CIEC Catalysis -- Asymmetric Catalysis

the importance of chirality

chiral catalysis

a catalyst which works

synthesis of menthol

Chiral Catalysis

To get some idea of their achievements we will look an example of the work done by Noyori.

How do chiral hydrogenation catalysts work?

They are transition metal complexes, often of rhodium or ruthenium.

1. The transition metal binds a chiral ligand to give a chiral catalyst

2. The catalyst simultaneously binds H₂ and the substrate.

3. The hydrogen can be added in two ways to the double bond in the substrate to give the different enantiomers.

4. The chiral product is released.

image: William S Knowles, Ryoji Noyori, K. Barry Sharpless

diagram: catalytic reactions and activation energies

Question

Inspect the energy diagrams for the two processes.

Which reaction will be faster, A or B?

After some time which product will be in excess, the one from A or B?

Answer

But why does it produce more of one enantiomer than the other?

The two pathways to the different isomers have different transition complexes, which are not mirror image forms and have slightly different energies.

The hands on the right symbolise the catalyst and the hands on the left the products. The match is better in the upper picture so the energy of the transition state is less than in the lower picture.

Answer

A The process has a lower activation energy, and so a faster rate. After a period of time with both competing reactions occurring there will be an excess of A over B.

How could we try to make the rate difference greater, to get a greater excess of one enantiomer? The answer is to find a way of increasing the difference in activation energies. Chemists achieved this by altering the ligands on transition metal catalysts.

Question

Identify the chiral centre in the product and draw the products as mirror image forms

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