Nitrogen nitrenoids

Two mechanisms are possible for the Cu-mediated aziridination using PhI=NTs as a nitrogen source (i) aziridination via Cu-nitrenoid species (L Cu=NTs) and (ii) aziridination via a L (Cu—PhI=NTs) adduct, in which the Cu complex functions as a Lewis acid catalyst. Jacobsen et al. demonstrated that the enantioselectivity of the aziridination using (48) as the chiral auxiliary did not depend on the nitrogen precursors.1 5 This supports the intermediacy of the Cu-nitrenoid... 

Nitrenes are neutral monovalent nitrogen analogs of carbenes. The term nitrenoid is applied to nitrene-like intermediates that transfer a mono substituted nitrogen fragment. 

A very reactive nitrogen atom is required to convert benzenes or naphthalenes into pyridines, and there are a number of such reactions which involve nitrenes or nitrenoid species. A number of substituted benzenes have been treated with sulfonyl diazide or carbonyl diazide and moderate yields of pyridines recorded (27CB1717). Thus p-xylene gives 2,5-dimethylpyridine there is no indication of the fate of the carbon atom which is lost. More controlled reaction is possible in intramolecular insertions. The examples in which o-nitrotoluene is converted into a derivative (759) of 2-acetylpyridine, and where 2,3-diazidonaphthalenes give 3-cyanoisoquinolines (760) are quoted in a review (81 AHC(28)231>. 

The prototype reactions for synthesis of pyrrole, indole and carbazole by processes involving electrophilic nitrogen are outlined in Scheme 3 with a nitrene as the electrophilic form of nitrogen. This conceptual scheme must be generalized to include nitrenoids and nitrene equivalents to accomodate the range of cyclizations which can proceed by this mechanism. In practice, this mechanistic pattern is rare for pyrroles, known but only of moderate utility for indoles, and among the most efficient methods for the synthesis of carbazoles. 

The nitrogen source for the aziridination of alkenes, a nitrene or nitrenoid, can be generated in various ways (1) oxidation of a primary amine (2) base-induced -elimination of HX from an amine or amide with an electronegative atom X (X = halogen, O) attached to the NH group or by -elimination of metal halides from metal A-arenesulfonyl-A-haloamides (3) metal-catalyzed reaction of [A-(alkane/arenesulfonyl)imino]aryliodanes (4) thermolytic or photolytic decomposition of organyl azides and (5) thermally induced cycloreversion reactions . 

The most important and well-defined reactions involving attack at nitrogen are those discovered by Schmitz and co-workers. A -Unsubstituted oxaziridines react with a variety of nucleophiles to effect transfer of the NH group from the oxa-ziridine to the nucleophile. Many of these reactions resemble nitrene or nitrenoid reactions although little mechanistic work has been reported, it seems better to classify them as involving nucleophilic substitution followed by elimination ... 

Recently, transition-metal-catalyzed reaction of iminoiodinanes 731 has been focused as a method of nitrogen transferring (Scheme 226). The iminoiodinanes 731 are readily synthesized from sulfor-amides or carbamates 730 by treatment with Phl-(OAc)2. The reaction of 731 with a transition-metal catalyst M produces the metal nitrenoid 732. The... 

Nitrenes are the nitrogenous isoelectronic analogs of carbenes. In the context of developing new protocols for asymmetric functionalization of C—H bonds, amination via metal nitrenoids species is direct and significantly important. In the past decade, remarkable progress has been made in this area. This section provides an overview of these developments. The reactions will be presented according to the type of the metal involved in these transformations. 

A last example of nitrenoid insertion has been reported and concerns the insertion reaction into a C-N bond of methano-Trogei bases (176). Under Cu(II)- and Rh(II)-catalysis, various nitrogenated substrates (176) have been found to react with imino phenyliodinane PhI=NTs to give their imino analogues (177) via a net C-N amination... 

A comparative study of the aziridination of styrene using a variety of arenesulfonyl azides and a Cu(acac>2 catalyst has shown that pyridine-2-sulfonyl azide and related snbstituted pyridines are particularly efficient. It seems likely that the nitrogen atom of the pyridine ring coordinates to the copper ion and drives the formation of an internally stabihzed nitrenoid intermediate. The method has been used to achieve aziridination of a range of substituted styrenes 29 in good yield and without the need for the alkene to be present in large excess (Scheme 6.15). 

Roizen JL, Harvey ME, Du Bois J (2012) Metal-catalyzed nitrogen-atom transfer methods for the oxidation of aliphatic C-H bonds. Acc Chem Res 45(6) 911-922 Davies HML, Manning JR (2008) Catalytic C-H functionalization by metal caibenoid and nitrenoid insertion. Nature 451 417 24... 

To expand the utility of the direct use of sulfonamide as a nitrogen source, several effective catalyst systems have been reported. Chang and coworkers developed the alkene aziridination using 5-methyl-2-pyridinesulfonamide and Phi (OAc)2 catalyzed by Cu(tfac)2 (tfac = trifluoroacetylaceto-nate) without external ligands or bases (Scheme 2.26) [39]. It was postulated that the coordination of pyridyl N atom to the copper center was the driving force for the formation of copper nitrenoid 20. Indeed, replacement of the pyridyl N atom to CH suppressed the reaction. 

Treatment of the nitrido manganese(V) derivative of 5,10,15,20-tetra-mesitylporphyrin with trifluoroacetic anhydride in the presence of an excess of cyclooctene resulted in the transfer of the nitrogen atom to the olefin. Conversion of cyclooctene to the corresponding aziridine proceeded via nitrenoid Mn porphyrin intermediate. During this process the nitrido Mn(V) porphyrin was reduced to the trifluoroacetato Mn(III) porphyrin. 

The results presented herein show that the redox reactivity of Mn(III) capped porphyrin is comparable to that previously observed for the unhindered analogues. All charge transfer processes involve a penta-coordinate metal center and, in some instances, involve ligand substitution reactions. The presence of the cap does not impede the generation of the high valent nitrido Mn(V) porphyrin. When compared to the reactivity of sterically unencumbered porphyrins, the presence of the cap actually facilitates the production of the nitrenoid complex, the key intermediate in the nitrogen transfer chemistry leading to aziridines. 

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