A Class of Ruthenium(II) Catalyst for Asymmetric Transfer Hydrogenations of Ketones

Abstract

Ruthenium dimer 6 (readily available in two steps from TsDPEN) is converted directly to monomeric asymmetric transfer hydrogenation catalyst 3 in situ under the conditions employed for ketone reduction. Catalyst 3 is a significantly more active catalyst for this application than the untethered derivative, exhibits higher enantioselectivities across a range of substrates, and appears to be highly stable to the reaction conditions. It is active at loadings of as low as 0.01 mol %, and reductions at the 0.1 mol % level are complete within 20 min at 80 degrees C without significant loss of enantioselectivity.

Transition Metal‐Catalyzed Homogeneous Asymmetric Hydrogenation

“…It is noteworthy that the tethered of Ru -TsDPEN catalysts such as ( R , R ) -142 and ( S , S ) -143 not only retained the same high level of enantioselectivities but more importantly, the catalytic activity was signifi cantly improved [333,352] . Related catalysts based on the Cp * Rh, Cp * Ir moiety were also reported to exhibit high enantioselectivity for the reduction of acetophenone derivatives [354,355,356] .…”

Applications of N′-alkylated derivatives of TsDPEN in the asymmetric transfer hydrogenation of CO and CN bonds

“…5d,5e Until recently, our own experience of ATH using a Ru(II)/ cymene complex formed in situ from N 0 -benzyl TsDPEN, that is, 7 indicated a poor level of catalytic activity. 7 Switching to the isolated catalysts 7-12, containing instead a benzene ring on the Ru(II), however, gave much better results. Indeed the N 0 -Me complex 7 exhibited a higher catalytic activity than the parent complex 2, for the ATH of both ketones and imines.…”

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

“…Very recently, Morris group [15,16] reported that chiral NNPP ligands and their iron(II) complexes were effective in the ATH of ketones under mild conditions, representing a rare case of using inexpensive iron as the active metal. 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].…”