Theoretical Mechanistic Study of Rhodium(I) Phosphine-Catalyzed H/D Exchange Processes in Aqueous Solutions

Cationic Rhodium(I)-Catalyzed Asymmetric Cyclohydroformylation of 1,6-Enynes with Formaldehyde

We report a rhodium(I)-catalyzed asymmetric cyclohydroformylation reaction of 1,6-enynes with formaldehyde. The reaction of 1,6-enynes with formaldehyde in the presence of a cationic Rh(I) catalyst, such as [Rh(cod)₂ ]⁺ OTf^- , and a chiral biaryl diphosphine led to asymmetric cyclohydroformylation to produce aldehydes with higher-order structures highly enantioselectively. This transformation procedure is applicable to a variety of enynes, with wide compatibility in various atoms liking between the alkyne and alkene parts, substituents at the alkyne terminus, and substituents at the alkene part, being converted to newly formed aldehydes in 14% to 90% yields with 50% to 98% ee. The products were further transformed with various nucleophiles to alcohols, an amine, and a diene without loss of chirality at their γ-position.

Chemoselectivity in Gold(I)-Catalyzed Propargyl Ester Reactions: Insights From DFT Calculations

Au-catalyzed propargyl ester reactions have been investigated by a comprehensive density functional theory (DFT) study. Our calculations explain the experimental observed chemoselectivity of Au-catalyzed propargyl ester reactions very well by considering all possible pathways both in the absence and presence of 1,2,3-triazole (TA). The “X-factor” of TA is disclosed to have triple effects on this reaction. First of all, it can stabilize and prevent rapid decomposition of the Au catalyst. Secondly, the existence of TA promotes the nucleophilic attack and alters the chemoselectivity of this reaction. Moreover, TA acts as a “relay” to promote the proton transfer.

Hydrogen and Dihydrogen Bonds in the Reactions of Metal Hydrides

The dihydrogen bond-an interaction between a transition-metal or main-group hydride (M-H) and a protic hydrogen moiety (H-X)-is arguably the most intriguing type of hydrogen bond. It was discovered in the mid-1990s and has been intensively explored since then. Herein, we collate up-to-date experimental and computational studies of the structural, energetic, and spectroscopic parameters and natures of dihydrogen-bonded complexes of the form M-H···H-X, as such species are now known for a wide variety of hydrido compounds. Being a weak interaction, dihydrogen bonding entails the lengthening of the participating bonds as well as their polarization (repolarization) as a result of electron density redistribution. Thus, the formation of a dihydrogen bond allows for the activation of both the MH and XH bonds in one step, facilitating proton transfer and preparing these bonds for further transformations. The implications of dihydrogen bonding in different stoichiometric and catalytic reactions, such as hydrogen exchange, alcoholysis and aminolysis, hydrogen evolution, hydrogenation, and dehydrogenation, are discussed.

Brønsted-Lowry Acid Strength of Metal Hydride and Dihydrogen Complexes

Transition metal hydride complexes are usually amphoteric, not only acting as hydride donors, but also as Brønsted-Lowry acids. A simple additive ligand acidity constant equation (LAC for short) allows the estimation of the acid dissociation constant Ka(LAC) of diamagnetic transition metal hydride and dihydrogen complexes. It is remarkably successful in systematizing diverse reports of over 450 reactions of acids with metal complexes and bases with metal hydrides and dihydrogen complexes, including catalytic cycles where these reactions are proposed or observed. There are links between pKa(LAC) and pKa(THF), pKa(DCM), pKa(MeCN) for neutral and cationic acids. For the groups from chromium to nickel, tables are provided that order the acidity of metal hydride and dihydrogen complexes from most acidic (pKa(LAC) -18) to least acidic (pKa(LAC) 50). Figures are constructed showing metal acids above the solvent pKa scales and organic acids below to summarize a large amount of information. Acid-base features are analyzed for catalysts from chromium to gold for ionic hydrogenations, bifunctional catalysts for hydrogen oxidation and evolution electrocatalysis, H/D exchange, olefin hydrogenation and isomerization, hydrogenation of ketones, aldehydes, imines, and carbon dioxide, hydrogenases and their model complexes, and palladium catalysts with hydride intermediates.

Formic acid dehydrogenation with bioinspired iridium complexes: a kinetic isotope effect study and mechanistic insight

Highly efficient hydrogen generation from dehydrogenation of formic acid is achieved by using bioinspired iridium complexes that have hydroxyl groups at the ortho positions of the bipyridine or bipyrimidine ligand (i.e., OH in the second coordination sphere of the metal center). In particular, [Ir(Cp*)(TH4BPM)(H2 O)]SO4 (TH4BPM: 2,2',6,6'-tetrahydroxyl-4,4'-bipyrimidine; Cp*: pentamethylcyclopentadienyl) has a high turnover frequency of 39 500 h(-1) at 80 °C in a 1 M aqueous solution of HCO2 H/HCO2 Na and produces hydrogen and carbon dioxide without carbon monoxide contamination. The deuterium kinetic isotope effect study clearly indicates a different rate-determining step for complexes with hydroxyl groups at different positions of the ligands. The rate-limiting step is β-hydrogen elimination from the iridium-formate intermediate for complexes with hydroxyl groups at ortho positions, owing to a proton relay (i.e., pendent-base effect), which lowers the energy barrier of hydrogen generation. In contrast, the reaction of iridium hydride with a proton to liberate hydrogen is demonstrated to be the rate-determining step for complexes that do not have hydroxyl groups at the ortho positions.

Synthesis, spectroscopy, and hydrogen/deuterium exchange in high-spin iron(II) hydride complexes

Very few hydride complexes are known in which the metals have a high-spin electronic configuration. We describe the characterization of several high-spin iron(II) hydride/deuteride isotopologues and their exchange reactions with one another and with H2/D2. Though the hydride/deuteride signal is not observable in NMR spectra, the choice of isotope has an influence on the chemical shifts of distant protons in the dimers through the paramagnetic isotope effect on chemical shift. This provides the first way to monitor the exchange of H and D in the bridging positions of these hydride complexes. The rate of exchange depends on the size of the supporting ligand, and this is consistent with the idea that H2/D2 exchange into the hydrides occurs through the dimeric complexes rather than through a transient monomer. The understanding of H/D exchange mechanisms in these high-spin iron hydride complexes may be relevant to postulated nitrogenase mechanisms.

Mechanism study of the intramolecular anti-Michael addition of N-alkylfurylacrylacetamides

The mechanism of the intramolecular nucleophilic addition of N-alkylfurylacrylacetamides is investigated by density functional theory calculations. Three possible reaction pathways have been considered based on possible conformations of the same reactant, which undergoes three stages, including hydrogen elimination by the base NaH, followed by the nucleophilic addition of N(-) on C(α) (C(β)) via an anti-Michael (Michael) mechanism, and then proton transfer affords the final product Pr-5 (Pr-6). The pathway corresponding to the reactant with the most stable conformation is found to be the most favorable one. The rate-determining step of the intramolecular nucleophilic addition is the nucleophilic addition of N(-) on C(α) (C(β)) featuring a cyclic ring transition state. Solvent effects are considered at the B3LYP/6-31G(d,p) level in solvent DMSO, and the results suggest that the relative reaction trends are consistent with the gas-phase reaction. Furthermore, the difference of the energy barriers explains the origin of the regioselectivity of the experiment. Finally, the effects of the substituent on N1 and C(β) to the regioselectivity have been discussed.

Microsolvated transition state models for improved insight into chemical properties and reaction mechanisms

Over the years, several methods have been developed to effectively represent the chemical behavior of solutes in solvents. The environmental effects arising due to solvation can generally be achieved either through inclusion of discrete solvent molecules or by inscribing into a cavity in a homogeneous and continuum dielectric medium. In both these approaches of computational origin, the perturbations on the solute induced by the surrounding solvent are at the focus of the problem. While the rigor and method of inclusion of solvent effects vary, such solvation models have found widespread applications, as evident from modern chemical literature. A hybrid method, commonly referred to as cluster-continuum model (CCM), brings together the key advantages of discrete and continuum models. In this perspective, we intend to highlight the latent potential of CCM toward obtaining accurate estimates on a number of properties as well as reactions of contemporary significance. The objective has generally been achieved by choosing illustrative examples from the literature, besides expending efforts to bring out the complementary advantages of CCM as compared to continuum or discrete solvation models. The majority of examples emanate from the prevalent applications of CCM to organic reactions, although a handful of interesting organometallic reactions have also been discussed. In addition, increasingly accurate computations of properties like pK(a) and solvation of ions obtained using the CCM protocol are also presented.

The nature of [PdCl(2)(C(2)H(4))(H(2)O)] as an active species in the Wacker process: new insights from ab initio molecular dynamics simulations

First-principles molecular dynamics coupled with metadynamics have been used to gain a deeper insight into the reaction mechanism of the Wacker process by determining the nature of the active species. An explicit and dynamic representation of the aqueous solvent, which was essential for modeling this reaction, was efficiently included into the simulations. Prompted by our earlier results, which showed that the configuration of the catalytically active species [PdCl(2)(H(2)O)(C(2)H(4))] was crucial in the subsequent steps of the Wacker process, herein we focused on the preceding equilibria that led to the formation of both the cis and trans isomers. Starting from the initial catalyst, [PdCl(4)](2-), the free-energy barriers for the forward and backward reactions were calculated. These results confirmed the relevance of the trans intermediate in the reaction mechanism, whilst conversely, they showed that the cis configuration played no role in it. This sole participation of the trans intermediate has some very important implications; besides the mechanistic interpretation of the initial steps in the Wacker reaction mechanism, the analysis of these equilibria provides additional information about the chemical nature of these ligand-substitution processes.

Nature of Cp*MoO2+ in Water and Intramolecular Proton-Transfer Mechanism by Stopped-Flow Kinetics and Density Functional Theory Calculations

A stopped-flow study of the Cp*MoO3- protonation at low pH (down to zero) in a mixed H2O-MeOH (80:20) solvent at 25 degrees C allows the simultaneous determination of the first acid dissociation constant of the oxo-dihydroxo complex, [Cp*MoO(OH)2]+ (pKa1 = -0.56), and the rate constant of its isomerization to the more stable dioxo-aqua complex, [Cp*MoO2(H2O)]+ (k-2 = 28 s-1). Variable-temperature (5-25 degrees C) and variable-pressure (10-130 MPa) kinetics studies have yielded the activation parameters for the combined protonation/isomerization process (k-2/Ka1) from Cp*MoO2(OH) to [Cp*MoO2(H2O)]+, viz., DeltaH++= 5.1 +/- 0.1 kcal mol-1, DeltaS++ = -37 +/- 1 cal mol-1 K-1, and DeltaV++ = -9.1 +/- 0.2 cm3 mol-1. Computational analysis of the two isomers, as well as the [Cp*MoO2]+ complex resulting from the dissociation of water, reveals a crucial solvent effect on both the isomerization and the water dissociation energetics. Introducing a solvent model by the conductor-like polarizable continuum model and especially by explicitly inclusion of up to three water molecules in the calculations led to the stabilization of the dioxo-aqua species relative to the oxo-dihydroxo isomer and to the substantial decrease of the energy cost for the water dissociation process. The presence of a water dissociation equilibrium is invoked to account for the unusually low effective acidity (pKa1' = 4.19) of the [Cp*MoO2(H2O)]+ ion. In addition, the computational study reveals the positive role of external water molecules as simultaneous proton donors and acceptors, having the effect of dramatically lowering the isomerization energy barrier.

Solution chemistry of a water-soluble eta2-H2 ruthenium complex: evidence for coordinated H2 acting as a hydrogen bond donor

The ability of an eta2-H2 ligand to participate in intermolecular hydrogen bonding in solution has long been an unresolved issue. Such species are proposed to be key intermediates in numerous important reactions such as the proton-transfer pathway of H2 production by hydrogenase enzymes. We present the synthesis of several new water-soluble ruthenium coordination complexes including an eta2-H2 complex that is surprisingly inert to substitution by water. The existence of dihydrogen hydrogen bonding (DHHB) was experimentally probed by monitoring the chemical shift of H-bonded Ru-(H2) complexes using NMR spectroscopy, by UV-visible spectroscopy, and by monitoring the rotational dynamics of a hydrogen-bonding probe molecule. The results provide strong evidence that coordinated H2 can indeed participate in intermolecular hydrogen bonding to bulk solvent and other H-bond acceptors.