Nitrenes high-spin

Photochemical properties of azido derivatives of six-member aza-heterocycles (pyridine, pyrimidine, triazine, quinoline, acridine) are discussed. Data on the stmcture of the reaction products formed under photolysis of azides in different conditions (solvent, temperature, additives), and also data on the matrix isolation spectroscopy of heterocyclic nitrenes, including high-spin nitrenes, produced by low-temperature photolysis of the corresponding azides are shortly examined. 

Azidopyridines are the simplest and obviously the most investigated heteroaromatic azides. In earlier investigations main attention was paid to identification of various reaction products. Recently, the quantitative structure-reactivity relationship was studied based on comparison of the experimental and quantum-chemical data. Many high-spin nitrenes were obtained by photolysis of poly azidopyridines. 

Low-temperature Photolysis of Heteroaromatic Azides. High-spin Nitrenes... 

The stabilizing effect of the chlorine atoms manifested itself also in the photolysis of 2,4,6-triazido-3,5-dichloropyridine TAP (methyltetrahydrofuran, 77 K), when one managed to successively observe and characterize mono-, di-, and trinitrene (dicyano derivative failed to produce high-spin nitrenes) [89, 95, 96]. It was found that, with an increase in the multiplicity (the number of the nitrene centers), the absorption spectrum shifts bathochromically, and the /)-parameter of splitting in zero field decreases (Table 17). 

S.V. Chapyshev, R. Walton, P.M. Lahti, Effect of substitution on the yield of high-spin nitrenes in the photolysis of 2,6-diazidopyridines, Mendeleev Commun., 2000, 114-115. 

T. Nakai, K. Sato, D. Shimoi, T. Takui, K. Itoh, M. Kozaki, K. Okada, High-Spin Nitrenes with s-Triazine Skeleton, Mo/. Cryst. Liq. Cryst., 1999, 334, 157-166. 

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]. 

Aryl nitrenes have been studied extensively over the last decades and are used in several industrial processes such as microlithography. Aryl nitrenes have also been used in photoaffinity labeling bioorganic molecules. The pursuit for organic magnetic material has sparked renewed interested in nitrene intermediates, which are ideal candidates for magnetic material because of their high spin properties. ... 

If this mechanism is really operative, the "abstraction product" formed in benzene is not necessarily due to a triplet nitrene precursor. Recently a careful study of the thermolysis of methylazide in substituted benzenes demonstrated that the unsubstituted primary sulfonamide is a product of hydrogen abstraction by the nitrene 72). On the other hand there are remarkable differences in isomer ratios (o m p) of the ring-substituted anilides formed depending on the spin state of the reacting nitrene. The triplet was shown to attach the aromatic nucleus mainly in the o-position, as is expected from a highly electrophilic diradical. Dehydrogenations by carbonylnitrenes have been reported by several authors for a variety of systems. In the direct photolysis of ethylazidoformate 29 in cyclohexene, the amide 30 and the bicyclohexenyl 31 were isolated 35b Both products result from an abstraction reaction. 

The parameter Z) is a measure of the interaction between the two unpaired spins of the triplet. The exceptionally high value of D in the spectra of the nitrenes (Table 4) indicates that the two spins are... 

The second parameter is indicative of the difference in spin density along the x and y axes of the molecule. The low value of E in the alkyl nitrenes is a consequence of the cylindrical symmetry of the / -orbitals about the C—N bond. In phenyl nitrene the / -orbital is locked in the plane of the phenyl ring and the equivalence of the two /(-orbitals is removed. The spin distribution is no longer symmetrical about the z-axis and the value of E is consequently quite high. 

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