Homochiral ligands

Another example of an achiral guest and a homochiral library was described in the work of Williams [9], which combined Co(II), bipy, and the homochiral ligand bipy. All possible combinations of A- and A-[Co(bipy ) (bipy)3 complexes were formed as confirmed by NMR and ES-MS. Treating with D-Cl was found to shift the speciation to only [Co(bipy)3] + and A-[Co(bipy )3] + (by NMR). Previous work on similar systems had shown that D-Cl deuterated the basic amine sites, and the resulting ammonium ions cooperatively bound two chloride ions [10]. In this example, addition of the achiral chloride ion amplified a single diastereomer of A-[Co(bipy )3] + (Fig. 5.9). 

Although modest, the results obtained with nonracemic lithium dialkylamides demonstrated the feasibility of such enantioselective transformations and important work has been undertaken from this date to improve both the yield and the ee values as well as developing a catalytic process. With this objective, both the use of homochiral lithium amide (HCLA) bases and organolithium-homochiral ligand complexes have been explored. This field has been extensively reviewed " and the following section presents a selection of the most outstanding results and recent developments. 

The RLi homochiral ligand complexes are seldom used for the base-promoted isomerization of oxiranes into allylic alcohols because their poor chemoselectivity lead to complex mixtures of products. As examples, the treatment of cyclohexene oxide by a 1 1 i-BuLi/(—)-sparteine mixture in ether at low temperature provides a mixture of three different products arising respectively from -deprotonation (75), a-deprotonation (76) and nucleophilic addition (77) (Scheme 32) . When exposed to similar conditions, the disubstituted cyclooctene oxide 78 affords a nearly 1 1 mixture of a- and -deprotonation products (79 and 80) with moderate ee (Scheme 32, entry 1). Further studies have demonstrated that the a//3 ratio depends strongly on the type of ligand used (Scheme 32, entry 1 vs. entry 2) . ... 

The use of a catalytic amount of the homochiral ligand is here allowed by the poor reactivity of aryllithium reagents toward oxiranes, the reaction being supposed to occur through the more reactive organolithium/Schiff base complex 109, as depicted in Scheme 49. 

The ligand-exchange process has been applied as a mobile-phase-additive technique for enantioseparations. It involves the formation of a dissociable diastereoisomeric complex between a homochiral additive and a heterochiral solute about a central metal ion (Fig. 28). The mobile phase contains both the homochiral ligand and the metal ion as additive components. These species probably exist as the fully complexed species with at least two molecules of the homochiral... 

Building on the excellent work of Itsuno [6a,b], who first described the use of oxazaborolidine as a homochiral ligand, and of Kraatz [7], Corey was the first to report the enantioselective reduction of ketones to chiral secondary alcohols in the presence of an oxazaborolidine in substoichiometric amounts [8a,b]. This general method was named the CBS method (Scheme 1). 

Fig. 49 Diastereoselective C=N bond reduction using a ruthenium catalyst containing a homochiral ligand...

Ionic liquids (room-temperature molten salts) with chiral cations were efficiently used as reaction media to prepare homochiral MOFs [33], But the use of homochiral ligands as precursors in the synthesis is even more efficient For instance, Lin et al. [34] prepared MOFs with BINOL (l,l -bi-2,2 -naphthol) and BINAP (2,2 -his(diphenylphosphino)-1,1 -binaphthyl) as chiral linkers. Unfortunately, attempts to use such chiral catalysts in asymmetric catalysis showed some but not high enantiomeric excess in the studied reactions, perhaps the flexibility of the ligands in the MOF framework is not sufficient to induce the chirality effect in the reactions. The most illustrative example was presented hy Lin [35] the reaction of asymmetric hydrogenation of aromatic ketones demonstrated a 99.2% ee for Ru-BINAP/MOF systems. 

Graf C-D, Malan C, Harms K, Knochel P. New homochiral ligands bearing nonstereogenic chirotopic centers. Lithiated N,N -dialkylureas as chiral bases and sterically crowded phosphines for asymmetric catalysis. J. Org. Chem. 1999 64 (15) 5581-5588. 

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