Contra-thermodynamic behavior in intermolecular hydrogen transfer of alkylperoxy radicals.

Quantum chemical investigation of bimolecular hydrogen transfer involving alkylperoxy radicals, a key reaction family in the free-radical oxidation of hydrocarbons, was performed to establish structure-reactivity relationships. Eight different reactions were investigated featuring four different alkane substrates (methane, ethane, propane and isobutane) and two different alkylperoxy radicals (methylperoxy and iso-propylperoxy). Including forward and reverse pairs, sixteen different activation energies and enthalpies of reaction were used to formulate structure-reactivity relationships to describe this chemistry. We observed that the enthalpy of formation of loosely bound intermediate states has a strong inverse correlation with the overall heat of reaction and that this results in unique contra-thermodynamic behavior such that more exothermic reactions have higher activation barriers. A new structure-reactivity relationship was proposed that fits the calculated data extremely well: E(A)=E(o)+alphaDeltaH(rxn) where alpha=-0.10 for DeltaH(rxn)<0, and alpha=1.10 for DeltaH(rxn)>0 and E(o)=3.05 kcal mol(-1).     

Killing three birds with one stone--simultaneous operando EPR/UV-vis/Raman spectroscopy for monitoring catalytic reactions

For the first time, three operando methods, namely EPR, UV-vis and laser-Raman spectroscopy have been applied in parallel on the same V/TiO2 catalyst and under identical reaction conditions during oxidative dehydrogenation of propane to elucidate structure-reactivity relationships in status operandi.     

Visualizing relative occurrences in metabolic transformations of xenobiotics using structure-activity maps.

Structure maps are presented as an efficient means of indicating structure-reactivity relationships in metabolic pathway databases. The relative occurrence of N-demethylation and N-oxidation of N-methyl tertiary amines was examined using the structure map methodology. A new family of reaction site representations, the n-level representations, was developed to describe the N-methyl reaction sites of the compounds in the data set. It was possible to differentiate N-demethylation and N-oxidation reaction sites using a structure map constructed from a 3-level representation of the reaction sites.     

Quantum chemical investigation of low-temperature intramolecular hydrogen transfer reactions of hydrocarbons

The B3LYP functional was evaluated as a method to calculate reaction barriers and structure-reactivity relationships for intramolecular hydrogen transfer reactions involving peroxy radicals. Nine different basis sets as well as five other MO/DFT and hybrid methods were used in comparing three reactions to available experimental data. It was shown that B3LYP/6-311+G(d,p) offers a good compromise between speed and accuracy for studies in which thermodynamic and kinetic data of many reactions are required. Sixteen reactions were studied to develop structure-reactivity relationships to correlate the activation energy with the heat of reaction. As long as no structural heterogeneities were present in the transition state ring, a simple Evans-Polanyí relationship was shown to capture the activation energy as a function of heat of reaction for reactions in the 1,5-hydrogen shift family. For peroxy radicals undergoing self-abstraction of a hydrogen atom in the 1,5-position, the activation energy was calculated as E(a) (kcal mol(-1)) = 6.3 + Delta H(rxn) (kcal mol(-1)). For reactions with a carbonyl group embedded in the ring of the transition state, the activation energy of peroxy radicals undergoing self-abstraction was correlated as E(a) (kcal mol(-1)) = 18.1 + 0.74 Delta H(rxn) (kcal mol(-1)). The impact of the size of the transition state ring on the activation energy and pre-exponential factor was also probed, and it was shown that these effects can be described using simple nonlinear and linear fits, respectively.     

Synthesis and kinetic investigation of the selective acidolysis of para-substituted N-benzyl- or N-phenyl-N-phenylacetyl-α,α-dialkylglycine cyclohexylamides

Several derivatives of N-phenylacetyl-N-benzyl-α,α-dimethylglycine cyclohexylamide and their α,α-dibenzylglycine analogues were synthesised by a Ugi-Passerini reaction. In addition, a few analogues of the former but having an N-phenyl instead of a benzyl group at the nitrogen atom were synthesised. The compounds in each of these three sets differed from each other at position 4 of the N-benzyl (and N-phenyl) group. These adducts were submitted to acidolysis with TFA to obtain the corresponding free acids, the reactions being monitored by HPLC and data collected for kinetic purposes. The kinetic data were submitted to Hammett uni- and biparametric relationships and the results were analysed in terms of structure-reactivity in connection with the sensitivity of the reaction rates to the electronic contributions of the various substituents at position 4 of the aromatic rings. The results allowed comparison with information obtained in previous investigations and rationalise the contribution of the substituent at the nitrogen atom to the lability of the C-terminal amide bond.     

Rhodium Complexes of Aminophosphines for 1-hexene Hydroformylation

Hydroformylation is one of the most versatile methods for the functionalization of C=C bonds. The rhodium phosphine catalysts, introduced by Wilkinson, have been shown to be more selective, and they give higher rates under milder conditions than the older cobalt carbonyl catalysts. The steric and electronic properties of the ligands have a dramatic influence on the reactivity of organo-metallic complexes. Despite recent extensive investigations on structure-reactivity correlation, the effects of electronic and steric parameters of P-liangds on the catalytic activity of rhodium catalysts, however, are not always predictable^[1,2] Here we present a study of the effect of a series of aminophosphines on the rhodium-catalyzed hydroformylation reaction.     

Amineborane dehydrogenation promoted by isolable zirconium sandwich, titanium sandwich and N(2) complexes

Catalytic dehydrogenation of R(2)NHBH(3) (R = Me, H) promoted by a family of bis(cyclopentadienyl)titanium and bis(indenyl)zirconium compounds is reported; structure-reactivity relationships as a function of cyclopentadienyl and indenyl substituents have been examined.     

Structure-reactivity correlations for solid-state enantioselective photochemical reactions established directly from powder X-ray diffraction

A prerequisite for the development of structure-reactivity correlations for photoreactive crystalline materials is to have detailed knowledge of the structural properties of the reactant crystalline phase. In some cases, however, the materials of interest can be prepared only as microcrystalline powders and are not amenable to structural characterization by single-crystal X-ray diffraction. This paper demonstrates the utility of modern powder X-ray diffraction techniques for obtaining structural understanding in such cases, leading to the development of structure-reactivity correlations. In particular, a series of three photoreactive organic salts are considered, which undergo the same photochemical asymmetric reaction but with high enantiomeric excess in two cases and low enantiomeric excess in the other case. The structural properties of the three salts determined from powder X-ray diffraction data are shown to provide a direct rationalization of these observations.     

Structural effects on the reactivity of substituted arylphosphines as potential oxygen-scavenging additives for future jet fuels

Efforts to develop oxygen-scavenging additives for future jet fuels (JP-900) are presented. Results from radiolytic experiments are used to develop structure-reactivity relationships for electron-transfer-initiated oxygenation (ETIO) of variously substituted arylphosphines. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:133–138, 1998     

An Analysis of the Relative Occurrence of N-Demethylation and N-Oxidation in Xenobiotic Metabolism Using Structure-Reactivity Maps

Abstract

The relative occurrence of N?demethylation and N?oxidation in a structurally diverse set of N?methyl tertiary amines was examined using structure-reactivity maps. A structure-reactivity map of the data indicated important structural features useful for predicting the relative occurrence of these reactions. A steric index which describes the degree of steric hinderance at the N?methyl reaction site is defined. A QSAR between the relative occurrence of N?demethylation and N?oxidation of N?methyl tertiary amines and the steric index was developed.     

有机磷化合物的研究XXIII:烷基膦酸二烷基酯的取代基结构参数与其水解速率常数的相关分析

为研究烷基膦酸酯在水解反应中的结构效应,合成了具有不同结构的烷基膦酸二烷基酯,并测得它们在不同温度时的碱性水解速率常数.     

Structural influences on the amperometric detection of opiates in high-performance liquid chromatography

The oxidation reactions of a series of opiates occurring at a glassy-carbon electrode in amperometric high-performance liquid chromatographic detection has been investigated. A structure-reactivity correlation has been drawn for morphone, morphinan and benzomorphan derivatives. Polarography and hydrodynamic voltammography were used to show the importance of phenolic groups to this reaction. Acyl substitution on the phenol did not prevent amperometric detection.     

On the use of multi-parameter free energy relationships: the rearrangement of (Z)-arylhydrazones of 5-amino-3-benzoyl-1,2,4-oxadiazole into (2-aryl-5-phenyl-2H-1,2,3-triazol-4-yl)ureas

By using a multi-parameter approach (a combination of Hammett/Ingold-Yukawa-Tsuno/Fujita-Nishioka free energy relationships) the mononuclear rearrangements of heterocycles (MRH) rates for five new ortho-substituted and ten new di-, tri-, or tetra-substituted (Z)-arylhydrazones of 5-amino-3-benzoyl-1,2,4-oxadiazole into the relevant (2-aryl-5-phenyl-2H-1,2,3-triazol-4-yl)ureas (in dioxane/water and in a large range of pS⁺ values) have been related to the electronic and proximity effects exerted by the present substituents, also considering previous results on some mono meta- and para-substituted (Z)-arylhydrazones. In every case, excellent correlation coefficients have been calculated (r² or R²≥0.996). Once more the study of MRH has furnished an interesting panel of different reactivity (three pathways of reaction have been evidenced: general-base-catalyzed, uncatalyzed, and specific-acid-catalyzed) and this has been useful in enlightening how polysubstitution can differently affect the MRH rates. Moreover 2,6-disubstitution on the (Z)-arylhydrazono moiety causes a significant increase of the reactivity in all of the three studied pathways. All of the collected data appear useful for understanding structure-reactivity/activity relationships in polysubstituted compounds.     

Click chemistry by microcontact printing on self-assembled monolayers: a structure-reactivity study by fluorescence microscopy

Microcontact printing (μCP) has developed into a powerful tool to functionalize surfaces with patterned molecular monolayers. μCP can also be used to induce a chemical reaction between a molecular ink and a self-assembled monolayer (SAM) in the nanoscale confinement between stamp and substrate. In this paper, we investigate the Huisgen 1,3-dipolar cycloaddition, the Diels-Alder cycloaddition and the thiol-ene/yne reaction induced by μCP. A range of fluorescent alkyne inks were printed on azide SAMs and fluorescence microscopy was used to monitor the extent of the 1,3-dipolar cycloaddition on a glass substrate. The rate of cycloaddition depends on the reactivity of the alkyne and on the presence of Cu(I). The cycloaddition is accelerated by Cu(I) but it also proceeds readily in the absence of Cu(I). In addition, a range of fluorescent diene inks were printed on alkene SAMs on glass. In this case, fluorescence microscopy was used to monitor the rate of the Diels-Alder cycloaddition as well as its retro-reaction. Finally, fluorescent thiol inks were printed on alkene SAMs on glass, and fluorescent alkenes and alkynes were printed on thiol SAMs. It is shown that reactions by μCP follow structure-reactivity relationships similar to solution reactions. Under optimized conditions all reactions lead to dense microarrays of addition products within minutes of printing time.     

Quantitative reactivity model for the hydration of carbon dioxide by biomimetic zinc complexes

A quantitative structure-reactivity relationship has been derived from the results of B3LYP/6-311+G calculations on the hydration of carbon dioxide by a series of zinc complexes designed to mimic carbonic anhydrase. The reaction mechanism found is general for all complexes investigated. The reaction exhibits a low (4-6 kcal/mol) activation energy and is exothermic by about 8 kcal/mol. The calculations suggest an equilibrium between Lipscomb and Lindskog intermediates. The effectiveness of the catalysis is a function of the nucleophilicity of the zinc-bound hydroxide and the nucleofugicity of the zinc-bound bicarbonate. Hydrogen bridging of the bicarbonate to NH moieties in the ligands also plays an important role.     

Role of noncovalent interactions in the enantioselective reduction of aromatic ketimines with trichlorosilane

[reaction: see text] Asymmetric reduction of ketimines 1 with trichlorosilane can be catalyzed by a new N-methyl L-valine derived Lewis basic organocatalyst, such as 4d, with high enantioselectivity. The structure-reactivity investigation suggests hydrogen bonding and arene-arene interactions between the catalyst and the incoming imine as the main factor determining the enantiofacial selectivity.     

The principle of imperfect synchronization: I. Ionization of carbon acids

The Principle of Imperfect Synchronization (PIS) states that a product stabilizing factor will lower the intrinsic rate of a reaction if it develops late, but increase the intrinsic rate if the factor develops early. It is shown in this paper that much of the structure-reactivity behavior of proton transfers involving carbon acids can be understood in the context of this principle. The factors discussed include the effect of resonance near the reaction site as well as in remote substituents, the polar effect of mote substituents, and the effects of solvation of reactants and products A sample matheatical formalism is developed to describe the contribution of each factor to the intrinsic rate constant. The possible reasons why there is a lack of synchronization between the various events which occur during the reaction are also discussed     

Reactions of perchlorofluoro compounds VIII. A structure-reactivity relationship of perchlorofluoroolefins based on MNDO calculations

The electronic structures of 17 perchlorofiuoroolefins and perfluoroolefins were calculated using the MNDO method. Based on these calculations combined with the perturbation theory, a good structure-reactivity relationship of perchlorofluoroolefins has been established. In the case of internal olefins, the direction of nucleophilic attack is governed not only by the perturbation energy of the ground state, but also by the stability of the anionic intermediate and the activation energy of the reaction.     

Electrochemical oxygen transfer reactions: electrode materials,surface processes,kinetic models,linear free energy correlations,and perspectives

This work summarizes progresses achieved in electrochemical oxygen transfer reactions from water to organic pollutant molecules on metal oxide electrodes during the past two decades. Fundamental understanding of the dynamics of the electrochemical oxygen transfer reaction is of crucial importance for the development of key concepts of electrocatalytic processes, leading to the implementation of environmental electrochemistry wastewater treatment schemes with rational design of the suitable electrocatalytic systems. We discuss the current knowledge on the electrochemical oxygen transfer reaction, emphasizing the importance of surface processes in order to generalize mechanistically the experimental results obtained on different electrode materials, describing also the practical kinetic models developed and their implications. From the information gathered in this review, it is apparent that explanations for the kinetics of the reactions in relation to the structure of the organic compounds involved is lacking, hence that new information about structure-reactivity relationships is needed. We show in particular that the open circuit decay of the concentration of radical cations, obtained from spectroelectrochemical data, allows correlating the structure of adsorbed states with reactivity during oxygen transfer reactions, pointing as well to research efforts required to understand the catalytic performance of metal oxide electrodes in decomposing organic compounds strongly adsorbed on their surfaces. Finally, some perspectives for future research in this area are briefly commented.