Barbara Gordillo

Relative reactivity of 2-diphenylphosphinoyl- and 2-diphenyl-thiophosphinoyl-2-[1,3]dithianyllithium as reagents Wittig-Horner/Corey-Seebach

Abstract The reaction of the title carbanions with several aldehydes and ketones to afford the corresponding ketene dithioketals was studied. It was found that the PO organolithium reagent reacted with both types of carbonyl substrates in good to excellent yields. On the other hand, the PS reagent reacted selectively with aldehydes although in poor yield. This contrasting behavior can not be ascribed to a greater stability of the latter carbanion since a ΔpK a measurement showed that the more stable organolithium (by 0.33 ± O.O8 a pK a units) is also the more reactive. The lower reactivity of the P=S analog can be explained in terms of kinetic inhibition in the formation of the oxaphosphetane intermediate, and/or thermodynamic instability of this pentacoordinated phosphorus species with sulfur (vis-a-vis oxygen) as ligand.

Electrochemical reduction of 1,4-benzoquinone. Interaction with alkylated thymine and adenine nucleobases

Abstract The electrochemical reduction of 1,4-benzoquinone (Q) in the presence of 1-octylthymine and 9-octyladenine has been performed in dimethylsulfoxide on glassy carbon electrodes. The electrochemical behaviour shows that the semiquinone (Q − ) and the quinone dianion (Q 2− ) interact with 1-octylthymine (TH) following a mechanism which involves association and protonation reactions. It is demonstrated by cyclic voltammetry and NMR experiments, that the protonated dianion (QH − ) is stabilised by an excess of TH, alternatively it disproportionates into an association complex of dianion-hydroquinone [Q,QH 2 ] 2− . The 1-octylthymine anion (T − ) produced in the proton transfer reactions activates a homogeneous chain mechanism allowing the consumption of benzoquinone. For the experiments carried out in the presence of 9-octyladenine (AH), it was observed that only a strong hydrogen bonding association takes place between the nucleobase and Q 2− .

Conformational analysis of highly hindered thiophosphoramidates

Abstract The conformational analyses in solution and in solid state of the highly crowded mobile systems, 2-dicyclohexylamino-2-thio-1,3,2λ 5 -dioxaphosphinane ( 1 ), 2-dicyclohexylamino-2-thio- trans -4,6-dimethyl-1,3,2λ 5 -dioxaphosphinane ( trans - 2 ) and their anancomeric analogs, ax -2-dicyclohexylamino-2-thio- cis -4,6-dimethyl-1,3,2λ 5 -dioxaphosphinane ( ax - cis - 2 ) and eq -2-dicyclohexylamino-2-thio- cis -4,6-dimethyl-1,3,2λ 5 -dioxaphosphinane ( eq - cis - 2 ) are informed. In accordance with proton–proton and proton–phosphorus coupling constants, the dioxaphosphinane ring adopts a chair conformation in all compounds except in the case of trans - 2 , which was found in a twisted conformation. Solid state structural analyses suggest that only the synaxial Me/NR 2 or Me/S interactions in trans - 2 , force the heterocycle ring to remain as a mixture of twisted boat and half-chair (chaise loungue) conformers.

A search for single-electron transfer (set) mechanisms in the reaction of 1,3-dithianyl- and 1,3-oxathianyllithiums with primary halides

Abstract The reaction of 1, 3-dithianyl-, 2-methyl-1, 3-dithianyl- and oxathianyllithium with 6-halo-1-hexenes affords the products expected from a S N 2 (rather than SET) mechanism.

Rapid synthesis of (±)-r-7-benzyloxymethyl-cyclopenta-cis-[4,5][1,3]-oxazolo[3,2-a]pyrimidinones versatile carbocyclic nucleoside precursors

Abstract (±)-r-7-Benzyloxymethyl-cyclopenta-cis-[4,5][1,3]-oxazolo[3,2-a]pyrimidinones were synthesized in two steps from 1-hydroxymethyl-3-cyclopentene. These compounds are versatile intermediates for the synthesis of carbocyclic nucleosides. The synthesis has been accomplished by the iodofunctionalization of olefins as a method of coupling the pyrimidine bases and the carbocycle.

Kinetic Studies of the Thermal cis-to-trans Isomerization of Dioxaphospholanes

By following a previously reported method,1 the synthesis of r-2-alkoxy-cis-4-cis-5-dimethyl-1,3,2-λ3-dioxaphospholanes ligands (1 and 3) was carried out. The purpose of this work is the kinetic study of the inversion barrier at phosphorus for 1 and 3 and the comparison with the already informed dioxaphospholane 2. The kinetic measurements of the thermal isomerization cis-to-trans were performed by 31P NMR spectroscopy, observing a first order kinetics for both compounds. The energy of activation (Ea) for the epimerization of compounds cis-1 and cis-3 was calculated to be 16.0 ± 0.6 and 11.8 ± 0.8 kcal/mol, respectively. The values of the thermodynamic parameters of the transition state (Δ H≠, Δ S≠, Δ G≠) suggest that the inversion at phosphorus not only depends on the spatial requirements of the alkoxy substituent but also on entropic effects. The thermodynamic parameters Δ H°, Δ S°, and Δ G° were also evaluated and they show that the cis isomers are preferred from enthalpic point of view, but entropic effects dominate the equilibrium trans ⇌ cis leading to the entropically favored trans isomers. Furthermore, the results are supported by density functional theory calculations of 1–4 at the B3LYP/6-31G** level.

The formation of open-chain thioesters in the reaction of 2-lithio-2-methyl- and 2-lithio-2-phenyl-1,3-dithiane with chlorodiphenylphosphane followed by oxidation ☆

The unexpected formation of open-chain thioesters (3) and (6) from the reaction of 2-lithio-r-2-t-4-t-6-trimethyl (1-Li) and 2-lithio-r-2-phenyl-t-4-t-6-dimethyl-1,3-dithiane (4-Li), respectively, with chlorodiphenylphosphane followed by oxidation was observed instead of the anticipated gem-derivatives. The X-ray diffraction analysis of (6) and the trapped intermediate (10) confirmed the structure and the proposed mechanism of formation of the open-chain products.

Synthesis and Study of the Thermal Epimerization of r -2-Ethoxy- cis -4- cis -5-Dimethyl-1,3,2-λ 3 -Dioxaphospholane Using 31 P NMR

The exclusive synthesis of r -2-ethoxy- cis -4- cis -5-dimethyl-1,3,2- u 3 -dioxaphospholane ( cis - 1 ) is reported. The kinetic measurements of the epimerization of cis - 1 to trans - 1 were performed by 31 P NMR from 50 to 80;C in toluene. The energy of activation (E a ) of the epimerization process was calculated from Arrhenius plot. The thermodynamic parameters of the transition state led us to conclude that the activation barrier of the inversion at phosphorus is driven by the enthalpy and by a large and negative entropy of activation.

THE LACK OF INFLUENCE OF ELECTRONIC EFFECTS ON THE STEREOCHEMISTRY OF THE OXIDATION OF ARYL THIOPHOSPHATES

A series of epimeric six-membered cyclic aryl thiophosphates 1–7 were oxidized to their corresponding phosphates by means of a mixture of aqueous hydrogen peroxide (30%) and hydrogen chloride. The oxidation, under these conditions, is highly stereospecific so that the products were obtained in quantitative yields with retention of configuration at the phosphorus center. Our results do not support the participation of pentacoordinate species as plausible intermediates of the oxidation reaction.

Investigación-docencia: ¿Un mito o una alternativa?

Soportando nuestro quehacer universitario se enraizan un sinnumero de mitos y creencias. Una de estas creencias dice mas o menos lo siguiente: para ser un profesor es necesario ser investigador y para serlo, se requiere de un doctorado. De otra manera: un mejor investigador es un mejor profesor. Una mas: para mejorar a las universidades es necesario vincular la docencia con la investigacion.