Regioselective N9 alkylation of purine rings assisted by β-cyclodextrin

Under the assistance of β-cyclodextrin, purine was effectively alkylated at N9 together with up to 99% conversion and good to excellent yield using water as the solvent. High regioselectivity-N9/N7 selectivity>99:1 was attributed to the β-cyclodextrin cavity blocked the N7 of purines ring.     

Synthesis of C8-alkyl-substituted purine analogues by direct alkylation of 8-H purines with tetrahydrofuran catalyzed by CoCl_2·6H_2O

C8-Alkyl-substituted purine analogues were synthesized through direct alkylation of 8-H purine with tetrahydrofuran in the presence of Co catalyst in one step. The reactions gave a series of novel C8-oxygen heterocyclic alkyl purine compounds in good yields under mild reaction conditions by the readily available alkylating reagents, providing a complementary route to the classical coupling reactions for the synthesis of C8-alkyl-substituted purine analogues.     

Why is alkylation of an enolate accompanied by so much polyalkylation?

[reaction: see text] The lithium enolate 1-Li of 6-phenyl-alpha-tetralone forms a monomer-tetramer equilibrium in THF at 25 degrees C with K(1,4) = 4.7E+10 M(-3). The lithium enolate 2-Li, however, forms a monomer-dimer equilibrium with K(1,2) = 3800 M(-1). In both cases reaction with benzyl bromide is dominantly with the monomer. The results support an earlier conjecture of House that alkylation of an enolate is frequently accompanied by extensive polyalkylation because the less substituted enolates are more aggregated.     

Study of model esterifications and of polyesterifications catalyzed by various organometallic derivatives—III. Kinetic and mechanistic study of eterifications catalyzed by tetrabutoxytitanium or zirconium

The kinetics of the esterification of octadecanoic acid in 1-octadecanol under 14 mmHg in the presence of either Ti(OnBu)₄ or Zr(OnBu)₄ are studied at various temperatures. It is shown that mechanisms taking only acid and alcohol into consideration do not account for experimental data. More complicated mechanisms are examined: existence of an equilibrium between the catalyst and the reaction species or successive and simultaneous reactions of the catalyst; the latter gives the more satisfactory results. A reaction scheme is proposed.     

Reactions of carbocations with unsaturated hydrocarbons: electrophilic alkylation or hydride abstraction?

Benzhydryl cations were used as reference electrophiles to determine the hydride donor reactivities of unsaturated hydrocarbons. The kinetics of the reactions were followed by UV-vis spectroscopy and conductivity measurements, and it was found that the second-order rate constants for the hydride transfer processes were almost independent of the solvents or counterions employed. The rate constants correlate linearly with the previously published empirical electrophilicity parameters E of the benzhydrylium ions. Therefore, the linear free energy relationship log k(20 degrees C) = s(E + N) could be employed to characterize the hydride reactivities of the hydrocarbons by the nucleophilicity parameters N and s. The similarity of the slopes s for hydride donors and pi-nucleophiles allows a direct comparison of the reactivities of these different functional groups based on their nucleophilicity parameters N. Since nucleophilicity parameters of -5 < N < 0 have been found for a large variety of allylic and bisallylic hydride donors, a rule of thumb is derived that hydride transfer processes may compete with carbon-carbon bond-forming reactions when carbocations are combined with olefins of pi-nucleophilicity N < 0.     

Asymmetric alkylation catalyzed by chiral alkali metal alkoxides of TADDOL. Synthesis of α-methyl amino acids

It is shown that sodium alkoxides formed from (4R,5R)-2,2-dimethyl-1,3-dioxolane-4,5-bis(diphenylmethanol) ((R,R)-TADDOL) and some of its derivatives can be used as chiral catalysts for enantioselective alkylation of Schiff's bases derived from alanine with reactive alkyl halides. Acid hydrolysis of the reaction products affords (R)-α-methylphenyl-alanine, (R)-α-allylalanine, and (R)-α-methylnaphthylalanine in 61–93% yields and withee 69–94%. When (S,S)-TADDOL is used, the (S)-amino acid is formed. A mechanism explaning the observed features of the reaction is proposed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 926–932, May, 1999.     

Enhancement of Selectivity in Capillary Zone Electrophoresis by Adding Ethanolamines or Ethylamines in the Running Buffer

IntroduCtionTheseleedvityandresolutionmCaPilwizoneeleCtrophoresis(CZE)areialluencedbymanyfacors,mcludingbuffercomposition,concentrationandpH,vanousadditives,eldricStrength,temperature,etc.ModulwitheseparametersInayleadtOtheenhancementofseleCtivityandresolution[l].However,optimhationoftheseparameterslsoftentime-consunung.onlydritedpapersinCZEhavedealtwiththeialluenceofcationicspeciesinthebuffertoWarseParationsofar.IsSaqetalI2,3]StUdiedtheeffeCtofvariousalkalineionsinaCetatebufferonthe…     

Elaboration of a Lewis acid-free protocol for the alkylation of silicon-containing π-donors by β-arylthioalkyl chlorides

Hexafluoroisopropanol (HFIP) was found to be a powerful electrophilic activator of covalent reagents. Due to this effect the title reaction was shown to proceed under mild conditions in the absence of any Lewis acid catalysts to give the respective products of C-alkylation in good to excellent yields.     

Buttressing effects on haloarene deprotonation: a merely kinetic or also thermodynamic phenomenon?

(2,6-Dichlorophenyl)- and (2,6-dibromophenyl)trialkylsilanes undergo hydrogen/metal interconversion preferentially at the 4- rather than 3-position. However, the organometallic species generated by such a "meta metalation" are thermodynamically less stable (i.e., more basic) than those that would result from an ordinary "ortho metalation". This was demonstrated by equilibration experiments based on permutational halogen/metal interconversion. A new buttressing effect can explain the unprecedented regioselectivity. It is supported by X-ray structures that reveal marked deformations of the benzene ring in halophenylsilanes. [structure: see text]     

Detection and Determination of Released Ions in the Presence of Nanoparticles: Selectivity or Strategy?

Metallic nanoparticles can release ionic species, but also both species can occur in the same samples. Therefore, there is a need of efficient and cost‐effective methods to determine these ionic species in the presence of the corresponding nanoparticles. Electroanalytical techniques open the door to this selective detection of NPs and their ions. In this work, a methodology that allows the direct determination of ionic silver (Ag⁺) in the presence of silver nanoparticles based on anodic stripping voltammetry was implemented. Silver nanoparticles were determined, after acidic digestion of the sample, by difference with respect to the total content of silver. The method was validated in terms of specific identification of silver ions, linearity, working range, limit of detection, limit of quantification, recovery, repeatability and ruggedness. All parameters are adequate for an analytical method following Eurachem recommendations. The validated method was used to determine the concentration of Ag⁺ and total silver in two commercial products of colloidal silver. The results were compared with those obtained by atomic absorption spectrometry in combination with an ultrafiltration step for isolation of ionic silver. There were no significant differences in the results. The proposed methodology benefits from the intrinsic selectivity of the electroanalysis methods, allowing to eliminate the steps of pretreatments of the samples, which are necessary in other techniques. The novelty of the article lies in the direct determination of Ag (I) ions in the presence of AgNPs, without the use of previous separation steps.     

What Determines the Selectivity of Arginine Dihydroxylation by the Nonheme Iron Enzyme OrfP?

The nonheme iron enzyme OrfP reacts with l -Arg selectively to form the 3R,4R-dihydroxyarginine product, which in mammals can inhibit the nitric oxide synthase enzymes involved in blood pressure control. To understand the mechanisms of dioxygen activation of l -Arg by OrfP and how it enables two sequential oxidation cycles on the same substrate, we performed a density functional theory study on a large active site cluster model. We show that substrate binding and positioning in the active site guides a highly selective reaction through C³−H hydrogen atom abstraction. This happens despite the fact that the C³−H and C⁴−H bond strengths of l -Arg are very similar. Electronic differences in the two hydrogen atom abstraction pathways drive the reaction with an initial C³−H activation to a low-energy ⁵σ-pathway, while substrate positioning destabilizes the C⁴−H abstraction and sends it over the higher-lying ⁵π-pathway. We show that substrate and monohydroxylated products are strongly bound in the substrate binding pocket and hence product release is difficult and consequently its lifetime will be long enough to trigger a second oxygenation cycle.     

Atroposelective Silylation of 1,1′-Biaryl-2,6-diols by a Chiral Counteranion Directed Desymmetrization Enhanced by a Subsequent Kinetic Resolution

A desymmetrizing silylation of aromatic diols is reported. The previously unknown asymmetric silyl ether formation of phenol derivatives is achieved by applying List's counteranion directed silylation technique. A silylium-ion-like silicon electrophile generated from an allylic silane paired with an imidodiphosphorimidate (IDPi) enables enantioselective discrimination of achiral 1,1′-biaryl-2,6-diols. The enantioselectivity of that desymmetrization is further improved by a downstream kinetic resolution, converting the monosilylated minor enantiomer into the corresponding, again achiral bissilylated diol.     

Fused vs. spiro: Kinetic,not thermodynamic preference may direct the reaction of α-carbonyl oxonium ylides

Rh(II)-catalyzed decomposition of certain cyclic α-diazocarbonyl compounds in the presence of cyclic ethers has been shown to give bicyclic ring expansion products. These are thought to arise from a [1,4]-alkyl shift toward the carbonyl oxygen atom and are in contrast with the recently observed spirocyclic products of a Stevens-type [1,2]-alkyl shift within the postulated oxonium ylide intermediate. Quantum chemical calculations performed at the B3LYP/6-31G* level of theory showed that the former reaction pathway (toward fused bicycles) is kinetically preferred.     

Impact of Zeolite Framework Composition and Flexibility on Methanol-To-Olefins Selectivity: Confinement or Diffusion?

The methanol-to-olefins reaction catalyzed by small-pore cage-based acid zeolites and zeotypes produces a mixture of short chain olefins, whose selectivity to ethene, propene and butene varies with the cavity architecture and with the framework composition. The product distribution of aluminosilicates and silicoaluminophosphates with the CHA and AEI structures (H-SSZ-13, H-SAPO-34, H-SSZ-39 and H-SAPO-18) has been experimentally determined, and the impact of acidity and framework flexibility on the stability of the key cationic intermediates involved in the mechanism and on the diffusion of the olefin products through the 8r windows of the catalysts has been evaluated by means of periodic DFT calculations and ab initio molecular dynamics simulations. The preferential stabilization by confinement of fully methylated hydrocarbon pool intermediates favoring the paring pathway is the main factor controlling the final olefin product distribution.     

Unprecedented Inversion of Selectivity and Extraordinary Difference in the Complexation of Trivalent f Elements by Diastereomers of a Methylated Diglycolamide

When considering f elements, solvent extraction is primarily used for the removal of lanthanides from ore and their recycling, as well as for the separation of actinides from used nuclear fuel. Understanding the complexation mechanism of metal ions with organic extractants, particularly the influence of their molecular structure on complex formation is of fundamental importance. Herein, we report an extraordinary (up to two orders of magnitude) change in the extraction efficiency of f elements with two diastereomers of dimethyl tetraoctyl diglycolamide (Me₂-TODGA), which only differ in the orientation of a single methyl group. Solvent extraction techniques, extended X-ray absorption fine structure (EXAFS) measurements, and density functional theory (DFT) based ab initio calculations were used to understand their complex structures and to explain their complexation mechanism. We show that the huge differences observed in extraction selectivity results from a small change in the complexation of nitrate counter-ions caused by the different orientation of one methyl group in the backbone of the extractant. The obtained results give a significant new insight into metal–ligand complexation mechanisms, which will promote the development of more efficient separation techniques.