Selective electrochemical CO bond cleavage of β-O-4 lignin model compounds mediated by iodide ion

The electrochemically oxidative cleavage of lignin β-O-4 model compounds mediated by iodide ion has been studied. The results indicate that electrolytic conditions play a predominant role in determining the distribution of cleavage products. The preparative-scale electrolysis proceeds in a simple undivided cell, employing a catalytic amount of NaI as the redox mediator and supporting electrolyte in methanol. Under these conditions, the C_βO bond is selectively cleaved with 2,2-dimethoxy-2-arylacetaldehyde being the main product. In some cases, the reaction gives a good yield of cleavaged products. The results further demonstrate that the indirect electrolysis mediated by halide is a versatile approach for chemical transformation.     

Formic acid oxidation on Pd/RuO2: does the RuO2 support enhance the electrocatalytic activity of Pd nanoparticles?

Journal of Solid State Electrochemistry - The aim of this work was to study the effect of a RuO2 support on the activity of Pd nanoparticles for formic acid oxidation, comparing the results with...     

Why does cyclopropene have the acidity of an acetylene but the bond energy of methane?

The gas-phase acidity of 3,3-dimethylcyclopropene (1) has been measured by bracketing and equilibrium techniques. Consistent with simple hybridization arguments, our value (deltaH degrees (acid) = 382.7 +/- 1.3 kcal mol(-)(1)) is indistinguishable from that for methylacetylene (i.e., deltadeltaH degrees (acid)(1 - CH(3)Ctbd1;CH) = 1.6 +/- 2.5 kcal mol(-)(1)). The electron affinity of 3,3-dimethylcyclopropenyl radical (1r) was also determined (EA = 37.6 +/- 3.5 kcal mol(-)(1)), and these quantities were combined in a thermodynamic cycle to afford the homolytic C-H bond dissociation energy. To our surprise, the latter quantity (107 +/- 4 kcal mol(-)(1)) is the same as that for methane, which cannot be explained in terms of the s-character in the C-H bonds. An orbital explanation (delocalization) is proposed to account for the extra stability of 1r. All of the results are supplemented with G3 and B3LYP computations, and both approaches are in good accord with the experimental values. We also note that for simple hydrocarbons which give localized carbanions upon deprotonation there is an apparent linear correlation between any two of the following three quantities: deltaH degrees (acid), BDE, and EA. This observation could be of considerable value in many diverse areas of chemistry.     

Why the model of non-interacting chains yields adequate description of spin crossover in space structures?

Considering many-body interactions in tetrahedral structures as perturbations of binary potentials by third bodies yields a free energy functional of the binary mixture equivalent to one earlier derived for spin crossover equilibrium in one-dimensional chains. Formal non-ideality parameters of this functional, the excess energy and asymmetries of splittings can be expressed in terms of molecular parameters based on binary potentials.     

Kinetic study of the oxidation of L-threonine by MnO4? ions

A self-catalytic effect attributed to Mn²⁺ ions was observed when studying the oxidation of L-threonine by permanganate ions. The process obeys the rate equation:

Does substrate oxidation determine the regioselectivity of cyclohexene and propene oxidation by cytochrome p450?

DFT and QM/MM computations of allylic C-H hydroxylation versus C=C epoxidation in cyclohexene and propene by Compound I of P450cam demonstrate that the relative barriers for the oxidative processes themselves are not good predictors of the observed selectivity. However, a kinetic expression previously developed (Kozuch, S.; Shaik, S. J. Am. Chem. Soc. 2006, 128, 3355) for catalytic cycles under steady-state conditions, predicts, in accord with experiment, that propene will undergo exclusive C=C epoxidation, while cyclohexene will undergo both reactions with a small preference for epoxidation. The model expression for the effective barrier of the cycle forms a general basis for understanding and predicting the selectivity of P450 isozymes.     

A synthesis of biaryl ketones via the C–S bond cleavage of thiol ester by a Cu/Ag salt

We report the synthesis of biaryl ketones via an unprecedented copper/silver catalyzed acylative cross-coupling of thiol esters with either an arylboronic acid or a potassium aryltrifluoroborate. This new method proceeds without a requisite Pd-catalyst and Cu(I)TC mediator, and is efficient, versatile, operationally simple, and accommodating functionally diverse thiol esters, arylboronic acids, and potassium aryltrifluoroborates.     

Is Se-Se bond cleavage the most favourable process in electron attachment to diselenides? The importance of asymmetry

Accurate ab initio calculations reveal that, in contrast with what has been commonly assumed up to now, electron attachment to diselenides does not always result in a Se-Se bond fission and, in general, the mechanism behind the bond fission may involve the crossing of several states.     

Why is the association of supramolecular assemblies different under homogeneous and heterogeneous conditions?

The origin of the change of the thermodynamic properties of association between homogeneous and heterogeneous systems is highlighted by numerical experiments.     

Metal-dependent stereochemistry of C—C bond formation under the asymmetric phase transfer catalysis by chiral salen complexes

The effect of the nature of the central metal atom in chiral salen type complexes on the stereodifferentiating capacity of these complexes as catalysts in phase transfer asymmetric alkylation of Schiff"s base derived from alanine isopropyl ester and benzaldehyde by benzyl bromide. The nature of the central metal atom in the complex has a pronounced influence on the stereochemistry of alkylation; copper(ii) complexes exhibit the highest activity combined with a high stereoselectivity.     

Why do the Abundances of Ions Generated by MALDI Look Thermally Determined?

In a previous study (J. Mass Spectrom. 48, 299–305, 2013), we observed that the abundance of each ion in a matrix-assisted laser desorption ionization (MALDI) spectrum looked thermally determined. To find out the explanation for the phenomenon, we estimated the ionization efficiency and the reaction quotient (Q_A) for the autoprotolysis of matrix, M + M → [M + H]⁺ + [M ? H]^?, from the temperature-controlled laser desorption ionization spectra of α-cyano-4-hydroxycinnamic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB). We also evaluated the equilibrium constants (K_A) for the autoprotolysis at various temperatures by quantum chemical calculation. Primary ion formation via various thermal models followed by autoprotolysis-recombination was compatible with the observations. The upper limit of the effective temperature of the plume where autoprotolysis-recombination occurs was estimated by equating Q_A with the calculated equilibrium constant.

X-ray scattering by the phases of 4-methoxybenzylidene-4′-n-butylaniline quenched under a magnetic field

Normally 4-methoxybenzylidene-4′-n-butylaniline exhibits only one liquid-crystalline phase; it is nematic at room temperature. However, after quenching it at liquid nitrogen temperature it is possible to reach four new phases by successive annealing. We report here an experiment aimed at providing a better understanding of these phases by studying their texture after quenching in a magnetic field large enough to orient the sample in the nematic phase.     

Selenium Blue-α and -β: turning on the fluorescence of a pyrenyl fluorophore via oxidative cleavage of the Se-C bond by reactive oxygen species

Rapid oxidation of nonfluorescent pyrenyl-CH₂SeAr (Ar = o-nitrophenyl) by hypochlorite yielded pyrenyl-CH₂Cl and pyrenyl-CH₂OH and turns on blue fluorescence, while slow oxidation of pyrenyl-CH₂SeAr with excess H₂O₂ leads to pyrenyl-CHO which emits a bluish-green fluorescence. The homolog, pyrenyl-CH₂CH₂SeAr′ (Ar′ = o-nitrophenyl) reacts slower with H₂O₂ and ClO⁻ giving the same product, vinyl pyrene.     

How are the charge transfer descriptors affected by the quality of the underpinning electronic density?

With the aim of investigating qualitatively and quantitatively the impact of using excited state relaxed or unrelaxed density for the estimation of nature and characteristic of electronic excited states, we analyzed the behavior of 52 exchange correlation functionals for the prediction of density‐based indexes such as those recently introduced in literature to evaluate the charge transfer distance (D_CT) (Le Bahers et al. J. Chem. Theory Comput. 2011, 7, 2498) in the case of a prototype family of push–pull dyes. Our results show that while a qualitatively consistent assessment of the nature of the excited states is obtained using either the unrelaxed or the relaxed density, from a quantitative standpoint we observe large discrepancies in the charge transfer distance for electronic transitions having substantial CT character. This behavior is independent of nature of the exchange–correlation functional used. © 2018 Wiley Periodicals, Inc.     

A new synthetic method for γ-butyrolactols by the paladium-catalyzed regioselective oxidation of 1-alken-4-ols

3-Substituted 1-alken-4-ols were oxidized with PdCl₂-benzoquinone regioselectively at the terminal carbon to afford cyclic hemiacetals (γ-butyrolactols), which were converted to γ-butyrolactones by the Jones oxidation. This reaction was applied to the syntheses of optically active deoxyribose and γ-butenolides.     

Effect of manganese(II) ions on the oxidation of malic and oxaloethanoic acids by aqueous HCrO−4

The catalytic effect of Mn^II ions on the pseudo-first-order rate constants (k _obs) for chromic acid oxidations of malic and oxaloethanoic acids (an oxidation product of malic acid) has been studied spectrophotometrically at 25 °C. The rates show a first-order dependence on the Cr^VI concentration for each reductant. The order with respect to [malic acid] was found to lie between 1 and 2, and 1 for [oxaloethanoic acid]. The rate increased markedly with increasing [Mn^II] in both the cases. The catalytic effects of Mn^II have been ascribed to a one-step three-electron process in which a termolecular complex is formed between the reductant, Mn^II and HCrO^– ₄. The intermediate Cr^IV is ruled out; details of such a process are discussed. Mechanisms in accordance with the experimental data are proposed for the reactions.