Construction of Highly Active Ruthenium(II) NNN Complex Catalysts Bearing a Pyridyl-Supported Pyrazolyl-Imidazolyl Ligand for Transfer Hydrogenation of Ketones

Abstract

A family of hemilabile ruthenium(II) NNN complexes bearing a unsymmetrical 2-(benzoimidazol-2-yl)-6-(pyrazol-1-yl)pyridine ligand has been synthesized and exhibited good to excellent catalytic activity in transfer hydrogenation of ketones in refluxing 2-propanol, reaching final TOFs up to 7.2 × 105 h−1 with 0.05 mol % loading. The γ-NH effect of the benzoimidazol-2-yl moiety in the ligand and coordination modes of the metal center in a Ru(II) NNN complex has great influence on the catalytic activity of the complex catalyst in transfer hydrogenation of ketones. It has been demonstrated that one of the structural prerequisites for an active Ru(II) complex catalyst is the coordinatively unsaturated environment around the metal center in the complex or the precatalyst, and the catalytic activity of a complex catalyst can be enhanced by making its metal center cationic. This paper presents a methodology to construct new types of efficient Ru(II) complex catalysts for transfer hydrogenation of ketones.

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).…”

β‐Protic Pyrazole and N‐Heterocyclic Carbene Complexes: Synthesis, Properties, and Metal–Ligand Cooperative Bifunctional Catalysis

“…[42] Notably, the related imidazole complex 34 bearing an NH group at a more remote g-position exhibits much higher catalytic activity. [43] This observation implies indirect participation of the b-protic group to the catalysis; the deprotonation may just increase the lability of the halide ligand to produce a vacant coordination site. The proposed mechanism of the transfer hydrogenation thus involves conventional b-elimination and migratory insertion steps without cooperation of the uncoordinated nitrogen atom and is not truly bifunctional.…”

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.…”