Cinchonidine catalyst interactions

Enantioselective hydrogenation of 2,3-butanedione and 3,4-hexanedione has been studied over different type of supported Pt catalysts (Pt/Al203, Pt/Si02, Pt/MCM-41) in the presence of cinchonidine (CD). Kinetic results confirmed that (i) 2,3-butanedione is more reactive than 3,4-hexanedione over all catalysts studied and (ii) both substrates have a strong poisoning effect. The kinetic results confirmed also that CD concentration close to 10"3 M is necessary to achieve both high reaction rate and enantioselectivity in the range of 55-65 %. NMR results confirmed that substrate-modifier interaction takes also place in the liquid phase. 

The enantioselective hydrogenation of prochiral substances bearing an activated group, such as an ester, an acid or an amide, is often an important step in the industrial synthesis of fine and pharmaceutical products. In addition to the hydrogenation of /5-ketoesters into optically pure products with Raney nickel modified by tartaric acid [117], the asymmetric reduction of a-ketoesters on heterogeneous platinum catalysts modified by cinchona alkaloids (cinchonidine and cinchonine) was reported for the first time by Orito and coworkers [118-121]. Asymmetric catalysis on solid surfaces remains a very important research area for a better mechanistic understanding of the interaction between the substrate, the modifier and the catalyst [122-125], although excellent results in terms of enantiomeric excesses (up to 97%) have been obtained in the reduction of ethyl pyruvate under optimum reaction conditions with these Pt/cinchona systems [126-128],... 

Figure 15. Interactions between the Cg-OH group and the C3-vinyl group in chinchonine (15) and cinchonidine (16) derived catalysts.

Theoretical calculations proved that the reaction intermediate leading to R-ethyl lactate on cinchonidine-modified Pt(lll) is energetically more stable than the intermediate leading to the S-ethyl lactate [147], However, the catalytic system is complex and the formation and breaking of intermediates are transient, so it is certainly difficult to obtain direct information spectroscopically. It is therefore advisable to use simplified model systems and investigate each possible pairwise interaction among reactants, products, catalyst, chiral modifier, and solvent separately [147, 148]. In order to constitute these model systems, it is important to get initial inputs from specific catalytic phenomena. 

Enantiomeric excess and catalytic activity of the asymmetric hydrogenation of ethyl pyruvate over (-)cinchonidine modified Pt/carrier catalysts depend significantly on the specific Pt surface area This is due to the morphology of the Pt particles and to surface chemical Pt/support interaction. Thus, reaction pathway control is possible by varying these parameters. 

This reaction over nonchiral catalysts when a ketone contains a prochiral center produces racemic mixtures of optical isomers. Cinchonidine, being a bulky molecule, reduces the accessible active platinum surface as it adsorbs and should cause some deactivation with respect to racemic hydrogenation. The decrease in formation rate of the main product after the maximum (Fig. 7.14) can be a result of poisoning by adsorbed spectator species, which inhibit enantiodifferentiating substrate-modifier interaction. 

The quantitative treatment of the hydrogenation kinetics requires to take into account two adsorption modes of the modifier, one mode being the parallel adsorbed species of (—)-cinchonidine involved in the enantiodifferentiation and the other being adsorbed in tilted mode only as a spectator on the catalyst surface. Adsorbed cinchonidine in parallel mode (active form) provides an enantioselective site (Fig. 7.16) and when the reactant is adsorbed in the vicinity, interaction between reactant and modifier leads to such... 

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