Juan E. Argüello

Photoremoval of protecting groups: mechanistic aspects of 1,3-dithiane conversion to a carbonyl group.

Photodeprotection of 1,3-dithianes in the presence of thiapyrylium was performed to return to the parent carbonyl compound, and the mechanism was studied by steady state photolysis, laser flash photolysis, and theoretical calculations. Electron transfer from dithianes to triplet sensitizers is extremely fast, and the decay of dithiane radical cations was not affected by the presence of water or oxygen as the consequence of a favorable unimolecular fragmentation pathway. Similar behaviors were observed for dithianes bearing electron-releasing or electron-withdrawing substituents on the aryl moiety, evidenced by C-S bond cleavage to form a distonic radical cation species. The lack of reaction under nitrogen atmosphere, requirement of oxygen for good conversion yields, inhibition of the photodeprotection process by the presence of p-benzoquinone, and absence of a labeled carbonyl final product when the reaction is performed in the presence of H2(18)O all suggest that the superoxide anion drives the deprotection reaction. Density functional theory computational studies on the reactions with water, molecular oxygen, and the superoxide radical anion support the experimental findings.

Photochemical and photophysical behavior of indolyl anions in photostimulated intramolecular arylation reactions

O-Arylation reaction is obtained when N-(2-halophenyl) indolo carboxamides are irradiated in a basic medium. On the basis of photochemical and photophysical experiments, we propose that 2-indolylbenzoxazole is formed by intramolecular electron transfer followed by fast dehalogenation of the halophenyl radical anion pendant moiety, finally a radical-radical collapse renders the observed product.

Breaking bonds with electrons: stepwise and concerted reductive cleavage of C–S, C–Se and Se–CN bonds in phenacylthiocyanates and phenacylselenocyanates

The mechanistic aspects of the electrochemical reduction of phenacylthio- and selenocyanates have been studied. With phenacylthiocyanates (1), a change in the reductive cleavage mechanism is observed as a function of the substituent on the phenyl ring. While a stepwise mechanism involving the intermediacy of a radical anion is followed for substrates bearing a strong electron withdrawing group, such as cyano and nitro substituent (1d, 1e), and a concerted mechanism is favoured for compounds bearing an electron-donating or no substituent on the phenyl ring (1a–c). A regioselective bond cleavage leads to the fragmentation of the CH2–S bond with all compounds 1a–e, further yielding the corresponding 1,4-diketone (3) as products. Contrastingly, with phenacylselenocyanates (2), two different reductive cleavages occur involving the breaking of both the CH2–Se and Se–CN bonds. Several products are obtained, all coming from nucleophilic attack at the α (phenacyl) carbon or the selenium atom.

Mechanistic Insight into the Cu-Catalyzed C–S Cross-Coupling of Thioacetate with Aryl Halides: A Joint Experimental–Computational Study

The mechanism of the Ullmann-type reaction between potassium thioacetate (KSAc) and iodobenzene (PhI) catalyzed by CuI associated with 1,10-phenanthroline (phen) as a ligand was explored experimentally and computationally. The study on C-S bond formation was investigated by UV-visible spectrophotometry, cyclic voltammetry, mass spectrometry, and products assessment from radical probes. The results indicate that under experimental conditions the catalytically active species is [Cu(phen)(SAc)] regardless of the copper source. An examination of the aryl halide activation mechanism using radical probes was undertaken. No evidence of the presence of radical species was found during the reaction process, which is consistent with an oxidative addition cross-coupling pathway. The different reaction pathways leading to the experimentally observed reaction products were studied by DFT calculation. The oxidative addition-reductive elimination mechanism via an unstable CuIII intermediate is energetically more feasible than other possible mechanisms such as single electron transfer, halogen atom transfer, and σ-bond methatesis.

Photochemical and thermal stability of some dihydroxyacetophenones used as UV-MALDI-MS matrices.

2,4‐, 2,5‐, 2,6‐ and 3,5‐dihydroxyacetophenone (DHA) used as matrices in matrix‐assisted ultraviolet laser desorption/ionization mass spectrometry (UV‐MALDI‐MS) were studied by steady‐state and transient absorption spectroscopy, together with DFT calculations at the B3LYP level of theory. All compounds have low fluorescence quantum yields, possibly due to an efficient excited‐state intramolecular proton transfer (ESIPT). Laser flash photolysis (LFP) results showed that, only for 2,4‐DHA, a phototautomer could be detected at λ = 400 nm. Their photochemical stability in solution at different wavelengths and conditions was analyzed by UV–Vis and 1H nuclear magnetic resonance spectroscopy (1H‐NMR), together with thin layer chromatography and ultraviolet laser desorption/ionization mass spectrometry (UV‐LDI‐MS). Only 3,5‐DHA showed decomposition when irradiated, probably because phototautomerization is not possible. Thermal stability studies of these compounds in solid state were also conducted.

Exploring the Photophysical and Photochemical Properties of N-(Thioalkyl)-saccharins as an Alternative Route to the Synthesis of Tricyclic Sultams

Photocyclization of N-(thioalkyl)-saccharin was carried out to obtain different polycyclic sultams in good yields. These photoreactions were efficient under inert atmosphere and acetone triplet-sensitized conditions indicating that the triplet excited state is directly involved in the formation of annulated products. The presence of molecular oxygen changes product distribution, and only photo-oxygenation products (sulfoxides and sulfones) were found. This result is especially valuable since, by simple changing from nitrogen- to oxygen-saturated solvent conditions, the reaction outcome can be tuned from cyclized to sulfur oxidation products. Additionally, steady-state photolysis, electrochemistry, and laser time-resolved spectroscopic studies confirmed that these reactions mainly proceeded by intramolecular electron transfer (ET) between the triplet excited saccharin moiety and sulfur atom.

Photoinduced One-Electron Oxidation of Aromatic Selenides: Effect of the Structure on the Reversible Dimerization Reaction

The photochemical one-electron oxidation of alkyl aryl selenides was studied by means of laser flash photolysis (355 nm). Quenching of the sensitizers in their excited state leads to selenide radical cation in the presence of selenium derivatives. The π-type dimer of methyl phenyl selenide radical cation was detected at 630 nm at expenses of the monomeric radical cation (530 nm). The effect of modification of the aryl and alkyl substituents was also studied, resulting that the formation of dimers depends on both, the electronic properties and steric hindrance of the substituents. Both effects, an increase in steric hindrance in the alkyl moiety or the presence of strongly electron donor groups in the aromatic substituent that stabilizes the radical cation, prevent the dimer formation.

A straightforward and sustainable synthesis of 1,4-disubstituted 1,2,3-triazoles via visible-light-promoted copper-catalyzed azide–alkyne cycloaddition (CuAAC)

A simple and green synthesis of 1,4-disubstituted 1,2,3-triazoles through the effective reduction of copper(II) assisted by organic dyes and promoted by visible light was developed. This reaction was performed under very mild conditions, using water as solvent, under a non-inert atmosphere, with low catalyst precursor loading, and in the absence of any additive. Copper and solvent recycling was successfully achieved at least three times without loss of efficiency. In addition, a safer one-step one-pot procedure was designed with very good yield.

Unexpected formation of 4,4-dimethyl-1,2-disubstituted-dicarbonyl cyclopentanes from ketone enolate anions and 1,3-diiodo-2,2-dimethylpropane

Cyclic 1,4-dicarbonyl compounds can be easily obtained by mixing a solution of aryl methyl or alkyl methyl ketone enolate anions with 1,3-diiodo-2,2-dimethylpropane in DMSO. This represents the first example of dimerization of ketone enolate anions using a simple diiodoalkane as a reagent followed by subsequent double alkylation with bis-iodide yielding a cyclopentane adduct. This methodology allows the use of a simple potassium tert-butoxide as a base at room temperature for the formation of three C–C bonds resulting in a relatively complex five-membered ring diketone structure under transition-metal-free conditions.

Daño mecánico en semillas de dos cultivares de poroto (Phaseolus vulgaris L.), impacto sobre la fisiología de la germinación

La calidad de la semilla decae como consecuencia del deterioro. Se analizo el impacto del dano inducido en semillas de poroto de los cultivares Perla INTA (blanco) y NAG 12 INTA (negro) sobre la fisiologia de su germinacion. Para ello, muestras de cada cultivar fueron deterioradas dejandolas caer sobre un plato metalico cero, dos y cuatro veces. Posteriormente, se evaluaron porcentajes de germinacion, conductividad electrica, parametros y tasas de crecimiento de plantulas; y la actividad de la a-amilasa en cotiledones y embriones durante los primeros tres dias de iniciada la imbibicion de la semilla. Si bien en poroto blanco y negro los porcentajes de germinacion no mostraron diferencias significativas respecto del testigo, solo el tratamiento con dos golpes en poroto blanco mostro incrementos significativos (p ≤ 0,05) en la conductividad electrica. En cuanto al crecimiento de plantulas, en poroto negro con cuatro golpes se observaron caidas significativas en la longitud y el peso fresco del vastago. Respecto de la valoracion enzimatica se evidenciaron mermas en la actividad de la a-amilasa asociadas a incrementos en los niveles de dano de la semilla, observandose que mientras en poroto blanco el deterioro promueve variaciones a nivel de cotiledones, en poroto negro el area critica de esas modificaciones le corresponderia al embrion. Los resultados indican que el dano mecanico promueve modificaciones diferenciales en la fisiologia de la germinacion de poroto blanco y de poroto negro y segun el nivel de deterioro que presente la semilla.