Enantioface binding selectivity

The high enantioselectivity again can be rationalized by enantioface-selective alkene coordination in 63 (Fig. 35). The olefin moiety is expected to bind trans to the upper imidazoline moiety [70,73] thereby releasing the catalyst strain. Coordination at this position may, in principal, afford four different isomers assuming the stereoelectronically preferred perpendicular orientation of the alkene and the Pt(II) square plane. In the coordination mode shown, steric repulsion between both olefin substituents and the ferrocene moiety is minimized. Outer-sphere attack of the indole core results in the formation of the product s stereocenter. 

Figure 7.4. Upper Quadrants model for two possible transition states, TS I (left) and TS II (right) for an olefin (R O CR3 4) insertion into the Rh-H bond of RhH(CO)[(R,S)-BINAPHOS]. Lower Sybyl (Cache) representation of the molecular mechanics calculations of transition states, TS I (left) and TS II (right), for styrene (grey) insertion into the Rh-H bond of RhH(CO)[(R,S)-BINAPHOS] (black). Re-face binding of styrene to Rh is calculated to be lower energy than that of si-face binding to both TS I (by 7.1 kcal/mol) and in TS II (by 1.8 kcal/mol). In both structures, the enantioface selection seems to arise from the steric repulsion between the phenyl group of styrene and one of the naphthyls of (R,S)-BINAPHOS (marked with rectangles).

Bach and coworkers reported a new strategy to achieve enantioselective photo actions in solution based on the use of a chiral host derived from Kemp s aci [134-138]. They designed and synthesized a hydrogen bonding chiral templal with a lactam functionality 60, which can bind prochiral amide substrates throuj two hydrogen bonds in an enantioface-selective fashion with the aid of a bul group fixed in the 1,3-axial position, as illustrated in Scheme 22. 

In any case, the asymmetric reduction surely takes place in a chiral environment of the protein. A factor determining the stereochemistry of the product must be an asymmetric binding so as to expose one of the enantiofaces of the carbonyl group selectively to the reductant. It is difficult to clarify the details of the interaction between BSA and the substrate and to elucidate the origin of selectivity. The data listed in Table 17 predicts that bulkiness of the substituent in the substrate is not the only factor for the asymmetric induction. 

On the basis of this experiment, Pino and coworkers were able to determine that catalysts derived from the (/ )-ethylenebis(tetrahydroindenyl)zirconium binaptholate preferentially selected the Re enantioface of propylene. These results led to a model for the transition state where the polymer chain is forced into an open region of the metallocene, thereby relaying the chirality of the metallocene to the incoming monomer through the orientation of the p-carbon of the aUcyl chain (Scheme IIA).43 Here, the role of the C2-symmetry of the catalyst site can be readily appreciated since as the polymer chain migrates to the coordinated olefin, the coordination site available for binding of the olefin alternates between two coordination sites (A -> B -> C). Because these two sites are related by a C2-symmetry axis, they are homotopic and therefore selective for the same olefin enantioface. The result is polymerization to yield an isotactic polyolefin. 

The NMR spectrum for polypropylene produced at 50°C with this catalyst reveals a high degree of stereocontrol (81% rrrr pentads). Moreover, analysis of the stereochemical defects (predominantly rmmr pentads) were indicative of a site control mechanism. For a site control mechanism to operate in syndiospecific polymerization, the olefin must alternately bind to coordination sites with opposite enantioface selectivity. The model for this polymerization is shown in Scheme HI. 

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