Home About us Contact
|
Home About us Contact | ||
|
|
||
Electrophilic Attack (electrophilic + attack)
Selected AbstractsElectrophilic Attack on Sulfur,Sulfur Bonds.CHEMINFORM, Issue 43 2004Part 1. Abstract For Abstract see ChemInform Abstract in Full Text. [source] Electrophilic Attack on Sulfur,Sulfur Bonds: Coordination of Lithium Cations to Sulfur-Rich Molecules Studied by Ab Initio MO MethodsCHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2005Yana Steudel Dr. Abstract Complex formation between gaseous Li+ ions and sulfur-containing neutral ligands, such as H2S, Me2Sn (n = 1,5; Me = CH3) and various isomers of hexasulfur (S6), has been studied by ab initio MO calculations at the G3X(MP2) level of theory. Generally, the formation of LiSn heterocycles and clusters is preferred in these reactions. The binding energies of the cation in the 29 complexes investigated range from ,88 kJ,mol,1 for [H2SLi]+ to ,189 kJ,mol,1 for the most stable isomer of [Me2S5Li]+ which contains three-coordinate Li+. Of the various S6 ligands (chair, boat, prism, branched ring, and triplet chain structures), two isomeric complexes containing the S5S ligand have the highest binding energies (,163±1 kJ,mol,1). However, the global minimum structure of [LiS6]+ is of C3v symmetry with the six-membered S6 homocycle in the well-known chair conformation and three LiS bonds with a length of 256 pm (binding energy: ,134 kJ,mol,1). Relatively unstable isomers of S6 are stabilized by complex formation with Li+. The interaction between the cation and the S6 ligands is mainly attributed to ion,dipole attraction with a little charge transfer, except in cations containing the six sulfur atoms in the form of separated neutral S2, S3, or S4 units, as in [Li(S3)2]+ and [Li(S2)(S4)]+. In the two most stable isomers of the [LiS6]+ complexes, the number of SS bonds is at maximum and the coordination number of Li+ is either 3 or 4. A topological analysis of all investigated complexes revealed that the LiS bonds of lengths below 280 pm are characterized by a maximum electron-density path and closed-shell interaction. [source] An Investigation of the Reactivity of [(diimine)(dithiolato)M] Complexes Using the Fukui Functions ConceptEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 3 2006Christodoulos Makedonas Abstract Fukui functions are widely used when investigating the reactivity of organic molecules, but rarely with metal complexes. Here, we investigate the reactivity of [(diimine)(dithiolato)M] complexes with different types of reagents and upon oxidation employing this concept. Mixed-ligand complexes of this type have a peculiar electronic description due to the mixed-metal-ligand-to-ligand charge-transfer band, which is why they are considered as very promising candidates for non-linear optical (NLO) materials and molecular photochemical devices (MPD). As a result, their reactivity is of crucial importance for their potential applications. The obtained results of f+ and f, for the neutral [(diimine)(dithiolato)M] complexes (M = Pd, Ni and Pt) not only predict that the sulfur atom is the preferable active site for electrophilic attack but also reveal the different tunability of these complexes when they are subjected to an oxidation process, in agreement with experimental results. Under the framework of the Fukui indices we also provide an alternative explanation for crystal packing that could find widespread application. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Synthesis of Substituted Oxazoles from N -Acyl-,-hydroxyamino Acid DerivativesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 27 2008Paula M. T. Ferreira Abstract Several N -acyl-,-hydroxyamino acids were prepared and treated with di- tert -butyl dicarbonate in the presence of 4-(dimethylamino)pyridine, followed by treatment with N,N,N,,N, -tetramethylguanidine to give the corresponding N -acyldehydroamino acids in good to high yields. These were then treated with I2/K2CO3 followed by 1,8-diazabicyclo[5.4.0]undec-7-ene. The methyl esters of N -acyldehydroaminobutyric acid gave the corresponding substituted oxazoles in good to high yields. The N -acyldehydrophenylalanines gave 5-phenyloxazole derivatives in low to moderate yields together with ,-iododehydrophenylalanines. Under the same conditions, N -acyldehydroalanines failed to give the corresponding oxazoles. However, when the reaction was carried out in the absence of DBU, it was possible to isolate the ,,,-diiododehydroalanine derivatives. Although the reason for the different reactivities of the N -acyldehydroamino acids is not completely clear to us, cyclic voltammetry studies showed that the less-reactive derivatives have higher reduction potentials. This suggests that the double bonds in dehydroaminobutyric acid derivatives are more susceptible to electrophilic attack by iodine.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] 2,3,4-Triphenyl-3-azabicyclo[3.2.0]hepta-1,4-diene , Facile Ring-Opening by Electrophiles and Novel Reactions with Dimethyl AcetylenedicarboxylateEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 22 2004Kiyoshi Matsumoto Abstract A 3-azabicyclo[3.2.0]hepta-1,4-diene with no substituent in the cyclobutene moiety has been prepared for the first time; it undergoes extremely facile electrophilic attack at the ,-position to give the ring-opened product, probably by a retro-Friedel,Crafts process. The title compound also undergoes a novel reaction with dimethyl acetylenedicarboxylate to afford the azepine. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] A New Class of Enehydroxylamino Ketones , (R)-2-(1-Hydroxy-4,4,5,5-tetraalkylimidazolidin-2-ylidene)ethanones: Synthesis and ReactionsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 4 2004Vladimir A. Reznikov Abstract Three approaches to the synthesis of (R)-2-(1-hydroxy-4,4,5,5-tetraalkylimidazolidin-2-ylidene)ethanones 1 are described: (a) condensation of 1,2-bishydroxylamines with ,-ketoaldehyde synthons, (b) treatment of metallated 1-hydroxy-2-methyl-4,5-dihydroimidazoles with esters, and (c) 1,3-dipolar cycloaddition between 1-hydroxy-4,5-dihydroimidazole-3-oxide and DMAD. The reactivity of 1 with electrophiles has been studied. The exocyclic methylene (enamine) carbon atom is shown to be the major site of electrophilic attack. Synthesized chloro-substituted 1-hydroxy-2-acetylideneimidazolidines react with sodium cyanide to form the corresponding nitriles. Oxidation of these nitriles occurs with formation of persistent vinyl nitroxides, which are of interest as potential paramagnetic ligands. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] The Reaction of Peroxynitrite with Morpholine (Secondary Amines) Revisited: The Overlooked Hydroxylamine FormationHELVETICA CHIMICA ACTA, Issue 10 2006Michael Kirsch Abstract The reaction of peroxynitrite/peroxynitrous acid with morpholine as a model compound for secondary amines is reinvestigated in the absence and presence of carbon dioxide. The concentration- and pH-dependent formation of N -nitrosomorpholine and N -nitromorpholine as reported in three previous papers ([25],[26],[14]) is basically confirmed. However, 13C-NMR spectroscopic product analysis shows that, in the absence of CO2, N -hydroxymorpholine is, at pH,,,7, the major product of this reaction, even under anaerobic conditions. The formation of N -hydroxymorpholine has been overlooked in the three cited papers. Additional (ring-opened) oxidation products of morpholine are also detected. The data account for radical pathways for the formation of these products via intermediate morpholine-derived aminyl and , -aminoalkyl radicals. This is further supported by EPR-spectrometric detection of morpholine-derived nitroxide radicals, i.e., morpholin-4-yloxy radicals. N -Nitrosomorpholine, however, is very likely formed by electrophilic attack of peroxynitrite-derived N2O4. 15N-CIDNP Experiments establish that, in the presence of CO2, N -nitro- and C -nitromorpholine are generated by radical recombination. The present results are in full accord with a fractional (28,±,2%) homolytic decay of peroxynitrite/peroxynitrous acid with release of free hydroxyl and nitrogen dioxide radicals. [source] Theoretical study of the substituent effect on the intramolecular hydrogen bonds in di(4-hydroxycoumarin) derivativesINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 6 2006Tzvetan Mihaylov Abstract Geometry optimization of ortho -, meta -, and para -pyridyl-substituted di(4-hydroxycoumarin) [di(4-HC)] was performed with the density functional theory (DFT) [B3LYP/6-31G(d)] method. Two asymmetrical intramolecular OH,O hydrogen bonds (HBs) stabilized the structures. The calculated single HB energies varied from ,62.56 to ,47.53 kJ mol,1 and pointed to a relative strong hydrogen bond in the systems studied. The 2- and 6-pyridyl substituents produced the largest geometrical changes in di(4-hydroxycoumarin) fragment. The highest total HB energy was found for 2-pyridyl-substituted and the lowest one for 6-pyridyl-substituted di(4-hydroxycoumarin). The HB energy variations were confirmed with rotational barrier method calculations. Both steric and electrostatic factors were found to be responsible for the HB asymmetry in the compounds studied. According to the molecular electrostatic potential (MEP) calculations the most preferred reactive site for electrophilic attack of pyridyl-substituted di(4-hydroxycoumarin)s are the pyridine nitrogen and the carbonyl oxygens, followed by the hydroxyl oxygens. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Influence of protein binding on acrolein turnover in vitro by oxazaphosphorines and liver microsomesJOURNAL OF CLINICAL LABORATORY ANALYSIS, Issue 3 2005Frank Baumann Abstract For a correct determination of acrolein amounts generated in in vitro turnover experiments with oxazaphosphorines, it is necessary to characterize the interaction of acrolein with liver microsomal proteins. Acrolein, a highly reactive metabolite of oxazaphosphorines, readily forms covalent adducts with proteins by electrophilic attack on nucleophiles, such as the sulfhydryl group of cysteine, imidazole group of histidine, and amino group of lysine. The current investigations were mainly directed toward determination of the degree of acrolein-protein binding under conditions of in vitro experiments with liver microsome preparations. The acrolein concentration in protein dilution was determined by a fluorescence method. Moreover, the influence of sucrose and glycerine on the extent of acrolein-protein binding commonly used for the stabilization of microsomal preparations during storage was investigated. The current investigations show evidence that the chemical reaction of acrolein with liver microsomal proteins strictly follows first order kinetics. The main part of the formed acrolein in the in vitro attempts is available as bound part. Results of these investigations indicate that the calibration should be carried out with mixtures from liver microsome preparations and known amounts of acrolein under the same conditions as the in vitro experiments to record the entirely formed acrolein part (free and bound) in oxazaphosphorine turnover experiments. Glycerine is recommended as a preservative to store liver microsomes instead of sucrose because the latter reacts with acrolein. J. Clin. Lab. Anal. 19:103,109, 2005. © 2005 Wiley-Liss, Inc. [source] Correlation of relative rates of chromyl chloride oxidation and chromic acid oxidation of acyclic alkenes versus alkene IPs and HOMOsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 11 2004Donna J. Nelson Abstract Plots of logarithms of relative reaction rates of chromyl chloride oxidation and of chromic acid oxidation of alkenes (log,krel values) versus alkene ionization potentials (IPs) and versus their highest occupied molecular orbital energy levels (HOMOs) demonstrate excellent correlations. Each plot has a similar appearance and shows a single line with a positive slope. The results indicate that the rate-determining step of each title reaction involves an electrophilic attack on the alkene ,-bond without significant steric effects; this supports a proposed 2,+,3 cycloaddition mechanism and disfavors a proposed stepwise 2,+,2 cycloaddition mechanism. Comparison is made with other d0 transition metal complexes that oxidize alkenes. Copyright © 2004 John Wiley & Sons, Ltd. [source] Transformation of organic molecules on the low-valent {M(Ph2PCH2CH2PPh2)2} moiety derived from trans -[M(N2)2(Ph2PCH2CH2PPh2)2] or related complexes (M = MO, W)THE CHEMICAL RECORD, Issue 5 2001Hidetake Seino Abstract A zero-valent {M(Ph2PCH2CH2PPh2)2} moiety (M = Mo, W) generated in situ by dissociation of the N2 ligands in trans -[M(N2)2(Ph2PCH2CH2PPh2)2] can activate ,-accepting organic molecules including isocyanides and nitriles, which undergo the electrophilic attack caused by a strong ,-donation from a zero-valent metal center. Cleavage of a variety of C,X bonds (X = H, C, N, O, P, halogen) also occurs at their electron-rich sites through oxidative addition to form reactive intermediates, which subsequently degradate to yield smaller molecules either bound to or dissociated from the metal center. The mechanism is substantiated unambiguously by isolation of numerous intermediate stages. © 2001 John Wiley & Sons, Inc. and The Japan Chemical Journal Forum Chem Rec 1:349,361, 2001 [source] The Interaction of Heteroaryl-Acrylates and Alanines with Phenylalanine Ammonia-Lyase from ParsleyCHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2006Csaba Paizs Dr. Abstract Acrylic acids and alanines substituted with heteroaryl groups at the ,-position were synthesized and spectroscopically characterized (UV, HRMS, 1H NMR, and 13C NMR spectroscopy). The heteroaryl groups were furanyl, thiophenyl, benzofuranyl, and benzothiophenyl and contained the alanyl side chains either at the 2- or 3-positions. While the former are good substrates for phenylalanine ammonia-lyase (PAL), the latter compounds are inhibitors. Exceptions are thiophen-3-yl-alanine, a moderate substrate and furan-3-yl-alanine, which is inert. Possible reasons for these exceptions are discussed. Starting from racemic heteroaryl-2-alanines their D -enantiomers were prepared by using a stereodestructive procedure. From the heteroaryl-2-acrylates, the L -enantiomers of the heteroaryl-2-alanines were prepared at high ammonia concentration. These results can be best explained by a Friedel,Crafts-type electrophilic attack at the aromatic part of the substrates as the initial step of the PAL reaction. [source] A comprehensive theoretical study on the hydrolysis of carbonyl sulfide in the neutral waterJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 3 2008Chao Deng Abstract The detailed hydration mechanism of carbonyl sulfide (COS) in the presence of up to five water molecules has been investigated at the level of HF and MP2 with the basis set of 6-311++G(d, p). The nucleophilic addition of water molecule occurs in a concerted way across the CS bond of COS rather than across the CO bond. This preferential reaction mechanism could be rationalized in terms of Fukui functions for the both nucleophilic and electrophilic attacks. The activation barriers, ,H, for the rate-determining steps of one up to five-water hydrolyses of COS across the CS bond are 199.4, 144.4, 123.0, 115.5, and 107.9 kJ/mol in the gas phase, respectively. The most favorable hydrolysis path of COS involves a sort of eight-membered ring transition structure and other two water molecules near to the nonreactive oxygen atom but not involved in the proton transfer, suggesting that the hydrolysis of COS can be significantly mediated by the water molecule(s) and the cooperative effects of the water molecule(s) in the nonreactive region. The catalytic effect of water molecule(s) due to the alleviation of ring strain in the proton transfer process may result from the synergistic effects of rehybridization and charge reorganization from the precoordination complex to the rate-determining transition state structure induced by water molecule. The studies on the effect of temperature on the hydrolysis of COS show that the higher temperature is unfavorable for the hydrolysis of COS. PCM solvation models almost do not modify the calculated energy barriers in a significant way. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] | ||||||||||||||||||||||