Conformation conformationally flexible catalysts

Sohtome, Nagasawa, and eo-workers developed a new strategy for asymmetric organocatalysis by introducing conformationally flexible catalysts. ... 

Along another line of work in our group (S,S)-l,4-bis(dimethylamino)-2,3-dimethoxy butane (DDB), which had been used as cosolvent in asymmetric synthesis [113], was tested as a core moiety for a dendritic amine catalyst. The conformationally flexible DDB-core, which has been synthesized in five steps from diethyl tartrate was coupled with different branches to give dendritically expanded diamines 90-92 (molecular weight 3800 Da) [114] (Fig. 32). 

DM-BIPHEP, a conformationally flexible diphosphine, exists as an equilibrium mixture of the R and S isomers (Fig. 32.50) [141]. Addition of an equimolar amount of (S,S)-DPEN to RuCl2(dm-biphep)(dmf) forms a 3 1 mixture of (S)-DM-BIPHEP/(S,S)-DPEN-RuC12 complex and the R,SS diastereoisomer. The major S,SS complex gives a more active and enantioselective catalyst for the hydro-... 

Maruoka and co-workers developed an elegant solution by creating phase-transfer-catalysts of type 30 [36], For example, the C2-symmetric N-spiro organocatalyst (S,S)-30, which contains a conformationally flexible biphenyl subunit, efficiently catalyzed the alkylation of glycinate 18 with benzyl bromide, with formation of the product (R)-20b in 95% yield and with 92% ee (Scheme 3.10) [36],... 

Three-dimensional alignment incorporates the problem of conformational flexibility. One possibility to address the problem is to pre-generate conformations (like in DISCO or Catalyst ), which makes the actual alignment less time-consuming, but the user has to ensure that all relevant conformations are included. On the other hand, there are in-process approaches that perform the pattern identification and conformational search simultaneously. 

Introduction. Chiral phosphines have played a crucial role in the development of catalytic asymmetric reactions. In particular, the coordination of a resolved, chiral phosphine to a transition metal center has been exploited to produce highly enan-tioselective catalysts for a variety of catalytic processes. Although there are many chiral monodentate and bidentate chiral phosphines available, (l/ ,2S, 4/ ,5S)-2,5-dimethyl-7-phenyl-7-phosphabicyclo[2.2.1 jheptane provides the advantage of a rigid-ified ring system that reduces the conformational flexibility present in many other phosphine ligands. 

Self-organization of ligands in multi-component titanium catalysts [69] with conformationally flexible biphenols [52] is also found in the enantioselective glyoxylate-ene reaction [60] to give the significantly high enantioselectivity (Eq. (7.25)) [70]. Some molecular modeling was reported that the hexacoordina-tion of the titanium atom would make the central titanium atom a center of chirality and that the A isomer is more favorable than the A isomer (Fig. 7-15). 

The PBO catalyst lc was recently applied to the kinetic resolution of allylic alcohols 2e-l by acylation with isobutyric anhydride [18]. The enantioselec-tivity depends strongly on the structure of allylic alcohols and increases with the conformational flexibility of the substrate. The dienyl alcohols 2i-l are good substrates for this kinetic resolution as shown by the high selectivity factors (30-80) obtained (Table 1). 

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