0000000001225738
AUTHOR
José Antonio López Sáez
Understanding the selectivity in the formation of δ-lactams vs. β-lactams in the Staudinger reactions of chloro-cyan-ketene with unsaturated imines. A DFT study
The reactions of chloro-cyan-ketene with two phenyl substituted unsaturated imines yielding β- or δ-lactams have been investigated using DFT methods at the MPWB1K/6-311G(d,p) level in diethyl ether. The reactions are initialised by the nucleophilic attack of the unsaturated imines on the ketene with formation of zwitterionic intermediates. The subsequent C–C single bond formation at the imine carbon or at the β-conjugated position enables the formation of β- or δ-lactams. Analysis of the energies involved in the two competitive channels explains the selectivity experimentally observed; in the absence of any steric hindrance, formation of δ-lactams is favoured over the formation of β-lactams…
The neolithic settlement of Limoneros (Elche, Alicante)
[EN] This paper shows the results of rescue archaeology campaigns undertaken on the site of Limoneros, an open-air settlement located to the south of Elche (Alicante), in the alluvial plain of the Vinalopó river and dated on the first half of the 5th millennium cal BC. The settlement entails different negative structures, including two stretches of ditch, silos and basins, structures finally reused as a waste area. The analysis of the recovered evidences allows the characterization of the activities developed by a farming community in a territory articulated by the river Vinalopó, where other settlements dated back to the same period have been documented. Therefore, Limoneros supplies relev…
Paleoecología y cultura material en el complejo tumular prehistórico del Castillejo del Bonete (Terrinches, Ciudad Real).
[EN] Castillejo del Bonete is a tumulus complex located on the southern edge the Iberian Plateau, occupied during Chalcolithic and Bronze Age periods, and linked to the Culture known as Motillas. Diverse archaeological objects have been recovered in association with their architectures (barrows, corridors, walls, etc.). Based on charcoal, pollen and microvertebrates a paleoecological study will be discussed. Furthermore, the analysis of different materials such as wood, shell, stone and ivory ornaments together with pottery vessels and metallic objets will also be carried out. All this evidence could be related to feasting practices and offerings
Understanding the regioselectivity in hetero Diels–Alder reactions. An ELF analysis of the reaction between nitrosoethylene and 1-vinylpyrrolidine
[EN] The regioselectivity in the hetero Diels-Alder reaction between nitrosoethylene 1 and 1-vinylpyrrolidine 2 has been studied by a comparative ELF bonding analysis along the IRC of the endo/ortho and endo/meta regioisomeric channels at the B3LYP/6-31G*. level. Along the most favorable endo/ortho regioisomeric channel, the C-C sigma bond is first formed by coupling of two pseudoradical centers located at the most electrophilic carbon of 1 and the most nucleophilic center of 2. Formation of these relevant pseudoradical centers, which depend on the total charge transfer process along the polar reaction, and not on the approach mode of each reagent, is well characterized by analysis of the a…
Understanding the formation of [3+2] and [2+4] cycloadducts in the Lewis acid catalysed reaction between methyl glyoxylate oxime and cyclopentadiene: a theoretical study
The formation of the formal [3+2] and [2+4] cycloadducts in the Lewis acid (LA) catalysed reaction of cyclopentadiene (Cp, 1) with methyl glyoxylate oxime (MGO, 2a) has been theoretically studied using DFT methods. Coordination of BF3 LA to the oxygen atom of MGO 2a not only increases the electrophilicity of the oxime, but also makes the corresponding tautomeric BF3:nitrone complex 8b the reactive species. The reaction is characterised by the nucleophilic attack of Cp 1 on the carbon atom of the corresponding BF3:nitrone complex 8b. The subsequent ring closure at the end of the reaction allows the formation of the [3+2] or [2+4] cycloadducts. ELF bonding analysis of selected points on the i…
Ring splitting of azetidin-2-ones via radical anions
The radical anions of azetidin-2-ones, generated by UV-irradiation in the presence of triethylamine, undergo ring-splitting via N-C4 or C3-C4 bond breaking, leading to open-chain amides. This reactivity diverges from that found for the neutral excited states, which is characterised by alpha-cleavage. The preference for beta-cleavage is supported by DFT theoretical calculations on the energy barriers associated with the involved transition states. Thus, injection of one electron into the azetidin-2-one moiety constitutes a complementary activation strategy which may be exploited to produce new chemistry.
Theoretical study on the molecular mechanism of the [5 + 2] vs. [4 + 2] cyclization mediated by Lewis acid in the quinone system
[EN] The thermal and Lewis acid (LA) catalyzed cyclizations of quinone 1 involved in the synthesis of Colombiasin A and Elipsaterosin B have been theoretically studied using DFT methods at the B3LYP/6-311G(d,p) computational level. B3LYP calculations suggest that the formal endo [4 + 2] cycloadduct allowing the synthesis of Colombiasin A is preferentially formed under thermal conditions, while in the presence of the BF3 LA catalyst the formal [5 + 2] cycloadduct is seen, allowing the synthesis of Elipsaterosin B. The BF3 LA catalyst not only accelerates the nucleophilic attack on the C2 carbon of the quinone framework through a more polar C-C bond formation, but also provokes a different el…
Understanding the mechanism of the Povarov reaction. A DFT study
The molecular mechanism of the Povarov reaction in acetonitrile has been studied at the MPWB1K/6-311G** level of theory. This reaction follows a domino process that comprises two sequential reactions: (i) a Lewis acid catalysed aza-Diels–Alder (A-DA) reaction between a N-aryl imine and a nucleophilic ethylene yielding a formal [4 + 2] cycloadduct; (ii) a stepwise 1,3-hydrogen shift at this intermediate affording the final tetrahydroquinoline. At this computational level, the Lewis acid catalysed A-DA reaction presents a two-step mechanism as a consequence of the large stabilisation of the corresponding zwitterionic intermediate. Our study allows establishing that the N-aryl substituent has …
Understanding the local reactivity in polar organic reactions through electrophilic and nucleophilic Parr functions
[EN] Building upon our recent studies devoted to the bonding changes in polar reactions [RSC Advances, 2012, 2, 1334 and Org. Biomol. Chem., 2012, 10, 3841], we propose herein two new electrophilic, P-k(+), and nucleophilic, P-k(-), Parr functions based on the spin density distribution at the radical anion and at the radical cation of a neutral molecule. These local functions allow for the characterisation of the most electrophilic and nucleophilic centres of molecules, and for the establishment of the regio- and chemoselectivity in polar reactions. The proposed Parr functions are compared with both, the Parr-Yang Fukui functions [J. Am. Chem. Soc. 1984, 106, 4049] based on frontier molecul…
A DFT study of the [3 + 2] versus [4 + 2] cycloaddition reactions of 1,5,6-trimethylpyrazinium-3-olate with methyl methacrylate.
The reaction between 1,5,6-trimethylpyrazinium-3-olate and methyl methacrylate (MMA) yielding a lactone-lactam has been studied using the DFT method at the B3LYP/6-31G(d) level. It is concluded that formation of the lactone-lactam is a domino process involving three consecutive reactions: (i) a 1,3-dipolar cycloaddition (13DC) reaction between the pyrazinium-3-olate and MMA yielding a [3 + 2] cycloadduct (CA); (ii) a skeletal rearrangement, which converts the [3 + 2] CA into a formal [4 + 2] CA, possessing a diazabicyclo[2.2.2]octane structure; and finally, (iii) an S(N)2 reaction, promoted by halide anion, with concomitant nucleophilic attack of the created carboxylate anion on an iminium …
Prehistoric land use at an archaeological hot-spot (the rock art park of Campo Lameiro, NW Spain) inferred from charcoal, synanthropic pollen and non-pollen palynomorph proxies
Pollen, non-pollen palynomorphs and charcoal from a colluvial soil surrounded by prehistoric petroglyphs (Campo Lameiro, NW Spain) were studied in order to assess the nature of human activities and their impact on Holocene vegetation patterns. Several phases of anthropogenic impact were observed. (i) Between 7.6 and 6.5 ka cal BP, synanthropic taxa (Urtica dioica type, Plantago lanceolata type) and coprophilous fungi (e.g. Sporormiella-type) are indicative of early (pre-agricultural) creation of small patches of pasture using fire, possibly for incipient animal husbandry or as part of a deliberate strategy to improve game availability. Such activities only had a minor effect on the deciduou…
Understanding the domino reaction between 3-chloroindoles and methyl coumalate yielding carbazoles. A DFT study.
The molecular mechanism of the reaction between N-methyl-3-chloroindole and methyl coumalate yielding carbazole has been studied using DFT methods at the MPWB1K/6-311G(d,p) level in toluene. This reaction is a domino process that comprises three consecutive reactions: (i) a polar Diels-Alder (P-DA) reaction between indole and methyl coumalate yielding two stereoisomeric [2 + 4] cycloadducts (CAs); (ii) the elimination of HCl from these CAs affording two stereoisomeric intermediates; and (iii) the extrusion of CO2 in these intermediates, finally yielding the carbazole. This P-DA reaction proceeds in a completely regioselective and slightly exo selective fashion. In spite of the highly polar …
Understanding C C bond formation in polar reactions. An ELF analysis of the Friedel Crafts reaction between indoles and nitroolefins
[EN] The Friedel-Crafts (FC) reaction of N-methyl indole 1 with nitroethylene 2 has been studied using DFT methods at the B3LYP/6-31+G** level in order to characterize the bonding changes along the C-C bond-formation process in polar reactions. For this FC reaction a two-step mechanism has been found. The first step is associated with the C-C bond formation between the most electrophilic centre of nitroethylene and the most nucleophilic centre of N-methyl indole, to yield a zwitterionic intermediate IN. The second step corresponds to an intramolecular proton transfer process at IN, regenerating the aromatic system present at the indole. Despite the high electrophilic character shown by nitr…
Understanding the Bond Formation in Hetero Diels-Alder Reactions. An ELF Analysis of the Reaction of Nitroethylene with Dimethylvinylamine
[EN] The bonding evolution in hetero-Diels-Alder (HDA) reactions has been studied by an ELF analysis of the electron reorganization along the HDA reaction between nitroethylene 6 and dimethylvinylamine (DMVA) 9 at the B3LYP/6-31G* level. This cycloaddition takes place along a two-stages one-step mechanism. In the first stage of the reaction, the C1-C6 bond is formed by coupling of two pseudoradical centers positioned at the most electrophilic carbon of nitroethylene 6 and the most nucleophilic center of DMVA 9. In the second stage, the formation of the second O4-C7 bond takes place between a pseudoradical center positioned at the C7 carbon of DMVA and some electron-density provided by the l…
Aportaciones hidrogeológicas al estudio arqueológico de los orígenes de la Edad del Bronce de La Mancha: la cueva monumentalizada de Castillejo del Bonete (Terrinches, Ciudad Real, España)
Estudios recientes indican que las motillas, asentamientos de la Edad del Bronce de La Mancha, pudieron ser las más antiguas captaciones de agua subterránea en la Península Ibérica. Pero ¿por qué no existen motillas en el Campo de Montiel, territorio ubicado tradicionalmente en esta área cultural? En Castillejo del Bonete, sitio arqueológico situado en esa comarca, existe una cueva que fue utilizada y sellada durante la Prehistoria Reciente. Se presenta ahora la primera investigación paleohidrogeológica interdisciplinar en La Mancha, que ha analizado manantiales y niveles de agua subterránea del acuífero de Campo de Montiel, así como el interior de la sima de Castillejo del Bonete. Las conc…
A DFT study on the NHC catalysed Michael addition of enols to α,β-unsaturated acyl-azoliums. A base catalysed C–C bond-formation step
The NHC catalysed nucleophilic additions of enols to α,β-unsaturated acyl-azolium intermediates have been investigated using DFT methods at the MPWB1K/6-31G** computational level. In the direct and the conjugate additions, formation of a hydrogen bond (HB) with the carboxyl oxygen is not sufficient to favour the C-C bond formation as a consequence of the low nucleophilic character of enols. Interestingly, when enols form a HB with the chloride counterion, the activation energies associated with the conjugate addition decrease as a consequence of the increased nucleophilic character of enols and the increased electrophilic character of the 'acyl-azolium + Cl' ion pair. Analysis of the DFT re…
Aportaciones hidrogeológicas al estudio arqueológico de los orígenes de la Edad del Bronce de La Mancha: la cueva monumentalizada de Castillejo del Bonete (Terrinches, Ciudad Real-España)
Estudios recientes indican que las motillas, asentamientos de la Edad del Bronce de La Mancha, pudieron ser las más antiguas captaciones de agua subterránea en la Península Ibérica. Pero ¿por qué no existen motivas en el Campo de Montiel, territorio ubicado tradicionalmente en esta área cultural? En Castillejo del Bonete, sitio arqueológico situado en esa comarca, existe una cueva que fue utilizada y sellada durante la Prehistoria Reciente. Se presenta ahora la primera investigación paleohidrogeológica interdisciplinar en La Mancha, que ha analizado manantiales y niveles de agua subterránea del acuífero de Campo de Montiel, así como el interior de la sima de Castillejo del Bonete. Las concl…