Distribution by Scientific Domains
Distribution within Chemistry

Kinds of Nucleophiles

  • carbon nucleophile
  • different nucleophile
  • nitrogen nucleophile
  • other nucleophile
  • various nucleophile

  • Selected Abstracts

    ChemInform Abstract: Enantiotopic Discrimination in Palladium-Mediated Nucleophilic Substitutions on Achiral Substrates: Chiral Ligand versus Chiral Nucleophile.

    CHEMINFORM, Issue 4 2010
    Olivier Jacquet
    Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

    ChemInform Abstract: Enantioselective Staudinger Synthesis of ,-Lactams Catalyzed by a Planar-Chiral Nucleophile.

    CHEMINFORM, Issue 28 2002
    Brian L. Hodous
    Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

    An HPLC/mass spectrometry platform for the development of multimodality contrast agents and targeted therapeutics: prostate-specific membrane antigen small molecule derivatives

    Valerie Humblet
    Abstract The production of disease-targeted agents requires the covalent conjugation of a targeting molecule with a contrast agent or therapeutic, followed by purification of the product to homogeneity. Typical targeting molecules, such as small molecules and peptides, often have high charge-to-mass ratios and/or hydrophobicity. Contrast agents and therapeutics themselves are also diverse, and include lanthanide chelates for MRI, 99mTc chelates for SPECT, 90Y chelates for radiotherapy, 18F derivatives for PET, and heptamethine indocyanines for near-infrared fluorescent optical imaging. We have constructed a general-purpose HPLC/mass spectrometry platform capable of purifying virtually any targeted agent for any modality. The analytical sub-system is composed of a single dual-head pump that directs mobile phase to either a hot cell for the purification of radioactive agents or to an ES-TOF MS for the purification of nonradioactive agents. Nonradioactive agents are also monitored during purification by ELSD, absorbance and fluorescence. The preparative sub-system is composed of columns and procedures that permit rapid scaling from the analytical system. To demonstrate the platform's utility, we describe the preparation of five small molecule derivatives specific for prostate-specific membrane antigen (PSMA): a gadolinium derivative for MRI, indium, rhenium and technetium derivatives for SPECT, and an yttrium derivative for radiotherapy. All five compounds are derived from a highly anionic targeting ligand engineered to have a single nucleophile for N -hydroxysuccinimide-based conjugation. We also describe optimized column/mobile phase combinations and mass spectrometry settings for each class of agent, and discuss strategies for purifying molecules with extreme charge and/or hydrophobicity. Taken together, our study should expedite the development of disease-targeted, multimodality diagnostic and therapeutic agents. Copyright 2006 John Wiley & Sons, Ltd. [source]

    Voltammetric Assay of Naproxen in Pharmaceutical Formulations Using Boron-Doped Diamond Electrode

    ELECTROANALYSIS, Issue 11 2005
    V. Suryanarayanan
    Abstract The electrooxidation of naproxen was studied, for the first time, using boron-doped diamond (BDD) electrode by cyclic and differential pulse voltammetry (CV and DPV) in nonaqueous solvent supporting electrolyte system. The results were also compared with glassy carbon electrode (GC) under the same conditions. Naproxen undergoes one electron transfer resulting in the formation of cation radical for the first electrooxidation step, which follows other chemical and electrochemical steps such as deprotonation, removal of another electron and the attack of nucleophile (ECEC mechanism). BDD electrode provided higher signal to background ratio, well resolved and highly reproducible cyclic voltammograms than the GC electrode. With a scan rate of 50,mV s,1 and pulse height of 50,ms, respectively, the DPV technique was able to determine the naproxen concentrations in the range of 0.5 to 50,,M with a detection limit of 30,nM. The influence of interference compounds namely 2-acetyl-6-methoxy naphthalene (AMN) on naproxen oxidation can also be followed successfully. Moreover, the percentage of AMN present in the standard chemical form of a mixture containing naproxen can be found accurately. Rapidity, precise and good selectivity were also found for the determination of naproxen in pharmaceutical formulations. [source]

    Better Performance of Monodentate P -Stereogenic Phosphanes Compared to Bidentate Analogues in Pd-Catalyzed Asymmetric Allylic Alkylations

    Arnald Grabulosa
    Abstract The cationic allylpalladium complexes 3a,3f, 4a, 4e, 5e of type [Pd(,3 -2-Me-C3H4)P2]PF6 were synthesized using a group of monodentate P -stereogenic phosphanes, P=PPhRR, (a,f) and diphosphanes (PhRPCH2)2 (1a, 1e) or PhRPCH2Si(Me)2CH2PPhR (2e). The analogous cationic complexes with the disubstituted allyl group (,3 -1,3-Ph2 -C3H3) and monodentate phosphanes were not isolated as stable solids; only [PdCl(,3 -1,3-Ph2 -C3H3)P] (6a, 6d) were obtained. Palladium allyl complexes were screened as precatalysts in the allylic substitution of rac -3-acetoxy-1,3-diphenyl-1-propene (I) and (E)-3-acetoxy-1-phenyl-1-propene (III) with dimethyl malonate as the nucleophile. The various catalytic precursors showed a wide range of activity and selectivity. The bismonodentate phosphane complexes 3 are more active than the bidentate analogues. With regard to the regioselectivity, precursors containing monodentate phosphanes favour the formation of the linear product in the allylic substitution of cinnamyl acetate (III) compared with those containing bidentate phosphanes. With substrate I, compounds with the diphosphanes 1a and 1e, containing a five-membered chelate ring, gave low enantioselectivities (less than 10,% ee), but those with the diphosphane 2e, forming a six-membered chelate ring or with two monodentate phosphanes, afforded products with moderate enantioselectivity under standard conditions (ee up to 74,%). The results show that the performance of precursors containing monodentate phosphanes was superior to those containing bidentate ligands in both activity and selectivity. [source]

    Ring Closure of Alkoxycarbonyl(tetracarbonyl)pyruvoyliron Complexes into Metallalactones Induced by Nucleophilic Attack of Carbanions

    Patrice Cabon
    Abstract The reaction of carbanions with the pyruvoyl-substituted iron complex [(CO)4Fe(CO2CH3){C(O)C(O)CH3}] (1) affords the anionic trifunctionalized metallalactones [(CO)3Fe{C(O)C(CH3)(CRR,R,)OC4(O)(Fe,C4)}(CO2CH3)], (3), whose formation results from the addition of the nucleophile to the , carbonyl of the pyruvoyl moiety, followed by attack of the oxygen of this , carbonyl on a terminal carbonyl ligand. These anionic lactones react, at low temperature, with HCl to give rise to the neutral lactones [(CO)4Fe{C(O)C(CH3)(CRR,R,)OC4(O)(Fe,C4)}] (2), which were previously obtained by addition of NuH nucleophiles to 1. Complex 3(3), whose lactonic ring formation has been performed using the diethyl malonate anion (R = R, = CO2C2H5; R, = H), and the dimethyl-substituted neutral lactone 2(1) (R = R, = R, = H) have been characterized by X-ray diffraction studies. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]

    The Reaction of (Bipyridyl)palladium(II) Complexes with Thiourea , Influence of DNA and Other Polyanions on the Rate of Reaction

    Matteo Cusumano
    Abstract [Pd(bipy)(py)2](PF6)2 reacts stepwise with excess thiourea to give [Pd(tu)4](PF6)2. The kinetics of the second step, which refers to the replacement of bipyridyl in [Pd(bipy)(tu)2](PF6)2, have been studied in water and in the presence of calf thymus DNA, sodium polyriboadenylate, sodium polyvinylsulfonate or sodium polymetaphosphate at 25 C and pH = 7 and a fixed sodium chloride concentration. The reaction follows a first order course and a plot of kobs against [thiourea]2 affords a straight line with a small intercept. DNA inhibits the process without altering the rate law. The kobs values decrease systematically on increasing the DNA concentration eventually tending to a limiting value. The values are larger at higher ionic strengths and the other polyanions show similar behaviour. The influence of DNA on the kinetics can be related to steric inhibition caused by noncovalent binding with the complex. Upon interaction with DNA, [Pd(bipy)(tu)2]2+ gives rise to immediate spectroscopic changes in the UV/Vis region as well as induced circular dichroism suggesting that the complex, like similar platinum(II) and palladium(II) species of bipyridyl, intercalates with the double helix. Such a type of interaction hampers the attack of the nucleophile at the metal centre inhibiting the reaction. The decrease in the rate of ligand substitution upon decreasing salt concentration but at a given DNA concentration is due to the influence of ionic strength on the complex,DNA interaction. The reactivity inhibition by single-stranded poly(A), polyvinylsulfonate or polymetaphosphate can be accounted for in terms of self-aggregation of the complex induced by the polyanion. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

    Gold(I) Catalysis: Selective Synthesis of Six- or Seven-Membered Heterocycles from Epoxy Alkynes

    Lun-Zhi Dai
    Abstract Gold(I)-catalyzed intramolecular cycloisomerization of ketone-substituted epoxides with alkynes to six- or seven-membered heterocyclic compounds is firstly introduced in this paper. This procedure involves a cascade isomerization of the ketone-substituted epoxides into 1,3-diketones in the presence of a Lewis acid and subsequent gold(I)-catalyzed selective intramolecular addition of an oxygen or a carbon nucleophile to the alkynes. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

    Multicomponent Access to Functionalized Mesoionic Structures Based on TFAA Activation of Isocyanides: Novel Domino Reactions

    Mara Jos Arvalo
    Abstract The reactions of azines (isoquinolines, pyridine) with TFAA and isocyanides in a new domino process yield mesoionic acid fluorides with an imidazo[1,2- a]azine core. This multicomponent reaction has a general character, tolerating a wide range of substitution patterns on each component, and displays an unprecedented arrangement of reaction pathways. The protocol allows the incorporation of a fourth synthetic input by the reaction of a suitable nucleophile (alcohols, thiols, amines) with the acid fluoride moiety.( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

    Ring Expansion of 2-(,-Hydroxyalkyl)azetidines: A Synthetic Route to Functionalized Pyrrolidines

    Franois Durrat
    Abstract A series of 2-(,-hydroxyalkyl)azetidines synthesized from enantiomerically pure ,-amino alcohols and presenting various patterns both on the four-membered ring and on the adjacent hydroxy group were treated with either thionyl chloride or methanesulfonyl chloride in the presence of triethylamine. The thus-prepared 2-(,-chloro- or ,-methanesulfonyloxyalkyl)azetidines were shown to rearrange stereospecifically into 3-(chloro- or methanesulfonyloxy)pyrrolidines. When this rearrangement is conducted in the presence of an added nucleophile (NaN3, KCN, KOH, or NaOAc), the produced pyrrolidine incorporates the added nucleophile at C-3 stereospecifically. The relative configuration of the substituents in the formed pyrrolidines is consistent with a mechanism involving the formation of an intermediate bicyclic aziridinium ion, which is opened regioselectively at the bridgehead carbon atom. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

    Nucleophilic Reactivities of Pyrroles,

    Tobias A. Nigst
    Abstract The second-order rate constants of the reactions of alkyl-substituted pyrroles with a series of benzhydrylium ions were determined in acetonitrile, and the reaction products were fully characterized by NMR spectroscopy and mass spectrometry. The formation of the , adducts is the rate-limiting step of these reactions. Because the second-order rate constants correlate linearly with the electrophilicity parameters of the benzhydrylium ions, the determination of the nucleophilicity parameters N and s according to the linear free energy relationship log k2 (20 C) = s(N + E) was achieved. With these findings, a direct comparison of the nucleophilic reactivities of these ,-excessive heterocycles with other nucleophiles became possible, and the pyrroles were integrated into the comprehensive scale of nucleophilicity, covering a range of 8,9 orders of magnitude from N -(triisopropylsilyl)pyrrole (N = 3.12), the weakest nucleophile of this series, to kryptopyrrole (3-ethyl-2,4-dimethylpyrrole, N = 11.63). Thus, highly reactive pyrroles show similar nucleophilic reactivities as enamines, whereas those of less-reactive pyrroles are comparable to allylsilanes or indoles. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

    Asymmetric Synthesis of Isoquinoline Derivatives from Amino Acids

    Oxana Sieck
    Abstract Reaction of isoquinolines 1 with N -arylsulfonylamino acid fluorides 2 provides a highly stereoselective access to new dihydroimidazo[2,1 -a]isoquinolin-3-ones 5 via intermediate N -acylisoquinolinium salts 3. Addition reactions to the en-amine double bond, such as hydrogenation or epoxidation with dimethyldioxirane, leads to tetrahydroimidazo[2,1 -a]isoquinoline-3-ones 6, 7 and oxiranes 8, respectively. Opening of the oxirane ring of the 8 with nucleophiles allows the synthesis of hydroxytetrahydroimidazo[2,1 -a]isoquinolin-3-ones 10 or 12 or of the polycyclic 1,4-dioxane 13 in high stereoselectivity. The regioselectivity of the oxiran ring opening depends on the kind of nucleophile and the conditions. Reaction of dihydroisoquinoline with O -TBDMS-mandelic acid chloride 15 leads to a tetrahydrooxazolo[2,3 -a]isoquinoline 17 with opposite facial selectivity as compared with dihydroimidazo[2,1 -a]isoquinolin-3-ones 5. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

    Why do Electron-Deficient Dienes React Rapidly in Diels,Alder Reactions with Electron-Deficient Ethylenes?

    A Density Functional Theory Analysis
    Abstract The Diels,Alder reaction of the electron-deficient (ED) dimethyl 2,3-dimethylenesuccinate with two electron-rich (ER) and two ED ethylenes has been studied at the B3LYP/6-31G* level of theory. The analysis of the geometry and electronic structure of the transition state of the reaction with the ED dimethyl 2-methylenemalonate along with the analysis of the global and local electrophilicity indices of the reagents provide an explanation of the participation of this ED diene as nucleophile against powerful electrophiles in polar Diels,Alder reactions. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

    A Combined Theoretical and Experimental Research Project into the Aminolysis of ,-Lactam Antibiotics: The Importance of Bifunctional Catalysis

    Natalia Daz
    Abstract This paper reports the results of experimental work on the aminolysis of penicillin (6-APA) and monobactam (aztreonam) antibiotics by propylamine or ethanolamine. In general, aztreonam is slightly more reactive than 6-APA, despite the common assumption that the amide bond should be less activated in monobactams. Intriguingly, when ethanolamine acts as the nucleophile, the corresponding rate law has a kinetic term proportional to [RNH2][RNH3+]. To complement the experimental observations, the rate-determining free energy barriers in aqueous solution for various mechanistic pathways were computed by standard quantum chemical methodologies. From previous theoretical work it was assumed that the aminolysis of ,-lactams proceeds through mechanisms in which either a water molecule or a second amine molecule may act as bifunctional catalysts, assisting proton transfer from the attacking amine molecule to the leaving amino group. The energy barriers as computed have moderate values (ca. 26,34 kcalmol,1) and reproduce most of the experimentally observed kinetic trends. Furthermore, the calculations predict that positively charged ethanolamine molecules can act as bifunctional catalysts as well, thus explaining the presence of the kinetic term proportional to [RNH2][RNH3+] in the rate law. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]

    C-Glycosidations of a 2-Ketohexosyl Bromide with Electrophilic, Radical, and Nucleophilic Anomeric Carbons

    Frieder W. Lichtenthaler
    Abstract The susceptibility of acylated 2-ketohexosyl halides to C-homologation is demonstrated with the easily accessible tri- O -benzoyl-,- D - arabino -hexos-ulosyl bromide 1 as the model compound. C-Glycosidation with an electrophilic anomeric carbon requires prior carbonyl protection, to avoid carbonyl addition by the C-nucleophile, for example, as the cyanohydrin. Silver triflate-promoted reaction with the silylenol ether of acetophenone then efficiently yields the ,-phenacyl product. With thermal (AIBN) or photochemical induction, 1 smoothly generates an anomeric radical , comparatively electrophilic, due to its capto-dative substitution , which exclusively traps hydrogen in the presence of tributyltin and electron-deficient alkenes. With allyltributylstannanes, however, it reacts with high stereoselectivity to afford ,- C -allyl glycosiduloses. The ,-bromoketone functionality in ulosyl bromide 1 is susceptible to Reformatsky conditions: treatment with zinc-copper couple readily generates the 1,2-enolate, a most simple anomeric nucleophile, which effectively adds to aldehydes to give ,- C -hydroxyalkyl glycosiduloses or ,- C -disaccharides (with sugar aldehydes) with a high degree of double stereoselection. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]

    The role of residues R97 and Y331 in modulating the pH optimum of an insect ,-glycosidase of family 1

    FEBS JOURNAL, Issue 24 2003
    Sandro R. Marana
    The activity of the digestive ,-glycosidase from Spodoptera frugiperda (Sf,gly50, pH optimum 6.2) depends on E399 (pKa = 4.9; catalytic nucleophile) and E187 (pKa = 7.5; catalytic proton donor). Homology modelling of the Sf,gly50 active site confirms that R97 and Y331 form hydrogen bonds with E399. Site-directed mutagenesis showed that the substitution of R97 by methionine or lysine increased the E399 pKa by 0.6 or 0.8 units, respectively, shifting the pH optima of these mutants to 6.5. The substitution of Y331 by phenylalanine increased the pKa of E399 and E187 by 0.7 and 1.6 units, respectively, and displaced the pH optimum to 7.0. From the observed ,pKa it was calculated that R97 and Y331 contribute 3.4 and 4.0 kJmol,1, respectively, to stabilization of the charged E399, thus enabling it to be the catalytic nucleophile. The substitution of E187 by D decreased the pKa of residue 187 by 0.5 units and shifted the pH optimum to 5.8, suggesting that an electrostatic repulsion between the deprotonated E399 and E187 may increase the pKa of E187, which then becomes the catalytic proton donor. In short the data showed that a network of noncovalent interactions among R97, Y331, E399 and E187 controls the Sf,gly50 pH optimum. As those residues are conserved among the family 1 ,-glycosidases, it is proposed here that similar interactions modulate the pH optimum of all family 1 ,-glycosidases. [source]

    Synthesis of 3-Acyl-4-alkenylpyrrolidines by Platinum-Catalyzed Hydrative Cyclization of Allenynes

    Takanori Matsuda
    Abstract A series of nitrogen-tethered allenynes (,5-aza-1,2-dien-7-ynes') 1 were transformed to the corresponding 3-acyl-4-alkenylpyrrolidines 3 when treated with a catalytic amount of PtCl2 in MeOH at 70. Initial Pt-promoted cyclization forms a nonclassical carbocationic intermediate. In contrast to the cycloisomerization in toluene, which produced the bicyclic cyclobutenes 2, the intermediate is intercepted by addition of an oxygen nucleophile to achieve the formal hydrative cyclization. [source]

    Controlled Release of Perfumery Alcohols by Neighboring-Group Participation.

    2-(Hydroxymethyl)-, 2-Carbamoylbenzoates, Comparison of the Rate Constants for the Alkaline Hydrolysis of 2-Acyl-
    A series of 2-acylbenzoates 1 and 2, 2-(hydroxymethyl)benzoates 3, 2-carbamoylbenzoates 4,6, as well as the carbamoyl esters 7 or 8 of maleate or succinate, respectively (see Fig.,2), were prepared in a few reaction steps, and the potential use of these compounds as chemical delivery systems for the controlled release of primary, secondary, and tertiary fragrance alcohols was investigated. The rate constants for the neighboring-group-assisted alkaline ester hydrolysis were determined by anal. HPLC in buffered H2O/MeCN solution at different pH (Table,1). The rates of hydrolysis were found to depend on the structure of the alcohol, together with the precursor skeleton and the structure of the neighboring nucleophile that attacks the ester function. Primary alcohols were released more rapidly than secondary and tertiary alcohols, and benzoates of allylic primary alcohols (e.g., geraniol) were hydrolyzed 2,4 times faster than their homologous saturated alcohols (e.g., citronellol). For the same leaving alcohol, 2-[(ethylamino)carbonyl]benzoates cyclized faster than the corresponding 2-(hydroxymethyl)benzoates, and much faster than their 2-formyl and 2-acetyl analogues (see, e.g., Fig.,4). Within the carbamoyl ester series, 2-[(ethylamino)carbonyl]benzoates were found to have the highest rate constants for the alkaline ester hydrolysis, followed by unsubstituted 2-(aminocarbonyl)benzoates, or the corresponding isopropyl derivatives. To rationalize the influence of the different structural changes on the hydrolysis kinetics, the experimental data obtained for the 2-[(alkylamino)carbonyl]benzoates were compared with the results of density-functional computer simulations (Table,2 and Scheme,4). Based on a preliminary semi-empirical conformation analysis, density-functional calculations at the B3LYP/6-31G** level were carried out for the starting precursor molecules, several reaction intermediates, and the cyclized phthalimides. For the same precursor skeleton, these simple calculations were found to model the experimental data correctly. With an understanding of the influence of structural parameters on the rate constants obtained in this work, it is now possible to influence the rates of hydrolysis over several orders of magnitude, to design tailor-made precursors for a large variety of fragrance alcohols, and to predict their efficiency as controlled-release systems in practical applications. [source]

    Kinetic and mechanistic investigation into the influence of chelate substituents on the substitution reactions of platinum(II) terpyridine complexes

    Deogratius Jaganyi
    The substitution kinetics of the complexes [Pt{4,-(o -CH3 -Ph)-terpy} Cl]SbF6 (CH3PhPtCl(Sb)), [Pt{4,-(o -CH3 -Ph)-terpy}Cl]CF3SO3 (CH3PhPtCl(CF)), [Pt(4,-Ph-terpy)Cl]SbF6 (PhPtCl), [Pt(terpy)Cl]Cl2H2O (PtCl), [Pt{4,-(o -Cl-Ph)-terpy}Cl]SbF6 (ClPhPtCl), and [Pt{4,-(o -CF3 -Ph)-terpy}Cl]SbF6 (CF3PhPtCl), where terpy is 2,2,:6,,2,-terpyridine, with the nucleophiles thiourea (TU), N,N,-dimethylthiourea (DMTU), and N,N,N,,N,-tetramethylthiourea (TMTU) were investigated in methanol as a solvent. The substitution reactions of the chloride displacement from the metal complexes by the nucleophiles were investigated as a function of nucleophile concentration and temperature under pseudo-first-order conditions using the stopped-flow technique. The reactions followed the simple rate law kobs = k2[Nu]. The results indicate that the introduction of substituents in the ortho position of the phenyl group on the ancillary ring of the terpy unit does influence the extent of ,-backbonding in the terpy ring. This controls the electrophilicity of the platinum center, which in turn controls the lability of the chloro-leaving group. The strength of the electron-donating or -withdrawing ability of the substituents correlates with the reactivity of the complexes. Electron-donating substituents decrease the rate of substitution, whereas electron-withdrawing substituents increase the rate of substitution. This was supported by DFT calculations at the B3LYP/LACVP+** level of theory, which showed that most of the electron density of the HOMO is concentrated on the phenyl ligand rather than on the metal center in the case of the strongest electron-withdrawing substituent in CF3PhPtCl. The opposite was found to be true with the strongest electron-donating substituent in CH3PhPtCl. Thiourea was found to be the best nucleophile with N,N,N,,N,-tetramethylthiourea being the weakest due to steric effects. The temperature dependence studies support an associative mode of activation. 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 808,818, 2008 [source]

    Estimation of homogeneous rate constants of reaction of electrochemically generated ortho -benzoquinones with 1,3-indandione

    Davood Nematollahi
    Electrochemical oxidation of some catechol derivatives has been studied in the presence of 1,3-indandione as nucleophile in aqueous solution, by means of cyclic voltammetry and controlled-potential coulometry. The results indicate the participation of electrochemically produced o -benzoquinones in the Michael reaction with 1,3-indandione to form the corresponding new catechol derivatives. On the basis of the EC mechanism, the observed homogeneous rate constants (kobs) of reaction of produced o -benzoquinones with 3-indandione were estimated by comparing the experimental cyclic voltammograms with the digitally simulated results. 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 605,613, 2007 [source]

    Spectrophotometric variable-concentration kinetic experiments applied to inorganic reactions

    Giuseppe Alibrandi
    The dependence of the observed rate constant of inorganic substitution reactions on the concentration of nucleophilic reagents was obtained by single variable-parameter kinetic runs. The experiments were carried out spectrophotometrically, varying the concentration of the nucleophile inside the reaction vessel. Software and apparatus were developed for an easy and rapid performance. The method gives accurate results and a saving in time by a factor of up to 100 compared to conventional methods. 2003 Wiley Periodicals, Inc. Int J Chem Kinet 35: 497,502, 2003 [source]

    Reaction of 4-benzylidene-2-methyl-5-oxazolone with amines, Part 2: Influence of substituents in para-position in the phenyl ring and a substituent on amine nitrogen atom on the reaction kinetics

    B. Bet, akowska
    An influence of a structure of the amine (benzylamine, N -methyl-benzylamine, N -isopropyl-benzylamine, N -methyl-butylamine, N -ethyl-butylamine, sec -butylamine, and tert -butylamine) on a rate constant of the ring-opening reaction of 4-benzylidene-2-methyl-5-oxazolone (Ox) was studied. The good correlation between logarithm of the rate constants and Charton's steric substituent constant , as well as good correlation with a form of the simple branching equation indicate that there is a steric effect because of substitution at C1 carbon atom of nucleophile which decreases the reaction rate. Additionally, an influence of a structure of the benzylidene moiety of Ox on a rate of the oxazolone ring-opening reaction was studied. The substituents (OH, OCH3, N(CH3)2, Cl, NO2) in para-position of the phenyl ring of Ox substantially modified the rate of the reaction with benzylamine in acetonitrile. The rate of the Ox ring-opening reaction decreased with increase of the electron-donating properties of the substituent. A good correlation between the rate constants of the reaction of 4-(4,-substituted-benzylidene)-2-methyl-5-oxazolones with benzylamine and the electron density at the reaction center (carbon C5 of the oxazolone ring), calculated using ab initio method, and the Hammett substituent constants, and CR equation were established. 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 148,155, 2002; DOI 10.1002/kin.10039 [source]

    An Efficient and General Microwave-Assisted Copper-Catalyzed Conia-Ene Reaction of Terminal and Internal Alkynes Tethered to a Wide Variety of Carbonucleophiles

    Sonia Montel
    Abstract This paper describes a highly efficient, microwave-assisted, Conia-ene reaction of alkynes bearing a stabilizing carbon nucleophile. The reaction, catalyzed by a commercially available copper catalyst, proceeds under neutral conditions and is generally applicable even to less reactive nucleophiles such as malonate, cyanoacetate, and sulfonylacetate derivatives. This copper-mediated cycloisomerization is also applicable to internal unactivated alkynes leading exclusively to the corresponding 5-membererd products having an E -olefinic chemistry. [source]

    Asymmetric Ring Opening of Benzo-7-oxabicyclo[2.2.1]heptadienes with Cationic Rhodium Complexes

    Angelika Preetz
    Abstract The efficient design of stereochemically challenging ring systems by ring opening of heterobicyclic alkenes has become a very important reaction in the chemistry of CC and CX bond formation. By using the hitherto applied in situ technique for the generation of the ,2 -bridged, dimeric neutral rhodium complexes, however, the catalytically active species and its concentration remained unidentified. Furthermore, the reaction temperature is at least 80,C. The application of cationic rhodium(I) solvate complexes (that no longer contain blocking diolefins) shows that a much greater activity and enantioselectivity for the synthesis of 1,2-dihydro-1-naphthols can be reached than was described so far, even at ambient temperature. NMR spectroscopy and X-ray analysis show that a product inhibition during the ring opening reaction takes place that is independent of the nucleophile. [source]

    A Simple, Effective Boron-Halide Ethoxylation Catalyst

    Abstract Boron esters B(OR)3, readily derived from boric acid and alcohols, combine with iodide or bromide to catalyze the ethoxylation of alcohols and phenols, giving good rates and narrow product distributions. The combined action of a weak electrophile [B(OR)3] and a weak nucleophile (halide) allows for the ethoxylation of base-sensitive alcohols. Experiment suggests a new mechanism for this commercially important reaction proceeding through key ,-haloalkoxy intermediates. [source]

    Enantioselective Aza-Morita,Baylis,Hillman Reaction Using Aliphatic ,-Amidosulfones as Imine Surrogates

    Nacim Abermil
    Abstract The bifunctional catalyst 6,-deoxy-6,-acylamino-,-isocupreidine (1) served both as a base to trigger the in situ generation of N -sulfonylimine from readily available ,-amidosulfones and as a chiral nucleophile to initiate the enantioselective aza-Morita,Baylis,Hillman (aza-MBH) reaction. ,-Methylene-,-amino-,-alkyl carbonyl compounds, difficultly accessible previously, can now be synthesized in excellent yields and enantioselectivities. [source]

    Correlating the positional reactivity of a masked electrophilic center to the topology of the electron density

    Harry L. Price
    Abstract Masking of electrophilic centers in biological molecules via structural modifications serves to control chemical reactivity and in some cases may affect the regioselectivity of a reaction. In the work presented the regioselective reactivity of the electrophilic cyclopropylpyrrololindole (CPI) center in the DNA alkylator CC-1065, a highly toxic antibiotic that has served as a structural template for the development of a series of novel anticancer drugs containing the CPI reactive center has been examined. The CPI reactive center is an interesting example of chemical masking as it relates to acid-dependent electrophilicity, and regioselective addition of a nucleophile to an electrophilic center. In an effort to better understand the reactivity of the CPI center, calculations using the B3LYP density functional theory (DFT) method, the 6-31G(d) basis set, and the atoms in molecules (AIM) theory were performed on unprotonated and protonated forms of the CPI reactive center. The results of these calculations demonstrate that activation of the CPI group via protonation induces significant changes in the electron density (,), the Laplacian of the density (,2,), and the bond ellipticity (,), and that these changes are linked to the observed reactivity of the CPI reaction center. 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source]

    Organocatalysis in Natural Product Synthesis: A Simple One-Pot Approach to Optically Active ,-Diols

    Nikolaj Rjkjr Andersen
    Abstract Optically active ,-diols have been prepared using an organocatalytic one-pot approach from ,,,-unsaturated aldehydes using (E) -benzaldehyde oxime as nucleophile in an oxa-Michael reaction with subsequent in situ reduction or Grignard addition. With this protocol at hand, two biologically active compounds, an insect sex pheromone and a glycerol kinase substrate have been synthesized. [source]

    Water versus Solvent-Free Conditions for the Enantioselective Inter- and Intramolecular Aldol Reaction Employing L -Prolinamides and L -Prolinethioamides as Organocatalysts

    Diana Alma
    Abstract Organocatalysts 1, derived from L -proline and (1S,2R)- cis -1-aminoindan-2-ol or (R)-1-aminoindane, are evaluated as promoters in the direct asymmetric aldol reaction between ketones and aromatic aldehydes in the presence of water and under solvent-free reaction conditions. L -Prolinethioamides 1c and 1d exhibited higher enantioselectivity than the corresponding prolinamides 1a and 1b in the model aldol reaction between cyclohexanone and 4-nitrobenzaldehyde in the presence of 4-nitrobenzoic acid as cocatalyst. In particular, L -prolinethioamide 1d (5,mol%), derived from L -proline and (R)-1-aminoindane, is shown as the most efficient organocatalyst studied promoting the direct aldol reaction of cycloalkyl, alkyl, and ,-functionalized ketones with aromatic aldehydes in the presence of water and under solvent-free reaction conditions employing only 2 equivalents of nucleophile. Generally, anti -aldol products are obtained in high yields and excellent diastereo- and enantioselectivities (up to >98/2 anti/syn, up to 98% ee). Solvent-free conditions give slightly higher dr and ee than using water as solvent. In addition, organocatalyst 1d can be easily recovered by extractive work-up and reused. Prolinethioamide 1d (5 mol%) in combination with 4-NO2C6H4CO2H (5 mol%) is also a very effective organocatalytic system for the asymmetric solvent-free intramolecular Hajos,Parrish,Eder,Sauer,Wiechert reaction with comparable or higher levels of enantioselectivity (up to 88% ee) to other reported catalysts in organic solvents. [source]

    Palladium(II)-Catalyzed Domino Reaction of 2-(1-Alkynyl)-2-alken-1-ones with Nucleophiles: Scope, Mechanism and Synthetic Application in the Synthesis of 3,4-Fused Bicyclic Tetrasubstituted Furans

    Yuanjing Xiao
    Abstract Described herein is the development of a palladium(II)-catalyzed two- or three-component reaction of 2-(1-alkynyl)-2-alken-1-ones with nucleophiles and allylic chlorides. Various types of nucleophiles such as O- , N- , C -based nucleophiles and olefin-tethered O- , N- , C -based nucleophiles were investigated. The scope, mechanism and application of this Pd(II)-catalyzed domino reaction were studied. In these transformations, the palladium catalyst exhibits a dual role, serving simultaneously as a Lewis acid and a transition metal. Two possible reaction pathways (cross-coupling reaction vs. Heck reaction) from the same intermediate furanylpalladium species were observed. The reaction pathway is dependent on the property of the nucleophile and the length of the tethered chain as well. When olefin-tethered O -based nucleophiles were used, only the cross-coupling reaction pathway was observed, in contrast, both reaction pathways were observed when olefin-tethered C -based nucleophiles were employed. The product ratio is dependent on the length of the tethered chain. Furthermore, ring-closing metathesis (RCM) of corresponding furans with CC bonds provides an easy method for the preparation of functionalized oxygen-heterocycles , 3,4-fused bicyclic furans. It is also noteworthy that allylic chloride can be as an oxidant besides its well known function as an alkylating reagent. [source]