A clock reaction based on molybdenum blue.

Clock reactions are rare kinetic phenomena, so far limited mostly to systems with ionic oxoacids and oxoanions in water. We report a new clock reaction in cyclohexanol that forms molybdenum blue from a noncharged, yellow molybdenum complex as precursor, in the presence of hydrogen peroxide. Interestingly, the concomitant color change is reversible, enabling multiple clock cycles to be executed consecutively. The kinetics of the clock reaction were experimentally characterized, and by adding insights from quantum chemical calculations, a plausible reaction mechanism was postulated. Key elementary reaction steps comprise sigmatropic rearrangements with five-membered or bicyclo[3.1.0] transition states. Importantly, numerical kinetic modeling demonstrated the mechanism's ability to reproduce the experimental findings. It also revealed that clock behavior is intimately connected to the sudden exhaustion of hydrogen peroxide. Due to the stoichiometric coproduction of ketone, the reaction bears potential for application in alcohol oxidation catalysis.

Molecule-induced peroxide homolysis.

The homolytic cleavage of peroxide bonds, leading to the formation of free radicals, plays an important role in the (spontaneous) oxidation of a wide variety of hydrocarbons in the presence of oxygen. Such aerobic oxidations can be desired (e.g. for industrially applied autoxidations) or undesired (e.g. food deterioration). In this contribution we provide experimental and computational evidence for a molecule-induced homolytic dissociation mechanism between alkyl peroxide and compounds featuring weakly bonded H atoms such as (di)unsaturated hydrocarbons.

Catalytic epoxidations with peroxides: molybdenum trioxide species as the origin of allylic byproducts.

Molybdenum(VI)peroxide species, formed in the reaction of Mo(VI) complexes with peroxides, are able to epoxidize >C=C< double bonds heterolytically. In this study, theoretical and experimental evidence is provided for a kinetically competing reaction reaction of such molybdenum(VI)peroxide species with additional peroxide reagent, leading to molybdenum(VI)trioxide species, which easily decompose into radicals. Under epoxidation conditions, those radicals will reduce the selectivity, due to the formation of allylic byproducts. The involved reaction pathways are characterized by DFT calculations, providing kinetic parameters that are in good agreement with the experimental observations.

Cavitation-induced radical-chain oxidation of valeric aldehyde.

The application of high-amplitude ultrasound to liquids triggers cavitation. By the collapse of the thereby appearing vacuum cavities, high temperatures can be reached in a transient manner. The high temperatures in these hot-spots can lead to homolytic scission of chemical bonds. The thereby generated radicals are usually utilized in aqueous systems for the degeneration of organic pollutants. In this contribution, we demonstrate that the radicals can also be used for synthetic purposes: under an oxygen atmosphere, they trigger the oxidation of an aldehyde substrate.

Origin of regioselectivity in α-humulene functionalization.

Humulene is a sesquiterpene with an important biochemical lead structure, consisting of an 11-membered ring, containing three nonconjugated C═C double bonds, two of them being triply substituted and one being doubly substituted. As observed by many groups, one of the two triply substituted C═C double bonds is significantly more reactive. In order to rationalize this peculiar regioselectivity, the conformational space of humulene has been explored computationally using various DFT functionals. Four different conformations were identified. Each conformation is chiral and has two enantiomeric forms, yielding a total of eight conformers. The potential energy surface for the interconversion of these conformers was characterized via intrinsic reaction coordinate analyses. Furthermore, an evaluation of the microcanonical partition functions allowed for a quantification of the entropy contributions and the calculation of the temperature dependent equilibrium composition. The results strongly suggest that the high regioselectivity is related to a strong, hyper-conjugative σ(Cα-Cß)-π(C═C) orbital overlap in the predominant conformations that discriminates one triply substituted double bond from the other. Furthermore, the order of magnitude of the calculated activation energies for the interconversions of the conformers is supported by NMR measurements at different temperatures.

The conformations of cyclooctene: consequences for epoxidation chemistry.

The conformational space of cyclooctene has been explored computationally in order to rationalize its high epoxidation selectivity. Four different conformations were identified. Each conformation is chiral and has two enantiomeric forms. The degeneracy is further increased by a ring-inversion process, yielding a total of 16 conformers. The potential energy surface for the interconversion of these conformers was characterized via intrinsic reaction coordinate analyses. Furthermore, an evaluation of the microcanonical partition functions allowed for a quantification of the entropy contributions and hence the calculation of the equilibrium composition at different temperatures. The results strongly suggest that the high epoxidation selectivity, typically observed for cyclooctene, is related to a poor σ(C-αH)-π(C═C) orbital overlap in the predominant conformation, disfavoring αH-abstraction by radical species and thus allylic byproduct formation via undesired homolytic side-reactions.

Peculiarities of ß-pinene autoxidation.

The thermal oxidation of the renewable olefin ß-pinene with molecular oxygen was experimentally and computationally investigated. Peroxyl radicals abstract weakly bonded allylic hydrogen atoms from the substrate, yielding allylic hydroperoxides (i.e., myrtenyl and pinocarvyl hydroperoxide). In addition, peroxyl radicals add to the C=C bond of the substrate to form an epoxide. It was found that a relatively high peroxyl radical concentration, together with the high rate of peroxyl cross-reactions, make radical-radical reactions surprisingly important for this particular substrate. Approximately 60 % of these peroxyl cross-reactions lead to termination (radical destruction), keeping a radical chain length of approximately 4 at 10 % conversion. Numerical simulation of the reaction-based on the proposed reaction mechanism and known or predicted rate constants-demonstrate the importance of peroxyl cross-reactions for the formation of alkoxyl radicals, which are the precursor of alcohol and ketone products.

Understanding selective oxidations.

Functionalizing organic molecules is an important value-creating step throughout the entire chemical value-chain. Oxyfunctionalization of e.g. C-H or C=C bonds is one of the most important functionalization technologies used industrially. The major challenge in this field is the prevention of side reactions and/or the consecutive over-oxidation of the desired products. Despite its importance, a fundamental understanding of the intrinsic chemistry, and the subsequent design of a tailored engineering environment, is often missing. Industrial oxidation processes are indeed to a large extent based on empirical know-how. In this mini-review, we summarize some of our previous work to help to bridge this knowledge gap and elaborate on our ongoing research.

Mechanism of the catalytic deperoxidation of tert-butylhydroperoxide with cobalt(II) acetylacetonate.

The Co(II)/Co(III)-induced decomposition of hydroperoxides is an important reaction in many industrial processes and is referred to as deperoxidation. In the first step of the so-called Haber-Weiss cycle, alkoxyl radicals and Co(III)-OH species are generated upon the reaction of the Co(II) ion with ROOH. The catalytic cycle is closed upon the regeneration of the Co(II) ion through the reaction of the Co(III)-OH species with a second ROOH molecule, thus producing one equivalent of the peroxyl radicals. Herein, the deperoxidation of tert-butylhydroperoxide by dissolved cobalt(II) acetylacetonate is studied by using UV/Vis spectroscopy in situ with a noninteracting solvent, namely, cyclohexane. Kinetic information extracted from experiments, together with quantum-chemical calculations, led to new mechanistic hypotheses. Even under anaerobic conditions, the Haber-Weiss cycle initiates a radical-chain destruction of ROOH propagated by both alkoxyl and peroxyl radicals. This chain mechanism rationalizes the high deperoxidation rates, which are directly proportional to the cobalt concentration up to approximately 75 µM at 333 K. However, at higher cobalt concentrations, a remarkable decrease of the rate is observed. The hypothesis put forward herein is that this remarkable autoinhibition effect could be explained by the hitherto overlooked chain termination of two Co(III)--OH species. The direct competition between the first-order Haber-Weiss initiation and the second-order termination can indeed explain this peculiar kinetic behavior of this homogeneous deperoxidation system.

Autoxidation of alpha-pinene at high oxygen pressure.

The liquid-phase oxidation of the renewable olefin alpha-pinene with molecular oxygen yields several valuable compounds for the fine-chemical industry. The most important products are verbenol/-one and alpha-pinene oxide. Following our previous work on the radical autoxidation at atmospheric pressure, this contribution addresses the influence of the oxygen pressure on the reaction mechanism and the product distribution. Trapping of the radical epoxide-precursor by O(2) causes a decrease of the epoxide selectivity, as well as the formation of a thermally unstable dialkylperoxide. This dialkylperoxide accelerates the rate significantly, due to an enhancement of the radical initiation. Although this causes a decrease of the radical chain-length, the amount of products produced in the chain-termination can still be neglected compared to the amount produced in the chain-propagations. Parallel to this, the ketone to alcohol ratio increases at higher oxygen pressure, due to the reaction of alkoxyl radicals with O(2), as well as a reaction of O(2) with the addition product of the alkoxyl radicals and the C=C double bond of the substrate. For O(2) partial pressures of 1 to 80 bar, rate constants of important reactions are extracted from the experimental observations via differential modelling, and confronted with literature values and/or quantum-chemical predictions. The derived mechanism is supported at the molecular level and provides a reliable description of the experimental observations.

Mechanism of the aerobic oxidation of alpha-pinene.

A combined experimental and theoretical approach is used to study the thermal autoxidation of alpha-pinene. Four different types of peroxyl radicals are generated; the verbenyl peroxyl radical being the most abundant one. The peroxyl radicals propagate a long radical chain, implying that chain termination does not play an important role in the production of the products. Two distinct types of propagation steps are active in parallel: the abstraction of allylic H atoms and the addition to the unsaturated C=C bond. The efficiency for both pathways appears to depend on the structure of the peroxyl radical. The latter step yields the corresponding epoxide product, as well as alkoxyl radicals. Under the investigated reaction conditions the alkoxyl radicals seem to produce both the alcohol and ketone products, the ketone presumably being formed upon the abstraction of the weakly bonded alphaH atom by O2. This mechanism explains the predominantly primary nature of all quantified products. At higher conversion, co-oxidation of the hydroperoxide products constitutes an additional, albeit small, source of alcohol and ketone products.