Meso-Epoxide ring opening chiral complex

Complexes of other metals such as gallium, indium, lead, and antimony have also been used as Lewis acids. Catalytic enantioselective meso-epoxide ring-opening reactions using a chiral gallium(III) catalyst (Ga-Li-linked-BINOL) have been reported (Scheme 84).348 The chemical yields are much improved by linking two BINOL units. 

Scheme 9.23 Enantioselective ring-opening reaction of meso-epoxides using a chiral vanadium complex, reported by Sun and Zhu. ...

Ogawa, C. Azoulay, S. Kobayashi, S. (2005) Bismuth triflate-chiral bipyridine complex catalyzed asymmetric ring opening reactions of meso-epoxide in water., Heterocycles, 66 201-206. 

Bruns and Haufe have described the first examples of a transition metal complex mediated asymmetric ring opening (ARO) of both meso- and racemic epoxides via formal hydro-fluorination [23]. Initial attempts with chiral Euln complexes led to very low asymmetric induction. Opening of cyclohexene oxide 30 with potassium hydrogendifluoride in the presence of 18-crown-6 and a stoichiometric amount of Jacobsens chiral chromium salen complex 29 [24a] finally yielded two products 31 and 32 in a 89 11 ratio and 92% combined yield, the desired product 31 being formed with 55% ee. Limiting 29 to a catalytic amount of 10 mol% led to an increase in the ratio of 31, however, with the enantiomeric excess dropping to 11% (Scheme 5). 

A tridentate Schiff base Cr(III) complex derived from l-amino-2-indanol catalyzes the enantioselective ring opening of meso A-2,4-dinitrobenzyl aziridines with TMSN3 (Sch. 18) [96]. The chiral (salen)metal complexes, used in the enantioselective ring opening of epoxides, were found to be much less effective (for Cr) or inactive (for Co). 

Along with the development of chiral Lewis acid catalysts, a chiral trialkanolamine (42) has been used to prepare the catalyst (43) (Eq. 19). By use of this zirconium complex as a catalyst, enantioselective addition of the azide to meso epoxides was achieved [20a]. Thus, the oxirane ring was opened by /-PrMe2SiN3 to give the adduct (44) with high enantioselectivity (Eq. 20). In another example, a diamide ligand (45), which behaves as a tetradentate ligand, was used to achieve a similar reaction (Eqs 21 and 22) [20b]. 

Catalysts formed from Me2Zn and binaphthol 3.7 (R = H) have been used for asymmetric ene-reactions [778]. Enantioselective ring opening of meso-epoxides by n-BuSH is catalyzed by a potassium tartrate/ZnCl2 complex [559, 778, 805]. Mukaiyama and coworkers have shown that reaction of Et2Zn with chiral sulfamides 3.15 (R = PI1CH2, r-Pr) generates Lewis acids [806] that catalyze asymmetric reactions of aldehydes with ketene acetals. 

Reversal of enantioselectivity in Cu-chiral bipyridine-catalyzed asymmetric ring-opening reactions of meso-epoxides with indole and aniline derivatives is observed compared to Sc-chiral bipyridine-catalyzed reactions, where the same chiral ligand is used (Scheme 32). " It was revealed from X-ray crystal structural analysis that a square pyramidal structure for the Cu° complex and a pentagonal bipyramidal... 

The chiral (salen)Co catalysts have also been applied to cyclization reaction and preparation of intermediates for natural product synthesis [85]. In addition, chiral (salen)Ru catalysts proved to be effective for kinetic resolution of racemic epoxides [86]. Tridentate Schiff base Cr(III) complex (201) derived l-amino-2-indanol acts as a potent catalyst for asymmetric ring-opening reaction of meso-aziridines with trimethylsilyl azide (Scheme 16.60) [87]. The aziridine (200) was readily converted at —30 °C to the corresponding amino-azide in 95% yield with 94% ee. 

Similar to the ring-opening reaction of epoxides, Cr(III) complexes (2) were reported to catalyze the enantioselective ringopening reaction of meso-azindines in 80-95% yields with 83-94% enantiomeric excess (eq 17). Other catalyst systems, including rare earth alkoxides with chiral ligands, have also proved efficient in this transformation. ... 

In 2010, a cooperative dual-catalyst system was reported to promote the highly enantioselective fluoride ring opening of various meso epoxides having alkene, ester, and protected amine functionalities. The reactions were conducted with a chiral (Salen)cobalt complex, (-)-tetramisole, benzoyl fluoride as a latent source of fluoride in the presence of HFIP. The efficient catalytic enantioselective reaction is explained by the generation of a (Salen)Co(III) fluoride under the cocatalytic conditions that occurred in good yields with up to 95% ee (Scheme 44.32). Racemic terminal epoxides, such as styrene oxide, were also studied, but they almost exclusively lead to the fluorine in the primary position therefore, the fluorine atom was not introduced on a stereogenic center. 

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