Mechanistic Study of the Ru-Catalyzed Asymmetric Hydrogenation of Nonchelatable and Chelatable tert-Alkyl Ketones Using the Linear Tridentate sp³P/sp³NH/sp²N-Combined Ligand PN(H)N: RuNH- and RuNK-Involved Dual Catalytic Cycle

The linear tridentate sp3P/sp3NH/sp2N ligand PN(H)N ((R)-2′-(diphenylphosphino)-N-(pyridin-2-ylmethyl)[1,1′-binaphthalen]-2-amine) exclusively forms fac-[Ru(PN(H)N)(dmso)3](BF4)2 over the mer isomer with the help of the three strongly π-accepting DMSO ligands. The three different ligating atoms exert a divergent effect on the trans-DMSO—Ru bond strengths, enabling the stereoselective generation of fac-RuH(CH3O)(PN(H)N)(dmso) (RuNH). RuNH efficiently hydrogenates both nonchelatable t-butyl methyl ketone (BMK) and chelatable t-butyl methoxycarbonylmethyl ketone (BMCK) in the presence of a catalytic amount of CH3OK. The reaction proceeds at the H—sp3N—Ru—H bifunctional reaction site of fac-RuH2(PN(H)N)(dmso), and high enantioselectivity is attained in a chiral 3D cavity constructed by the sp3N trans DMSO, the conformation of which is fixed by a PyC(6)H—O═S hydrogen bond. We determined the structures of RuNH, the K amide RuNK, Ru dihydride, and Ru amido species by detailed NMR analysis using 15N-labeled PN(H)N and C(3)-Ph-substituted PN(H)N. The rate of BMK hydrogenation is significantly affected by [CH3OK]0, showing a characteristic curve with a peak followed by a pseudo-minus-first-order decay. The RuNH is easily deprotonated by CH3OK to generate RuNK, which is less reactive but has the same enantioface discrimination ability. Increased contribution of the slow RuNK cycle decreases the rate at higher [CH3OK]0. The RuNH- and RuNK-involved dual catalytic cycle is supported by curve-fitting analyses and K+ trapping experiments. In hydrogenation of BMCK, only the RuNH cycle operates because BMCK is preferentially deprotonated over RuNH.