Si-enantiofaces

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... 

In the stereochemical model of the catalyst-aldehyde chelate complex, the square pyramidal complex 28 [39], the re aldehyde enantioface is shielded by the ligand phenyl group exposing the si enantioface to nucleophilic attack (Fig. 1-9). Since enantioselective formation of (5)-/(-hydroxy esters is observed (si face attack), the absolute stereochemistry of the products is consistent with the proposed coordination model. 

In every case for copper catalyst 31, the absolute stereochemistry of the cycloadducts is accounted for by the intervention of the substrate-catalyst complex depicted in Fig. 23, in which the s-cis configured dienophile is bound to the catalyst in the plane of the ligand in a bidentate fashion. The ferf-butyl group shields the top face and cycloaddition occurs from the exposed si enantioface. Support for this model derives from X-ray structures of aquo complexes of catalysts 31a and 31b which show that the complex possesses a distorted square planar geometry EPR spectroscopy on the binary catalyst-dienophile complex indicates that this geometry carries over from the solid state into solution. Calculations at the PM3 level of theory further favor the indicated reactive assembly [85]. 

It should be noted that a mixture of active catalyst centers that insert either only the re or si enantioface will still produce iPP. Although the chain start and chain end of each polymer chain are generally not the same group, when iPP molecular weight is high, a pseudo plane of symmetry exists... 

This mechanism will produce perfectly isotactic polypropylene as long as there are no enantiofacial misinsertions. However, the difference in energy between insertion of the re and si enantiofaces is fairly small ( 2 kcal/mol) for the chain-end control mechanism. The stereocenter resulting from each misinsertion will cause the opposite enantioface of the alkene to be preferred for the next insertion the result is a stereoblock structure with one isolated r defect, mmmmmrmmmm (Figure 1.6). The signature for a chain-end control mechanism is therefore the presence of the mmrm and mmmr pentads in a 1 1 ratio in the NMR of the polypropylene synthesized. ... 

The five different monomer insertions shown in Figure 12.4 (ethylene and the four different propylene combinations of 1,2- or 2,1-insertion with the re or si enantioface) can be considered to be five different repeat units (five different comonomer units) present in the macromolecular chain. Scheme 12.1 summarizes these five comonomer insertions for an EP copolymer. 

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. 

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. 

The MacMillan laboratory has produced an interesting study on the reductive amination of a broad scope of aromatic and aliphatic methyl ketones catalyzed by ent-lk, utilizing Hantzsch ester as a hydride source (Scheme 5.26) [48]. Apphcation of corresponding ethyl ketones gave very low conversions. Computational studies indicated that while catalyst association with methyl ketones exposes the C=N Si face to hydride addition, substrates with larger alkyl groups are forced to adopt conformations where both enantiofaces of the iminium ir... 

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).

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. 

We can generalize, based on our understanding of the different approach modes of the alkene to the metal, by saying that isotactic polymers result from multiple insertions of the alkene to the same enantioface (re or si.) and syn-diotactic chains form from regular alternations for complexation and insertion... 

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