Ligand asymmetric influence, trans

Asymmetric Influence from the trans Ligand. As shown later, when an asymmetric nitrogen is in the trans position of the complexes of the type trans N,//)-[PtCl(L-am)(ethylene)], asymmetry is introduced by the substitution of tbn for the ethylene in organic solvents. 10.) L-Alanine, L-phenylalanine, and L-valine fail to introduce asymmetry on the incoming tbn in the equilibrated state. However, L-proline, N- and C-substituted L-proline and even -substituted L-valine induce asymmetry. (Table V) There must be some electronic interaction from the asymmetric nitrogen upon the olefin through platinum(II). 

In this study the authors also noted that (A8-2-me(hylallyl) PdCKPPhs) exhibited two CH2 resonances (C(l) and C(3)). This observation is consistent with the known crystal structure of this compound (155) which shows the presence of an asymmetrically bound /i -allyl ligand due to the differing trans-influences of Cl and PPhs. 

As expected, increasing the size of the R group increases enantioselection, and the buttressing effect on the bis-oxazoline ring caused by the geminal disubstitution in 10 provides further enhancement of enantiocontrol. From the results in Table S.4, however, the ligand s R substituent has only a minor influence on the trans.cis of cyclopropane products. To increase product diastereoselectivity, Evans [40] increased the size of the ester substituent from ethyl to te/t-butyl and then to the bulky BHT ester, previously reported by Doyle [43] to provide exceptional diastereocontrol in catalytic cyclopropanation reactions. Applications of these catalysts to alkenes other than styrene have demonstrated the potential generality of their uses for asymmetric intermolecular cyclopropanation (Table 5.5). 

---