Thermal C-C bond cleavage reactions allow the construction of structurally diverse molecular skeletons via predictable and efficient bond reorganizations. Visible light photoredox-catalyzed ...radical-mediated C-C bond cleavage reactions have recently emerged as a powerful alternative method for overcoming the thermodynamic and kinetic barrier of C-C bond cleavage in diverse molecular scaffolds. In recent years, a plethora of elegant and useful reactions have been invented, and the products are sometimes otherwise inaccessible by classic thermal reactions. Considering the great influence and synthetic potential of these reactions, we provide a summary of the state of art visible light-driven radical-mediated C-C bond cleavage/functionalization strategies with a specific emphasis on the working models. We hoped that this review will be useful for medicinal and synthetic organic chemists and will inspire further reaction development in this interesting area.
A room‐temperature, visible‐light‐driven N‐centered iminyl radical‐mediated and redox‐neutral C−C single bond cleavage/radical addition cascade reaction of oxime esters and unsaturated systems has ...been accomplished. The strategy tolerates a wide range of O‐acyl oximes and unsaturated systems, such as alkenes, silyl enol ethers, alkynes, and isonitrile, enabling highly selective formation of various chemical bonds. This method thus provides an efficient approach to various diversely substituted cyano‐containing alkenes, ketones, carbocycles, and heterocycles.
A visible‐light‐driven room‐temperature N‐centered iminyl radical‐mediated and redox‐neutral C−C single bond cleavage/radical addition cascade reaction of oxime esters and unsaturated systems has been accomplished. The strategy tolerates a wide range of O‐acyl oximes and alkenes, silyl enol ethers, alkynes, and isonitrile. This method allows access to various cyano‐containing alkenes, ketones, carbocycles, and heterocycles.
Replacement of CC unit with its isoelectronic BN unit in aromatics provides a new class of molecules with appealing properties, which have attracted great attention recently. In this Concept, we ...focus on BN‐substituted polycyclic aromatics with fused structures, and review their synthesis, photophysical, and redox properties, as well as their applications in organic electronics. We also present challenging synthetic targets, large BN‐ substituted polycyclic aromatics, such as regioregular BN heterosuperbenzenes, which can be viewed as BN‐doped nanographenes. Finally, we propose an atomically precise bottom‐up synthesis of structurally well‐defined BN‐doped graphenes.
A new super hero! BN substitution in aromatic systems could provide a new family of interesting compounds. In this Concept, the development of BN‐substituted polycyclic aromatics is reported, and their synthesis, properties and electronic applications are summarized. From monocyclic BN‐substituted benzene to polycyclic BN heteroaromatics (like BN heterosuperbenzene), the possible ways to structurally well‐defined BN‐doped graphenes are proposed.
A visible‐light‐driven, copper‐catalyzed three‐component radical cross‐coupling of oxime esters, styrenes, and boronic acids has been developed. Key steps of this protocol involve catalytic ...generation of an iminyl radical from a redox‐active oxime ester and subsequent C−C bond cleavage to generate a cyanoalkyl radical. Upon its addition to styrene, the newly formed benzylic radical undergoes coupling with a boronic‐acid‐derived ArCuII complex to achieve 1,1‐diarylmethane‐containing alkylnitriles.
Key steps of the visible‐light‐driven title reaction involve catalytic generation of an iminyl radical from a redox‐active oxime ester and subsequent C−C bond cleavage to give a cyanoalkyl radical. Upon its addition to styrene, a benzylic radical is formed that undergoes coupling with a boronic‐acid‐derived ArCuII complex to give 1,1‐diarylmethane‐containing alkylnitriles. The method features the use of readily available substrates and high functional‐group tolerance.
New advances in nanographene chemistry Narita, Akimitsu; Wang, Xiao-Ye; Feng, Xinliang ...
Chemical Society reviews,
09/2015, Letnik:
44, Številka:
18
Journal Article
Recenzirano
Odprti dostop
Nanographenes, or extended polycyclic aromatic hydrocarbons, have been attracting renewed and more widespread attention since the first experimental demonstration of graphene in 2004. However, the ...atomically precise fabrication of nanographenes has thus far been achieved only through synthetic organic chemistry. The precise synthesis of quasi-zero-dimensional nanographenes,
i.e.
graphene molecules, has witnessed rapid developments over the past few years, and these developments can be summarized in four categories: (1) non-conventional methods, (2) structures incorporating seven- or eight-membered rings, (3) selective heteroatom doping, and (4) direct edge functionalization. On the other hand, one-dimensional extension of the graphene molecules leads to the formation of graphene nanoribbons (GNRs) with high aspect ratios. The synthesis of structurally well-defined GNRs has been achieved by extending nanographene synthesis to longitudinally extended polymeric systems. Access to GNRs thus becomes possible through the solution-mediated or surface-assisted cyclodehydrogenation, or "graphitization," of tailor-made polyphenylene precursors. In this review, we describe recent progress in the "bottom-up" chemical syntheses of structurally well-defined nanographenes, namely graphene molecules and GNRs.
This review discusses recent advancements in nanographene chemistry, focusing on the bottom-up synthesis of graphene molecules and graphene nanoribbons.
Conspectus Nanographenes, which are defined as nanoscale (1–100 nm) graphene cutouts, include quasi-one-dimensional graphene nanoribbons (GNRs) and quasi-zero-dimensional graphene quantum dots ...(GQDs). Polycyclic aromatic hydrocarbons (PAHs) larger than 1 nm can be viewed as GQDs with atomically precise molecular structures and can thus be termed nanographene molecules. As a result of quantum confinement, nanographenes are promising for next-generation semiconductor applications with finite band gaps, a significant advantage compared with gapless two-dimensional graphene. Similar to the atomic doping strategy in inorganic semiconductors, incorporation of heteroatoms into nanographenes is a viable way to tune their optical, electronic, catalytic, and magnetic properties. Such properties are highly dependent not only on the molecular size and edge structure but also on the heteroatom type, doping position, and concentration. Therefore, reliable synthetic methods are required to precisely control these structural features. In this regard, bottom-up organic synthesis provides an indispensable way to achieve structurally well-defined heteroatom-doped nanographenes. Polycyclic heteroaromatic compounds have attracted great attention of organic chemists for decades. Research in this direction has been further promoted by modern interest in supramolecular chemistry and organic electronics. The rise of graphene in the 21st century has endowed large polycyclic heteroaromatic compounds with a new role as model systems for heteroatom-doped graphene. Heteroatom-doped nanographene molecules are in their own right promising materials for photonic, optoelectronic, and spintronic applications because of the extended π conjugation. Despite the significant advances in polycyclic heteroaromatic compounds, heteroatom-doped nanographene molecules with sizes of over 1 nm and their relevant GNRs are still scarce. In this Account, we describe the synthesis and properties of large heteroatom-doped nanographenes, mainly summarizing relevant advances in our group in the past decade. We first present several examples of heteroatom doping based on the prototypical nanographene molecule, i.e., hexa-peri-hexabenzocoronene (HBC), including nitrogen-doped HBC analogues by formal replacement of benzene with other heterocycles (e.g., aromatic pyrimidine and pyrrole and antiaromatic pyrazine) and sulfur-doped nanographene molecules via thiophene annulation. We then introduce heteroatom-doped zigzag edges and a variety of zigzag-edged nanographene molecules incorporating nitrogen, boron, and oxygen atoms. We finally summarize heteroatom-doped GNRs based on the success in the molecular cases. We hope that this Account will further stimulate the synthesis and applications of heteroatom-doped nanographenes with a combined effort from different disciplines.
A benzo‐fused double 7carbohelicene (D7H) was synthesized through a regioselective cyclodehydrogenation of a tetranaphthyl‐p‐terphenyl‐based precursor. The twisted (D7H‐1) and anti‐folded (D7H‐2) ...conformers of D7H were separated by recrystallization, and their double helicene structures with overlapping terminal benzene rings were unambiguously elucidated by X‐ray crystallography. A record‐high isomerization barrier (46.0 kcal mol−1) in double helicenes was estimated based on density functional theory (DFT) calculation, which resulted in the excellent conformational stability of D7H. The physicochemical properties of D7H‐1 and D7H‐2 were investigated by UV/Vis absorption spectroscopy and cyclic voltammetry, displaying the variation of electronic structure upon conformational changes. The optical resolution of the racemic D7H‐1 was carried out by chiral HPLC, offering enantiopure D7H‐1‐(P,P) and D7H‐1‐(M,M), which were further characterized by circular dichroism spectroscopy.
Carbon‐rich compounds: An all‐carbon double 7helicene has been synthesized by regioselective cyclodehydrogenation. The compound represents the highest homolog of the double carbohelicenes. The twisted conformers D7H‐1 and D7H‐2 were separated by recrystallization, and their double helicene structures were elucidated by X‐ray crystallography.
The Friedel–Crafts acylation reaction, which belongs to the class of electrophilic aromatic substitutions is a highly valuable and versatile reaction in synthesis. Regioselectivity is predictable and ...determined by electronic as well as steric factors of the (hetero)arene substrate. Herein, a radical approach for the acylation of arenes and heteroarenes is presented. C−H acylation is achieved through mild cooperative photoredox/NHC radical catalysis with the cross‐coupling of an arene radical cation with an NHC‐bound ketyl radical as a key step. As compared to the classical Friedel–Crafts acylation, a regiodivergent outcome is observed upon switching from the ionic to the radical mode. In these divergent reactions, aroyl fluorides act as the acylation reagents in both the ionic as well as the radical process.
Regiodivergent benzoylation of 2‐methoxynaphthalene with benzoyl fluoride is presented using either Friedel–Crafts‐type chemistry to give selectively the 1‐isomer or cooperative NHC/photoredox catalysis to afford the 4‐isomer exclusively. Along with arenes, regiodivergent aroylation also works for heteroarenes.
A photoredox-catalyzed iminyl radical-triggered C-C bond cleavage/addition/Kornblum oxidation cascade of cycloketone oxime esters and styrenes in DMSO is described. This three-component, one-pot ...procedure features mild conditions, a broad substrate scope, and high functional group tolerance, providing an efficient approach to access diversely functionalized ketonitriles.
Conspectus Nitrogen-centered radicals (NCRs) are a versatile class of highly reactive species that have a longer history than the classical carbon-based radicals in synthetic chemistry. Depending on ...the N-hybridization and substitution patterns, NCRs can serve as electrophiles or nucleophiles to undergo various radical transformations. Despite their power, progress in nitrogen-radical chemistry is still slow compared with the popularity of carbon radicals, and their considerable synthetic potential has been largely underexplored, which is, as concluded by Zard, mainly hampered by “a dearth of convenient access to these species and a lack of awareness pertaining to their reactivity”. Over the past decade, visible-light photoredox catalysis has been established as a powerful toolbox that synthetic chemists can use to generate a diverse range of radical intermediates from native organic functional groups via a single electron transfer process or energy transfer under mild reaction conditions. This catalytic strategy typically obviates the need for external stoichiometric activation reagents or toxic initiators and often enables traditionally inaccessible ionic chemical reactions. On the basis of our long-standing interest in nitrogen chemistry and catalysis, we have emphasized the use of visible-light photoredox catalysis as a tactic to discover and develop novel methods for generating NCRs in a controlled fashion and synthetic applications. In this Account, we describe our recent advances in the development of visible-light-driven photoredox-catalyzed generation of NCRs and their synthetic applications. Inspired by the natural biological proton-coupled electron transfer (PCET) process, we first developed a strategy of visible-light-driven photoredox-catalyzed oxidative deprotonation electron transfer to activate the N–H bonds of hydrazones, benzamides, and sulfonamides to give the corresponding NCRs under mild reaction conditions. With these reactive species, we then achieved a range of 5-exo and 6-endo radical cyclizations as well as cascade reactions in a highly regioselective manner, providing access to a variety of potentially useful nitrogen heterocycles. To further expand the repertoire of possible reactions of NCRs, we also revealed that iminyl radicals, derived from O-acyl cycloalkanone oxime esters, can undergo facile ring-opening C–C bond cleavage to give cyanoalkyl radicals. These newly formed radical species can further undergo a variety of C–C bond-forming reactions to allow the synthesis of diverse distally functionalized alkyl nitriles. Stimulated by these studies, we further developed a wide variety of visible-light-driven copper-catalyzed radical cross-coupling reactions of cyanoalkyl radicals. Because of their inherent highly reactive and transient properties, the strategy of heteroatom-centered radical catalysis is still largely underexplored in organic synthesis. Building on our understanding of the fundamental chemistry of NCRs, we also developed for the first time the concept of NCR covalent catalysis, which involves the use of in situ-photogenerated NCRs to activate allyl sulfones, vinylcyclopropanes, and N-tosyl vinylaziridines. This catalytic strategy has thus enabled efficient difunctionalization of various alkenes and late-stage modification of complex biologically active molecules. In this Account, we describe a panoramic picture of our recent contributions since 2014 to the development and application of the visible-light-driven photoredox systems in the field of NCR chemistry. These studies provide not only efficient methods for the synthesis of functionally rich molecules but also some insight into the exploration of new reactivity or reaction modes of NCRs.
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