RESUMEN
The Ni-catalyzed cross-coupling of aryl ethers is a powerful synthetic tool to transform widely available phenol derivatives into functionalized aromatic molecules. Recent theoretical and experimental mechanistic studies have identified the involvement of heterobimetallic nickelates as key intermediates that facilitate the challenging transformation under mild conditions and often without the need for external ligands or additives. In this work, based on calculations performed at the density functional theory (DFT) level and by comparison with spectroscopic and kinetic data, we investigate the mechanism of the Ni(COD)2-catalyzed cross-coupling of 2-methoxynaphthalene with PhLi and assess the speciation of lithium nickelate intermediates. The crucial role of solvent on the reaction is explained, and the multiple roles played by lithium are unveiled. Experimental studies have identified key lithium nickelate species which support and help to evolve the calculated reaction mechanism and ultimately complete the catalytic cycle. Based on this new mechanistic knowledge, a well-known experimental challenge of these transformations, the so-called "naphthalene problem" which restricts the use of electrophilic coupling partners to π-extended systems, can be addressed to enable the cross-coupling of unbiased aryl ethers under mild conditions.
RESUMEN
Halide salts facilitate the oxidative addition of organic halides to Pd(0). This phenomenon originates from a combination of anionic, cationic, and Pd-Pd cooperative effects. Exhaustive computational exploration at the density functional theory level of the complexes obtained from [Pd0(PPh3)2] and a salt (NMe4Cl or LiCl) showed that chlorides promote phosphine release, leading to a mixture of mononuclear and dinuclear Pd(0) complexes. Anionic Pd(0) dinuclear complexes exhibit a cooperativity between Pd(0) centers, which favors the oxidative addition of iodobenzene. The higher activity of Pd(0) dimers toward oxidative addition rationalizes the previously reported kinetic laws. In the presence of Li+, the oxidative addition to mononuclear [Pd0L(Li2Cl2)] is estimated barrierless. LiCl coordination polarizes Pd(0), enlarging both the electrophilicity and the nucleophilicity of the complex, which promotes both coordination of the substrate and the subsequent insertion into the C-I bond. These conclusions are paving the way to the rational use of the salt effects in catalysis for the activation of more challenging bonds.
RESUMEN
The Au-catalyzed [2 + 2 + 2] cycloadditions of ynamides with two discrete nitriles were theoretically studied with the aid of DFT calculations. The reaction under consideration is found to start from binding of the catalyst with the ynamide rather than with the nitrile. The Au(i)-ynamide species can effectively induce dimerization of two nitrile molecules while the catalyst only cannot. The Au(i)-ynamide species () is revealed to be more reactive than the Au(i)-nitrile species. Also, the regioselectivity and the influence of EWG vs. EDG involved in the reaction were also rationalized.
RESUMEN
With the aid of DFT calculations, the FeBr3-catalyzed skeletal rearrangements of 2-cyclohexanal,2-p-C6H4OMe-propylaldehyde (1A) and 2-phenyl,2-p-C6H4OMe-propylaldehyde (1B) were investigated theoretically. As compared to mono-FeBr3 as a catalyst, the bis-FeBr3 serving as a catalyst is found to be not only enhancing the catalytic efficiency but also improving the product selectivity. For the reaction starting from 1A, the [1,2]-group shift (first step) is rate-determining, and why the Cy shift is the most favored is rationalized in comparison with the p-C6H4OMe and Me shifts. For the reaction starting from 1B, the [1,2]-H shift (second step) is rate-determining although the [1,2]-p-C6H4OMe shift is favored over the [1,2]-phenyl shift. In contrast to the experimental proposal, the newly established H2O/Br(-) joint-assisted H-shift mechanism explains the partial α-H source of the [1,2]-Cy shift product. In addition, we discussed the inherent mechanism that explains why both the [1,2]-p-C6H4OMe and [1,2]-p-C6H4CF3 shifts are more facile than the [1,2]-phenyl shift although the substituents -OMe and -CF3 have opposite electronic behaviors.
