Earth-Abundant Mixed-Metal Catalysts for Hydrocarbon Oxygenation

Abstract

The oxygenation of aliphatic and aromatic hydrocarbons using earth-abundant Fe and Cu catalysts and "green" oxidants such as hydrogen peroxide is becoming increasingly important to atom-economical chemical processing. In light of this, we describe that dinuclear Cu complexes of pyrrolic Schiff-base macrocycles, in combination with ferric chloride (FeCl), catalyze the oxygenation of π-activated benzylic substrates with hydroperoxide oxidants at room temperature and low loadings, representing a novel design in oxidation catalysis. Mass spectrometry and extended X-ray absorption fine structure analysis indicate that a cooperative action between Cu and Fe occurs, most likely because of the interaction of FeCl or FeCl with the dinuclear Cu macrocycle. Voltammetric measurements highlight a modulation of both Cu and Fe redox potentials in this adduct, but electron paramagnetic resonance spectroscopy indicates that any Cu-Fe intermetallic interaction is weak. High ketone/alcohol product ratios, a small reaction constant (Hammett analysis), and small kinetic isotope effect for H-atom abstraction point toward a free-radical reaction. However, the lack of reactivity with cyclohexane, oxidation of 9,10-dihydroanthracene, oxygenation by the hydroperoxide MPPH (radical mechanistic probe), and oxygenation in dinitrogen-purge experiments indicate a metal-based reaction. Through detailed reaction monitoring and associated kinetic modeling, a network of oxidation pathways is proposed that includes "well-disguised" radical chemistry via the formation of metal-associated radical intermediates.

Visible‐Light‐Triggered Quantitative Oxidation of 9,10‐Dihydroanthracene to Anthraquinone by O2 under Mild Conditions

“…[14] Therefore, it is of great practical importance to developacost-effective and environmentally friendlym ethod for the synthesis of AQ. Recently,aseries of works have been reported to replacet he conventional oxidative systems of AQ synthesis, such as metal (Fe, Cu, Co, Ru, Mn) complexes, [15][16][17][18][19][20][21][22][23][24][25] Ru(tmp)(O) 2 /N 2 O( tmp:5 ,10,15,20-tetramesitylporphyrin), [26] air/oxygen/nitrica cid, [13] H 5 PV 2 Mo 10 O 40 /O 2 , [27] molecular sieve catalysts, [28][29][30] and so on. The use of dinuclear Cu II complexes of pyrrolic Schiff-base macrocycles in combination with FeCl 3 (Cu II -FeCl 3 )a sa ne ffective catalyst for the oxidation of 9,10-dihydroanthracene (DHA) to AQ was reported by Pankhurst et al,g iving ah igherD HA conversion (100 %) and 87.0 %AQyield, but high cost severely restricted its application for the synthesis of AQ.…”

“…The use of dinuclear Cu II complexes of pyrrolic Schiff-base macrocycles in combination with FeCl 3 (Cu II -FeCl 3 )a sa ne ffective catalyst for the oxidation of 9,10-dihydroanthracene (DHA) to AQ was reported by Pankhurst et al,g iving ah igherD HA conversion (100 %) and 87.0 %AQyield, but high cost severely restricted its application for the synthesis of AQ. [18] Selvaraj and Kawi reportedt hat CrSBA-15 was also an efficient solid catalyst for the liquidphase oxidation of AN to AQ, and achieved 90.6 %A Nc onversion and 100 %A Qs electivity. [28] Unfortunately,n ot only are the synthesis routes of the catalyst rather complex, but it is also very expensive to use al arge amounto fc atalyst (100 wt %) and tert-butyl hydroperoxide (TBHP) as an oxidant.…”

“…Recently, a series of works have been reported to replace the conventional oxidative systems of AQ synthesis, such as metal (Fe, Cu, Co, Ru, Mn) complexes, Ru(tmp)(O) 2 /N 2 O (tmp: 5,10,15,20‐tetramesitylporphyrin), air/oxygen/nitric acid, H 5 PV 2 Mo 10 O 40 /O 2 , molecular sieve catalysts, and so on. The use of dinuclear Cu II complexes of pyrrolic Schiff‐base macrocycles in combination with FeCl 3 (Cu II ‐FeCl 3 ) as an effective catalyst for the oxidation of 9,10‐dihydroanthracene (DHA) to AQ was reported by Pankhurst et al., giving a higher DHA conversion (100 %) and 87.0 % AQ yield, but high cost severely restricted its application for the synthesis of AQ . Selvaraj and Kawi reported that CrSBA‐15 was also an efficient solid catalyst for the liquid‐phase oxidation of AN to AQ, and achieved 90.6 % AN conversion and 100 % AQ selectivity .…”

Nitric‐Acid‐Catalyzed Aerobic Conversion of Benzyl Ethers to Benzaldehydes at Room Temperature

“…[3] By comparison, procedures for the preparation of aryl aldehydes with benzyl ethers are a more challenging goal. In early literatures, such a transformation relies on the use of various oxidants such as H 2 O 2 /Br 2 , [4] t-BuOOH, [5] Ca(ClO) 2 , [6] HOF⋅CH 3 CN, [7] N-bromobutanimide, [8] HNO 3 , [9] DMSO/HBr, [10] H 2 O 2 , [11] NO 2 [12] and so on. Compared with above-mentioned oxidizing agents, the oxygen is a more attractive alternative due to its inexpensive and green characters, [13] which impels chemist to develop various methods with oxygen as the oxidant.…”

Pressure-induced inclusion of neon in the crystal structure of a molecular Cu2(pacman) complex at 4.67 GPa

“…For example, the active dicobalt pacman complex contains O 2 coordinated within the cleft of the pacman ligand forming the intermediate in the reduction of dioxygen to water. 19 The most obvious hydrostatic medium to use for this study would have been dioxygen itself, but as a liquid at high pressure this is extremely reactive, and rapidly oxidises the complex. We turned instead to neon, which is inert, and is hydrostatic to 15 GPa.…”