Nitrene radical intermediate

A combined theoretical and experimental investigation has been conducted to further rationalize the mechanism of cobalt porphyrin-catalysed nitrene C—H insertion to benzyUc C—H bonds using diverse organic azides as nitrene sources.Results of DFT calculations suggest that the insertion proceeds via a multi-step radical-type mechanism involving notably an unusual nitrene radical intermediate, which was confirmed by EPR spectroscopy. 

A more practical, atom-economic and environmentally benign aziridination protocol is the use of chloramine-T or bromamine-T as nitrene source, which leads to NaCl or NaBr as the sole reaction by-product. In 2001, Gross reported an iron corrole catalyzed aziridination of styrenes with chloramine-T [83]. With iron corrole as catalyst, the aziridination can be performed rmder air atmosphere conditions, affording aziridines in moderate product yields (48-60%). In 2004, Zhang described an aziridination with bromamine-T as nitrene source and [Fe(TTP)Cl] as catalyst [84]. This catalytic system is effective for a variety of alkenes, including aromatic, aliphatic, cyclic, and acyclic alkenes, as well as cx,p-unsaturated esters (Scheme 28). Moderate to low stereoselectivities for 1,2-disubstituted alkenes were observed indicating the involvement of radical intermediate. 

Asymmetric amidation of sp C—H bonds was reported in good yields and moderate enantioselectivities (Scheme 5.27)." ° When benzylic or allylic C—H bonds were used, similar results were also obtained." In these reactions the prepared nitrenes, PhI=NTs, and/or PhI(OAc)2+NH2Ts were used as nitrogen atom transfer sources. The studies showed that Ru=NTs was formed in situ and acted as a possible active intermediate when a ruthenium catalyst was used (Figure 5.12), whereas a radical intermediate might be involved when a manganese catalyst was used. 

Anodic oxidation of benzenesulphenanilides 56 leads to cleavage of the nitrogen-sulphur bond in the radical-cation with the formation of a nitrenium ion, which deprotonates to the nitrene. The intermediate dimerises to a phenazine [168]. 

The para-nitro ester 71d generated only 4-nitroaniline (70%) and 4,4 -dinitroazoxybenzene (10%) when it underwent decomposition (Scheme 29). These products could have been derived from either a triplet nitrene or a triplet nitrenium ion precursor. Homolysis of the N—O bond to generate radical intermediates was ruled out because of the nearly quantitative yield of pivalic acid derived from 71d. The pivaloxy radical would have undergone rapid decarboxylation to generate CO2 and the rert-butyl radical under these conditions. Since no rearrangement product was observed, it was tentatively concluded that this ester underwent direct decomposition to 4-nitrophenyl-nitrene without the intermediacy of a nitrenium ion. ... 

The addition of a singlet nitrene to a double bond is supposed to occur concert-eddy, while the triplet nitrene is supposed to add in two discrete steps via a 1.3-di-radical intermediate. The rate of ring closure of the diradical is supposed to be smaller than that for the rotation around the C—C-bond, therefore the stereochemistry of the olefin will be lost. 

Decomposition of azides very often begins with formation of nitrenes which contain uncharged nitrogen radical intermediate according to reaction (4)... 

Many methodologies have been developed for the ring expansion of heterocycles involving different reaction intermediates, such as carbenes, nitrenes, radicals and a large number of anionic species these processes being promoted thermically, photo-chemically or by means of transition metals. 

Quintet m-phenylenedinitrenes have been generated in low-temperature matrices by photolysis of the corresponding diazide precursors, and their secondary photochemical transformations studied. Two competing ring-opening pathways were identified, and it was also concluded that quintet dinitrenes are much more photochemically reactive than triplet nitrenes. An EPR study of radical intermediates formed in the photooxidation of 4,4 -diazidodiphenyl in benzene and toluene has also been published The radicals apparently arise by abstraction of H atoms from the solvent by a triplet nitrene-02 complexes. 

The second mechanism worthy of consideration involves initial abstraction of one hydrogen by triplet nitrene to form an anilino radical which may subsequently either dimerize or abstract another hydrogen. This is illustrated for the thermal decomposition of p-anisyl azide in cumene. Indirect support for the hydrazo intermediate 37 comes from the observation that thermolysis of phenyl azide in decalin affords hydrazobenzene and benzidine as well as azobenzene and aniline. Anilino radical intermediates also are implicated in the oxidation... 

It is interesting to note that whilst the overall yield of triplet derived products remains the same at higher temperature, a second hydrogen abstraction, to give amine, predominates over the more selective dimerizations. It would be bold to conclude from the above that anilino radical intermediates are involved in arylnitrene dimerizations but they do seem more likely than attack of nitrene on a molecule of starting azide. 

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. 

Time-resolved spectroscopic techniques are important and effective tools for mechanistic photochemical studies. The most widely used of these tools, time-resolved UV-VIS absorption spectroscopy, has been applied to a variety of problems since its introduction by Norrish and Porter almost 60 years ago. Although a great deal of information about the reactivity of organic photochemical intermediates (e.g., excited states, radicals, carbenes, and nitrenes) in solution at ambient temperatures has been amassed with this technique, only limited structural information can be extracted from... 

The rate of decomposition of benzenesulphonyl azide to benzene-sulphonamide is said to be accelerated appreciably by thiophenol 18> in a radical-catalyzed process probably not involving a free nitrene intermediate. 

These three types, radicals, carbocations and carbanions, by no means exhaust the possibilities of transient intermediates in which carbon is the active centre others include the electron-deficient species carbenes, R2C (p. 266), nitrenes, RN (p. 122) and also arynes (p. 174). 

This chapter has to do with reactions wherein the photochemical event is the breaking of a bond in a molecule. For a single bond this results in the formation of a pair of radicals or a diradical. For a double bond as in diazo compounds or in azides a carbene or a nitrene and nitrogen are formed. All these intermediates will then undergo further mono- or bi-molecular dark reactions or eventually recombine to ground state starting materials. 

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