Ruthenium‐Catalyzed Enantioselective Hydrogenation of 9‐Phenanthrols

The enantioselective hydrogenation of arenols to corresponding chiral cyclic alcohols remains a challenge because of their aromaticity and the difficulty in controlling the regio‐, chemo‐, and stereoselectivity. In this work, the first highly efficient ruthenium‐catalyzed enantioselective hydrogenation of 9‐phenanthrols has been successfully realized under mild conditions via trapping the unstable keto tautomers. The method provides a facile access to a range of chiral 9,10‐dihydrophenanthren‐9‐ols with up to 98 % yield and >99 % ee. The hydrogenation pathway includes base‐promoted tautomerization of 9‐phenanthrols and Ru‐catalyzed asymmetric hydrogenation of the in situ generated unstable keto tautomers. The first highly enantioselective hydrogenation of 9‐phenanthrols promoted by a base has been successfully realized using a chiral Ru(diamine) catalyst, affording a facile access to the optically active 9,10‐dihydrophenanthren‐9‐ols with excellent results (up to 98 % yield, >99 % ee and >20 : 1 dr).