Hydrogenation of carene

Carene is a natural product extracted from the resin of some conifers. It has the structure shown:

One of the methyl groups attached to the cyclopropane ring "covers" one of the faces of the double bond (in green). Only the face opposite the cyclopropane can approach the catalyst surface without hindrance, so hydrogenation takes place exclusively from that face and gives cis-carane. This product is relatively unstable, because it has an interaction between the two methyl groups shown.

trans-Carane, more stable because it lacks that interaction, is not formed: the approach of carene to the catalyst through the other face is strongly hindered by one of the cyclopropane methyl groups.

The qualitative energy diagram of the reaction is the following:

• Pathway A: reasonable activation energy, because carene approaches the catalyst through its less hindered face. It leads to the less stable cis isomer.
• Pathway B: an activation energy so high that the reaction proceeds at a negligible rate, even though it would lead to the more stable trans product.

The reaction is under kinetic control: the way the olefin approaches the catalyst, and not the stability of the products, determines the stereochemical outcome.

The fusion of the cyclopropane to the six-membered ring creates two stereocentres of fixed configuration, so the two faces of the double bond have very different steric surroundings. In carene the difference is so large, and the molecule so rigid, that hydrogen adds from one face only: a case of complete facial selectivity.