Aziridination silver

Similarly, another accident occurred when metallic silver came into contact with aziridine. According to the authors of the report, the accident was interpreted by the formation of an aziridine silver derivative. Comparing this behaviour with the one of ethylene oxide when silver is present, a danger which is of the same nature is demonstrated. The interpretation that had been given at the time was based on the presence of acetylene in ethylene oxide, whose silver derivatives are very sensitive explosives. It may be that acetylene traces were present in aziridine although none of the authors mentioned such as possibility as far as we know. 

A number of catalysts have been used to promote ring opening of A-tosyl-aziridines, such as phospho-molybdic acid and silica gel, for azide, cyanide and alcohols " and tri-n-butylphosphine for thiols and amines. Opening with iodide occurs at room temperature with iodine and thiophenol in the presence of air. In the nucleophilic ring opening of A-tosyl-aziridines, silver ion catalysis facilitates reactions with electron-rich arenes or hetarenes. ... 

Several reviews on the synthesis of aziridines have been published in the previous year. These publications include a review on the silver catalyzed addition of nitrenes (among other intermediates such as carbene) across a double bond <06EJOC4313> a review on sulfur ylide addition to imines to form aziridines <06SL181> a review on nitrogen addition across double bonds <06ACR194> a general review on functionalization of a,p-unsaturated esters with some discussion of aziridination <06TA1465>... 

Other polymerisation incidents are f Acrylaldehyde, 1145 Acrylamide, 1180 Acrylic acid, 1148 Acrylic acid, Initiator, Water, 1148 f Acrylonitrile, 1107 f Acrylonitrile, Initiators, 1107 f Acrylonitrile, Silver nitrate, 1107 f Acryloyl chloride, 1093 Allyl 4-toluenesulfonate, 3315 Aluminium chloride, Alkenes, 0062 3 - Aminopropiononitrile f Aziridine, Acids, 0863... 

Addition of an excess of anhydrous aziridine to silver chloride resulted in dissolution and the formation of a colourless solution. Attempts to isolate a complex were unsuccessful and addition of water, ethanol or ether caused reprecipitation of silver chloride. 

With silver nitrate, colourless sheets could be precipitated by the addition of anhydrous ether to an aziridine solution. It was noticed that on standing for a few days in aqueous solution the salt decomposed and deposited metallic silver. The formation constants in aqueous solution were determined at 16.5 °C (/ = 1.0 M NaN03) log 8i = 2.40, log /S2 = 5.40.54... 

In the presence of a range of metal ions, including Ag1, aziridine was found to dimerize to l-(2-aminoethyl)aziridine (equation 4). A bis complex of this ligand was isolated as the silver nitrate salt. Characteristic JHNMR spectral data are given in Table 10.55... 

Table 10 HNMR Spectral Data for l-(2-Aininoethyl)aziridine and its Silver(I) Complex (p.p.m.)ss...

Silver is often used as a halophile. In the context of six-electron species, the role of silver atoms in carbene, nitrene, and silylene transfer reactions, including aziridination, CH insertion, ring expansion, and silacyclopropanation, has been reviewed.9... 

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. 

Silver has three synthetically useful oxidation states Ag(I), Ag(II), and Ag(III).16 Free Ag(III) is very unstable, and most currently known Ag(III) complexes are stablized with electron-donating and/or sterically demanding ligands.17 It is known that Ag(I) can be oxidized to Ag(II) with strong oxidants such as persulfates. Nitrogen-based ligands such as pyridines are commonly used to stabilize high-valence metal ions.18 In 2003, He and coworkers utilized a pyridine-supported silver catalyst and reported the first silver-catalyzed aziridination of olefins.19... 

The aziridination catalysis was carried out in situ with mixed ligand and silver salt, which gives results similar to those obtained using the pure [Ag2(iBu3tpy)2(N03)] (N03) crystal. The (Bu3tpy ligand shows superior reactivity over other pyridine... 

The aziridination works for both aromatic and aliphatic olefins, including less active linear terminal olefins. Most reactions proceed in good yield at room temperature. The use of ci.v-stilbene at 0°C gives predominately cis aziridine product in about 90 10 cis trails ratio (Table 6.1). The conservation of cis structure suggests that a discrete silver nitrene intermediate is involved in the reaction path. Because of the unique disilver structure and unlikely formation of a silver(III) species, the authors suspect that a bridged nitrene intermediate between the two silver atoms may be responsible for this transformation in which each silver atom donates one electron to the nitrenoid. However, further research is necessary to prove this hypothesis and a fast radical reaction mechanism cannot be eliminated on the basis of current evidence. 

TABLE 6.1. Substrate Scope for Olefin Aziridination by Silver-Terpyridine Catalyst... 

Scheme 6.3. Silver-terpyridine catalyst can catalyze olefin aziridination using an in situ prepared nitrene.

After successful application of the silver catalyst shown in olefin aziridination (Section 6.1.1), He and coworkers showed that intramolecular amidation was possible with both hydrocarbon-tethered carbamates and sulfamate esters.24 They found that only the Bu3tpy silver complex could catalyze efficient intramolecular amidation, while other pyridine ligands gave either dramatically lower yields or complicated product mixtures. In an interesting control study, both copper and gold were also tested in this reaction. Both the copper and gold Bu tpy complexes can mediate olefin aziridination, but only silver can catalyze intramolecular C-H amidation, indicating that the silver catalyst forms a more reactive metal nitrene intermediate. 

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). 

J0rgensen and coworkers reported the preparation of A-tosyl aziridines 19-20 by the net carbene addition (via a diazo compound) to A-tosyl iminoesters with either copper or silver catalysts.13,14 It was noted that the copper catalysts were generally superior, although a catalyst derived from AgSbF6 and (R)-Tol-BINAP provided the corresponding aziridine 19 from 16 and trimethylsilyl diazomethane 17 (R = TMS) in excellent chemical yield with high levels of diastereoselectivity, but unfortunately the enantioselectivity was poor (Scheme 8.3). This success with trimethylsilyldiazo-... 

In 2001, Rai and co-workers (114) reported a silver-mediated aziridination of olefins in THF with Chloramine-T. In their case, aprotic solvents gave better yields versus protic solvents. Then, in 2003, Komatsu and co-workers (115) used similar conditions and found no reaction in THF (solvent) while they detected 70% conversion in CH2CI2. Silver nitrate (AgNOs) was required stoichiometrically in this transformation. Komatsu proposed a nitrene-radical mechanism based on the fact that the reaction shut down in the presence of oxygen. They designed a model reaction using 1,6-dienes, and as they expected, bicyclic pyrrolidines were isolated as products instead of aziridines. The role of silver in this reaction is not clear and most likely a free nitrene radical is released with the precipitation of silver(I) chloride (Fig. 18). 

A more promising olefin aziridination protocol with silver in catalytic amounts was developed in 2003 (112). A novel Ag2(l) compound was prepared from the reaction of 1 equiv of different silver(l) salts and 1 equiv of 4,4, 4"-tri-ferf-butyl-2,2 6,2 -terpyridine(f-Bu3tpy)inMe3CN at room temperature (Fig. 19). 

Figure 20. Olefin aziridination with Phi — NTs utilizing the silver catalyst shown in Figure 19.

---