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

Abstract

In this work, it has been shown that aliphatic C−H oxidations by bioinspired catalyst systems Mn aminopyridine complex/H 2 O 2 /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 H 2 O 2 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.

Resolving Oxygenation Pathways in Manganese-Catalyzed C(sp3)–H Functionalization via Radical and Cationic Intermediates

“…An analogous competition has been recently proposed by Bryliakov and co-workers in the benzylic C−H bond oxygenation of cumene with H 2 O 2 catalyzed by manganese complexes. 26 In this study, the hydroxylation/esterification product ratio was observed to be unaffected by the carboxylic acid loading, increasing increasing steric bulk of the carboxylic acid additive, supporting the involvement of competitive hydroxyl and carboxylate rebound pathways in these reactions.…”

“…Within this mechanistic picture, the steric hindrance of the carboxylic acid additive plays a crucial role in determining the contribution of the carboxylate rebound pathway, with the relative importance of this pathway, quantified by the P1-O / P1a-OX n ratio, that decreases with increasing steric bulk of the acid, being completely suppressed when employing the very bulky Phth-Tle-OH (Scheme a). An analogous competition has been recently proposed by Bryliakov and co-workers in the benzylic C–H bond oxygenation of cumene with H 2 O 2 catalyzed by manganese complexes . In this study, the hydroxylation/esterification product ratio was observed to be unaffected by the carboxylic acid loading, increasing with increasing steric bulk of the carboxylic acid additive, supporting the involvement of competitive hydroxyl and carboxylate rebound pathways in these reactions.…”

“…Iron and manganese complexes containing tetradentate aminopyridine ligands are powerful C–H oxidation catalysts that are able to promote selective aliphatic C–H bond hydroxylation. Mechanistic studies point toward a common mechanistic scenario for iron and manganese catalysts where C–H hydroxylation proceeds via an enzymatic-like HAT/rebound mechanism executed by high-valent metal-oxo species. − , Alternative reactions entailing the transfer of the ligand cis to oxo − and desaturation have been observed only in selected cases where, however, these pathways are always accompanied by the canonical hydroxylation reaction, which typically dominates the reactivity. A general understanding of the factors that govern these divergent reactivities is lacking and evidence in favor of cationic paths is scarce.…”

Theoretical Insight into the Mechanism and Selectivity in Manganese-Catalyzed Oxidative C(sp3)–H Methylation

“…Once formed, IM10 might evolve along the three potential branches: , OH-rebound, AcO-rebound, and desaturation. The OH-rebound branch involves the recombination of two fragments in IM10 , leading to the hydroxylation product complex IM11 , which then dissociates into the hydroxylated heterocycle intermediate IM12 and the Mn III –OAc species.…”

Reactivity vs. Selectivity of Biomimetic Catalyst Systems of the Fe(PDP) Family through the Nature and Spin State of the Active Iron‐Oxygen Species

“…To this end, DFT calculations have been performed at the B3LYP/def2‐TZVPP (for Fe)/6‐311G(d) (for other atoms) level of theory (see Supporting Information for details), which approach has been recently shown to adequately predict the gaps between different spin states of transition metal oxo species [12a,b] . Crucially, the results witness that the difference between the energies of the high‐ and low‐spin isomers decreases with increasing electron‐donating properties of the substituents (H, OMe, NMe 2 ), such that high‐spin structures become preferred in the case of very strong electron donors NMe 2 (Table 2).…”