Selective C–H Bond Oxidation Catalyzed by the Fe-bTAML Complex: Mechanistic Implications

Nonheme iron complexes bearing tetradentate N-atom-donor ligands with cis labile sites show great promise for chemoselective aliphatic C–H hydroxylation. However, several challenges still limit their widespread application. We report a mechanism-guided development of a peroxidase mimicking iron complex based on the bTAML macrocyclic ligand framework (Fe-bTAML: biuret-modified tetraamido macrocyclic ligand) as a catalyst to perform selective oxidation of unactivated 3° bonds with unprecedented regioselectivity (3°:2° of 110:1 for adamantane oxidation), high stereoretention (99%), and turnover numbers (TONs) up to 300 using <i>m</i>CPBA as the oxidant. Ligand decomposition pathways involving acid-induced demetalation were identified, and this led to the development of more robust and efficient Fe-bTAML complexes that catalyzed chemoselective C–H oxidation. Mechanistic studies, which include correlation of the product formed with the Fe^V(O) reactive intermediates generated during the reaction, indicate that the major pathway involves the cleavage of C–H bonds by Fe^V(O). When these oxidations were performed in the presence of air, the yield of the oxidized product doubled, but the stereoretention remained unchanged. On the basis of ¹⁸O labeling and other mechanistic studies, we propose a mechanism that involves the dual activation of <i>m</i>CPBA and O₂ by Fe-bTAML, leading to formation of the Fe^V(O) intermediate. This high-valent iron oxo remains the active intermediate for most of the reaction, resulting in high regio- and stereoselectivity during product formation

Formation of a Room Temperature Stable Fe^V(O) Complex: Reactivity Toward Unactivated C–H Bonds

An Fe^V(O) complex has been synthesized from equimolar solutions of (Et₄N)₂[Fe^III(Cl)­(biuret-amide)] and <i>m</i>CPBA in CH₃CN at room temperature. The Fe^V(O) complex has been characterized by UV–vis, EPR, Mössbauer, and HRMS and shown to be capable of oxidizing a series of alkanes having C–H bond dissociation energies ranging from 99.3 kcal mol^–1 (cyclohexane) to 84.5 kcal mol^–1 (cumene). Linearity in the Bell–Evans–Polayni graph and the finding of a large kinetic isotope effect suggest that hydrogen abstraction is engaged the rate-determining step

Tuning the Reactivity of Fe^V(O) toward C–H Bonds at Room Temperature: Effect of Water

The presence of an Fe^V(O) species has been postulated as the active intermediate for the oxidation of both C–H and CC bonds in the Rieske dioxygenase family of enzymes. Understanding the reactivity of these high valent iron–oxo intermediates, especially in an aqueous medium, would provide a better understanding of these enzymatic reaction mechanisms. The formation of an Fe^V(O) complex at room temperature in an aqueous CH₃CN mixture that contains up to 90% water using NaOCl as the oxidant is reported here. The stability of Fe^V(O) decreases with increasing water concentration. We show that the reactivity of Fe^V(O) toward the oxidation of C–H bonds, such as those in toluene, can be tuned by varying the amount of water in the H₂O/CH₃CN mixture. Rate acceleration of up to 60 times is observed for the oxidation of toluene upon increasing the water concentration. The role of water in accelerating the rate of the reaction has been studied using kinetic measurements, isotope labeling experiments, and density functional theory (DFT) calculations. A kinetic isotope effect of ∼13 was observed for the oxidation of toluene and <i>d</i>₈-toluene showing that C–H abstraction was involved in the rate-determining step. Activation parameters determined for toluene oxidation in H₂O/CH₃CN mixtures on the basis of Eyring plots for the rate constants show a gain in enthalpy with a concomitant loss in entropy. This points to the formation of a more-ordered transition state involving water molecules. To further understand the role of water, we performed a careful DFT study, concentrating mostly on the rate-determining hydrogen abstraction step. The DFT-optimized structure of the starting Fe^V(O) and the transition state indicates that the rate enhancement is due to the transition state’s favored stabilization over the reactant due to enhanced hydrogen bonding with water

Abstracts of National Conference on Research and Developments in Material Processing, Modelling and Characterization 2020

This book presents the abstracts of the papers presented to the Online National Conference on Research and Developments in Material Processing, Modelling and Characterization 2020 (RDMPMC-2020) held on 26th and 27th August 2020 organized by the Department of Metallurgical and Materials Science in Association with the Department of Production and Industrial Engineering, National Institute of Technology Jamshedpur, Jharkhand, India. Conference Title: National Conference on Research and Developments in Material Processing, Modelling and Characterization 2020Conference Acronym: RDMPMC-2020Conference Date: 26–27 August 2020Conference Location: Online (Virtual Mode)Conference Organizer: Department of Metallurgical and Materials Engineering, National Institute of Technology JamshedpurCo-organizer: Department of Production and Industrial Engineering, National Institute of Technology Jamshedpur, Jharkhand, IndiaConference Sponsor: TEQIP-