Issue 23, 2021

Exploration of highly electron-rich manganese complexes in enantioselective oxidation catalysis; a focus on enantioselective benzylic oxidation

Abstract

The direct enantioselective hydroxylation of benzylic C–H bonds to form chiral benzylic alcohols represents a challenging transformation. Herein, we report on the exploration of new biologically inspired manganese and iron complexes bearing highly electron-rich aminopyridine ligands containing 4-pyrrolidinopyridine moieties ((S,S)-1, (R,R)-1, 2 and 5) in combination with chiral bis-pyrrolidine and N,N-cyclohexanediamine backbones in enantioselective oxidation catalysis with aqueous H2O2. The current manganese complexes outperform the analogous manganese complexes containing 4-dimethylaminopyridine moieties (3 and 4) in benzylic oxidation reactions in terms of alcohol yield while keeping similar ee values (∼60% ee), which is attributed to the higher basicity of the 4-pyrrolidinopyridine group. A detailed investigation of different carboxylic acid additives in enantioselective benzylic oxidation provides new insights into how to rationally enhance enantioselectivities by means of proper tuning of the environment around the catalytic active site, and has resulted in the selection of Boc-L-tert-leucine as the preferred additive. Using these optimized conditions, manganese complex 2 was shown to be effective in the enantioselective benzylic oxidation of a series of arylalkane substrates with up to 50% alcohol yield and 62% product ee. A final set of experiments also highlights the use of the new 4-pyrrolidinopyridine-based complexes in the asymmetric epoxidation of olefins (up to 98% epoxide yield and >99% ee).

Graphical abstract: Exploration of highly electron-rich manganese complexes in enantioselective oxidation catalysis; a focus on enantioselective benzylic oxidation

Supplementary files

Article information

Article type
Paper
Submitted
09 Sep 2021
Accepted
19 Oct 2021
First published
20 Oct 2021
This article is Open Access
Creative Commons BY-NC license

Catal. Sci. Technol., 2021,11, 7751-7763

Exploration of highly electron-rich manganese complexes in enantioselective oxidation catalysis; a focus on enantioselective benzylic oxidation

E. Masferrer-Rius, F. Li, M. Lutz and R. J. M. Klein Gebbink, Catal. Sci. Technol., 2021, 11, 7751 DOI: 10.1039/D1CY01642C

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