8.15: Reducción de alquenos: Hidrogenación catalítica asimétrica
Catalytic hydrogenation of alkenes is a transition-metal catalyzed reduction of the double bond using molecular hydrogen to give alkanes. The mode of hydrogen addition follows syn stereochemistry.
The metal catalyst used can be either heterogeneous or homogeneous. When hydrogenation of an alkene generates a chiral center, a pair of enantiomeric products is expected to form. However, an enantiomeric excess of one of the products can be facilitated using an enantioselective reaction or an asymmetric hydrogenation process using chiral homogeneous catalysts. The chiral catalysts are designed such that the metal coordinates to a chiral ligand. The most frequently used chiral ligand is BINAP [(2,2'-bis(diphenylphosphino)-1,1'-binaphthyl] — a chelating diphosphine. The metal coordinates to the two phosphorus atoms of BINAP, creating a chiral environment for itself. Such chiral catalysts have tremendous applications in pharmaceutical industries, such as the asymmetric synthesis of (S)-naproxen, an anti-inflammatory drug molecule, and the synthesis of L-dopa, a drug used to treat patients with Parkinson's disease.
Asymmetric hydrogenation is specific to the type of double bond undergoing reduction. The presence of a functional group directly adjacent to the target double bond is essential for the hydrogenation process as it aids with effective coordination of the metal.