8.15 : اختزال الألكينات: الهدرجة الحفزية غير المتماثلة

Catalytic hydrogenation of an alkene is the reduction of the C=C bond using molecular hydrogen to give an alkane in the presence of a transition-metal catalyst.

The catalyst used for hydrogenation can be heterogeneous, that is, finely divided metal over charcoal, or homogeneous, like Wilkinson's catalyst, giving a syn addition product.

Since the reaction results in a new chiral center in the product, a pair of enantiomeric products is anticipated.

Now the question arises — is it possible to produce a single enantiomeric product instead of a pair of enantiomers?

For an alkene of this type, an enantiomeric excess of one of the products can be achieved through hydrogenation, using a chiral homogeneous catalyst.

This is called asymmetric hydrogenation. In this reaction, the chiral catalyst significantly reduces the activation energy for the formation of one enantiomer over the other. 

The catalysts generally used are ruthenium and rhodium complexes coordinated to a chiral phosphine ligand like "BINAP".

Although the ligand itself has no chiral centers, its chirality arises from the restricted rotation of the two rings about the single bond. 

The chelating diphosphine provides a chiral catalyst with the metal anchored to the two phosphorus atoms in a chiral atmosphere. 

The catalyst has several industrial applications; for instance, it catalyzes the asymmetric synthesis of the anti-inflammatory (S)-naproxen with more than 98% ee.

Another application is towards the asymmetric hydrogenation of geraniol, a natural product isolated from rose oil.

Interestingly, although geraniol contains two double bonds, the one nearer to the –OH group undergoes reduction. 

Therefore, for substrates undergoing asymmetric hydrogenation, a neighboring functional group close to a double bond is essential for coordinating to the metal catalyst.