Ruthenium-Catalyzed\nCarbocycle-Selective Hydrogenation\nof Fused Heteroarenes

The homogeneous catalytic hydrogenation of benzo-fused\nheteroarenes\ngenerally provides partially hydrogenated products wherein the heteroaryl\nring is preferentially reduced, such as quinoline hydrogenation, leading\nto 1,2,3,4-tetrahydroquinoline. Herein, we report a carbocycle-selective\nhydrogenation of fused <i>N</i>-heteroarenes (quinoline,\nisoquinoline, quinoxaline, etc.) using the Ru complex of a chiral\nspiroketal-based diphosphine (SKP) as the catalyst, affording the\ncorresponding 5,6,7,8-tetrahydro products in high chemoselectivities.\nThis catalytic system is also effective for the asymmetric carbocycle\nhydrogenation of fused heteroarenes bearing a boryl or amino group.\nExperimental studies provided a strong support for the homogeneous\nnature of the catalysis, and an inner-sphere mechanism was proposed\nfor the hydrogenation. DFT calculations indicated that the hydrogenation\nis initiated by η<sup>4</sup>-coordinative activation of quinoline\ncarbocycle to Ru dihydride complex of SKP, followed by metal-to-ligand\nhydride transfer. Subsequent carbocycle reduction proceeds via consecutive\nsteps of the H<sub>2</sub> oxidative addition and C–H reductive\nelimination.