Asymmetric synthesis-Basics

Asymmetric Synthesis -Transition state Energies

Enantioselectivity-Basic Principles-page2

On the other hand if ligand (L) is chiral the two transition states are no longer enantiotropic but diasteretopic.
There is a difference in energies of the two transition states.
The reduction now is enantioselective that is one of the alcohols is obtained in higher proportion.
Hence the reaction is enantioselective.
In the diagram it is clear that the transition state energy is greater for formation of (S) compound. Hence it's product proprtion is lower.
So [R] is greater than [S].

The scheme on the right illustrates the formation of the enantiomeric alcohols.
When the hydride is delivered from a one side it leads to a specific alcohol. When the reagent that is the metal hydride is not chiral, the hydride is delivered from both sides with equal ease
Because the transition state energies for both processes is the same. Resulting in an almost equal mixture of enantiomers.
The situation changes when the metal hydride is chiral.
Now the tranition state energies are different for the two processes.
This results in greater proportion of an alcohol which has less energy barrier.
Thus leading to enantioselectivity.