Oxaziridiniums

Yang/Shi (1996), Denmark (1997) chiral dioxiranes and chiral oxaziridinium ions H-Bond Cataiysis... 

Asymmetric epoxidation of olefins is an effective approach for the synthesis of enan-tiomerically enriched epoxides. A variety of efficient methods have been developed [1, 2], including Sharpless epoxidation of allylic alcohols [3, 4], metal-catalyzed epoxidation of unfunctionalized olefins [5-10], and nucleophilic epoxidation of electron-deficient olefins [11-14], Dioxiranes and oxazirdinium salts have been proven to be effective oxidation reagents [15-21], Chiral dioxiranes [22-28] and oxaziridinium salts [19] generated in situ with Oxone from ketones and iminium salts, respectively, have been extensively investigated in numerous laboratories and have been shown to be useful toward the asymmetric epoxidation of alkenes. In these epoxidation reactions, only a catalytic amount of ketone or iminium salt is required since they are regenerated upon epoxidation of alkenes (Scheme 1). 

Isolated or in situ generated oxaziridinium salt 67 efficiently epoxidizes various olefins in good yields [133-135],... 

In 1993, Bohe and coworkers synthesized enantiomerically pure oxaziridinium salt 71 by methylation with Meerwein s salt and oxidation with mCPBA from dihydr-oisoquinoline 68 (Scheme 13) [136,137], Alternatively, 71 could also be produced by switching the reaction order. Epoxidations were carried out with either... 

Epoxidation reactions have also been performed by the oxidizing reagents, generated in situ by reacting Oxone and an iminium salt, a modified iminium salt, a oxaziridinium... 

The epoxidation of olefins catalyzed by iminium salts and amines (or ammonium salts) is emerging as a new technique for the functionalization of simple aUcenes. These catalysts have relatively simple structures and hence are easily produced at scale they offer potential as green oxidation catalysts. These organic salts are effective oxygen transfer reagents towards electron-rich unfunctionalized olefins. For the iminium salt systems oxone oxidizes an iminium salt to the oxaziridi-nium intermediate, which then transfers oxygen to the olefin and as oxone reacts readily with iminium ions to regenerate the oxaziridinium species catalyti-cally, efficient oxidation is possible. 

Recently, oxaziridinium salts derived in situ from chiral iminium salts (1 and 2) and Oxone were found to catalyze epoxidation with moderate-to-good enantioselectivity (up to 73% ee) (Scheme 6B.4) [6], Although the substrates are limited to conjugated olefins, this reaction has an advantage in being catalytic with respect to chiral iminium salts. 

Imines and iminiums salts are oxidized by peracids into oxazidines and oxaziridinium ions, which are good reagents for oxidation of sulfides. The imine or iminium salt can, in principle,... 

The reaction mechanism for A-oxidation by performic acid has been studied by AMI calculation methods.174 The iminium salt A-mcthyl-3,4-dihydroisoquinolinium p-toluenesulfonate has been used to catalyse the oxidation of the azo dye calmagite by peracetic acid. The mechanism at pH 10 involves peracid oxidation of the quinolinium ion to form an oxaziridinium salt, which then acts as an oxygen transfer agent for oxidation of cahnagite.175 The presence of lithium salts affects the course of the reaction determining the formation of benzoyl peroxide and benzoic acid as final products in the oxidation of benzaldehyde by perbenzoic acid.176,177... 

C chiral iminium cation, D chiral oxaziridinium cation... 

As shown in cycle (b) in Scheme 10.1, the iminium-oxaziridinium pair can also effect catalytic asymmetric epoxidation of alkenes. Early work in this field by Bohe et al. included investigation of the norephedrine-derived oxaziridinium salt 34 (33% ee in the catalytic epoxidation of traws-stilbene [41] ee up to 61% was achieved when 34 was employed stoichiometrically [42]), or the L-proline-derived material 35 (39% ee in the epoxidation of trans-3-phenyl-2-propenol [43]). Rapid... 

As discussed in Section 10.1, asymmetric epoxidation of C=C double bonds usually requires electrophilic oxygen donors such as dioxiranes or oxaziridinium ions. The oxidants typically used for enone epoxidation are, on the other hand, nucleophilic in nature. A prominent example is the well-known Weitz-Scheffer epoxidation using alkaline hydrogen peroxide or hydroperoxides in the presence of base. Asymmetric epoxidation of enones and enoates has been achieved both with metal-containing catalysts and with metal-free systems [52-55]. In the (metal-based) approaches of Enders [56, 57], Jackson [58, 59], and Shibasaki [60, 61] enantiomeric excesses > 90% have been achieved for a variety of substrate classes. In this field, however, the same is also true for metal-free catalysts. Chiral dioxiranes will be discussed in Section 10.2.1, peptide catalysts in Section 10.2.2, and phase-transfer catalysts in Section 10.2.3. 

Closely related to the ketone/Oxone epoxidation system is the use of iminium salts as promoters. As isolated oxaziridinium salts are known to effect alkene epoxidation [38], these are presumed to be the reactive intermediates in this catalytic system (see Scheme 12.1 X = NR.2+). The first asymmetric example used the dihydroisoquinolinium-based system 15 (Fig. 12.7), which afforded 33% ee for the epoxidation of F-stilbene [39]. 

Calculations [46] and studies of intramolecular oxaziridinium epoxidations [47] suggest that, like their dioxirane counterparts, these epoxidation processes proceed via spiro-transition states. However, the iminium epoxidations are generally more substrate-specific than those using dioxiranes, and models to explain the observed trends in stereocontrol have proved more difficult to construct. One complication is the possibility of formation of diastereomeric oxaziridinium salts from most of the iminium catalysts. Houk has rationalized computationally the observed enantioselectivity with Aggarwal s catalyst 16 [46]. The results of a recent study by Breslow suggest that hydrophobic interactions are important in these processes [48], and aromatic-aromatic interactions between catalyst and substrate may also play a role. 

A particularly complicated reaction is reported to occur43,47 with a steroidal enamine. Formation of the products can be explained in terms of the intermediacy of an oxaziridinium salt. 

A number of new oxaziridinium epoxidation reagents have been reported. A new axially chiral epoxidation catalyst 4 has been reported <070BC501>. These catalysts, as are others, are converted to an oxaziridinium with Oxone, which then epoxidizes the olefin. This study examined several chiral groups on the nitrogen as well as both atropisomers. The (S,F)-isomer 4 provided the (1R,2R) epoxide with moderate enantioselectivity and 82% conversion. The (.V,A/)-isomcr of 4 provided the (lS,2S)-epoxide in slightly lower enantiomeric excess (76%) and lower conversion as well. 

The first use of an enantiomerically pure oxaziridinium salt to catalyze asymmetric epoxidation (trans stilbene oxide produced with 33% ee using 61 (Figure 13)) was reported by Lusinchi and co-workers in 1993 <1993TL7271> Subsequently, it was reported that phenylcyclohexene is converted to the corresponding epoxide with just 5% ee using stoichiometric quantities of 61 <1999T141>. 

This remains a developing area for the synthesis of chiral oxiranes and has attracted interest from several research groups. As with the use of dioxiranes (above), it is not necessary to form the reactive oxaziridinium salt rather, the epoxidation reaction can be mediated by the corresponding iminium salt and Oxone. 

Brief reports have been made about two unusual oxaziridine derivatives. A quaternary oxaziridinium salt apparently reacts with nucleophiles at oxygen in any case, this salt suffered deoxygenation by some process. Finally, the per-fluorinated oxaziridine 59, undergoes attack exclusively at nitrogen to produce amide derivatives ... 

Bohe, L., Lusinchi, M., Lusinchi, X. Oxygen atom transfer from a chiral oxaziridinium salt. Asymmetric epoxidation of unfunctionalized olefins. Tetrahedron 1999, 55,141-154. 

The formation and the oxidative properties of an oxaziridinium salt have been reported. With methyl fluorosulphonate the oxaziridine (30) led to the oxaziridinium salt (36) which was stable as a crystalline product at room temperature, but unstable in solution, giving (37), also obtained from (38) with methyl fluorosulphonate. NaBHt reduction of (36) led to (35). Peroxidic reactivity of (36) was shown by oxygen transfer from (36) to the imine (38) with formation of the nitrone (39) (Scheme 5). 

Enantiomerically pure oxaziridinium salt (187) was prepared as shown in Scheme (34) <93TL727i>. Oxidation of (185) with MCPBA in MeOH gave a 9 1 mixture of oxaziridine diastereoisomers of which (186) was the major one as determined by x-ray diffraction. 

In the book, the section on homogeneous catalysis covers soft Pt(II) Lewis acid catalysts, methyltrioxorhenium, polyoxometallates, oxaziridinium salts, and N-hydroxyphthalimide. The section on heterogeneous catalysis describes supported silver and gold catalysts, as well as heterogenized Ti catalysts, and polymer-supported metal complexes. The section on phase-transfer catalysis describes several new approaches to the utilization of polyoxometallates. The section on biomimetic catalysis covers nonheme Fe catalysts and a theoretical description of the mechanism on porphyrins. 

In the case of iminium salts [38,204], the group of Page has shown that the intermediate is an oxaziridinium salt generated by a single oxygen atom transfer from a TPPP as determined by H NMR [510] (see Chapter 5). Calculations of the transition state indicate that the transition state is synchronous [511]. 

TiO2/SiO2, Ag-Cu alloys, soft Pt(II) Lewis acids, MTO, oxaziridinium salts, NHPI, polyoxometallates, phase-transfer systems, polymer-supported complexes, calixarene complexes, and biologically based systems. 

A. Picot, P. Millet, X. Lusinchi, Formation d un sel d oxaziridinium quaternaire par methylation d un oxaziranne—mise en evidence de ses proprieties oxydantes. Tetrahedron Lett. 17 (1976) 1573. 

M. R. Biscoe, R. Breslow, Oxaziridinium salts as hydrophobic epoxidation reagents Remarkable hvdrophobically-directed selectivity in olefin epoxidation, ]. Am. Chem. Soc. 127 (2005) 10812. 

Oxaziridinium Salt-Mediated Catalytic Asymmetric Epoxidation Philip C. Bulman Page and Benjamin R. Buckley ... 

Key Words Iminium, Oxaziridinium, Oxaziridine, Ketiminium, Oxone, Tetra-phenylphosphonium monoperoxysulphate, Isopinocampheylamine, Alkene, Epoxide, Enantiomeric excess. Asymmetric synthesis, Organocatalysis, 2-(2-Bromoethyl)benzaldehyde, Levcromakalim, Dihydroisoquinolinium, Spiro, Azepinium, Benzopyran, Dielectric constant, Binol. 2008 Elsevier B.v. 

Oxaziridinium salts are the quartemized analogues of oxaziridines, and as a result of being more electrophilic, transfer oxygen more efficiently to nucleophilic substrates. The first oxaziridinium salt, described by Lusinchi in 1976 [1-3], was based on a steroidal pyrrolinic skeleton. Through peracid oxidation of the steroidal imine and quatemization using methylfluorosulphonate, it was shown that an oxaziridinium species could be formed (Scheme 5.1). This new species was rather unstable, and upon decomposition reverted to an iminium salt, which could be directly prepared from the imine. However, it was not until some 11 years later that the potential of this type of system to transfer oxygen was realized [4,5]. 

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