Ring-opening azidation azide complex

The analogous chromium complex was used in the asymmetric ring opening of meso epoxides with trimethylsilyl azide [15] (Scheme 4). In this case a strong dependence on the anion of the ionic hquid was observed. Anions leading to hydrophobic ionic hquids, such as PFe" and SbFe", led to high... 

Method B was also used in the preparation of occluded (salen)Cr complexes. ligands Ih and li were prepared within the pores of Cr -exchanged EMT and Y zeolites, respectively [25]. These complexes were tested as catalysts in the ring opening of meso-epoxides with trimethylsilyl azide (Scheme 4). The occluded complexes showed a dramatic decrease in catalytic... 

A rather complex microwave-assisted ring-opening of chiral difluorinated epoxy-cyclooctenones has been studied by Percy and coworkers (Scheme 6.131) [265]. The epoxide resisted conventional hydrolysis, but reacted smoothly in basic aqueous media (ammonia or N-methylimidazole) under microwave irradiation at 100 °C for 10 min to afford unique hemiacetals and hemiaminals in good yields. Other nitrogen nucleophiles, such as sodium azide or imidazole, failed to trigger the reaction. The reaction with sodium hydroxide led to much poorer conversion of the starting material. 

Scheme 7. Catalytic enantioselective ring opening of cyclohexene oxide with trimethylsilyl azide using the chiral Cr(lll) - salen complex 1 [26],...

Because the two regioisomeric products 8a and 8b have almost the same molecular dimensions, it is difficult to discriminate between the two isomers with the geometric constraints imposed by the zeolite pores. Considering that calcium ions are apt to form mainly five-membered chelate complexes with polyhydroxy compounds (Fig. 4b) 32,33) and that calcium zeolites have also been employed as sorbents in carbohydrate separations (ii), it is possible to speculate that in the CaY-supported NaN3 system the epoxy alcohol first forms a coordinated structure around a calcium ion, as shown in Fig. 4a, followed by ring opening with an azide anion at the C-3 position of the epoxy alcohol, giving a stable, five-membered chelate complex with the calcium ion. 

MTO [methyltrioxorhenium(VII), cf. Chapter 3.3.13] can be used as a catalyst for the epoxidation of olefins with urea hydroperoxide in [EMIMJBF4 [19]. The activity is reported to be comparable with the reaction in organic solvents but side reactions are suppressed. The use of an ionic liquid as a co-solvent in CH2CI2 for the enantioselective Mn-salen complex-catalyzed epoxidation of olefins with Na(OCl) was reported to result in enhanced reaction rates at no loss of enantioselectivity [20]. Cr-salen complexes can further be used for the asymmetric kinetic resolution of epoxides by ring-opening with azide [21]. 

Ytterbium triisopropoxide (10-20 mol%) catalyzes the ring-opening of epoxides with trimethylsilyl azide at ambient temperature to yield vicinal azide alcohols (eq. (25)) [191]. Complexation/chelation interactivity of functionalized substrates affects the regioselectivity in the product. 

The asymmetric ring-opening (ARO) reachon of epoxides by trimethylsilyl azide (TMSN3) catalyzed by the chiral Cr(salen) complex has been recognized as an attrachve approach to the synthesis of ophcally enriched P-amino alcohols [47]. In parhcular, the chiral Cr(salen) catalyst 34 exhibits remarkable stabihty under catalyhc conditions, which allows its repeated recycling. Jacobsen and co workers reported that this reachon could be run without solvents, and that the catalyst could be recycled several hmes without any loss of activity or enanhoselectivity... 

Highly enantioselective catalytic desymmetrization of meso-epoxides through nucleophihc ring opening was first effectively demonstrated by Nugent, who found that a zirconium trialkanolamine complex catalyzed the addition of azi-dosilanes to meso-epoxides (Scheme 1) [3]. Azide has been the most widely explored nitrogen nucleophile [4,5,6,7], in part due to its utiHty as an amine sur-... 

Scheme 8.11. (a) Group-selective ring-opening of meso epoxides by nucleophiles leads to enantioselective syntheses of 1,2-difunctionalized compounds, (b) Azido alcohol synthesis from epoxides and trimethylsilyl azide as catalyzed by (salen)CrCl complexes (see Scheme 8.6a for general structures of salen catalysts)... 

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