Materiales carbonosos nanoestructurados para la preparación de catalizadores híbridos mediante el sistema SILP

The main objective of this Doctoral thesis is the preparation of hybrid active catalysts using the SILP (Supported Ionic Liquid Catalysis) methodology and employing carbon materials as support. For that, in first place, SILP samples have been prepared and characterized. In second place, hybrid the SILP catalysts have been prepared, characterized and tested in some hydrogenation reactions, including an asymmetric hydrogenation. The investigated variables are mainly the amount and kind of ionic liquid and the physical, chemical and morphological properties of the support.

Estudio de complejos metal-carbeno mesoiónicos como catalizadores en condiciones medioambientalmente sostenibles

Los carbenos, conocidos como intermedios lábiles de reacción debido a su alta reactividad, son un grupo de compuestos con propiedades muy interesantes. Su uso está extendido en química organometálica, puesto que la mayoría de aplicaciones de estas especies implican su coordinación con metales de transición para formar complejos metal-carbeno. Existen diferentes tipos que de complejos en base a su configuración electrónica y los sustituyentes presentes en la molécula. Uno de los grupos de carbenos con mayor interés en catálisis organometálica son los complejos derivados de carbenos N-heterocíclicos (NHC). Entre ellos se encuentran los carbenos normales (nNHC), los anormales (aNHC) y los mesoiónicos (MIC). Cada uno de estos tipos presenta propiedades físicas y químicas diferentes, debido a la diferente disposición de los heteroátomos y el modo de enlace en la estructura. Los complejos metal-carbeno mesoiónicos están empezando a usarse en catálisis homogénea por su facilidad de formación y su alta estabilidad en condiciones normales de reacción. Además, se ha probado su utilidad como catalizadores de reacciones orgánicas en medios no convencionales, tales como el agua. Entre los medios no convencionales que pueden ser utilizados para llevar a cabo reacciones en condiciones más benignas para el medio ambiente, se encuentran los disolventes eutécticos (DES). Estos disolventes son formados a partir de la mezcla de dos sólidos, una sal cuaternaria (HBA) y una especie donante de hidrógenos (HBD), que dan lugar una disolución líquida a temperatura ambiente. Las propiedades de esta mezcla eutéctica se pueden ajustar en función de la relación HBA/HBD, con el fin de adaptarse a diferentes tipos de reacciones orgánicas. En este trabajo se estudia la reacción de Suzuki-Miyaura, un acoplamiento cruzado de dos especies orgánicas, tanto en agua como en un líquido eutéctico usando un complejo derivado de una carbeno mesoiónico, obteniéndose un mejor rendimiento de la reacción en el disolvente eutéctico. Además, el uso de un DES como medio de reacción permitiría recuperar el catalizador utilizado en la reacción, de modo que se reduciría el número de residuos generados en el proceso.

Spin alignment of extra electrons in K-phenanthrene clusters taken from the crystalline tripotassium-intercalated phenanthrene structure

The appearance of ferromagnetic correlations among π electrons of phenanthrene (C14H10) molecules in the herringbone structure is proven for K doped clusters both by ab initio quantum-chemistry calculations and by the direct solution of the many-body Pariser-Parr-Pople Hamiltonian. Magnetic ground states are predicted for one or three additional electrons per phenanthrene molecule. These results are a consequence of the small overlap between the lowest unoccupied molecular orbitals (and lowest unoccupied molecular orbitals + 1) of neutral neighboring phenanthrene molecules, which makes the gain in energy by delocalization similar to the corresponding increase due to the Coulomb interaction.

Exponential decay of spin-spin correlation between distant defect states produced by contour hydrogenation of polycyclic aromatic hydrocarbon molecules

The first few low-lying spin states of alternant polycyclic aromatic hydrocarbon (PAH) molecules of several shapes showing defect states induced by contour hydrogenation have been studied both by ab initio methods and by a precise numerical solution of Pariser-Parr-Pople (PPP) interacting model. In accordance with Lieb's theorem, the ground state shows a spin multiplicity equal to one for balanced molecules, and it gets larger values for imbalanced molecules (that is, when the number of π electrons on both subsets is not equal). Furthermore, we find a systematic decrease of the singlet-triplet splitting as a function of the distance between defects, regardless of whether the ground state is singlet or triplet. For example, a splitting smaller than 0.001 eV is obtained for a medium size C46H28 PAH molecule (di-hydrogenated [11]phenacene) showing a singlet ground state. We conclude that π electrons unbound by lattice defects tend to remain localized and unpaired even when long-range Coulomb interaction is taken into account. Therefore they show a biradical character (polyradical character for more than two defects) and should be studied as two or more local doublets. The implications for electron transport are potentially important since these unpaired electrons can trap traveling electrons or simply flip their spin at a very small energy cost.

Palladium and organocatalysis: an excellent recipe for asymmetric synthesis

The dual activation of simple substrates by the combination of organocatalysis and palladium catalysis has been successfully applied in a variety of different asymmetric transformations. Thus, the asymmetric a-allylation of carbonyl compounds, a-fluorination of acyl derivatives, decarboxylative protonation of β-dicarbonyl compounds, cyclization reactions of alkynyl carbonyl compounds and β-functionalization of aldehydes have been efficiently achieved employing this double-catalytic methodology.

Three-way catalysts: past, present and future

In this article, the past and the state-of-the-art in Three-Way Catalyst (TWC) technology are reviewed. The main chemical reactions occurring in a gasoline engine are discussed and also the main reactions taking place in a TWC placed in the tailpipe, namely CO and hydrocarbons oxidation and nitrogen oxides reduction to molecular nitrogen. The main components of a TWC (substrates, noble metals and cerium oxides) and their role in the different chemical reactions occurring in a TWC are described. Finally, the problem of diesel vehicles gas aftertratment is described, and the current state-of-the art in catalytic converters for these vehicles are commented.

Isoprene-mediated lithiation of imidazole derivatives: mechanism considerations

The isoprene-mediated lithiation, with lithium metal, of different imidazole derivatives is an interesting methodology for their functionalization. Studies of different possible intermediates involved in the reaction employing density functional theory calculations, at the B3LYP/6-311++G(d,p) level are considered. A plausible mechanism is described, in which isoprene is reduced, to the corresponding radical anion, in the presence of Li(s), acting then as a base deprotonating N-methylimidazole (NMI) and producing the 1,1-dimethylallyl radical. This radical is further reduced by the excess of lithium proceeding once more as a base. This final step produces stable final products that compensate the previous equilibriums, making favourable the whole process.

Enantioselective catalytic lithiation using a chiral binaphthyl derivative as electron carrier

The lithiation, of the secondary chloride 2, catalyzed by binaphthyl derivatives, i.e. BINAM 4, BINOL 5, BINAP 6, H8-BINAP 7, Tol-BINAP 8, 2,2’-bis(pyrrolidin-1-yl)-1,1’-binaphthalene 9, and 2,2’-dimethyl-1,1’-binaphthalene 11, in the presence of different ketones has been studied, yielding the corresponding alcohol derivatives 3 and 12-16 in moderate to good yields. Binaphthyl derivative 11 has revealed to be very active as catalyst in the lithiation process at room temperature, and has allowed the preparation of the alcohol derivatives with enantioselectivities up to 50%.

Heterocyclic carbene–metal-catalyzed Csp2–Csp2 and Csp–Csp2 couplings using nonmetallic substrates

The use of carbene ligands for transition-metal complexes has been developed in the last decades, being of special interest those carbenes derived from a nitrogen-containing heterocyclic system. An interesting variety of carbene-metal complexes has been tested in the Mizoroki-Heck reaction. In comparison, few examples can be found for the Matsuda-Heck version of this coupling reaction. Additionally, the Sonogashira coupling has been also catalyzed with different carbene-metal catalysts.

Multicomponent Azide-Alkyne Cycloaddition Catalyzed by Impregnated Bimetallic Nickel and Copper on Magnetite

A new bimetallic catalyst derived from nickel and copper has been used successfully for the first time in the multicomponent reaction of terminal alkynes, sodium azide, and benzyl bromide derivatives. The presence of both metallic species on the surface of magnetite seems to have a positive and synergetic effect. The catalyst loading is the lowest ever published for a catalyst of copper anchored on any type of iron support. The catalyst could be easily removed from the reaction media just by magnetic decantation and it could be reused up to ten times without any negative effect on the initial results.

Reductive electrochemical formation of 6H-dibenzo[b,d]pyran-6-one and 2-benzopyran-1(1H)-one

In the present Letter several carbolactones (oxidative products) are obtained under aprotic cathodic conditions in the preparative scaled electrolysis of 1,2-quinones in a divided electrochemical cell and in the presence of oxygen. When 9,10-phenanthrenequinone is reduced 6H-dibenzo[b,d]pyran-6-one and [1,1′-biphenyl]-2,2′-dicarboxylic acid are obtained as major products. In the reduction of 1,2-naphthoquinone, 2-benzopyran-1(1H)-one, and 2-(2-carboxyethenyl)-benzoic acid were formed as main products. The proposed mechanism to explain the formation of these and other products, that involves an electron-transfer reaction to the oxygen in air, is now discussed.

Solvent- and Ligand-free Diboration of Alkynes and Alkenes Catalyzed by Platinum Nanoparticles on Titania

Platinum nanoparticles supported on titania efficiently catalyzed the diboration of alkynes and alkenes under solvent- and ligand-free conditions in air. The cis-1,2-diborylalkenes and 1,2-diborylalkanes were obtained in moderate to excellent yields following, in most cases, a simple filtration workup protocol. The versatility of the cis-1,2-diboronvinyl compounds was demonstrated in a series of organic transformations, including the Suzuki–Miyaura cross coupling and the boron–halogen exchange.

Recent Advances in the Catalytic Enantioselective Reformatsky Reaction

This paper reviews the present state of the catalytic enantioselective Reformatsky reaction. Advancements in asymmetric versions of this reaction have recently led to a considerable extension of its scope and applicability, principally due to the use of highly active chiral ligands and very specific reaction conditions.

Catalyzed addition of acid chlorides to alkynes by unmodified nano-powder magnetite: synthesis of chlorovinyl ketones, furans, and related cyclopentenone derivatives

Inexpensive and commercially available nano-powder magnetite is an excellent catalyst for the addition of acid chlorides to internal and terminal alkynes, yielding the corresponding chlorovinyl ketones in good yields. The process has been applied to the synthesis of 5-chloro-4-arylcyclopent-2-enones, 3-aryl-1H-cyclopenta[a]naphthalen-1-ones, and (E)-3-alkylidene-2,3-dihydro-1H-cyclopenta[a]naphthalen-1-ones, just by changing the nature of the starting acid chloride or the alkyne. All tested processes elapse with an acceptable or excellent regio- and stereo-selectivity. Moreover, the use of the iridium impregnated on magnetite catalyst permits the integration of the chloroacylation process with a second dehydrochlorination–annulation process to yield, in one-pot, 1-aryl-2,4-dialkylfurans in good yields, independently of the nature of the starting reagents, and including the heteroaromatic ones.