Nitrene ligand

A related transformation to the previous carbene transfer reaction involves a nitrene ligand bonded to the metal center, in a metallonitrene intermediate in situ generated upon the appropriate selection of the catalyst and the nitrene precursor. As shown in Scheme 17, some transition metal complexes react with such a precursor to generate an unsaturated intermediate, generally electrophilic in nature, which might react with olefins or C—H bonds affording aziridines or amines in a catalytic manner. The most employed nitrene sources are hypervalent I(III) compounds such as PhI=NTs, chloramine-T or organic azides. 

A five-coordinate nitrene complex has been reported where the nitrene ligand is (2,2,6,6-tetramethyl-l-piperidyl)nitrene. [Fe(Tp-ClPP)(nitrene)] has Fe-N(Ax) =... 

Fe4metal atoms[623. Moreover, even the J -coordination of the EtNO ligand is very unusual. 

In this situation the nitrene ligand is rather inert[230,23lJ. However the addition o-f an appropriate ligand L can induce the trans-formation into a bent, doubly bonded nitrene species, which must be much more reactive. A Lewis acid, which can bind to the lone pair on nitrogen, can also induce this trans-formation. 

The fact that this kind of reaction is rare might reflect the requirement for high carbon monoxide pressure in order to occur. On the other hand, it has been demonstrated that appropriated co-reagents can induce an unreactive nitrene ligand to become accessible to attack by carbon monoxide (Chapter 6) [23], In mononuclear complexes, this should correspond to the transformation of a linear, triply bonded species to a bent, doubly bonded nitrene ligand (eq. 5) [14, 15] ... 

The excess of electron density carried by the entering ligand L to the metal should induce the lone pair to be free and accessible by carbon monoxide. An entering ligand could also help the opening of an arm of a triply-bridging nitrene ligand, which otherwise would not have positions of attack by carbon monoxide (eq. 6) ... 

Thus the urea and other by-products would not be formed from an amine derived from reaction of the intermediate isocyanate with adventitious moisture, but via side reactions of nitrene-like intermediates. However, this view considers the nitrene species as a pure organic entity, whereas, in the presence of transition metals, nitrene complexes can be formed (Chapter 1) and the reactivity of the bound nitrene ligand is entirely different from the one of the free species (Chapter 6) [14-16]. 

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