Pd-catalyzed asymmetric hydrogenation of indoles

Chiral indolines represent useful building blocks in pharmaceuticals, herbicides and insecticides. Asymmetric hydrogenation of unprotected indoles with molecular hydrogen represents the most straightforward and atom-economy method to produce these compounds.Although the asymmetric (up to 98% ee) hydrogenation of N-protected or N-Ts indoles with is known since 2000 (R. Kuwano, Kyushu 

University) or since 2010 (A. Pfaltz, University of Basel), the very 

The overall mechanism thus is outer-sphere and ionic consisting of H+ substrate protonation from acid and then H transfer from the catalyst (H+/H sequence). 

E Osborn, J. A. Jardine, R H. Wilkinson, G. Hydride intermediates in homogeneous hydrogenation reactions of olefins and acetylenes using rhodium catalysts. Chem. Commun. 1965,131-132. 

---