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Systematic structural variation and visualization of chemical shielding tensors (VIST) provide unique insights into the properties of π-conjugated macrocycles

π-Conjugated macrocycles are molecules with unique properties that are increasingly exploited for applications. Their study began in the early 1960s and many π-conjugated macrocycles have been synthesized since. However, only recently the field has moved towards making use of the unique properties of these cyclic molecules for applications. π-Conjugated macrocycles are now being investigated in organic solar cells, photodetectors, field-effect transistors, light-emitting diodes, and battery electrodes. They can also be used in bioimaging, as templates for the growth of carbon nanotubes, and as molecular nanoreactors.

Figure 1. (a) Reversible two-electron reduction of a π-conjugated macrocycle for charge storage in organic battery electrodes. (b) Visualization of chemical shielding tensors (VIST) helping to rationalize the optoelectronic properties.

Despite this recent interest in π-conjugated macrocycles, there are only a small number of experimental studies that investigate how the properties of π-conjugated macrocycles evolve with systematic structural changes. Recently, a team around Florian Glöcklhofer has reported such a systematic experimental study and combined it with an in-depth computational analysis. The study reveals the central role of local and global aromaticity for rationalizing the optoelectronic properties of the macrocycles. A recently developed computational method for the visualization of chemical shielding tensors (VIST) was applied to provide unique insight into local and global ring currents occurring in different planes along the macrocycles.

Figure 2. Functional group introduction and aromatic unit variation in a set of π-conjugated macrocycles.

The study makes a significant contribution to the development of structure–property relationships and molecular design guidelines and will help to understand, rationalize, and predict the properties of other π-conjugated macrocycles. Furthermore, it shows that cyclophanetetraenes, the investigated class of macrocycles, provide versatile scaffolds for applications due to their unusual properties along with their high tunability. Their remarkable optoelectronic properties, in particular their large Stokes shifts and the accessibility of a variety of charged states, were traced back to their formal ground state antiaromaticity along with high structural flexibility.

Rimmele, M.;  Nogala, W.;  Seif-Eddine, M.;  Roessler, M. M.;  Heeney, M.;  Plasser, F.; Glöcklhofer, F., Functional group introduction and aromatic unit variation in a set of π-conjugated macrocycles: revealing the central role of local and global aromaticity, Organic Chemistry Frontiers 2021, Advance Article. https://doi.org/10.1039/D1QO00901J

Author’s Biography

Florian Glöcklhofer is an Erwin Schrödinger Fellow in the Department of Chemistry at Imperial College London. He received his PhD in 2017 at TU Wien (Vienna) for developing a reaction for the conversion of quinones into cyanated aromatic compounds. His research in the field of π-conjugated organic compounds focuses on the design and synthesis of π-conjugated macrocycles for battery electrodes and organic electronics and on the development of new synthetic approaches to aromatic organic compounds.

https://www.gloecklhofer-research.com/

 

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Recent advance in the C-F bond functionalization of trifluoromethyl-containing compounds

Organofluorine compounds play an important role in pharmaceuticals, agrochemicals, and functional materials, due to their special chemical, physical and biological properties, such as increased electronegativity, hydrophobicity, bioavailability and metabolic stability. Therefore, great efforts have recently been devoted to the development of new methods for the synthesis of fluorinated compounds. Conventional strategies mainly focus on the selective introduction of fluorine atom or fluorine-containing moiety into organic molecules. Alternatively, the development of novel synthetic methodologies via the selective activation of C-F bond is of vital importance, which could allow for the synthesis of partially fluorinated synthetic intermediates from readily available polyfluorinated starting materials. The research of C-F bond activation is the most challenging task in organic synthesis, which has recently drawn increasing attention from chemists. In this review, we mainly focus on the C-F bond activation of CF3 groups for the synthesis of fluorinated compounds as well as discussion of their mechanisms(Scheme 1).

Scheme 1 The C-F bond functionalization of trifluoromethyl-containing compounds.

Due to the electron-deficient property, α-trifluoromethylstyrenes exhibit unique reactivities in their C-F bonds activation, mainly including three kinds of reactions (SN2′-type, SNV and ipso-substitution reactions, Scheme 2).

Scheme 2 Allylic and vinylic C-F bond activation in SN2’-type, SNV and ipso-sunstitution reactions

The cleavage of C-F bond in trifluoromethylated aromatic (Scheme 3) and alkyl compounds (Scheme 4) could be achieved through using transition metal complexes, main-group Lewis acids and base-conditions. An alternative strategy mainly relied on the single-electron transfer (SET) via Low-valent metals, irradiation with visible light.

Scheme 3 Photoredox-catalyzed defluoroalkylation of ArCF3 with unactivated alkenes.

Scheme 4 Pd-catalyzed defluorination/arylation reaction of α-trifluoromethyl ketones with aryl boronic acids.

Trifluoromethyl ketones (Scheme 5) and their corresponding diazo compounds (Scheme 6) , N‑tosylhydrazones (Scheme 7) have been utilized as the coupling partners in modern synthetic organic chemistry. Recently, the C-F bond cleavage of these compounds has also been reported through transition-metal catalysts.

Scheme 5 Cu-catalyzed reaction of trifluoromethylketones with aldehydes.

Scheme 6 Cu-catalyzed gem-difluoroolefination of diazo compounds.

Scheme 7 Cu-catalyzed cross-coupling of N-tosylhydrazones with terminal alkynes.

Although great progress has been made in this rapidly developing area, these reactions still suffer from some issues of the limitation of substrates and poor regioselectivity. To overcome these central challenges, it is more important to extend the substrate scope of the existing methodologies, especial trifluoromethylated alkyl compounds. Furthermore, the robust catalytic systems will be developed for the selective activation of a single C-F bond in CF3 groups.

 Guobing Yan

Zhejiang A&F University

Guobing Yan is a Professor in college of Jiyang at Zhejiang A&F University. He obtained B.Sc. degree from Jinggangshan Normal University, his M.Sc. degree from Suzhou University, and his Ph.D. degree from Tongji University in 2010. He spent two years in 2008 and 2009 as visiting student in professor Jianbo Wang’s laboratory at Peking University. In 2013, He joined Dr. Dong’s group at the University of Texas at Austin as a visiting professor. His current research interests focus on the transition-metal-catalyzed activation of inert chemical bonds and green synthetic chemistry. He is the author of 6 patents and more than 70 articles indexed by SCI.

 

Recent Advance in the C-F bond functionalization of trifluoromethyl-containing compounds

Guobing Yan, Kaiying Qiu and Ming Guo

https://doi.org/10.1039/D1QO00037C

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C(sp3)–H functionalization with isocyanides

Isocyanides have proven to be versatile building blocks in organic synthesis and medicinal chemistry due to their synthetic possibilities capable of reacting with electrophiles, nucleophiles and radicals. Multi-component reactions involving isocyanides have been well developed with the advantages of diverse skeletons of products, functional group compatibility, high chemo-, regio and stereoselectivities, and atom economy. With the continuous research on isocyanides, C(sp3)–H functionalization with isocyanides has made significant progresses with the advancement of mechanistic studies, new techniques and novel strategies in recent years. In this review, the group of Xu and coworkers of Shanghai University of China highlights the most recent advances of isocyanide chemistry in the C(sp3)–H functionalization since 2015 by elaborating the strategies of state-of-the-art synthetic routes. The synergistic combination of the isocyanide insertion and the C(sp3)–H bond activation offers a novel and efficient route to establish complicated reactions and provides an effective strategy for the synthesis of various nitrogen- and oxygen-containing compounds.

Figure 1.C(sp3)–H functionalization with isocyanides

Bin Xu received his bachelor’s degree from Shanghai University of Science and Technology in 1992. He conducted his PhD research with Professor Shengming Ma at Shanghai Institute of Organic Chemistry and completed his degree in 2000. After a two-year postdoctoral training at National Institutes of Health (NIH) as a Visiting Fellow (Mentor: Dr Kenneth A. Jacobson), he joined VivoQuest as a staff scientist (2002–2005). He began his independent academic career at Shanghai University in the end of 2005 as a Full Professor. His current research interests span the development of new methodology for nitrogen-containing heterocycles and their biological applications.

C(sp3)–H Functionalization with Isocyanides

Org. Chem. Front., 2021, Accepted Manuscript

https://pubs.rsc.org/en/content/articlelanding/2021/qo/d1qo00153a#!divAbstract

 

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Efficient stereoselective synthesis of chiral 3,3′-dimethyl-(2,2′-bipyridine)-diol ligand and applications in Fe(II)-catalysis

Metal-assisted asymmetric catalysis has proved to be one of the most efficient strategies to obtain high stereoselectivities in organic synthesis. High levels of chiral induction in a chemical transformation, together with high turn-overs of the catalyst, arise from the best match between the transition metal and the ligand. The widespread use of N-containing ligand precursors, as enantiopure building blocks, facilitates the synthesis of a broad scope of heterocyclic ligands, such as (bis)oxazolines, salens, and NHCs. The strong chelation properties of the bipyridine core with various metal ions made chiral 2,2′-bipyridines a very promising class of ligand to be used in asymmetric catalysis.

 

A first N2O2 tetradentate 2,2′-bipyridinediol was first synthesized in the early 90’s by Bolm and efficiently used in asymmetric catalysis. Later, the stability of Lewis acid complexes with this ligand was highlighted in highly enantioselective reactions run in aqueous media, where easily hydrolysable metal salts were transformed into water-compatible metal-complexes. With the objective of optimizing chiral inductions, tremendous efforts have been invested to design new C2-symmetric 2,2′-bipyridinediol derivatives.

 

Recently, the group of Thierry Ollevier and collaborators of Laval University in Québec, Canada, have disclosed the total synthesis of 2,2′-bipyridinediol (S,S)-1 in seven steps starting from commercially available 2-bromo-5-methylpyridine, with 25% overall yield and stereoselectivities up to 99% de and >99.5% ee (Scheme). The most crucial step for high levels of stereoenrichment of the ligand was demonstrated to be the oxidative homocoupling reaction, where the physical properties of the 2,2′-bipyridine N,N′-dioxides allowed removal of undesired diastereoisomers by silica gel column chromatography. X-ray studies revealed a favored complexation of (S,S)-1 that reaches heptacoordination of FeII.

 

The potential of (S,S)-1 for asymmetric induction for the FeII-catalyzed Mukaiyama aldol and thia-Michael reactions was highlighted. An increase of the chiral induction was demonstrated using the FeII catalyst made from newly synthesized ligand vs Bolm’s ligand.

Thierry Ollevier

Laval University

Thierry Ollevier is currently Full Professor in the Department of Chemistry at Laval University in Québec, Canada. He obtained his B.Sc. (1991) and Ph.D. (1997) at the Université of Namur (Belgium), and was post doctorate fellow at the Université catholique de Louvain (Belgium), under István E. Markó (1997), NATO postdoctorate fellow at Stanford University under Barry M. Trost (1998–2000), then post doctorate fellow at the Université de Montréal under André B. Charette (2000–2001). Current research in his group aims at designing novel catalysts, developing catalytic reactions and applying these methods to chemical synthesis. He is active in the areas of iron catalysis, diazo chemistry, asymmetric catalysis, and synthetic green chemistry. He is the author of more than 70 articles indexed by SCI and cited more than 2000 times.

http://www2.chm.ulaval.ca/tollevier/

 

Efficient stereoselective synthesis of chiral 3,3′-dimethyl-(2,2′-bipyridine)-diol ligand and applications in FeII-catalysis 

S. Lauzon, L. Caron and T. Ollevier, Organic Chemistry Frontiers2021.

https://doi.org/10.1039/D1QO00188D 

 

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