Nitrenes transfer processes

The oxidative imination of sulfides and sulfoxides via nitrene transfer processes leads to N-substituted sulfilimines and sulfoximines. This reaction is interesting as chiral sulfoximines are efficient chiral auxiliaries in asymmetric synthesis, a promising class of chiral ligands for asymmetric catalysis and key intermediates in the synthesis of pseudopeptides [169]. However, very few examples of such iron-catalyzed transformations have been described. 

This complex catalyzes the aziridination of a variety of different olefins including simple terminal aliphatic olefins with Phl=NTs (Ts=/ -methylbenze-nesulfonyl) in good-to-excellent yields. Notably, cw-stilbene was converted to cis-product (90%) with 86% isolated yield when the reaction was carried out at 0°C. The retention of the cis configuration indicates that a nitrene-transfer process may dominate at lower temperature with this catalyst (Fig. 20). 

The first metal-catalyzed nitrogen atom-transfer process was reported by Kwart and Khan, who demonstrated that copper powder promoted the decomposition of benzenesulfonyl azide when heated in cyclohexene.280 Evans has demonstrated that Cu(i) and Cu(n) triflate and perchlorate salts are efficient catalysts for the aziridination of olefins employing TsN=IPh as the nitrene precursor.281 Subsequent to this finding, intensive effort has focused on the identification of... 

Preliminary efforts to examine the mechanism of C-H amination proved inconclusive with respect to the intermediacy of carbamoyl iminoiodinane 12. Control experiments in which carbamate 11 and PhI(OAc)2 were heated in CD2CI2 at 40°C with and without MgO gave no indication of a reaction between substrate and oxidant by NMR. In Hne with these observations, synthesis of a carbamate-derived iodinane has remained elusive. The inability to prepare iminoiodinane reagents from carbamate esters precluded their evaluation in catalytic nitrene transfer chemistry. By employing the PhI(OAc)2/MgO conditions, however, 1° carbamates can now serve as effective N-atom sources. The synthetic scope of metal-catalyzed C-H amination processes is thus expanded considerably as a result of this invention. Details of the reaction mechanism for this rhodium-mediated intramolecular oxidation are presented in Section 17.8. 

P, Miiller, Transition Metal-Catalyzed Nitrene Transfer Aziridination and Insertion, in Advances in Catalytic Processes, Vol. 2, M. P. Doyle, Ed., JAI Press, Greenwich, CT, pp. 113/. 

Nitrene transfer to selenide is also possible. Catalytic asymmetric induction in this process has been studied with Cu(OTf)/bis(oxazoline) catalyst (Scheme 22). When prochiral selenide 206 and TsN=IPh are allowed to react in the presence of Cu(OTf)/chiral bis(oxazoline) 122b, selenimide 207 is obtained with enantioselectivity of 20-36% ee. When arylcinnamyl selenide 208 is applied to this reaction, corresponding selenimide 209 is produced which undergoes [2,3]-sigmatropic rearrangement to afford chiral allylic amides 211 in up to 30% ee. 

A carbene or nitrene transfer reaction to a carbon-carbon or carbon-heteroatom double bond system leads to the formation of three-membered rings, such as a cyclopropane, an aziridine or an epoxide. These processes can be catalyzed by applying iron catalysts and the different cyclic systems are discussed here. 

In addition to rhodium and ruthenium, silver catalysts have also been investigated, and this chapter discusses these silver-based nitrene transfer reactions.2 1 In discussing the work chronologically, we hope that the readers can get an idea of the evolution of thinking in the research process. 

Reviews have also featured transfer processes of nitrenes arising from chloramine-T and iminoindane catalysed by silver,C-H insertion of metallonitrene as an efficient process to perform enantioselective aminations, and silver-catalysed silylene transfers. " ... 

The direct copper-catalyzed iodosyl-mediated nitrogen transfer to olefins compares with the parent rhodium-catalyzed process that is made possible by the combination of iodosylbenzene diacetate, magnesium oxide, and sulfamates. Other recent promising nitrene transfer methods involve the bromine-catalyzed aziridination of olefins using chloramine-T and the direct electrochemical aziridination with TV aminophthalimide. ... 

C-H alkylation and amination reactions involving metal-carbenoid and metal-nitrenoid species have been developed for many years, most extensively with (chiral) dirhodium(ll) carboxylate and carboxamidate complexes as catalysts [45]. When performed in intramolecular settings, such reactions offer versatile methods for the (enantioselective) synthesis of hetero- and carbocy-cles. In the past decade, Zhang and coworkers had explored the catalysis of cobalt(II)-porphyrin complexes for carbene- and nitrene-transfer reactions [46] and revealed a radical nature of such processes as a distinct mechanistic feature compared with typical metal (e.g., rhodium)-catalyzed carbenoid and nitrenoid reactions [47]. Described below are examples of heterocycle synthesis via cobalt(II)-porphyrin-catalyzed intramolecular C-H amination or C-H alkylation. 

Among these, a variety of oxygen atom transfer reactions have been described [la,b] and highly stereoselective reactions have been reported [2]. Although the formation of aziridines by the reaction of nitrenes with olefins is well known, the efficiency is moderate, because of the competition between hydrogen abstraction and insertion processes [3]. A typical example is shown (Eq. 2) [3d]. 

When the triplet is an excited state, energy transfer occurs to form singlet oxygen. Ground state triplets react with oxygen by a spin-allowed process which, for carbenes in particular, produce carbonyl oxides [64], It seems that triplet nitrenes react with oxygen slowly. This will be discussed more fully later. Here we examine the products formed from reaction of photolysis of phenyl azide in the presence of oxygen. 

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