Influence of Solvents on Catalytic C–H Bond Oxidation by a Copper(II)–Alkylperoxo Complex

Abstract

Oxidation of unactivated alkanes, which requires substantial energy for conversion to valuable organic chemicals, is a major challenge in both industry and academia. Herein, we describe how solvents affect and improve the catalytic oxidation ability of a mononuclear copper(II)–alkylperoxo complex, [Cu II ( i Pr 3 -tren)(OOC(CH 3 ) 2 Ph)] + ( 1, i Pr 3 -tren = tris[2-(isopropylamino)ethyl]amine), toward hydrocarbon substrates. 1 was prepared by adding cumene hydroperoxide and triethylamine to the solution of [Cu( i Pr 3 -tren)(CH 3 CN)] 2+, which was characterized using various physicochemical methods. Product analyses, along with theoretical calculations, indicate that homolytic O–O bond cleavage occurs during the thermal decomposition of 1 at 60 °C in various solvents such as CH 3 CN, CH 3 COCH 3, C 6 H 5 CF 3, and C 6 H 6 . Both experimental results and density functional theory (DFT) calculations supported variations in the catalytic activity of 1 depending on solvents. In CH 3 CN and CH 3 COCH 3, 1 activates weak C–H bonds (bond dissociation energy (BDE) ≤ ∼81.6 kcal mol –1 ), while 1 in C 6 H 5 CF 3 and C 6 H 6 can oxidize slightly stronger C–H bonds with a BDE of up to 84.5 kcal mol –1 . In supercritical carbon dioxide (SC-CO 2 ), 1 can oxidize alkanes with strong C–H bonds, such as cyclohexane (99.5 kcal mol –1 ). The enhanced C–H bond oxidation of 1 in C 6 H 5 CF 3, C 6 H 6, and SC-CO 2 was generally attributed to two different factors: (a) the nonpolarity of the solvent and (b) the absence of C(sp 3 )–H bonds in the solvent. Interestingly, in CH 2 Cl 2, a nonpolar solvent with C(sp 3 )–H bonds, 1 exhibited similar reactivity to that in C 6 H 5 CF 3, indicating that nonpolar solvents enhance the catalytic ability of copper(II)–cumylperoxo complex to abstract hydrogen atoms from substrates, regardless of the presence of C(sp 3 )–H bonds in solvent molecules. DFT calculations employing an implicit solvent model further supported the enhanced reactivity, without the need to account for the presence of a C(sp 3 )–H bond. The reactivity of the different possible reactive intermediates arising from the catalytic oxidation was also explored using DFT calculations. This study provides a perspective on how solvents can be utilized to modulate the catalytic effects on C–H bond activation.

Topics & Concepts