Experimental Study of the Mechanism of the Pd-catalyzed Aryl-alkyl Negishi Coupling using Hybrid Phosphine- Electron Withdrawing Olefin Ligands

Producción CientíficaA detailed study of the Negishi cross-coupling reaction of ArI (Ar = 2-C6H4CO2Et) and ZnEt2 with palladium catalysts containing conventional phosphines versus one using a chelating hybrid phosphine–electron-withdrawing olefin (P-EWO) ligand reveals that for conventional phosphines (e.g., PPh3) β-H elimination from intermediate [PdArEt(PPh3)2] is competitive with Ar–Et reductive elimination and is responsible for part of the undesired reduction product ArH. In contrast, with the EWO phosphine, the β-H elimination from intermediate [PdArEt(P-EWO)] is slow compared to the fast Ar–Et reductive elimination, and the undesired reduction product ArH observed proceeds in this case of hydrolysis of ZnArEt, formed in transmetalations where Ar is transferred from Pd to Zn. The rate of these transmetalations is comparable to the rate of reductive eliminations. Consequently, undesired transmetalations affording [PdEt2(P-EWO)] and ZnArEt are more effective at early stages of the reactions and less effective when the ethylating agent becomes poorer in ZnEt2 and richer in ZnEtX (X = I), as the reaction proceeds. Careful analysis of the experiments reveals the detailed changing evolution of the reaction, not only providing the main features of the catalytic cycle but also deducing how the reagents in the system change with time and what the effects on the products of these changes are.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. GR169 and VA256U13)Ministerio de Economía, Industria y Competitividad (CTQ2013-48406-P

Triblock copolyesters derived from lactic acid and glucose: synthesis, nanoparticle formation and simulation

ABA triblock copolyesters were synthesized by ring-opening polymerization (ROP) of l-lactide in solution initiated by a telechelic d-glucose-based polyester macroinitiator. The macroinitiator with a number-average molecular weight about 2500 g mol-1 was synthesized by non-stoichiometric polycondensation in the melt of 2,4:3,5-di-O-methylene-d-glucitol and dimethyl succinate. Two triblock copolyesters of Mn ranging between ~6000 and ~9000 g mol-1, and differing in the length of the polylactide blocks were prepared. These copolyesters started to decompose when heated at ~220 °C and degraded slowly upon aqueous incubation under physiological conditions. They did not display any perceivable crystallinity and showed a single glass transition temperature (Tg) around 60 °C with the higher value corresponding to the larger content in glucitol units. The copolyesters were able to form nanoparticles with average diameters of ~100–130 nm and satisfactory dispersity. The effect of the block lengths on size, ¿-potential values and physical stability of the nanoparticles was evaluated. A molecular dynamics simulation study allowed modelling the two-phase structure of the nanoparticles and evidenced the preference of the glucose-based block to be peripherally located.Peer ReviewedPostprint (author's final draft

Isohexide and sorbitol-derived, enzymatically synthesized renewable polyesters with enhanced Tg

Sugar-based polyesters derived from sorbitol and isohexides were obtained via solvent-free enzymatic catalysis. Pendant hydroxyl groups, coming from the sorbitol units, were present along the polyester backbone, whereas the two isohexides, namely, isomannide and isoidide dimethyl ester monomers, were selected to introduce rigidity into the polyester chains. The feasibility of incorporating isomannide as a diol compared to the isoidide dimethyl ester as acyl-donor via lipase-catalyzed polycondensation was investigated. The presence of bicyclic units resulted in enhanced Tg with respect to the parent sorbitol-containing polyester lacking isohexides. The different capability of the two isohexides to boost the thermal properties confirmed the more flexible character provided by the isoidide diester derivative. Solvent-borne coatings were prepared by cross-linking the sugar-based polyester polyols with polyisocyanates. The increased rigidity of the obtained sugar-based polyester polyols led to an enhancement in hardness of the resulting coatings.Peer ReviewedPostprint (author's final draft

Fully bio-based aromatic–aliphatic copolyesters: poly(butylene furandicarboxylate-co-succinate)s obtained by ring opening polymerization

Poly(butylene 2,5-furandicarboxylate-co-succinate) copolyesters (coPBFxSy) have been synthesized by ring opening polymerization (ROP). Cyclic butylene 2,5-furandicarboxylate and butylene succinate oligomer mixtures, to be used as monomers for ROP, were prepared by high dilution condensation and enzymatic cyclization reactions, respectively. Two different catalytic systems, tin dioctanoate and supported Candida antarctica lipase B (CALB), were used for polymerization. Thus two series of copolyesters covering the whole range of compositions were prepared and their properties comparatively examined. In both cases, random copolyesters with compositions close to those used in their respective feeds were obtained. The influence of composition on reaction kinetics with respect to time and temperature was evaluated for the two series. Chemically catalyzed ROP rendered copolyesters with Mw in the ~50 000–65 000 g mol-1 range, whereas values between 15 000 and 45 000 g mol-1 were attained when the ROP reaction was assisted by CALB. The thermal behavior of coPBFxSy obtained by ROP was similar to that reported for such copolymers prepared by melt polycondensation. They all start to decompose above 300 °C and display melting enthalpy and temperatures that decrease with copolymerization, attaining minimum values when the comonomer contents are approximate to balance. On the contrary, the glass-transition temperature increased almost linearly with the content of butylene furandicarboxylate units, covering the whole range of values between those of the two parent homopolyesters. Small deviations in thermal properties observed between the two series could be attributed to their differences in molecular weights. Hydrolytic and enzymatic degradation studies revealed that coPBFxSy became more degradable with increasing content of succinic units, whereas the homopolyester PBF remained practically unaffected when incubated under similar conditions.Peer ReviewedPostprint (author's final draft

Crystalline structure and thermotropic behavior of alkyltrimethylphosphonium amphiphiles

Quaternary organophosphonium salts bearing long alkyl chains are cationic surfactants of interest owing to their physical and biological properties. In the present work, the crystal structure and thermotropic behavior of the homologous series of alkyltrimethylphosphonium bromides (nATMP·Br), with the alkyl chain containing an even number (n) of carbon atoms from 12 to 22, have been examined within the 0–300 °C range of temperatures. These compounds were shown to be resistant to heat up to ~390 °C. The phases adopted at different temperatures were detected by DSC, and the structural changes involved in the phase transitions have been characterized by simultaneous WAXS and SAXS carried out in real-time, and by polarizing optical microscopy as well. Three or four phases were identified for n = 12 and 14 or n = 16, respectively, in agreement with the heat exchange peaks observed by DSC. The phase existing at room temperature (Ph-I) was found to be fully crystalline and its crystal lattice was determined by single-crystal X-ray diffraction methods. Ph-II consisted of a semicrystalline structure that can be categorized as Smectic-B with the crystallized ionic pairs hexagonally arranged in layers and the molten alkyl chain confined in the interlayer space. Ph-II of 12ATMP·Br and 14ATMP·Br directly isotropicized upon heating at ~220 °C, whereas for n = 16, it converted into a Smectic-A phase (Ph-III) that needed to be heated above ~240 °C to become isotropic (Ph-Is). The correlation existing between the thermal behavior, phase structure and length of the alkyl side chain has been demonstrated.Peer ReviewedPostprint (published version

Códigos correctores óptimos mediante técnicas elementales

Los códigos correctores de errores se utilizan cuando se quiere enviar una información a través de un canal sujeto a ruido. Un código corrector añade información extra al mensaje que se quiere transmitir con el fin de recuperar la información enviada. En este trabajo se tratan los códigos en bloque y en concreto los códigos lineales. Se presentan diferentes métodos sencillos para la obtención de códigos lineales óptimos y se estudia el código simplex ternario y el código Reed Muller ternario.Grado en Matemática

Controlling the isothermal crystallization of isodimorphic PBS-ran-PCL random copolymers by varying composition and supercooling

In this work, we study for the first time, the isothermal crystallization behavior of isodimorphic random poly(butylene succinate)-ran-poly(e-caprolactone) copolyesters, PBS-ran-PCL, previously synthesized by us. We perform nucleation and spherulitic growth kinetics by polarized light optical microscopy (PLOM) and overall isothermal crystallization kinetics by differential scanning calorimetry (DSC). Selected samples were also studied by real-time wide angle X-ray diffraction (WAXS). Under isothermal conditions, only the PBS-rich phase or the PCL-rich phase could crystallize as long as the composition was away from the pseudo-eutectic point. In comparison with the parent homopolymers, as comonomer content increased, both PBS-rich and PCL-rich phases nucleated much faster, but their spherulitic growth rates were much slower. Therefore, the overall crystallization kinetics was a strong function of composition and supercooling. The only copolymer with the eutectic composition exhibited a remarkable behavior. By tuning the crystallization temperature, this copolyester could form either a single crystalline phase or both phases, with remarkably different thermal propertiesPeer ReviewedPostprint (published version

Sugar-based bicyclic monomers for aliphatic polyesters: a comparative appraisal of acetalized alditols and isosorbide

Three series of polyalkanoates (adipates, suberates and sebacates) were synthesized using as monomers three sugar-based bicyclic diols derived from D-glucose (Glux-diol and isosorbide) and D-mannose (Manx-diol). Polycondensations were conducted in the melt applying similar reaction conditions for all cases. The aim was to compare the three bicyclic diols regarding their suitability to render aliphatic polyesters with enhanced thermal and mechanical properties. The ensuing polyesters had molecular weights (Mw) in the 25,000–50,000 g mol-1 range with highest values being attained for Glux-diol. All the polyesters started to decompose above 300 °C and most of them did not display perceivable crystallinity. On the contrary, they had glass transition temperatures much higher than usually found in homologous polyesters made of alkanediols, and showed a stress– strain behavior consistent with their Tg values. Glux-diol was particularly effective in increasing the Tg and to render therefore polyesters with high elastic modulus and considerable mechanical strength.Peer ReviewedPostprint (published version

The stille reaction, 38 years later

Producción CientíficaThe first now named Stille reaction was published 38 years ago, and the last comprehensive revision of this catalysis was in 2004. Since then the knowledge of the different steps of the three possible (and sometimes competing) reaction pathways (cyclic, open, and ionic) has been almost completed by synergistic experimental and theoretical studies: the Stille reaction is perhaps the best characterized catalytic process if we consider the number of intermediates that have been detected. This review concentrates on the mechanistic new knowledge, and on important aspects as the revolution with the use of bulky phosphines, the bimetallic alternative of the Stille reaction, the enantioselectivity in Stille and palladium free Stille processes, the meaning of copper effect, or the possible approaches to make Stille coupling a greener process.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. GR 169 and VA256U13)Ministerio de Economía, Industria y Competitividad (CTQ2013-48406-P

Salinas costeras y gestión integrada de espacios naturales protegidos: el caso del litoral atlántico de la Península ibérica

International audienceThe aims of the paper, presented in the framework of the ECOSAL ATLANTIS project (INTERREG IV B Atlantic Space EU programme) are to: i) analyse the protection of coastal saltpans along the Atlantic coast of the Iberian peninsula, ii) evaluate their insertion in management plans for natural areas, and iii) suggest actions favouring the enhancement of ecotourism and heritage in the interests of sustainable development. The saltpans have a marked ecological, landscape and social value and most of them are part of larger natural protected areas. The present situation of the saltpans and their differences in terms of levels of protection (regional, national, European and international) are summarised in a table. Management plans in Spain and Portugal are analysed to evaluate the level of integration of salt sites and artisan salt making in the overall policies of a given area. Ownership of the salt site is also crucial, especially in Portugal. Measures designed to preserve the traditional salt site landscapes and encourage ecotourism are suggested in order to enhance the integration of the natural and cultural heritage.El objetivo de esta comunicación, que se enmarca en el proyecto ECOSAL ATLANTIS (programa europeo INTERREG IV B Espacio Atlántico), es (1) analizar la protección de las salinas costeras atlánticas de la Península Ibérica, (2) valorar su inserción en los planes de gestión de los espacios naturales y (3) proponer actuaciones que favorezcan una valorización ecoturística y patrimonial enaras de un desarrollo sostenible. Las salinas costeras poseen un alto valor ecológico, paisajístico y social, y forman parte, en su granmayoría, de espacios naturales protegidos. Según la figura de protección aplicada tanto de carácter regional (Paraje Natural, Parque Natural, Reserva Natural, Espacio Natural, Paisaje Protegido...), como nacional (Parque nacional,...) e internacional (Natura 2000, convenio RAMSAR...) existengrandes diferencias ‐no sólo entre países, sino incluso dentro de un mismo estado‐ en cuanto a la extensión y valores del área protegida, así como en relación con las actividades permitidas o autorizadas. En este sentido, se elaborará un cuadro sintético de la situación actual de las salinas incluidas en espacios naturales protegidos. En función de estas figuras, las estrategias de gestión en lo referente a la actividad salinera, ya sea tradicional, ya industrial, difieren en gran medida. Por ello, se propone analizar los diferentes planes de gestión en España y Portugal ‐Planes de Ordenación de los Recursos Naturales (PORN), Planes Rectores de Uso y Gestión (PRUG), Planos de Ordenamento de Parques e Reservas Naturais... - con objeto de valorar el grado de integración de las salinas y de la actividad salinera en la política general del espacio natural protegido. Un aspecto importante que se tendrá en consideración es la propiedad de las salinas, especialmente en Portugal, donde, a partir de 2014, toda salina cuya propiedad (antes de 1864) no se haya probado pasará a manos públicas. Esta circunstancia es fundamental a la hora de implementar medidas de gestión y protección. Sin embargo, la gestión de estos espacios está basada principalmente en la riqueza faunística y florística asociada a los humedales costeros. Las autoridades tienen muy presentes los valores estrictamente naturales, olvidando otros elementos patrimoniales y culturales que les confieren identidad y singularidad. A pesar de su apariencia natural, no hay que olvidar que las salinas son áreas creadas por el hombre con fines económicos, que deben su valor a las tareas extractivas tradicionales y que, precisamente, es el mantenimiento de esas actividades, y del sistema hidráulico asociado, lo que permite un entorno adecuado para el desarrollo de la flora y la fauna. Desde esta perspectiva integradora del patrimonio natural y cultural, se propone una serie de medidas destinadas a conservar el paisaje salinero tradicional y a fomentar el ecoturismo