Nitrogen nitrenes

The main example of a category I indole synthesis is the Hemetsberger procedure for preparation of indole-2-carboxylate esters from ot-azidocinna-mates[l]. The procedure involves condensation of an aromatic aldehyde with an azidoacetate ester, followed by thermolysis of the resulting a-azidocinna-mate. The conditions used for the base-catalysed condensation are critical since the azidoacetate enolate can decompose by elimination of nitrogen. Conditions developed by Moody usually give good yields[2]. This involves slow addition of the aldehyde and 3-5 equiv. of the azide to a cold solution of sodium ethoxide. While the thermolysis might be viewed as a nitrene insertion reaction, it has been demonstrated that azirine intermediates can be isolated at intermediate temperatures[3]. 

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

However, in some cases carboxylic acid-derived groups can participate in ring fission-reclosure reactions. Thus photolysis of 1,5-disubstituted tetrazole (399) gives nitrogen and appears to involve the amino-nitrene intermediate (400), which reacts further to give (401) (77AHC(21)323). 

The most important chemistry of azidoazoles is the fragmentation of derived nitrenes of which the prototypes are (453) (454) and (455) (456). Thus 5-azido-l,4-diphenyltriazole (457) evolves nitrogen at 50 °C (70JOC2215). 4-Azido-pyrazoles and -1,2,3-triazoles (458) undergo fragmentation with formation of unsaturated nitriles (8lAHC(28)23l). 

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

Aziridines have been prepared stereospecifically by the nucleophilic addition of the nitrogen residue to alkenes <80T73). Introduction of the nitrene is accomplished readily via a Michael-type addition with free diphenylsulfilimine (Scheme 12), and where a chiral sulfilimine is used the chirality is transferred to the aziridine with optical yields in excess of 25%. 

The third class of compounds to be discussed in this chapter are those in which an RE group (E = S, Se, Te) is attached to a nitrogen centre. This category includes amines of the type (REfsN and the related radicals [(RE)2N] , as well as organochalcogen(ir) azides, REN3, and nitrenes REN (E = S, Se). Covalent azides of the type RTe(N3)3 and R2Te(N3)2, in which the chalcogen is in the +4 oxidation state, have also been characterized. 

The possible routes leading to azides 2 are outlined in Scheme 1. The only pathway which we can exclude the 2=>C=>D=>E=>2 sequence since neither intermediate E nor its possible derivative E could be detected in the reaction mixture. (P-Azido a,P-unsaturated ketones are known to afford isoxazoles via nitrenes derived by loss of nitrogen (refs. 4,10). Azides 2 may form either via intermediate A (Sn + E route) or via allyl bromide-type intermediate B (E -f Sn route), both routes may operate on the basis of experimental results obtained so far. 

There are four types of organic species in which a carbon atom has a valence of only 2 or 3/ They are usually very short lived, and most exist only as intermediates that are quickly converted to more stable molecules. However, some are more stable than others and fairly stable examples have been prepared of three of the four types. The four types of species are carhocations (A), free radicals (B), carbanions (C), and carbenes (D). Of the four, only carbanions have a complete octet around the carbon. There are many other organic ions and radicals with charges and unpaired electrons on atoms other than carbon, but we will discuss only nitrenes (E), the nitrogen analogs of carbenes. 

Nitrenes (R—N) are the nitrogen analogs of carbenes, and most of what we have said about carbenes also applies to them. Nitrenes are too reactive for isolation under ordinary conditions, although ab initio calculations show that nitrenes are more stable than carbenes with an enthalpy difference of 25-26kcal moP... 

Though we have presented this mechanism as taking place in three steps, and some reactions do take place in this way, in many cases two or all three steps are simultaneous. For instance, in the nitrene example above, as the R migrates, an electron pair from the nitrogen moves into the C—N bond to give a stable isocyanate ... 

Iron-nitrene/imido complexes are proposed to be the reaction intermediates in nitrogen group transfer reactions. The nitrene group can be transferred to organic substrates. Aziridination and amination are the well-known nitrogen atom/group... 

Modification of [Fe(TTP)Cl] due to intramolecular nitrene insertion to iron-nitrogen bond... 

The nitrogen analogs of carbenes are called nitrenes. As with carbenes, both singlet and triplet electronic states are possible. 

The characteristic reaction of an alkyl nitrene is migration of one of the substituents to nitrogen, giving an imine. 

N-benzylaniline with phosgene, and then with sodium azide to produce carbonyl azide 52. On heating, nitrogen is evolved and a separable mixture of nitrene insertion product 53 and the desired ketoindazole 54 results. The latter reaction appears to be a Curtius-type rearrangement to produce an N-isocyanate (54a), which then cyclizes. Alkylation of the enol of 54... 

Azides are known to undergo photoelimination of nitrogen to produce univalent nitrogen intermediates (nitrenes). These electron-deficient intermediates can then undergo a variety of intermolecular and intramolecular... 

Uncatalyzed thermal decomposition of sulphonyl azides is believed to give nitrene intermediates and nitrogen, in general ... 

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