Robby Vroemans

Synthesis
DOI: 10.1055/a-1482-2486

An efficient nickel-catalyzed vicinal thiosulfonylation of 1-bromoalkynes with thiosulfonates in the presence of cesium carbonate is described. An operationally simple and highly regioselective atom transfer radical addition (ATRA) of alkynyl bromides provides a wide range of (E)-1,2-thiosulfonylethenes (α-aryl-β-thioarylvinyl sulfones) in moderate to high yields. The extensive substrate scope of both alkynyl bromides and thiosulfonates is explored with a broad range of functional groups. Indole-derived 1,1-bromoalkenes were also successfully explored in this 1,2-thiosulfonylation process. Moreover, the nickel-catalyzed geminal-dithiolation of alkynyl bromides with N-arylthio succinimides provides 1,1-dithioalkenes in high yields. The present protocol is reliable on gram scale, and a sequential one-pot bromination and thiosulfonylation of phenylacetylene is achieved in a scale-up synthesis. Following control experiments, a plausible mechanism is proposed to rationalize the experimental outcome and the vicinal thio­sulfonylation.
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Abstract

Several new heterocyclic systems based on a hydroxybenzofuro[2,3-b]pyridine building block were prepared. This benzofuropyridine is easily available from the Meerwein reaction of benzoquinone and a heterocyclic diazonium salt, followed by reduction and cyclization. Electrophilic substitution and further condensations give polycyclic systems, including oxazolo- and chromeno-fused analogues.

Beilstein J. Org. Chem. 2021, 17, 977–982. doi:10.3762/bjoc.17.79

A palladium-catalyzed asymmetric hydrogenation of levulinic acid has been successful developed. By using Zn(OTf)₂ as co-catalyst, the present method has allowed the preparation of a wide range of chiral γ-valerolactones in good yields with excellent enantioselectivities.

Abstract

A palladium-catalyzed asymmetric hydrogenation of levulinic acid has been successful developed by using Zn(OTf)₂ as co-catalyst. The present method not only has provided a strategy in the palladium-catalyzed asymmetric hydrogenation of ketone, but also allowed the preparation of a wide range of chiral γ-valerolactones in good yields with excellent enantioselectivities.

A closer look: Computational chemistry is able to grind out the mechanisms of ball milling for a better understanding of the mechanochemical environment. Two reactions reported in previous experimental publications are studied: i) a series of Diels-Alder reactions and ii) the synthesis of sulfonylguanidines.

Abstract

Mechanochemistry is an emerging field with many potential applications in sustainable chemistry. But despite the growing interest in the field, its underlying mechanistic foundations are not fully understood yet. This work presents the application of computational tools, such as DFT calculations in continuum and microkinetic modeling, to the analysis of mechanically activated procedures. Two reactions reported in previous experimental publications were studied: (i) a series of Diels-Alder reactions and (ii) the synthesis of sulfonylguanidines. Calculations succeed in reproducing experimentally reported reaction times. The procedures were mostly standard, coupled with some sensitive choices in terms of starting concentrations and dielectric constant. This means that these particular reactions accelerated by ball milling followed the same mechanism as the equivalent reactions in solution. The implications of this result on the general picture of mechanochemical processes are discussed.

This Review outlines the strategic use of electrophilic aminating agents in total synthesis. A wide range of C−N bond-forming methods are highlighted, including polar, radical and transition-metal-catalyzed processes.

Abstract

Classical amination methods involve the reaction of a nitrogen nucleophile with an electrophilic carbon center; however, in recent years, umpoled strategies have gained traction where the nitrogen source acts as an electrophile. A wide range of electrophilic aminating agents are now available, and these underpin a range of powerful C−N bond-forming processes. In this Review, we highlight the strategic use of electrophilic aminating agents in total synthesis.

An iron(II)‐catalyzed aerobic biomimetic oxidation of amines has been developed. The principle is based on biological oxidation in the respiratory chain in living organisms. The electron transfer from the amine to molecular oxygen occurs through coupled redox reactions, mediated by a bifunctional hydroquinone/cobalt Schiff base as hybrid catalyst. This hybrid catalyst was found to significantly shield the iron catalyst from deactivation.

Abstract

Herein we report the first Fe^II‐catalyzed aerobic biomimetic oxidation of amines. This oxidation reaction involves several electron transfer steps and is inspired by biological oxidation in the respiratory chain. The electron transfer from the amine to molecular oxygen is aided by two coupled catalytic redox systems, which lower the energy barrier and improve the selectivity of the oxidation reaction. An iron hydrogen transfer complex was utilized as the substrate‐selective dehydrogenation catalyst along with a bifunctional hydroquinone/cobalt Schiff base complex as a hybrid electron transfer mediator. Various primary and secondary amines were oxidized in air to their corresponding aldimines or ketimines in good to excellent yield.

Twelve steps to sustainability: Mechanochemistry is commonly associated to green chemistry for its ability to reduce waste production in chemical synthesis. This Review exemplifies how mechanochemistry touches upon all of the twelve principles of green chemistry.

Abstract

In recent years, mechanochemistry has been growing into a widely accepted alternative for chemical synthesis. In addition to their efficiency and practicality, mechanochemical reactions are also recognized for their sustainability. The association between mechanochemistry and Green Chemistry often originates from the solvent-free nature of most mechanochemical protocols, which can reduce waste production. However, mechanochemistry satisfies more than one of the Principles of Green Chemistry. In this Review we will present a series of examples that will clearly illustrate how mechanochemistry can significantly contribute to the fulfillment of Green Chemistry in a more holistic manner.

Abstract

Several new heterocyclic systems based on a hydroxybenzofuro[2,3-b]pyridine building block were prepared. This benzofuropyridine is easily available from the Meerwein reaction of benzoquinone and a heterocyclic diazonium salt, followed by reduction and cyclization. Electrophilic substitution and further condensations give polycyclic systems, including oxazolo- and chromeno-fused analogues.

Beilstein J. Org. Chem. 2021, 17, 977–982. doi:10.3762/bjoc.17.79

The photoredox-catalyzed win–win coupling strategy, in which CO₂ valorization is integrated with organic synthesis in one reaction system, provides a promising approach that enables sufficient utilization of excited electrons and holes to achieve feasible and sustainable light-mediated artificial photosynthesis.

Abstract

Photocatalytic reduction of CO₂ to solar fuels and/or fine chemicals is a promising way to increase the energy supply and reduce greenhouse gas emissions. However, the conventional reaction system for CO₂ photoreduction with pure H₂O or sacrificial agents usually suffers from low catalytic efficiency, poor stability, or cost-ineffective atom economy. A recent surge of developments, in which photocatalytic CO₂ valorization is integrated with selective organic synthesis into one reaction system, indicates an efficient modus operandi that enables sufficient utilization of photogenerated electrons and holes to achieve the goals for sustainable economic and social development. In this Review we discuss current advances in cooperative photoredox reaction systems that integrate CO₂ valorization with organics upgrading based on heterogeneous photocatalysis. The applications and virtues of this strategy and the underlying reaction mechanisms are discussed. The ongoing challenges and prospects in this area are critically discussed.

Seamless low-temperature syntheses, crystallization and spectroscopies provide structural details of the heterobimetallic reagent routinely used for regioselective carbon−carbon bond forming reactions with carbonyl compounds. ⁷Li NMR spectroscopy takes on an important role here, tracking down n-BuLi dissociation.

Abstract

CeCl₃(thf) reacts at low temperatures with MeLi, t-BuLi, and n-BuLi to isolable organocerium complexes. Solvent-dependent extensive n-BuLi dissociation is revealed by ⁷Li NMR spectroscopy, suggesting “Ce(n-Bu)₃(thf)_x” or solvent-separated ion pairs like “[Li(thf)₄][Ce(n-Bu)₄(thf)_y]” as the dominant species of the Imamoto reagent. The stability of complexes Li₃Ln(n-Bu)₆(thf)₄ increases markedly with decreasing Ln^III size. Closer inspection of the solution behavior of crystalline Li₃Lu(n-Bu)₆(thf)₄ and mixtures of LuCl₃(thf)₂/n-BuLi in THF indicates occurring n-BuLi dissociation only at molar ratios of <1:3. n-BuLi-depleted complex LiLu(n-Bu)₃Cl(tmeda)₂ was obtained by treatment of Li₂Lu(n-Bu)₅(tmeda)₂ with ClSiMe₃, at the expense of LiCl incorporation. Imamoto's ketone/tertiary alcohol transformation was examined with 1,3-diphenylpropan-2-one, affording 99 % of alcohol.

Journal of Porphyrins and Phthalocyanines, Volume 25, Issue 07n08, Page 623-629, July & August 2021.
We report here the synthesis and structural characterization of a novel expanded Schiff base oligopyrrolic macrocycle TxLH ([math] compound 2) along with its smaller congener hemi-TxLH ([math] compound 1). The solid-state structure of TxLH is reminiscent of the shape of a Texas Longhorn[math]. It thus defines a new architectural form for porphyrin analogues. The present study thus underscores the potential of using functionalized oligopyrroles as readily accessible molecular building blocks for the construction of structurally non-trivial molecules.

Mechanically interlocked ligands enable predictable metal ion coordination, offering a new strategy to engineer their properties. A series of field-induced Co^II single-ion magnets illustrates this principle. The predictable coordination behaviour of the rotaxane enables remarkably accurate calculation of their zero-field splittings, which are in agreement with experimental observations, and can be rationalized in terms of metal–ligand bonding.

Abstract

Mechanically chelating ligands have untapped potential for the engineering of metal ion properties. Here we demonstrate this principle in the context of Co^II-based single-ion magnets. Using multi-frequency EPR, susceptibility and magnetization measurements we found that these complexes show some of the highest zero field splittings reported for five-coordinate Co^II complexes to date. The predictable coordination behaviour of the interlocked ligands allowed the magnetic properties of their Co^II complexes to be evaluated computationally a priori and our combined experimental and theoretical approach enabled us to rationalize the observed trends. The predictable magnetic behaviour of the rotaxane Co^II complexes demonstrates that interlocked ligands offer a new strategy to design metal complexes with interesting functionality.

Investigations on Lewis- and Brønsted acid-mediated cyclizations of trienones revealed a novel Nazarov cyclization/ene reaction tandem reaction. This reaction allows the stereoselective construction of quaternary spirocenters, with a β-hydroxyl group acting as superb stereodirecting element. The produced spirocycles possess useful functionality for further modifications, making them potentially valuable synthetic intermediates.

Abstract

The different reactivity of trienones under Lewis and Brønsted acids catalysis was investigated, resulting in distinct cyclization products and carbon backbones that originated either from a conjugate Prins cyclization or an interrupted Nazarov cyclization. In particular, an unprecedented Nazarov cyclization tandem reaction is presented, terminating the oxyallyl cation by an ene-type reaction, and leading stereoselectively to bicyclic spiro compounds. The terminal olefin of this motif represents a useful handle for further functionalization, making it a strategic intermediate in total syntheses. The tandem Nazarov/ene cyclization was shown to be preferred over a Nazarov/[3+2] tandem reaction for all our substrates, independent of chain length. Deuteration studies further support the mechanistic hypothesis of the terminating ene reaction.