Aziridination, of olefins

Bromine-catalyzed Aziridination of Olefins with Chloramines... 

Table 12.3 Aziridination of olefins with TsNCINa/PTAB. a ...

Non-metal catalyzed aziridinations have also been reported. These methods are often more broadly applicable than the metal-catalyzed methods. The use of iV-methylpyrrolidine-2-one hydrotribromide (MPHT) and chloramine-T is an effective route for the synthesis of iV-tosyl aziridines <06MI16>. The aziridination of olefins using i-BuOI and sulfonamides appears to be a general method for aziridination <06CC3337>. The i-BuOI is prepared in situ from t-BuOCl and Nal. This is a broadly applicable method in that a wide variety of sulfonamides (tosyl, nosyl, SES) can be used with roughly equivalent yields. 

Aziridines, the smallest heterocycles, are an important class of compounds in organic chemistry. Interesting access [8] to aziridines by using Bromamine-T as a source of nitrogen in the copper (Il)-catalyzed aziridination of olefins in MeCN was recently reported. Application of microwaves has resulted in enhanced yields for aziridines [9] with short reaction times (Scheme 8.5). 

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... 

Bis(oxazoline)-copper complexes 158 have been used by Evans group as chiral catalysts for the enantioselective aziridination of olefins.116 Aryl-substituted olefins have been found to be particularly suitable substrates, which can be efficiently converted to A-tosylaziridines with ee of up to 97% (R = Ph... 

TABLE 4-23. Bromide Catalyzed Aziridination of Olefins with TsNCINa s==h TsNCINa. 10 mol% PTAB... 

Benzylidene derivatives of the enantiomers of 1,2-diaminocyclohexane are also excellent ligands for the Cu(I)-catalyzcd asymmetric aziridination of olefins with 64, but the enantioselectivities using acyclic alkenes were about the same as those using ligand (S, S )-6658. When (5, 5 )-bis-(2,4-dichlorobenzylidenediamino)cyclohexane [(S,S)-67] was employed with C.u(I) triflate, 6-cyano-2,2-dimethylchromene (68) was converted to (R,R) 69 in a 75% yield with an ee greater than 98%58. 

Apart from Cu(I) and Cu(II) catalysts, Fe(III)- and Mn(III)-derived porphyrin catalysts have also been used in the aziridination of olefins with 19. However, the latter approach suffers from several drawbacks. For in-... 

Copper complexes catalyze formally related aziridination of olefins with ]7V-(p-toluenesulfonyl)imino]phenyliodinane, a nitrene precursor (219b). As exemplified in Scheme 98, catalysts formed from Cu(I) tri-flate and optically active bis(oxazolines) effect enantioselective reaction of styrene (Scheme 98) (218b, 219a). 

Another recent innovation is the use of [(2-trimethylsilylethanesulfonylimi-no)iodo]benzene (43), Phi = NSes, for Cu(I) and Cu(II)-catalyzed aziridinations of olefinic compounds (Scheme 67) [180]. This reagent was deliberately designed for facile deprotection of the AT-sulfonylaziridines and products derived therefrom. For example, several N-(Ses)aziridines were converted to aziridines with TASF [(Me2N)3S+ SiMe3F2] in MeCN or DMF at room temperature. 

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... 

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... 

Nitrogen Atom Transfer Aziridination of Olefins with Other Nitrogen Sources... 

Pellacani and co-workers reported the aziridination of olefins by using car-boethoxynitrene generated from ethyl A-[(4-nitrophenyl)sulfonyl]oxycarbamate (also known as 4-nitrobenzenesulfonoxyurethane or the Lwowski reagent) and bases, such as triethylamine, K2C03 or CaO [13] (Scheme 5). In some cases, allylic animation products were obtained in low yields. 

More recently, chloramine-T (CT) was found to be an efficient nitrogen source for the aziridination of olefins by our group [7a], Among the transition metals, copper(I) chloride was the most suitable catalyst for the aziridination of olefins with CT. For example, frans-P-methylstyrene was successfully aziridinated with CT at 25 °C in acetonitrile in the presence of a catalytic amount of CuCl (Scheme 6). Other olefins have also been aziridinated by the reaction, whose products are shown in Scheme 7. 

Following the publication of our method, similar procedures were reported by two other groups. One involves the use of bromamine-T as a nitrogen source instead of CT for the aziridination of olefins [14] the other is the copper(I) triflate-catalyzed aziridination and allylic amination with CT trihydrate [7d] (Schemes 8 and 9). 

Scheme 11. Iodine-catalyzed aziridination of olefins with CT.

Asymmetric Aziridination of Olefins with Chiral Nitridomanganese Complexes... 

In order to increase the yield and/or the enantioselectivity of the reaction, the reaction temperature and additives were examined. Although aziridination was found to proceed smoothly at 0 °C, the product was not obtained at lower temperatures. Katsuki and co-workers have reported that pyridine /V-oxide is an effective additive for the asymmetric epoxidation catalyzed by salen-manganese(IH) complexes [24], and applied these findings to the asymmetric aziridination of olefins with Phi = NTs [9f]. Thus, the addition of pyridine /V-oxide at 0°C improved the enantioselectivity and allowed the reaction to proceed even at -20 °C (Table 6.1). Other additives, such as 4-phenylpyridine IV-oxide, 4-methylmorphorine N-oxide and 1-methylimidazole were used in the place of pyridine JV-oxide, but positive effects were not observed. 

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). 

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