Nitrenes transfer

Aziridines are versatile intermediates in organic synthesis and commonly found in bioactive molecules. The transition metal-catalyzed nitrene transfer to alkenes is an attractive method for the synthesis of aziridines [7]. In 1984, Mansuy and coworkers reported the first example of an iron-catalyzed alkene aziridination in which iron porphyrin [Fe(TTP)Cl] was used as catalyst and PhINTs was used as nitrene source [30]. Subsequently, the same authors demonstrated that [Fe(TDCPP) (CIO4)] is a more efficient and selective catalyst than [Fe(TTP)Cl] (Scheme 20). 

Three different side reactions are found in [Fe(TTP)Cl]-catalyzed nitrene transfer reaction including... 

The synthesis of sulfoximides and sulfimides has attracted considerable attention in recent years due to the potential utility of these compounds as efficient auxiliaries and chiral ligands in asymmetric synthesis (reviews [86-88]). Transition metal-catalyzed nitrene transfer to sulfoxides and sulfides is an efficient and straightforward way to synthesize sulfoximides and sulfimides, respectively. Bach and coworkers reported the first iron-catalyzed imination of sulfur compounds with FeCl2 as catalyst and B0CN3 as nitrene source. Various sulfoxides and sulfides were... 

A first terminal imido complex of nickel (121) was prepared according to Equation (3).468 The synthesis goes via the Ni11 amido compound (122) and uses the steric bulk of the arylimido group for stabilization. The Ni11 center in (121) is planar and three-coordinate. Reaction of (121) with CO or benzyl isocyanide leads to formal nitrene transfer with formation of (124) and (125), respectively. Further reaction with CO liberates the isocyanate and carbodiimide (Equation (4)). 69... 

Metal-oxenoid (oxo metal) species and metal-nitrenoid (imino metal) species are isoelectronic and show similar reactivity both species can add to olefins and be inserted into C—H bonds. Naturally, the study of nitrene transfer reactions began with metalloporphyrins, which were originally used as the catalysts for oxene transfer reactions. 

Reaction of metal nitrosyls with azide ion proceeds with formation of N2 and N20 (56). This can be viewed as the result of a nitrene transfer reaction in analogy with the Curtius rearrangement (62) and its organome-tallic counterpart (63). 

FeCl2 has been used to catalyse nitrene transfer from l-butyloxycarbonyl azide to sulfoxides (to form sulfoximides), sulfides (to give sulfimides), and a ketene acetal (to form an a-amino ester). ... 

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. 

Summary and Outlook for Rhodium(ll)-Catalyzed Nitrene Transfer... 

Catalytic methods are suitable for nitrene transfer," and many of those found to be effective for carbene transfer are also effective for these reactions. However, 5- to 10-times more catalyst is commonly required to take these reactions to completion, and catalysts that are sluggish in metal carbene reactions are unreactive in nitrene transfer reactions. An exception is the copper(ll) complex of a 1,4,7-triaza-cyclononane for which aziridination of styrene occurred in high yield, even with 0.5 mol% of catalyst. Both addition and insertion reactions have been developed. 

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/. 

In 1993, Jacobsen and Evans simultaneously reported that [7V-(p-tolylsulfonyl)imino]phenyliodinane (TsN=IPh, 195) is an efficient asymmetric nitrene transfer reagent to alkenes in the presence of a catalytic amount of a copper(i) salt and a chiral diimine ligand or a chiral bis(oxazoline) ligand (Equation (31)). Mechanistic study by Jacobsen and co-workers suggests that a discrete copper(iii) nitrene complex is an intermediate responsible to the reaction. ... 

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. 

As discussed above, iodosylbenzene (Phl=0) oxidizes various transition-metal ions (Mn+) such as manganese, iron, ruthenium, and chromium ions to the corresponding oxo-metal species (0=Mtransfer agents. Likewise, fM-(p-toluenesulfonyl)imino]-phenyliodinane(PhI=NTs) also oxidizes these metal ions to give the corresponding tosylimino-metal species (TsN=M(n+2)+) that undergo nitrene-transfer reaction such as aziridination (Scheme 6B.28) [73],... 

In 1991, Evans et al. reported that cationic Cu(I) ions catalyzed the nitrene-transfer reaction smoothly (Scheme 6B.29) [74]. Since then, many studies on asymmetric aziridination have been carried out with chiral copper(I) complexes as catalysts. 

The enol form of mandelic acid (101) has been generated by flash photolysis of phenyldiazoacetic acid in aqueous solution.101 The enol forms by hydration of the intermediate carbene (102). The reaction of chloramine-T (TsNClNa O) with methyl p-tolyl sulfide to give the corresponding sulfimide (103) appears to proceed via a nitrene-transfer mechanism in the presence of copper(I) and a second nitrogen ligand (such as acetonitrile).102... 

The iron-catalyzed nitrene transfer to alkenes is a challenging field of research. Remarkable results have been reported but much more effort has to be made to develop further this area of catalysis. 

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. 

Bach and Korber reported in 1998that iron(I I) chloride could be used in combination with tert-butyloxycarbonyl azide (BocN3) for nitrene transfer to sulfides and sulfoxides [170], Whereas usually 1 equiv. of FeCl2 was employed in the imination of sulfoxides, catalytic amounts of this iron salt (25mol%) could also be applied (Table 3.13). 

Bolm and coworkers discovered that inexpensive and easy to handle iron(III) acetylacetonate [Fe(acac)3] was also capable of catalyzing nitrene transfer to sulfides and sulfoxides using various sulfonyl amides in combination with iodosylbenzene (Scheme 3.58) [174]. 

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. 

The nitrene transfer from PhI=NTos to alkenes catalyzed by the CpFe(CO)2+ fragment gave better results (85% for styrene) [25], but the characteristics of the chemistry of the cationic intermediates as postulated by the reaction mechanism are closely connected to the alternative formation of aziridines by a carbene transfer... 

The use of sulfonylimino(aryl)iodanes, especially [(tosylimino)iodo]benzene (41), as nitrene-transfer agents has undergone considerable development during the past decade. Much of this effort is based on the finding in the early 1990s that tosylaziridinations of alkenes with Phi = NTs, previously demonstrated with Mn(III)- and Fe(III)-porphyrin catalysts, can be achieved more generally and efficiently with copper(I) and copper(II) salts i.e., the Evans aziridination reaction [172,173]. Standard conditions for preparative aziridinations of this type were developed, and applied to cyclic and acyclic alkenes, arylalkenes, and a, / -... 

The aziridination of olefins, which forms a three-membered nitrogen heterocycle, is one important nitrene transfer reaction. Aziridination shows an advantage over the more classic olefin hydroamination reaction in some syntheses because the three-membered ring that is formed can be further modified. More recently, intramolecular amidation and intermolecular amination of C-H bonds into new C-N bonds has been developed with various metal catalysts. When compared with conventional substitution or nucleophilic addition routes, the direct formation of C-N bonds from C-H bonds reduces the number of synthetic steps and improves overall efficiency.2 After early work on iron, manganese, and copper,6 Muller, Dauban, Dodd, Du Bois, and others developed different dirhodium carboxylate catalyst systems that catalyze C-N bond formation starting from nitrene precursors,7 while Che studied a ruthenium porphyrin catalyst system extensively.8 The rhodium and ruthenium systems are... 

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. 

As stated in the introduction, chloramine-T (where T denotes three crystalline water molecules) is a commonly used nitrene precursor, which is commercially available and costs less than do most other nitrene sources. The benefit of a silver salt in nitrene transfer reactions with chloramine-T is surprisingly simple. Because silver chloride is insoluble in most solvents, substoichiometric amounts of silver salts (like silver nitrate) can be used to remove the chloride from chloramine to facilitate the release of a free nitrene radical, which can aziridinate olefins. Since the amount of silver is near stoichiometric, it should not be called silver-based catalysis, although turnover numbers (TONs) higher than 1 have been observed in some cases. 

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