Re-enantiofaces

The energy minima labeled as a, b, and c in Figure 1.9a, corresponding to 0O 0°, are coincident with those labeled with the same letters in the energy plot of Figure 1.7a. As already cited, the absolute minimum energy (labeled a) corresponds to the preinsertion intermediate for propene primary insertion with re enantioface, which is sketched in Figure 1.4. 

C2-symetric initiators have a pair of equivalent homotopic sites, both of which prefer the same monomer enantioface, that is, both sites prefer the re enantioface or both prefer the si face. Isoselective propagation proceeds with or without migratory insertion since coordination and insertion of monomer at either site give the same stereochemical result. 

For instance, when a-[2H]-styrene is hydroformylated with the Rh/(—)-DIOP catalytic system, the two reaction products obtained have almost identical optical purity and opposite absolute configuration (Fig. 2)51). Therefore, in this case, the enantiomeric excess measured indicates both the type (the re-re enantioface reacts preferentially) and extent of enantioface discrimination ( 15%) occurring during the reaction. 

The investigation of platinum(II)-chiral olefin complexes has shown that, when the diastereomeric equilibrium is reached, which diastereoface of the olefin is preferentially bound to the metal depends on the type of chirality of the olefin used61-63. When an optically active asymmetric ligand is present in the complex and a racemic olefin, is used, one diastereoface will be preferred for complexation and correspondingly one of the antipodes is preferentially complexed61 63). Let us suppose that with a certain catalytic system (e.g., Rh/(—)-DIOP), the re-re enantioface of a prochiral a-olefin reacts preferentially. With the same catalytic system the same face of all a-olefins, including the racemic a-olefins, is expected to react preferentially. However, when a racemic olefin is used, two diastereomeric transition states (e.g. a and b in Fig. 11) can form for each of the transition states shown in Fig. 7, depending on which one of the antipodes of the racemic monomer approaches the catalyst. 

For the chiral racemic pair A and B, the two sites are equivalent (homotopic) through the C2 axis of symmetry. Both sites will orient the growing polymer chain away from the benzene portion of the indene ligand. This causes the (/ ,/ )-enantiomer, A, to prefer insertion of the re enantioface of propylene and the (5,5)-enantiomer, B, to prefer the si face. The coordination of the two enantiofaces of propylene (re and si) to metallocenes A and B is shown in Figure 1.8. 

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. 

Isospecificity derived from racemic enantiomers can be understood by considering that the approach of the incoming prochiral propylene to the reactive metal-carbon bond must be controlled by the chiral structure of reactive centers (enantiomorphic site control). Pino and coworkers obtained (S)-2,4-dimethylheptane with high optical purity by trimerizing propene in the presence of (—)(R) -bis(l-tetrahydroindenyl)ethane zirconium dimethyl/MAO in the presenee of H2, whieh causes very fast chain transfer. As insertion oceurs with cis stereochemistry, they supposed the prevailing chirality of the hydro-oligomers is supposed to indicate that the Re enantioface of propylene preferentially approached Zr—carbon bond (Fig. 15) [64, 65]. 

The heterobimetallic asymmetric catalyst, Sm-Li-(/ )-BINOL, catalyzes the nitro-aldol reaction of ot,ot-difluoroaldehydes with nitromethane in a good enantioselective manner, as shown in Eq. 3.78. In general, catalytic asymmetric syntheses of fluorine containing compounds have been rather difficult. The S configuration of the nitro-aldol adduct of Eq. 3.78 shows that the nitronate reacts preferentially on the Si face of aldehydes in the presence of (R)-LLB. In general, (R)-LLB causes attack on the Re face. Thus, enantiotopic face selection for a,a-difluoroaldehydes is opposite to that for nonfluorinated aldehydes. The stereoselectivity for a,a-difluoroaldehydes is identical to that of (3-alkoxyaldehydes, as shown in Scheme 3.19, suggesting that the fluorine atoms at the a-position have a great influence on enantioface selection. 

It is worth noting that the calculations on models with 3-f-butyl-l-indenyl ligand (16) predict a substantial stereoselectivity but in favor of the opposite monomer enantioface [si and re for (R, R)- and (S,. S)-coordinated K-ligand, respectively],... 

The chiral sites which are able to rationalize the isospecific polymerization of 1-alkenes are also able, in the framework of the mechanism of the chiral orientation of the growing polymer chain, to account for the stereoselective behavior observed for chiral alkenes in the presence of isospecific heterogeneous catalysts.104 In particular, the model proved able to explain the experimental results relative to the first insertion of a chiral alkene into an initial Ti-methyl bond,105 that is, the absence of discrimination between si and re monomer enantiofaces and the presence of diastereoselectivity [preference for S(R) enantiomer upon si (re) insertion]. Upon si (re) coordination of the two enantiomers of 3-methyl-l-pentene to the octahedral model site, it was calculated that low-energy minima only occur when the conformation relative to the single C-C bond adjacent to the double bond, referred to the hydrogen atom bonded to the tertiary carbon atom, is nearly anticlinal minus, A- (anticlinal plus, A+). Thus one can postulate the reactivity only of the A- conformations upon si coordination and of the A+ conformations upon re coordination (Figure 1.16). In other words, upon si coordination, only the synperiplanar methyl conformation would be accessible to the S enantiomer and only the (less populated) synperiplanar ethyl conformation to the R enantiomer this would favor the si attack of the S enantiomer with respect to the same attack of the R enantiomer, independent of the chirality of the catalytic site. This result is in agreement with a previous hypothesis of Zambelli and co-workers based only on the experimental reactivity ratios of the different faces of C-3-branched 1-alkenes.105... 

The driving force for isoselective propagation results from steric and electrostatic interactions between the substituent of the incoming monomer and the ligands of the transition metal. The chirality of the active site dictates that monomer coordinate to the transition metal vacancy primarily through one of the two enantiofaces. Actives sites XXI and XXII each yield isotactic polymer molecules through nearly exclusive coordination with the re and si monomer enantioface, respectively, or vice versa. That is, we may not know which enantio-face will coordinate with XXI and which enantioface with XXII, but it is clear that only one of the enantiofaces will coordinate with XXI while the opposite enantioface will coordinate with XXn. This is the catalyst (initiator) site control or enantiomorphic site control model for isoselective polymerization. 

Although the same combination of enantiofaces (e.g., Re + Re, 1/10) leads to identical stereoisomers 2 in both orientation, the stcric interaction of the substituents is different. Therefore, the ratio of the diastereomers (2 and 8) (4 and 6) crucially depends on the formation of an open or closed transition state. 

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).

Similarly to the hydroformylation, under certain reaction conditions the formation of the intermediate palladium-alkyl complex can be practically irreversible as shown by the different prevailing chirality of the 2-methylbutanoic acid ester obtained from 1-butene and (Z)-2-butene, as well as from ( )- and (Z)-2-butene. Therefore, re-gioselection and enantioface selection must occur, as in hydroformylation, during or before the formation of the postulated palladium-alkyl intermediate (see Scheme IV). 

Chiral quaternary alkanes do not have any m-electrons or a heteroatom. Thus, the present chiral discrimination may involve the CH-7T interactions between the CH group of the chiral hydrocarbon and the m-electrons of the pyrimidine-5-carbaldehyde 11. This CH-jt interaction [109] maybe used to discriminate between the Re and Si enantiofaces of aldehyde 11, because the... 

It is clear that in the asymmetric epoxidation of allyl alcohols, coordination by the OH functionality to the titanium center plays a crucial role. Such a coordination has a favorable effect on the entropy of activation, that is, the rate constant (see Question 14 of Chapter 8). It also helps to orient only one of the two possible enantiofaces for a facile oxygen atom transfer. With alkenes that do not have any such functional groups, the titanium tartarate system gives poor enantioselectivities. The precatalyst that has been found to exhibit re-... 

The Cp() ,f )-Ti[All] and Cp(S,S)-Ti[All] reagents have been condensed with a variety of aldehydes always with good enantios-electivities (eqs 4-11). The degree of enantioface discrimination of these allyltitanium reagents is very high. The Si face attack is preferred for the Cp(/f,/f)-Ti[All] reagent and the Re face attack is preferred for the Cp(S,S)-Ti[All] reagent. 

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