species themselves. We may view phosphinidines as the phosphorus analogues of nitrenes (R-N), highly reactive monovalent nitrogen species formed as transient intermediates in the thermal decomposition of azides and in the a-elimination reaction of A-tosylates. 

The following isomers are represented 2H, 4H and 3H, 4H. A benzo-1,2-thiazete 447 has been proposed as an intermediate in the reaction of phenyl nitrene (triplet) with the thiazo derivative 446 a sulfurane structure 448 was suggested for one product of the reaction of perfluoropropyliminosulfur difluoride with per-fluoropropene. Stable benzo- and naphthyl-l,2-thiazetyl radicals (e.g., 452) are obtained by thermolysis of 5-amino compounds 449, 450, and 451. Electron-spin resonance studies established the structures. The magnitude of the spin density on nitrogen in derivatives of 452, which carry a substituent para to the nitrogen atom, depends on the nature of the substituent. Loss of sulfur from the naphthyl derivative 453 occurs at temperatures above 150°C. ... 

Compounds of type 303 were isolated by Atkinson and Kelly on cautious oxidation of 3-amino-2-ethylquinazolone-4 [Eq. (87)] (87CC1362). N-Acetoxyamino derivative 305 is stable only at temperatures below 0 C, and at -40°C it reacts with styrene to give aziridine 307, supposedly via intermediate 306. These data were interpreted as a proof of the possibility of forming aziridines without N-nitrene participation. Notice, however, that in principal there is no great difference between the nitrene mechanism and mechanisms presented in Eqs. (86) and (87), since if elimination of X from 304 and of MeCOj from 305 is a little ahead of the following reaction, then, in fact, the masked nitrene mechanism is described. 

In these reactions the 2-acyIoxaziridines do not act as nitrene precursors. The amination reactions proceed under mild conditions, at temperatures of 0° or slightly above, in the course of a few minutes or even seconds. The starting compounds are quite stable under these conditions in the absence of a nucleophile. Even hydrazines can be aminated to form very unstable triazanes. So cyclohexylhydrazine forms crystalline 60. ... 

The relatively stable trifluoromethanesulfinic acid azide,14 synthesized from trifluoromethane-sulfinyl chloride and sodium azide, begins to decompose at — 10 C to give the extremely reactive sulfur oxide nitrene 4. The intermediate 4 oligomerizes spontaneously, but in the presence of acetonitrile compound 4 forms the stable dithiatriazine 1,3-dioxide 5. cis- and ri-mw-Isomers of compound 5 can be detected by gas chromatography (ratio 8 10). 

The observation that hetarylcarbenes alwa3rs rearrange to nitrenes, when possible, suggests that nitrenes are thermodynamically more stable than carbenes (cf. Section V.2). That this is so can be seen by comparing the heats of formation of CH2 and NH (triplet ground states) with those of some "normal molecules (Table 26). Normal nitrogen containing compounds have heats of formation... 

As it is known that in the flash thermolysis of tetrazole (5.80) in the vapor, cyanamide, diazomethane, and dinitrogen are formed, Guimon et al. (1989) calculated with ab initio (3-21G ) and with MNDO methods the potential energy surface including the compounds mentioned and diazirine (5.59), carbodiimide (5.77), isodiazirine (5.78), and the nitrene 5.82. Tetrazole is calculated slightly more stable than its isomer 5.81. 

We thus investigated the photochemical behavior of the azidophosphine 9, possessing bulky diisopropylamino substituents. Unfortunately, the desired compound 10 was not stable enough to be isolated. We also realized that the chemical behavior of 10 was very different from that of a nitrene, but rather close to to that expected for a phosphonitrile. [14] Of particular interest, it did not dimerize to give the corresponding azo compound 12, as for the diazene 7, but to give the cyclic [2-1-2] head-to-tail dimer 11. [15]... 

In contrast to the N-MEM (N-alkyl) analogue (see Sect. 2.2.2), N-aryl-aza-fiilleroids, obtained from the thermal reaction of aryl azides with 50, can be photochemically isomerized to azirenofullerenes [51]. This closely resembles the difference in the photochemical behavior of C-aryl- and C-alkyl fulleroids obtained from the reaction of diazo compounds with Qo (e.g. [5,6] CgiHj is photochemically stable). After some initial studies (see for example [52-55]), the addition of azides and nitrenes to C o has been investigated and used for the preparation of a series of functional fullerene derivatives by a number of other investigators, but their work is not relevant in relation to the preparation of aza[60] fullerene. The addition of azides to fullerenes has been brie% reviewed previously [56-58]. 

Another mechanistic problem concerned the involvement of the bicyclic intermediates, 160 and 162 (Scheme 30). Compound 162 has been postulated for many years as the intermediate which is trapped by nucleophiles in the thermolysis or photolysis of aryl azides, leading to azepines (this reaction is described in detail in Section VIII,C). However, the present author pointed out that there was no compelhng evidence for 162, and that the trappable intermediate could equally well be the seven-membered ring 161. The question then arises is 163 161 a one-step reaction, or does it take place via 162 (see Scheme 30). CNDO/2 and extended HUckel calculation failed to answer this question unambiguously, but both methods showed that the nitrene N in 163 must move out of the plane of the ring en route to 161. The calculations by Shillady and Trindle favored 162 as a stable intermediate. There is no doubt of the existence of carbocyclic analogs of 162, which can be trapped in solution [e.g., Eq. (45)]. ... 

For example, 2-quinolylnitrene rearranges thermally to 1-isoquinolyl-nitrene, from which the products 166a-c arise (Scheme 33) 221,222,222a cyclic carbodiimide 165 (R = H) was isolable at - 196°C and directly observable by IR spectroscopy (2000 cm following flash vacuum pyrolysis of either tetrazolo[ 1,5-a] quinoline or tetrazolo [5,1-a] isoquinoline at 490°C. Compound 165 dimerized on warming to — 55°C. This intermediate had originally been postulated as the mesomeric form 165a (Scheme 33). The bicyclic azirines 167 and 168 (Scheme 33) can be written as intermediates or transition states en route to 165, but they are not expected to be stable intermediates under these reaction conditions, and no evidence for their formation has been obtained. 

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