We are proud to announce the three best covers of Organic Chemistry Frontiers in 2022! The awarded work was chosen by our readers through a worldwide vote. To learn more about the science behind the winning pieces, read the cover articles below.
We are proud to announce the three best covers of Organic Chemistry Frontiers in 2022! The awarded work was chosen by our readers through a worldwide vote. To learn more about the science behind the winning pieces, read the cover articles below.
Emerging Investigator: Meng Li
2009–2012 University of Chinese Academy of Sciences M.Sc.
2005–2009 Shandong Normal University B.Sc.
Read Meng Li’s Emerging Investigator Series article in Organic Chemistry Frontiers and learn more about him.
|Advances in circularly polarized electroluminescence based on chiral TADF-active materials|
This review summarizes the development status of chiral TADF-active materials with CPEL, covering chiral perturbed TADF molecules, intrinsically chiral TADF molecules, and TADFsensitized fluorescent enantiomers.
|From the themed collection: Frontiers Emerging Investigator Series|
|The article was first published on 24 Sep 2022|
|Org. Chem. Front., 2022, 9, 6441-6452|
My research interests
|Key words: organic synthesis, circularly polarized luminescence, organic light-emitting diodes|
|Meng Li’s research focuses on developing chiral optoelectronic materials for photoelectric conversion devices. He is also interested in understanding the structure–activity relationship between chiral molecules and their circularly polarized luminescence properties, as well as developing circularly polarized organic light-emitting diodes for display device of low power consumption.|
10 Facts about me
|My favourite published academic article is the one on circularly polarized electroluminescence (CPEL) materials in Angew. Chem. Int. Ed. in 2018. In this work, we created a new system of CPEL materials based on chiral thermally activated delayed fluorescence (TADF) materials, providing an original research idea to solve the key problem of low device efficiency in the field of CPEL materials.
The academic group that helped me most is the Youth Innovation Promotion Association CAS. The members of this academic group are all outstanding young scientists under the age of 35 in CAS. They are active in various academic fields, and are ready to help others. Becoming a member of the group has greatly helped my scientific research.
The most important questions to be asked/answered in my research field include: (1) How to design and construct chiral luminescent materials with large asymmetry factor and high efficiency? (2) How to realize the transfer, amplification and regulation of chiral optoelectronic properties with multi-level chiral structures? (3) How to realize the creation of high-performance circularly polarized light emitting devices?
The most challenging work about my research is the application of chiral luminescent materials. I think this will require interdisciplinary collaborations between different academic fields, partnerships with industrial stakeholders, etc.
If I were not a scientist, I would be a secondary school teacher.
In my spare time, I enjoy reading biographical novels and playing Chinese chess.
One piece of career-related wisdom I would like to share with other early career scientists: be passionate about your research, and keep your curiosity.
The next big goal of my research is to improve the asymmetry factor of circularly polarized electroluminescence of chiral TADF-active materials.
My favourite book is Journey to the West.
My favourite time of the day is on the way to work in the morning. At that time, I am full of expectations for the whole day’s experiments.
Carbon–phosphorus (C–P) bond cleavage reactions are one of the most important transformations because organophosphorus compounds are widely utilized as pharmaceuticals, phosphine ligands, organocatalysts, and functional materials. Therefore, C–P bond cleavage reactions have been developed using various approaches such as radicals produced using photolysis or peroxides, reduction using transition metal catalysts and alkali metals, and via P(V) intermediates (Scheme 1A). However, these synthetic methodologies require relatively harsh conditions, leading to low functional group tolerance. In addition, they require the use of precious transition metals and the formation of highly polarized C–P bonds in activated species.
Recently, the group of Prof. Shinobu Takizawa and Mohamed S. H. Salem as a collaborator from SANKEN, Osaka University have developed a metal-free C(aryl)–P bond cleavage reaction starting via the reaction of triarylphosphines and alkynyl esters using water as a nucleophile and electrophile to yield propanoic acid/phosphine oxide derivatives (Scheme 1B), which have applications in pharmaceuticals and catalysis.
The reaction proceeded under mild and neutral conditions (10 °C, 24 h, H2O as a nucleophile) without using any metal catalyst, which is consistent with the concepts of green chemistry. In addition, they have carried out the full optimization of the reaction system to obtain high yields (up to 98%). The scopes of phosphine reagents and activated alkynes were investigated under the optimal conditions to disclose a broad spectrum with high yields in most of the cases.
Takizawa, Salem and coworkers have complemented their experimental work with computational calculations to understand and verify their proposed reaction mechanism. DFT calculations and some control experiments using (allenic esters, or vinyl ketone instead of propiolate esters), and (D2O, or H218O instead of water) indicated that the rapid formation of hydroxy-λ5-phosphane as a key intermediate plays a crucial role in smooth C(aryl)–P bond cleavage (Scheme 2).
The development of this new synthetic method using metal-free conditions is crucial for the development of greener chemical practices. Further, the understanding of the mechanism of these synthetic reactions has a key value in expanding their applications, as well as the synthesis of industrially important phosphorus-containing molecules.
Mohamed Salem is a Ph.D. student in the SANKEN at Osaka University (Japan). He received his M.Sc. in Medicinal Chemistry at the Faculty of Pharmacy, Suez Canal University in 2017. Subsequently, he received a Japanese Government Scholarship (MEXT) to study for a Ph.D. in 2019 with Professor Hiroaki SASAI and Shinobu TAKIZAWA. The research field of Mohamed Salem is asymmetric catalysis using first-row organometallic complexes such as Cobalt and Vanadium and the study and development of new green and cutting edge electrochemical transformations.
Shinobu TAKIZAWA is an associate professor in the Institute of Scientific and Industrial Research (SANKEN) and AI Research Center at Osaka University (Japan). He received his Ph.D. in chemistry at the Graduate School of Pharmaceutical Science, Osaka University in 2000. From 2006 to 2008 he did postdoctoral studies at The Scripps Research Institute (USA) with Professor Dale L. Boger. The research field of Professor Shinobu TAKIZAWA is asymmetric catalysis using either organo-or organometallic catalysts and the study and development of new green and cutting edge chemical transformations either electrochemically or photochemically guided by AI and ML. He is the author of more than 118 articles indexed by SCI and cited more than 4000 times with an index H = 38.
Moving from single molecules to dynamic molecular systems and responsive materials requires control of molecules that can induce motion and enable machine-like functions. Light-driven rotary molecular motors are compounds that can undergo unidirectional rotational motion, using light as a power source. Their 360° rotation cycle is driven by the successive interconversion of four unique isomers of the motor, undergoing two energetically uphill photochemical E-Z isomerisation (PEZ) steps and two downhill thermal helix inversion (THI) steps. Thanks to this unidirectional rotational behaviour, molecular motors can be used to produce useful physical work on the nanoscale – finding a multitude of applications in fields of catalysis, smart materials, and nanotechnology.
The physical characteristics governing the rotational motion of molecular motors include their rotational speed, the wavelength of light which the motors can consume to power their motion, and their photochemical efficiency (or quantum yield). Investigating these characteristics will allow us to understand the dynamic behaviour of molecular motors – and ultimately how to tune this behaviour – will allow molecular motors to be used as tuneable actuators and molecular machines.
Molecular motors containing the heterocyclic oxindole moiety were discovered in 2019 and it was found that they have desirable rotational properties – they can be driven with benign visible light, and they can be synthesised easily and quickly, relative to traditional hydrocarbon-based molecular motors. Due to these advantages, more research into the functionalisation of oxindole-based motors could help to further improve these promising motors.
Recently, researchers in the group of Ben Feringa have carried out a systematic benchmark study on oxindole-based molecular motors; through functionalisation with cyano or methoxy groups at three different positions along the motor backbone, the rotational properties of oxindole-based molecular motors can be fine-tuned (Figure 1).
By functionalising the motors at these positions (Figure 1, R1, R2 and R3), the wavelength of light used to power the molecular motors can be further red-shifted into the visible region of the electromagnetic spectrum, which is a lower energy source of light. In some cases, the use of longer wavelengths even improved the photochemical efficiency of the motors. In addition, the photochemical quantum yields of the motors could be tuned, with electron withdrawing cyano groups improving the quantum yields of the photochemical isomerisation steps of the motor rotation cycle.
The favourable properties fulfilled by the oxindole-based molecular motors investigated in this work show the great potential of these molecules to be used as viable visible light-driven actuators, which can be fine-tuned to accurately control nanoscale motion in light responsive systems.
Ben Feringa obtained his PhD degree in 1978 at the University of Groningen in the Netherlands under the guidance of Prof. Hans Wynberg. After working as a research scientist at Shell he was appointed full professor at the University of Groningen in 1988 and named the distinguished Jacobus H. van’t Hoff Professor of Molecular Sciences in 2004. He was elected foreign member of the American Academy of Arts and Sciences, and the Royal Society and is member of the Royal Netherlands Academy of Sciences. His research interests include stereochemistry, organic synthesis, asymmetric catalysis, molecular switches and motors, photopharmacology, self-assembly and nanosystems.
Indian scientist, Dr Harapriya Rath and her collaborator Dr. Dandamudi Usharani experimentally isolated and theoretically supported the first ever s-aromatic doubly N-confused porphyrinoid. Unique π-reconstructions owing to sequential C-oxygenation of N-confused N-methyl pyrrole rings leading to the genesis of 18π aromatic doubly N-confused monooxo porphyrinoid, 16π antiaromatic doubly N-confused diioxo porphyrinoid and σ-aromatic doubly N-confused tetraoxo isophlorinoid via chemical transformation(s) of doubly N-confused porphodimethene.
Harapriya Rath received her PhD degree from the Indian Institute of Technology, Kanpur, India on the Nonlinear Optical studies of Core-modified Aromatic Expanded porphyrinoids under the supervision of Prof. T. K. Chandrashekar in 2006. In 2007, she joined the group of Prof. Hiroshi Shinokubo at the Kyoto University, Japan as JSPS Postdoctoral fellow and focused on the subject of Möbius aromaticity (antiaromaticity) and excited state aromaticity of metallated all-aza expanded porphyrinoids. In 2009, she moved to Prof. Richard EP Winpenny’s lab at the University of Manchester, United Kingdom as a Royal Society Newton International postdoctoral fellow focusing on hybrid Organic-Inorganic Rotaxanes. In 2012, she started an independent academic career at Indian Association for the Cultivation of Science, Kolkata, India as an Assistant professor and Ramanujan Fellow. Since 2017, she is Associate Professor in the School of Chemical Sciences, IACS, Kolkata, India.
Diborons can directly be transformed into diverse organoboron compounds of high synthetic significance via a variety of transition metal-catalyzed and -free borylation reactions. Besides commonly used symmetrical diborons such as (pin)B–B(pin), recently attention has also been paid to the use of unsymmetrical ones, especially those having different boron-Lewis acidities.
Numerous attention have been directed toward developing unsymmetrical diborons with different boron-Lewis acidities including (pin)B–B(aam) (aam = anthranilamidato), (neop)B–B(dan) (dan = naphthalene-1,8-diaminato) and (pin)B–B(mdan) (mdan = N,N′-dimethyl-naphthalene-1,8-diaminato), and their application in catalytic borylation reactions via σ-bond metathesis as a key elementary step; palladium-catalyzed Miyaura–Ishiyama-type borylation of aryl halides and the copper-catalyzed internal-selective hydroboration of terminal alkynes show the synthetic versatility of (pin)B–B(aam).
Recently, the group of Hiroto Yoshida from Hiroshima University has demonstrated that (pin)B–B(aam) can also be catalytically activated by oxidative addition to a platinum complex, leading to syn-diboration of terminal alkynes, and that a highly electron-deficient triarylphosphine developed by Korenaga, P(BFPy)3, is effective for regiocontrol (Figure 1).
A broad substrate scope of the regioselective diboration has been demonstrated (Figure 2): a variety of aromatic and aliphatic terminal alkynes bearing an electron-donating or electron-withdrawing substituent smoothly underwent the diboration to afford the respective products in high yields without damaging the reactive functionalities. Internal alkynes participating in this reaction also gave syn-products with a high regiocontrol, whereas the reaction of 2-octyne (2x) led to the formation of a mixture of regioisomers (3x and 3′x). Finally, a conjugated diene (4) was also readily transformable into a 1,4-diborated product (5) in a stereoselective fashion.
The synthetic utility of the resulting diborylalkene was demonstrated by site-selective cross-coupling to construct a triarylalkene efficiently (Figure 3). In the plausible catalytic cycle, the present diboration could be initiated by the oxidative addition of the boron–boron bond of 1 to a Pt(0) catalyst (Figure 4). The resulting oxidative adduct (8) then accepts the insertion of an alkyne at the Pt–B (pin) bond and/or –B(aam) bond to provide an alkenyl platinum intermediate (9 and/or 9′), which finally affords the diboration product (3) through reductive elimination.
(pin)B–B(aam) with different boron-Lewis acidities can be activated by oxidative addition to a platinum(0) complex, leading to regio- and stereo-selective diboration of unsaturated carbon linkages including terminal alkynes, internal alkynes and a conjugated diene. Moreover, the use of a highly electron-deficient triarylphosphine ligand, P(BFPy)3, is indispensable for the regiocontrol, and electron-deficiency in ligands has been proven to be closely correlated with the regioselectivity.
Hiroto Yoshida graduated from Kyoto University in 1996 and received his Ph.D. from Kyoto University under the supervision of Professors Tamejiro Hiyama and Eiji Shirakawa in 2001. He then became an Assistant Professor in 2001 and an Associate Professor in 2006 at Hiroshima University. From 2020, he has been a Full Professor at the same University. His research interests include (1) development of new methods for synthesis of main group organometallics containing boron, tin or silicon, (2) carbon–carbon bond-forming reactions with the main group organometallics, and (3) aryne-based organic synthesis. He is the author of more than 110 articles indexed by SCI and cited more than 5200 times with an index H = 45.
Web of Science ResearcherID: B-7954-2011 (https://publons.com/researcher/1189090/hiroto-yoshida/)
We are delighted to announce the Best Organic Chemistry Frontiers Covers of 2021!
Read below the scientific papers.
Congratulations to the winners of Best Organic Chemistry Frontiers Covers of 2021!
We expressed our sincere appreciation for all the support and contributions from our authors, reviewers, and readers in the past 2021.
Looking forward to receiving your high-quality work in 2022.
Happy Chinese New Year!
Among small drugs and pharmaceutically relevant molecules, alkaloids have a leading role and those related to pyrrolidine and piperidine are the most common five- and six-membered aliphatic N-heterocycles. Differently from their aromatic counterparts, there are limited strategies to access substituted saturated N-heterocycles with ease and efficiency which fall into two main categories: the cyclization of suitable precursors and the modification of readily available pyrrolidines and piperidines.
Recently, many researchers have dedicated their attention to alternative conditions in order to lower the reaction temperature while keeping the efficiency high in term of both yield and selectivity. In 2010, the group of Köhler observed, for the first time, the acceleration of a Heck reaction under UV-visible light using both homogeneous or heterogeneous Pd(II) pre-catalysts. Few years later, Gevorgyan disclosed an original visible light-induced room temperature Heck reaction between functionalised alkyl halides and styrenes. That was the starting point for the employment of transition metal complexes, especially those containing Pd, as unconventional photocatalysts and remarkable results have been reported by the research groups of Gevorgyan, Fu, Königs, Glorius and Rueping.
Recently, Renzi et al. presented a synthetic strategy to obtain arylated vinyl pirrolidines and piperidines starting from N-tosylaminoallenes. Key to the process was the usage of the blue light which allowed the efficient exploitation of the simple catalytic system Pd(OAc)2/2PPh3 at room temperature. More than fifty Electron-donating, electron-withdrawing aryl and heteroaryl bromides were coupled with allenes in a Pd(0) catalysed cross coupling. A subsequent domino cyclisation is triggered by the tosylamino functionality. The mechanistic investigation, both experimental and computational, highlighted three main aspects: no radicals seems to be involved in this mechanism, the light is not needed in the oxidative addition of Pd(0) to the aryl bromide because of the associated low energy barrier. On the contrary, the light plays its main role in the carbo-palladation step, but its influence in making easier the formation of the active Pd(0) species cannot be excluded (figure 1).
Annamaria Deagostino is associate professor of organic chemistry at the University of Torino. She graduated in chemistry at the University of Torino and after a fellowship at the University of Padova, in 1998 she received her Ph.D. in chemistry at the University of Torino, supervisor Prof. Paolo Venturello, and in the same year she obtained a postdoctoral fellowship at the University of Caen, under the supervision of Prof. Marie-Claire Lasne. In 1999, she became assistant professor of organic chemistry at the University of Torino. The main interests of her research group are in the field of synthetic organic chemistry, mainly focused on organopalladium chemistry, visible light photo-catalysis and the synthesis of BNCT (Boron Neutron Capture Therapy) theranostic agents.
Giovanni Ghigo is assistant professor of organic chemistry at the University of Torino since 2005. He graduated in chemistry 1994 and in 1999 received his Ph.D. in Chemistry at the University of Torino, supervisor Prof. Glauco Tonachini. He obtained postdoctoral fellowships at the University of Torino then at the University of Lund.
GG is a computational organic chemist interested in the study of the mechanisms of a large type of organic reactions spanning from the generation and oxidation of organic pollutants and, more recently, metal-free, copper or palladium catalyzed and photocatalyzed reactions with synthetic applications.
Among various oxidation reactions, olefin epoxidation is undoubtedly one of the most studied organic conversion reactions. The target product epoxide is amongst extensively used chemicals and has important applications in the synthesis of polymers, food additives and drugs. Therefore, efficient synthesis of epoxides holds a significant place for academic and industrial research perspectives. Traditional olefin epoxidation methods generally require toxic and harmful, high-priced oxidants with harsh reaction conditions and difficult separation. The most commonly used process for the industrial synthesis of epoxides is the halogenated alcohol (Br, Cl) method (HALCON process). However, this process produces a large amount of chlorine (bromine) wastewater during the production process, causing serious pollution to the environment. The epoxidation of olefins by electrochemical oxidation of bromide ions is an emerging and promising strategy (Scheme 1). The choice of the electrode is very important for electro-epoxidation process. The common electrodes currently being used in the scientific community are platinum electrodes and carbon electrodes, however they all have certain limitations. Although the platinum electrode has good catalytic activity for the oxidation of bromide ions, but it can be easily corroded by bromide ions, which affect its catalytic activity and causes the waste of precious metal. In addition, carbon electrodes are also known to possess low catalytic activity. Therefore, the development of an economical and efficient electrode that could promote the oxidation of bromide ions for the facile epoxidation of olefins still needs to be endeavoured.
Recently, Zai’s group has reported a strategy for the electrochemical, efficient and green synthesis of styrene epoxide. First, the team synthesized a series of metal sulfide electrodes by solvothermal method, and analysed the catalytic activity of metal sulfide electrodes for bromide ion oxidation via linear sweep voltammetry (LSV). The team noticed that cobalt-based sulfides, especially heterostructure CoS2/CoS, can effectively promote the oxidation of bromide ions on the anode. Subsequently, the electrochemical epoxidation of styrene was carried out with metal sulfides as the anode and carbon rod as the cathode, and a series of optimizations were carried out for the optimal reaction solvent, time and temperature. Under the optimal reaction conditions, the platinum electrode (Pt) displayed a selectivity of 55% for styrene epoxide. This may be due to the surface corrosion of platinum electrode by bromide ions during the reaction and the other reason could be the smaller electrochemical surface area (ECSA) of the platinum electrode. Moreover, the metal sulfide electrodes Cu7S4, CdS, MnS, FeS2, NiS2 have 73%, 76%, 78%, 80%, and 81% selectivity to Styrene epoxide, respectively. It is worth noting that the selectivity of CoS and CoS electrodes towards styrene epoxide reached 88% and 90%, respectively. When CoS and CoS2 were combined to form heterojunction CoS2/CoS, Highly selective synthesis of styrene epoxide (97%) was attained with GF-CoS2/CoS as anode and carbon rod as cathode. The applied voltage was reduced to 4-5 V at 30 mA cm-2 compared to 7.8-9.3 V on Pt cathode, which effectively reduces the energy consumption of the reaction. Subsequently, the team conducted in-depth studies on the mechanism (Scheme 2) involved in the electrochemical epoxidation of styrene by conducting experiments in different electrolytes, different atmospheres, different reaction cells (single, H-type), and different free radical quenchers (Figure 1).
The mechanistic investigations inferred the generation of oxygen redox species (such as OH•, H2O2, O2•¯) besides the olefin-based radicals and bromine radical, facilitating the epoxide formation. The synergistic cooperation between the bromine redox species and oxygen redox species (generated on graphite rod cathode) can establish a benign electro-epoxidation system for olefin substrates in an ambient environment (Scheme 3). The group demonstrated that the proposed strategy could also be applied to other olefins. As this strategy is simple, sustainable, and economical so, it could be of great significance for technical aspects.
Jiantao Zai is associate professor in the school of chemistry and chemical engineering in Shanghai Jiaotong University. He is engaged in the design, synthesis and performance research of inorganic solid materials, especially micro-nano, multi-component functional materials. He has published 106 papers in journals such as Nat Commun, Adv Energy Mater, Nano Energy, etc. The papers have been cited 4559 times, and the H index is 40. In 2015, he won the first prize of Shanghai Natural Science (No.5), and was selected as Shanghai in 2018. Youth Science and Technology Venus (Class A).
Selective C-H aromatization of aromatic compounds has long been a challenging issue. Directing group strategies can only make the ortho- or meta-sites of the aromatic compounds activate in past years. Recently, an elegant approach was reported to successfully achieved the para-arylation of arene by means of a cooperative action of directing group with norbornene relay (Scheme 1). This strategy first implements meta-C-H activation through directing group guidance, subsequently, the transient mediator norbornene relay Pd to the para-position to achieve precise siteselectivity. Understanding this novel relay mechanism is of great significance for achieving more extensive and accurate site selection.
Dezhan Chen and collaborators explored in detail the mechanism of the arylation of arenes by means of norbornene relay palladation through meta- to para-selectivity using DFT calculations. The results revealed that the reaction was initiated by a [mono N-protected amino acid ligand-Pd] complex to activate firstly the meta-C-H guided by the directing group. The para-arylation was subsequently achieved by NBE relay palladation from meta- to para-position (Figure 1). Significantly, the palladium/norbornene cooperative relay was realized by a bimetallic Pd-Ag complex. The authors demonstrated that the Pd-Ag bimetallic complex play a significant stabilization role in secondary para-C-H activation rather than the intuitive tether length (Figure 2).
The calculated energy profiles of the NBE relay Pd through meta- to para-position to produce para-arylated product is summarized in Figure 3a (black pathway). The author further optimized the energy profiles of NBE relay palladation through para- to meta-position (blue pathway), while this pathway was kinetically unfavorable. The calculation results shown that directing group assisted primary C-H activation was the rate-determining step for the overall catalytic cycle, also the key step of determining siteselectivity. The primary meta-activation was favorable in energy due to less ring strain in the cyclic nitrile-coordinated C-H transition states in meta-position. As illustrated by the Gibbs energy profile in Figure 3b, norbornene insertion, as the key step to achieve the expected Pd relay, can take place spontaneously with exothermic of 7.1 kcal/mol, which can compensates for the energy needed to cross the barrier. It follows that the palladium relay process is driven thermodynamically to achieve the activation from meta- to para-position.
The perfect cooperation of a remote directing group and a transient mediator NBE through the alternating association with the Pd center achieved the active site relay through meta- to para-position. The present results provided a reasonable insight into the para-C-H arylation by Pd/MPAA/NBE cooperative catalysis in conjunction with a precise directing group and Ag(Ⅰ) additive and would have implications for understanding C-H functionalization chemistry by norbornene relay palladation.
Dezhan Chen is professor in College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University (China). He received his master’s degree in Department of Chemistry of Shandong University in 1989. He studied in UCLA at Houk group as a visiting scholar in 1996-1997. The research interest of Professor Chen is the exploration of theoretical mechanism of chemical reaction and its thermodynamic and kinetic processes.