Jonas Wuyts

Many chemical investigations are supported by routine calculations of molecular structures, reaction energies, barrier heights, and spectroscopic properties. Most of these quantum-chemical calculations apply various combinations of DFT-based methods. This Scientific Perspective provides best-practice protocols and guidance in the choice of robust method combinations to deal with many day-to-day challenges in computational chemistry and discusses representative examples.

Abstract

Nowadays, many chemical investigations are supported by routine calculations of molecular structures, reaction energies, barrier heights, and spectroscopic properties. The lion's share of these quantum-chemical calculations applies density functional theory (DFT) evaluated in atomic-orbital basis sets. This work provides best-practice guidance on the numerous methodological and technical aspects of DFT calculations in three parts: Firstly, we set the stage and introduce a step-by-step decision tree to choose a computational protocol that models the experiment as closely as possible. Secondly, we present a recommendation matrix to guide the choice of functional and basis set depending on the task at hand. A particular focus is on achieving an optimal balance between accuracy, robustness, and efficiency through multi-level approaches. Finally, we discuss selected representative examples to illustrate the recommended protocols and the effect of methodological choices.

The Cover Feature artistically shows that when 5 wt% acid water is added to a conventional ionic liquid, such as 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]), with a simple pipette, water microdroplets of ∼1 μm spontaneously form at room temperature. In their Research Article, A. Leyva-Pérez and co-workers describe that the same effect occurs when basic water is added, and also when both acidic and basic water are added sequentially. The microdroplets form without mixing. The microstructured acid/base liquid system performs as a catalyst/solvent system for one-pot organic reactions, such as the rearrangement of stilbene oxide, one-pot ketal hydrolysis/Knoevenagel condensation, or Pd/Cu-catalysed one-pot Sonogashira coupling/silane deprotection/Au-catalysed alkyne hydration. This extremely simple acid/base system is a rare example of co-existing simple acids (i.e., HCl) and bases (i.e., KOH). More information can be found in the Research Article by A. Leyva-Pérez and co-workers.

Single-atom photocatalysts (SACs) are emerging as promising platforms for the development of novel organic transformations. In this context, carbon nitrides (CNs) are ideal photoactive supports capable of effectively stabilizing the single metal sites. In this Concept, the recent advances in the synthesis of CN-based SACs and their applications in synthesis are discussed.

Abstract

In recent years, the field of dual photocatalysis has become an increasingly popular tool for the functionalization of organic substrates under mild operative conditions. Single-atom heterogeneous catalysts (SACs), where the metal atoms are stabilized by means of properly structured photoactive supports, are currently one of the frontiers of this research field. To this end, Carbon Nitrides (CNs) have emerged as ideal two-dimensional semiconducting supports, capable of stabilizing single metal sites (for instance: nickel, iron, among other) through nitrogen-rich structures. This Concept highlights the recent advances in the synthesis of carbon nitride-based SACs and their applications in light-driven dual-catalytic processes, also providing forward-looking opportunities within this research area.

Lignin valorization: Technical lignin is a large-volume and low-cost industrial resource for making renewable aromatic chemicals. Oxidative depolymerization with heterogeneous catalysts allows viable upgrading to value-added aromatic monomers and more sustainable biorefineries. This Review summarizes and discusses performance–reactivity patterns and product analytics and recovery for the efficient oxidative valorization of technical lignin.

Abstract

The efficient valorization of lignin is crucial if we are to replace current petroleum-based feedstock and establish more sustainable and competitive lignocellulosic biorefineries. Pulp and paper mills and second-generation biorefineries produce large quantities of low-value technical lignin as a by-product, which is often combusted on-site for energy recovery. This Review focuses on the conversion of technical lignins by oxidative depolymerization employing heterogeneous catalysts. It scrutinizes the current literature describing the use of various heterogeneous catalysts in the oxidative depolymerization of lignin and includes a comparison of the methods, catalyst loadings, reaction media, and types of catalyst applied, as well as the reaction products and yields. Furthermore, current techniques for the determination of product yields and product recovery are discussed. Finally, challenges and suggestions for future approaches are outlined.

The new metal-free cross-coupling of carboxylic acids and sulfinates provides a direct and modular access to sulfoxides by a photocatalytic process that has unveiled the unexplored chemoselective radicalophilic reactivity of intermediate sulfinyl sulfones and enabled the conjunctive coupling of two carboxylic acids, revealing broad new chemical space accessible by the merger of the two coupling partner classes of emergent synthetic importance.

Abstract

The intermediate oxidation state of sulfoxides is central to the plethora of their applications in chemistry and medicine, yet it presents challenges for an efficient synthetic access, limiting the structural diversity of currently available sulfoxides. Here, we report a data-guided development of direct decarboxylative sulfinylation that enables the previously inaccessible functional group interconversion of carboxylic acids to sulfoxides in a reaction with sulfinates. Given the broad availability of carboxylic acids and the growing synthetic potential of sulfinates, the direct decarboxylative sulfinylation is poised to improve the structural diversity of synthetically accessible sulfoxides. The reaction is facilitated by a kinetically favored sulfoxide formation from the intermediate sulfinyl sulfones, despite the strong thermodynamic preference for the sulfone formation, unveiling the previously unknown and chemoselective radicalophilic sulfinyl sulfone reactivity.

Nature Chemistry, Published online: 26 August 2022; doi:10.1038/s41557-022-01028-6

Complete utilization of lignin: the methoxy groups in lignin react with carboxylic acids to generate methyl carboxylates (applied to the synthesis of pharmaceuticals), and the remaining carbon atoms of lignin react with oxygen to form CO predominantly, which may be used directly in carbonylation reactions.

Abstract

Lignin is an abundant renewable carbon source. Due to its complex structure, utilization of lignin is very challenging. Herein, we describe an efficient strategy for the simultaneous utilization of lignin, in which the methoxy groups in lignin react with carboxylic acids to generate methyl carboxylates and the other alkyl and phenyl carbons react with oxygen to predominantly form CO that can be used directly in carbonylation reactions. The method was applied to the methylation of various functionalized aryl and alkyl carboxylic acids, including natural compounds, to produce valuable chemicals, including pharmaceuticals. No solid or liquid residues remain after the reaction. Mechanistic studies demonstrate that a well-ordered C−C and C−O bond activation sequence takes place to realize total transformation of lignin. This work opens a way for transformation of the entire lignin polymer into valuable products, exemplified by the synthesis of the pharmaceutical, Ramipril, on a gram scale.

Size effects: Crystalline porous MoVBiO was tuned in the range of 30–300 nm using organic additives. Methanol could decrease the crystal size to 30 nm while tartaric acid increased the size to 300 nm. The MoVBiO with small size showed higher methyl nitrite conversion and methyl formate selectivity than that with large size, as the result of more accessible surface active sites.

Abstract

Crystalline porous Mo-V-Bi oxides (MoVBiO) were prepared by hydrothermal synthesis approach. The effects of organic additives (methanol and tartaric acid) on the morphology and particle size of the MoVBiO crystals were investigated. The size of MoVBiO crystals was tuned in the range of 30–300 nm. It was found that methanol accelerated the formation of crystal nuclei resulting in the crystallization of small particles (30 nm), while tartaric acid inhibited the nucleation thus resulting in MoVBiO crystals with a size of 300 nm. The as-synthesized MoVBiO samples were used in the indirect oxidation of methanol to methyl formate. The MoVBiO crystals with a size of 30 nm, synthesized with methanol, showed a high methyl nitrite conversion of 92 % and methyl formate selectivity of 94 % at 140 °C. The catalyst exhibited a relatively slow but almost constant deactivation over 50 h due to deposition of reaction species. The small MoVBiO crystals (30 nm) with more surface active sites and accessibility showed high performance in the oxidation of methanol to methyl formate in contrast to the big crystals (300 nm).

Photocatalytic water-donating transfer hydrogenation can be achieved over a palladium single-atom catalyst supported on mesoporous carbon nitride, in which hydrogen protons generated from photocatalytic water splitting are sequentially added to various unsaturated bonds. In-depth characterization confirms the direct hydrogenation mechanism and links the outstanding performance to reduced barriers for hydrogenation and accelerated proton supply from water.

Abstract

Solar-driven transfer hydrogenation of unsaturated bonds has received considerable attention in the research area of sustainable organic synthesis; however, water, the ultimate green source of hydrogen, has rarely been investigated due to the high barrier associated with splitting of water molecules. We report a carbon-nitride-supported palladium single-atom heterogeneous catalyst with unparalleled performance in photocatalytic water-donating transfer hydrogenation compared to its nanoparticle counterparts. Isotopic-labeling experiments and operando nuclear magnetic resonance measurements confirm the direct hydrogenation mechanism using in situ-generated protons from water splitting under visible-light irradiation. Density functional theory calculations attribute the high activity to lower barriers for hydrogenation, facilitated desorption of ethylbenzene, and facile hydrogen replenishment from water on the atomic palladium sites.