Mechanistic insight into stereoselective carbolithiation

This article addresses the mechanistic features of asymmetric carbolithiation of β‐methylstyrenes. While often the presence of functional groups is required to obtain high enantioselectivities in carbolithiation reactions, simple β‐methylstyrene also gives high selectivities in (?)‐sparteine‐mediated addition of alkyl lithium compounds. Computational studies on the carbolithiation of β‐methylstyrene with (?)‐sparteine show that the observed selectivities are the result of repulsion effects in the diastereomeric transition states between the (?)‐sparteine ? alkyl lithium adduct and the β‐methylstyrene, upon approximation of the two reactants. In contrast, for the ortho‐amino β‐methylstyrene (E)‐benzyl(2‐propenylphenyl)amine ( 4 ) X‐ray structure analyses of intermediate lithium amides indicate a carbolithiation mechanism in which one side of the double bond is shielded by the amide moiety, leaving only one side free for approach of the chiral alkyl lithium adduct.     

Mechanistic insight into the preactivation of a modern palladium catalyst precursor in phosphine-free Heck reactions

Abstract  The use of well-defined trans-dichlorobis(diethanolamine-N)palladium(II) complex (trans-[PdCl₂(DEA)₂]) as a new, modern, and effective Heck precatalyst is reported. The mechanism of the key, initial steps of the reaction, i.e., the formation of the Pd(0) complex, is examined using density functional theory. Graphical abstract        

Mechanistic insight into the reactivity of oxotransferases by novel asymmetric dioxomolybdenum(VI) model complexes

The asymmetric molybdenum(VI) dioxo complexes of the bis(phenolate) ligands 1,4‐bis(2‐hydroxybenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐4‐methylbenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐3,5‐dimethylbenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐3,5‐di‐tert‐butylbenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐4‐flurobenzyl)‐1,4‐diazepane, and 1,4‐bis(2‐hydroxy‐4‐chlorobenzyl)‐1,4‐diazepane (H₂(L1)–H₂(L6), respectively) have been isolated and studied as functional models for molybdenum oxotransferase enzymes. These complexes have been characterized as asymmetric complexes of type [MoO₂(L)] 1–6 by using NMR spectroscopy, mass spectrometry, elemental analysis, and electrochemical methods. The molecular structures of [MoO₂(L)] 1–4 have been successfully determined by single‐crystal X‐ray diffraction analyses, which show them to exhibit a distorted octahedral coordination geometry around molybdenum(VI) in an asymmetrical cis‐β configuration. The Mo? O_oxo bond lengths differ only by ≈0.01 Å. Complexes 1 , 2 , 5 , and 6 exhibit two successive Mo^VI/Mo^V (E_1/2, ?1.141 to ?1.848 V) and Mo^V/Mo^IV (E_1/2, ?1.531 to ?2.114 V) redox processes. However, only the Mo^VI/Mo^V redox couple was observed for 3 and 4 , suggesting that the subsequent reduction of the molybdenum(V) species is difficult. Complexes 1 , 2 , 5 , and 6 elicit efficient catalytic oxygen‐atom transfer (OAT) from dimethylsulfoxide (DMSO) to PMe₃ at 65 °C at a significantly faster rate than the symmetric molybdenum(VI) complexes of the analogous linear bis(phenolate) ligands known so far to exhibit OAT reactions at a higher temperature (130 °C). However, complexes 3 and 4 fail to perform the OAT reaction from DMSO to PMe₃ at 65 °C. DFT/B3LYP calculations on the OAT mechanism reveal a strong trans effect.     

Mechanistic insight into the chemiluminescent decomposition of firefly dioxetanone

The peroxide decomposition that generates the excited-state carbonyl compound is the key step in most organic chemiluminescence, and chemically initiated electron exchange luminescence (CIEEL) has been widely accepted for decades as the general mechanism for this decomposition. The firefly dioxetanone, which is a peroxide, is the intermediate in firefly bioluminescence, and its decomposition is the most important step leading to the emission of visible light by a firefly. However, the firefly dioxetanone decomposition mechanism has never been explored at a reliable theoretical level, because the decomposition process includes biradical, charge-transfer (CT) and several nearly degenerate states. Herein, we have investigated the thermolysis of firefly dioxetanone in its neutral (FDOH) and anionic (FDO(-)) forms using second-order multiconfigurational perturbation theories in combination with the ground-state intrinsic reaction coordinate calculated via the combined hybrid functional with Coulomb attenuated exchange-correlation, and considered the solvent effect on the ground-state reaction path using the combined hybrid functional with Coulomb attenuated exchange-correlation. The calculated results indicate that the chemiluminescent decomposition of FDOH or FDO(-) does not take place via the CIEEL mechanism. An entropic trap was found to lead to an excited-state carbonyl compound for FDOH, and a gradually reversible CT initiated luminescence (GRCTIL) was proposed as a new mechanism for the decomposition of FDO(-).     

铂铜合金催化甲醇电氧化的机理研究

直接甲醇燃料电池(DMFC)可以将甲醇的化学能转化为电能.甲醇在室温下是一种液体,很容易运输和低风险储存.在常用燃料中,甲醇热值较高且价格便宜,其单位价格热值甚至高于汽油.更重要的是,甲醇可以通过二氧化碳催化加氢制得.因此可以将可再生能源转化为氢气,并高效地存储在甲醇分子中.而燃料电池消耗甲醇后,产物只有二氧化碳和无污...     

Mechanistic insight into the aromatic hydroxylation by high-valent iron(IV)-oxo porphyrin pi-cation radical complexes

Mechanistic studies of the aromatic hydroxylation by high-valent iron(IV)-oxo porphyrin pi-cation radicals revealed that the aromatic oxidation involves an initial electrophilic attack on the pi-system of the aromatic ring to produce a tetrahedral radical or cationic sigma-complex. The mechanism was proposed on the basis of experimental results such as a large negative Hammett rho value and an inverse kinetic isotope effect. By carrying out isotope labeling studies, the oxygen in oxygenated products was found to derive from the iron-oxo porphyrin intermediates.     

Mechanistic insight into olefin epoxidation catalyzed by rhenium(V) oxo complexes that contain pyridazine-based ligands

Three novel tridentate pyridazine phenolate ligands were prepared in high yields by Schiff-base condensation of salicylic aldehyde with various pyridazine hydrazines (substituent R in the 6 position: R = Cl (HL(Cl)), (t)Bu (HL((t)Bu)), or tol (HL(tol))). They react with [ReOCl(3)(OPPh(3))(SMe(2))] to form rare mononuclear trans-dichloro oxo complexes of general formula [ReOCl(2)(L(R))] with R = tol (1), (t)Bu (2), or Cl (3) as confirmed by single-crystal X-ray diffraction analyses of 1 and 2. They were found to be catalysts for oxidation of cyclooctene to the corresponding epoxide by tert-butyl hydroperoxide (TBHP). Extensive UV-vis and NMR spectroscopic investigations followed by evaluation using the powerful Mauser method revealed mechanistic details. This showed the catalyst precursor [ReOCl(2)(L)] (2) to be transformed into the rhenium(VII) compound [ReO(3)L] (4) in a two-step reaction via intermediate INT which is tentatively assigned to [ReO(2)L]. Confirmation gave the isolation of 4 by reaction of 2 with excess of TBHP. Monitoring the catalytic oxidation reaction by UV-vis spectroscopy clearly excludes the two rhenium(V) compounds 2 and INT from being the catalytically active species as their formation is several orders of magnitude faster than the observed catalytic epoxidation reaction.     

Mechanistic insight into circularly polarized photoluminescence from a chiral-nematic film

Circularly polarized photoluminescence (CPPL) was characterized for rod-like molecules of Exalite 428 helically arranged in a chiral-nematic liquid crystalline film. With an unpolarized excitation at 370nm, CPPL intensities measured at 428nm, and a selective reflection wavelength ranging from 7.2 to 72.6mum, the observed dissymmetry factor, g, was found to be in excellent agreement with theoretical prediction, without resorting to adjustable parameters. As a result, new insight into liquid crystal-induced CPPL has emerged. Specifically, it was found that circular dichroism and circular polarization of the excitation beam prior to inducing linearly polarized photoluminescence (LPPL) at quasi-nematic layers play an insignificant role. The relatively large g value in the spectral region far removed from the selective reflection band was attributed to the circular polarization of LPPL emanating from all quasi-nematic layers comprising the chiral-nematic film. In the absence of a fluorescent dye, the propagation of unpolarized light through the chiral-nematic film under otherwise identical conditions results in no circular polarization. The present study has provided a foundation for generating circularly polarized light by way of photo-excitation with unpolarized light of a chiral-nematic film containing a fluorescent dye. e e     

Mechanistic insights into electrocatalytic reactions provided by SERS

In situ vibrational spectroscopy can provide molecular-level mechanistic insights missing from purely electrochemical measurements. Surface enhanced Raman spectroscopy (SERS) is a particularly promising method and is used in aqueous and nonaqueous studies of a variety of electrode reactions. Enhancement of the weak Raman signal is achieved by structuring the electrode surface or by use of SERS probes. This review article highlights the recent use of SERS to study several important electrode reactions: oxygen reduction and evolution, carbon monoxide oxidation and carbon dioxide reduction and the electrocatalytic oxidation of small organic molecules such as formic acid.     

Further insight into the mass-spectral fragmentation of tetracyclone. Tetraphenyltetrahedrane ion

The p-fluoro labeling technique is used to demonstrate that the ions formed by the loss of CO from tetracyclones pass through a tetrahedral transition state or intermediate before they fragment.     

Mechanistic insight into hydroxamate transfer reaction mimicking the inhibition of zinc-containing enzymes

A hydroxamate transfer reaction between metal complexes has been investigated by a combination of experimental and theoretical studies. A hydroxamate-bound cobalt(ii) complex bearing a tetradentate macrocyclic ligand, [Co^II(TBDAP)(CH₃C(–NHO)O)]⁺ (1), is prepared by the reduction of a hydroximatocobalt(iii) complex with a biological reductant. Alternatively, 1 is accessible via a synthetic route for the reaction between the cobalt(ii) complex and acetohydroxamic acid in the presence of a base. 1 was isolated and characterized by various physicochemical methods, including UV-vis, IR, ESI-MS, and X-ray crystallography. The hydroxamate transfer reactivity of 1 was examined with a zinc complex, which was followed by UV-vis and ESI-MS. Kinetic and activation parameter data suggest that the hydroxamate transfer reaction occurs via a bimolecular mechanism, which is also supported by DFT calculations. Moreover, 1 is able to inhibit the activity against a zinc enzyme, i.e., matrix metalloproteinase-9. Our overall investigations of the hydroxamate transfer using the synthetic model system provide considerable insight into the final step involved in the inhibition of zinc-containing enzymes.

A hydroxamate transfer reaction between metal complexes has been investigated by a combination of experimental and theoretical studies.     

Mechanistic insight into the cleavage of an aromatic C-C bond by tungsten

The pathway for the cleavage of an aromatic C-C bond in quinoxaline by a tungsten(II) complex [W(PMe(3))(4)(η(2)-CH(2)PMe(2))H] is explored by performing detailed DFT calculations. The real active complex was found to be [W(PMe(3))(2)(η(2)-CH(2)PMe(2))H] rather than [W(PMe(3))(4)]. The key step in the whole reaction is the reductive elimination of two hydrides that are located originally on quinoxaline (see scheme).     

Mechanistic insight on the catecholase activity of dinuclear copper complexes with distant metal centers

The catecholase activity of two dinuclear Cu(II) complexes with distant metal centers is discussed together with solid state and solution studies. The crystal structure for one of them, [Cu(2)(diep)(H(2)O)(4)](ClO(4))(4)·2H(2)O, is described, showing the two copper ions are 7.457 ? apart and in a square pyramidal coordination. Both complexes display a weak antiferromagnetic coupling in the solid state that is manifest in the dimer EPR spectra obtained in frozen solution. The pH-potentiometric speciation performed in 1:1 MeOH-H(2)O allowed the assignment of hydrolyzed copper species as those catalytically active in the oxidation of 3,5-di-tert-butylcatechol (DTBC). The kinetic measurements led us to propose behavior consistent with Michaelis-Menten plus a linear dependence of the initial rate on [DTBC]. This can be associated with the presence of more than one catalytically active species, which is consistent with the evidence of several differently hydrolyzed species shown in the predominance diagrams. Product characterization studies led to establishing the formation of hydrogen peroxide during the catalytic cycle, while semiquinone and superoxide radicals were detected by EPR spectroscopy, supporting one-electron transference at each of the copper centers.     

Mechanistic insight into water exchange and aqua/fluoride ligand substitution reactions on aqueous species of Al,Ga and In

Abstract

Two ligand substitution reactions on aqueous complexes of Al, Ga and In, water-exchange and aqua/fluoride substitution reactions, were investigated using density functional theory (DFT), and the preferred substituted mechanisms were estimated based on the activated energy barriers. A series of mechanistic changeovers were found and interpreted at the molecular level. Four factors influencing substitution mechanisms were summarized, i.e. the size of central metal ions, the volume of entering ligands, the charge of clusters, and structural rigidity. The present study provides an approach to probe the ligand substitution mechanism of some reactions inaccessible experimentally.     

Mechanistic insight into domain formation and growth in a phase-separated Langmuir-Blodgett monolayer

The mechanism of the formation and growth of phase-separated domains in mixed arachidic acid (C19H39COOH) (AA) and perfluorotetradecanoic acid (C13F27COOH) (PA) monolayer films was investigated through a combination of surface pressure-area isotherm measurements and atomic force microscope (AFM) imaging. In the mixed AA-PA monolayer system, distinct discontinuous domains consisting primarily of AA form spontaneously in a surrounding continuous matrix enriched in PA. By varying the monolayer deposition conditions, including temperature, surface pressure, and the mechanical agitation of sample solutions, it was determined that phase-separated nuclei are formed initially in the bulk sample solution and further growth of domains proceeds on the subphase surface via an Ostwald ripening process involving the diffusion of AA from the matrix to the discontinuous domains. In addition, selective dissolution of the arachidic acid followed by in situ AFM imaging has allowed the visualization of the fusion of AA to the phase-separated domains and has highlighted some unusual pattern formation that occurs at low subphase temperatures.     

Mechanistic insight into direct arylations with ruthenium(II) carboxylate catalysts

Mechanistic studies revealed ruthenium-catalyzed direct arylations to proceed through reversible C-H bond activation and subsequent rate-limiting oxidative addition with aryl halides, which led to the development of widely applicable well-defined ruthenium(II) carboxylate catalysts.     

Co-complexes derived from alkene insertion to alkyne-dicobaltpentacarbonyl complexes: insight into the regioselectivity of pauson-khand reactions of cyclopropenes

Described are the X-ray crystallographic and spectral properties of Co-complexes that were isolated from two Pauson-Khand reactions of chiral cyclopropenes. These are the first examples of isolated Co-complexes derived from the putative alkene-insertion intermediates of Pauson-Khand reactions. The binuclear Co-complexes are coordinated to mu-bonded, five-carbon "flyover" carbene ligands. It is proposed that the complexes result from cyclopropane fragmentation subsequent to alkene insertion. The observation of these metal complexes provides a rationale for the origin of regioselectivity in Pauson-Khand reactions of cyclopropenes.