Exceptionally Efficient Unsymmetrical Ruthenium(II) NNN Complex Catalysts Bearing a Pyridyl-Based Pyrazolyl−Imidazolyl Ligand for Transfer Hydrogenation of Ketones

Abstract

Rare ruthenium(II) complexes bearing a pyridylbased pyrazolyl-imidazolyl ligand were synthesized and exhibited exceptionally high catalytic actiVity in the transfer hydrogenation of ketones in 2-propanol at 82 °C or room temperature, reaching 100% conVersion of the substrates and final TOFs up to 7.2 × 10 5 h -1 with 0.05 mol % catalyst at 82 °C and 55 800 h -1 with 0.1 mol % catalyst at room temperature.

New N,N,N‐Donors Resulting in Highly Active Ruthenium Catalysts for Transfer Hydrogenation at Room Temperature

“…Typical for octahedral ruthenium(II) monophosphane complexes, the 31 P NMR resonances of 3a,b are observed a δ = 44.20 and 42.71 ppm. [18] We recently published the reaction of the N,N,N-ligand 1b (see Scheme 1), with N-H instead of N-allyl functions, with [RuCl 2 (PPh 3 ) 3 ]. [19] This leads to a cationic ruthenium(II) complex of the type [RuCl(1b)(PPh 3 ) 2 ]Cl with the two phosphane ligands trans to each other ( 31 P NMR: δ = 25.5 ppm).…”

Ru(II) pyridyl-based NNN complex catalysts for (asymmetric) transfer hydrogenation of ketones at room temperature

“…As the bulkiness of the substituents decreased from bromo to chloro, the resultant enantioselectivity decreased from 95% to 93% (entries 4 and 5, Table 3). We have previously shown that a benzimidazolyl N-H in a ligand can accelerate the TH and ATH of ketones [28][29][30], and an imidazolinyl N-H functionality has also been reported as improving the enantioselectivity in ATH [38]. In our case, the combination of the two N-H functionalities may provide the complex catalysts with high catalytic activity and good selectivity for ATH of ketones under mild conditions.…”

“…Following the procedure previously reported from our laboratory [21][22][23][24][25][26][27][28][29][30][31][32], complex 6a was tested as a catalyst for the transfer hydrogenation of acetophenone in the presence of i-PrOK. To our surprise, 6a exhibited excellent catalytic activity for the reduction of acetophenone in 2-propanol even at room temperature (28 °C).…”

“…Although new ligands and their transition metal complexes have been established in this area [17][18][19][20], the desire for more highly active catalytic systems with better stereoselectivity and broader substrate scopes under mild conditions is still strong. We have recently documented that highly active transition metal complex catalysts could be rationally constructed by introducing a benzimidazolyl or pyrazolyl coordinating arm on a pyridyl-based ligand framework [21][22][23][24][25][26][27][28][29][30]. A series of highly active Ru(II) complexes have been obtained for the transfer hydrogenation of ketones [31,32], achieving >99% yield and final TOFs up to 720 000 h -1 at 82 °C, and 99% ee and final TOFs up to 55 800 h -1 at 28 °C in ATH (Scheme 1).…”

A family of novel cationic ruthenium pincer complexes: Synthesis, characterization and catalytic activity in the transfer hydrogenation of ketones

“…Compared with the commonly used reduction processes which involve high hydrogen pressure or hazardous reducing reagents [2][3][4], transfer hydrogenation has emerged as a safe, ecofriendly and versatile tool for the reduction of carbonyl compounds. Recently, the Ru(II) complexes bearing planar tridentate N 3 ligands and unsymmetrical planar tridentate ligands have been successfully developed and explored to construct the effective catalyst systems for the transfer hydrogenation of ketones [5][6][7][8][9][10]. In the study of the transfer hydrogenation mechanism, several ruthenium hydride complexes are considered as active species [11][12][13][14][15][16][17][18], but some ruthenium hydride complexes, such as RuHCl(PPh 3 ) 3 [11], are not the really active species.…”