Hydrogen atom abstraction from hydrocarbons by a copper(III)-hydroxide complex

Debanjan Dhar, William B. Tolman

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Abstract

With the aim of understanding the basis for the high rate of hydrogen atom abstraction (HAT) from dihydroanthracene (DHA) by the complex LCuOH (1; L = N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide), the bond dissociation enthalpy of the reaction product LCu(H2O) (2) was determined through measurement of its pKa and E1/2 in THF solution. In so doing, an equilibrium between 2 and LCu(THF) was characterized by UV-vis and EPR spectroscopy and cyclic voltammetry (CV). A high pKa of 18.8 ± 1.8 and a low E1/2 of -0.074 V vs Fc/Fc+ in THF combined to yield an O-H BDE for 2 of 90 ± 3 kcal mol-1 that is large relative to values for most transition metal oxo/hydroxo complexes. By taking advantage of the increased stability of 1 observed in 1,2-difluorobenzene (DFB) solvent, the kinetics of the reactions of 1 with a range of substrates with varying BDE values for their C-H bonds were measured. The oxidizing power of 1 was revealed through the accelerated decay of 1 in the presence of the substrates, including THF (BDE = 92 kcal mol-1) and cyclohexane (BDE = 99 kcal mol -1). CV experiments in THF solvent showed that 1 reacted with THF via rate-determining attack at the THF C-H(D) bonds with a kinetic isotope effect of 10.2. Analysis of the kinetic and thermodynamic data provides new insights into the basis for the high reactivity of 1 and the possible involvement of species like 1 in oxidation catalysis.

Original languageEnglish (US)
Pages (from-to)1322-1329
Number of pages8
JournalJournal of the American Chemical Society
Volume137
Issue number3
DOIs
StatePublished - Jan 28 2015

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