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Selected AbstractsExotic plant species invade diversity hot spots: the alien flora of northwestern KenyaECOGRAPHY, Issue 2 2000J. Stadler We analysed the distribution of native and alien plant species across 20 ecogeographic zones of northwestern Kenya. The source pool for the majority of aliens was Europe and America. Thus, the source pool has a biogeographic bias which explains the low proportion of aliens in the tropics: most species in the European or American source pool are not well adapted to tropical conditions. As expected, native and alien plant species showed an area effect. Correcting for this area effect. species rich zones showed a higher proportion of alien plant species in their flora. At the analysed scale, species richness of native plant communities does not increase the resistance to invasions and alien plant species invade diversity hotspots. Compared to the other ecogeographic zone, the urban area around Nairobi showed an increased richness in alien and native plant species. This is very similar to findings in Europe, although the history of urbanisation is much shorter in Kenya. The species turnover between zones (,-diversity) shows a similar pattern in native and alien plant species. Within a very short time scale the alien plant species mapped the biogeographic patterns of natives, although the geography of human activities influences the propagule pressure. [source] A new method for fabrication of an integrated indium tin oxide electrode on electrophoresis microchips with amperometric detection and its application for determination of synephrine and hesperidin in pericarpium citri reticulataeELECTROPHORESIS, Issue 21 2006Wei Wang Abstract A new, simple, and fast method to integrate indium tin oxide electrode in an amperometric detection (AD) microchip is introduced. Without the help of photoresist and complicated apparatus, the microchip could be fabricated in most laboratories in a very short time by this method. The experiment indicated that the microchip was stable and had good reproducibility. On this microchip, a new method was established to separate and determine synephrine and hesperidin, which are the main electroactively bioactive ingredients of pericarpium citri reticulatae, by AD. Under the optimal conditions, the two compounds could be completely separated within 5.5,min and the detection limits were 0.13 and 0.57,,g/mL, respectively. The proposed method has been successfully used to determine synephrine and hesperidin in real pericarpium citri reticulatae sample, and the results show that the proposed method is sensitive, reliable, fast, and economical. [source] SEQUENTIAL RAPID ADAPTATION OF INDIGENOUS PARASITOID WASPS TO THE INVASIVE BUTTERFLY PIERIS BRASSICAEEVOLUTION, Issue 8 2007Shingo Tanaka The introduction of a new species can change the characteristics of other species within a community. These changes may affect discontiguous trophic levels via adjacent trophic levels. The invasion of an exotic host species may provide the opportunity to observe the dynamics of changing interspecific interactions among parasitoids belonging to different trophic levels. The exotic large white butterfly Pieris brassicae invaded Hokkaido Island, Japan, and quickly spread throughout the island. Prior to the invasion, the small white butterfly P. rapae was the host of the primary parasitoid Cotesia glomerata, on which both the larval hyperparasitoid Baryscapus galactopus and the pupal hyperparasitoid Trichomalopsis apanteroctena depended. At the time of the invasion, C. glomerata generally laid eggs exclusively in P. rapae. During the five years following the invasion, however, the clutch size of C. glomerata in P. rapae gradually decreased, whereas the clutch size in P. brassicae increased. The field results corresponded well with laboratory experiments showing an increase in the rate of parasitism in P. brassicae. The host expansion of C. glomerata provided the two hyperparasitoids with an opportunity to choose between alternative hosts, that is, C. glomerata within P. brassicae and C. glomerata within P. rapae. Indeed, the pupal hyperparasitoid T. apanteroctena shifted its preference gradually to C. glomerata in P. brassicae, whereas the larval hyperparasitoid B. galactopus maintained a preference for C. glomerata in P. rapae. These changes in host preference may result from differential suitability of the two host types. The larval hyperparasitoid preferred C. glomerata within P. rapae to C. glomerata within P. brassicae, presumably because P. brassicae larvae attacked aggressively, thereby hindering the parasitization, whereas the pupal hyperparasitoid could take advantage of the competition-free resource by shifting its host preference. Consequently, the invasion of P. brassicae has changed the host use of the primary parasitoid C. glomerata and the pupal hyperparasitoid T. apanteroctena within a very short time. [source] Expert system for nuclear power plant accident diagnosis using a fuzzy inference methodEXPERT SYSTEMS, Issue 4 2002rey Lee Huge and complex systems such as nuclear power generating stations are likely to cause the operators to make operational mistakes for a variety of inexplicable reasons and to produce ambiguous and complicated symptoms in the case of an emergency. Therefore, a safety protection system to assist the operators in making proper decisions within a limited time is required. In this paper, we develop a reliable and improved diagnosis system using the fuzzy inference method so that the system can classify accident symptoms and identify the most probable causes of accidents in order for appropriate actions to be taken to mitigate the consequences. In the computer simulation, the proposed system proved to be able to classify accident types within only 20,30 s. Therefore, the corresponding operation guidelines can be determined in a very short time to put the nuclear power plant in a safe state immediately after the accident. [source] Colonization of beech leaves by two endophytic fungi in northern JapanFOREST PATHOLOGY, Issue 2 2000N. Sahashi Summary Leaves of Japanese beech (Fagus crenata) were collected monthly during the vegetation period at five sites in the Tohoku district in Japan to isolate endophytic fungi. Leaves were also collected only once at two additional sites. Two endophytic fungi were dominant, a Discula species and a sterile mycelium. This result strongly suggests that these two fungi are generally associated with leaves of the Japanese beech at different sites. At most sites the isolation frequency of Discula sp. was greatest in June and gradually decreased from July to October whereas the isolation frequency of the sterile mycelium increased during the vegetation period and remained at a high isolation frequency in October. Spores of Discula sp. were released for a very short time in late May, just after the disappearance of the snow cover on the forest floor. These spores may be important for the infection of newly sprouting leaves. [source] GPSPA: a new adaptive algorithm for maintaining shortest path routing trees in stochastic networksINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 10 2004Sudip Misra Abstract This paper presents a new efficient solution to the Dynamic Shortest Path Routing Problem, using the principles of Generalized Pursuit Learning. It proposes an efficient algorithm for maintaining shortest path routing trees in networks that undergo stochastic updates in their structure. It involves finding the shortest path in a stochastic network, where there are continuous probabilistically based updates in link-costs. In vast, rapidly changing telecommunications (wired or wireless) networks, where links go up and down continuously and rapidly, and where there are simultaneous random updates in link costs, the existing algorithms are inefficient. In such cases, shortest paths need to be computed within a very short time (often in the order of microseconds) by scanning and processing the minimal number of nodes and links. The proposed algorithm, referred to as the Generalized Pursuit Shortest Path Algorithm (GPSPA), will be very useful in this regard, because after convergence, it seems to be the best algorithm to-date for this purpose. Indeed, it has the advantage that it can be used to find the shortest path within the ,statistical' average network, which converges irrespective of whether there are new changes in link-costs or not. Existing algorithms are not characterized by such a behaviour inasmuch as they would recalculate the affected shortest paths after each link-cost update. The algorithm has been rigorously evaluated experimentally, and it has been found to be a few orders of magnitude superior to the algorithms available in the literature. Copyright © 2004 John Wiley & Sons, Ltd. [source] Microwave accelerated synthesis and characterization of poly(acrylamide)JOURNAL OF APPLIED POLYMER SCIENCE, Issue 6 2007Vandana Singh Abstract Poly(acrylamide) (PAM) was efficiently synthesized under microwave (MW) irradiation using catalytic amount of potassium persulfate. The synthesis does not require any inert atmosphere and could be accomplished in very short time. Microwave power, exposure time, concentration of persulfate, and concentration of the acrylamide were varied to optimize the polymerization in terms of the % conversion (%C). The maximum %C that could be achieved was in 98.5%. The average molecular weight of the synthesized PAM samples ranged from 4.11 × 104 to 1.30 × 105, depending upon the MW power used for their synthesis. The representative PAM was characterized by Fourier transform-infrared, SEM, and X-ray diffraction studies. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3702,3707, 2007 [source] Adsorptive removal of textile dyes from aqueous solutions by dead fungal biomassJOURNAL OF BASIC MICROBIOLOGY, Issue 1 2006Dilek Asma Dead fungal biomass prepared from Phanerochaete chrysosporium and Funalia trogii was tested for their efficiency in removal of textile dyes. The effects of contact time, initial dye concentration, amount of dead biomass and agitation rate on dye removal have been determined. Removal of all dyes required a very short time (60 min). Experimental results show that, P. chrysosporium was more effective than F. trogii . An increase in the amount of dead biomass positively affected of the dye removal. The removal efficiency of different amount of biomass was in order 1 g > 0.5 g > 0.2 g > 0.1 g. The highest removal was obtained at 150,200 rpm. Slightly lower removing activities were found at lower agitation rates. This study showed that it was possible to remove textile dyes by dead biomass of P. chrysosporium . (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Towards stereoselective radiosynthesis of ,-[11C]methyl-substituted aromatic ,-amino acids , a challenge of creation of quaternary asymmetric centre in a very short time,JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS, Issue 5-6 2007Alexander Popkov Abstract In positron emission tomography (PET) , -methyl amino acids have two potential applications: As analogues of neutransmitter precursors for the study of neurodegenerative diseases; as non-metabolised analogues of proteinogenic amino acids for the study of amino acid uptake into normal and cancer cells. Clinical applications of such amino acids are strongly limited due to their poor availability. We carried out [11C]methylation of metalocomplex synthons derived from protected DOPA or tyrosine. For [11C]methylation, sodium hydroxide (5 mg of fine dry powder) was sealed in a vial, which was flushed with dry nitrogen before addition of a solution of the complex (10 mg) and 11CH3I in 1,3-dimethylimidazolidin-2-one (300 µl). After 10 min at 25°C, a 9% radiochemical yield (decay-corrected) of a mixture of the diastereomeric , -[11C]methylDOPA complexes or a 7% radiochemical yield of a mixture of the diastereomeric , -[11C]methyltyrosine complexes was achieved. Individual diastereomers were successfully separated by preparative HPLC, diluted with excess of water and extracted on C18 cartridges. Optimisation of the procedure including hydrolysis of the complexes (hydrolytic deprotection of enantiomerically pure amino acids) and subsequent purification of the enantiomers of , -[11C]methylDOPA and , -[11C]methyltyrosine is underway. Copyright © 2007 John Wiley & Sons, Ltd. [source] Titanium Diboride,Tungsten Diboride Solid Solutions Formed by Induction-Field-Activated Combustion SynthesisJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2003Masachika Shibuya Solid solutions of titanium diboride,tungsten diboride (TiB2,WB2) were synthesized by induction-field-activated combustion synthesis (IFACS) using elemental reactants. In sharp contrast to conventional methods, solid solutions could be formed by the IFACS method within a very short time, ,2 min. Solutions with compositions ranging from 40,60 mol% WB2 were synthesized with a stoichiometric ratio (Ti + W)/B =½; however, samples with excess boron were also made to counter the loss of boron by evaporation. The dependence of the lattice constants of the resulting solid solutions on composition was determined. The "a" parameter decreased only slightly with an increase in the WB2 content, whereas the "c" parameter exhibited a significant decrease over the range 40,60 mol% WB2. Solid-solution powders formed by the IFACS method were subsequently sintered in a spark plasma sintering (SPS) apparatus. After 10 min at 1800°C, the samples densified to relative density 86%. XRD analysis showed the presence of only the solid-solution phase. [source] Microstructures and Pyroelectric Properties of Multicomposition 0.9PbZrO3·xPbTiO3·(0.1,x)Pb(Zn1/3Nb2/3)O3 CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2002Yong Jun Wu The microstructures and pyroelectric properties of multicomposition 0.9PbZrO3·xPbTiO3·(0.1,x)Pb(Zn1/3Nb2/3)O3 (PZ,PT,PZN) ceramics were investigated. The PZ,PT,PZN ceramics with nearly theoretical density were prepared by spark plasma sintering at low temperature (800°C) for a very short time (10 min) from two original compositions with x= 0.025 and x= 0.050. The heat treatment was successfully used to control the diffusion between the different compositions in such ceramics. For ceramics heat-treated at 900°C, two pyroelectric peaks corresponding to the original compositions were observed. When the heat-treatment temperature was increased to 1200°C, these two pyroelectric peaks combined into one sharp pyroelectric peak, which corresponds to average composition. When the spark-plasma-sintered ceramics were heat-treated at 950°C, a high and stable pyroelectric coefficient (>100 nC·cm,2·K,1) over a wide temperature range (23°,47°C) was obtained. It was found that the pyroelectric properties strongly depended on the microstructures. [source] Whole genome amplification from a single cell: a new era for preimplantation genetic diagnosisPRENATAL DIAGNOSIS, Issue 4 2007Serdar Coskun Abstract Preimplantation genetic diagnosis (PGD) is a technique used for determining the genetic status of a single cell biopsied from embryos or oocytes. Genetic analysis from a single cell is both rewarding and challenging, especially in PGD. The starting material is very limited and not replaceable, and the diagnosis has to be made in a very short time. Different whole genome amplification (WGA) techniques have been developed to specifically increase the DNA quantities originating from clinical samples with limited DNA contents. In this review, currently available WGA techniques are introduced and, among them, multiple displacement amplification (MDA) is discussed in detail. MDA generates abundant assay-ready DNA to perform broad panels of genetic assays through its ability to rapidly amplify genomes from single cells. The utilization of MDA for single-cell molecular analysis is expanding at a high rate, and MDA is expected to soon become an integral part of PGD. Copyright © 2007 John Wiley & Sons, Ltd. [source] Cell Subpopulation-related Volumetric Parameters: a Complementary Tool of the Modified Hypo-osmotic Swelling Test on Model of Boar SpermatozoaREPRODUCTION IN DOMESTIC ANIMALS, Issue 5 2000A. Petrounkina Content It is a general property of the intact animal cell to swell rapidly in response to hypo-osmotic conditions. The modified hypo-osmotic swelling test (HOS-test) is an indicative test to evaluate the integrity of the plasma membrane by means of an electronic cell counter, based on the relative increase of the cell volume in response to hypo-osmotic conditions. In this study the relationships between the osmotically induced changes of the cell volume of boar spermatozoa as determined by cell counter and the integrity of the membrane as determined by propidium iodide staining (PI) were studied. Boar sperm cell volume distributions were measured under iso-osmotic (300 mosmolar) conditions and after a hypo-osmotic stress (150 mosmolar). The relative volume shift of mean and modal volume were calculated as a proportion coefficient of modal and mean values of the cell volume distributions by transition from iso-osmotic to hypo-osmotic conditions. The volumetric parameters related to the different cell subpopulations were derived from the different peaks of cell volume distributions. PI-staining techniques were used for comparison. The values of the volume shift and of derived percentages of the osmotically inactive cells were correlated negatively and positively, respectively (p < 0.05) with the percentage of the PI-stained cells. This correlation indicates that a relationship exists between membrane functions of the different cell compartments (sperm head and tail) due to the circumstance that the increase of the cell volume in the HOS-test is associated with the morphological changes in the tail and the PI-staining is associated with the membrane integrity and permeability of the head region. The advantage of computer-assisted volume measurement is that a large number of cells (5000,50 000 spermatozoa) can be measured and evaluated during one procedure and in a very short time. The relative volume shift is a quantitative continuous parameter characterizing the osmotic reactivity and membrane functional competence of a cell population and of subpopulations within one ejaculate. This parameter could be useful to evaluate membrane functional competence rapidly and sensitively. Inhalt Es ist eine generelle Eigenschaft membranintakter tierischer Zellen, mit einer Volumenzunahme auf eine hypoosmotische Belastung zu reagieren. Der auf der relativen Vergrößierungdes Zellvolumens basierende modifizierte hypoosmotischeSchwelltest ist ein indikativer Test zur Beurteilung der Membranintegrität mittels eines elektronischen Partikelzählers. In dieser Studie wurden die Zusammenhänge zwischen der mittels der Propidiumjodid-Färbung bestimmten Zellmembranintegrität und den osmotisch induzierten Veränderungen des Zellvolumens von Eberspermien untersucht. Volumenverteilungen von Eberspermien wurden unter isoosmotischen (300 mosmolar) und hypoosmotischen (150 mosmolar) Bedingungen gemessen. Die relative Volumenverschiebung der modalen und mittleren Werte der Volumenverteilung wurde als Quotient aus Modalwerten der Zellvolumenverteilungen und des mittleren Zellvolumens beim Übergang von isotonen zu hypotonen Bedingungen berechnet. Die auf verschiedene Subpopulationen bezogenen volumetrischen Parameter werden aus den originalen Volumenverteilungen berechnet. Der Betrag der Zellvolumenzunahme und die aus den Volumenverteilungen bestimmten Anteile an Zellen mit beschädigter Geißielmembran korrelierten signifikant negativ bzw. positiv (p < 0,05) mit dem Anteil an den Zellen mit beschädigter Kopfmembran, der sich aus der Propidiumjodid-Färbung ergab. Es wird geschlossen, daßi im Verhalten zwischen den Membranen der verschiedenen Zellkompartimente (Spermienkopf und-Geißiel) ein Zusammenhang besteht. Die beschriebene Methode ermöglicht die Analyse großier Zellpopulationen (5.000,50.000 Zellen). Die relative Volumenverschiebung stellt einen quantitativen kontinuierlichen Parameter dar, der den Membranzustand der Eberspermien einer Spermatozoenpopulation und Subpopulationen innerhalb eines Ejakulates charakterisiert. Diese Parameter können zur schnellen und sensitiven Beurteilung der Membranzustandes eingesetzt werden. [source] Accumulation of cytoplasmic calcium, but not apamin-sensitive afterhyperpolarization current, during high frequency firing in rat subthalamic nucleus cellsTHE JOURNAL OF PHYSIOLOGY, Issue 3 2008Mark Teagarden The autonomous firing pattern of neurons in the rat subthalamic nucleus (STN) is shaped by action potential afterhyperpolarization currents. One of these is an apamin-sensitive calcium-dependent potassium current (SK). The duration of SK current is usually considered to be limited by the clearance of calcium from the vicinity of the channel. When the cell is driven to fire faster, calcium is expected to accumulate, and this is expected to result in accumulation of calcium-dependent AHP current. We measured the time course of calcium transients in the soma and proximal dendrites of STN neurons during spontaneous firing and their accumulation during driven firing. We compared these to the time course and accumulation of AHP currents using whole-cell and perforated patch recordings. During spontaneous firing, a rise in free cytoplasmic calcium was seen after each action potential, and decayed with a time constant of about 200 ms in the soma, and 80 ms in the dendrites. At rates higher than 10 Hz, calcium transients accumulated as predicted. In addition, there was a slow calcium transient not predicted by summation of action potentials that became more pronounced at high firing frequency. Spike AHP currents were measured in voltage clamp as tail currents after 2 ms voltage pulses that triggered action currents. Apamin-sensitive AHP (SK) current was measured by subtraction of tail currents obtained before and after treatment with apamin. SK current peaked between 10 and 15 ms after an action potential, had a decay time constant of about 30 ms, and showed no accumulation. At frequencies between 5 and 200 spikes s,1, the maximal SK current remained the same as that evoked by a single action potential. AHP current did not have time to decay between action potentials, so at frequencies above 50 spikes s,1 the apamin-sensitive current was effectively constant. These results are inconsistent with the view that the decay of SK current is governed by calcium dynamics. They suggest that the calcium is present at the SK channel for a very short time after each action potential, and the current decays at a rate set by the deactivation kinetics of the SK channel. At high rates, repetitive firing was governed by a fast apamin-insensitive AHP current that did not accumulate, but rather showed depression with increases in activation frequency. A slowly accumulating AHP current, also insensitive to apamin, was extremely small at low rates but became significant with higher firing rates. [source] Me3SiCl and Et3SiI-promoted one-pot synthesis of 1,4-dihydropyridine derivativesAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 7 2009Maryam Mirza-Aghayan Abstract An efficient synthesis of 1,4-dihydropyridine derivatives has been achieved by the one-pot cyclocondensation reaction of methyl 3-aminocrotonate and a range of aldehydes in the presence of chlorotrimethylsilane as a promoter under solvent-free conditions. The cyclocondenstion reaction requires a very short time and takes place in good to excellent yields. Furthermore iodotriethylsilane, generated in situ by the reaction of triethylsilane and methyl iodide in the presence of palladium chloride, has been investigated for the synthesis of 1,4-dihydropyridine derivatives. This facile and efficient method affords high yields for the preparation of 1,4-dihydropyridines at room temperature and short reaction times. Copyright © 2009 John Wiley & Sons, Ltd. [source] An Efficient Solid Acid Promoted Synthesis of Quinoxaline Derivatives at Room TemperatureCHINESE JOURNAL OF CHEMISTRY, Issue 6 2007Shaabani Ahmad Abstract Quinoxaline derivatives have been synthesized in a very short time with excellent yields by the condensation of 1,2-diamines with aliphatic or aromatic 1,2-dicarbonyl compounds or benzilmonoxime in the presence of silica sulfuric acid as a very inexpensive solid acid catalyst at room temperature. The recovery and reuse of the catalyst are also satisfactory. [source] Rapid and accurate quantitative phase analysis using a fast detectorJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2004A. F. Gualtieri The accuracy of the weight fractions calculated with the Rietveld method for various polycrystalline systems using data collected for very short times (5,45,min) with an RTMS (real-time multiple strip) detector was verified. The weight estimates were compared with those obtained using the same conventional Bragg,Brentano geometry, a gas proportional detector and a 13,h data acquisition. The analysed samples were monophasic and polyphasic mixtures with different degrees of complexity: the standard corundum NIST 676; a sample (labelled 1g) provided as a standard sample for the IUCr CPD Quantitative Phase Analysis Round Robin; a natural pyroclastic rock from Riano (Rome, Italy) containing zeolitic minerals and a glass phase; and a hydraulic lime. The results of the refinements show estimated weights consistent with both those obtained with a gas proportional detector and with the nominal values, indicating a very good accuracy. Only when variable slits are used, the accuracy of the estimated weights slightly decreases. The outcome of this work is a very important step forward towards fast and accurate QPA for production control and quality management, obtained by combining the use of a rapid detector and existing user-friendly software. [source] Hybrid organic/inorganic epoxy resins prepared by frontal polymerizationJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2010Sergio Scognamillo Abstract The syntheses of hybrid epoxy resins made from different ratios among bisphenol-A diglycidyl ether, 3-glycidoxypropyltrimethoxysilane (GPTMS), and diethylenetriamine were successfully performed by using frontal polymerization. Conversions were always almost quantitative, and, because of the use of this alternative convenient technique, materials were prepared in very short times. Samples were characterized by DSC, TGA, IR spectroscopy, and solvent extraction. It was found that those materials containing a relatively high-Si amount exhibit two different transition temperatures, with the highest one that increases as the content of GPTMS raises. The analogies and the differences with the analogous samples prepared by the classical batch technique are discussed. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010 [source] Palladium-Catalyzed Cross-Coupling Reactions of Amines with Alkenyl Bromides: A New Method for the Synthesis of Enamines and IminesCHEMISTRY - A EUROPEAN JOURNAL, Issue 2 2004José Barluenga Prof. Dr. Abstract The palladium-catalyzed cross-coupling reaction of alkenyl bromides with secondary and primary amines gives rise to enamines and imines, respectively. This new transformation expands the applicability of palladium-catalyzed CN bond forming reactions (the Buchwald,Hartwig amination), which have mostly been applied to aryl halides. After screening of different ligands, bases, and solvents, the catalytic combination [Pd2(dba)3]/BINAP in the presence of NaOtBu in toluene gave the best results in the cross-coupling of secondary amines with 1-bromostyrene (dba=dibenzylideneacetone, BINAP=2,2,-bis(diphenylphosphino)-1,1,-binaphthyl). The corresponding enamines are obtained cleanly and in nearly quantitative yields. However, steric hindrance seems to be a limitation of the reaction, as amines carrying large substituents are not well converted. The same methodology can be applied to the coupling of secondary amines with 2-bromostyrene. Moreover, the reaction with substituted 2-bromopropenes allows regioselective synthesis of isomerizable terminal enamines without isomerization of the double bond. The best catalytic conditions for the cross-coupling of 1-bromostyrene with primary amines include again the use of the Pd0/BINAP/NaOtBu system. The reaction gives rise to the expected imines in very short times and with low catalyst loadings. A set of structurally diverse imines can be prepared by this methodology through variations in the structure of both coupling partners. However, 2-bromostyrene failed to give good results in this coupling reaction, probably due to product inhibition of the catalytic cycle. Competition experiments of vinyl versus aryl amination reveal that the reaction occurs preferentially on vinyl bromides. La reacción de acoplamiento cruzado de bromuros de alquenilo con aminas secundarias y primarias da lugar a enaminas e iminas respectivamente. Esta nueva transformación expande las aplicaciones de la reacción de formación de enlaces CN catalizada por paladio (aminación Buchwald,Hartwig), que se había limitado fundamentalmente a haluros de arilo. Después de un estudio de diferentes ligandos. bases y disolventes, los mejores resultados en la reacción de acoplamiento cruzado de aminas secundarias con 1-bromoestireno, se obtuvieron para el catalizador constituido por la combinación Pd/BINAP en presencia de NaOtBu en tolueno. Las correspondientes enaminas se obtienen limpiamente y con rendimientos prácticamente cuantitativos. Sin embargo, los impedimentos estéricos en la amina parecen ser una limitación de la reacción, puesto que aminas con sustituyentes voluminosos proporcionan conversiones bajas. La misma metodología puede aplicarse al acoplamiento de aminas secundarias con 2-bromoestireno. Además, la reacción con 2-bromopropenos sustituidos permite obtener de forma regioselectiva enaminas terminales isomerizables, sin que la isomerización del doble enlace tenga lugar. Las mejores condiciones catalíticas para el acoplamiento de aminas primarias con 1-bromoestireno incluyen de nuevo la utilización del sistema Pd(0)/BINAP/NaOtBu. La reacción proporciona las iminas esperadas en tiempos de reacción muy cortos y con baja carga del catalizador. Mediante esta metodología pueden prepararse un conjunto de iminas de gran diversidad estructural, permitiendo variaciones en ambos reactivos de acoplamiento. Sin embargo, la reacción con 2-bromoestireno no produce buenos resultados en este acoplamiento, probablemente debido a inhibición del ciclo catalítico por parte del producto de reacción. Finalmente, experimentos de competencia de aminación vinílica frente a aminación arílica, ponen de manifiesto que esta reacción se produce de forma preferente sobre los bromuros de vinilo. [source] | |||||||||||||||||||