Aerobic intramolecular carbon-hydrogen bond oxidation promoted by Cu(I) complexes

Photoredox Oxidation of Alkanes by Monometallic Copper-Oxygen Complexes Using Visible Light Including One Sun Illumination

Oxygenation of hydrocarbons offers versatile catalytic routes to more valuable compounds, such as alcohols, aldehydes, and ketones. Despite the importance of monometallic copper-oxygen species as hydroxylating agents in biology, few synthetic model compounds are known to react with hydrocarbons, owing to high C-H bond dissociation energies. To overcome this challenge, the photoredox chemistry of monometallic copper (pyrazolyl)borate complexes coordinated by chlorate has been explored in the presence of C1-C6 alkanes with BDEs ≥ 93 kcal/mol. Ethane is photooxidized at room temperature under N2 with yields of 15-30%, which increases to 77% for the most oxidizing tris(3,5-trifluoromethyl-pyrazolyl)borate complex (Cu-3). This complex also promotes the photooxidation of methane to methanol in significant yield (38%) when the photoredox reaction is run under aerobic conditions. Ligand modification alters the reaction selectivity by tuning the redox potential. The ability to activate 1° C-H bonds of C1-C6 alkanes using visible light is consistent with the photogeneration of a powerfully oxidizing copper-oxyl, which is supported by photocrystallographic studies of the tris(3,4,5-tribromopyrazolyl)borate chlorate complex. Mechanistic studies are consistent with the hydrogen atom abstraction of the C-H bond by the copper-oxyl intermediate. We demonstrate for Cu-3 with hexane as an exemplar, that the photoredox chemistry may be achieved under solar conditions of one-sun illumination.

Recent advances in computational studies on Cu-catalyzed aerobic reactions: cooperation of copper catalysts and dioxygen

O2, one of the ideal oxidants, suffers from low solubility, low oxidizability, low selectivity and a triplet ground state when applied in organic synthesis. Biomimetic copper catalysis has been demonstrated to be a powerful method for activating and transforming O2 to conduct aerobic reactions for a long time. On the other hand, the structures of Cu-O2 complexes are complex with diverse downstream reactions, whereas active copper intermediates were rarely identified by experimental methods, making the mechanisms of many Cu-catalyzed aerobic reactions far from clear. In this context, computational studies emerged as an effective alternative to mechanistic studies on Cu-catalyzed aerobic reactions. This review introduces the relevant computational studies since 2012, focusing on showing the cooperation of copper catalysts and O2 in dehydrogenation, oxygenation and coupling reactions.

Direct Benzene Hydroxylation with Dioxygen Induced by Copper Complexes: Uncovering the Active Species by DFT Calculations

The direct oxidation of benzene into phenol using molecular oxygen at very mild temperatures can be promoted in the presence of the copper complex Tp^Br3Cu(NCMe) in the homogeneous phase in the presence of ascorbic acid as the source of protons and electrons. The stoichiometric nature, relative to copper, of this transformation prompted a thorough DFT study in order to understand the reaction pathway. As a result, the dinuclear species Tp^Br3Cu^II(μ-O^•)(μ-OH)Cu^IITp^Br3 is proposed as the relevant structure which is responsible for activating the arene C–H bond leading to phenol formation.

Mechanistic Studies on the Synthesis of Pyrrolidines and Piperidines via Copper-Catalyzed Intramolecular C–H Amination

We have recently developed a method for the synthesis of pyrrolidines and piperidines via intramolecular C–H amination of N-fluoride amides using [Tp^xCuL] complexes as precatalysts [Tp^x = tris(pyrazolyl)borate ligand and L = THF or CH₃CN]. Herein, we report mechanistic studies on this transformation, which includes the isolation and structural characterization of a fluorinated copper(II) complex, [(Tp^iPr2OH)CuF] [Tp^iPr = hydrotris(3,5-diisopropylpyrazolyl)borate], pertinent to the mechanistic pathway. The effects of the nature of the Tp^x ligand in the copper catalyst as well as of the halide in the N–X amides employed as reactants have been investigated both from experimental and computational perspectives.

Transmetalation Reactions Triggered by Electron Transfer between Organocopper Complexes

[Cu(bipy)(C₆F₅)] reacts with most aryl iodides to form heterobiphenyls by cross-coupling, but when Rf–I is used (Rf = 3,5-dicholoro-2,4,6-trifluorophenyl), homocoupling products are also formed. Kinetic studies suggest that, for the homocoupling reaction, a mechanism based on transmetalation from [Cu(bipy)(C₆F₅)] to Cu(III) intermediates formed in the oxidative addition step is at work. Density functional theory calculations show that the interaction between these Cu(III) species and the starting Cu(I) complex involves a Cu(I)–Cu(III) electron transfer concerted with the formation of an iodine bridge between the metals and that a fast transmetalation takes place in a dimer in a triplet state between two Cu(II) units.

In copper-catalyzed cross-coupling reactions, electron-transfer processes between Cu(I) and Cu(III) species are overlooked behind RDS (C−X activation). Density functional theory studies considering two molecules of the catalyst and two spin states throughout the course of the reaction have revealed the feasibility of such a process and the transmetalation between Cu(II) species, justifying in this way the formation of homocoupling products.