Ring 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... 

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

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... 

Scheme 11.26 Chiral phosphoric acid catalyzed asymmetric ring opening of meso-aziridines.

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). 

Some uses of sugars in novel resolutions of enantiomers have been reported. The ferrocene derivative 225 could be isolated from reaction of a mixture of d,l-and meso-isomers of the corresponding ferrocene dicarbonyl chloride with methyl 4,6-0-benzylidene-a-D-glucopyranoside. This could then be converted by methanolysis into the enantiomerically-pure (R,/ )-ferrocene derivative 226. When the chiral pyridinium salt 227 was photolysed, equimolar amounts of the diastereomers 228 and 229 were obtained. Although no chirality transfer occurred, the two isomers could be separated after acetylation, and some further chemistry was performed on the fused aziridine ring of one of the dia-... 

Antilla et al. have described the phosphoric acid (234) that catalyzed, highly enantioselective ring-opening of meso A-acyl aziridines (235) with a series of functionalized aromatic thiols as nucleophiles (Scheme 65). ... 

Bicyclic meso-N-ary aziridines are ring-opened by anilines, giving trans-1,2-diamines in the presence of a Ti(0-Bu-t)4/(l )-BlN0L catalyst in CH2Cl2." The reactions occur in yields between 65 and 96% with 57-99% ee. Lower enantioselectivities were found when electron-withdrawing and orifto-substituents were on the aniline. Alkyl amines failed to react. ESI-MS experiments indicate that the catalyst contains two titanium and two linked bis-BINOL ligands. 

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