Olalla Nieto Faza

Mechanism of the Gold-Catalyzed Rearrangement of (3-Acyloxyprop-1-ynyl)oxiranes : A Dual Role of the Catalyst

The three competing paths for the rearrangement of 1 (involving 1,2- and 1,3-ester migration with alkyne or oxirane activation) evidence the multifaceted character of gold as a catalyst. The most favorable mechanism for this useful synthetic transformation involves a cascade of more than eight steps. All the functional groups in the substrate play a crucial and synergistic role, and sequential gold coordination to both the pi-system and the lone pairs of oxygen is needed. Exploration of these three paths suggests the use of a nonalkynophilic Lewis acid (BF(3)) as a possible synthetic alternative for this transformation.

Noyori hydrogenation: aromaticity, synchronicity, and activation strain analysis.

By means of density functional theory calculations, we have computationally explored the intimacies of the crucial step of Noyori hydrogrogenation reactions of multiple bonds. This process can be considered analogous to the so-called double group transfer reactions. Both kinds of transformations proceed concertedly via the simultaneous migration of two hydrogen atoms/groups in a pericyclic [σ2s + σ2s + π2s] reaction through six-membered transition structures. Despite the structural resemblances of both types of saddle points, significant differences are found in terms of synchronicity and in-plane aromaticity. In addition, the activation strain model has been used to get quantitative insight into the factors which control the corresponding barrier heights. It is found that the presence of a heteroatom in the acceptor moiety is responsible for a remarkable increase of the interaction energy between the reactants which can compensate the destabilizing effect of the strain energy associated with the deformation of the initial reagents leading to low reaction barriers.

On the memory of chirality in gold(I)-catalyzed intramolecular carboalkoxylation of alkynes.

A computational study of the mechanism of the intramolecular carboalkoxylation of alkynes reported by Toste et al. allows the characterization of the chirality transfer process that makes this reaction enantioselective. Memory of chirality is preserved up until the stereocenter-generating iso-Nazarov cyclization through the synergy between the helicity of a pentadienyl cation intermediate and the control in the conformation of the allyl group, both elements defined upon alkoxy migration. The high barriers to conformational scrambling relative to those corresponding to chemical steps ensure the robustness of the chirality transfer mechanism and result in an unusual importance of conformational changes in reactivity that seems to be common in gold-catalyzed transformations.

Hg2+ Detection by New Anthracene Pendant-Arm Derivatives of Mixed N/S- and N/S/O-Donor Macrocycles: Fluorescence, Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry and Density Functional Theory Studies

The optical response of four new anthracenylmethyl pendant-arm derivatives (L1-L4) of the macrocyclic ligands [12]aneNS(3), [12]aneNS(2)O, [15]aneNS(2)O(2), and [12]aneN(2)SO toward the metal ions Zn(2+), Cd(2+), Pb(2+), Cu(2+), Hg(2+), Ag(+), Fe(2+), Co(2+), Ni(2+), Mn(2+), Ca(2+), Na(+), Mg(2+), and K(+) was investigated in 1:1 (v/v) MeCN/H(2)O solutions. A strong chelation enhancement of quenching effect was observed on the fluorescent emission intensity of L2 as a consequence of the host-guest interaction with Hg(2+) and the formation of a 1:2 metal-to-ligand complex. Density functional theory calculations confirmed the formation of a sandwich-type complex between L2 and Hg(2+) as a favorable process. A matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectrometry study using the four ligands as active MALDI probes was also performed. L1-L4 have also been explored as fluorescence chemosensors in microsamples using NANODROP technology.

Allenyl Azide Cycloaddition Chemistry. Photochemical Initiation and CuI Mediation Leads to Improved Regioselectivity

Irradiation of 2-(3-alkenyl)allenylphenyl azides in the presence of excess CuI furnished functionalized 2,3-cyclopentenylindoles in good yield with only trace amounts of competitive C-N-bonded regioisomeric products. These results represent a significant departure from the modest-to-nonexistent regioselectivity that attended thermal cyclization of these allenyl azide substrates.

Allenyl Azide Cycloaddition Chemistry. 2,3-Cyclopentennelated Indole Synthesis through Indolidene Intermediates

The thermal, photochemical, and photochemical/CuI-mediated cascade cyclizations of a range of substituted 1-(2-azidophenyl)-3-alkenylallenes are described. These reactions provide both 1,2- and 2,3-cyclopentennelated indole products in varying ratios. In most cases, high regioselectivity for the 2,3-annelated isomer can be achieved under the hnu/CuI conditions. Computational studies of this multistep reaction support the intermediacy of indolidene intermediates whose electrocyclizations (with or without copper present) define the regioselectivity branch point in the sequence.

Normal-to-Abnormal NHC Rearrangement of Al(III) , Ga(III) , and In(III) Trialkyl Complexes: Scope, Mechanism, Reactivity Studies, and H2 Activation.

The present contribution reports experimental and theoretical mechanistic investigations on a normal-to-abnormal (C2-to-C4-bonded) NHC rearrangement processes occurring with bulky group 13 metal NHC adducts, including the scope of such a reactivity for Al compounds. The sterically congested adducts (nItBu)MMe3 (nItBu=1,3-di-tert-butylimidazol-2-ylidene; M=Al, Ga, In; 1 a-c) readily rearrange to quantitatively afford the corresponding C4-bonded complexes (aItBu)MMe3 (4 a-c), a reaction that may be promoted by THF. Thorough experimental data and DFT calculations were performed on the nNHC-to-aNHC process converting the Al-nNHC (1 a) to its aNHC analogue 4 a. A nItBu/aItBu isomerization is proposed to account for the formation of the thermodynamic product 4 a through reaction of transient aItBu with THF-AlMe3 . The reaction of benzophenone with (nItBu)AlMe3 afforded the zwitterionic species (aItBu)(CPh2 -O-AlMe3 ) (6), reflecting the unusual reactivity that such bulky adducts may display. Interestingly, the nItBu/Al(iBu)3 Lewis pair behaves like a frustrated Lewis pair (FLP) since it readily reacts with H2 under mild conditions. This may open the way to future reactivity developments involving commonly used trialkylaluminum precursors.

Computation of vertical excitation energies of retinal and analogs: Scope and limitations

A comprehensive survey of computational methods: semiempirical (ZINDO/S), Time‐Dependent Hartree–Fock (TD‐HF), Configuration Interaction Singles (CIS), and several approximate functionals within the Time‐Dependent Density Functional Theory (TD‐DFT) has been carried out for the description of vertical excitation energies and oscillator strengths of retinal and related polyenals. ZINDO and TD‐DFT computations showed the best agreement with the experimental data. In particular, hybrid functionals including approximately 25% of exact exchange (B3LYP, B3P86, and PBE0) were found to perform best with these highly conjugated polyenes. A systematic average error of 0.18–0.22 eV has been found after a simple one‐parameter correction. Thus, 0.18 eV might be considered the upper limit of accuracy for current one‐determinant methods in the computation of vertical excitation energies. The consideration of adiabatic excitations, conformational sampling, solvation, and nondynamic correlation should describe this processes more accurately, but this leads to highly demanding methods beyond feasibility for these large polyenes. The trends observed, particularly the good performance of the ZINDO/S method, should pave the way for the prediction of excited states properties in natural and artificial photoreceptor proteins, thus advancing towards the description of their light‐transducing biological role in Nature.

Theoretical and experimental exploration of the photochemistry of resveratrol: beyond the simple double bond isomerization

The photochemical isomerization of resveratrol has been the subject of recent studies in which contradictory results were reported. The photoproduct mixture of this reaction needs to be considered more complex than the coexistence of cis and trans isomers. An unidentified third product, at least, has been detected in various studies although its nature was unknown. In this work, we aim to provide a thorough description of the photochemical course of this reaction through experimental and computational approaches working in a synergetic association.

Allenyl Azide Cycloaddition Chemistry: Exploration of the Scope and Mechanism of Cyclopentennelated Dihydropyrrole Synthesis through Azatrimethylenemethane Intermediates

Detailed studies of the thermal conversion of 1-azidohepta-3,4,6-trienes into cyclopentennelated dihydropyrroles are presented. High levels of diastereoselectivity and regioselectivity are documented. A mechanistic proposal that accounts for all of the diverse results is developed through the use of density functional calculations. These calculations provide support for the intervention of unexpected mechanistic subtleties, such as the planarity of an azatrimethylenemethane diyl intermediate and an apparent Woodward-Hoffmann-type electrocyclization of a five-atom diyl array.