Enantioselective Transfer Hydrogenation of Ketones Catalyzed by a Manganese Complex Containing an Unsymmetrical Chiral PNP′ Tridentate Ligand

Abstract

Manganese complexes of the types [Mn(PNP′)(Br)(CO)2] and [Mn(PNP′)(H)(CO)2] containing a tridentate ligand with a planar chiral ferrocene and a centro chiral aliphatic unit were synthesized, characterized, and tested in the enantioselective transfer hydrogenations of 13 ketones. The catalytic reactions proceeded with conversions up to 96 % and ee values up to 86 %. The absolute configuration of all products was determined to be (S). Notably, the presence of dihydrogen (up to 20 bar) did not affect the reduction. On the basis of DFT calculations, preliminary mechanistic details including the origin of the (S) selectivity are presented. The molecular structure of [Mn(PNP′)(Br)(CO)2] was studied by X‐ray diffraction.

Retracted: The Manganese(I)‐Catalyzed Asymmetric Transfer Hydrogenation of Ketones: Disclosing the Macrocylic Privilege

“…Other aryl substituents ( 5 d – f ) also give good enantioselectivities (93 % ee ). This reaction is the first example of asymmetric Mn I ‐catalyzed hydrogenation of ketones containing large aryl groups such as 1‐acetylpyrene . Several acyl‐substituted heterocycles such as acetylthiophenes ( 5 g – h ), furan ( 5 i ), pyrrole ( 5 j ), indole ( 5 k ), and pyridines ( 5 l – m ) were reduced to the corresponding alcohols without catalyst poisoning.…”

“…The catalyst is less active with ortho ‐substituted substrates ( 3 b , 3 e , 3 i ), probably because of steric effects, but increasing the reaction time from 3 to 6 hours gives good yields (88–98 % yield). Notably, the 2,6‐methoxy‐disubstituted ketone 3 h is reduced at 80 °C within 9 hours with 61 % yield and 97 % ee , which is unprecedented with Mn I . In addition to alkyl and alkoxy substituents, halogen substituents ( 3 i – n ) are tolerated (90–97 % ee ).…”

“…Herein, we report the most effective Mn I catalyst for the transfer hydrogenation of a broad number of substrates (43) with enantioselectivity ranging from 90 to 99 % ee that is based on the previously developed chiral (NH) 2 P 2 macrocycle 1 (Scheme ) . The catalyst 2 outperforms both C and the asymmetric transfer hydrogenation catalysts E and F , which give low to moderate enantioselectivities (20–85 ee %) with aryl alkyl ketones …”

“…Tertiary amines ( 3 r – u ) are also well tolerated (90–96 % ee ). The complex 2 is the first chiral Mn I catalyst that hydrogenates acetylanilines ( 3 r , s ) and 4′‐(imidazol‐1‐yl)acetophenone ( 3 u ) . Ketones are reduced chemoselectively in the presence of C≡C triple bonds ( 3 v ) and sulfone ( 3 w ).…”

Aminotriazole Mn(I) Complexes as Effective Catalysts for Transfer Hydrogenation of Ketones

“…Taken together, the results summarized in Table and together with previous reports from the literature are consistent with a reaction mechanism as suggested in Scheme . First, the active manganese hydride I could be generated in presence of i PrOH and the base.…”