Meso-Aziridines

Ring opening of meso aziridines has also been investigated. Jacobsen and coworkers found that tridentate Schiff base Cr(III) complex 62 is efficient at inducing good levels of enantioselectivity in the azidolysis of aziridines 60 [Eq. (10.15)]. Alkyl-substituted aziridines were found to work best with the electron-deficient dinitrophenylmethyl providing optimal results ... 

Lastly, Antilla has disclosed a novel asymmetric desymmetrization of a wide range of aliphatic, aromatic, and heterocyclic meso-aziridines with TMS-N3 promoted by 11 and related 12 (Scheme 5.31) [56]. Uniquely, this is one of only several reports of electrophilic activation of nonimine substrates by a chiral phosphoric acid. Mechanistic studies suggest that silylation of 11 or 12 by displacement of azide generates the active catalytic species A. Consequently, the aziridine is activated through coordination of it carbonyl with chiral silane A to produce intermediate B. Nucleophilic ring opening by azide furnishes the desymmetrized product and regenerates 11 or 12. 

New ligands for a polymetallic Gd catalyst used in the ring opening of meso-aziridines with trimethylsilyl cyanide and 2,6-dimethylphenol have been developed.44... 

Tridentate Schiff base chromium(III) complexes were identified as the optimal catalysts for the enantioselective ring opening of meso-aziridines by TMSN3.51 Indeed, preliminary studies have shown that, although the (salen)chromium complexes catalyzed the reaction to some extent, they consistently led to low enantioselectivities (<14% ee). It was rationalized that the diminished reactivity and selectivity of the salen complexes with aziridines compared to epoxides was a result of the steric hindrance created by the /V-substituent of the coordinated aziridine. As expected, improved results were observed using tridentate ligands on the chromium center because they offer a less-hindered coordination environment (Figure 17.7).51... 

Shortly thereafter, Aiken and coworkers also reported that quinine (4) could be used as a catalyst (50mol%) to promote the methanolytic desymmetrization of the meso-epoxy anhydride 8a to give the lactone 9a in 57% yield and 76% ee (Scheme 11.6) [4]. Lowering the reaction temperature to 0 or —30 °C did not result in any increase in selectivity. meso-Aziridine anhydride 8b was also tested under similar reaction conditions, but a lower enantioselectivity (40% ee) was obtained (Scheme 11.6). 

Scheme 3.57 Enantioselective desymmetrization of meso aziridines with azide.

Antilla and co-workers reported desymmetrization of meso-aziridines catalyzed by the (R)-VAPOL-derived phosphoric acid 26 (Equation 10.50, Figure 10.22) [96],... 

Lattanzi and coworkers disclosed that eommercially available diphenyl prolinol lb served as an organoeatalyst in the desymmetrisation of meso-aziridines with thiols as pronueleophiles working at room temperature (Scheme 7.12). ... 

Scheme 7.12 Desyiiunetrisation of meso-aziridines with thiols catalysed by prolinol lb.

Based on this study, the authors extended the magnesium-catalysed reaction to the asymmetric [3 -I- 2]-cycloaddition of meso-aziridines 75 and C3-allq lindoles 127. In the presence of a quinine-magnesium complex and an achiral additive, the reaction proceeded smoothly to give pyrroloindolines 128 with high diatereo- and enantioselectivily (Scheme 3.39). 

Scheme 3.39 Cycloaddition of meso-aziridines and C3-allg lindoles.

Scheme 9.26 Enantioselective ring-opening reaction of meso-epoxides 53 and meso-aziridines 63 using a chiral niobium complex, reported by Kobayashi. "...

Wu et al. developed the quinine-catalysed enantioselective desymmetrisation of meso-aziridines with benzenethiols to afford aminosulfides in 30-87% yield and in 19-72% ee (Scheme 15.14). ... 

Scheme 15.14 Quinine catalysed desymmetrisation of meso-aziridines with benzenethiols.

Seki et al. also used zirconium and hafnium terf-butoxide-BINOL, 108-109, for the asymmetric meso-aziridine, 110, ring-opening reactions (reaction 7.22) with aniline derivatives. 111 [72]. Ihey reported high yield and acceptable enantioselectivity for these reactions with this catalytic system. 

Asymmetric Ring Opening of meso-Epoxides and meso-Aziridines... 

A similar catalyst system was also effective for asymmetric ring opening of meso-aziridines with TMSN3. The reaction provided a direct method for the synthesis of optically active 1,2-diamines. In the reaction, bimetallic lanthanide... 

Table 13.31 Catalytic enantioselective meso-aziridine ring-opening with TMSN3...

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. 

Thus, the asymmetric catalysis of cyanoethoxycarbonylation, cyanophosphoryla-tion, epoxidation of electron-deficient olefins, Michael reactions of malonates and (3-keto-esters, Strecker reaction of keto-imines, conjugate addition of cyanide to a, (3-unsaturated pyrrole amides, ring opening of meso aziridines with TMSCN and cyanosilylation of ketones (example shown below) have been successfully carried out using these complexes as asymmetric catalysts. 

Jacobsen et al. described the enantioselective ring opening of meso aziridines by TMSN3 catalysed by tridentate Schiff base chromium complexes. Using 133 as the catalyst, high yields of conversion to azidoamines and high levels of enantioselectivity are... 

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