Initial Adduct Research Articles

Theoretical Kinetic and Mechanistic Studies on the Reactions of CF₃CBrCH₂ (2-BTP) with OH and H Radicals.

CF3CBrCH2 (2-bromo-3,3,3-trifluoropropene, 2-BTP) is a potential replacement for CF3Br; however, it shows conflicted inhibition and enhancement behaviors under different combustion conditions. To better understand the combustion chemistry of 2-BTP, a theoretical study has been performed on its reactions with OH and H radicals. Potential energy surfaces were exhaustively explored by using B3LYP/aug-cc-pVTZ for geometry optimizations and CCSD(T)/aug-cc-pVTZ for high level single point energy refinements. Detailed kinetics of the major pathways were predicted by using RRKM/master-equation methodology. The present predictions imply that the –C(Br)=CH2 moiety of 2-BTP is most likely to be responsible for its fuel-like property. For 2-BTP + OH, the addition to the initial adduct (CF3CBrCH2OH) is the dominant channel at low temperatures, while the substitution reaction (CF3COHCH2 + Br) and H abstraction reaction (CF3CBrCH + H2O) dominates at high temperatures and elevated pressures. For 2-BTP + H, the addition to the initial adduct (CF3CBrCH3) also dominates the overall kinetics at low temperatures, while Br abstraction reaction (CF3CCH2 + HBr) and β-scission of the adduct forming CF3CHCH2 + Br dominates at high temperatures and elevated pressures. Compared to 2-BTP + OH, the 2-BTP + H reaction tends to have a larger effect on flame suppression, given the fact that it produces more inhibition species.

Anthracene and phenanthrene tropospheric oxidation promoted by the nitrate radical in the gas-phase. Theoretical modelistic study

Polycyclic aromatic hydrocarbons and their oxidized derivatives are ubiquitous environmental pollutants that are toxic to different degrees. The NO3 radical is known to be an important actor in fostering nighttime atmospheric chemistry; hence it could elicit some nocturnal PAH-loss processes. In this study the gas phase pathways open to the initial anthracene and phenanthrene NO3 π−radical adducts in the presence of O2, NO and NO2 are examined by Density Functional Theory to ascertain the mechanistic features of their NO3-initiated oxidative degradation. Unimolecular steps involving the initial adducts (ring closures or fragmentation) present rather high free energy barriers and seem unlikely. Regarding bimolecular reactions, any radical present in the tropospheric environment can give an intrinsically fast radical coupling with the initial adducts, thus producing bifunctional closed shell species. The addition of ground state dioxygen is a relatively fast step that is reversible in some cases, less so in others. It entails subsequent reaction steps, which involve the peroxyl radical intermediate and addition and loss of small species as NO, NO3, NO2 and O2. These steps can either trigger a β−fragmentation with formation of a closed shell dialdehyde or formation of a closed shell nitroxy ketone, bifunctional species that appear to be the most likely products when typical tropospheric concentrations are taken into account.

Time-matched analysis of DNA adduct formation and early gene expression as predictive tool for renal carcinogenesis in methylazoxymethanol acetate treated Eker rats.

Genotoxic carcinogens pose great hazard to human health. Uncertainty of current risk assessment strategies and long latency periods between first carcinogen exposure and diagnosis of tumors have raised interest in predictive biomarkers. Initial DNA adduct formation is a necessary step for genotoxin induced carcinogenesis. However, as DNA adducts not always translate into tumorigenesis, their predictive value is limited. Here we hypothesize that the combined analysis of pro-mutagenic DNA adducts along with time-matched gene expression changes could serve as a superior prediction tool for genotoxic carcinogenesis. Eker rats, heterozygous for the tuberous sclerosis (Tsc2) tumor suppressor gene and thus highly susceptible towards genotoxic renal carcinogens, were continuously treated with the DNA alkylating carcinogen methylazoxymethanol acetate (MAMAc). Two weeks of MAMAc treatment resulted in a time-dependent increase of O6-methylguanine and N7-methylguanine adducts in the kidney cortex, which was however not reflected by significant expression changes of cyto-protective genes involved in DNA repair, cell cycle arrest or apoptosis. Instead, we found a transcriptional regulation of genes involved in the tumor-related MAPK, FoxO and TGF-beta pathways. Continuous MAMAc treatment for up to 6months resulted in a mild but significant increase of cancerous lesions. In summary, the combined analysis of DNA adducts and early gene expression changes could serve as a suitable predictive tool for genotoxicant-induced carcinogenesis.

Reaction mechanism of cyanoethynyl radical (C3N) with ethylene (C2H4) to form C5H3N and H: a theoretical investigation

The reaction mechanisms of cyanoethynyl radical (C3N) with ethylene (C2H4) to form C5H3N and H were investigated using density functional theory and a high-level ab initio method such as the coupled-cluster singles and doubles including a perturbative estimate of triples (CCSD(T)). The reaction pathways can be categorized into three types. The first pathway includes only chain intermediates (IMs). The second pathway includes ring IMs except for pyridine-like structures. The final pathway includes the pyridine-like structures. On the basis of the calculated results, the most favorable reaction pathway is the first type; the H elimination takes place from the initial chain adduct of C3N with C2H4. Because C3N and butadinyl (C4H) radical are isoelectronic, the most favorable reaction pathway and barrier energy of the reaction of C3N with C2H4 are almost identical to those of the reaction of C4H with C2H4.

Through-Space Charge Interaction Substituent Effects in Molecular Catalysis Leading to the Design of the Most Efficient Catalyst of CO2-to-CO Electrochemical Conversion.

The starting point of this study of through-space substituent effects on the catalysis of the electrochemical CO2-to-CO conversion by iron(0) tetraphenylporphyrins is the linear free energy correlation between through-structure electronic effects and the iron(I/0) standard potential that we established separately. The introduction of four positively charged trimethylanilinium groups at the para positions of the tetraphenylporphyrin (TPP) phenyls results in an important positive deviation from the correlation and a parallel improvement of the catalytic Tafel plot. The assignment of this catalysis boosting effect to the Coulombic interaction of these positive charges with the negative charge borne by the initial Fe0-CO2 adduct is confirmed by the negative deviation observed when the four positive charges are replaced by four negative charges borne by sulfonate groups also installed in the para positions of the TPP phenyls. The climax of this strategy of catalysis boosting by means of Coulombic stabilization of the initial Fe0-CO2 adduct is reached when four positively charged trimethylanilinium groups are introduced at the ortho positions of the TPP phenyls. The addition of a large concentration of a weak acid-phenol-helps by cleaving one of the C-O bonds of CO2. The efficiency of the resulting catalyst is unprecedented, as can be judged by the catalytic Tafel plot benchmarking with all presently available catalysts of the electrochemical CO2-to-CO conversion. The maximal turnover frequency (TOF) is as high as 106 s-1 and is reached at an overpotential of only 220 mV; the extrapolated TOF at zero overpotential is larger than 300 s-1. This catalyst leads to a highly selective formation of CO (practically 100%) in spite of the presence of a high concentration of phenol, which could have favored H2 evolution. It is also very stable, showing no significant alteration after more than 80 h of electrolysis.

Multiconfigurational and DFT analyses of the electromeric formulation and UV–vis absorption spectra of the superoxide adduct of ferrous superoxide reductase

The putative initial adduct of ferrous superoxide reductase (SOR) with superoxide has been alternatively formulated as ferric-peroxo or ferrous-superoxo. The ~600-nm UV–vis absorption band proposed to be assigned to this adduct (either as sole intermediate in the SOR catalytic cycle, or as one of the two intermediates) has recently been interpreted as due to a ligand-to-metal charge transfer, involving thiolate and superoxide in a ferrous complex, contrary to an alternative assignment as a predominantly cysteine thiolate-to-ferric charge transfer in a ferric-peroxo electromer. In an attempt to clarify the electromeric formulation of this adduct, we report a computational study using a multiconfigurational complete active space self-consistent field (MC-CASSCF) wave function approach as well as modelling the UV–vis absorption spectra with time-dependent density functional theory (TD-DFT). The MC-CASSCF calculations disclose a weak interaction between iron and the dioxygenic ligand and a dominant configuration with an essentially ferrous-superoxo character. The computed UV–vis absorption spectra reveal a marked dependence on the choice of density functional – both in terms of location of bands and in terms of orbital contributors. For the main band in the visible region, besides the recently reported thiolate-to-superoxide charge transfer, a more salient, and less functional-dependent, feature is a thiolate-to-ferric iron charge transfer, consistent with a ferric-peroxo electromer. By contrast, the computed UV–vis spectra of a ferric-hydroperoxo SOR model match distinctly better (and with no qualitative dependence on the DFT methodology) the 600-nm band as due to a mainly thiolate-to-ferric character – supporting the assignment of the SOR “600-nm intermediate” as a S=5/2 ferric-hydroperoxo species.

The Myeloablative Drug Busulfan Converts Cysteine to Dehydroalanine and Lanthionine in Redoxins.

The myeloablative agent busulfan (1,4-butanediol dimethanesulfonate) is an old drug that is used routinely to eliminate cancerous bone marrow prior to hematopoietic stem cell transplant. The myeloablative activity and systemic toxicity of busulfan have been ascribed to its ability to cross-link DNA. In contrast, here we demonstrate that incubation of busulfan with the thiol redox proteins glutaredoxin or thioredoxin at pH 7.4 and 37 °C results in the formation of putative S-tetrahydrothiophenium adducts at their catalytic Cys residues, followed by β-elimination to yield dehydroalanine. Both proteins contain a second Cys, in their catalytic C-X-X-C motif, which reacts with the dehydroalanine, the initial Cys adduct with busulfan, or the S-tetrahydrothiophenium, to form novel intramolecular cross-links. The reactivity of the dehydroalanine (DHA) formed is further demonstrated by adduction with glutathione to yield a lanthionine and by a novel reaction with the reducing agent tris(2-carboxyethyl)phosphine (TCEP), which yields a phosphine adduct via Michael addition to the DHA. Formation of a second quaternary organophosphonium salt via nucleophilic substitution with TCEP on the initial busulfan-protein adduct or on the THT(+)-Redoxin species is also observed. These results reveal a rich potential for reactions of busulfan with proteins in vitro, and likely in vivo. It is striking that several of the chemically altered protein products retain none of the atoms of busulfan, in contrast to typical drug-protein adducts or traditional protein modification reagents. In particular, the ability of a clinically used drug to convert Cys to dehydrolanine in intact proteins, and its subsequent reaction with biological thiols, is unprecedented.

A Novel Abnormal Michael Reaction of 2-Acylmethyl-4,4-dimethyl-2-oxazolines with Acetylenic Ketones and Esters

Abstract The first example of abnormal Michael reaction of an active methylene compound, 2-acylmethyl-4,4-dimethyl-2-oxazoline with acetylenic ketone in acetonitrile is reported. The reaction accompanies 1,3-migration of the acyl group of the substrate to give 2-(3-acyl-1-buten-4-on-1-yl)-2-oxazoline, which was easily cyclized to 5-acyl-2-pyridone derivatives by treatment with silica gel. Selectivity of the reaction depends on bulkiness of all the substituents of both the substrate and the reagent. The selectivity is interpreted in terms of reduced kinetic acidity of an initial anionic adduct intermediate by both steric and electronic factors.

Theoretical kinetics of O + C2H4

The reaction of atomic oxygen with ethylene is a fundamental oxidation step in combustion and is prototypical of reactions in which oxygen adds to double bonds. For 3O+C2H4 and for this class of reactions generally, decomposition of the initial adduct via spin-allowed reaction channels on the triplet surface competes with intersystem crossing (ISC) and a set of spin-forbidden reaction channels on the ground-state singlet surface. The two surfaces share some bimolecular products but feature different intermediates, pathways, and transition states. The overall product branching is therefore a sensitive function of the ISC rate. The 3O+C2H4 reaction has been extensively studied, but previous experimental work has not provided detailed branching information at elevated temperatures, while previous theoretical studies have employed empirical treatments of ISC. Here we predict the kinetics of 3O+C2H4 using an ab initio transition state theory based master equation (AITSTME) approach that includes an a priori description of ISC. Specifically, the ISC rate is calculated using Landau–Zener statistical theory, consideration of the four lowest-energy electronic states, and a direct classical trajectory study of the product branching immediately after ISC. The present theoretical results are largely in good agreement with existing low-temperature experimental kinetics and molecular beam studies. Good agreement is also found with past theoretical work, with the notable exception of the predicted product branching at elevated temperatures. Above ∼1000K, we predict CH2CHO+H and CH2+CH2O as the major products, which differs from the room temperature preference for CH3+HCO (which is assumed to remain at higher temperatures in some models) and from the prediction of a previous detailed master equation study.

Peptide Reactivity of Isothiocyanates--Implications for Skin Allergy.

Skin allergy is a chronic condition that affects about 20% of the population of the western world. This disease is caused by small reactive compounds, haptens, able to penetrate into the epidermis and modify endogenous proteins, thereby triggering an immunogenic reaction. Phenyl isothiocyanate (PITC) and ethyl isothiocyanate (EITC) have been suggested to be responsible for allergic skin reactions to chloroprene rubber, the main constituent of wetsuits, orthopedic braces, and many types of sports gear. In the present work we have studied the reactivity of the isothiocyanates PITC, EITC, and tetramethylrhodamine-6-isothiocyanate (6-TRITC) toward peptides under aqueous conditions at physiological pH to gain information about the types of immunogenic complexes these compounds may form in the skin. We found that all three compounds reacted quickly with cysteine moieties. For PITC and 6-TRITC the cysteine adducts decomposed over time, while stable adducts with lysine were formed. These experimental findings were verified by DFT calculations. Our results may suggest that the latter are responsible for allergic reactions to isothiocyanates. The initial adduct formation with cysteine residues may still be of great importance as it prevents hydrolysis and facilitates the transport of isothiocyanates into epidermis where they can form stable immunogenic complexes with lysine-containing proteins.

Au-Catalysed oxidative cyclisation.

The two main strategies of gold-catalysed oxidative cyclisation are discussed in this tutorial. The first one employs nucleophilic oxidants as either internal or external nucleophiles. The inherently weak O-heteroatom bond in the oxidant enables the versatile reactivities of the initial gold-promoted adduct of the oxidant to alkyne, including its fragmentation into a highly reactive α-oxo gold carbene intermediate. The second features external oxidant-powered Au(i)/Au(iii) catalysis, where the metal oxidation state changes during the catalytic cycle. These strategies have been applied toward the development of a variety of valuable synthetic transformations.

Reliable Potential Energy Surfaces for the Reactions of H2O with ThO2, PaO2(+), UO2(2+), and UO2(.).

The potential energy surfaces for the reactions of H2O with ThO2, PaO2(+), UO2(2+), and UO2(+) have been calculated at the coupled cluster CCSD(T) level extrapolated to the complete basis set limit with additional corrections including scalar relativistic and spin-orbit. The reactions proceed by the formation of an initial Lewis acid-base adduct (H2O)AnO2(0/+/2+) followed by a proton transfer to generate the dihydroxide AnO(OH)2(0/+/2+). The results are in excellent agreement with mass spectrometry experiments and prior calculations of hydrolysis reactions of the group 4 transition metal dioxides MO2. The differences in the energies of the stationary points on the potential energy surface are explained in terms of the charges on the system and the populations on the metal center. The use of an improved starting point for the coupled cluster CCSD(T) calculations based on density functional theory with the PW91 exchange-correlation functional or Brueckner orbitals is described. The importance of including second-order spin-orbit corrections for closed-shell molecules is also described. These improvements in the calculations are correlated with the 5f populations on the actinide.

Mechanistic Studies of the Cathodic Cleavage of Diphenylacetaldehyde Derivatives

At the 2014 Electrochemical Society Orlando meeting, we reported the electrochemical oxidation of 1,1-diphenylacetaldehyde (1) in the presence of an equimolar quantity of any of several alcohols (2) in acetonitrile in an undivided cell containing two carbon electrodes (Scheme 1). When the electrolysis was interrupted at its mid-point it was possible to isolate a-alkoxy aldehydes (3) in good yield. When the electrolysis was continued past this point, compounds 3 were further converted to benzhydryl ethers (4)in excellent yields. Since 1 is converted to 3 at the anode via 2, we initially assumed that the ethers 4 are also produced at the anode by subsequent oxidation of 3 to 4.However, two control experiments disproved this hypothesis. First, 3 (independently synthesized by reaction of alpha-bromo-diphenylacetaldehyde2 with cyclohexanol in the presence of silver tetrafluoroborate3) was placed in the anode compartment of a divided cell. It was found to be stable to anodic oxidation. On the other hand, when it was placed in the cathode compartment, it was quickly decarbonylated to 4a . Likewise, when electrolysis of 1 was carried out in the anode compartment of a divided cell, 3 was produced but was found to be stable to further oxidation. On the other hand, when 3 was placed in the cathode compartment of the divided cell it was readily converted to 4 (Scheme 1). The mechanism of this unusual mild cathodic cleavage reaction is the subject of this report. We hypothesized that the reaction involves initial attack on the carbonyl group by a nucleophilic component of the medium (X), followed by ejection of a benzhydryl anion (6) from the initial adduct 5. Protonation of 6 by solvent would then afford 4 We believe that X is superoxide ion, generated by reduction of cathodic reduction of traces of oxygen in the medium. Indeed, electrolysis of 3 in the cathode compartment while passing oxygen through the solution quickly converted 3 to 4. Furthermore, this suggests that it should be possible to convert 3 to 4 in a separate non-electrochemical experiment. Indeed, addition of potassium superoxideto a solution of 3 in acetonitrile was found to produce 4 in high yield (Scheme 2). Interestingly, benzophenone (8) is produced when 1 is either (a) electrolyzed in the cathode compartment with oxygen or (b) treated with potassium superoxide. This conversion probably involves initial hydroperoxidation of 3 at the a-position, followed by nucleophilic addition to the carbonyl group and fragmentation. Experiments testing the further scope of these cathodic decarbonylative cleavages will be described. Figure 1

Mutagenic effects induced by the attack of NO2 radical to the guanine-cytosine base pair.

We investigate the attack of the nitrogen dioxide radical (NO•2) to the guanine—cytosine (GC) base pair and the subsequent tautomeric reactions able to induce mutations, by means of density functional theory (DFT) calculations. The conducted simulations allow us to identify the most reactive sites of the GC base pair. Indeed, the computed relative energies demonstrate that the addition of the NO•2 radical to the C8 position of the guanine base forms to the most stable adduct. Although the initial adducts might evolve to non-canonical structures via inter-base hydrogen bonds rearrangements, the probability for the proton exchange to occur lies in the same range as that observed for undamaged DNA. As a result, tautomeric errors in NO2-attacked DNA arises at the same rate as in canonical DNA, with no macroscopic impact on the overall stability of DNA. The potential mutagenic effects of the GC–NO•2 radical adducts likely involve side reactions, e.g., the GC deprotonation to the solvent, rather than proton exchange between guanine and cytosine basis.

Radical-based route to 2-(trifluoromethyl)-1,3,4-oxadiazoles and trifluoromethyl-substituted polycyclic 1,2,4-triazoles and dihydrofurans.

O-Ethyl S-[5-(trifluoromethyl)-1,3,4-oxadiazol-2-yl]methyl xanthate was readily prepared on a large scale and shown to undergo very efficient intermolecular radical additions to unactivated alkenes. The products were further elaborated by exploiting both radical and ionic processes to provide a variety of trifluoromethyl-substituted derivatives, including medicinally relevant triazoles. In particular, the application of a radical allylation on the initial adducts leads to structures that are able to undergo intramolecular [4 + 2] cycloaddition reactions.

The reaction of aluminium clusters with water: a quantum chemical investigation

The reaction of charged aluminium clusters with water in the gas phase is investigated theoretically. To this end, the doublet potential energy surface for the Aln+ + H2O → AlnO+ + H2 reaction with n=6 is explored using density functional theory. The calculated reaction pathways include the initial adduct ion formation, the O-H bond activation and H2 elimination steps, and are consistent with the recent experimental report (Ref.2). The results of the current quantum-chemical study are relevant to the issues of catalytic role of the main group metal clusters and H2 generation.

Accidental exposure to gas emissions from transit goods treated for pest control.

BackgroundInternational phytosanitary standards ISPM 15 require (since 2007) fumigation or heat treatment for shipping and storage. Those dealing with fumigated freight might be accidentally exposed. In this paper we report a series of three accidents of six storage room workers in a medium sized company regularly importing electronic production parts from abroad.MethodsPatients (n = 6, aged from 32–54 yrs.) and control group (n = 30, mean 40 yrs.) donated blood and urine samples. The fumigants: ethylene oxide, methyl bromide, chloropicrin, ethylene dichloride, other halo-alkanes and solvents were analyzed by headspace gas chromatography/mass spectrometry (GCMS). For the quantitation of long term exposure/s, macromolecular reaction products (hemoglobin adducts) were used (with GCMS) as molecular dosimeter; additionally 8-OHdG and circulating mtDNA (cmtDNA) were analyzed as nonspecific biological effect markers.ResultsThe hemoglobin adducts N-methyl valine (MEV) and N-(2-hydroxy ethyl) valine (HEV) were elevated after exposure to the alkylating chemicals methyl bromide and ethylene oxide. Under the consideration of known elimination kinetics and the individual smoking status (biomonitored with nicotine metabolite cotinine and tobacco specific hemoglobin adduct: N-(2 cyan ethyl) valines, CEV), the data allow theoretical extrapolation to the initial protein adduct concentrations at the time of the accident (the MEV/CEV levels were from 1,616 pmol/g globin to 1,880 pmol/g globin and HEV/CEV levels from 1,407 pmol/g globin to 5,049 pmol/g globin, and correlated with inhaled 0.4-1.5 ppm ethylene oxide. These integrated, extrapolated internal doses, calculated on the basis of biological exposure equivalents, confirmed the clinical diagnosis for three patients, showing severe intoxication symptoms. Both, cmtDNA and 8-OHdG, as non-specific biomarkers of toxic effects, were elevated in four patients.ConclusionThe cases reported here, stress the importance of a suitable risk assessment and control measures. We put emphasis on the necessity of human biomonitoring guidelines and the urgency for the relevant limit values.

B(C6F5)3-Mediated Transformations and Hydrogenation of Carbodiimides

The reactivity of a series of carbodiimides RNCNR (R=tBu (1 a), iPr (1 b), SiMe3 (1 c), and Dipp (2,6-di-iso-propylphenyl (1 d)) with B(C6 F5 )3 was investigated. After initial adduct formation, several distinct reaction pathways were identified. These pathways involve either isomerization of the carbodiimide to cyanamide derivatives or insertion of a carbodiimide into a BC bond of B(C6 F5 )3 to yield four-membered heterocycles. In the presence of dihydrogen, stepwise hydrogenation of the heteroallene moiety in the carbodiimides was achieved, which yielded the respective amidine-B(C6 F5 )3 adducts and amidinium borate salts upon reaction with one or two equivalents of H2 , respectively.

A convergent route to substituted azetidines and to Boc-protected 4-aminomethylpyrroles

The radical addition of xanthates to Boc-protected azetine gives adducts, which can be reductively dexanthylated to furnish variously substituted azetidines. In the case of α-xanthyl ketones, treatment of the initial adducts with ammonia or primary amines, furnishes 2,4-disubstituted, 2,3,4-trisubstituted, and polycyclic pyrroles having a protected aminomethyl group at position-4. An unusual ring opening was observed in the case of a cyclobutanone precursor. It also proved possible to add an azetidinyl radical to an indole ring. Most of these products would be very difficult to obtain by more conventional routes.

Oxidative esterification of aldehydes using mesoionic 1,2,3-triazolyl carbene organocatalysts.

The synthesis and catalytic activity of a new class of 1,2,3-triazolyl N-heterocyclic carbene organocatalysts is described. These new catalysts chemoselectively facilitate the oxidative esterification of aldehydes. NMR acidity studies show an inverse correlation between triazolium acidity and reactivity. Kinetic studies show that the resting state of the catalyst involves a NHC-aldehyde adduct. A catalytically active intermediate was synthesized and characterized by X-ray diffraction as the initial carbene-aldehyde adduct.