Activation parameters as mechanistic probes in the TAML iron(V)-oxo oxidations of hydrocarbons.

Soumen Kundu, Jasper Van Kirk Thompson, Longzhu Q Shen, Matthew R Mills, Emile L Bominaar, Alexander D Ryabov, Terrence J Collins

Chemistry (Weinheim an der Bergstrasse, Germany) 2015 Jan 19

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The results of low-temperature investigations of the oxidations of 9,10-dihydroanthracene, cumene, ethylbenzene, [D10]ethylbenzene, cyclooctane, and cyclohexane by an iron(V)-oxo TAML complex (2; see Figure 1) are presented, including product identification and determination of the second-order rate constants k2 in the range 233-243 K and the activation parameters (ΔH(≠) and ΔS(≠)). Statistically normalized k2 values (log k2') correlate linearly with the C-H bond dissociation energies DC-H, but ΔH(≠) does not. The point for 9,10-dihydroanthracene for the ΔH(≠) vs. DC-H correlation lies markedly off a common straight line of best fit for all other hydrocarbons, suggesting it proceeds via an alternate mechanism than the rate-limiting C-H bond homolysis promoted by 2. Contribution from an electron-transfer pathway may be substantial for 9,10-dihydroanthracene. Low-temperature kinetic measurements with ethylbenzene and [D10]ethylbenzene reveal a kinetic isotope effect of 26, indicating tunneling. The tunnel effect is drastically reduced at 0 °C and above, although it is an important feature of the reactivity of TAML activators at lower temperatures. The diiron(IV) μ-oxo dimer that is often a common component of the reaction medium involving 2 also oxidizes 9,10-dihydroanthracene, although its reactivity is three orders of magnitude lower than that of 2. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Citation

Soumen Kundu, Jasper Van Kirk Thompson, Longzhu Q Shen, Matthew R Mills, Emile L Bominaar, Alexander D Ryabov, Terrence J Collins. Activation parameters as mechanistic probes in the TAML iron(V)-oxo oxidations of hydrocarbons. Chemistry (Weinheim an der Bergstrasse, Germany). 2015 Jan 19;21(4):1803-10