Nitrenes electrophiles

Electrophilic nitrogen compounds, such as arenesulfonyloxyamines, can convert alkenes to aziridines without the intervention of free nitrenes (80CC560). 

The [l,2,3]triazolo[2,l- ]benzotriazole (74 2,3-benzo-l,3a,4,6a-tetraazapentalene) heterocyclic ring system is obtained by an electrophilic attack of singlet nitrene, generated by heating the corresponding nitrophenyl triazole 251 in trialkyl phosphite, on the triazole nitrogen (Equation 39) <1998JOC3352>. 

Pyrazolo[l,2- ][l,2,3]triazole mesomeric betaines are generally available by an electrophilic attack of singlet nitrenes on the pyrazole nitrogen atom. When phthalazone derivative 252 is used and the nitrene is generated by reduction with triethyl phosphite, 59% yield of mesomeric betaine 253 is obtained (Equation 40) <2000T5523>. 

A 7r-bond can react with various active species, such as the electrophile oxene and its isoelec-tronic species (nitrenes and carbenes) and radicals. A 7r-bond can also react with a nucleophile, when it is conjugated with an electron-withdrawing group. In these reactions O, N, or C atom(s) are transferred from the active species to the olefins, forming two tr-bonds, such as C—O, C—N, and C—C, at the expense of the 7r-bond. If the 7r-bond is prochiral, chiral center(s) are... 

The aziridines are the nitrogen analogs of the epoxides and undergo similar electrophilic reactions. No biological data were obtained for these compounds nor were they used as precursors to any CA-4, 7, analogs. They have been included since the synthesis is noteworthy, and they could be interesting intermediates. Xu et al. stereoselectively aziridinated chalcones using the nitrene precursor (PhINTS) and a copper catalyst to form compound 141 (Scheme 36) [82],... 

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]. 

Besides the applications of the electrophilicity index mentioned in the review article [40], following recent applications and developments have been observed, including relationship between basicity and nucleophilicity [64], 3D-quantitative structure activity analysis [65], Quantitative Structure-Toxicity Relationship (QSTR) [66], redox potential [67,68], Woodward-Hoffmann rules [69], Michael-type reactions [70], Sn2 reactions [71], multiphilic descriptions [72], etc. Molecular systems include silylenes [73], heterocyclohexanones [74], pyrido-di-indoles [65], bipyridine [75], aromatic and heterocyclic sulfonamides [76], substituted nitrenes and phosphi-nidenes [77], first-row transition metal ions [67], triruthenium ring core structures [78], benzhydryl derivatives [79], multivalent superatoms [80], nitrobenzodifuroxan [70], dialkylpyridinium ions [81], dioxins [82], arsenosugars and thioarsenicals [83], dynamic properties of clusters and nanostructures [84], porphyrin compounds [85-87], and so on. 

Accordingly, many reactions can be performed on the sidewalls of the CNTs, such as halogenation, hydrogenation, radical, electrophilic and nucleophilic additions, and so on [25, 37, 39, 42-44]. Exhaustively explored examples are the nitrene cycloaddition, the 1,3-dipolar cycloaddition reaction (with azomethinylides), radical additions using diazonium salts or radical addition of aromatic/phenyl primary amines. The aryl diazonium reduction can be performed by electrochemical means by forming a phenyl radical (by the extrusion of N2) that couples to a double bond [44]. Similarly, electrochemical oxidation of aromatic or aliphatic primary amines yields an amine radical that can be added to the double bond on the carbon surface. The direct covalent attachment of functional moieties to the sidewalls strongly enhances the solubility of the nanotubes in solvents and can also be tailored for different... 

The fact that the ratio (55 45) of 1-methylcarbazole to 3-methylcarbazole was effectively the same when obtained either via the triethylphosphite (or trisdimethylaminophosphine) deoxygenation of 2-nitro-3 -methylbiphenyl or of 2-nitroso-3 -methylbiphenyl or via the thermal or photochemical decomposition of the corresponding azides is taken as strong circumstantial evidence that each process proceeds via a common intermediate-the nitrene. The electrophilic nature of such a nitrene was well illustrated by the formation of 275 and not 276 from the nitrene 277. ... 

Substituted 3,5-dihydro-4//-pyridazino[4,5-, ]indol-4-ones 50 <2001H(55)1105, 2002T10137> and 2,5-dihydro-l/7-pyridazino[4,5-7]indol-l-ones 52 <2006T121> have been synthesized from 5-(2-aminophenyl)pyridazin-3(2/0-ones 49 and 4-(2-aminophenyl)pyridazin-3(27/)-ones 51, respectively. For this purpose diazotization of the amino groups was followed by a nucleophilic substitution with sodium azide affording aryl azides. Upon heating of these compounds, the ring-closed products were obtained most probably via the formation of an electrophilic nitrene (Scheme 10). 

Formation of l-aryl-l//-azepines is rare and occurs only with those arylnitrenes made sufficiently electrophilic by an electron-withdrawing, e.g. CN, N02 or CF3, ortho or para substituent. Even so, these docile nitrenes attack only electron-enriched arenes (e.g. mesity-lene or Af,Af-dimethylaniline) and are of minor synthetic importance (B-73MI51600). More reactive are 7r-deficient heteroarylnitrenes, and moderate yields (15-40%) of 1-heteroaryl-l//-azepines, e.g. (228 R=4,6-dimethoxy-l,3,5-triazin-2-yl), may be obtained by the photodecomposition of 2-azido-4,6-dimethoxy-l,3,5-triazine in a variety of aromatic substrates (81BCJ301). Interestingly, intramolecular insertion of arylnitrenes into arenes is more common and has been used for the synthesis of fused azepines, e.g. the azepinoindoles (229) from o-azidodiphenylmethanes (81JCS(P1)1132). 

Other tetraazapentalenes have been produced by photochemical or thermal decomposition of azidophenyl-l - and -2//-benzotri-azoles.339-341 Hall, Stephanie, and Nordstrom341 found that decomposition of the azide 259 in decalin at 170° led to a mixture of 260 and 261 in the ratio 1 9. The preference for cyclization to the position para to the methyl group to give 261 is consistent with the known electrophilic character of the intermediate nitrene. 

Electrophilic nitrogen compounds, such as arenesulfonyloxyamines, can convert alkenes to aziridines without the intervention of free nitrenes (80CC560). The ylide Ph2S+-NH adds stereospecifically to E and Z conjugated alkenes, and chiral sulfimides can transfer chirality to the aziridines formed (80T73). These reactions are often named aziridinations . 

Most monoheterocycles with one cyclic double bond have been prepared by C-Z bond formation in which the Z atom acts as the nucleophile. However, for six-membered rings of this type, Diels-Alder reactions are especially important. Three-membered rings are also atypical azirines are often made by C-N bond formation from precursors in which N is electrophilic or has nitrene character, while oxirenes are but fleeting intermediates (CHEC 5.05.6.3). 

At room or higher temperature they can behave as diradicals (triplet) as well as electrophilic (singlet) species, depending on the nature of the nitrene and on the reaction conditions.18- ... 

The prototype reactions for synthesis of pyrrole, indole and carbazole by processes involving electrophilic nitrogen are outlined in Scheme 3 with a nitrene as the electrophilic form of nitrogen. This conceptual scheme must be generalized to include nitrenoids and nitrene equivalents to accomodate the range of cyclizations which can proceed by this mechanism. In practice, this mechanistic pattern is rare for pyrroles, known but only of moderate utility for indoles, and among the most efficient methods for the synthesis of carbazoles. 

The comparison of thiophene with thioethers on the one hand and with enol thioethers on the other, in regard to its behaviour towards conventional electrophiles, has been made in Section 3.02.2.3. Attack on carbon is the predominant mode of reaction (Section 3.14.2.4) reaction at sulfur is relatively rare (Section 3.14.2.5). Carbenes are known to act as electrophiles attack at both carbon and sulfur of thiophene has been reported. The carbene generated from diazomalonic ester by rhodium(II) catalysis attacks the sulfur atom of thiophene, resulting in an ylide. It has also been shown that the carbenoid species derived by thermolysis of such an ylide functions as an electrophile, attacking the a-carbon of a second molecule of thiophene (Section 3.14.2.9). Singlet nitrene is electrophilic. However, in contrast to carbenes, it invariably attacks only the carbon atom (Section 3.14.2.9). 

Thermolysis or sensitized photolysis of the azide (272) leads to the nitrene-inserted product (273). Here electrophilic attack at C-3 by singlet nitrene is prevented by the presence of a substituent. Apparently attack at sulfur is either unfavourable or reversible. 

---