Mechanism of the Hydrogenation of Ketones Catalyzed by trans-Dihydrido(diamine)ruthenium(II) Complexes

Top Cited Papers
Publisher Website
Google Scholar
Abstract
The complexes trans-RuH(Cl)(tmen)(R-binap) (1) and (OC-6−43)-RuH(Cl)(tmen)(PPh3)2 (2) are prepared by the reaction of the diamine NH2CMe2CMe2NH2 (tmen) with RuH(Cl)(PPh3)(R-binap) and RuH(Cl)(PPh3)3, respectively. Reaction of KHBsecBu3 with 1 yields trans-Ru(H)2(R-binap)(tmen) (5) while reaction of KHBsecBu3 or KOtBu with 2 under Ar yields the new hydridoamido complex RuH(PPh3)2(NH2CMe2CMe2NH) (4). Complex 4 has a distorted trigonal bipyramidal geometry with the amido nitrogen in the equatorial plane. Loss of H2 from 5 results in the related complex RuH(R-binap)(NH2CMe2CMe2NH) (3). Reaction of H2 with 4 yields the trans-dihydride (OC-6−22)-Ru(H)2(PPh3)2(tmen) (6). Calculations support the assignment of the structures. The hydrogenation of acetophenone is catalyzed by 5 or 4 in benzene or 2-propanol without the need for added base. For 5 in benzene at 293 K over the ranges of concentrations [5] = 10-4 to 10-3 M, [ketone] = 0.1 to 0.5 M, and of pressures of H2 = 8 to 23 atm, the rate law is rate = k[5][H2] with k = 3.3 M-1 s1, ΔH‡ = 8.5 ± 0.5 kcal mol-1, ΔS‡ = −28 ± 2 cal mol-1 K-1. For 4 in benzene at 293 K over the ranges of concentrations [4] = 10-4 to 10-3 M, [ketone] 0.1 to 0.7 M, and of pressures of H2 = 1 to 6 atm, the preliminary rate law is rate = k[4][H2] with k = 1.1 × 102 M-1 s-1, ΔH‡ = 7.6 ± 0.3 kcal mol-1, ΔS‡ = −23 ± 1 cal mol-1 K-1. Both theory and experiment suggest that the intramolecular heterolytic splitting of dihydrogen across the polar RuN bond of the amido complexes 3 and 4 is the turn-over limiting step. A transition state structure and reaction energy profile is calculated. The transfer of Hδ+/Hδ- to the ketone from the RuH and NH groups of 5 in a Noyori metal−ligand bifunctional mechanism is a fast process and it sets the chirality as (R)-1-phenylethanol (62−68% ee) in the hydrogenation of acetophenone. The rate of hydrogenation of acetophenone catalyzed by 5 is slower and the ee of the product is low (14% S) when 2-propanol is used as the solvent, but both the rate and ee (up to 55% R) increase when excess KOtBu is added. The formation of ruthenium alkoxide complexes in 2-propanol might explain these observations. Alkoxide complexes {RuP2}H(OR)(tmen), {RuP2} = Ru(R-binap) or Ru(PPh3)2, R= i Pr, CHPhMe, tBu, are observed by reacting the alcohols iPrOH, phenylethanol, and tBuOH with the dihydrides 5 and 6, respectively, under Ar. In the absence of H2, the amido complexes 3 and 4 react with acetophenone to give the ketone adducts {RuP2}H(OCPhMe)(NH2CMe2CMe2NH) in equilibrium with the enolate complexes trans-{RuP2}(H)(OCPh=CH2)(tmen) and eventually the decomposition products {RuP2}H(η5-CH2CPhCHCPhO), with the binap complex characterized crystallographically. In general, proton transfer from the weakly acidic molecules dihydrogen, alcohol, or acetophenone to the amido nitrogen of complexes 3 and 4 is favored in two ways when the molecule coordinates to ruthenium: (1) an increase in acidity of the molecule by the Lewis acidic metal and (2) an increase in the basicity of the amido nitrogen caused by its pyramidalization. The formato complexes trans-{RuP2}H(OCHO)(tmen) were prepared by reacting the respective complex 4 or 5 with formic acid. The crystal structure of RuH(OCHO)(PPh3)2(tmen) displays similar features to the calculated transition state for Hδ+/Hδ- transfer to the ketone in the catalytic cycle.

This publication has 36 references indexed in Scilit: