Homochiral ligand complexes

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) . ... 

As mentioned earlier, the ligand Hbpa (8) reacts with [NEt4]2 [ClsFeOFeCla] to form a homochiral ferric complex [Fe(bpa )(Cl)2... 

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). 

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... 

Ahn et al. [20] reported the synthesis of homochiral bis(oxazolinyl)-biferrocene ligands (structure 11 in Scheme 7), which also have both planar and central chirality. With these complexes, 2-(phenyl)cyclopropane carboxy-lates were obtained in up to 99% ee and a trans/cis ratio of 88/12. 

There are very few examples of asymmetric synthesis using optically pure ions as chiral-inducing agents for the control of the configuration at the metal center. Chiral anions for such an apphcation have recently been reviewed by Lacour [19]. For example, the chiral enantiomerically pure Trisphat anion was successfully used for the stereoselective synthesis of tris-diimine-Fe(ll) complex, made configurationally stable because of the presence of a tetradentate bis(l,10-phenanthroline) ligand (Fig. 9) [29]. Excellent diastereoselectivity (>20 1) was demonstrated as a consequence of the preferred homochiral association of the anion and the iron(ll) complex and evidence for a thermodynamic control of the selectivity was obtained. The two diastereoisomers can be efficiently separated by ion-pair chromatography on silica gel plates with excellent yields. 

The neutral, homochiral complex (SS,SS-9) is formed by stirring two equivalents of (4S,4, S)-7 with Zn(OAc)2 (Figure 9.5). Zn(OAc)2 serves a dual role in the readion, simultaneously delivering the metal center and the required base. When a racemic mixture of box ligands [i.e., one equivalent each of (4S,4 S)-7 and (4R,4 R)-7] is combined with Zn(OAc)2, three complexes could form, the homochiral complexes SS,SS)- and RR,RR)-9 (i.e., chiral self-recognition) and the heterochiral complex SS,RR)- (i.e., chiral self-discrimination). The tetrahedral coordination geometry strongly favors self-discrimination only the heterochiral complex SS,RR)- is... 

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