Ruthenium(II) complex catalysts bearing a pyridyl-supported pyrazolyl-imine ligand for transfer hydrogenation of ketones

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

“…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.…”

“…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. For the Ru(II) complexes bearing planar tridentate N 3 ligands, although some researchers regard the ruthenium hydride complexes as the real active species in the transfer hydrogenation of ketones [5][6][7][8][9][10], no ruthenium hydride complex has been isolated. Hence, to obtain ruthenium hydride complex bearing N 3 ligand is still strongly desired in this area.…”

“…Furthermore, complex 2 was also an efficient catalyst for the transfer hydrogenation of alkyl ketones and cyclic alkyl ketones (entries [11][12][13][14]. Compared to the reported Ru(II) N 3 pincer complexes [5][6][7][8][9][10], the activity of complex 2 is relatively low for substituted acetophenone. To our delight, cyclopentanone was quantitatively reduced to its corresponding alcohol within 10 min and the [26], and rigid diphosphine arene-ruthenium(II) complex [27].…”