Heterochiral ligands

These ligand combinations were then applied to the asymmetric addition of aryl boronic acid derivatives to 2-cyclohexenone. The results of this study are illustrated in Figure 4. From this data it can be seen that the heterochiral ligand combination LL/L2 exhibited the highest conversion during the 3h evaluation period. Furthermore, the heterochiral complexes LL/L2 and LL/L3 are significantly more enantioselectivity than the homochiral combinations. 

The three water ligands located at meridional positions of the J ,J -DBFOX/Ph aqua complexes may be replaced by another molecule of DBFOX/Ph ligand if steric hindrance is negligible. Based on molecular model inspection, the hetero-chiral enantiomer S,S-DBFOX/Ph looks like a candidate to replace the water ligands to form the heterochiral meso-2 l complex J ,J -DBFOX/Ph-S,S-DBFOX/... 

On the other hand, in the single crystals prepared from equivalent amounts of heterochiral 1 1 complexes, a pair of two heterochiral 1 1 complexes are incorporated in a unit cell to form a layered structure with alternate layer distances of 7.33 and 7.6 A. Two perchlorate ions stay in the narrower gap, and two additional acetone molecules as crystallization solvent occupy the wider gap. The perchlorate ions interact with two axial water ligands by hydrogen bonds (3.71 and 3.77 A) to construct a layered structure. The adjacent two molecules of heterochiral 1 1 com-... 

In addition, we investigated a nonlinear-like effect (NLE), activity, temperature dependence, and kinetics of hydroxy[2.2]paracyclophane ketimine ligands with the 1,2-addition reaction of diethylzinc to cyclohexylcarbaldehyde. A linear correlation between the enantiomeric excess of AHPC ketimine ligands bearing a phenylethyl side group and the product was observed with 0.5 mol% of catalyst loading. When the catalyst loading of (Sp,S)/(Rp,R)-4a was increased to 4 mol%, a precipitate of the inactive heterochiral species was formed and resulted in a positive nonlinear like effect (Fig.2.1.3.6), while a linear behavior is observed with 5b (Fig. 2.1.3.7). The enantiomeric ratio was found to have linear temperature dependence. [16]... 

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... 

These deviations from linearity indicate the existence of an oligomeric distribution of chiral ligands. Noyori proposed a rationale as follows Due to the different dissociability (stability) of homochiral and heterochiral dimer, the enantiopurity of the remaining reactive catalyst (monomer) is improved as compared with that of the submitted chiral ligand 6 (Scheme 9.5) [11]. Heterochiral dimer is thermodynamically more stable than homochiral dimer, which is consistent with Noyori s rationale mentioned above [12a]. An ab initio molecular orbital study was also reported in a simplified model reaction between formaldehyde and dimethylzinc catalyzed by achiral 2-aminoethanol [12b]. 

It was soon recognized that in specific cases of asymmetric synthesis the relation between the ee of a chiral auxiliary and the ee of the product can deviate from linearity [17,18,72 - 74]. These so-called nonlinear effects (NLE) in asymmetric synthesis, in which the achievable eeprod becomes higher than the eeaux> represent chiral amplification while the opposite case represents chiral depletion. A variety of NLE have been found in asymmetric syntheses involving the interaction between organometallic compounds and chiral ligands to form enantioselective catalysts [74]. NLE reflect the complexity of the reaction mechanism involved and are usually caused by the association between chiral molecules during the course of the reaction. This leads to the formation of diastereoisomeric species (e.g., homochiral and heterochiral dimers) with possibly different relative quantities due to distinct kinetics of formation and thermodynamic stabilities, and also because of different catalytic activities. 

Chiral discrimination in binuclear homo- and heterochiral complexes formed by hydrazine (Fig. 3.23), a metal atom of group 10 (Ni, Pd, Pt), and halogen atoms (F, Cl, Br, I), depends on the electronic properties arising from the metal atoms and the ligand atoms attached to them [36]. The complexes were studied by quantum chemical methods by applying DFT methods at the B3LYP/LANL2DZ level of theory. 

In order to achieve an amplification of chirality, it requires that/> 1. If P = 0 (no meso catalyst) or g = 1 (same reactivity of meso and homochiral catalysts), then/= 1. The condition/> 1 is achieved for 1 + p > 1 + g ), or g < 1. Thus the necessary condition for asymmetric amplification in the above model is for the heterochiral or meso catalyst to be less reactive than the homochiral catalyst. If the meso catalyst is more reactive, then/< 1, and hence a negative nonlinear effect is observed. The size of the asymmetric amplification is regulated by the value off, which increases as K does. The more meso catalyst (of the lowest possible reactivity) there is, the higher will be eeproduct. This is well illustrated by computed curves in Scheme 11. The variation of eeproduct with eeaux is represented for various values of g (the relative reactivity of the meso complex) with K = 4 (corresponding to a statistical distribution of ligands Scheme 11, top). The variation in the relative amounts of the three complexes with eeaux is also represented for a statistical distribution of ligands (Scheme 11, bottom). 

The reactivity of the encounter complexes between protonated and acetylated (R)-and (5)-2-butyl acetate, (CH3COOsBu) M+ (sBu = (/ )-, (5)-, or ( )-2-butyl M = H (n= 1,2) CH3CO (n = 1)), and (S, S,, S )-tri-sec-buty I borate has been measured by FT-ICR-MS. The relevant ion patterns are shown in Scheme 12.475 The kinetic data of Table 20 reveal some differences in both the overall reactivity of chiral (CH3COOsBu) M+ ions toward (iS, iS, S )-tri-sec-butylborate (kUtl, k M, and k5) and the relative extent of the competing addition/elimination (k and k4), proton transfer (k2), and ligand exchange (fc3) channels. They clearly indicate that (S,S,S) tri-sec-butylborate reacts more efficiently with the homochiral (5)-2-butyl acetate ions, than with the heterochiral (R)-2-butyl acetate ones. As expected, the reaction efficiency of the racemate ( )-2-butyl acetate ions falls in between. 

Figure 3.60 The heterochiral and homochiral palladium(ll) carbene complexes of a cydohex-anolyl functionalised NHC ligand.

These catalysts are likely to have different stabihties and be formed in unequal amounts. Both the homochiral complexes (Lx )2Rh and (Ly )2Rh are accessible by combining a chiral ligand with the rhodimn precursor. The heterochiral complex, however, is new. If the heterochiral complex exhibits greater activity and enantioselectivity than the homochiral complexes, not only will the resulting combination lead to improved... 

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... 

The use of enantiomerically pure ligands excludes the presence of symmetry elements such as centres of inversion or planes in the final assembled structure. With racemic ligands the assembly can follow either a homochiral path, in which ligands of the same chirality bind to the same metal, or a heterochiral path as in [Ag2((i , )-... 

---