Ru-TsDPEN catalysts and derivatives in asymmetric transfer hydrogenation reactions

Ru(II)-Catalyzed Enantioselective Transfer Hydrogenation of α-Ketophosphonates: Straightforward Access to Valuable Chiral α-Hydroxy Phosphonates

Asymmetric transfer hydrogenation (ATH) is arguably one of the most powerful tools for the synthesis of chiral compounds. Despite tremendous advances in this field, the reduction of α-ketophosphonates remains largely unexplored. Herein, we report an efficient Ru-catalyzed ATH on a broad range of α-ketophosphonates. Compared with existing methods, our approach offers as advantages mild conditions, operational simplicity, limited waste generation, broad substrate scope (26 examples), good to excellent yields (75-93%), and excellent levels of stereoinduction (from 90% to >99% ee).

Ru(II)-Catalyzed Asymmetric Transfer Hydrogenation of Chalcones in Water: Application to the Enantioselective Synthesis of Flavans BW683C and Tephrowatsin E

The oxo-tethered-Ru(II) precatalyst promoted the one-pot C═C/C═O reduction of chalcones using sodium formate as the hydrogen source in water through asymmetric transfer hydrogenation. Twenty-seven 1,3-diarylpropan-1-ols were obtained in good to excellent yields (up to 96%) and enantiomeric purities (up to 98:2). Our data suggested that the enones are first reduced to the corresponding dihydrochalcones (1,4-selectivity) and then into 1,3-diarylpropan-1-ols (C═O reduction). The stereoelectronic effects of electron-donating and electron-withdrawing groups at the ortho, meta and para positions of both aromatic rings were evaluated. The 2-OH group at the B ring was well tolerated, allowing a straightforward enantioselective synthesis of two flavans through the Mitsunobu cyclization, the antiviral (S)-BW683C and the natural flavan (S)-tephrowatsin E.

Asymmetric transfer hydrogenation of boronic acid pinacol ester (Bpin)-containing acetophenones

A series of Bpin-containing acetophenone derivatives were reduced by asymmetric transfer hydrogenation (ATH), using Noyori-Ikariya catalysts, with formic acid/triethylamine, to alcohols in high ee when the Bpin is in the para- or meta-position. Substrates containing ortho-Bpin groups were reduced in lower ee, with formation of a cyclic boron-containing group. The products were converted to substituted derivatives using Pd-catalysed coupling reactions. The results represent the first examples of ATH of Bpin-containing ketones.

Diastereoselective synthesis of a cyclic diamide-bridged biphenyl as chiral atropos ligand

Chiral compounds with a 1,2-diamine structure motif and their derivatives are of great interest in organic chemistry and are broadly used in asymmetric transformations, as chiral auxiliaries, (co)ligands, and ligand core structure. Here, we present a straightforward, diastereoselective synthesis for a diamide-bridged biaryl ligand. The ring closing reaction of the racemic atropos biphenyl 6,6'-dimethoxy-[1,1'-biphenyl]-2,2'-dicarboxylic acid with (R,R)-diaminocyclohexane yields the diasteromerically and enantiomerically pure cyclic (S_ax ,R,R)-BIPOL, which can be used as a versatile chiral ligand. By NMR spectroscopy, we observed the formation of intermolecular aggregates of the diamide-bridged BIPOL with anhydrous DMSO-d₆ . DFT calculations at the B3LYP/6-31G* level of theory corroborate the high interconversion barrier for the biaryl axis of ΔG^ǂ  = 148.7 kJ mol^-1 and the favoured formation of (S_ax ,R,R)-BIPOL as single stereoisomer.

Recent Developments in Enantio- and Diastereoselective Hydrogenation of N-Heteroaromatic Compounds

The enantioselective and diastereoselective hydrogenation of N-heteroaromatic compounds is an efficient strategy to access chirally enriched cyclic heterocycles, which often possess highly bio-active properties. This strategy, however, has only been...

Asymmetric hydrogenation and transfer hydrogenation in the enantioselective synthesis of flavonoids

In this review, we explore the applications of Asymmetric Hydrogenation (AH) and Asymmetric Transfer Hydrogenation (ATH) in the total synthesis of natural flavonoids and their analogues, highlighting the limitations and opportunities in the field.

Asymmetric Transfer Hydrogenation of α-Keto Amides; Highly Enantioselective Formation of Malic Acid Diamides and α-Hydroxyamides

The asymmetric transfer hydrogenation (ATH) of α-keto-1,4-diamides using a tethered Ru/TsDPEN catalyst was achieved in high ee. Studies on derivatives identified the structural elements which lead to the highest enantioselectivities in the products. The α-keto-amide reduction products have been converted to a range of synthetically valuable derivatives.