Nitrenes reactions

Transition metal complex-catalyzed carbon-nitrogen bond formations have been developed as fundamentally important reactions. This chapter highlights the allylic amination and its asymmetric version as well as all other possible aminations such as crosscoupling reactions, oxidative addition-/3-elimination, and hydroamination, except for nitrene reactions. This chapter has been organized according to the different types of reactions and references to literature from 1993 to 2004 have been used. 

Work originally described by Kwart and Khan is often cited as the first example of a nitrene reaction performed under the influence of a transition metal ion [13, 14]. In these experiments, benzenesulfonyl azides were heated in the presence of copper pow-... 

When an azole carbene is formed, spontaneous ring fission can occur. The prototypes for these reactions are shown (565 — 566), (567 — 568) cf. corresponding nitrene reactions (Section 3.4.3.6.2). 

Stabilization of Intermediate Nitrenes Nitrenes Reaction products... 

The photochemical decomposition of acyl azides, on the other hand, proceeds by an intermediate nitrene whicn, in many cases, leads to typical nitrene reaction products. - 1- 111 -14 This is not unreasonable since a photochemical reaction provides enough energy to break the N-N, bond without alkyl or aryl participation. Homer, for example, has proved the existence of benzoylnitrene as the primary cleavage product of the photolysis of benzoyl azide in the presence of trapping reagents (Scheme II).1 7... 

In contrast to the nitrene reactions with aliphatic hydrocarbons, where only little amide formation occurs 25>69), in aromatic hydrocarbons much better yields of sulphonylamides are obtained 10.68,70), F0r example, thermolysis of benzenesulphonylazide and methanesulphonylazide in benzene gave 18% of benzenesulphonamide 16> and 14% ethanesulphonylamide, respectively 68). In contrast to the nitrene reactions in aliphatic hydrocarbons no coupling product such as biphenyl-derivatives was detected. This should have been the case however if abstraction of a single hydrogen atom did occur thus giving rise to an aryl radical as shown below. 

Arylazides normally add to olefinic double bonds faster (forming the corresponding triazolines) than their thermal decomposition to nitrenes. These triazolines however loose nitrogen more easily 75-78) (thermally or photolytically) than the corresponding azides and lead to aziridines, which should also arise from a nitrene reaction. 

The approximately comparable overall yield shows that the nonstereospecifity is increased by decreasing the olefine concentration. This is in agreement with the theory of collisional deactivation, which predicts that dilution increases the ratio of triplet nitrene. Thermolysis of NCN3 in 1,2-dimethylcyclohexane 53 is another example of the dilution effect in nitrene reactions 94>. 

From the data indicating partial loss of stereospecifity of the nitrene reaction in CH2CI2 and the complete one in CH2Br2, it can be concluded that NCN-reacts partly as triplet in CH2CI2 (dilution effect) and exclusively as triplet in CH2Br2 (dilution and heavy atom effect). Similiar results were obtained in the thermolysis of NCN3 in cyclooctatetraene 49 93) (see Scheme p. 108). 

Symmetry Allowed Cycloaddition of SO to a diene Cheletropic Reactions involving 4 Electrons Addition of Carbenes or Nitrenes Reactions involving 6n Electrons Cycloaddition and Elimination Reactions involving 8ji Electroncs... 

Levya, E. Sagredo, R. Photochanistiy of fluorophenyl azides in diethylamine. Nitrene reaction vCTsns ring expansion, Tetrahedron 1998,54, 7367-7374. 

The ring contraction of 2-azido-3-methylquinoxaline 1,4-dioxide (1) to 2 is believed to proceed by an assisted non-nitrene reaction because it proceeds in solution below 100 °C. If the reaction is conducted in toluene at 90 C, however, only compound 3 is formed in 94% yield. ... 

The vapor-phase pyrolysis of phenyl azide yields azobenzene in 72 % yield . The pyrolysis in aniline solution leads to the production of dibenzamil , identified as an azepine by modern techniques in benzene and p-xylene, the major products are azobenzene and aniline, presumably originating from reactions of PhN in hydrocarbon solvents the thermolysis produces aniline, alkyl-anilines, azobenzene and polymer . It has been shown that aniline and iV-alkylanilines arise from triplet state nitrene reactions with the solvent. In the gas-phase thermolysis of phenyl azide at low pressures aniline and azobenzene are the major products whereas at higher pressures the formation of l-cyano-1,3-cyclopentadiene (or 2-anilino-7H-azepine in the presence of aniline) predominates . It was deduced that aniline and azobenzene arise from triplet nitrene reactions and that at high pressures, a hot singlet nitrene is formed, which undergoes intramolecular insertion to form an azocyclopropene intermediate, viz. 

The Curtius rearrangement also occurs in the direct photolysis of acylazides, but additional products have been isolated and have been shown to arise from typical nitrene reactions . For example, Horner et have trapped benzoyl nitrene with a variety of scavengers. Similarly to the Wolff rearrangement (vide infra) it has been shown ° that the Curtius rearrangement occius via a singlet intermediate this intermediate can be a nitrene or an excited azide molecule in which a concerted decomposition leads to isocyanate derivatives. This reaction could occur in competition with nitrene formation or, alternatively, originate from a vibrationally-excited ground state azide molecule. 

When the azomethine group is part of an electron-deficient ring, such as pyridine, pyrimidine or thiazole, the compounds exist as tetrazoles in the solid state, and at equilibrium with the azido form in solution . The equilibrium constants depend on the solvent, the nature of the substituents and the temperature . 2-Azido-4,6-dimethylpyrimidine (288a) thus exists in equilibrium with tetrazolo-pyrimidine (288b). Its chemical behaviour is, however, in accord with the azide structure 288a, including dipolar addition reactions and nitrene reactions . 

Addition to aromatic systems is probably the primary step in a number of more complex nitrene reactions such as the formation of azepines from carbethoxynitrene and benzene and a number of... 

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