Heterogeneous asymmetric catalysis chirally modified catalysts

The first reported attempts of what was then called "absolute or total asymmetric synthesis" with chiral solid catalysts used nature (naturally ) both as a model and as a challenge. Hypotheses of the origin of chirality on earth and early ideas on the nature of enzymes strongly influenced this period [15]. Two directions were tried First, chiral solids such as quartz and natural fibres were used as supports for metallic catalysts and second, existing heterogeneous catalysts were modified by the addition of naturally occuring chiral molecules. Both approaches were successful and even if the optical yields were, with few exceptions, very low or not even determined quantitatively the basic feasibility of heterogeneous enantioselective catalysis was established. 

The desire to produce enantiomerically pure pharmaceuticals and other fine chemicals has advanced the field of asymmetric catalytic technologies. Since the independent discoveries of Knowles and Homer [1,2] the number of innovative asymmetric catalysis for hydrogenation and other reactions has mushroomed. Initially, nature was the sole provider of enantiomeric and diastereoisomeric compounds these form what is known as the chiral pool. This pool is comprised of relatively inexpensive, readily available, optically active natural products, such as carbohydrates, hydroxy acids, and amino acids, that can be used as starting materials for asymmetric synthesis [3,4]. Before 1968, early attempts to mimic nature s biocatalysis through noble metal asymmetric catalysis primarily focused on a heterogeneous catalyst that used chiral supports [5] such as quartz, natural fibers, and polypeptides. An alternative strategy was hydrogenation of substrates modified by a chiral auxiliary [6]. 

A Baiker. Progress in asymmetric heterogeneous catalysis Design of novel chirally modified platinum metal catalysts. J Mol Catal A Chem 115 473-493,1997. 

DFT calculations are also used in asymmetric heterogeneous catalysis, for example, to address interaction between a reactant and a chiral modifier adsorbed on the metal catalyst surface to rationalize the mechanism of heterogeneous enantioselective [146] and diastereoselective [147] hydrogenations. 

Nickel and other transition metal catalysts, when modified with a chiral compound such as (R,R)-tartaric acid 5S), become enantioselective. All attempts to modify solid surfaces with optically active substances have so far resulted in catalysts of only low stereoselectivity. This is due to the fact that too many active centers of different structures are present on the surface of the catalysts. Consequently, in asymmetric hydrogenations the technique of homogeneous catalysis is superior to heterogeneous catalysis56). However, some carbonyl compounds have been hydrogenated in the presence of tartaric-acid-supported nickel catalysts in up to 92% optical purity55 . 

If an asymmetric catalyst acts perfectly well from the point of view of enanti-oselectivity and activity, there remains one difficulty to overcome, namely the recovery and the reuse of the catalyst. This can be achieved by asymmetric heterogeneous catalysis. This old approach, where a metal was modified by a chiral additive [12,14], has been reappraised recently, giving interesting results, but the scope of the reaction remains very narrow (see the review in Ref. [ 122]). 

---