The journal Advanced Synthesis & Catalysis features on its front cover the recently published paper “Aromatic C?H Hydroxylation Reactions with Hydrogen Peroxide Catalyzed by Bulky Manganese Complexes”.
The oxidation of aromatic substrates to phenols with H2O2 as a benign oxidant remains an ongoing challenge in synthetic chemistry. Herein, we successfully achieved to catalyze aromatic C?H bond oxidations using a series of biologically inspired manganese catalysts in fluorinated alcohol solvents. While introduction of bulky substituents into the ligand structure of the catalyst favors aromatic C?H oxidations in alkylbenzenes, oxidation occurs at the benzylic position with ligands bearing electron-rich substituents. Therefore, the nature of the ligand is key in controlling the chemoselectivity of these Mn-catalyzed C?H oxidations. We show that introduction of bulky groups into the ligand prevents catalyst inhibition through phenolate-binding, consequently providing higher catalytic turnover numbers for phenol formation. Furthermore, employing halogenated carboxylic acids in the presence of bulky catalysts provides enhanced catalytic activities, which can be attributed to their low pKa values that reduces catalyst inhibition by phenolate protonation as well as to their electron-withdrawing character that makes the manganese oxo species a more electrophilic oxidant. Moreover, to the best of our knowledge, the new system can accomplish the oxidation of alkylbenzenes with the highest yields so far reported for homogeneous arene hydroxylation catalysts. Overall our data provide a proof-of-concept of how Mn(II)/H2O2/RCO2H oxidation systems are easily tunable by means of the solvent, carboxylic acid additive, and steric demand of the ligand. The chemo- and site-selectivity patterns of the current system, a negligible KIE, the observation of an NIH-shift, and the effectiveness of using tBuOOH as oxidant overall suggest that hydroxylation of aromatic C?H bonds proceeds through a metal-based mechanism, with no significant involvement of hydroxyl radicals, and via an arene oxide intermediate.
The cover is designed by Eila Serrano-Hervás, illustrates a new methodology for the direct oxidation of aromatic substrates to the corresponding phenol products with aqueous hydrogen peroxide, using a combination of a bulky manganese catalyst, a halogenated carboxylic acid, and a fluorinated alcohol solvent. Like in a recipe for an ice-cream flavor, each and every ingredient of this reaction has been carefully selected and is crucial for achieving the final aromatic product/flavor.
The corresponding paper was recently published in Advanced Synthesis & Catalysis:
E. Masferrer?Rius, M. Borrell, M. Lutz, M. Costas, and R.J.M. Klein Gebbink
“Aromatic C?H Hydroxylation Reactions with Hydrogen Peroxide Catalyzed by Bulky Manganese Complexes”
Adv. Synth. Catal. 2021, ASAP-
DOI: 10.1002/adsc.202001590
Girona, July 29, 2021
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