Robby Vroemans

Abstract

This review deals with the historical development of all N-F fluorinating agents developed so far. The unique properties of fluorine make fluorinated organic compounds attractive in many research areas and therefore fluorinating agents are important. N-F agents have proven useful by virtue of their easy handling. This reagent class includes many types of N-F compounds: perfluoro-N-fluoropiperidine, N-fluoro-2-pyridone, N-fluoro-N-alkylarenesulfonamides, N-fluoropyridinium salts and derivatives, N-fluoroquinuclidium salts, N-fluoro-trifluoromethanesulfonimide, N-fluoro-sultams, N-fluoro-benzothiazole dioxides, N-fluoro-lactams, N-fluoro-o-benzenedisulfonimide, N-fluoro-benzenesulfonimide, 1-alkyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane salts, N-fluoropyridinium-2-sulfonate derivatives, 1-fluoro-4-hydroxy-1,4-diazoniabicyclo[2.2.2]octane salts, N-fluorodinitroimidazole, N-fluoro-trichloro-1,3,5-triazinium salt, N-F ethano-Tröger’s base derivatives, N-fluoro-methanesulfonimide, N-fluoro-N-arylarenesulfonamides, bisN-F salts such as N,N’-difluorobipyridinium salts and N,N’-difluoro-1,4-diazoniabicyclo[2.2.2]octane salts, and their many derivatives and analogs, including chiral N-F reagents such as optically active N-fluoro-sultam derivatives, N-fluoro-alkaloid derivatives, DABCO-based N-F derivatives, and N-F binaphthyldisulfonimides. The synthesis and reactions of these reagents are described chronologically and the review also discusses the relative fluorination power of each reagent and their mechanisms chronicling developments from a historical perspective.

Beilstein J. Org. Chem. 2021, 17, 1752–1813. doi:10.3762/bjoc.17.123

Catalytic conversion of methane to methanol remains an economically tantalizing but fundamentally challenging goal. Current technologies based on zeolites deactivate too rapidly for practical application. We found that similar active sites hosted in different zeolite lattices can exhibit markedly different reactivity with methane, depending on the size of the zeolite pore apertures. Whereas zeolite with large pore apertures deactivates completely after a single turnover, 40% of active sites in zeolite with small pore apertures are regenerated, enabling a catalytic cycle. Detailed spectroscopic characterization of reaction intermediates and density functional theory calculations show that hindered diffusion through small pore apertures disfavors premature release of CH₃ radicals from the active site after C-H activation, thereby promoting radical recombination to form methanol rather than deactivated Fe-OCH₃ centers elsewhere in the lattice.

A rhodium-catalyzed hydroxylation of aryl fluorides via η⁶-coordination was achieved. This method was also applicable in the alkoxylation. A Meisenheimer-type intermediate was successfully isolated and well characterized. Density functional theory calculations and mechanistic studies suggested a stepwise or stepwise-like energy profile for the S_NAr reaction.

Abstract

Nucleophilic aromatic substitution (S_NAr) is a powerful strategy for incorporating a heteroatom into an aromatic ring by displacement of a leaving group with a nucleophile, but this method is limited to electron-deficient arenes. We have now established a reliable method for accessing phenols and phenyl alkyl ethers via catalytic S_NAr reactions. The method is applicable to a broad array of electron-rich and neutral aryl fluorides, which are inert under classical S_NAr conditions. Although the mechanism of S_NAr reactions involving metal arene complexes is hypothesized to involve a stepwise pathway (addition followed by elimination), experimental data that support this hypothesis is still under exploration. Mechanistic studies and DFT calculations suggest either a stepwise or stepwise-like energy profile. Notably, we isolated a rhodium η⁵-cyclohexadienyl complex intermediate with an sp³-hybridized carbon bearing both a nucleophile and a leaving group.

The correct assessment of the aromaticity of a 78π-electron six-porphyrin nanoring hinges upon the choice of the computational method. Density functional approximations with a low percentage of long-range Hartree–Fock exchange commit large delocalization errors and suggest the presence of an aromatic ring current. The transition from the porphyrins to the bridging butadiyne linkers is responsible for the absence of an aromatic ring current.

Abstract

Large conjugated rings with persistent currents are novel promising structures in molecular-scale electronics. A six-porphyrin nanoring structure that allegedly sustained an aromatic ring current involving 78π electrons was recently synthesized. We provide here compelling evidence that this molecule is not aromatic, contrary to what was inferred from the analysis of ¹H-NMR data and computational calculations that suffer from large delocalization errors. The main reason behind the absence of an aromatic ring current in these nanorings is the low delocalization in the transition from the porphyrins to the bridging butadiyne linkers, which disrupts the overall conjugated circuit. These results highlight the importance of choosing a suitable computational method to study large conjugated molecules and the appropriate aromaticity descriptors to identify the part of the molecule responsible for the loss of aromaticity.

This review article describes the ruthenium-catalyzed C−X (X=C, N, O, S) bond formation via intramolecular C−H functionalization that leads to the generation of various cyclic and heterocyclic molecules in good to excellent yield. Additionally, the substrate scope and brief mechanistic discussions of these reactions were also covered in this article.

Abstract

Ruthenium catalyzed C−H activation is well known for its high tolerance towards the functional group and broad applicability in organic synthesis and molecular sciences, with significant applications in pharmaceutical industries, material sciences, and polymer industry. In the last few decades, enormous progress has been observed with ruthenium-catalyzed C−H activation chemistry. Notably, the vast majority of the C−H functionalization known in the literature are intermolecular, although the intramolecular variant provides fascinating new structural facet starting from the simple molecular scaffolds. Intramolecular C−H functionalization is atom economical and step efficient, results in less formation of undesired products which is easy to purify. This has created a lot of interest in organic chemistry in developing new synthetic strategies for such functionalization. The focus of this review is to present the relatively unexplored intramolecular functionalization of C−H bonds into C−X (X=C, N, O, S) bonds utilizing versatile ruthenium catalysts, their scope, and brief mechanistic discussion.

Journal of Porphyrins and Phthalocyanines, Volume 25, Issue 10n12, Page 958-965, October & December 2021.
Two BOPYIN dyes (W-1 and W-2) were synthesized, and their structural and photophysical properties were investigated. The absorption bands of both dyes located at ca. 350 and 500 nm in diverse solvents. The absorption profiles are a linear combination of those of the BOPYIN core and the boranil or anil subunits. Their emission spectra were observed at ca. 565 and 540 nm with 14.2% and 77% quantum yield in CHCl3, respectively. The absorption, excitation and emission spectra for both dyes confirm that the boranil or anil moieties could be used as an input energy antenna for cascade energy transfer to the fluorescent BOPYIN residue at 540 nm. Subsequently, ESIPT process of W-1 is fast and more likely to occur in non-polar solvent investigated by DFT calculation.

ARKIVOC Volume 2021 - Part (ix): Special Issue 'Reviews and Accounts'
A concise review on the stereoselective synthesis of chiral ?-bisamides using stereoselective Ugi-type reaction (21-11519FR)
Ramakrishna D. S. and Ankita Pradhan
Full Text: PDF (3,552K)
Supplementary Material: PDF (0K)
pp. 1 - 41

received Mar 23 2021; accepted May 29 2021; published Jul 6 2021;

Herein the synthesis of two novel tetra-BODIPY tetraphenylethylene (TPE) conjugates is presented. We find differing AIE responses dependent upon the linkage between the TPE and BODIPY cores, as well as differing singlet oxygen generation of the synthesized conjugates.

Abstract

Tetraphenylethylene (TPE) and its derivatives exhibit excellent aggregation-induced emission (AIE) properties. The TPE unit is easily accessible, and many functional groups can be introduced in a facile manner to yield effective luminescent materials in both solution and the solid-state. It is because of this, several TPE-based compounds have been developed and applied in many areas, such as OLEDs and chemical sensors. Boron dipyrromethenes (BODIPYs) are a class of pyrrolic fluorophore of great interest with myriad application in both material science and biomedical applications. Through the combination of Pd-catalyzed cross-coupling reactions and traditional dipyrromethene chemistry, we present the syntheses of novel tetra-BODIPY-appended TPE derivatives with different distances between the TPE and BODIPY cores. The TPE-BODIPY arrays 6 and 9 show vastly differing AIE properties in THF/H₂O systems, with 9 exhibiting dual-AIE, along with both conjugates being found to produce singlet oxygen (¹O₂). We presume the synthesized BODIPY-appended TPE scaffolds to be utilized for potential applications in the fields of light-emitting systems and theranostics.

Molecules, Vol. 26, Pages 4139: Ni Oxidation State and Ligand Saturation Impact on the Capability of Octaazamacrocyclic Complexes to Bind and Reduce CO2

Molecules doi: 10.3390/molecules26144139

Authors: Barbora Vénosová Ingrid Jelemenská Jozef Kožíšek Peter Rapta Michal Zalibera Michal Novotný Vladimir B. Arion Lukáš Bučinský

Two 15-membered octaazamacrocyclic nickel(II) complexes are investigated by theoretical methods to shed light on their affinity forwards binding and reducing CO2. In the first complex 1[NiIIL]0, the octaazamacrocyclic ligand is grossly unsaturated (π-conjugated), while in the second 1[NiIILH]2+ one, the macrocycle is saturated with hydrogens. One and two-electron reductions are described using Mulliken population analysis, quantum theory of atoms in molecules, localized orbitals, and domain averaged fermi holes, including the characterization of the Ni-CCO2 bond and the oxidation state of the central Ni atom. It was found that in the [NiLH] complex, the central atom is reduced to Ni0 and/or NiI and is thus able to bind CO2 via a single σ bond. In addition, the two-electron reduced 3[NiL]2− species also shows an affinity forwards CO2.

Nerve agents are lethal organophosphorus compounds (OPs), posing a serious threat to military and civilians. In spite of decades invested toward developing detoxifying systems acting as prophylactics or therapeutics, there has been insufficient progress. In this critical review, we posit that examining molecular recognition of nerve agents shall help constructing more effective scavengers of OPs.

Abstract

Nerve agents are tetrahedral organophosphorus compounds (OPs) that were developed in the last century to irreversibly inhibit acetylcholinesterase (AChE) and therefore impede neurological signaling in living organisms. Exposure to OPs leads to a rapid development of symptoms from excessive salivation, nasal congestion and chest pain to convulsion and asphyxiation which if left untreated may lead to death. These potent toxins are prepared on a large scale from inexpensive staring materials, making it feasible for terrorist groups or states to use them against military and civilians. The existing antidotes provide limited protection and are difficult to apply to a large number of affected individuals. While new prophylactics are currently being developed, there is still need for therapeutics capable of both preventing and reversing the effects of OP poisoning. In this review, we describe how the science of molecular recognition can expand the pallet of tools for rapid and safe sequestration of nerve agents.