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.