Mechanism‐Based Enantiodivergence in Manganese Reduction Catalysis: A Chiral Pincer Complex for the Highly Enantioselective Hydroboration of Ketones

Abstract

A manganese alkyl complex containing a chiral bis(oxazolinyl‐methylidene)isoindoline pincer ligand is a precatalyst for a catalytic system of unprecedented activity and selectivity in the enantioselective hydroboration of ketones, thus producing preparatively useful chiral alcohols in excellent yields with up to greater than 99 % ee. It is applicable for both aryl alkyl and dialkyl ketone reduction under mild reaction conditions (TOF >450 h−1 at −40 °C). The earth‐abundant base‐metal catalyst operates at very low catalyst loadings (as low as 0.1 mol %) and with a high level of functional‐group tolerance. There is evidence for the existence of two distinct mechanistic pathways for manganese‐catalyzed hydride transfer and their role for enantiocontrol in the selectivity‐determining step is presented.

Manganese‐Catalyzed Asymmetric Formal Hydroamination of Allylic Alcohols: A Remarkable Macrocyclic Ligand Effect

“…Among them, Mn I ‐catalyzed asymmetric hydrogenation [10] and transfer hydrogenation [11] are much more attractive, and expected to acquire more eye‐catching achievements. Considering the wide applications of multidentate PNP, [9a–f, 10a–c, 11a,f] PNN, [9g–i, 10d–g, 11b] and NNN [9j–l] pincer ligands in manganese catalysis and based on the continuing efforts of our group in asymmetric hydrogenation, [12] we envisioned that the above designed chiral N 6 ‐macrocyclic ligands could be applied in Mn I ‐catalyzed asymmetric catalysis. To the best of our knowledge, there is no report on Mn I ‐catalyzed enantioselective reactions by utilizing a chiral peraza macrocyclic ligand so far.…”

Half‐Sandwich Manganese Complex Bearing Fused Oxazoline‐NHC Ligand: Conformational Analysis and Evaluation in Asymmetric Ketone Hydrosilylation

“…In sharp contrast to the multitude of identified Mn‐based catalytic systems for ketone hydrosilylation, asymmetric version of this transformation remains very scarce. In 2017, Gade mentioned that the Mn(II) alkyl complex 1 supported by a pincer‐type bis(oxazolinylmethylidene)isoindoline scaffold (Scheme 1) was capable to reduce acetophenone using various silanes with 70–71 % ee [5] . In parallel, the group of Huang showed that the Mn(II) dichloride complex 2 (Scheme 1) bearing a bulky iminopyridine‐oxazoline ligand was active in hydrosilylation of diverse alkyl(aryl)ketones with PhSiH 3 in the presence of NaBHEt 3 as activator affording the corresponding alcohols in 80–92 % ee [6] …”

Manganese‐Catalyzed Asymmetric Formal Hydroamination of Allylic Alcohols: A Remarkable Macrocyclic Ligand Effect

“…Among them, Mn I ‐catalyzed asymmetric hydrogenation [10] and transfer hydrogenation [11] are much more attractive, and expected to acquire more eye‐catching achievements. Considering the wide applications of multidentate PNP, [9a–f, 10a–c, 11a,f] PNN, [9g–i, 10d–g, 11b] and NNN [9j–l] pincer ligands in manganese catalysis and based on the continuing efforts of our group in asymmetric hydrogenation, [12] we envisioned that the above designed chiral N 6 ‐macrocyclic ligands could be applied in Mn I ‐catalyzed asymmetric catalysis. To the best of our knowledge, there is no report on Mn I ‐catalyzed enantioselective reactions by utilizing a chiral peraza macrocyclic ligand so far.…”