Robby Vroemans

This review presents an overview of the last ten years of advances in organic synthesis using calixarenes as organocatalysts. Calixarenes display a three-dimensional structure of a macrocycle that can be chemically modified to better act as a catalyst. They also are tolerant to aqueous or solvent-free systems, being recyclable and easy to separate from the reaction medium.

Abstract

The use of calix[n]arenes as organocatalysts has increasingly been considered a prime focus due to their adjustable three-dimensional molecular structure, easy synthesis, and versatile structural modification at the upper and lower edges. For instance, these macromolecules have been described as efficient catalysts for asymmetric aldol reactions, asymmetric Michael addition, esterification and transesterification, dehydration, nucleophilic aromatic substitution, and multicomponent reactions. Furthermore, calix[n]arenes also tolerate aqueous or solvent-free reaction systems and can be supported on solid and/or magnetic particles, showing cost, time and energy savings, easier experimental procedures, and chemical waste reduction.

Perfluoroalkylated (hetero)arenes are commonly utilized in many areas such as medicinal chemistry, agrochemistry and material sciences. This review article discusses methods for the direct perfluoroalkylation of C(sp²)-H bonds in (hetero)arenes, one of the most attractive and straightforward entry to perfluoroalkylated (hetero)arenes, with a special focus on the reagents utilized, reaction mechanisms and their regioselectivity.

Michael J. S. Dewar, John D. Roberts, and Andrew Streitwieser, Jr. were three organic chemists who were also experts in molecular orbital theory in the 1950s and 1960s. They, and many of their colleagues could have, but did not, discover orbital symmetry control of pericyclic reactions.

Abstract

It is a reasonable question to ask, why, as of 1965 when the five Woodward-Hoffmann communications appeared, did no other organic chemist discover the orbital symmetry rules for pericyclic reactions? The previous two papers in this 27-paper series on the history of the Woodward-Hoffmann rules discussed the physical chemists, chemical physicists, and theoretical chemists who could have solved the pericyclic no-mechanism problem; and the organic chemists in whose laboratory many of the key hints to this problem were found but still did not solve the problem. The stories of 16 other chemists who knew of (at least portions of) the pericyclic no-mechanism problem are presented in this paper. Social, political, and scientific explanations are presented as partial rationalizations as to why none of these individuals – except Woodward with Hoffmann – solved the pericyclic no-mechanism problem.

Double trouble: Dual Ni/photoredox catalysis is a prominent approach to forge valuable C−C and C−heteroatom bonds of synthetic relevance. This Review summarizes the most relevant examples within this field, highlighting the great evolution of the different photocatalytic systems synergistically employed together with the Ni catalyst. These span from metal-based complexes to organic dyes, along with heterogeneous semiconductors.

Abstract

Recently, the field of dual photocatalysis has grown rapidly, to become one of the most powerful tools for the functionalization of organic molecules under mild conditions. In particular, the merging of Earth-abundant nickel-based catalytic systems with visible-light-activated photoredox catalysts has allowed the development of a number of unique green synthetic approaches. This goes in the direction of ensuring an effective and sustainable chemical production, while safeguarding human health and environment. Importantly, this relatively new branch of catalysis has inspired an interdisciplinary stream of research that spans from inorganic and organic chemistry to materials science, thus establishing itself as one dominant trend in modern organic synthesis. This Review aims at illustrating the milestones on the timeline evolution of the photocatalytic systems used, with a critical analysis toward novel applications based on the use of photoactive two-dimensional carbon-based nanostructures. Lastly, forward-looking opportunities within this intriguing research field are discussed.

Nature, Published online: 19 July 2022; doi:10.1038/d41586-022-01966-8

The Cover Feature shows the upcycling of various plastic wastes such as polyethylene (PE), polyethylene terephthalate (PET), and polylactic acid (PLA) into high-value chemicals, lubricants, fuels, and carbon materials. Heterogeneous catalysis plays a determining role in the product distribution and reaction efficiency. More information can be found in the Review by T. Tan et al.

Single atom catalysts consist of a transition metal embedded in a supporting matrix. We show that in determining the final catalytic activity in Hydrogen Evolution (HER) Reaction the transition metal and the neighbouring atoms are equally important and that comparable changes in reactivity can be induced by changing the metal or by replacing the atoms bound to it.

Abstract

Single atom catalysts (SACs) consist of isolated metal atoms stabilized on a solid support. The name suggests that the catalytic activity is due to the nature of the metal atom, but of course the interaction with the substrate plays a role as well. But what is more important? The metal atom or its surrounding? To answer this question, we have performed numerical experiments based on density functional theory (DFT). 24 transition metal atoms have been incorporated in Nitrogen-doped graphene, and the catalytic activity in the hydrogen evolution reaction (HER) has been studied changing the metal and keeping the N-doped graphene matrix fixed. Then, one specific atom, Pt, has been embedded in the same matrix but the nitrogen neighbors of Pt have been systematically replaced by carbon or oxygen atoms generating more than 20 structures. The HER has thus been studied by keeping the metal center fixed but changing the surrounding. It turns out that the same great variability in chemical behavior can be achieved by changing the active site or the surrounding environment. This shows the importance of the local coordination in determining the catalytic activity. The consequences of this conclusion for modeling studies of SACs are discussed.

Structured catalysis: The highly stable Pt single atoms on Ni surface as single-atom alloys (SAAs) are prepared by reducing Pt single atoms supported on NiO in single-atom catalysts (SACs). The Pt/Ni atomic ratio of 0.008 produces well-distributed Pt−Ni SAA structured catalysts investigated by scanning transmission electron microscopy (STEM) and X-ray absorption spectroscopy (XAS).

Abstract

The well-dispersed Pt atoms on Ni have shown excellent coke resistance during methane reforming. In this work, we performed detailed characterization experiments using aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM), and X-ray absorption spectroscopy (XAS) to elucidate the formation of different Pt structures with respect to Pt/Ni atomic ratios. The Pt−Ni single-atom alloys (SAAs) in Pt/Ni-CeO₂@SiO₂ yolk-shell nanotube structures are synthesized by reducing Pt single atoms supported on the NiO surface as single-atom catalysts (SACs). Pt−Ni SAAs show Pt−Ni contribution in the first shell with the coordination number of 5 without any Pt−Pt and Pt−O interactions, implying Pt atoms are isolated on the outermost layer of Ni species. However, Pt clusters can be detected at Pt/Ni atomic ratios higher than 0.008. The well-dispersed Pt single atoms on Ni show high activity while maintaining their structure in dry reforming of methane condition at 773 K, verifying the excellent stability of the Pt−Ni SAA morphology.

Nickel catalysis: Nickel-catalyzed cross-coupling reactions have become a powerful methodology to construct C-heteroatom bonds but often require non-equimolar amounts of coupling partners. In this study we reveal the competition between productive catalysis and deleterious pathways (comproportionation and protodehalogenation) in the commonly proposed self-sustained NiI/NiIII catalytic cycle. Thereby we show that for productive nickel-catalyzed carboxylate O-arylation a choice must be made between either mild conditions or equimolar ratios of substrates.

Abstract

Nickel-catalyzed cross-coupling reactions have become a powerful methodology to construct C-heteroatom bonds. However, many protocols suffer from competitive off-cycle reaction pathways and require non-equimolar amounts of coupling partners to suppress them. Here, we report on mechanistic examination of carboxylate O-arylation under thermal conditions, in both the presence and absence of an exogeneous bipyridine-ligand. Furthermore, spectroscopic studies of the novel ligand-free carboxylate O-arylation reaction unveiled the resting state of the nickel catalyst, the crucial role of the alkylamine base and the formation of an off-cycle Ni^I−Ni^II dimer upon reduction. This study provides insights into the competition between productive catalysis and deleterious pathways (comproportionation and protodehalogenation) in the commonly proposed self-sustained Ni^I/Ni^III catalytic cycle. Thereby we show that for productive nickel-catalyzed carboxylate O-arylation a choice must be made between either mild conditions or equimolar ratios of substrates.

Supported catalysts: This review provides the catalytic performance of different forms of Pd (metal salts or metal nanoparticles) grafted to metal-organic frameworks for C−C coupling reactions under heterogeneous conditions.

Abstract

Among the various cross coupling reactions, C−C cross coupling reaction has attracted many researchers to investigate in the last four decades. The continuous, constant, and consistent progress in this field fetched a Noble prize in 2010, showing the importance of this reaction in diversified fields. Among the various transition metals studied for this reaction, Pd is one of the metals that has exhibited the highest activity due to its unique features and reactivity. Although Pd-based homogeneous catalyst was the preferred choice for many researchers, the field slowly diverted towards the development of Pd-based heterogeneous catalysts for C−C coupling reactions. This is obviously due to the high cost of Pd precursor, its self-deactivation. In this context, metal organic frameworks (MOFs) are one of the solid catalysts frequently employed as host matrix for the deposition of Pd(II) salts and Pd nanoparticles. This is due to the widespread possibilities available for the functionalization of MOFs and their stability during the synthetic procedure. This review provides the design and development of Pd-based MOFs as heterogeneous catalysts for the C−C cross coupling reactions. After a brief introduction on MOFs and the importance of C−C coupling reactions, the main part of the review summarizes the catalytic data with respect to Pd(II) and Pd nanoparticles as active sites. The last section provides our views in this field for the further development in the near future.

Aerobic oxidative coupling is the one of the most straightforward reactions involving dehydrogenative bond formations. This review highlights recent advances in aerobic oxidative couplings combining palladium with other transition metal catalysts to achieve the synergic catalysis.

Abstract

Aerobic oxidative coupling is a dehydrogenative organic transformation in which molecular oxygen acts as a re-oxidant for the catalyst system. Pd has emerged as one of the most popular transition metal catalysts for such reactions. Combinations of Pd with other transition metal catalysts can also provide effective catalytic cycles together with highly atom- and step-economical organic transformations. A synergic Pd catalyst system can promote such reactions with a high degree of efficiency based on a re-oxidation step exhibiting improved kinetics in which two electrons are transferred from another transition metal. This occurs in conjunction with a lower energy barrier than that associated with direct re-oxidation by molecular oxygen. This review highlights recent developments in aerobic oxidative coupling reactions catalyzed by synergic combinations of Pd with other transition metals. Highlighted reactions include dehydrogenative C−C bond formation such as arylation of arenes, heteroarenes, alkenes and dehydrogenative C−N and C−O bond formations with unsaturated hydrocarbons such as alkenes, 1,3-dienes and allenes. Moreover, mechanistic insights into these aerobic reactions based on experimental, computational and optical studies are detailed.

An operationally simple and robust methodology towards a direct condensation of carboxylic acids and amines has been realized in a versatile electrochemical approach, using a combination of triphenylphosphine and iodide as coupling reagent. A broad scope of different amides and dipeptides were accessible by the presented method, without the need for pre-functionalization of the starting materials.

Abstract

The ubiquity of amide bonds, present in natural products and common pharmaceuticals renders this functional group one of the most prevalent in organic chemistry. Despite its importance and a wide variety of existing methods for its formation, the latter still can be a challenge for classical activating reagents such as chloridating agents or carbodiimides. As the spent reagents often cannot be recycled, the development of more sustainable methods is highly desirable. Herein, we report an operationally simple and mild indirect electrochemical protocol to effect the condensation of carboxylic acids with amines, forming a wide variety of carboxamides.

Catalysts, Vol. 12, Pages 783: Selective Synthesis of Levulinic Ester from Furfural Catalyzed by Hierarchical Zeolites

Catalysts doi: 10.3390/catal12070783

Authors: Sancler C. Vasconcelos Luiz F. C. Pinhel Vinicius G. C. Madriaga Vinicius Rossa Leyliane G. S. Batinga Domingos S. A. Silva Rodrigo D. dos Santos André V. H. Soares Ernesto A. Urquieta-González Fabio Barboza Passos Rajender S. Varma Thiago M. Lima

Furfural is a platform molecule that can be catalytically converted using a cascade series of reactions into levulinic esters, essential compounds used as fuel additives. Bifunctional catalysts containing Lewis and Brønsted acid sites such as zeolites are commonly used for these conversions. However, microporous zeolites often present diffusional restriction due to the size similarity of furfural and other molecules to the zeolites’ micropores. Thus, incorporating mesopores in these materials through post-synthetic protocols is a promising pathway to circumventing these limitations. This study presents the creation of hierarchical beta and mordenite using Si or Al removal and their employment in the furfural conversion to isopropyl levulinate (PL). Mordenite zeolite did not produce satisfactory mesopores, while the beta was more efficient in generating them by both acid and alkaline treatments. Beta zeolite treated in an alkaline solution presented larger mesopores (14.9 and 34.0 nm), maintaining a total acidity value close to its parent zeolite and a higher Lewis/Brønsted ratio. The combination of these features led to an improved diffusion of bulkier products and the highest furfural conversion (94%) and PL selectivity (90%), suggesting that a post-modification of beta zeolites produced efficient catalysts for upgrading abundantly available furfural.

Nature, Published online: 13 July 2022; doi:10.1038/d41586-022-01900-y

Nature Reviews Chemistry, Published online: 11 July 2022; doi:10.1038/s41570-022-00403-8

Efficient and convenient palladium-catalyzed alkoxy- and aminocarbonylations of cinnamoyl chloride and aliphatic allyl chlorides to synthesize β,γ-unsaturated esters/amides under mild condition were developed.

Abstract

Improved procedures for carbonylative transformations (alkoxy- and aminocarbonylation) of cinnamyl chloride to synthesize β,γ-unsaturated esters/amides have been developed. Studying critical reaction parameters (palladium precursors, solvents and bases) enabled the practical preparation of diverse β,γ-unsaturated esters/amides under mild conditions (low Pd catalyst loading, phosphine-free, 2 bar CO, 60 °C). The optimal catalytic system shows excellent chemo- and regioselectivity for the activation of the allylic C−Cl bonds.

Nature, Published online: 04 July 2022; doi:10.1038/s41586-022-04899-4