Induction chiral

Slgmatropic rearrangement with H-transler and C-C txjrxJ formation (inter or intramolecular) and chiral induction... 

Chiral boron(III) Lewis acid catalysts have also been used for enantioselective cycloaddition reactions of carbonyl compounds [17]. The chiral acyloxylborane catalysts 9a-9d, which are also efficient catalysts for asymmetric Diels-Alder reactions [17, 18], can also catalyze highly enantioselective cycloaddition reactions of aldehydes with activated dienes. The arylboron catalysts 9b-9c which are air- and moisture-stable have been shown by Yamamoto et al. to induce excellent chiral induction in the cycloaddition reaction between, e.g., benzaldehyde and Danishefsky s dienes such as 2b with up to 95% yield and 97% ee of the cycloaddition product CIS-3b (Scheme 4.9) [17]. 

The assumed transition state for this reaction is shown in Scheme 5.5. The two bulky t-butoxy groups are expected to locate at the two apical positions. One of the 3,3 -phenyl groups would effectively shield one face of an imine, and consequently, a diene attacks from the opposite side. Judging from this model, similar selectivities were expected in the Mannich-type reactions of imines with silyl eno-lates. Actually, when ligand 10 was used in the reaction of imine la with S-ethyl-thio-l-trimethylsiloxyethene, the corresponding / -amino thioester was obtained in 84% ee (Scheme 5.6). As expected, the sense of the chiral induction in this case was the reverse of that observed when using catalyst 6 [12, 25]. 

The complexation procedure included addition of an equimolar amount of R,R-DBFOX/Ph to a suspension of a metal salt in dichloromethane. A clear solution resulted after stirring for a few hours at room temperature, indicating that formation of the complex was complete. The resulting solution containing the catalyst complex was used to promote asymmetric Diels-Alder reactions between cyclopen-tadiene and 3-acryloyl-2-oxazolidinone. Both the catalytic activity of the catalysts and levels of chirality induction were evaluated on the basis of the enantio-selectivities observed for the endo cycloadduct. 

In total, 29 pbospborus-containing chiral ligands of various structures were screened under tlie optimized ji-selective condhions, but most of tlieni gave little or no chiral induction. Hie four ligands 38a-d, all derived from i-)-TADDOL, depicted in Fig. 8.4 gave tts in excess of 3096 in the reaction between etliyl magnesium bromide and cinnamyl cliloride. 

Table 1.11 based 80. Chiral induction through the use of the valine ... 

Pitfalls are encountered when allowing chiral nonracemic aldehydes to react with chiral, but racemic, reagents having a stereogenic center at the metal-bearing carbon atom, since its chiral induction usually overrides that of the substrate leading to mixtures of two diastereomers in essentially equal amounts26,27 (Sections D.1.3.3.1.4.1., D.1.3.3.3.3.3.2. and D.1.3.3.3.8.2.3.1.). 

The reaction of allyl-9-BBN with the chiral imine 1 gives the Cram isomer 2 predominantly5,6. The 1,3-asymmetric induction is developed by a transition state 3, in which the 1,2-axial-equa-torial interaction between R and the ligand (L) plays an important role in the high chiral induction. 

From this discussion it is clear, that, independently of their redox properties, suitably modified electrodes offer themselves for the introduction of diastereo- or enantioselectivity into electrochemistry. Early reports of chiral inductions at modified electrodes include reactions at graphite and SnO surfaces derivatized with monolayers of (S)-(—)-phenylalanine. Asymmetric inductions at the chiral graphite electrode could, however, not be verified in other laboratories even after great efforts... 

Subsequently, a number of reactions at poly-L-valine coated carbon electrodes 237-243) gj.g reported to yield optically active products. Reductions, e.g. of citraconic acid or l,l-dibromo-2,2-diphenylcyclopropane as well as the oxidation of aryl-alkyl sulfides proceeded with chiral induction at such electrodes... 

Recent studies on isolated BVMOs using Rh-complexes as NADPH substitutes for facile cofactor recycling suggested a pivotal role of the native cofactor to generate the proper environment within chiral induction in sulfoxidation reactions. While biooxidation was still observed in the presence of the metal complex, stereoselectivity of the enzyme was lost almost completely [202]. 

Uechi, 1., Katsuki, A., Dunin-Barkovskiy, L. and Tanimoto, Y. (2004) 3D-morphological chirality induction in zinc silicate membrane tube using a high magnetic field. J. Phys. Chem. B, 108,... 

Chiral and achiral Jacobsen s catalysts exhibit similar diatereomeric excesses during the diastereoselective epoxidation of R-(+)-limonene using in situ prepared oxidizing agents. Therefore, the chiral center of the substrate appears to govern the chiral induction. In contrast, the chirality of the Jacobsen s catalyst appears to be responsible for the chiral induction when commercially available oxidants were used. 

Catalytic enantioselective synthesis of vzc-diols is a challenging issue. Chiral induction using chiral ligands is difficult to achieve. The moderately enantioselective pinacolization of benzaldehyde is demonstrated to be performed by the chiral titanocene catalysts 15 and 16 [42,43]. 

In the case of 67g which has a chiral alkyl group, optically pure 67g was included at the complexation process with 2a, and (—)-67g of 100% ee [[a]D —66.8° (c 0.22, CHClj)] was obtained. Irradiation of the 1 1 inclusion complex of 2a and (—)-67g of 100% ee gave 69g of 100% ee which has three optically pure chiral centers 40). This is not the result of a chiral induction by the optically active alkyl group, since irradiation of 67g of 100% ee in benzene gave 69g of only 12% de (diastereomeric excess). 

Recently, with the aid of computational techniques, enough understanding of the mechanism of chiral induction at surfaces has been developed that new chiral modifiers have been synthesized.40,41 Their success at chiral induction confirms the theoretical understanding upon which they were created. 

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