Mechanism of enantioselection

There can be significant differences in the detailed structure and mechanism of these catalysts. For example, the geometry of the phosphine ligands may affect the reactivity at the metal ion, but the basic elements of the mechanism of enantioselection are similar. The phosphine ligands establish a chiral environment and provide an appropriate balance of reactivity and stability for the metal center. The reactants bind to the metal through the double bond and at least one other functional group, and mutual interaction with the chiral environment is the basis for enantioselectivity. The new stereocenters are established under the influence of the chiral environment. 

The mechanism of enantioselective hydrogenation by rhodium complexes has been reviewed on several occasions, including a recent detailed publication by the present author [14]. In addition, much of the contemporary work by Gridnev and Imamoto has been reviewed, as described below. Consequently, details of the older studies will be cited only briefly to provide the necessary context, after which the post-1998 developments will be discussed in detail. For a mature field, it is surprising how much new and significant information has been reported during the past five years. 

Figure 17.15. Mechanism of enantioselection with 5R-MEPY-rhodium...

The mechanism of enantioselective reduction here can be described in simplified form as follows ... 

DFT calculations have been used to probe the mechanism of enantioselective addition of diphenylzinc to aldehydes, and in particular the enhanced selectivity which accompanies addition of diethylzinc to the system.245... 

Casella L, Gullotti M, Ghezzi R et al (1992) Mechanism of enantioselective oxygenation of sulfides catalyzed by chloroperoxidase and horseradish peroxidase. Spectral studies and characterization of enzyme-substrate complexes. Biochemistry 31 9451-9459... 

NMR data indicate the ratio of the concentrations of the major and the minor isomer to be approximately 10 1. Since the concentration of one of the isomers is almost ten times the other, if the rate constants for oxidative additions of dihydrogen are approximately the same, the major diastereomer should undergo conversion to the dihydride ten times faster. This, however, is not the mechanism for enantioselection. The mechanism of enantioselection is the much larger rate constant (—600 times) for the reaction between the minor isomer and dihydrogen ... 

Job A, Janeck CF, Bettray W, Peters R, Enders D (2002) Tetrahedron 58 2253 Josephsohn NS, Kuntz KW, Snapper ML, Hoveyda AH (2001) Mechanism of enantioselective Ti-catalyzed Strecker reaction peptide-based metal complexes as bifunctional catalysts. J Am Chem Soc 123 11594—11599 Juhl K, Gathergood N, Jprgensen KA (2001) Catalytic asymmetric direct Man-nich reactions of carbonyl compounds with alpha-imino esters. Angew Chem Int Ed Engl 40 2995-2997... 

Studies aimed at clarifying the mechanism of enantioselection associated with 41, making use of a combination of molecular dynamics calculations and NMR techniques <2001J(P2)1685>, as well as X-ray diffraction data <20040BC3470>, have led to a better comprehension of the subtle interactions that characterize and differentiate the diastereomeric complexes of tetra-acid 41 with the D,L-amino acid pairs. 

The Claisen rearrangement has attracted much attention as an attractive tool for the construction of new carbon-carbon bonds. Taguchi et al. reported the enantioselective and regioselective aromatic Claisen rearrangement of catechol mono allylic ether derivatives by means of Corey s chiral boron reagent (Eq. 70) [53a,54]. The mechanism of enantioselectivity is that a rigid five-membered cyclic intermediate is formed by reaction of catechol mono allylic ethers with the chiral boron reagent and this is fol-... 

Although these reactions obviously involve intramolecular oxygen transfer within a titanium(IV) tartrate-ally lie alcohol-alkyl hydroperoxide complex, analogous to the vanadium-catalyzed epoxidations discussed above, the exact nature of the catalytic species and the mechanism of enantioselection remain controversial [39, 40],... 

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