CNN Pincer Ruthenium Catalysts for Hydrogenation and Transfer Hydrogenation of Ketones: Experimental and Computational Studies

Abstract

Reaction of [RuCl(CNN)(dppb)] (1-Cl) (HCNN=2-aminomethyl-6-(4-methylphenyl)pyridine; dppb=Ph2 P(CH2 )4 PPh2 ) with NaOCH2 CF3 leads to the amine-alkoxide [Ru(CNN)(OCH2 CF3 )(dppb)] (1-OCH2 CF3 ), whose neutron diffraction study reveals a short RuO⋅⋅⋅HN bond length. Treatment of 1-Cl with NaOEt and EtOH affords the alkoxide [Ru(CNN)(OEt)(dppb)]⋅(EtOH)n (1-OEt⋅n EtOH), which equilibrates with the hydride [RuH(CNN)(dppb)] (1-H) and acetaldehyde. Compound 1-OEt⋅n EtOH reacts reversibly with H2 leading to 1-H and EtOH through dihydrogen splitting. NMR spectroscopic studies on 1-OEt⋅n EtOH and 1-H reveal hydrogen bond interactions and exchange processes. The chloride 1-Cl catalyzes the hydrogenation (5 atm of H2 ) of ketones to alcohols (turnover frequency (TOF) up to 6.5×10(4) h(-1) , 40 °C). DFT calculations were performed on the reaction of [RuH(CNN')(dmpb)] (2-H) (HCNN'=2-aminomethyl-6-(phenyl)pyridine; dmpb=Me2 P(CH2 )4 PMe2 ) with acetone and with one molecule of 2-propanol, in alcohol, with the alkoxide complex being the most stable species. In the first step, the Ru-hydride transfers one hydrogen atom to the carbon of the ketone, whereas the second hydrogen transfer from NH2 is mediated by the alcohol and leads to the key "amide" intermediate. Regeneration of the hydride complex may occur by reaction with 2-propanol or with H2 ; both pathways have low barriers and are alcohol assisted.

Ruthenium and osmium complexes containing 2-(aminomethyl)pyridine (Ampy)-based ligands in catalysis

“…Density functional theory (DFT) calculations carried out on model complexes RuX(CNN)(dmpb) (X = H, OR) (HCNN = 2-aminomethyl-6-(phenyl)pyridine, dmpb = Me 2 P(CH 2 ) 4 PMe 2 ) in the presence of added molecule of 2-propanol showed that the reaction of the Ru hydride with the ketone substrate leads to the most stable Ru alkoxide, the species observed in solution by NMR [54].…”

“…EtOH is kept under H 2 (5 atm), the hydride 38a, which is present in trace, starts to form through hydrogen splitting, affording a mixture 38a/123 in 2:3 ratio. Catalytic investigations on the HY of acetophenone with 36a in ethanol show that high activity is observed in a defined "pH window" [54]. As a matter of fact, under acidic conditions the Ru-H would be protonated resulting in H 2 formation, whereas in basic conditions the presence of a large amount of alkoxide may prevent H 2 coordination to the Ru center.…”

“…Addition of EtOH (3 equiv. ) produces the alcohol adduct 123 (122/123 = 1:3), which equilibrates with the hydride 38a (38a/123 = 3:4) at 25 • C [54]. When a greater amount of EtOH (10 equiv.)…”

“…A clear-cut evidence accounting for the stability of these species has been provided by the alkoxide 114, prepared in 93% yield by treatment of 36a with NaOCH 2 CF 3 stabilized through an intramolecular O· · ·H N hydrogen bonding inferred from NMR spectroscopic and neutron diffraction studies (Scheme 75) [54].…”

“…In addition, this hydrogen bonding network leads to highly labile alkoxide complexes, which easily convert to the hydride species. The catalytically active hydride 38a can be formed from the Ru-alkoxide through two pathways: (1) a fast ␤-hydrogenelimination (path a, Scheme 78) and (2) [54]. be in equilibrium with the Ru-amide-alcohol adduct 127 (path c, Scheme 78).…”