Login In
2019 Volume 35 Issue 9
2019, 35(9): 903-904
doi: 10.3866/PKU.WHXB201903058
Abstract:
2019, 35(9): 905-905
doi: 10.3866/PKU.WHXB201903057
Abstract:
2019, 35(9): 906-912
doi: 10.3866/PKU.WHXB201811038
Abstract:
Pyridones represent an important family of heterocycles that exhibit a wide range of biological activities. They are often found in pharmaceutical agents and biomolecules. Several transition-metal-catalyzed transformations have been developed to access this family of heterocycles. Among them, C―H bond activation has recently emerged as a general strategy for the construction of substituted pyridones. In most cases, the core nitrogen-containing heterocycle is assembled via the dehydrogenative annulation of α, β-unsaturated amides and alkynes. Such processes involve a cascade sequence of N―H cleavage, sp2 C―H activation, and annulation. Despite this progress, the more readily available α, β-saturated amides are rarely used. Ideally, tethering the direct dehydrogenation of an amide with the above-mentioned C―H annulation cascade would give a more practical synthesis of pyridones. Nevertheless, the dehydrogenation of amides under mild conditions is a synthetic challenge due to their intrinsic weak α-acidity. Recently, we have reported a general protocol for the aerobic dehydrogenation of γ, δ-unsaturated amides, acids, and ketones. A key Ir―allyl intermediate was believed responsible for enhancing the α-acidity of the amides studied, which enables the dehydrogenation step to occur under mild reaction conditions. Herein, we describe a new method for the synthesis of polysubstituted pyridones using γ, δ-unsaturated amides and alkynes. In the presence of [RhCp*Cl2]2, the dehydrogenation step occurs via β-C―H bond activation. The resulting π-allyl―Rh intermediate undergoes an accelerated dehydrogenation reaction to afford the doubly unsaturated amide. This in-situ generated dienamide undergoes sp2 C―H activation at the β-position and a subsequent alkyne insertion/cyclization reaction to yield the target heterocycle. Regeneration of the Rh catalyst is accomplished using an external oxidant and completes the streamlined double C―H activation and double dehydrogenation catalytic cycle. Various functional groups are well tolerated. The γ-alkenyl moiety not only facilitates the direct dehydrogenation of amides, but also serves as a handle for further derivatization of the as-obtained products. To gain a mechanistic insight into the reaction cascade, a set of control experiments were carried out. The results demonstrate that the dienamide is one of the key reaction intermediates. NMR experiments confirmed that the fast dehydrogenation process occurs during the early stage of the reaction. The alkyne insertion is believed to be the rate-determining step in the reaction cascade, as suggested by competition experiments. ![]()
Pyridones represent an important family of heterocycles that exhibit a wide range of biological activities. They are often found in pharmaceutical agents and biomolecules. Several transition-metal-catalyzed transformations have been developed to access this family of heterocycles. Among them, C―H bond activation has recently emerged as a general strategy for the construction of substituted pyridones. In most cases, the core nitrogen-containing heterocycle is assembled via the dehydrogenative annulation of α, β-unsaturated amides and alkynes. Such processes involve a cascade sequence of N―H cleavage, sp2 C―H activation, and annulation. Despite this progress, the more readily available α, β-saturated amides are rarely used. Ideally, tethering the direct dehydrogenation of an amide with the above-mentioned C―H annulation cascade would give a more practical synthesis of pyridones. Nevertheless, the dehydrogenation of amides under mild conditions is a synthetic challenge due to their intrinsic weak α-acidity. Recently, we have reported a general protocol for the aerobic dehydrogenation of γ, δ-unsaturated amides, acids, and ketones. A key Ir―allyl intermediate was believed responsible for enhancing the α-acidity of the amides studied, which enables the dehydrogenation step to occur under mild reaction conditions. Herein, we describe a new method for the synthesis of polysubstituted pyridones using γ, δ-unsaturated amides and alkynes. In the presence of [RhCp*Cl2]2, the dehydrogenation step occurs via β-C―H bond activation. The resulting π-allyl―Rh intermediate undergoes an accelerated dehydrogenation reaction to afford the doubly unsaturated amide. This in-situ generated dienamide undergoes sp2 C―H activation at the β-position and a subsequent alkyne insertion/cyclization reaction to yield the target heterocycle. Regeneration of the Rh catalyst is accomplished using an external oxidant and completes the streamlined double C―H activation and double dehydrogenation catalytic cycle. Various functional groups are well tolerated. The γ-alkenyl moiety not only facilitates the direct dehydrogenation of amides, but also serves as a handle for further derivatization of the as-obtained products. To gain a mechanistic insight into the reaction cascade, a set of control experiments were carried out. The results demonstrate that the dienamide is one of the key reaction intermediates. NMR experiments confirmed that the fast dehydrogenation process occurs during the early stage of the reaction. The alkyne insertion is believed to be the rate-determining step in the reaction cascade, as suggested by competition experiments.
2019, 35(9): 913-922
doi: 10.3866/PKU.WHXB201809036
Abstract:
The strategy of transition-metal-catalyzed C―H activation has been greatly developed in recent years. Direct transformations of inert C―H bonds undoubtedly provide powerful ways to construct various C―C and C―X (X = heteroatom) bonds, with enhanced atom- and step-economy. Impressive efforts have been devoted to this research all along. However, concerns about reactivity and selectivity remain to be tackled, due to their strong dependence on directing groups and acidic reactive sites. In this regard, more effective catalytic systems are of great importance and therefore in high demand. Bimetallic C―H activation, by virtue of the cooperative effect, has emerged as a promising solution to this issue. The intriguing interactions between two metals with substrates afford exceptional reaction efficiency and selectivity. Intensive interest in both experimental and computational studies has been recently triggered. In this minireview, diverse bimetallic catalytic reactions are summarized into three categories according to the initiator in the C―H activation step, namely, bimetallic catalyses based on palladium, nickel, and other metals. Experimental results as well as density functional theory (DFT) calculations are invoked in the plausible mechanistic considerations. In the first part, collaborative modes based on palladium are described, in which magnesium, chromium, cobalt, and silver are successfully engaged as accessory partners. Most of them stabilize the C―H activation transition states by decreasing the energy, thus facilitating the cleavage of C―H bonds. Notably, some reactions previously reported as examples of monomeric palladium catalysis are now reinvestigated as bimetallic scenarios, in light of computational discussions. In the second part, reactions based on the synergy of nickel, and zinc or aluminum, are generalized, in which zinc or aluminum acts as a Lewis acid to increase the acidity of C―H bonds. It has been shown that the choice of different kinds of Lewis acids and ligands has a great influence on the reaction chemo-, regio-, and stereoselectivity. Gratefully, even enantioselective transformations can be achieved using the cooperation of nickel and aluminum. Moreover, a key reaction intermediate in the bimetallic C―H activation by nickel and aluminum has been isolated, providing guidance for this bimetallic catalytic system in further mechanistic studies and applications. In the last part, synergetic catalysis based on various other metals is presented. Bimetallic regimes of ruthenium/copper, rhodium/bismuth, iridium/aluminum, manganese/zinc, and zirconium/aluminum have been elegantly applied to C―H activation reactions. Multifarious action modes are proposed on account of the mechanistic research. ![]()
The strategy of transition-metal-catalyzed C―H activation has been greatly developed in recent years. Direct transformations of inert C―H bonds undoubtedly provide powerful ways to construct various C―C and C―X (X = heteroatom) bonds, with enhanced atom- and step-economy. Impressive efforts have been devoted to this research all along. However, concerns about reactivity and selectivity remain to be tackled, due to their strong dependence on directing groups and acidic reactive sites. In this regard, more effective catalytic systems are of great importance and therefore in high demand. Bimetallic C―H activation, by virtue of the cooperative effect, has emerged as a promising solution to this issue. The intriguing interactions between two metals with substrates afford exceptional reaction efficiency and selectivity. Intensive interest in both experimental and computational studies has been recently triggered. In this minireview, diverse bimetallic catalytic reactions are summarized into three categories according to the initiator in the C―H activation step, namely, bimetallic catalyses based on palladium, nickel, and other metals. Experimental results as well as density functional theory (DFT) calculations are invoked in the plausible mechanistic considerations. In the first part, collaborative modes based on palladium are described, in which magnesium, chromium, cobalt, and silver are successfully engaged as accessory partners. Most of them stabilize the C―H activation transition states by decreasing the energy, thus facilitating the cleavage of C―H bonds. Notably, some reactions previously reported as examples of monomeric palladium catalysis are now reinvestigated as bimetallic scenarios, in light of computational discussions. In the second part, reactions based on the synergy of nickel, and zinc or aluminum, are generalized, in which zinc or aluminum acts as a Lewis acid to increase the acidity of C―H bonds. It has been shown that the choice of different kinds of Lewis acids and ligands has a great influence on the reaction chemo-, regio-, and stereoselectivity. Gratefully, even enantioselective transformations can be achieved using the cooperation of nickel and aluminum. Moreover, a key reaction intermediate in the bimetallic C―H activation by nickel and aluminum has been isolated, providing guidance for this bimetallic catalytic system in further mechanistic studies and applications. In the last part, synergetic catalysis based on various other metals is presented. Bimetallic regimes of ruthenium/copper, rhodium/bismuth, iridium/aluminum, manganese/zinc, and zirconium/aluminum have been elegantly applied to C―H activation reactions. Multifarious action modes are proposed on account of the mechanistic research.
2019, 35(9): 923-939
doi: 10.3866/PKU.WHXB201810002
Abstract:
With the increasing energy demands and the limited petroleum reserves, it is highly desirable to produce fuels and chemicals from non-petroleum feedstocks, such as coal, natural gas and biomass. Catalytic conversion of C1 resources (CO, CO2, CH3OH, CH4, etc.) affords various products and attracts increasing attention from both academia and industries. Methane and methanol are important C1 feedstocks in the production of fuels and chemicals. In order to obtain high selectivity for the target product, it is necessary to control the activation of C―H bonds in methane and methanol. However, this remains a great challenge. Although the traditional thermal catalytic conversion of methane and methanol has been developed over decades, there are still some disadvantages associated with the catalytic process, such as harsh reaction conditions, high energy consumption, and low selectivity. Photocatalysis, which is driven by photoenergy, can compensate for the Gibbs free energy. In the photocatalytic reactions, semiconductor photocatalysts absorb photons and generate electrons and holes in their conduction and valence bands, respectively, to accelerate the reaction rate. The position of the conduction band determines the oxidation capacity, and the bandgap determines the light absorption property. Normally, the oxidation capacity of photocatalysts is regulated by choosing semiconductors with a suitable bandgap or anions/cations doping. Fabrication of heterojunction and loading metalsare recognized as effective methods to promote the separation of electron-hole pairs and improve the photocatalytic efficiency. In contrast to thermal catalysis, photocatalysis can be carried out under mild reaction conditions with low energy consumption. Recently, photocatalysis has been considered an attractive route for the efficient conversion of methane and methanol to fuels and chemicals. Partial oxidation of methane, which is necessary to avoid the formation of byproducts, can be achieved by adjusting the wavelength and intensity of the light and the oxidation capacity of the photocatalysts. In addition, light-induced plasmon resonance improves the efficiency of methane conversion by forming an intrinsic high-energy magnetic field that can polarize methane. In methanol conversion, the C―H bond can be selectively activated, instead of the O―H bond, by light irradiation. Therefore, C―C coupling can be realized for the production of various value-added chemicals from methanol. This review summarizes the recent advances in the photocatalytic conversion of methane and methanol including the reactions of reforming, oxidation, and coupling. Perspectives and challenges for further research on the photocatalytic conversion of methane and methanol are also discussed. ![]()
With the increasing energy demands and the limited petroleum reserves, it is highly desirable to produce fuels and chemicals from non-petroleum feedstocks, such as coal, natural gas and biomass. Catalytic conversion of C1 resources (CO, CO2, CH3OH, CH4, etc.) affords various products and attracts increasing attention from both academia and industries. Methane and methanol are important C1 feedstocks in the production of fuels and chemicals. In order to obtain high selectivity for the target product, it is necessary to control the activation of C―H bonds in methane and methanol. However, this remains a great challenge. Although the traditional thermal catalytic conversion of methane and methanol has been developed over decades, there are still some disadvantages associated with the catalytic process, such as harsh reaction conditions, high energy consumption, and low selectivity. Photocatalysis, which is driven by photoenergy, can compensate for the Gibbs free energy. In the photocatalytic reactions, semiconductor photocatalysts absorb photons and generate electrons and holes in their conduction and valence bands, respectively, to accelerate the reaction rate. The position of the conduction band determines the oxidation capacity, and the bandgap determines the light absorption property. Normally, the oxidation capacity of photocatalysts is regulated by choosing semiconductors with a suitable bandgap or anions/cations doping. Fabrication of heterojunction and loading metalsare recognized as effective methods to promote the separation of electron-hole pairs and improve the photocatalytic efficiency. In contrast to thermal catalysis, photocatalysis can be carried out under mild reaction conditions with low energy consumption. Recently, photocatalysis has been considered an attractive route for the efficient conversion of methane and methanol to fuels and chemicals. Partial oxidation of methane, which is necessary to avoid the formation of byproducts, can be achieved by adjusting the wavelength and intensity of the light and the oxidation capacity of the photocatalysts. In addition, light-induced plasmon resonance improves the efficiency of methane conversion by forming an intrinsic high-energy magnetic field that can polarize methane. In methanol conversion, the C―H bond can be selectively activated, instead of the O―H bond, by light irradiation. Therefore, C―C coupling can be realized for the production of various value-added chemicals from methanol. This review summarizes the recent advances in the photocatalytic conversion of methane and methanol including the reactions of reforming, oxidation, and coupling. Perspectives and challenges for further research on the photocatalytic conversion of methane and methanol are also discussed.
2019, 35(9): 940-953
doi: 10.3866/PKU.WHXB201810052
Abstract:
Transition-metal-catalyzed C―H bond activation, which has been widely applied to construct new covalent bonds, has emerged as one of the most effective strategies in synthetic chemistry due to atom economy and simple procedure. In this review, we have summarized the recent reports on the theoretical mechanistic study of transition-metal-catalyzed C―H bond cleavage. Based on these comprehensive theoretical studies, we have systematically discussed the general modes of C―H bond activation, which involves oxidative addition, base-assisted deprotonation, σ-metathesis, Friedel-Crafts-type electrophilic aromatic substitution, α- or β-hydrogen elimination, and hydrogen atom abstraction. From a mechanistic point of view, C―H bond activation by oxidative addition generally involves a zero-valent transition metal catalyst with strong reducibility, which requires a low activation barrier. The concerted metalation-deprotonation (CMD)-type C―H bond cleavage often occurs via a six-membered cyclic transition state using transition metal carboxylate as the catalyst with a directing group, which is a common mechanism for transition metals with high oxidation states. Base-assisted internal electrophilic substitution (BIES)-type C―H bond activation is commonly performed in the presence of cationic transition metal catalysts, in which electron-rich arenes react preferentially compared to electron-deficient arenes. In some other cases, outer-sphere base-assisted deprotonation can also result in C―H activation, which is dependent on the strength of the base used. The stronger the base used, the lower the energy barrier, and thus, the easier it is to protonate. The σ-metathesis pathway, which could occur via a four-membered cyclic transition state, is often considered an alternative for concerted metalation-deprotonation. If the aromatic hydrocarbon is attacked by electrophiles, the C―H bond can be activated by Friedel-Crafts-type electrophilic aromatic substitution. Elimination of α- or β-hydrogen is also frequently proposed for transition-metal-catalyzed C―H functionalization. Hydrogen atom abstraction could achieve C―H bond activation via a free radical process. Moreover, the C―H bonds of hydrocarbons can be considered weak nucleophiles because the electronegativity of carbon is higher than that of hydrogen, and they could be converted to strong nucleophiles (C-M) in the presence of transition metal catalysts via the different pathways mentioned above. It enables further functionalization with electrophiles or nucleophiles to construct complex molecular skeletons. Summarizing the general modes of C―H bond activation will increase our understanding of the associated chemical mechanism and will pave the way for new synthetic strategies. This review aims to offer theoretical guidance for experimental studies and inspire new reaction design by summarizing the modes of transition-metal-catalyzed C―H bond activation. ![]()
Transition-metal-catalyzed C―H bond activation, which has been widely applied to construct new covalent bonds, has emerged as one of the most effective strategies in synthetic chemistry due to atom economy and simple procedure. In this review, we have summarized the recent reports on the theoretical mechanistic study of transition-metal-catalyzed C―H bond cleavage. Based on these comprehensive theoretical studies, we have systematically discussed the general modes of C―H bond activation, which involves oxidative addition, base-assisted deprotonation, σ-metathesis, Friedel-Crafts-type electrophilic aromatic substitution, α- or β-hydrogen elimination, and hydrogen atom abstraction. From a mechanistic point of view, C―H bond activation by oxidative addition generally involves a zero-valent transition metal catalyst with strong reducibility, which requires a low activation barrier. The concerted metalation-deprotonation (CMD)-type C―H bond cleavage often occurs via a six-membered cyclic transition state using transition metal carboxylate as the catalyst with a directing group, which is a common mechanism for transition metals with high oxidation states. Base-assisted internal electrophilic substitution (BIES)-type C―H bond activation is commonly performed in the presence of cationic transition metal catalysts, in which electron-rich arenes react preferentially compared to electron-deficient arenes. In some other cases, outer-sphere base-assisted deprotonation can also result in C―H activation, which is dependent on the strength of the base used. The stronger the base used, the lower the energy barrier, and thus, the easier it is to protonate. The σ-metathesis pathway, which could occur via a four-membered cyclic transition state, is often considered an alternative for concerted metalation-deprotonation. If the aromatic hydrocarbon is attacked by electrophiles, the C―H bond can be activated by Friedel-Crafts-type electrophilic aromatic substitution. Elimination of α- or β-hydrogen is also frequently proposed for transition-metal-catalyzed C―H functionalization. Hydrogen atom abstraction could achieve C―H bond activation via a free radical process. Moreover, the C―H bonds of hydrocarbons can be considered weak nucleophiles because the electronegativity of carbon is higher than that of hydrogen, and they could be converted to strong nucleophiles (C-M) in the presence of transition metal catalysts via the different pathways mentioned above. It enables further functionalization with electrophiles or nucleophiles to construct complex molecular skeletons. Summarizing the general modes of C―H bond activation will increase our understanding of the associated chemical mechanism and will pave the way for new synthetic strategies. This review aims to offer theoretical guidance for experimental studies and inspire new reaction design by summarizing the modes of transition-metal-catalyzed C―H bond activation.
2019, 35(9): 954-967
doi: 10.3866/PKU.WHXB201810044
Abstract:
Thioesters, which are essential sulfur-containing organic molecules, are indispensable in natural products, pharmaceuticals, and organic light-emitting materials. Efficient synthesis of thioethers has received considerable attention due to the widespread applications of these compounds, and many fundamental approaches for C-S bond formation have been proposed. However, most of them construct C-S bonds by employing organic halides/organic boronic acid. These methodologies generally suffer from a pre-functionalized starting material. Recently, selective C-H functionalization emerged as a powerful tool for the synthesis of C-N, C-O, C-C, and C-halogen bonds. Nevertheless, C-S bond formation via C-H functionalization has only recently been given more importance because organosulfur compounds are believed to inactivate catalysts. In contrast to traditional cross-coupling reactions, direct functionalization of C-H bonds for the synthesis of thioethers can shorten the reaction steps and minimize the amount of waste formed. In this review, which is divided into several parts, we describe C-H functionalization strategies for the construction of thioethers. In Part Ⅰ, we introduce the importance and widespread applications of thioethers in daily life. For example, Lissoclibadin 6 is a polysulfur aromatic alkaloid that shows antimicrobial activity. Seroquel is an antipsychotic medicine. It is used to treat bipolar disorder and schizophrenia in adults, and children who are at least 10 years old. Tazarotene is approved for the treatment of psoriasis, acne, and sun-damaged skin. Furthermore, a comparison between conventional synthesis methods and C-H thiolation is discussed. In Part Ⅱ, we introduce copper-catalyzed or copper-mediated C-H thiolation. Along with the direct functionalization of sp2 and sp C-H for the synthesis of aryl sulfides, some significant and challenging thiolations of sp3 C-H are included. In addition to copper, palladium is an excellent catalyst for C-H functionalization. In Part Ⅲ, we elucidate palladium-catalyzed C-H thiolation and discuss many proposed mechanisms. Nickel, which is a first-flow, low-cost, and earth-abundant metal catalyst, has increasingly gained attention. In contrast to copper and palladium, despite its late start, several remarkable reports on nickel-catalyzed C-H thiolation were published by several groups. Rhodium plays a key role in selective C-H functionalization. Some published results proved the capacity of rhodium catalysts to promote C-S construction via C-H functionalization. In Part Ⅳ, we introduce rhodium-catalyzed C-H thiolation. In recent years, metal-free C-H functionalization has been quite attractive. In Part Ⅴ, some C-S construction strategies via metal-free C-H functionalization are presented. In the last part, the conclusion discusses the limitations and possible development directions of these advances in the construction of thioethers. ![]()
Thioesters, which are essential sulfur-containing organic molecules, are indispensable in natural products, pharmaceuticals, and organic light-emitting materials. Efficient synthesis of thioethers has received considerable attention due to the widespread applications of these compounds, and many fundamental approaches for C-S bond formation have been proposed. However, most of them construct C-S bonds by employing organic halides/organic boronic acid. These methodologies generally suffer from a pre-functionalized starting material. Recently, selective C-H functionalization emerged as a powerful tool for the synthesis of C-N, C-O, C-C, and C-halogen bonds. Nevertheless, C-S bond formation via C-H functionalization has only recently been given more importance because organosulfur compounds are believed to inactivate catalysts. In contrast to traditional cross-coupling reactions, direct functionalization of C-H bonds for the synthesis of thioethers can shorten the reaction steps and minimize the amount of waste formed. In this review, which is divided into several parts, we describe C-H functionalization strategies for the construction of thioethers. In Part Ⅰ, we introduce the importance and widespread applications of thioethers in daily life. For example, Lissoclibadin 6 is a polysulfur aromatic alkaloid that shows antimicrobial activity. Seroquel is an antipsychotic medicine. It is used to treat bipolar disorder and schizophrenia in adults, and children who are at least 10 years old. Tazarotene is approved for the treatment of psoriasis, acne, and sun-damaged skin. Furthermore, a comparison between conventional synthesis methods and C-H thiolation is discussed. In Part Ⅱ, we introduce copper-catalyzed or copper-mediated C-H thiolation. Along with the direct functionalization of sp2 and sp C-H for the synthesis of aryl sulfides, some significant and challenging thiolations of sp3 C-H are included. In addition to copper, palladium is an excellent catalyst for C-H functionalization. In Part Ⅲ, we elucidate palladium-catalyzed C-H thiolation and discuss many proposed mechanisms. Nickel, which is a first-flow, low-cost, and earth-abundant metal catalyst, has increasingly gained attention. In contrast to copper and palladium, despite its late start, several remarkable reports on nickel-catalyzed C-H thiolation were published by several groups. Rhodium plays a key role in selective C-H functionalization. Some published results proved the capacity of rhodium catalysts to promote C-S construction via C-H functionalization. In Part Ⅳ, we introduce rhodium-catalyzed C-H thiolation. In recent years, metal-free C-H functionalization has been quite attractive. In Part Ⅴ, some C-S construction strategies via metal-free C-H functionalization are presented. In the last part, the conclusion discusses the limitations and possible development directions of these advances in the construction of thioethers.
2019, 35(9): 968-976
doi: 10.3866/PKU.WHXB201810007
Abstract:
Methane, the most abundant constituent of natural gas, is a potential substitute for the dwindling petroleum resources for the chemical industry as a carbon-based feedstock. Over the last two decades, global research endeavors have focused on the development of more efficient and selective catalysts for the conversion of ubiquitous but inert methane. In addition, the transportation of gaseous methane in pipelines is unavoidably accompanied by leakage, and methane is recognized as a potent greenhouse gas (20 times more powerful than carbon dioxide per molecule). Thus, the conversion of methane into heavier derivatives is also of crucial environmental concern. Unfortunately, there is still a lack of economical and practical routes for methane conversion. Currently, the major route for methane conversion is the steam reforming of methane into synthetic gases, which is a multistep and energy-consuming route. Another option is to use photoenergy to drive the conversion of methane, which has significant advantages such as the capacity to minimize coking by running at room temperature. A promising approach to photocatalytic methane conversion is the photo-powered direct coupling or oxidation of methane to form ethane, methanol and hydrogen. The ethane or methanol produced can, in turn, be converted into ethene or liquid fuels through metathesis or dehydrogenation, respectively. Furthermore, the direct dehydrogenation of methane is the best way to produce clean H2 energy from fossil fuels since methane has the highest H/C ratio among hydrocarbons. However, the methane conversion efficiency of previously reported photocatalysts is low. Furthermore, the wavelength of light used in previously reported photocatalytic systems usually needs to be less than 270 nm, which is beyond the range of the solar spectrum (wavelength λ > 290 nm) reaching the Earth's surface. To achieve substantial yield and selectivity, and to exploit solar energy effectively, the development of photocatalytic systems with distinctly higher activity, higher selectivity, and lower photon energy threshold is desired. Over the past decades, many efforts have been made to activate the strong C―H bond in methane by light at room temperature. Based on the current state of research on photocatalytic methane conversion, we have focused our review on the following aspects: non-oxidative coupling of methane, dehydroaromatization of methane, and total and partial oxidation of methane. Finally, we summarize the difference between photocatalysis and thermal catalysis in the methane conversion reaction. ![]()
Methane, the most abundant constituent of natural gas, is a potential substitute for the dwindling petroleum resources for the chemical industry as a carbon-based feedstock. Over the last two decades, global research endeavors have focused on the development of more efficient and selective catalysts for the conversion of ubiquitous but inert methane. In addition, the transportation of gaseous methane in pipelines is unavoidably accompanied by leakage, and methane is recognized as a potent greenhouse gas (20 times more powerful than carbon dioxide per molecule). Thus, the conversion of methane into heavier derivatives is also of crucial environmental concern. Unfortunately, there is still a lack of economical and practical routes for methane conversion. Currently, the major route for methane conversion is the steam reforming of methane into synthetic gases, which is a multistep and energy-consuming route. Another option is to use photoenergy to drive the conversion of methane, which has significant advantages such as the capacity to minimize coking by running at room temperature. A promising approach to photocatalytic methane conversion is the photo-powered direct coupling or oxidation of methane to form ethane, methanol and hydrogen. The ethane or methanol produced can, in turn, be converted into ethene or liquid fuels through metathesis or dehydrogenation, respectively. Furthermore, the direct dehydrogenation of methane is the best way to produce clean H2 energy from fossil fuels since methane has the highest H/C ratio among hydrocarbons. However, the methane conversion efficiency of previously reported photocatalysts is low. Furthermore, the wavelength of light used in previously reported photocatalytic systems usually needs to be less than 270 nm, which is beyond the range of the solar spectrum (wavelength λ > 290 nm) reaching the Earth's surface. To achieve substantial yield and selectivity, and to exploit solar energy effectively, the development of photocatalytic systems with distinctly higher activity, higher selectivity, and lower photon energy threshold is desired. Over the past decades, many efforts have been made to activate the strong C―H bond in methane by light at room temperature. Based on the current state of research on photocatalytic methane conversion, we have focused our review on the following aspects: non-oxidative coupling of methane, dehydroaromatization of methane, and total and partial oxidation of methane. Finally, we summarize the difference between photocatalysis and thermal catalysis in the methane conversion reaction.
2019, 35(9): 977-988
doi: 10.3866/PKU.WHXB201811045
Abstract:
Normal alkyl sp3C―H bonds are ubiquitous in compounds such as methane, linear alkanes, and cycloalkanes that are not linked directly to heteroatoms or other functional groups. These unactivated bonds are not broken readily under mild conditions because their bond dissociation energy values are high and acidity values are low. Moreover, in the radical processes at high temperatures, reaction selectivity is not good for an alkane substrate with various alkyl sp3C―H bonds, which is commonly methyl < 1° < 2° < 3°. In the past five decades, C―H activation by transition-metal species to give C-metal bonds under mild conditions was intensively studied; all efforts were undertaken to provide new methods that can be applied in both chemical synthesis and chemical industry. However, the effective transformations of inert C―H bonds, particularly alkyl sp3C―H bonds, without the assistance of directing groups have been rarely investigated. This review focuses on the functionalization of normal alkyl sp3C―H bonds, such as methyl and primary sp3C―H bonds, via electrophilic activation or oxidative addition by using homogenous transition-metal catalysts, which are two main strategies in the study of inert C―H activation. The selectivity on C―H bond is methyl > 1° > 2° > 3° in both the reactions. Neither heterogeneous catalysis nor biocatalysis is mentioned in this review. Some remarkable progress is described on the study of reaction mechanisms and the establishment of novel reactions. For example, several selective oxidations of methane or linear alkanes have been introduced to afford new C―O, C―Cl, or even C―C bonds in the presence of Pt or Pd catalysts. The Shilov chemistry, which combines electrophilic activation of the C―H bond by the transition-metal complex, oxidation of the transition-metal intermediate, and nucleophilic substitution of organometallic species, has been emphasized in these reactions. Other transition-metal catalysts including Rh, Ir, Re, and W have been employed successfully in the carbonylation, borylation, and dehydrogenation of alkanes at moderate temperatures. The reaction pathways normally involve oxidative addition of the C―H bond with the transition-metal complex followed by insertion-elimination, reductive elimination, or β-H elimination. In the cascade reactions consisting of dehydrogenation of alkanes and addition of alkenes, new C―C or C―Si bonds can also be formed at terminal sites of linear alkanes. However, most of the above-mentioned reactions are still under investigation because of limited scope of the substrate, excess loading of the alkane, low efficiency of the catalyst, and high cost of the reaction operation. Breakthroughs in this promising field of alkane functionalization are possible when new concepts and technology are realized and applied. ![]()
Normal alkyl sp3C―H bonds are ubiquitous in compounds such as methane, linear alkanes, and cycloalkanes that are not linked directly to heteroatoms or other functional groups. These unactivated bonds are not broken readily under mild conditions because their bond dissociation energy values are high and acidity values are low. Moreover, in the radical processes at high temperatures, reaction selectivity is not good for an alkane substrate with various alkyl sp3C―H bonds, which is commonly methyl < 1° < 2° < 3°. In the past five decades, C―H activation by transition-metal species to give C-metal bonds under mild conditions was intensively studied; all efforts were undertaken to provide new methods that can be applied in both chemical synthesis and chemical industry. However, the effective transformations of inert C―H bonds, particularly alkyl sp3C―H bonds, without the assistance of directing groups have been rarely investigated. This review focuses on the functionalization of normal alkyl sp3C―H bonds, such as methyl and primary sp3C―H bonds, via electrophilic activation or oxidative addition by using homogenous transition-metal catalysts, which are two main strategies in the study of inert C―H activation. The selectivity on C―H bond is methyl > 1° > 2° > 3° in both the reactions. Neither heterogeneous catalysis nor biocatalysis is mentioned in this review. Some remarkable progress is described on the study of reaction mechanisms and the establishment of novel reactions. For example, several selective oxidations of methane or linear alkanes have been introduced to afford new C―O, C―Cl, or even C―C bonds in the presence of Pt or Pd catalysts. The Shilov chemistry, which combines electrophilic activation of the C―H bond by the transition-metal complex, oxidation of the transition-metal intermediate, and nucleophilic substitution of organometallic species, has been emphasized in these reactions. Other transition-metal catalysts including Rh, Ir, Re, and W have been employed successfully in the carbonylation, borylation, and dehydrogenation of alkanes at moderate temperatures. The reaction pathways normally involve oxidative addition of the C―H bond with the transition-metal complex followed by insertion-elimination, reductive elimination, or β-H elimination. In the cascade reactions consisting of dehydrogenation of alkanes and addition of alkenes, new C―C or C―Si bonds can also be formed at terminal sites of linear alkanes. However, most of the above-mentioned reactions are still under investigation because of limited scope of the substrate, excess loading of the alkane, low efficiency of the catalyst, and high cost of the reaction operation. Breakthroughs in this promising field of alkane functionalization are possible when new concepts and technology are realized and applied.
2019, 35(9): 989-1004
doi: 10.3866/PKU.WHXB201812016
Abstract:
Transition-metal-catalyzed C―H functionalization reactions, assisted by directing groups (DGs), have become some of the most powerful strategies to form C―C and C―X (X = O, N, S, etc.) bonds. It has brought about a revolution in the synthesis of drugs and natural products, and the method is widely applicable in the fields of material chemistry and pharmaceutical industry. This strategy has mainly focused on regioselective C―H functionalization of amides, esters, carbamates, and enamides with DGs to form C―C and C―X bonds. Since these DGs are relatively stable, they must be removed by other methods when the reaction is completed. Therefore, the use of a traceless DG is one of the important challenges for transition-metal-catalyzed C―H functionalization. Recently, N-phenoxyamide has been attracting significant research attention as a versatile DG. Oxyacetamide (O―NHAc) is one of the most versatile functionalities for directed C―H functionalization cascades, such as the internal oxidation with N―O bond cleavage. The O―NHAc has been reported as a superb DG for redox-neutral C―H activation/annulation cascade reactions to synthesize phenol and complex heterocyclic scaffolds by coupling with alkynes, alkenes, heteroarenes, and diazo compounds. However, for the external oxidation with preservation of the N―O bond, e.g. when a stoichiometric external oxidant is present, N-phenoxyamides could react with aldehydes or α, β-unsaturated aldehydes. In addition, the solvent can control the chemoselectivity. In this minireview, the C―H bond functionalization of N-phenoxyamide is divided into five categories according to the different substrates, viz. alkenes, alkynes, diazo, and other compounds and intramolecular C―H bond activation reactions. Based on experimental and theoretical research results, the reaction mechanism was discussed. In the first part, we summarize the ortho-alkylation, alkenylation, and cyclization of N-phenoxyamide with olefins. In the second part, we present the Rh- and Ir-catalyzed C―H activation or cyclization of N-phenoxyamide with alkanes to synthesize phenol or benzofuran compounds. In the third part, we describe the synthesis of phenolic compounds functionalized by Rh-catalyzed diazo compounds by carbene intermediates and N-phenoxyamides. The forth part summarizes the C―H activation/annulation reaction using aldehydes, heterocyclic aromatic, and sulfur reagents as substrates. The last part of the paper generalizes the intramolecular ortho-hydroxylation and ortho, para-amidation reactions. ![]()
Transition-metal-catalyzed C―H functionalization reactions, assisted by directing groups (DGs), have become some of the most powerful strategies to form C―C and C―X (X = O, N, S, etc.) bonds. It has brought about a revolution in the synthesis of drugs and natural products, and the method is widely applicable in the fields of material chemistry and pharmaceutical industry. This strategy has mainly focused on regioselective C―H functionalization of amides, esters, carbamates, and enamides with DGs to form C―C and C―X bonds. Since these DGs are relatively stable, they must be removed by other methods when the reaction is completed. Therefore, the use of a traceless DG is one of the important challenges for transition-metal-catalyzed C―H functionalization. Recently, N-phenoxyamide has been attracting significant research attention as a versatile DG. Oxyacetamide (O―NHAc) is one of the most versatile functionalities for directed C―H functionalization cascades, such as the internal oxidation with N―O bond cleavage. The O―NHAc has been reported as a superb DG for redox-neutral C―H activation/annulation cascade reactions to synthesize phenol and complex heterocyclic scaffolds by coupling with alkynes, alkenes, heteroarenes, and diazo compounds. However, for the external oxidation with preservation of the N―O bond, e.g. when a stoichiometric external oxidant is present, N-phenoxyamides could react with aldehydes or α, β-unsaturated aldehydes. In addition, the solvent can control the chemoselectivity. In this minireview, the C―H bond functionalization of N-phenoxyamide is divided into five categories according to the different substrates, viz. alkenes, alkynes, diazo, and other compounds and intramolecular C―H bond activation reactions. Based on experimental and theoretical research results, the reaction mechanism was discussed. In the first part, we summarize the ortho-alkylation, alkenylation, and cyclization of N-phenoxyamide with olefins. In the second part, we present the Rh- and Ir-catalyzed C―H activation or cyclization of N-phenoxyamide with alkanes to synthesize phenol or benzofuran compounds. In the third part, we describe the synthesis of phenolic compounds functionalized by Rh-catalyzed diazo compounds by carbene intermediates and N-phenoxyamides. The forth part summarizes the C―H activation/annulation reaction using aldehydes, heterocyclic aromatic, and sulfur reagents as substrates. The last part of the paper generalizes the intramolecular ortho-hydroxylation and ortho, para-amidation reactions.
2019, 35(9): 1005-1013
doi: 10.3866/PKU.WHXB201809006
Abstract:
The activation of methane (CH4) is a key step in its conversion to more valuable products. The activation mechanisms of CH4 on catalyst surfaces have been widely studied using gas-phase cluster models, which can be operated on systems with a precise number of atoms and determined structures. Herein, we have used MV3Oyq (M = Au/Ag, y = 6–8, q = 0 or ±1) clusters, in which a single Au or Ag atom was supported on vanadium oxide clusters, as simple models to mimic the properties of newly developed single-atom catalysts. The adsorption and activation of CH4 on these MV3Oyq clusters were systematically studied via density functional theory calculations at the B3LYP/Def2-TZVP level, which provided insights into the geometric structures, adsorption energies, and charge distributions of the adsorption systems. Five Au-containing clusters, AuV3O6, AuV3O7, AuV3O8, AuV3O6+, and AuV3O7+, were able to activate CH4, while other clusters, including all Ag-containing clusters, were inert. In the active clusters, all Au atoms were adsorbed on the O-atom sites of the supporting V3Oyq cluster and served as the active sites for CH4 activation. The activation of CH4 was characterized by the lengthened C―H bond (approximately 115 pm), short distances between CH4 and Au (approximately 184 pm), relatively high adsorption energies of CH4 (~0.590–1.145 eV), and significant electron transfer from CH4 to the clusters (above 0.08e). In particular, AuV3O8, which is a neutral cluster with a close-shell electronic state, can activate CH4 with a C―H bond length of 115 pm, Au―H bond length of 183 pm, the adsorption energy of CH4 of 0.853 eV, and the charge on CH4 of +0.088e. The charge state of the cluster has a significant effect on the activation ability: cationic clusters are the most active, followed by neutral clusters, while anionic clusters have the lowest activities toward CH4. Consistently, the local charge on the M atom has a positive correction with the activation ability of MV3Oyq clusters with a certain M. However, as compared to Au-containing clusters, Ag-containing clusters have lower activities despite the higher local charges on Ag in each MV3Oyq cluster. The results indicate that the inclusion of D3 dispersion correction has a small effect on structures and energies. This study may serve as a foundation for further research on the activation of CH4 on single-atom catalysts and provides useful information on rational designing of single-atom catalysts for CH4 conversion at low temperatures. ![]()
The activation of methane (CH4) is a key step in its conversion to more valuable products. The activation mechanisms of CH4 on catalyst surfaces have been widely studied using gas-phase cluster models, which can be operated on systems with a precise number of atoms and determined structures. Herein, we have used MV3Oyq (M = Au/Ag, y = 6–8, q = 0 or ±1) clusters, in which a single Au or Ag atom was supported on vanadium oxide clusters, as simple models to mimic the properties of newly developed single-atom catalysts. The adsorption and activation of CH4 on these MV3Oyq clusters were systematically studied via density functional theory calculations at the B3LYP/Def2-TZVP level, which provided insights into the geometric structures, adsorption energies, and charge distributions of the adsorption systems. Five Au-containing clusters, AuV3O6, AuV3O7, AuV3O8, AuV3O6+, and AuV3O7+, were able to activate CH4, while other clusters, including all Ag-containing clusters, were inert. In the active clusters, all Au atoms were adsorbed on the O-atom sites of the supporting V3Oyq cluster and served as the active sites for CH4 activation. The activation of CH4 was characterized by the lengthened C―H bond (approximately 115 pm), short distances between CH4 and Au (approximately 184 pm), relatively high adsorption energies of CH4 (~0.590–1.145 eV), and significant electron transfer from CH4 to the clusters (above 0.08e). In particular, AuV3O8, which is a neutral cluster with a close-shell electronic state, can activate CH4 with a C―H bond length of 115 pm, Au―H bond length of 183 pm, the adsorption energy of CH4 of 0.853 eV, and the charge on CH4 of +0.088e. The charge state of the cluster has a significant effect on the activation ability: cationic clusters are the most active, followed by neutral clusters, while anionic clusters have the lowest activities toward CH4. Consistently, the local charge on the M atom has a positive correction with the activation ability of MV3Oyq clusters with a certain M. However, as compared to Au-containing clusters, Ag-containing clusters have lower activities despite the higher local charges on Ag in each MV3Oyq cluster. The results indicate that the inclusion of D3 dispersion correction has a small effect on structures and energies. This study may serve as a foundation for further research on the activation of CH4 on single-atom catalysts and provides useful information on rational designing of single-atom catalysts for CH4 conversion at low temperatures.
2019, 35(9): 1014-1020
doi: 10.3866/PKU.WHXB201811039
Abstract:
Methane activation by transition metal species has been extensively investigated over the past few decades. It is observed that ground-state monocations of bare 3d transition metals are inert toward CH4 at room temperature because of unfavorable thermodynamics. In contrast, many mono-ligated 3d transition metal cations, such as MO+ (M = Mn, Fe, Co, Cu, Zn), MH+ (M = Fe, Co), and NiX+ (X = H, CH3, F), as well as several bis-ligated 3d transition metal cations including OCrO+, Ni(H)(OH)+, and Fe(O)(OH)+ activate the C―H bond of methane under thermal collision conditions because of the pronounced ligand effects. In most of the above-mentioned examples, the 3d metal atoms are observed to cooperate with the attached ligands to activate the C―H bond. Compared to the extensive studies on active species comprising of middle and late 3d transition metals, the knowledge about the reactivity of early 3d transition metal species toward methane and the related C―H activation mechanisms are still very limited. Only two early 3d transition metal species HMO+ (M = Ti and V) are discovered so far to activate the C―H bond of methane via participation of their metal atoms. In this study, by performing mass spectrometric experiments and density functional theory calculations, we have identified that the diatomic vanadium boride cation (VB+) can activate methane to produce a dihydrogen molecule and carbon-boron species under thermal collision conditions. The strong electrostatic interaction makes the reaction preferentially proceed the V side. To generate experimentally observed product ions, a two-state reactivity scenario involving spin conversion from high-spin sextet to low-spin quartet is necessary at the entrance of the reaction. This result is consistent with the reported reactions of 3d transition metal species with CH4, in which the C―H bond cleavage generally occurs in the low-spin states, even if the ground states of the related active species are in the high-spin states. For VB+ + CH4, the insertion of the synergetic V―B unit (rather than a single V or B atom) into the H3C―H bond causes the initial C―H bond activation driven by the strong bond strengths of V―CH3 and B―H. The mechanisms of methane activation by VB+ discussed in this study may provide useful guidance to the future studies on methane activation by early transition metal systems. ![]()
Methane activation by transition metal species has been extensively investigated over the past few decades. It is observed that ground-state monocations of bare 3d transition metals are inert toward CH4 at room temperature because of unfavorable thermodynamics. In contrast, many mono-ligated 3d transition metal cations, such as MO+ (M = Mn, Fe, Co, Cu, Zn), MH+ (M = Fe, Co), and NiX+ (X = H, CH3, F), as well as several bis-ligated 3d transition metal cations including OCrO+, Ni(H)(OH)+, and Fe(O)(OH)+ activate the C―H bond of methane under thermal collision conditions because of the pronounced ligand effects. In most of the above-mentioned examples, the 3d metal atoms are observed to cooperate with the attached ligands to activate the C―H bond. Compared to the extensive studies on active species comprising of middle and late 3d transition metals, the knowledge about the reactivity of early 3d transition metal species toward methane and the related C―H activation mechanisms are still very limited. Only two early 3d transition metal species HMO+ (M = Ti and V) are discovered so far to activate the C―H bond of methane via participation of their metal atoms. In this study, by performing mass spectrometric experiments and density functional theory calculations, we have identified that the diatomic vanadium boride cation (VB+) can activate methane to produce a dihydrogen molecule and carbon-boron species under thermal collision conditions. The strong electrostatic interaction makes the reaction preferentially proceed the V side. To generate experimentally observed product ions, a two-state reactivity scenario involving spin conversion from high-spin sextet to low-spin quartet is necessary at the entrance of the reaction. This result is consistent with the reported reactions of 3d transition metal species with CH4, in which the C―H bond cleavage generally occurs in the low-spin states, even if the ground states of the related active species are in the high-spin states. For VB+ + CH4, the insertion of the synergetic V―B unit (rather than a single V or B atom) into the H3C―H bond causes the initial C―H bond activation driven by the strong bond strengths of V―CH3 and B―H. The mechanisms of methane activation by VB+ discussed in this study may provide useful guidance to the future studies on methane activation by early transition metal systems.
2019, 35(9): 1021-1026
doi: 10.3866/PKU.WHXB201811044
Abstract:
The broad existence of the biaryl linkage in bioactive organic molecules and functional materials makes it an attractive synthesis target via construction of aryl-aryl carbon bonds. Transition metal catalyzed cross-coupling reactions of two pre-functionalized aryl partners, e.g., Suzuki-Miyaura cross-coupling and Negishi cross-coupling reactions, are the main methods typically used for the construction of biaryl linkages. Since the end of the last century, transition metal catalyzed direct C-H arylation of unactivated arenes has emerged as a practical alternative to the well-established cross-coupling strategies. However, the use of transition metal catalysts and/or organometallic reagents would lead to problems, such as the disposal of waste from large-scale syntheses and the removal of heavy metal contaminants from pharmaceutical intermediates. In this regard, the base-promoted homolytic aromatic substitution (BHAS) reaction of aryl halides with unactivated arenes offers a simpler strategy for the synthesis of biaryl scaffolds, and avoids the use of transition metals. Although the BHAS reaction can proceed to a small extent without any additives, particularly at elevated temperatures, the addition of organic promoters would significantly accelerate the reaction rate and improve the overall efficiency of the process. Over the past ten years, a wide variety of N- and O-based organic promoters have been developed to promote the BHAS reaction in the presence of the tert-butoxide base. The mechanism of the BHAS reaction has been studied extensively, and is accepted as occurring via a radical chain process involving an aryl radical. However, the role and mode of initiation of most organic promoters studied remain controversial. The development of more and varied organic promoters will surely promote the mechanistic understanding and further development of the BHAS reaction. Herein, we report that 1, 1'-bis(diphenylphosphino)ferrocene (dppf, or DPPF) can act as a P-based promoter to facilitate the direct arylation of unactivated arenes with aryl iodides using potassium tert-butoxide as the base and electron donor. A broad range of aryl iodides and arenes reacted smoothly under the optimized reaction conditions, giving arylated products in good yields and with high regio-selectivity. Intramolecular C-H arylation also worked well following a sequence of single electron transfer (SET)/initiation, 5-exo-trig aryl radical addition, ring expansion, deprotonation, and re-aromatization/propagation. A mechanistic study indicated that the diphenylphosphino group of dppf played a vital role in the initiation step by enhancing the SET-inhibiting ability of the tert-butoxide anion. A primary kinetic isotope effect was observed in the parallel reactions between 4-methoxy-iodobenzene with benzene and deuterated benzene, implying that the deprotonation of the cyclohexadienyl radical intermediate by tert-butoxide was the rate-determining step in the radical chain pathway. ![]()
The broad existence of the biaryl linkage in bioactive organic molecules and functional materials makes it an attractive synthesis target via construction of aryl-aryl carbon bonds. Transition metal catalyzed cross-coupling reactions of two pre-functionalized aryl partners, e.g., Suzuki-Miyaura cross-coupling and Negishi cross-coupling reactions, are the main methods typically used for the construction of biaryl linkages. Since the end of the last century, transition metal catalyzed direct C-H arylation of unactivated arenes has emerged as a practical alternative to the well-established cross-coupling strategies. However, the use of transition metal catalysts and/or organometallic reagents would lead to problems, such as the disposal of waste from large-scale syntheses and the removal of heavy metal contaminants from pharmaceutical intermediates. In this regard, the base-promoted homolytic aromatic substitution (BHAS) reaction of aryl halides with unactivated arenes offers a simpler strategy for the synthesis of biaryl scaffolds, and avoids the use of transition metals. Although the BHAS reaction can proceed to a small extent without any additives, particularly at elevated temperatures, the addition of organic promoters would significantly accelerate the reaction rate and improve the overall efficiency of the process. Over the past ten years, a wide variety of N- and O-based organic promoters have been developed to promote the BHAS reaction in the presence of the tert-butoxide base. The mechanism of the BHAS reaction has been studied extensively, and is accepted as occurring via a radical chain process involving an aryl radical. However, the role and mode of initiation of most organic promoters studied remain controversial. The development of more and varied organic promoters will surely promote the mechanistic understanding and further development of the BHAS reaction. Herein, we report that 1, 1'-bis(diphenylphosphino)ferrocene (dppf, or DPPF) can act as a P-based promoter to facilitate the direct arylation of unactivated arenes with aryl iodides using potassium tert-butoxide as the base and electron donor. A broad range of aryl iodides and arenes reacted smoothly under the optimized reaction conditions, giving arylated products in good yields and with high regio-selectivity. Intramolecular C-H arylation also worked well following a sequence of single electron transfer (SET)/initiation, 5-exo-trig aryl radical addition, ring expansion, deprotonation, and re-aromatization/propagation. A mechanistic study indicated that the diphenylphosphino group of dppf played a vital role in the initiation step by enhancing the SET-inhibiting ability of the tert-butoxide anion. A primary kinetic isotope effect was observed in the parallel reactions between 4-methoxy-iodobenzene with benzene and deuterated benzene, implying that the deprotonation of the cyclohexadienyl radical intermediate by tert-butoxide was the rate-determining step in the radical chain pathway.
2019, 35(9): 1027-1036
doi: 10.3866/PKU.WHXB201902004
Abstract:
The selective oxidation of methane to basic petrochemicals (ethylene and ethane) is desirable and has attracted extensive research attention. The oxidative coupling of methane (OCM) is considered a promising one-step route for the production of C2 compounds (ethylene and ethane) from methane, and has been the focus of industrial and fundamental studies. It is widely accepted that the composition is a crucial factor governing the activity of a catalyst system. It was found that the phase structures, basicity, existing status and distribution of the active components, oxygen species, and chemical states of the catalyst were influenced by the composition and ratio, resulting in different catalytic performances for the OCM. In this study, a series of solid acid WO3/TiO2-supported lithium-manganese oxide catalysts for OCM were synthesized via the impregnation method. The impacts of diverse compositions, such as the individual contents (Li and Mn) and dual contents (Li-Mn), on the OCM were investigated in detail, using inductively coupled plasma optical emission spectrometry, X-ray diffraction, high-resolution transmission electron microscopy, CO2-temperature-programmed desorption, O2-temperature-programmed desorption, H2-temperature-programmed reduction, Raman spectroscopy, X-ray photoelectron spectroscopy, and CH4-temperature-programmed surface reaction. The addition of Li content to the catalyst not only led to the anatase-to-rutile crystal structure transformation of TiO2, and the reduction of the high-valence-state Mn species to low-valence-state Mn, but also increased the content of surface lattice oxygen and decreased the surface basicity. The observed effects on the structures and catalytic performance suggest that the Li content is helpful in suppressing the formation of completely oxidized CO2, and increases the C2 selectivity. Moreover, increasing the Li content of the catalyst facilitated the mobility of the lattice oxygen, which triggered the promotion of CH4 activation, thereby enhancing the OCM catalytic performance. The Mn content acted as the active sites for OCM; therefore, the performance of the catalyst was closely related to the Mn concentration and valence state. However, the WO3/TiO2-supported catalyst with excessive Mn content exhibited a high surface basicity, high valence state of Mn, and low abundant lattice oxygen, which was unfavorable for C2 selectivity. The Raman spectroscopy results revealed that MnTiO3 was formed due to the co-existence of Li and Mn on WO3/TiO2, and played an essential role in improving the low-temperature OCM performance. There was a synergic effect of the Li and Mn components on the OCM. The optimal performance (16.3% C2 yield) was achieved over the WO3/TiO2-supported lithium-manganese catalyst with n(Li) : n(Mn) = 2 : 1 at 750 ℃. ![]()
The selective oxidation of methane to basic petrochemicals (ethylene and ethane) is desirable and has attracted extensive research attention. The oxidative coupling of methane (OCM) is considered a promising one-step route for the production of C2 compounds (ethylene and ethane) from methane, and has been the focus of industrial and fundamental studies. It is widely accepted that the composition is a crucial factor governing the activity of a catalyst system. It was found that the phase structures, basicity, existing status and distribution of the active components, oxygen species, and chemical states of the catalyst were influenced by the composition and ratio, resulting in different catalytic performances for the OCM. In this study, a series of solid acid WO3/TiO2-supported lithium-manganese oxide catalysts for OCM were synthesized via the impregnation method. The impacts of diverse compositions, such as the individual contents (Li and Mn) and dual contents (Li-Mn), on the OCM were investigated in detail, using inductively coupled plasma optical emission spectrometry, X-ray diffraction, high-resolution transmission electron microscopy, CO2-temperature-programmed desorption, O2-temperature-programmed desorption, H2-temperature-programmed reduction, Raman spectroscopy, X-ray photoelectron spectroscopy, and CH4-temperature-programmed surface reaction. The addition of Li content to the catalyst not only led to the anatase-to-rutile crystal structure transformation of TiO2, and the reduction of the high-valence-state Mn species to low-valence-state Mn, but also increased the content of surface lattice oxygen and decreased the surface basicity. The observed effects on the structures and catalytic performance suggest that the Li content is helpful in suppressing the formation of completely oxidized CO2, and increases the C2 selectivity. Moreover, increasing the Li content of the catalyst facilitated the mobility of the lattice oxygen, which triggered the promotion of CH4 activation, thereby enhancing the OCM catalytic performance. The Mn content acted as the active sites for OCM; therefore, the performance of the catalyst was closely related to the Mn concentration and valence state. However, the WO3/TiO2-supported catalyst with excessive Mn content exhibited a high surface basicity, high valence state of Mn, and low abundant lattice oxygen, which was unfavorable for C2 selectivity. The Raman spectroscopy results revealed that MnTiO3 was formed due to the co-existence of Li and Mn on WO3/TiO2, and played an essential role in improving the low-temperature OCM performance. There was a synergic effect of the Li and Mn components on the OCM. The optimal performance (16.3% C2 yield) was achieved over the WO3/TiO2-supported lithium-manganese catalyst with n(Li) : n(Mn) = 2 : 1 at 750 ℃.
