Aziridination alkenes,

Although an efficient reaction, the Rees-Atkinson aziridination method suffers from two drawbacks the necessity for an N-phthalimido or N-quinazolinonyl substituent and the use of a highly toxic oxidant. Thus, recent efforts (especially in these green times) have focussed upon more benign methods for generation of the key nitrenoids. Yudin demonstrated the power of electrochemistry with a novel method that removes the need for an added metal oxidant, demonstrating an unusually and impressively broad substrate tolerance compared to many alkene aziridination reactions (Scheme 4.14) [10]. 

Since the mid-1990s, synthetic attention has been directed more towards the use of metal-stabilized nitrenes as synthetic effectors of alkene aziridination. In 1969 it was reported that Cu(i) salts were capable of mediating alkene aziridination when treated with tosyl azide, but the method was limited in scope and was not adopted as a general method for the synthesis of aziridines [12]. Metaloporphyrins [13] were shown to be catalysts for the aziridination of alkenes in the presence of the nitrene precursor N-tosyliminophenyliodinane [14] in the early 1980s, but the reaction did... 

The metal catalyst is not absolutely required for the aziridination reaction, and other positive nitrogen sources may also be used. After some years of optimization of the reactions of alkenes with positive nitrogen sources in the presence of bromine equivalents, Sharpless et al. reported the utility of chloramine-T in alkene aziridinations [24]. Electron-rich or electron-neutral alkenes react with the anhydrous chloramines and phenyltrimethylammonium tribromide in acetonitrile at ambient temperature, with allylic alcohols being particularly good substrates for the reaction (Schemes 4.18 and 4.19). 

Another conceptually unique approach in alkene aziridination has come from Johnston s labs. These workers shrewdly identified organic azides as nitrene equivalents when these compounds are in the amide anion/diazonium resonance form. Thus, when a range of azides were treated with triflic acid and methyl vinyl ketone at 0 °C, the corresponding aziridines were obtained, in synthetically useful yields. In the absence of the Bronsted acid catalyst, cycloaddition is observed, producing triazolines. The method may also be adapted, through the use of unsaturated imi-des as substrates, to give anti-aminooxazolidinones (Scheme 4.25) [32]. 

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). 

Addition to alkenes. Aziridines add to a wide variety of alkenes, -with or without the use of a basic catalyst. The mode ol... 

Although it is not the intent of this review to provide a comprehensive discussion of metal-mediated alkene aziridination, the mechanistic parallels between the... 

Cu-promoted reaction and Cu-catalyzed C-H amination merit some analysis of the former process. From the standpoint of reaction utility, the work of Dauban and Dodd [31, 32] and, more recently, Lebel [33] has advanced Cu-catalyzed alkene aziridination to a state-of-the-art synthetic method. In particular, Dauban and Dodd have shown that pre-formation of sulfonyliodoimines is not required and that such species can be generated in situ from the corresponding sulfonamide and Phl=0. These conditions greatly expand the number and type of sulfonamide derivatives that will engage in olefin aziridination processes. 

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... 

MPEG with Ru-porphyrin- complexes =0.06-0.14" (via H-NMR) CH2CI2" Alkene epoxidation Alkene cycloprop an ation Cyclisation of a-diazo-carbonyl compounds Alkene aziridination Precipitation (r) Precipitation (diethyl ether) Precipitation (diethyl ether) [141]... 

ILL. Zhen and K. R. Conser, Asymmetric Alkene Aziridination and Readily Available Chiral Diimine-Based Catalysts, J. Am. Chem. Soc., 115(1993)5326. 

A theoretical approach to understanding the nature of intramolecular allylic C-H amination versus alkene aziridination reaction pathways was presented, where the reaction of 4-pentenylsulfamate 192 was investigated using three different dirhodium catalysts (14JOC9799). The 1/A ratios of products for the three catalysts range from 1 4 to 1 1 to >20 1 and the theoretical observations presented are consistent with experimental results. 

In fact, the stepwise nature of [Ru2(hp)4Cl] catalyzed reaction accounts for preferential allylic C-H bond functionalization over alkene aziridination which is the favored product when dirhodium catalysts are used. The presence of a discrete diradical intermediate further assists the selectivity for allylic C-H amination using a diruthenium catalyst. 

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