Olefins nitrene transfer

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. 

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... 

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. 

Zraras-aziridine products were still detected from r/.v-olefin substrates, and sometimes as the predominant product. Current results on silver-catalyzed nitrene transfer reactions, indicate that silver probably can interact with iminoiodanes to generate a silver nitrene precursor. This precursor can lead to reactions via either a concerted metal nitrene or a stepwise radical pathway, depending on the substrate and reaction conditions (Scheme 6.8). 

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 easiest reactions are those in which the nucleophile is the gold-activated species. Examples of this are Au(I)-catalyzed carbene and nitrene transfers (equations 142 and 143) that convert olefins into cyclopropanes or aziridines, respectively. In the carbene transfer, ethyl diazoacetate is the source of carbene and the active NHC-gold cationic catalyst is generated by chloride abstraction with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate NaBAT4. The cyclopropanation is competitive with other carbene insertions with active C H or N H bonds present in the substrate. For the aziridinations of olefins, nitrene formation is accomplished by the oxidation of sulfonamides with PhI(OAc)2 and the catalyst of choice is a gold-(I) triflate with a terpyridine ligand. 

There are reports on the use of a Cu(l) complex of ferrocenyldiimine 465 to facilitate nitrene transfer to olefins <1998SL617>. Of particular interest for industrial applications, these reactions can also be efficiently catalyzed by... 

Methyltrioxorhenium has been found to be a universal catalyst for a number of [2-1-1] cycloaddition reactions, including nitrene, carbene, or oxo-atom addition to olefins <2001GC235>. Typically, to increase the chemical yield of the reaction, at least 5 equiv of an olefin is required. As with most nitrene transfer reactions, simple cyclic olefins such as cyclohexene produce a low chemical yield of aziridine. The authors assume that the intermediate of the reaction is a reactive rhenoxaziridine intermediate. 1,2-Dihydronaphthalene provides aziridine 28 in 43% chemical yield under these reaction conditions (Equation 11). 

Scheme 17 The catalytic nitrene transfer reaction to olefins and C—H bonds...

Apart from cyclopropenation, catalytic aziridination with nitrene transfer to olefins is generally considered an analogue reaction of metal-catalyzed carbene-transferred cyclopropanation. Aziridination and cyclopropanation are proposed to share fundamental mechanistic features. Many of the catalysts that were successfully applied in aziridination are also efficient catalysts for cyclopropanation. For... 

An interesting application of copper-catalyzed aziridination is the preparation of ( )-olefin dipeptide isosteres based on a diastereoselective nitrene transfer onto chiral ( )-crotylsilanes (eq 86)7 CuOTf catalyzes the formation of an aziridine whose rearrangement after spontaneous desilylation affords allylamines. Excellent levels of acyclic stereocontrol can be achieved via a hydroxyl-assisted aziridination. Copper-catalyzed aziridination of enol ethers also leads to aziridines that undergo spontaneous rearrangement. Thus, CuOTf and particularly CUCIO4 mediate the formation of an a-methylserinal derivative from a 5-methyl-4//-1,3-dioxin (eq 87). ... 

Catalytic Nitrene Transfer to Heteroatoms. The experimental procedure described above for the copper-catalyzed aziridination of olefins can be applied to the imidation of sulfides, where CuOTf in conjunction with PhI=NTs mediates the formation of siilfimides in good yields (eq 95). Spontaneous [2,3] sigma-tropic rearrangements occur in the case of allylic sulfides. Chiral bis(oxazoline)-CuOTf complexes catalyze both reactions with acceptable enantioselectivities (eq 96). Chloratnine-T is also a suitable but less efficient nitrene precursor. Selenides undergo the same catalytic asymmetric imidation to afford selenimides albeit with lower yields and enantioselectivities. ... 

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. ... 

Diazo compounds are commonly used as a carbene source in organic chemistry. A few systems based on metals such as Ru and Rh have been reported for the transfer of carbenes from diazo compounds. P6rez and coworkers reported NHC-copper systems for carbene and nitrene transfer. In the first report, [Cu (Cl)(IPr)] was used for the transfer to olefins, amines, and alcohob [64]. The main transformation was the cyclopropanation of styrene with ethyl diazoacetate (Scheme 8.24). Monitoring of the reaction showed a fast formation of the product (90% conversion in 6 h). The absence of styrene does not lead to the decomposition of EDA even after a long period of time (13 h). Decent stereoselectivity was obtained with styrene (cisitrans 32/68) and cyclooctene exolendo 73/27). 

A 7r-bond can react with various active species, such as the electrophile oxene and its isoelec-tronic species (nitrenes and carbenes) and radicals. A 7r-bond can also react with a nucleophile, when it is conjugated with an electron-withdrawing group. In these reactions O, N, or C atom(s) are transferred from the active species to the olefins, forming two tr-bonds, such as C—O, C—N, and C—C, at the expense of the 7r-bond. If the 7r-bond is prochiral, chiral center(s) are... 

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... 

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]. 

In the carbene insertions to alkane C-H bonds the same concept as for the olefin cyclopropanations is applied. In that work, the gold-catalyzed carbene transfer is now used for insertions into C-H bonds (equation 150), with a selectivity that is influenced by the electronic properties of the ligand and the counterion employed. The carbene and nitrene insertion is not limited to Csp H bonds, but N H (equation 151), O H (equation 152), and aryl Csp -H bonds react as well (equation 153). ... 

The transition metal-mediated nitrenoid transfer to olefins represents a very concise route to the aziridine structure very often, however, an excess of the olefinic substrate is required for preparatively useful yields. In this arena, Andersson and co-workers have studied the copper-catalyzed aziridination of olefins using [A -(arenesulfonyl)imino]phenyliodinanes 446 as nitrene precursors, and have reported on conditions which give good to excellent yields of aziridines 447 without the constraint of having to use an excess of alkene (Scheme 116). 

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. 

Perchloro-2//-pyrrole (164) combines with styrene to yield the rearranged adduct (165)." Irradiation of the enamine (166) in the presence of benzo-phenone as a sensitizer results in hydrogen transfer from the pyrrolidine ring to the olefinic group to yield the pyrrole (167)." The direct observation of the nitrene (168) by i.r. and u.v. spectroscopy has been accomplished. ... 

Imido and 0x0 compounds are intermediates in many of the transfers of oxygen atoms and nitrene units to olefins to form epoxides and aziridines, and they are intermediates in many of the insertions of oxygen atoms and nitrene units into the C-H bonds of hydrocarbons to form alcohols and amine derivatives. The enantioselective epoxidation of allylic alcohols (Scheme 13.22) " is the most widely used epoxida-tion process, and the discovery and development of this process was one of the sets of chemistry that led K. Barry Sharpless to receive the Nobel Prize in Chemistry in 2001. The mechanism of this process is not well established, despite the long time since its discovery and development. Nevertheless, most people accept that transfer of the oxygen atom occurs from a titanium-peroxo complex - rather than from an 0x0 complex. Jacobsen s and Katsuki s - manganese-salen catalysts for the enantioselective epoxidations of unfunctionalized olefins, which were based on Kochi s achiral chromium- and manganese-salen complexes, are a second set of... 

Chloramine-T and bromamine-T can also be used as the nitrene source in the copper-catalyzed aziridination but the transfer to olefins occurs with less efficiency. In this context, bromine and iodine are better catalysts. However, CuOTf... 

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