text: In 1956 a heterogeneous catalyst made of palladium deposited on silk was shown to effect asymmetric hydrogenation. Later, in 1968, the groups of William Knowles and Leopold Horner independently published the examples of asymmetric hydrogenation using a homogeneous catalysts. While exhibiting only modest enantiomeric excesses, these early reactions demonstrated feasibility. By 1972, enantiomeric excess of 90% was achieved, and the first industrial synthesis of the Parkinson's drug L-DOPA commenced using this technology.
  The field of asymmetric hydrogenation continued to experience a number of notable advances. Henri Kagan developed DIOP, an easily prepared C-symmetric diphosphine that gave high ees in certain reactions. Ryōji Noyori introduced the ruthenium-based catalysts for the asymmetric hydrogenated polar substrates, such as ketones and aldehydes. Robert H. Crabtree demonstrated the ability for Iridium compounds to catalyse asymmetric hydrogenation reactions in 1979 with the invention of Crabtrees catalyst. In the early 1990's, the introduction of P,N ligands by several groups independently then further expanded the scope of the C-symmetric ligands, although they are not fundamentally superior to chiral ligands lacking rotational symmetry.  
  Today, asymmetric hydrogenation is a routine methodology in laboratory and industrial scale organic chemistry. The importance of asymmetric hydrogenation was recognized by the 2001 Nobel Prize in Chemistry awarded to William Standish Knowles and Ryōji Noyori.