Conspectus Transition-metal catalyzed cross-coupling reactions have emerged as a powerful tool for constructing biaryl compounds. Aryl halides and aryl metallic reagents (typically prepared from aryl ...halides) are used as coupling partners. It would be desirable to replace either aryl halide or aryl metallic reagents used in cross-couplings reactions with more readily available surrogates. Oxidative dehydrogenative cross-coupling between two different “inert” aryl C–H bonds represents an ideal system that would revolutionize cross-coupling chemistry. Furthermore, cross-coupling reactions might be improved by developing new catalytic protocols based on cheap transition-metal catalysts or even transition-metal-free systems to decrease costs and avoid the use of heavy metal and noble transition metals. It would be desirable to promote both catalytic systems and replace either or both coupling partners. We have used different strategies to improve cross-coupling reactions for constructing biaryls, which we categorized into four groups as follows. First, we focused on developing methodologies to be applied to easily produced and naturally abundant arenol-based electrophiles in cross-coupling via C–O activation. We have extended coupling partners to aryl carboxylates and arenols. Direct application of arenes as surrogates for organohalides and organometallic reagents avoids the tedious preparation of these reagents from arenes and considerably reduces the cost of starting materials. We have also explored cross-coupling reactions of arenes with various organometallic reagents, such as arylboronic acids, arylsilanes, and aryl Grignard reagents. Second, we summarize oxidative cross-coupling reactions based on C–H activation with aryl metallic reagents. On the basis of the reactivity patterns of different organometallic reagents, we adapted different catalytic systems to achieve effective cross-coupling reactions. Third, we improved a well-developed cross-coupling between arenes and organohalides through a strategy of replacing one coupling partner and using a new catalytic system. We have applied earth-abundant transition metals, such as Fe, and Co, and even developed transition-metal-free catalytic systems. Finally, our ultimate goal is to construct biaryls by cross dehydrogenative arylation between two different arenes. Owing to the structural similarity of both arenes, in particular two substituted benzenes, the greatest challenges are not only achieving regio- and chemo-selective C–H activation reactions but also matching both the reactivities and selectivities of both substrates to avoid homocouplings of either arene. Through our efforts, we have developed and applied four different strategies by introducing directing groups, controlling electronic and steric properties, and using dual directing strategies. We hope our studies will stimulate interest and new thinking on cross-couplings reactions for building carbon–carbon bonds from readily available and inexpensive chemicals from basic petroleum chemistry and nature.
Lithium‐rich layered oxides with the capability to realize extraordinary capacity through anodic redox as well as classical cationic redox have spurred extensive attention. However, the ...oxygen‐involving process inevitably leads to instability of the oxygen framework and ultimately lattice oxygen release from the surface, which incurs capacity decline, voltage fading, and poor kinetics. Herein, it is identified that this predicament can be diminished by constructing a spinel Li4Mn5O12 coating, which is inherently stable in the lattice framework to prevent oxygen release of the lithium‐rich layered oxides at the deep delithiated state. The controlled KMnO4 oxidation strategy ensures uniform and integrated encapsulation of Li4Mn5O12 with structural compatibility to the layered core. With this layer suppressing oxygen release, the related phase transformation and catalytic side reaction that preferentially start from the surface are consequently hindered, as evidenced by detailed structural evolution during Li+ extraction/insertion. The heterostructure cathode exhibits highly competitive energy‐storage properties including capacity retention of 83.1% after 300 cycles at 0.2 C, good voltage stability, and favorable kinetics. These results highlight the essentiality of oxygen framework stability and effectiveness of this spinel Li4Mn5O12 coating strategy in stabilizing the surface of lithium‐rich layered oxides against lattice oxygen escaping for designing high‐performance cathode materials for high‐energy‐density lithium‐ion batteries.
A heterostructured spinel Li4Mn5O12 encapulated lithium‐rich layered oxide cathode is designed by the controlled KMnO4 oxidiation strategy. Spinel Li4Mn5O12 is chosen due to its lattice stability against oxygen release as well as a 3D lithium diffusion framework with minimal Jahn–Teller distortion. Such uniform coating can suppress lattice oxygen release, associated phase transformation, and catalytic side reactions, consequently ensuring improved electrochemical performance.
Activation of inert chemical bonds, such as C–H, C–O, C–C, and so on, is a very important area, to which has been drawn much attention by chemists for a long time and which is viewed as one of the ...most ideal ways to produce valuable chemicals. Under modern chemical bond activation logic, many conventionally viewed “inert” chemical bonds that were intact under traditional conditions can be reconsidered as novel functionalities, which not only avoids the tedious synthetic procedures for prefunctionalizations and the emission of undesirable wastes but also inspires chemists to create novel synthetic strategies in completely different manners. Although activation of “inert” chemical bonds using stoichiometric amounts of transition metals has been reported in the past, much more attractive and challenging catalytic transformations began to blossom decades ago. Compared with the broad application of late and noble transition metals in this field, the earth-abundant first-row transition-metals, such as Fe, Co, and Ni, have become much more attractive, due to their obvious advantages, including high abundance on earth, low price, low or no toxicity, and unique catalytic characteristics. In this Account, we summarize our recent efforts toward Fe, Co, and Ni catalyzed “inert” chemical bond activation. Our research first unveiled the unique catalytic ability of iron catalysts in C–O bond activation of both carboxylates and benzyl alcohols in the presence of Grignard reagents. The benzylic C–H functionalization was also developed via Fe catalysis with different nucleophiles, including both electron-rich arenes and 1-aryl-vinyl acetates. Cobalt catalysts also showed their uniqueness in both aromatic C–H activation and C–O activation in the presence of Grignard reagents. We reported the first cobalt-catalyzed sp2 C–H activation/arylation and alkylation of benzohquinoline and phenylpyridine, in which a new catalytic pathway via an oxidative addition process was demonstrated to be much preferable. Another interesting discovery was the Co-catalyzed magnesiation of benzylic alcohols in the presence of different Grignard reagents, which proceeded via Co-mediated selective C–O bond activation. In C–O activation, Ni catalysts were found to be most powerful, showing the high efficacy in different kinds of couplings starting form “inert” O-based electrophiles. In addition, Ni catalysts exhibited their power in C–H and C–C activation, which have been proven by us and pioneers in this field. Notably, our developments indicated that the catalytic efficacy in cross coupling between aryl bromides and arenes under mild conditions was not the privilege of several noble metals; most of the transition metals exhibited credible catalytic ability, including Fe, Co, and Ni. We hope our studies inspire more interest in the development of first row transition metal-catalyzed inert chemical bond functionalization.
Tea is the world＇s oldest and most popular caffeine-containing beverage with immense economic, medicinal, and cultural importance. Here, we present the first high-quality nucleotide sequence of the ...repeat-rich （80.9%）, 3.02-Gb genome of the cultivated tea tree Camellia sinensis. We show that an extraordinarily large genome size of tea tree is resulted from the slow, steady, and long-term amplification of a few LTR retrotransposon families. In addition to a recent whole-genome duplication event, lineage-specific expansions of genes associated with flavonoid metabolic biosynthesis were discovered, which enhance catechin production, terpene enzyme activation, and stress tolerance, important features for tea flavor and adaptation. We demonstrate an independent and rapid evolution of the tea caffeine synthesis pathway relative to cacao and coffee. A comparative study among 25 Camellia species revealed that higher expression levels of most flavonoid- and caffeinebut not theanine-related genes contribute to the increased production of catechins and caffeine and thus enhance tea-processing suitability and tea quality. These novel findings pave the way for further metabolomic and functional genomic refinement of characteristic biosynthesis pathways and will help develop a more diversified set of tea flavors that would eventually satisfy and attract more tea drinkers worldwide.
Since their development in the 1970s, cross-coupling reactions catalyzed by transition metals have become one of the most important tools for constructing both carbon−carbon and carbon−heteroatom ...bonds. Traditionally, organohalides were widely studied and broadly used as the electrophile, both in the laboratory and in industry. Unfortunately, the high cost, environmental toxicity, and sluggish preparation often associated with aryl halides can make them undesirable for the large-scale syntheses of industrial applications. However, with the further development of catalytic systems, and particularly of the ligands contained therein, a variety of electrophiles have now been successfully applied to cross-coupling reactions. Oxygen-based electrophiles have attracted much attention due to their ready availability from phenol and carbonyl compounds. Initially, aryl and alkenyl triflates were used in cross-coupling reactions due to their high reactivity; however, low moisture stability and high cost hampered their application. Later, with the development of highly efficient catalytic systems, the less reactive sulfonates and phosphates were successfully employed in cross-coupling reactions. Although they have higher stability and can be easily prepared, low atom economy remains an obstacle to their broader utility. Our group has worked to directly apply the abundant and readily available oxygen-containing compounds, such as phenols, alcohols, ethers, and carbonyl compounds, to cross-coupling reactions. In this Account, we describe our recent efforts in transition-metal-catalyzed cross-coupling reactions of new O-based electrophiles via C−O bond activation. We began by developing the methylation of aryl methyl ethers and benzyl methyl ethers via Ni-catalyzed selective C−O bond cleavage. With the refined Ni-based catalytic system, we further applied aryl/alkenyl carboxylates and carbamates to Suzuki−Miyaura, Negishi, and Kumada−Tamao−Corriu reactions to construct various biaryl scaffolds and highly substituted alkenes. To further improve the carbon atom economy, we developed the diaryl sulfates as one-by-one electrophiles (that is, both aryl groups are used in the reaction). Most recently, we have achieved the first successful cross-coupling reaction of magnesium naphtholates with aryl Grignard reagents. These results extend aryl and benzyl ethers, aryl and alkenyl carboxylates/carbamates, and magnesium naphtholates as novel electrophiles in cross-coupling reactions. More importantly, these studies contribute to our better understanding the intrinsic nature of C−O bonds, which were traditionally considered “inert” but clearly show enormous synthetic potential with the proper catalysts.
Enol and phenol functionalities are very common in organic molecules. Utilization of these materials is very appealing in organic synthesis because they are important alternatives to organohalides in ...cross‐coupling reactions. In this review, we summarize the transition‐metal‐catalyzed cross‐coupling of enol‐ and phenol‐based electrophiles, including phosphates, sulfonates, ethers, carboxylates, and phenolates.
Coupled up! Protected enol and phenol compounds are important alternatives to organohalides in cross‐coupling reactions. The transition‐metal‐catalyzed cross‐coupling of enol‐ and phenol‐based electrophiles, including phosphates, sulfonates, ethers, carboxylates, and phenolates, have been summarized (see scheme; PG=protecting group).
Organoborane compounds are among the most commonly employed intermediates in organic synthesis and serve as crucial precursors to alcohols, amines, and various functionalized molecules. A simple ...palladium‐based system catalyzes the conversion of primary C(sp3)H bonds in functionalized complex organic molecules into alkyl boronate esters. Amino acids, amino alcohols, alkyl amines, and a series of bioactive molecules can be functionalized with the use of readily available and removable directing groups in the presence of commercially available additives, simple ligands, and oxygen (O2) as the terminal oxidant. This approach represents an economic and environmentally friendly method that could find broad applications.
Crucial additives: A simple palladium‐based system catalyzes the conversion of primary C(sp3)H bonds in complex organic molecules into alkyl boronate esters. Amino acids, amino alcohols, alkyl amines, and a series of bioactive molecules that are modified with readily available directing groups are functionalized in the presence of commercially available additives, simple ligands, and oxygen as the terminal oxidant.
Dibenzopyranones were synthesized by a palladium‐catalyzed phenol‐directed C–H activation/carbonylation of 2‐phenylphenol derivatives in the presence of CO. Pd(OAc)2 was used as a catalyst and ...Cu(OAc)2 as a catalytic oxidant in the presence of air.
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