To Rebound or...Rebound? Evidence for the “Alternative Rebound” Mechanism in C–H Oxidations by the Systems Nonheme Mn Complex/H2O2/Carboxylic Acid

• Published in: ACS catalysis 2021-05, Vol.11 (9), p.5517-5524
• Main Authors: Ottenbacher, Roman V, Bryliakova, Anna A, Shashkov, Mikhail V, Talsi, Evgenii P, Bryliakov, Konstantin P
• Format: Article
• Language: English
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.1c00811

In this work, it has been shown that aliphatic C–H oxidations by bioinspired catalyst systems Mn aminopyridine complex/H2O2/carboxylic acid in acetonitrile afford predominantly a mixture of the corresponding alcohol and the ester. The alcohol/ester ratio is higher for catalysts bearing electron-donating groups at the aminopyridine core. Isotopic labeling studies witness that the oxygen atom of the alcohol originates from the H2O2 molecule, while the ester oxygen comes exclusively from the acid. Oxidation of ethylbenzene in the presence of acetic acid affords enantiomerically enriched 1-phenylethanol and 1-phenyl acetate, with close enantioselectivities and the same sign of absolute chirality. Experimental data and density functional theory calculations provide evidence in favor of the rate-limiting benzylic H atom abstraction by the high-spin (S = 1) [LMnV(O)­OAc]2+ active species followed by competitive OH/OC­(O)­R rebound. This mechanism has been unprecedented for C–H oxidations catalyzed by bioinspired Mn complexes. The trends governing the alcohol/ester ratios have been rationalized in terms of steric properties of the catalyst, acid, and substrate.