Nitrenes 2-pyridyl

As shown in Table 3, triplet lb is computed to be 25-26 kcal/mol lower in enthalpy than triplet lc.77 Table 3 also shows that radicals 8b and 8c, formed by adding a hydrogen atom to lb and lc, respectively, differ in enthalpy by only 1-3 kcal/mol. Therefore, the large enthalpy difference between 3lb and 3lc is not due to a difference between the abilities of the phenyl and pyridyl groups to stabilize an unpaired tt electron. Instead it must reflect an intrinsic enthalpy difference between arylnitrenes and arylcarbenes. Table 3 also shows that aniline (9b) and fl-picoline (9c) are also predicted to have very similar enthalpies, thus providing further evidence that the large enthalpy difference between lb and lc is, indeed, due to the fact that lb is a nitrene, while lc is a carbene. 

In contrast, the much lower enthalpy computed for 3 lb, compared to 3lc, means that the N-H BDE of the anilinyl radical 8b is much lower than the C-H BDE of the 3-pyridylmethyl radical 8c. The results in Table 5 show that this is indeed the case, not only for R=Ph and R =3-pyridyl, but also for R=R =Ph and R=R —H.77 The data in Table 5 indicate that, not just for lb and lc but in general, triplet nitrenes are ca. 20 kcal/mol more thermodynamically stable than comparably substituted triplet carbenes. 

Chapman et al. [79] analyzed an argon matrix sample known by IR spectroscopy to contain the dehydroazepine, by EPR spectroscopy. They detected not only the known spectrum of triplet phenyl nitrene, but also the EPR spectrum of 2-pyridyl carbene. 

Extended photolysis of the carbene produced triplet phenyl nitrene. It was clear from EPR studies that 2-pyridyl carbene and triplet phenyl nitrene could be photochemically interconverted in argon at 10 K, presumably via the dehydroazepine. 

The products from the thermolysis of anthranils bearing a heteroaryl residue at position 3 are less predictable. 3-(2-Pyridyl)anthranils in boiling trichlorobenzene yield pyrido[l,2-h]cinnolin-6-iums (e.g., 139) (cyclization of the initially formed nitrene taking place only at the electron-rich nitrogen center) and no rearranged products.191 In contrast, the 3-(2-imidazolyl)-anthranil 108 (X = N, Y = CH) yields a mixture of the nitrene-induced rearrangement product (157) (63%) and the nitrene insertion product (158) (3%).131 The 3-(5-pyrazolyl)anthranil 108 (X = CH, Y = N) behaves similarly. 

Chemical activation in azide and nitrene chemistry Methyl azide, phenyl azide, naphthyl azides, pyridyl azides, benzotriazoles, and triazo-lopyridines 137yC825. 

The signals of two possible isomers, 2-nitreno and 4-nitreno derivatives, should not coincide in the ESR spectra. To discriminate between these isomers, the observed ESR spectra were compared with the spectra of 2-pyridyl nitrenes obtained from 2-(mono)azidopyridines additionally, the experimental D-parameters were correlated with the C-N bond lengths in nitrenes calculated by PM3 method. The authors [39] came to conclusion that 2-nitreno derivatives matched better to the spectral line positions observed. 

This effeet was aeeounted for by the fact that the chlorine atoms on the ortho-position increased the stability of the pyridine ring to photoisomerization, as was shown by the example of the triplet perchloro-substituted pyridyl-4-nitrene [90]. 

Radical Reactions.—Thiazolyl, pyridyl, and other heteroaryl radicals formed by aprotic diazotization of the corresponding heterocyclic amines substitute homolytically on thiophen with the formation of 2-heteroaryl-thiophens as the main products in 20—50% yield. The results of competitive experiments indicate that the reactivity of thiophen in this reaction at 70—80 °C is slightly higher than that of benzene. The currently accepted mechanism of the decomposition of benzoyl peroxide in thiophen has been criticized on the basis of new experimental results. No free thienyl radicals are involved in the reaction, as demonstrated by scavenging experiments, and the bithienyls formed are probably derived from dimerization of a benzoyloxythiophen radical a-complex, with subsequent loss of benzoic acid. Nitrene insertion into the thiophen ring has been observed in the thermal decomposition of 2-(2-azidobenzyl)thiophen and similar compounds, leading to thieno[3,2-6]quinoline derivatives. ... 

A number of trifluoromethyl-substituted 2-pyridyl azides, e.g., 97, have been matrix-isolated and photolyzed in Ar at 12-18 Like other 2-pyridyl azides, 97 exists in equilibrium with the tetrazole valence tautomer 98, but sublimation for the purpose of matrix isolation usually causes significant ring opening to the azido forms. In any case, the tetrazole forms as weU as the azides eliminate Nj when photolyzed. All the 2-pyridyl azides or tetrazoles investigated underwent conversion into the corresponding l,3-diazacyclohepta-l,2,4,6-tetraenes, e.g., 99 from 97, and these ring expanded products, which are cycHc carbodiimides, had v(N=C=N)35 bands at about 2000 cm" in their IR spectra. In the case of 97, the triplet nitrene was also observed by both IR and EPR spectroscopy, and it was seen to give 99 on further photolysis. 

Enantioselective imidation of alkyl aryl sulfides with A -alkoxycarbonyl azides as a nitrene precursor is effected by using (OC)Ru(salen) complex as catalyst. The steric and electronic nature of the Af-alkoxycarbonyl group strongly affect the enantioselectivity and the reaction rate. In a systematic and well-executed study of ligand effects on Lewis-acid-catalyzed Diels-Alder reaction, it has been shown that the attachment of aromatic a-amino acid ligands to copper(II) ions leads to an increase in the overall rate of the Diels-Alder reaction between 3-phenyl-l-(2-pyridyl)-2-propene-l-one (Din) and cyclopentadiene... 

---