Cross-coupling of C(sp)-H Bonds with Organometallic Reagents via Pd(II)/Pd(0) Catalysis**

Palladium-catalyzed C-H activation/C-C bond-forming reactions have emerged as a promising class of synthetic tools in organic chemistry. Among the many different means of forging C-C bonds using Pd-mediated C-H activation, a new horizon in this field is Pd(II)-catalyzed cross-coupling of C-H bonds with organometallic reagents via a Pd(II)/Pd(0) catalytic cycle. While this type of reaction has proven to be effective for the selective functionalization of aryl C(sp(2))-H bonds, the focus of this review is on Pd(II)-catalyzed C(sp(3))-H activation/C-C cross-coupling, a topic of particular importance because reactions of this type enable fundamentally new methods for bond construction. Since our laboratory's initial report on cross-coupling of C-H bonds in 2006, this area has expanded rapidly, and the unique ability of Pd(II) catalysts to cleave and functionalize alkyl C(sp(3))-H bonds has been exploited to develop protocols for forming an array of C(sp(3))-C(sp(2)) and C(sp(3))-C(sp(3)) bonds. Furthermore, enantioselective C(sp(3))-H activation/C-C cross-coupling has been achieved through the use of chiral amino acid-derived ligands, offering a novel technique for producing enantioenriched molecules. Although this nascent field remains at an early stage of development, further investigations hold the potential to revolutionalize the way in which chiral molecules are synthesized in industrial and academic laboratories.     

Catalytic sequential reactions involving palladacycle-directed aryl coupling steps

Catalytic methods are important tools for the synthesis of C-C bonds under mild and ambient conditions. Palladium chemistry predominates in this area because it offers the opportunity to form several different types of bonds in one pot. Palladium can also tolerate a variety of functional groups. Among the many investigations of catalytic aryl-aryl couplings, the most successful technique has been the Suzuki reaction, which uses an arylboronic acid to attack an aryl-Pd bond. This Account reports our methodology, based on the cooperative action of Pd and norbornene, that achieves selective aryl-aryl coupling through C-halide and C-H activation. We are primarily interested in Pd-catalyzed sequential reactions. These reactions combine palladium as an inorganic catalyst and a strained olefin such as norbornene as an organic catalyst and can lead to biphenyl derivatives. While the palladium facilitates C-C bond formation through C-halide and C-H activation, the norbornene contributes to the construction of a palladacycle, an intermediate structure that controls and directs the subsequent reaction steps selectively. To achieve regioselective arylation at the carbon ortho to the original C-halide bond, palladacycles require an additional ortho substituent (R(1)). The palladacycle opens, giving rise to a biphenylylnorbornylpalladium complex. Because of the steric hindrance exerted by the two ortho groups, norbornene deinsertion readily occurs to form a biphenylylpalladium complex. Thus, norbornene acts as a removable scaffold. We used this biphenylylpalladium species to form C-C (with olefins, alkynes, or arylboronic acids) or C-H bonds (by hydrogenolysis). Using nonidentical aryl or heteroaryl halides, we also formed a biaryl-bonded Pd species able to undergo the final termination reaction (C-C, C-N, or C-O bond formation) either inter- or intramolecularly. We used this method to synthesize a variety of aromatic and heteroaromatic compounds. We also obtained the key metallacycle able to selectively direct the reactions by replacing norbornene with an aryl-bonded aminocarbonyl group. This method provided a diverse series of condensed heterocycles.     

Catalytic C-h/olefin coupling

The cleavage and addition of ortho C-H bonds in various aromatic compounds such as ketones, esters, imines, imidates, nitrile, and aldehydes to olefins and acetlylenes can be achieved catalytically with the aid of ruthenium catalysts. The reaction is generally highly efficient and useful in synthetic methods. The coordination to the metal center by a heteroatom in directing groups such as carbonyl and imino groups is the key. The reductive elimination to form a C-C bond is the rate-determining step.     

C-F and C-H bond activation of fluorobenzenes and fluoropyridines at transition metal centers: how fluorine tips the scales

In this Account, we describe the transition metal-mediated cleavage of C-F and C-H bonds in fluoroaromatic and fluoroheteroaromatic molecules. The simplest reactions of perfluoroarenes result in C-F oxida tive addition, but C-H activation competes with C-F activation for partially fluorinated molecules. We first consider the reactivity of the fluoroaromatics toward nickel and platinum complexes, but extend to rhenium and rhodium where they give special insight. Sections on spectroscopy and molecular structure are followed by discussions of energetics and mechanism that incorporate experimental and computational results. We highlight special characteristics of the metal-fluorine bond and the influence of the fluorine substituents on energetics and mechanism. Fluoroaromatics reacting at an ML(2) center initially yield η(2)-arene complexes, followed usually by oxidative addition to generate MF(Ar(F))(L)(2) or MH(Ar(F))(L)(2) (M is Ni, Pd, or Pt; L is trialkylphosphine). The outcome of competition between C-F and C-H bond activation is strongly metal dependent and regioselective. When C-H bonds of fluoroaromatics are activated, there is a preference for the remaining C-F bonds to lie ortho to the metal. An unusual feature of metal-fluorine bonds is their response to replacement of nickel by platinum. The Pt-F bonds are weaker than their nickel counterparts; the opposite is true for M-H bonds. Metal-fluorine bonds are sufficiently polar to form M-F···H-X hydrogen bonds and M-F···I-C(6)F(5) halogen bonds. In the competition between C-F and C-H activation, the thermodynamic product is always the metal fluoride, but marked differences emerge between metals in the energetics of C-H activation. In metal-fluoroaryl bonds, ortho-fluorine substituents generally control regioselectivity and make C-H activation more energetically favorable. The role of fluorine substituents in directing C-H activation is traced to their effect on bond energies. Correlations between M-C and H-C bond energies demonstrate that M-C bond energies increase far more on ortho-fluorine substitution than do H-C bonds. Conventional oxidative addition reactions involve a three-center triangular transition state between the carbon, metal, and X, where X is hydrogen or fluorine, but M(d)-F(2p) repulsion raises the activation energies when X is fluorine. Platinum complexes exhibit an alternative set of reactions involving rearrangement of the phosphine and the fluoroaromatics to a metal(alkyl)(fluorophosphine), M(R)(Ar(F))(PR(3))(PR(2)F). In these phosphine-assisted C-F activation reactions, the phosphine is no spectator but rather is intimately involved as a fluorine acceptor. Addition of the C-F bond across the M-PR(3) bond leads to a metallophosphorane four-center transition state; subsequent transfer of the R group to the metal generates the fluorophosphine product. We find evidence that a phosphine-assisted pathway may even be significant in some apparently simple oxidative addition reactions. While transition metal catalysis has revolutionized hydrocarbon chemistry, its impact on fluorocarbon chemistry has been more limited. Recent developments have changed the outlook as catalytic reactions involving C-F or C-H bond activation of fluorocarbons have emerged. The principles established here have several implications for catalysis, including the regioselectivity of C-H activation and the unfavorable energetics of C-F reductive elimination. Palladium-catalyzed C-H arylation is analyzed to illustrate how ortho-fluorine substituents influence thermodynamics, kinetics, and regioselectivity.     

Borylation and silylation of C-H bonds: a platform for diverse C-H bond functionalizations

Methods that functionalize C-H bonds can lead to new approaches for the synthesis of organic molecules, but to achieve this goal, researchers must develop site-selective reactions that override the inherent reactivity of the substrates. Moreover, reactions are needed that occur with high turnover numbers and with high tolerance for functional groups if the C-H bond functionalization is to be applied to the synthesis of medicines or materials. This Account describes the discovery and development of the C-H bond functionalization of aliphatic and aromatic C-H bonds with borane and silane reagents. The fundamental principles that govern the reactivity of intermediates containing metal-boron bonds are emphasized and how an understanding of the effects of the ligands on this reactivity led us to broaden the scope of main group reagents that react under mild conditions to generate synthetically useful organosilanes is described. Complexes containing a covalent bond between a transition metal and a three-coordinate boron atom (boryl complexes) are unusually reactive toward the cleavage of typically unreactive C-H bonds. Moreover, this C-H bond cleavage leads to the formation of free, functionalized product by rapid coupling of the hydrocarbyl and boryl ligands. The initial observation of the borylation of arenes and alkanes in stoichiometric processes led to catalytic systems for the borylation of arenes and alkanes with diboron compounds (diborane(4) reagents) and boranes. In particular, complexes based on the Cp*Rh (in which Cp is the cyclopentadienyl anion) fragment catalyze the borylation of alkanes, arenes, amines, ethers, ketals, and haloalkanes. Although less reactive toward alkyl C-H bonds than the Cp*Rh systems, catalysts generated from the combination of bipyridines and iridium(I)-olefin complexes have proven to be the most reactive catalysts for the borylation of arenes. The reactions catalyzed by these complexes form arylboronates from arenes with site-selectivity for C-H bond cleavage that depends on the steric accessibility of the C-H bonds. These complexes also catalyze the borylation of heteroarenes, and the selectivity for these substrates is more dependent on electronic effects than the borylation of arenes. The products from the borylation of arenes and heteroarenes are suitable for a wide range of subsequent conversions to phenols, arylamines, aryl ethers, aryl nitriles, aryl halides, arylboronic acids, and aryl trifluoroborates. Studies of the electronic properties of the ancillary ligand on the rate of the reaction show that the flat structure and the strong electron-donating property of the bipyridine ligands, along with the strong electron-donating property of the boryl group and the presence of a p-orbital on the metal-bound atom, lead to the increased reactivity of the iridium catalysts. Based on this hypothesis, we studied catalysts containing substituted phenanthroline ligands for a series of additional transformations, including the silylation of C-H bonds. A sequence involving the silylation of benzylic alcohols, followed by the dehydrogenative silylation of aromatic C-H bonds, leads to an overall directed silylation of the C-H bond ortho to hydroxyl functionality.     

Catalytic coupling of sp2- and sp-hybridized carbon-hydrogen bonds with vinylmetalloid compounds

In the Account given herein, it has been shown that silylative coupling of olefins, well-recognized as a new catalytic route for the activation of double bond C-H bond of olefins and double bond C-Si bond of vinylsilicon compounds with ethylene elimination, can be extended over both other vinylmetalloid derivatives (double bond C-E) (where E = Ge, B, and others) as well as the activation of triple bond C-H, double bond C aryl-H, and -O-H bond of alcohols and silanols. This general transformation is catalyzed by transition-metal complexes (mainly Ru and Rh) containing or initiating TM-H and/or TM-E bonds (inorganometallics). This new general catalytic route for the activation of double bond C-H and triple bond C-H as well as double bond C-E bonds called metallative coupling or trans-metalation (cross-coupling, ring-closing, and polycondensation) constitutes an efficient method (complementary to metathesis) for stereo- and regioselective synthesis of a variety of molecular and macromolecular compounds of vinyl-E (E = Si, B, and Ge) and ethynyl-E (E = Si and Ge) functionality, also potent organometallic reagents for efficient synthesis of highly pi-conjugated organic compounds. The mechanisms of the catalysis of this deethenative metalation have been supported by equimolar reactions of TM-H and/or TM-E with initial substances and reactions with deuterium-labeled reagents.     

Organometallic-like C-H bond activation and C-S bond formation on the disulfide bridge in the RuSSRu core complexes

C-S bond formations on the disulfide bridge in the dinuclear Ru(III) complexes have been found in the reaction with unsaturated organic molecules, such as alkenes and ketones. The reactions are initiated by the organometallic-like C-H bond activation. Through the mechanistic study of these novel reactions, the specific nature of the disulfide bridging ligand has been unveiled: (i) the double bond character of the sulfur-sulfur bond, which allows the Diels-Alder-type [4 + 2] cycloaddition reaction with butadiene to form a C(4)S(2) ring, (ii) the C-H bond activation on the S-S bond forming a C-S bond. All of the C-H activation reactions and the ensuing C-S bond formation reactions suggest that the disulfide ligand can act in a manner similar to the transition metal centers of the organometallic complexes.     

Rhodium catalyzed chelation-assisted C-H bond functionalization reactions

Over the last several decades, researchers have achieved remarkable progress in the field of organometallic chemistry. The development of metal-catalyzed cross-coupling reactions represents a paradigm shift in chemical synthesis, and today synthetic chemists can readily access carbon-carbon and carbon-heteroatom bonds from a vast array of starting compounds. Although we cannot understate the importance of these methods, the required prefunctionalization to carry out these reactions adds cost and reduces the availability of the starting reagents. The use of C-H bond activation in lieu of prefunctionalization has presented a tantalizing alternative to classical cross-coupling reactions. Researchers have met the challenges of selectivity and reactivity associated with the development of C-H bond functionalization reactions with an explosion of creative advances in substrate and catalyst design. Literature reports on selectivity based on steric effects, acidity, and electronic and directing group effects are now numerous. Our group has developed an array of C-H bond functionalization reactions that take advantage of a chelating directing group, and this Account surveys our progress in this area. The use of chelation control in C-H bond functionalization offers several advantages with respect to substrate scope and application to total synthesis. The predictability and decreased dependence on the inherent stereoelectronics of the substrate generally result in selective and high yielding transformations with broad applicability. The nature of the chelating moiety can be chosen to serve as a functional handle in subsequent elaborations. Our work began with the use of Rh(I) catalysts in intramolecular aromatic C-H annulations, which we further developed to include enantioselective transformations. The application of this chemistry to the simple olefinic C-H bonds found in α,β-unsaturated imines allowed access to highly substituted olefins, pyridines, and piperidines. We observed complementary reactivity with Rh(III) catalysts and developed an oxidative coupling with unactivated alkenes. Further studies on the Rh(III) catalysts led us to develop methods for the coupling of C-H bonds to polarized π bonds such as those in imines and isocyanates. In several cases the methods that we have developed for chelation-controlled C-H bond functionalization have been applied to the total synthesis of complex molecules such as natural products, highlighting the utility of these methods in organic synthesis.     

Copper, silver, and gold complexes in hydrosilylation reactions

The reduction of diverse functional groups is an essential protocol in organic chemistry. Transition-metal catalysis has been successfully applied to the reduction of olefins, alkynes, and many carbonyl compounds via hydrogenation or hydrosilylation; the latter presenting several advantages over hydrogenation. Notably, hydrosilylation generally occurs under mild reaction conditions, and consequently over-reduced products are rarely detected. Moreover, the great majority of hydrosilanes employed in this reaction are easily handled, inexpensive, or both. A large number of multiple bonds can be involved in this context, and the hydrosilylation reaction can be regarded as a useful method for the synthesis of silicon-containing organic molecules or a convenient way of reducing organic compounds. Furthermore, the silyl group can also be retained as a protecting group, a strategy that can be of great usefulness in organic synthesis. Since the first Wilkinson's catalyst-mediated hydrosilylation of ketones in 1972, metals such as rhodium and iridium have attracted most of the attention in this area. A wide array of catalytic systems for hydrosilylation reactions is nowadays available, which has allowed for a great expansion of the synthetic scope of this transformation. After having been overlooked in the early years, group 11 metals (Cu, Ag, and Au), especially copper, have emerged as appealing alternatives for hydrosilylation. The use of a stabilized form of copper hydride, the hexameric [(Ph3P)CuH]6, by Stryker represented a breakthrough in copper-catalyzed reduction reactions. Nowadays, several copper-based catalytic systems compare well with a variety of reported rhodium-based catalysts, which generally suffer from the high cost of the catalyst. Tertiary phosphine ligands are the most widely used in these transformations. However, other families such as N-heterocyclic carbenes (NHCs) have shown promising activities. Compared with copper, little attention has been paid to silver- or gold-based catalysts. Silver salts have been considered inert towards hydrosilylation, and they are often employed as innocent anion exchange reagents for the in situ generation of cationic transition metal catalysts. Despite the rare reports available, they have already shown interesting reactivity profiles, for example, in the chemoselective reduction of aldehydes in the presence of ketones. Furthermore, 1,2-hydride delivery is favored over 1,4-reductions for alpha,beta-unsaturated carbonyl compounds, in contrast with most copper-based systems.     

Ag掺杂V2O5/TiO2催化氧化3-甲基吡啶 脱甲基效应

浸渍法制备了不同Ag掺杂的Ag-V/TiO2催化剂,考察其催化部分氧化3-甲基吡啶脱甲基性能。采用X射线衍射（XRD）,X射线光电子能谱（XPS）,H2程序升温还原（H2-TPR）对催化剂进行了表征。表征表明,Ag的掺杂提高催化剂中表面氧和 4价态的钒物种的含量,有效的加快了催化剂的还原过程,提高了氧化活性。3-甲基吡啶氧化反应表明,Ag掺杂使反应选择性发生根本性变化,主产物由烟酸变成吡啶,选择性84.7%,烟酸几乎不产生。该脱甲基氧化反应的过程是V4 /V5 （O2-）在攻击侧链的C-H键的同时Ag在O2的作用下形成了含有亲电子型氧（O-和O2-）的Agp O活性物种,攻击侧链和苯环相连的C-C键,Ag和V共同作用将侧链的甲基以氧化形成COx的形式脱除。     

Weak coordination as a powerful means for developing broadly useful C-H functionalization reactions

Reactions that convert carbon-hydrogen (C-H) bonds into carbon-carbon (C-C) or carbon-heteroatom (C-Y) bonds are attractive tools for organic chemists, potentially expediting the synthesis of target molecules through new disconnections in retrosynthetic analysis. Despite extensive inorganic and organometallic study of the insertion of homogeneous metal species into unactivated C-H bonds, practical applications of this technology in organic chemistry are still rare. Only in the past decade have metal-catalyzed C-H functionalization reactions become more widely utilized in organic synthesis. Research in the area of homogeneous transition metal-catalyzed C-H functionalization can be broadly grouped into two subfields. They reflect different approaches and goals and thus have different challenges and opportunities. One approach involves reactions of completely unfunctionalized aromatic and aliphatic hydrocarbons, which we refer to as "first functionalization". Here the substrates are nonpolar and hydrophobic and thus interact very weakly with polar metal species. To overcome this weak affinity and drive metal-mediated C-H cleavage, chemists often use hydrocarbon substrates in large excess (for example, as solvent). Because highly reactive metal species are needed in first functionalization, controlling the chemoselectivity to avoid overfunctionalization is often difficult. Additionally, because both substrates and products are comparatively low-value chemicals, developing cost-effective catalysts with exceptionally high turnover numbers that are competitive with alternatives (including heterogeneous catalysts) is challenging. Although an exciting field, first functionalization is beyond the scope of this Account. The second subfield of C-H functionalization involves substrates containing one or more pre-existing functional groups, termed "further functionalization". One advantage of this approach is that the existing functional group (or groups) can be used to chelate the metal catalyst and position it for selective C-H cleavage. Precoordination can overcome the paraffin nature of C-H bonds by increasing the effective concentration of the substrate so that it need not be used as solvent. From a synthetic perspective, it is desirable to use a functional group that is an intrinsic part of the substrate so that extra steps for installation and removal of an external directing group can be avoided. In this way, dramatic increases in molecular complexity can be accomplished in a single stroke through stereo- and site-selective introduction of a new functional group. Although reactivity is a major challenge (as with first functionalization), the philosophy in further functionalization differs; the major challenge is developing reactions that work with predictable selectivity in intricately functionalized contexts on commonly occurring structural motifs. In this Account, we focus on an emergent theme within the further functionalization literature: the use of commonly occurring functional groups to direct C-H cleavage through weak coordination. We discuss our motivation for studying Pd-catalyzed C-H functionalization assisted by weakly coordinating functional groups and chronicle our endeavors to bring reactions of this type to fruition. Through this approach, we have developed reactions with a diverse range of substrates and coupling partners, with the broad scope likely stemming from the high reactivity of the cyclopalladated intermediates, which are held together through weak interactions.     

N-heterocyclic carbene gold(I) and copper(I) complexes in C-H bond activation

Environmental concerns have and will continue to have a significant role in determining how chemistry is carried out. Chemists will be challenged to develop new, efficient synthetic processes that have the fewest possible steps leading to a target molecule, the goal being to decrease the amount of waste generated and reduce energy use. Along this path, chemists will need to develop highly selective reactions with atom-economical pathways producing nontoxic byproduct. In this context, C-H bond activation and functionalization is an extremely attractive method. Indeed, for most organic transformations, the presence of a reactive functionality is required. In Total Synthesis, the "protection and deprotection" approach with such reactive groups limits the overall yield of the synthesis, involves the generation of significant chemical waste, costs energy, and in the end is not as green as one would hope. In turn, if a C-H bond functionalization were possible, instead of the use of a prefunctionalized version of the said C-H bond, the number of steps in a synthesis would obviously be reduced. In this case, the C-H bond can be viewed as a dormant functional group that can be activated when necessary during the synthetic strategy. One issue increasing the challenge of such a desired reaction is selectivity. The cleavage of a C-H bond (bond dissociation requires between 85 and 105 kcal/mol) necessitates a high-energy species, which could quickly become a drawback for the control of chemo-, regio-, and stereoselectivity. Transition metal catalysts are useful reagents for surmounting this problem; they can decrease the kinetic barrier of the reaction yet retain control over selectivity. Transition metal complexes also offer important versatility in having distinct pathways that can lead to activation of the C-H bond. An oxidative addition of the metal in the C-H bond, and a base-assisted metal-carbon bond formation in which the base can be coordinated (or not) to the metal complexes are possible. These different C-H bond activation modes provide chemists with several synthetic options. In this Account, we discuss recent discoveries involving the versatile NHC-gold(I) and NHC-copper(I) hydroxide complexes (where NHC is N-heterocyclic carbene) showing interesting Br?nsted basic properties for C-H bond activation or C-H bond functionalization purposes. The simple and easy synthesis of these two complexes involves their halide-bearing relatives reacting with simple alkali metal hydroxides. These complexes can react cleanly with organic compounds bearing protons with compatible pK(a) values, producing only water as byproduct. It is a very simple protocol indeed and may be sold as a C-H bond activation, although the less flashy "metalation reaction" also accurately describes the process. The synthesis of these complexes has led us to develop new organometallic chemistry and catalysis involving C-H bond activation (metalation) and subsequent C-H bond functionalization. We further highlight applications with these reactions, in areas such as photoluminescence and biological activities of NHC-gold(I) and NHC-copper(I) complexes.     

Low-Oxidation State Indium-Catalyzed C-C Bond Formation

The development of innovative metal catalysis for selective bond formation is an important task in organic chemistry. The group 13 metal indium is appealing for catalysis because indium-based reagents are minimally toxic, selective, and tolerant toward various functional groups. Among elements in this group, the most stable oxidation state is typically +3, but in molecules with larger group 13 atoms, the chemistry of the +1 oxidation state is also important. The use of indium(III) compounds in organic synthesis has been well-established as Lewis acid catalysts including asymmetric versions thereof. In contrast, only sporadic examples of the use of indium(I) as a stoichiometric reagent have been reported: to the best of our knowledge, our investigations represent the first synthetic method that uses a catalytic amount of indium(I). Depending on the nature of the ligand or the counteranion to which it is coordinated, indium(I) can act as both a Lewis acid and a Lewis base because it has both vacant p orbitals and a lone pair of electrons. This potential ambiphilicity may offer unique reactivity and unusual selectivity in synthesis and may have significant implications for catalysis, particularly for dual catalytic processes. We envisioned that indium(I) could be employed as a metallic Lewis base catalyst to activate Lewis acidic boron-based pronucleophiles for selective bond formation with suitable electrophiles. Alternatively, indium(I) could serve as an ambiphilic catalyst that activates both reagents at a single center. In this Account, we describe the development of low-oxidation state indium catalysts for carbon-carbon bond formation between boron-based pronucleophiles and various electrophiles. We discovered that indium(I) iodide was an excellent catalyst for α-selective allylations of C(sp(2)) electrophiles such as ketones and hydrazones. Using a combination of this low-oxidation state indium compound and a chiral semicorrin ligand, we developed catalytic highly enantioselective allylation, crotylation, and α-chloroallylation reactions of hydrazones. These transformations proceeded with rare constitutional selectivities and remarkable diastereoselectivities. Furthermore, indium(I) triflate served as the most effective catalyst for allylations and propargylations of C(sp(3)) electrophiles such as O,O-acetals, N,O-aminals, and ethers, and we applied this methodology to carbohydrate chemistry. In addition, a catalyst system composed of indium(I) chloride and a chiral silver BINOL-phosphate facilitated the highly enantioselective allylation and allenylation of N,O-aminals. Overall, these discoveries demonstrate the versatility, efficiency, and sensitivity of low-oxidation state indium catalysts in organic synthesis.     

Activation of propane C-h and C-C bonds by gas-phase pt atom: a theoretical study

The reaction mechanism of the gas-phase Pt atom with C(3)H(8) has been systematically investigated on the singlet and triplet potential energy surfaces at CCSD(T)//BPW91/6-311++G(d, p), Lanl2dz level. Pt atom prefers the attack of primary over secondary C-H bonds in propane. For the Pt + C(3)H(8) reaction, the major and minor reaction channels lead to PtC(3)H(6) + H(2) and PtCH(2) + C(2)H(6), respectively, whereas the possibility to form products PtC(2)H(4) + CH(4) is so small that it can be neglected. The minimal energy reaction pathway for the formation of PtC(3)H(6) + H(2), involving one spin inversion, prefers to start at the triplet state and afterward proceed along the singlet state. The optimal C-C bond cleavages are assigned to C-H bond activation as the first step, followed by cleavage of a C-C bond. The C-H insertion intermediates are kinetically favored over the C-C insertion intermediates. From C-C to C-H oxidative insertion, the lowering of activation barrier is mainly caused by the more stabilizing transition state interaction ΔE(≠) (int), which is the actual interaction energy between the deformed reactants in the transition state.     

Ullmann偶联反应催化剂研究进展

Ullmann偶联反应是典型的碳碳键偶联反应,反应合成的联苯类化合物是重要有机化工原料,应用前景广阔。初期采用均相Pd催化剂,不能重复利用,工业化生产受到限制。改用多相Pd催化剂催化反应,需要添加剂导致产物分离困难。多相Au催化剂适用性受到限制,反应底物局限于碘代芳烃,双金属催化剂在催化活性与选择性方面均有较好的优势。综述Ullmann-type偶联反应中均相Pd催化体系、多相Pd催化体系、多相Au催化体系以及多相双金属催化体系催化剂的研究进展,阐述反应机理,并对Ullmann偶联反应研究进行展望。     

亚砜配体在金属催化烯丙基官能团化反应中的应用

过渡金属催化的烯丙基官能团化反应在有机合成中具有重要作用，是构建碳碳键和碳杂原子键的重要方法。亚砜配体以其独特的立体化学性质，不仅在传统的（Tsuji-Trost）烯丙基取代反应中取得优异结果，而且在烯丙基碳氢活化反应中展现出巨大潜力。本文根据产生烯丙基钯中间体的催化反应机理及亚砜配体配位模式的不同，总结了近些年亚砜配体在传统的烯丙基取代、烯丙基碳氢官能团化及不对称烯丙基碳氢官能团化领域的最新研究进展，指出影响各个催化体系的关键因素及所存的问题并对今后发展做出了展望。     

煤催化加氢气化研究进展

煤加氢气化制天然气技术具有工艺路径短、热效率高等优点,其应用基础研究备受关注。但煤中存在部分致密的芳香碳结构,加氢反应性较差,即使在苛刻的反应条件下(~1 000℃、~7 MPa H_2),仍难以转化。通过引入催化剂,进行煤催化加氢气化可在温和的反应条件下实现煤的碳转化率和CH_4收率的同步提高。论述了碱金属(K、Na等)、碱土金属(Ca)和过渡金属(Fe、Co、Ni等)催化剂对模型碳加氢气化的催化作用原理。探讨了反应温度、氢气压力、和碳结构对C-H_2催化反应的影响规律,分析了适用于原煤催化加氢气化的最佳催化剂及工艺条件,并从CH_4和轻质液体焦油等产物生成规律、煤中碳结构随着反应进行的衍变过程等角度,讨论了催化剂分别对煤加氢热解和热解半焦加氢气化的催化作用行为。提出了煤催化加氢气化联产CH_4和轻质液体焦油技术从基础走向应用的进一步研究建议。现有研究结果表明,过渡金属与碱土金属组成的二元催化剂(Fe/Co/Ni-Ca)对煤加氢气化的活性较高。过渡金属元素在反应过程中主要提供C-H_2反应所需的活性氢,并削弱C—C键的键能;碱土金属元素Ca主要促进Fe/Co/Ni的分散,防止其发生硫中毒失活,并增强Fe/Co/Ni与碳之间的相互作用。温度升高一方面为化学键断裂过程提供了更高能量,加速C-H_2反应,另一方面促进催化剂在煤结构中扩散,提升催化剂的供氢和断键效率。升高压力促进了活性氢的供应,同时CH_4浓度得到稀释,反应向生成CH_4的方向移动。以5%Co-1%Ca为催化剂,在850℃、3 MPa H_2反应条件下,30 min内可同时达到90.0%的碳转化率和77.3%的CH_4收率。Co-Ca催化剂在煤加氢热解过程中具有催化解聚和催化加氢的作用,提高焦油和CH_4收率,同时催化剂在煤加氢热解过程中对煤结构产生催化活化作用,使得生成的半焦具有较高的气化活性。煤催化加氢气化的机理研究目前仍处于推测阶段,另外,该技术气化剂、煤种的适应性,催化剂循环利用性能有待进一步阐明。     

NHPI催化分子氧选择性氧化木质素模型分子中的C-C键

以1,2-二苯乙烷为模型分子,考察了N-羟基邻苯二甲酰亚胺（NHPI）对分子氧氧化断裂木质素分子中C-C键的催化作用,比较了Cu、Co和Mn的多种可溶性盐对NHPI催化活性的促进作用,探究了工艺条件对1,2-二苯乙烷氧化的影响规律。结果表明：NHPI能够在温和条件下催化分子氧氧化1,2-二苯乙烷中的C-C键断裂生成苯甲醛和苯甲酸。可溶性的Cu、Co和Mn盐都具有助催化作用,其中乙酸锰的助催化作用最强。确定了二苯基乙酮为氧化的初级反应产物。发现提高反应温度、增加乙酸锰用量以及延长反应时间,有利于C-C键的氧化断裂和提高小分子产物的选择性。优化的反应条件为NHPI为底物物质的量的10%,乙酸锰和NHPI之比为1：10,反应温度90℃,氧气分压1.0 MPa,反应时间8 h,C-C键的氧化断裂产物苯甲醛和苯甲酸的总选择性可达76.5%。     

Remote C-H bond activation/transformations: A continuous growing synthetic tool; Part II

The selective functionalization of inert C-H bonds is always challenging due to their abundance and large bond dissociation energies. Despite recent advancements, the engagement of inert building blocks for distant functionalization is the most appealing approach for the past decade for the construction of complex molecules. Along with the upsurge of proximal C-H bond activation methods, the presence of directing group or participation of ligand surmounts the challenge of regioselective remote C-H bond transformation. Remote C-H functionalization has emerged as an important tool for the direct synthesis of a variety of natural as well as pharmaceutical products. In this area, chemists are continuously designing and exploring new catalysts, ligands and directing group for the functionalization of C-H bonds which are beyond proximity. Earlier success in this area was limited to meta-position, but recently scientists have come out with new templates which can reach even para-position. The developed catalytic transformations provide access for production of a wide range of value-added products without using classical methods such as Friedel-Craft reactions, Heck coupling, etc., providing atom economical alternate and avoiding the toxic waste generation. On this topic, we have recently published a review article entitled “Distant C-H Activation/Functionalization: A New Horizon of Selectivity beyond Proximity” in the same journal, i.e., Catalysis Reviews: Science and Engineering, 2015, 57(3), 345. In continuation of this article, the present review article will cover the catalytic processes on the mentioned topic mainly developed from 2014 to 2017. The main focus will be on mechanistic pathways and the critical role of template as well as ligands. The purpose of this review is to highlight the recent advancements in remote C-H catalysis and a path ahead.