Asymmetric Transfer Hydrogenation with a Bifunctional Iron(II) Hydride: Experiment Meets Computation

Abstract

Hydride cis-β-[FeH(CNCEt 3 )(1)]BF 4 (5) (1 is a chiral N 2 P 2 macrocycle) is the catalytically active species in the asymmetric transfer hydrogenation of ketones formed upon reaction of [Fe(CNCEt 3 ) 2 ( 1)](BF 4 ) 2 (3) with base. Stoichiometric reactions show that hydride 5 is formed by H-elimination from the 2-propoxo complex [Fe(O i Pr)(CNCEt 3 )(1)]BF 4 (8a) and inserts the CO bond of acetophenone to give the diastereoisomeric alcoholato complexes [Fe(OCH(Me)Ph)-(CNCEt 3 )(1)]BF 4 (10R and 10S). Complexes 5, 8a, and 10 were characterized by NMR spectroscopy, and their structures were calculated by DFT. The DFT study supports a bifunctional mechanism with the alkoxo complexes 8a and 10 as resting species. The stereochemical model reproduces the high enantioselectivity with acetophenone, which results from the combination of the rigid macrocyclic scaffold with the bulky, yet conformationally flexible isonitrile.

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

“…Our previous studies have shown that the bulky isonitrile in the iron(II) analogue [Fe(CNCEt 3 ) 2 ( 1 )](BF 4 ) 2 is beneficial to enantioselectivity . The present DFT study reveals that attractive CH–π interactions between the substrate and 1 induce high enantioselectivity (≥90 % ee ) even without the assistance of bulky ancillary ligands and explain why aryl alkyl ketones are the substrates of choice.…”

Isonitrile Ruthenium and Iron PNP Complexes: Synthesis, Characterization and Catalytic Assessment for Base-Free Dehydrogenative Coupling of Alcohols

“…This has been recently observed by Mezetti in a related context (ironmediated transfer hydrogenation of ketones using a secondary alcohol as hydrogen source), though basic conditions were required for the carbene formation to proceed. 30 As alkoxide species of the type [Ru(PNP H )(OR)(H)(CNR)] are most likely formed and involved in the catalytic cycle for alcohol dehydrogenative coupling, as in the case of carbonyl-based systems, 16d one may also envision an intramolecular deactivation pathway to access such carbene species. Alternatively, when investigating iron isonitrile complexes as catalysts for ketones transfer hydrogenation, Reiser and coworkers have also proposed that an intermediate iron hydride moiety reacts with isonitrile ligand, affording iminoformyl species 31 , similar to hydride migratory insertion into a carbonyl to form a acyl group.…”

Catalytic Strategies to Enantiopure Benzoins: Past and Future

“…95 Indeed, a mechanistic study shows that, also for this catalytic system, the active species is hydride [FeH(CNCEt 3 )(N 2 P 2 )]BF 4 , which is formed from the bis(isonitrile) complex in basic 2-propanol. 96…”