Tuning the reduction potentials of benzoquinone through the coordination to Lewis acids

Electron transfer promoted by the coordination of a substrate molecule to a Lewis acid or hydrogen bonding group is a critical step in many biological and catalytic transformations. This computational study investigates the nature of the interaction between benzoquinone and one and two Lewis acids by examining the influence of Lewis acid strength on the ability to alter the two reduction potentials of the coordinated benzoquinone molecule. To investigate this interaction, the coordination of the neutral (Q), singly reduced ([Q]˙-), and doubly reduced benzoquinone ([Q]2-) molecule to eight Lewis acids was analyzed. Coordination of benzoquinone to a Lewis acid became more favorable by 25 kcal mol-1 with each reduction of the benzoquinone fragment. Coordination of benzoquinone to a Lewis acid also shifted each of the reduction potentials of the coordinated benzoquinone anodically by 0.50 to 1.5 V, depending on the strength of the Lewis acid, with stronger Lewis acids exhibiting a larger effect on the reduction potential. Coordination of a second Lewis acid further altered each of the reduction potentials by an additional 0.70 to 1.6 V. Replacing one of the Lewis acids with a proton resulted in the ability to modify the pKa of the protonated Lewis acid-Q/[Q]˙-/[Q]2- adducts by about 10 pKa units, in addition to being able to alter the ability to transfer a hydrogen atom by 10 kcal mol-1, and the capacity to transfer a hydride by about 30 kcal mol-1.

London Dispersion Interactions Rather than Steric Hindrance Determine the Enantioselectivity of the Corey-Bakshi-Shibata Reduction

The well-known Corey-Bakshi-Shibata (CBS) reduction is a powerful method for the asymmetric synthesis of alcohols from prochiral ketones, often featuring high yields and excellent selectivities. While steric repulsion has been regarded as the key director of the observed high enantioselectivity for many years, we show that London dispersion (LD) interactions are at least as important for enantiodiscrimination. We exemplify this through a combination of detailed computational and experimental studies for a series of modified CBS catalysts equipped with dispersion energy donors (DEDs) in the catalysts and the substrates. Our results demonstrate that attractive LD interactions between the catalyst and the substrate, rather than steric repulsion, determine the selectivity. As a key outcome of our study, we were able to improve the catalyst design for some challenging CBS reductions.

Thermodynamic and kinetic hydricity of transition metal hydrides

This review of thermodynamic and kinetic hydricity provides conceptual overviews, tutorials on how to determine hydricity both experimentally and computationally, and salient case studies.

Electrostatic polarization of nonpolar substrates: a study of interactions between simple cations and Mo-bound N2

Although a great deal of catalytic studies have focused on covalent interactions between substrates and catalyst centers, recognition of the importance of noncovalent and ionic interactions is driving new approaches to catalyst design. Electrostatic interactions with simple cations (those with little covalency, such as alkali metals) play crucial roles in many catalytic processes, but these effects are challenging to study due to their complicated solvation and speciation behaviour. These effects are particularly difficult to study during cation-mediated reactions with weakly-polar or non-polar substrates. Dinitrogen is one of the most nonpolar substrates known to be affected by electrostatic interactions in both heterogeneous and homogeneous reactions but understanding the significance of these effects requires further exploration. To examine these effects systematically, a new multidentate ligand framework bearing pendent crown ethers has been developed and incorporated into a series of Mo(0)-based dinitrogen complexes. Prepared via both reduction and ligand substitution routes, the strength and impact of cation-N₂ interactions have been studied experimentally (IR spectroscopy) and computationally. Although the smallest cation (Li⁺) has the largest impact on the ground-state heterobimetallic activation of N₂, solvation interactions are highly competitive and result in low Li⁺-(N₂)Mo binding affinities. Thus, although smaller cations can have the largest electronic impact on substrates, these interactions are also the least persistent.

Investigation of main group promoted carbon dioxide reduction

The reduction of carbon dioxide (CO₂) is of interest to the chemical industry, as many synthetic materials can be derived from CO₂. To help determine the reagents needed for the functionalization of carbon dioxide this experimental and computational study describes the reduction of CO₂ to formate and CO with hydride, electron, and proton sources in the presence of sterically bulky Lewis acids and bases. The insertion of carbon dioxide into a main group hydride, generating a main group formate, was computed to be more thermodynamically favorable for more hydridic (reducing) main group hydrides. A ten kcal/mol increase in hydricity (more reducing) of a main group hydride resulted in a 35% increase in the main group hydride's ability to insert CO₂ into the main group hydride bond. The resulting main group formate exhibited a hydricity (reducing ability) about 10% less than the respective main group hydride prior to CO₂ insertion. Coordination of a second identical Lewis acid to a main group formate complex further reduced the hydricity by about another 20%. The addition of electrons to the CO adduct of ^t Bu₃P and B(C₆F₅)₃ resulted in converting the sequestered CO₂ molecule to CO. Reduction of the CO₂ adduct of ^t Bu₃P and B(C₆F₅)₃ with both electrons and protons resulted in only proton reduction.

Clustering of HClO4 with Brønsted (H2SO4, HClO4, HNO3) and Lewis acids BX3 (X = H, F, Cl, Br, OH): a DFT study

Interaction of HClO₄ with Lewis and Brønsted acids leads to a variety of clusters exhibiting a wide range of acidity.