Mechanistic Study of Unprecedented Highly Regioselective Hydrocyanation of Terminal Alkynes: Insight into the Origins of the Regioselectivity and Ligand Effects.

Density functional theory (DFT) calculations were performed to gain insight into the mechanism of the nickel-catalyzed hydrocyanation of terminal alkynes with Zn(CN)2 and water to exclusively generate the branched nitrile with excellent Markovnikov selectivity. After precatalyst activation to give the LNi(0) active species, the transformation proceeds via the following steps: (1) oxidative addition of H2 O to the LNi(0) provides the intermediate LNi(II)H(OH); (2) ligand exchange of LNi(II)H(OH) with Zn(CN)2 gives the intermediate LNi(II)H(CN); (3) alkyne insertion to the LNi(II)H(CN) forms the alkenyl nickel complex, followed by the reductive elimination step reaching the final product. This mechanism is kinetically and thermodynamically more favorable than that of the experimental proposed ones. On the basis of the experimental observations, more water molecules cannot further improve the reaction as it has also been rationalized. Furthermore, the origin of the high regioselectivity of the product, the variable effectiveness of the metal mediator as function of ligands, as well as the high yield of the alkyl-substituted alkynes substrates, is analyzed in detail. © 2019 Wiley Periodicals, Inc.

Bonding Analysis of the Shortest Bond between Two Atoms Heavier than Hydrogen and Helium: O22.

Quantum chemical calculations using ab initio methods at the CCSD(T) level with large basis sets and DFT calculations using the BP86 functional have been carried out for O22+ and N2. An energy decomposition analysis of the chemical bonds suggests that the shorter bond in O22+ compared with isoelectronic N2 is due to the weaker Pauli repulsion in the dication, which overcompensates the weakening of attractive interactions that are operative in N2. At the equilibrium distance of N2, the orbital (covalent) bonding in O22+ is weaker than in N2, and the attractive Coulomb interactions in the neutral diatomic system become repulsive in the dication, but the weakening of the Pauli repulsion caused by the shrinking of the orbitals in O22+ compensates for these forces and leads to a shortening of the bond. The results also show that the bond dissociation energy is not a reliable indicator for the strength of bond, which is more faithfully given by the (local) force constant.

Isolation of Transient Acyclic Germanium(I) Radicals Stabilized by Cyclic Alkyl(amino) Carbenes.

Despite the notable progress in the stabilization of main group radicals by NHCs and cAACs, no germanium radicals have been isolated so far due to synthetic challenges. Stabilization of neutral [:EIR]• (E = Si, Ge) radicals is an uphill task, as these reactive transient species are highly susceptible to dimerization. Herein, we report the synthesis of acyclic neutral germanium(I) radicals Cy-cAAC:GeN(SiMe3)Dip (1) and Me-cAAC:GeN(SiPh3)Mes (2) obtained by the reduction of [Ar(SiR3)NGeCl3] with KC8 in the presence of cAAC. Compounds 1 and 2 are well characterized by single crystal X-ray structural analysis, cyclic voltammetry, and EPR spectroscopy. Furthermore, the structure and bonding of compounds 1 and 2 have been investigated by theoretical methods.

Aluminum alkoxy-catalyzed biomass conversion of glucose to 5-hydroxymethylfurfural: Mechanistic study of the cooperative bifunctional catalysis.

Density functional theory calculations were performed to understand the detailed reaction mechanism of aluminum alkoxy-catalyzed conversion of glucose to 5-hydroxymethylfurfural (HMF) using Al(OMe)3 as catalyst. Potential energy surfaces were studied for aggregates formed between the organic compounds and Al(OMe)3 and effects of the medium were considered via continuum solvent models. The reaction takes place via two stages: isomerization from glucose to fructose (stage I) and transformation of fructose to HMF (stage II). Stage II includes three successive dehydrations, which begins with a 1,2-elimination to form an enolate (i.e., B), continues with the formation of the acrolein moiety (i.e., D), and ends with the formation of the furan ring (i.e., HMF). All of these steps are facilitated by aluminum alkoxy catalysis. The highest barriers for stage I and stage II are 23.9 and 31.2 kcal/mol, respectively, and the overall catalytic reaction is highly exothermic. The energetic and geometric results indicate that the catalyzed reaction path has feasible kinetics and thermodynamics and is consistent with the experimental process under high temperature (i.e., 120 °C). Remarkably, the released water molecules in stage II act as the product, reactant, proton shuttle, as well as stabilizer in the conversion of fructose to HMF. The metal-ligand functionality of the Al(OMe)3 catalyst, which combines cooperative Lewis acid and Lewis base properties and thereby enables proton shuttling, plays a crucial role in the overall catalysis and is responsible for the high reactivity. © 2019 Wiley Periodicals, Inc.

Computational Insights into the Catalytic Mechanism of Bacterial Carboxylic Acid Reductase.

Multidomain carboxylic acid reductases (CARs) can reduce a wide range of carboxylic acids to the corresponding aldehydes in the presence of ATP and NADPH. Recent X-ray structures of the individual (di)domains of Segniliparus rugosus CAR (SrCAR) shed light on the catalysis mechanism and revealed domain dynamics during the different states of the reaction. However, the details of the catalytic mechanism of each step operated by the corresponding domains are still elusive. Toward this end, several models based on the crystal structures were constructed, and molecular dynamics simulations along with density functional theory (DFT) calculations were employed to elucidate the conformational dynamics and catalytic mechanism of SrCAR concealed to static crystallography. We investigated the roles of the key residues in the substrate binding pocket involved in the adenylation and thiolation reactions and especially determined the roles played by a nonconserved Lys528 residue in the thiolation step, which were further verified by site-directed mutagenesis. The reduction mechanism of SrCAR, including the natures of the transition states for hydride and proton transfer, was also explored theoretically using the DFT method B3LYP. The information presented here is useful as a guide for the future rational design of CARs.