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

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) [LMn^V(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.