Binaphthol backbone

In an effort to impart a stronger twist sense bias on the mPE helix, a chiral binaphthol derivative was placed directly into the mPE backbone [64]. The chiral binaphthol unit was placed both at the center (47) and terminus (48) of the oligomer in order to determine how the position of the chiral unit would affect the twist sense bias and folded state stability. Solvent denaturation studies (UV... 

Terada and co-workers reported a novel guanidine catalyst with a chiral binaphthol backbone for the asymmetric addition of dicarbonyl compounds to nitro-olefins [126]. Substitution on the binaphthol backbone dramatically increased enantioselectivity. 

More specifically, 3,5-di-ferf-butylphenyl substitution on the 3,3 -position of the binaphthol backbone (260) provided overall best yields and selectivities. Using catalyst 260, the authors expanded the scope of substrates to include aliphatic and aromatic nitro-alkenes, and a-substituted P-ketoesters, while maintaining good yields and enantiomeric ratios (Scheme 71). 

Obviously, catalyst structure is the key to determining enantiocontrol in the phospho-aldol reaction. In the lanthanide heterobimetaUic systems, backbone chirality within the binaphthol leads to generation of a stereocentre at the metal itself and does so with complete diastereospecificity. This is yet another example of the concept of chirality tra s/er, wherein fixed stereochemistry at one site leads to control over stereochemistry at a more remote site and is one of the most powerful concepts in catalyst design. It is the presence of such a highly... 

Shibaski and co-workers have focused very strongly on the binaphthol ligand framework, which they have foimd to be extremely powerful in this and related catalytic processes. They have been able to manipulate the Ugand backbone ster-ics and electronics to some degree although the synthetic chemistry involved is not trivial [41]. Moreover, there have been some issues over catalyst performance being linked to the synthetic history of the catalyst itself. It was thought possible therefore that complexes could be prepared which possessed... 

In 2002, Hoveyda and coworkers introduced an alternative concept to install chirality in ruthenium olefin metathesis complexes through a Ci-symmetric bidentate NHC ligand, bearing binaphtholate moieties (Figure 11.33). The NHCs in this type of complexes lacked backbone substitution and it was chelation that prevented free rotation of the ligand [118]. In this case, chiral information installed within the A-substituent was transferred directly to the ruthenium center. Unfortunately, these complexes were found to be less active because of the reduced Lewis acidity at the metal center, mainly due to the exchange of Cl... 

In 1998, Du Pont claimed the synthesis of bis(pyrrolyl) and bis(indolyl) ligands 1-3 characterized by a binaphthol backbone (Scheme 2.124) [10]. Unfortunately, the synthesis was detailed only for ligand 1. It was tested in the hydroformylation of methyl 3-pentenoate at 5 bar syngas pressure, where almost perfect selectivity for the formation of the terminal aldehyde was observed. The ligand was also used to convert 1- and 2-hexene. In comparison to a corresponding diphosphite with electron-withdrawing 3,5-CFg-phenyl groups, improved 1-heptanal selectivity was achieved. In the reaction with 1,3-butadiene, mainly pentenal was formed. 

Several aliphatic or aromatic diols were employed for the construction of the backbone, such as l-benzyl-3,4-dihydroxypyrrolidine-2,5-dione, enan-tiopure binaphthols or achiral biphenols, brenzcatechol, aliphatic diols, or spiro[4.4]nonane-1.6-diol (see also Section 2.2.2) [44-47]. Three-carbon bridged diols proved to be more effective compared to two-carbon or four-carbon bridged analogs, which has been explained by a counterproductive fluxional behavior in the corresponding rhodium complexes [48]. Diphosphites based on 1,3-diphenylpropane also produced iirferior enantioselectivities in comparison to the 2,4-pentanediol backbone of Chiraphite. For the design of both side arms, it was concluded that a diphosphite based on the unsubstituted biphenol is less suited... 

Alternatively, atropisomery can be introduced in Kelliphite-type ligands by using an enantiopure binaphthol backbone (Figure 2.45) [44, 54]. Substituents in the 3,3 -positions of the binaphthyl unit enhanced the rate and enantiomeric excess values in the hydroformylation of vinyl acetate. After the best ligand was chosen and the reaction conditions were optimized, up to 80% ee could be realized. 

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