Nitrenoid catalysts

With the iron complex [Fe(Cl3terpy)2]( 104)2 (Clsterpy = 4,4, 4"-trichloro-2,2 6, 2"-terpyridine) as catalyst, sulfamate esters react with Phl(OAc)2 to generate iminoiodanes in situ which subsequently undergo intramolecular nitrenoid C-H insertion to give amidation products in good yields (Scheme 30) [48]. 

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. 

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... 

Recently, Scott et al. have reported that a Cu complex bearing an axially chiral ligand (49) is an excellent catalyst for aziridination of 2,2-dimethylchromene and cinnamate esters (Scheme 36), though it is also less efficient for the reactions of simple olefins.157,158 On the basis of DFT investigation of the nitrenoid intermediate (50), one of the oxygen atoms of the A -sulfonyl group has been proposed to be interacting with the nitrene N-atom.158... 

Various approaches to epoxide also show promise for the preparation of chiral aziridines. Identification of the Cu(I) complex as the most effective catalyst for this process has raised the possibility that aziridination might share fundamental mechanistic features with olefin cyclopropanation.115 Similar to cyclo-propanation, in which the generally accepted mechanism involves a discrete Cu-carbenoid intermediate, copper-catalyzed aziridation might proceed via a discrete Cu-nitrenoid intermediate as well. 

Decomposition of sulfonyl azides was shown to be catalyzed by copper in 1967 (72, 73). In the presence of alkenes, the reaction provides both aziridines and the C-H insertion products, albeit in low yields (73). In 1991, Evans et al. (74, 75) illustrated that both Cu(I) and Cu(II) salts were effective catalysts for nitrenoid transfer from [A-(/Moluenesulfonyl)imino]phenyliodinane (PhI=NTs) to a variety of acceptor alkenes. In the absence of ancillary ligands, reactions proceed best in polar aprotic solvents such as acetonitrile. Similar results are observed using both Cu(MeCN)4C104 and Cu(acac)2 as precatalysts, Eq. 53. 

The groundwork for this study was laid in the bis(oxazoline)-copper-catalyzed cyclopropanation reaction reported by Evans, Masamune, Pfaltz, and their coworkers (32-34) (cf. Section II.A.6). Indeed, two of these early papers reported that the same catalysts were capable of effecting nitrenoid transfer to acceptor alkenes in moderate ee. 

PhI=NTs in MeCN affords a copper species that is indistinguishable by ultraviolet-visible (UV-vis) spectroscopy from an identical solution derived from Cu(OTf)2. Given the strong oxidizing nature of PhI=NTs, it seems likely that both catalysts proceed through a Cu(II) species. Beyond this, little can be said with certainty. If nitrenoid formation proceeds by a two-electron oxidation of the catalyst, one would need to invoke Cu(IV) as an intermediate in this process (77). This issue is resolved if one invokes the intervention of a bimetallic complex in the catalytic cycle. However, attempted observation of a nonlinear effect revealed a linear relationship between ligand enantiopurity and product ee (77, 78). 

The initial screen of potential catalysts by these workers revealed that several Lewis acids are capable of effecting nitrenoid transfer to alkenes. In particular, SmLOf-Bu, a species that is unlikely to participate in redox processes, was found to work well for 7ra s-p-methylstyrene aziridination. Although the generality of this catalyst fell far short of the copper system, it raises the intriguing possibility that the Cu(II) species formed in the aziridination acts at least in part as a Lewis acid. The considerable Lewis acidity of cationic Cu(II) complexes has since been extensively exploited (cf. Section V). 

Investigations into the mechanism of this reaction revealed several interesting facts (61). Compelling evidence was presented that a discreet Cu nitrenoid was involved in the catalytic cycle. Photolysis of a solution of tosyl azide and styrene in the presence of the catalyst afforded aziridine with the same enantioselectivity as obtained from the PhI=NTs reaction, Eq. 69. Since photolysis of tosyl azide is known to extrude dinitrogen and form the free nitrene, the authors argue that this is indicative of a common Cu-nitrenoid intermediate in this reaction. 

Uemura and co-workers (91) demonstrated that copper catalysts effectively transfer nitrenoid groups to sulfides generating chiral sulfimides. A complex obtained from CuOTf and 55d catalyzes nitrenoid transfer to prochiral sulfides to afford products such as 139 in moderate to poor enantioselectivities (<71% ee, Eq. 78). Nitrenoid transfer occurs selectively to the sulfur atom of allylic sulfides generating allylic sulfenamide (140) in moderate selectivity, after [2,3] sigmatropic rearrangement of the initial sulfimide 141, Eq. 79. 

Recently Liu and coworkers used (porphyrin)iron(III) chloride complex 96 to promote 1,5-hydrogen transfer/SHi reactions of aryl azides 95, which provided indolines or tetrahydroquinolines 97 in 72-82% yield (Fig. 24) [148]. The reaction starts probably with the formation of iron nitrenoids 95A from 95. These diradicaloids undergo a 1,5- or 1,6-hydrogen transfer from the benzylic position of the ortho-side chain. The resulting benzylic radicals 95B react subsequently with the iron(IV) amide unit in an Sni reaction, which liberates the products 97 and regenerates the catalyst. /V,/V-Dialkyl-w// o-azidobenzamides reacted similarly in 63-83% yield. For hydroxy- or methoxy-substituted indolines 97 (R2=OH or OMe) elimination of water or methanol occurred from the initial products 97 under the reaction conditions giving indoles 98 in 74—78% yield. 

One of the attractions of dirhodium paddelwheel complexes is their ability to catalyse a wide variety of organic transformations such as C-H insertions, cyclopropanations and ylide formation. A review on the application of high symmetry chiral Rh2(II,II) paddlewheel compounds highlights their application as catalysts for asymmetric metal carbenoid and nitrenoid reactions, and as Lewis acids.59 Their impressive performance as catalysts in C-H functionalisation reactions has been exploited in the synthesis of complex natural products and pharmaceutical agents. A recent review on catalytic C-H functionalisation by metal carbenoid and nitrenoid insertion demonstrates the important role of dirhodium species in this field.60... 

Electrophilic, nitrenoid-mediated amination processes are often challenged to discriminate between alkene aziridination and C-H insertion in substrates possessing some degree of unsaturation. Alkene aziridination is typically favored owing to the greater nucleophilicity of an ordinary n bond vis-a-vis a ct-C-H center. White and coworkers have found an elegant solution to this difficult problem of chemo-selectivity with the advent of a selective Pd(II) catalyst for allylic C-H functionalization [138, 139]. In these examples, allylic C-H activation of terminal alkenes... 

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... 

Direct preparation of an aziridine from an alkene is possible by reaction of the alkene with a nitrene or metal nitrenoid species. Nitrenes can be generated thermally or photochemically from azides, although their reaction with alkenes to give aziridines is often low yielding and is complicated by side reactions. Oxidation of iV-amino-phthalimide or related hydrazine compounds (e.g. with Pb(OAc>4 or by electrolysis) and reaction with an alkene has found some generality. The metal-catalysed reaction of nitrenes with alkenes has received considerable study. A variety of metal catalysts can be used, with copper(II) salts being the most popular. For example, styrene was converted to its A-tosyl aziridine 72 by reaction with [A-(tosyl)imino]phenyliodinane (PhI=NTs) and copper(II) triflate (5.75). ... 

In the context of C—H bond insertion reactions involving metal nitrenoids, Mn and Ru catalysts hold important positions. These catalysts often contain multidentate ligands, such as porphyrins and salen ligands. With these catalysts, the enantioselectivity can be well controlled in many cases. 

Nevertheless, there still exist some obstacles to be overcome. Generally, the diazo compounds as carbene precursors require a slow addition technique to avoid potential side reactions such as dimerization. For C—H bond insertion by metal nitrenoids, and metal oxo species, only a handful of substrates can lead to products in satisfactory yield and enantioselectivity. Under these circumstances, albeit in limited cases, new carbene precursors are still in great demand to make the reaetion more operative. Since Rh catalysts are not so eheap, reduetion in the eatalyst loading, recovery of eatalyst, simplified syn-thetie routes for the eatalyst, and other cheap and efficient metal catalysts might be the potential solutions to solve this issue. Furthermore, C—H bond insertion by metal nitrenoids, and metal oxo species are largely unexplored. 

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