Analysis of Hydrogen Atom Abstraction from Ethylbenzene by an Fe^VO(TAML) Complex

It was shown previously (<i>Chem. Eur. J</i>. <b>2015</b>, 21, 1803) that the rate of hydrogen atom abstraction, <i>k</i>, from ethylbenzene (EB) by TAML complex [Fe^V(O)­B*]⁻ (<b>1</b>) in acetonitrile exhibits a large kinetic isotope effect (KIE ∼ 26) in the experimental range 233–243 K. The extrapolated tangents of ln­(<i>k</i>/<i>T</i>) vs <i>T</i>^–1 plots for EB-<i>d</i>₁₀ and EB gave a large, negative intercept difference, Int­(EB) – Int­(EB-<i>d</i>₁₀) = −34.5 J mol^–1 K^–1 for <i>T</i>^–1 → 0, which is shown to be exclusively due to an isotopic mass effect on tunneling. A decomposition of the apparent activation barrier in terms of electronic, ZPE, thermal enthalpic, tunneling, and entropic contributions is presented. Tunneling corrections to Δ<i>H</i>^⧧ and Δ<i>S</i>^⧧ are estimated to be large. The DFT prediction, using functional B3LYP and basis set 6-311G, for the electronic contribution is significantly smaller than suggested by experiment. However, the agreement improves after correction for the basis set superposition error in the interaction between EB and <b>1</b>. The kinetic model employed has been used to predict rate constants outside the experimental temperature range, which enabled us to compare the reactivity of <b>1</b> with those of other hydrogen abstracting complexes

Revised Mechanism for a Ruthenium-Catalyzed Coupling of Aldehyde and Terminal Alkyne

Ruthenium catalysts have been found to be of great use for many kinds of reactions. Understanding the details of the catalytic cycle allows to not only rationalize experimental results but also to improve upon reactions. Herein, we present a detailed computational study of a ruthenium-catalyzed coupling between a terminal alkyne and an aldehyde. The reaction under examination facilitates novel access to olefins with the concurrent loss of a single carbon as carbon monoxide. The reaction was first developed in 2009, but the tentative mechanism initially proposed was proven to be contradictory to some experimental data obtained since then. Using a combination of computational investigations and isotope-labeling experiments, several potential mechanisms have been studied. In contrast to the [2+2] cycloaddition mechanism suggested for similar catalysts, we propose a new consensus pathway that proceeds through the formation of a ruthenium–vinylidene complex that undergoes an aldol-type reaction with the aldehyde to yield the product olefins. Computational insights into the influence of different reagents used to optimize reaction conditions and the intricacies of decarbonylation of a Ru–CO complex affecting catalyst turnover are highlighted

TAML Activator/Peroxide-Catalyzed Facile Oxidative Degradation of the Persistent Explosives Trinitrotoluene and Trinitrobenzene in Micellar Solutions

TAML activators are well-known for their ability to activate hydrogen peroxide to oxidize persistent pollutants in water. The trinitroaromatic explosives, 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitrobenzene (TNB), are often encountered together as persistent, toxic pollutants. Here we show that an aggressive TAML activator with peroxides boosts the effectiveness of the known surfactant/base promoted breakdown of TNT and transforms the surfactant induced nondestructive binding of base to TNB into an extensive multistep degradation process. Treatment of basic cationic surfactant solutions of either TNT or TNB with TAML/peroxide (hydrogen peroxide and <i>tert</i>-butylhydroperoxide, TBHP) gave complete pollutant removal for both in <1 h with >75% of the nitrogen and ≥20% of the carbon converted to nitrite/nitrate and formate, respectively. For TNT, the TAML advantage is to advance the process toward mineralization. Basic surfactant solutions of TNB gave the colored solutions typical of known Meisenheimer complexes which did not progress to degradation products over many hours. However with added TAML activator, the color was bleached quickly and the TNB starting compound was degraded extensively toward minerals within an hour. A slower surfactant-free TAML activator/peroxide process also degrades TNT/TNB effectively. Thus, TAML/peroxide amplification effectively advances TNT and TNB water treatment giving reason to explore the environmental applicability of the approach