Laura Estévez

Facile oxidative addition of aryl iodides to gold(I) by ligand design: bending turns on reactivity.

Thanks to rational ligand design, the first gold(I) complexes to undergo oxidative addition of aryl iodides were discovered. The reaction proceeds under mild conditions and is general. The ensuing aryl gold(III) complexes have been characterized by spectroscopic and crystallographic means. DFT calculations indicate that the bending induced by the diphosphine ligand plays a key role in this process.

Activation of Aryl Halides at Gold(I): Practical Synthesis of (P,C) Cyclometalated Gold(III) Complexes

Taking advantage of phosphine chelation, direct evidence for oxidative addition of Csp(2)-X bonds (X = I, Br) to a single gold atom is reported. NMR studies and DFT calculations provide insight into this unprecedented transformation, which gives straightforward access to stable (P,C) cyclometalated gold(III) complexes.

Oxidative Addition of Carbon–Carbon Bonds to Gold

The oxidative addition of strained CC bonds (biphenylene, benzocyclobutenone) to DPCb (diphosphino-carborane) gold(I) complexes is reported. The resulting cationic organogold(III) complexes have been isolated and fully characterized. Experimental conditions can be adjusted to obtain selectively acyl gold(III) complexes resulting from oxidative addition of either the C(aryl)C(O) or C(alkyl)C(O) bond of benzocyclobutenone. DFT calculations provide mechanistic insight into this unprecedented transformation.

Enhanced π‐Backdonation from Gold(I): Isolation of Original Carbonyl and Carbene Complexes

The specific electronic properties of bent o-carborane diphosphine gold(I) fragments were exploited to obtain the first classical carbonyl complex of gold [(DPCb)AuCO](+) (ν(CO)=2143 cm(-1) ) and the diphenylcarbene complex [(DPCb)Au(CPh2 )](+) , which is stabilized by the gold fragment rather than the carbene substituents. These two complexes were characterized by spectroscopic and crystallographic means. The [(DPCb)Au](+) fragment plays a major role in their stability, as substantiated by DFT calculations. The bending induced by the diphosphine ligand substantially enhances π-backdonation and thereby allows the isolation of carbonyl and carbene complexes featuring significant π-bond character.

A Computational Study on the Acidity Dependence of Radical-Scavenging Mechanisms of Anthocyanidins

On the basis of quantum chemical calculations, the radical-scavenging property attributed to anthocyanidins was analyzed considering three mechanisms: hydrogen atom transfer (HAT), stepwise electron-transfer-proton-transfer (ET-PT), and sequential proton loss electron transfer (SPLET). We found that the activity of anthocyanidins and the mechanism through which they react are pH-dependent, because the diverse colorful forms in which anthocyanidins may exist in prototropic equilibria (cationic, neutral, anionic) are susceptible to experience each of the mechanisms proposed. According to redox parameters calculated, we can conclude that HAT is always the most favored of the generally accepted mechanisms to scavenge reactive oxygen species (ROS) by the three colored forms. Nevertheless, only neutral and anionic forms are found to be able to scavenge 1,1-diphenyl-2-picrylhydrazyl (DPPH.) radical through HAT and SPLET mechanisms from a thermodynamical point of view, whereas ET-PT is only feasible for anions. Sequential proton loss hydrogen atom transfer (SPL-HAT) is proposed as the only pathway for the reaction between anthocyanidin cations and the DPPH. radical. It should be viable according to our quantum mechanical calculations and even competitive with typical HAT, ET-PT, and SPLET.

Computational Study on the Stacking Interaction in Catechol Complexes

The stability and electron density topology of catechol complexes (dimers and tetramer) were studied using the MPW1B95 functional. The QTAIM analysis shows that both dimers (face to face and C-H/pi one) display a different electronic origin. The formation of the former is accompanied by a significant change in the values of atomic electron dipole and quadrupole components, flattening the most diffuse part of the electron density distribution toward the molecular plane. A small electron population transfer is observed between catechol monomers connected by C-H/pi interactions, whose QTAIM characterization does not differ from that of a weak hydrogen bond. Cooperative effects in the tetramer on binding energies are small and negligible for bond properties and charge transfer. Nevertheless, they are significant on atomic electron populations.

β-Hydride Elimination at Low-Coordinate Gold(III) Centers

This Article reports the first comprehensive study of β-hydride elimination at gold(III). The stability/fate of gold(III) alkyl species have been investigated experimentally and computationally. A series of well-defined cationic cyclometalated gold(III) alkyl complexes [(P,C)gold(III)(R)][NTf2] [(P,C) = 8-diisopropylphosphino-naphthyl; R = Me, nPr, nBu] have been synthesized and spectroscopically characterized. While the cationic gold(III) methyl derivative 3c is stable for days at room temperature, the gold(III) n-propyl and n-butyl complexes 3a,b readily undergo β-hydride elimination at low temperature to generate propylene and 2-butenes, respectively. The formation of internal olefins from the gold(III) n-butyl complex 3b shows that olefin isomerization takes place after β-hydride elimination. Computational studies indicate that this isomerization proceeds through a chain-walking mechanism involving a highly reactive gold(III) hydride intermediate and a sequence of β-hydride elimination/reinsertion into the Au-H bond. The reaction of the cationic gold(III) methyl complex 3c with ethylene was also explored. According to (1)H and (13)C NMR spectroscopy, a mixture of propylene, 1-butene, and 2-butenes is formed. DFT calculations provide detailed mechanistic insights and support the occurrence of migratory insertion of ethylene, β-hydride elimination, and olefin exchange at gold(III).

Influence of the Solvent on the Charge Distribution of Anomeric Compounds

Conformational preferences of methanediol, dimethoxymethane, methanediamine, and fluoromethanol in the presence of solvents of diverse polarity (water, acetone, and chloroform), modeled with the polarizable continuum model, were analyzed within the framework of the Quantum Theory of Atoms in Molecules. The results indicate that the hydrogens bonded to the anomeric carbon experience the largest reorganization of electron density upon conformational change, as was obtained from previous calculations in the gas phase. When the water solvation is simulated by explicit inclusion of water molecules, the electron density reorganization involved in the cluster formation is significantly different for each conformer of methanediol. As a consequence, similar depletions of electron population are displayed by the hydrogens of hydroxyl and methylene groups in the cluster obtained for the most stable conformer of methanediol, with regard to that built for the completely antiperiplanar conformer.

Rational development of catalytic Au(I)/Au(III) arylation involving mild oxidative addition of aryl halides

The reluctance of gold to achieve oxidative addition reaction is considered as an intrinsic limitation for the development of gold-catalyzed cross-coupling reactions with simple and ubiquitous aryl halide electrophiles. Here, we report the rational construction of a Au(I)/Au(III) catalytic cycle involving a sequence of Csp2–X oxidative addition, Csp2–H auration and reductive elimination, allowing a gold-catalyzed direct arylation of arenes with aryl halides. Key to this discovery is the use of Me-Dalphos, a simple ancillary (P,N) ligand, that allows the bottleneck oxidative addition of aryl iodides and bromides to readily proceed under mild conditions. The hemilabile character of the amino group plays a crucial role in this transformation, as substantiated by density functional theory calculations.Catalysis involving Au(I)/Au(III) cycles are notoriously hampered by the reluctance of Au(I) towards oxidative addition. Here, the authors show that an hemilabile bidentate ligand promotes oxidative addition of aryl halides to Au(I) and the catalytic formation of biaryl coupling products.

Combined DFT and experimental studies of C–C and C–X elimination reactions promoted by a chelating phosphine–alkene ligand: the key role of penta-coordinate PdII

A combined computational and experimental study of the coordination chemistry of phosphine-alkene ligand L1 (N-diphenylphosphino-7-aza-benzobicyclo[2.2.1]hept-2-ene) with Pd(0) and Pd(II) is presented. Experimentally it is established that ligand L1 promotes direct alkyl-alkyl and indirect alkyl-halide reductive elimination from Pd(II) species, affording the palladium(0) complex [Pd(κ(2)-P,C-L1)2] (2) in each case. The effectiveness of L1 in promoting these reactions is attributed to the initial formation of a penta-coordinate intermediate [PdMe(X)(κ(1)-P-L1)(κ(2)-P,C-L1)] (X = Me, Cl) coupled with the ease with which it transforms to 2. From computation, a lower activation barrier for C(sp(3))-C(sp(3)) coupling and subsequent elimination has been computed for a stepwise associative pathway involving the initial formation of [PdMe2(κ(1)-P-L1)(κ(2)-P,C-L1)], compared to that computed for direct elimination from its parent, cis-[PdMe2(κ(2)-P,C-L1)]. Moreover, the C(sp(3))-C(sp(3)) coupling reaction has been found to be primarily under thermodynamic control. It has also been demonstrated computationally that the methyl group of penta-coordinate [PdCl(Me)(κ(1)-P-L1)(κ(2)-P,C-L1)] is susceptible to nucleophilic attack by the phosphorus lone pair of a further equivalent of ligand L1, which proceeds through an SN2-like transition state. This initiates an unusual, indirect intermolecular reductive elimination process, resulting in the formation of equimolar quantities of the methyl phosphonium chloride salt of L1 and complex 2, in agreement with experimental observations. In contrast to the C(sp(3))-C(sp(3)) coupling, computation shows that this indirect C(sp(3))-Cl reductive elimination process is essentially under kinetic control.