Methane functionalization

Last updated

Methane functionalization is the process of converting methane in its gaseous state to another molecule with a functional group, typically methanol or acetic acid, through the use of transition metal catalysts.

Contents

In the realm of carbon-hydrogen bond activation and functionalization (C-H activation/functionalization), many recent efforts have been made in order to catalytically functionalize the C-H bonds in methane. The large abundance of methane in natural gas or shale gas deposits presents a large potential for its use as a feedstock in modern chemistry. However, given its gaseous natural state, it is quite difficult to transport economically. Its ideal use would be as a raw starting material for methanol or acetic acid synthesis, with plants built at the source to eliminate the issue of transportation. [1] Methanol, in particular, would be of great use as a potential fuel source, and many efforts have been applied to researching the feasibilities of a methanol economy.

The challenges of C-H activation and functionalization present themselves when several factors are taken into consideration. Firstly, the C-H bond is extremely inert and non-polar, with a high bond dissociation energy, making methane a relatively unreactive starting material. Secondly, any products formed from methane would likely be more reactive than the starting product, which would be detrimental to the selectivity and yield of the reaction. [1]

The main strategy currently used to increase the reactivity of methane uses transition metal complexes to activate the carbon-hydrogen bonds. In a typical C-H activation mechanism, a transition metal catalyst coordinates to the C-H bond to cleave it, and convert it into a bond with a lower bond dissociation energy. By doing so, the product can be used in further downstream reactions, since it will usually have a new functional group attached to the carbon. It is also important to note the difference between the terms "activation" and "functionalization," since both terms are often used interchangeably, but should be held distinct from each other. Activation refers to the coordination of a metal center to the C-H bond, whereas functionalization occurs when the coordinated metal complex is further reacted with a group "X" to result in the functionalized product. [1]

Methane activation

The four most common methods of transition metal catalyzed methane activation are the Shilov system, sigma bond metathesis, oxidative addition, and 1,2 addition reactions.

The mechanism for the Shilov system (Figure 3) Shilov mechanism.png

The Shilov system involves platinum based complexes to produce metal alkyls. It was first discovered when a hydrogen-deuterium exchanged was observed in a deuterated solution with the platinum tetrachloride anion. [2] Shilov et al. then was able to catalytically convert methane into methanol or methyl chloride when a Pt(IV) salt was used as a stoichiometric oxidant. The process is simplified down into three main steps: (1) C-H activation, (2) a redox reaction to form an octahedral intermediate, followed by (3) the formation of the carbon-oxygen bond to form methanol ( Figure 3 ). [3]

Methane activation via sigma-bond metathesis (Figure 4) Sigma bond metathesis.png

Sigma bond metathesis involves the formation of new C-H and metal-carbon bonds, where the metals are typically in the d0 configuration. Starting with a metal alkyl, a C-H bond coordinates with the metal complex via sigma bonding. A four-member transition state is created, where a new metal-carbon bond is formed, and the former C-H linkage is broken ( Figure 4 ). [1]

Simple mechanism of using oxidative addition for methane activation (Figure 5) Ox add.png

In oxidative addition, the metal center's oxidation state increases by 2 units during the process. First, the metal center coordinates with a sigma C-H bond to form an intermediate called a sigma-methane complex. The C-H linkage is then broken, as the metal becomes covalently bonded each to the carbon and the hydrogen ( Figure 5 ). [1]

A simplified mechanism of 1,2 addition of methane to unsaturated metal-ligand bonds. (Figure 6) 1 2 addition.png

Similar to sigma bond metathesis is the 1,2 addition reaction, where a four-membered transition state is also formed. However, a polarized double or triple metal-ligand bond is required in order to favor the formation of the desired product ( Figure 6 ). [1]

Methane functionalization

Once the C-H bond of methane is activated by bonding to a transition metal complex, the net functionalization of the alkyl metal complex into another hydrocarbon containing a functional group is actually much harder to achieve. In general, alkanes of various lengths have typically been functionalized by a number of more commonly known reactions: electrophilic activation (Shilov system, see above), dehydrogenation, borylation, hydrogen-deuterium exchange, and carbene/nitrene/oxo insertion. [1] The functionalization of methane in particular has been reported in four different methods that use homogeneous catalysts rather than heterogeneous catalysts. Heterogeneous systems, using copper- and iron exchanged Zeolite, are also investigated. In these systems, reactive oxygen species such as Alpha-Oxygen are generated which can perform a hydrogen atom abstraction. [4]

The Catalytica system

In 1993, Periana et al. reported a synthesis of methyl bisulfate from methane using a mercury catalyst at 180 °C. [5] Mercuric bisulfate activates methane electrophilically to form a methyl-complex, which then reacts with sulfuric acid to produce methyl bisulfate. The resulting mercury complex Hg2(OSO3)2 is re-oxidized by sulfuric acid to regenerate the catalyst and restart the catalytic cycle ( Figure 7 ).

Electrophilic functionalization of methane with Hg(II). (Figure 7) Pre catalytica.png

This method of functionalizing methane preceded the 1998 discovery by the same group of the so-called Catalytica system, the most active cycle to date in terms of turnover rate, yields, and selectivity. [6] Performing the reaction in sulfuric acid at 220 °C means that the catalyst must be able to withstand these harsh conditions. A platinum-bipyrimidine complex serves as the catalyst. The mechanism for this system is similar to the one described above, where methane is first activated electrophilically to form a methyl-platinum intermediate. The Pt(II) complex is then oxidized to Pt(IV) as two sulfuric acid groups are added to the complex. The reductive elimination of methyl bisulfate transforms the Pt(IV) species back to Pt(II) to regenerate the catalyst ( Figure 8 ).

The Catalytica system with a platinum catalyst. (Figure 8) Catalytica.png

In a hypothetical combined process, the Catalytica system could be used in a net conversion of methane to methanol. The methyl bisulfate produced in the cycle could be converted to methanol by hydrolysis, and the sulfur dioxide generated could be converted back to sulfuric acid. [1]

Conversion to acetic acid

Periana's group was also able to convert methane into acetic acid using similar conditions to the Catalytica system. Palladium(II) salts were used in this process, and the products formed were a mixture of methanol and acetic acid, along with side products of carbon monoxide and possibly carbon dioxide due to over-oxidation. [7] The mechanism of reaction involves another electrophilic activation of methane, and when carbon monoxide is incorporated, the acetic acid derivative is generated through its activation to an acyl intermediate ( Figure 9 ).

Synthesis of acetic acid from methane using palladium salts. (Figure 9) Periana acetic acid.png

Another example of acetic acid synthesis was demonstrated by Pombeiro et al., which used vanadium-based complexes in trifluoroacetic acid with peroxodisulfate as the oxidant. [8] The proposed mechanism involves a radical mechanism, where methane is the methyl source and trifluoroacetic acid is the carbonyl source. Minor side products were formed, including methyltrifluoroacetate and methylsulfate.

Dehydrogenative silation and olefin hydromethylation

T. Don Tilley and coworkers were able to use the process of sigma-bond metathesis to design catalytic systems that work by the formation of carbon-carbon bonds. [9] They first demonstrated an example using a scandium-based system, where methane is dehydrogenated and silated. Starting from phenyl silane, methane pressure converts it into Ph2MePhH using a Cp*ScMe catalyst. The scandium complex then transfers the methyl group to the silane by sigma-bond metathesis to form the product and the Cp*2ScH intermediate. The favorable formation of hydrogen gas combined with methane will regenerate the methyl complex from the hydride derivative ( Figure 10 ).

Silation of methane using a scandium catalyst. (Figure 10) Methane silation.png

Cp*2ScMe was also used as a catalyst in the formation of isobutane by adding methane to the double bond of propene. This was achieved when propene and methane were combined in the presence of the scandium catalyst and heated to 80 °C. [10]

Carbene insertion

Carbene insertion use a different strategy for the functionalization of methane. A strategy using metallocarbenes has been shown with several linear and branched alkanes with rhodium, silver, copper, and gold-based catalysts. [11] With a carbene ligand attached to a metal center, it can be transferred from the coordination sphere and inserted into an activated C-H bond. In this case, there is no interaction between the metal center and the alkane in question, which separates this method from the other methods mentioned above. The general mechanism for this cycle begins with the reaction of an electron-poor metal center with a diazocompound to form a metallo-carbene intermediate. In order for this reaction to occur, the diazocompound must be very electrophilic, since the C-H bond is such a poor nucleophile as well as being an unactivated alkane. The reaction then proceeds in a concerted manner, where the C-H bond of the incoming molecule coordinates with the carbene carbon of the metallocarbene complex. The hydrocarbon then dissociates from the metal center to regenerate the catalyst and free the newly formed carbon-carbon bond ( Figure 11 ).

Metal-catalyzed carbene insertion to functionalize alkanes from diazocompounds. (Figure 11) Carbeneinsertion.png

This route is very successful for higher order alkanes due to the fact that there is no formation of strong metal-carbon or metal-hydrogen bonds that could prevent any intermediates from reacting further. The reactions also take place in room temperature under mild conditions. However, when applying this method to methane specifically, the gaseous nature of methane requires an appropriate solvent. Reactions with other alkanes usually have the alkane in question be the solvent itself; however, any C-H bond with a lower BDE or higher polarity than methane will react first and prevent methane functionalization. Therefore, Pérez, Asensio, Etienne, et al. developed a solution to use supercritical carbon dioxide as the solvent, which is formed under the critical pressure of 73 bar and a temperature of 31 °C. [12] In these conditions, scCO2 behaves as a liquid, and since fluorinated compounds can dissolve easily in scCO2, highly fluorinated silver-based catalysts were developed and tested with methane and ethyl diazoacetate. However, under the reaction conditions, only 19% yield of ethyl propionate was able to be achieved. The reaction depends on a delicate balance between methane pressure and catalyst concentration, and consequently more work is being done to further improve yields.

Related Research Articles

<span class="mw-page-title-main">Alkane</span> Type of saturated hydrocarbon compound

In organic chemistry, an alkane, or paraffin, is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which all the carbon–carbon bonds are single. Alkanes have the general chemical formula CnH2n+2. The alkanes range in complexity from the simplest case of methane, where n = 1, to arbitrarily large and complex molecules, like pentacontane or 6-ethyl-2-methyl-5-(1-methylethyl) octane, an isomer of tetradecane.

<span class="mw-page-title-main">Alkene</span> Hydrocarbon compound containing one or more C=C bonds

In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or in the terminal position. Terminal alkenes are also known as α-olefins.

<span class="mw-page-title-main">Alkyne</span> Hydrocarbon compound containing one or more C≡C bonds

In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and no other functional groups form a homologous series with the general chemical formula CnH2n−2. Alkynes are traditionally known as acetylenes, although the name acetylene also refers specifically to C2H2, known formally as ethyne using IUPAC nomenclature. Like other hydrocarbons, alkynes are generally hydrophobic.

<span class="mw-page-title-main">Organometallic chemistry</span> Study of organic compounds containing metal(s)

Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and sometimes broadened to include metalloids like boron, silicon, and selenium, as well. Aside from bonds to organyl fragments or molecules, bonds to 'inorganic' carbon, like carbon monoxide, cyanide, or carbide, are generally considered to be organometallic as well. Some related compounds such as transition metal hydrides and metal phosphine complexes are often included in discussions of organometallic compounds, though strictly speaking, they are not necessarily organometallic. The related but distinct term "metalorganic compound" refers to metal-containing compounds lacking direct metal-carbon bonds but which contain organic ligands. Metal β-diketonates, alkoxides, dialkylamides, and metal phosphine complexes are representative members of this class. The field of organometallic chemistry combines aspects of traditional inorganic and organic chemistry.

A transition metal carbene complex is an organometallic compound featuring a divalent carbon ligand, itself also called a carbene. Carbene complexes have been synthesized from most transition metals and f-block metals, using many different synthetic routes such as nucleophilic addition and alpha-hydrogen abstraction. The term carbene ligand is a formalism since many are not directly derived from carbenes and most are much less reactive than lone carbenes. Described often as =CR2, carbene ligands are intermediate between alkyls (−CR3) and carbynes (≡CR). Many different carbene-based reagents such as Tebbe's reagent are used in synthesis. They also feature in catalytic reactions, especially alkene metathesis, and are of value in both industrial heterogeneous and in homogeneous catalysis for laboratory- and industrial-scale preparation of fine chemicals.

<span class="mw-page-title-main">Methyl bisulfate</span> Chemical compound

Methyl bisulfate is a chemical compound with the molecular formula (CH3)HSO4. This compound is the mono-methyl ester of sulfuric acid. Its structure is CH3−O−S(=O)2−OH. The significance of methyl bisulfate is that it is an intermediate in the hydrolysis of the important reagent dimethyl sulfate, (CH3)2SO4:

In organic chemistry and organometallic chemistry, carbon–hydrogen bond activation is a type of organic reaction in which a carbon–hydrogen bond is cleaved and replaced with a C−X bond. Some authors further restrict the term C–H activation to reactions in which a C–H bond, one that is typically considered to be "unreactive", interacts with a transition metal center M, resulting in its cleavage and the generation of an organometallic species with an M–C bond. The intermediate of this step could then undergo subsequent reactions with other reagents, either in situ or in a separate step, to produce the functionalized product.

In chemistry, carbonylation refers to reactions that introduce carbon monoxide (CO) into organic and inorganic substrates. Carbon monoxide is abundantly available and conveniently reactive, so it is widely used as a reactant in industrial chemistry. The term carbonylation also refers to oxidation of protein side chains.

In organometallic chemistry, a migratory insertion is a type of reaction wherein two ligands on a metal complex combine. It is a subset of reactions that very closely resembles the insertion reactions, and both are differentiated by the mechanism that leads to the resulting stereochemistry of the products. However, often the two are used interchangeably because the mechanism is sometimes unknown. Therefore, migratory insertion reactions or insertion reactions, for short, are defined not by the mechanism but by the overall regiochemistry wherein one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:

<span class="mw-page-title-main">Shilov system</span>

The Shilov system is a classic example of catalytic C-H bond activation and oxidation which preferentially activates stronger C-H bonds over weaker C-H bonds for an overall partial oxidation.

Catalytic oxidation are processes that rely on catalysts to introduce oxygen into organic and inorganic compounds. Many applications, including the focus of this article, involve oxidation by oxygen. Such processes are conducted on a large scale for the remediation of pollutants, production of valuable chemicals, and the production of energy.

Organoplatinum chemistry is the chemistry of organometallic compounds containing a carbon to platinum chemical bond, and the study of platinum as a catalyst in organic reactions. Organoplatinum compounds exist in oxidation state 0 to IV, with oxidation state II most abundant. The general order in bond strength is Pt-C (sp) > Pt-O > Pt-N > Pt-C (sp3). Organoplatinum and organopalladium chemistry are similar, but organoplatinum compounds are more stable and therefore less useful as catalysts.

Georgiy Borisovich Shul’pin was born in Moscow, Russia. He graduated with a M.S. degree in chemistry from the Chemistry Department of Moscow State University in 1969. Between 1969 and 1972, he was a postgraduate student at the Nesmeyanov Institute of Organoelement Compounds under the direction of Prof. A. N. Nesmeyanov and received his Ph.D. in organometallic chemistry in 1975. He received his Dr. of Sciences degree in 2013.

In chemistry, mesoionic carbenes (MICs) are a type of reactive intermediate that are related to N-heterocyclic carbenes (NHCs); thus, MICs are also referred to as abnormal N-heterocyclic carbenes (aNHCs) or remote N-heterocyclic carbenes (rNHCs). Unlike simple NHCs, the canonical resonance structures of these carbenes are mesoionic: an MIC cannot be drawn without adding additional charges to some of the atoms.

<span class="mw-page-title-main">Roy A. Periana</span> Guyanese-American organometallic chemist (born 1957)

Roy A. Periana is a Guyanese-American organometallic chemist.

<span class="mw-page-title-main">White–Chen catalyst</span> Chemical compound

The White–Chen catalyst is an Iron-based coordination complex named after Professor M. Christina White and her graduate student Mark S. Chen. The catalyst is used along with hydrogen peroxide and acetic acid additive to oxidize aliphatic sp3 C-H bonds in organic synthesis. The catalyst is the first to allow for preparative and predictable aliphatic C–H oxidations over a broad range of organic substrates. Oxidations with the catalyst have proven to be remarkably predictable based on sterics, electronics, and stereoelectronics allowing for aliphatic C–H bonds to be thought of as a functional group in the streamlining of organic synthesis.

In organometallic chemistry, sigma-bond metathesis is a chemical reaction wherein a metal-ligand sigma bond undergoes metathesis (exchange of parts) with the sigma bond in some reagent. The reaction is illustrated by the exchange of lutetium(III) methyl complex with a hydrocarbon (R-H):

Karen Ila Goldberg is an American chemist, currently the Vagelos Professor of Energy Research at University of Pennsylvania. Goldberg is most known for her work in inorganic and organometallic chemistry. Her most recent research focuses on catalysis, particularly on developing catalysts for oxidation, as well as the synthesis and activation of molecular oxygen. In 2018, Goldberg was elected to the National Academy of Sciences.

In organic chemistry, hydrovinylation is the formal insertion of an alkene into the C-H bond of ethylene :

Concerted metalation-deprotonation (CMD) is a mechanistic pathway through which transition-metal catalyzed C–H activation reactions can take place. In a CMD pathway, the C–H bond of the substrate is cleaved and the new C–Metal bond forms through a single transition state. This process does not go through a metal species that is bound to the cleaved hydrogen atom. Instead, a carboxylate or carbonate base deprotonates the substrate. The first proposal of a concerted metalation deprotonation pathway was by S. Winstein and T. G. Traylor in 1955 for the acetolysis of diphenylmercury. It was found to be the lowest energy transition state in a number of computational studies, was experimentally confirmed through NMR experiments, and has been hypothesized to occur in mechanistic studies.

References

  1. 1 2 3 4 5 6 7 8 Caballero, Ana; Pérez, Pedro J. (2013-11-04). "Methane as raw material in synthetic chemistry: the final frontier". Chemical Society Reviews. 42 (23): 8809–8820. doi:10.1039/c3cs60120j. PMID   23954933.
  2. ↑ N. F. Gol’dshleger, M. B. Tyabin, A. E. Shilov and A. A. Shteinman, Russ. J. Phys. Chem., 1969, 43, 1222–1223.
  3. ↑ N. F. Gol’dshleger, A. A. Shteinman, A. E. Shilov and V. V. Es’kova, Russ. J. Phys. Chem., 1972, 46, 785.
  4. ↑ Snyder, Benjamin E. R.; Bols, Max L.; Schoonheydt, Robert A.; Sels, Bert F.; Solomon, Edward I. (2017-12-19). "Iron and Copper Active Sites in Zeolites and Their Correlation to Metalloenzymes". Chemical Reviews. 118 (5): 2718–2768. doi:10.1021/acs.chemrev.7b00344. ISSN   0009-2665. PMID   29256242.
  5. ↑ R. A. Periana, D. J. Taube, E. R. Evitt, D. G. Löfler, P. R. Wentrcek, G. Voss and T. Masuda, Science, 1993, 250, 340–343.
  6. ↑ Periana, Roy A.; Taube, Douglas J.; Gamble, Scott; Taube, Henry; Satoh, Takashi; Fujii, Hiroshi (1998-04-24). "Platinum Catalysts for the High-Yield Oxidation of Methane to a Methanol Derivative". Science. 280 (5363): 560–564. Bibcode:1998Sci...280..560P. doi:10.1126/science.280.5363.560. ISSN   0036-8075. PMID   9554841.
  7. ↑ Periana, Roy A.; Mironov, Oleg; Taube, Doug; Bhalla, Gaurav; Jones, C. J. (2003-08-08). "Catalytic, Oxidative Condensation of CH4 to CH3COOH in One Step via CH Activation". Science. 301 (5634): 814–818. Bibcode:2003Sci...301..814P. doi:10.1126/science.1086466. ISSN   0036-8075. PMID   12907796. S2CID   1388751.
  8. ↑ Kirillova, Marina V.; Kuznetsov, Maxim L.; Reis, Patrícia M.; Silva, José A. L. da; Silva, João J. R. Fraústo da; Pombeiro, Armando J. L. (2007-08-04). "Direct and Remarkably Efficient Conversion of Methane into Acetic Acid Catalyzed by Amavadine and Related Vanadium Complexes. A Synthetic and a Theoretical DFT Mechanistic Study". Journal of the American Chemical Society. 129 (34): 10531–10545. doi:10.1021/ja072531u. PMID   17676842.
  9. ↑ Sadow, Aaron D.; Tilley, T. Don (2003-02-17). "Catalytic Functionalization of Hydrocarbons by σ-Bond-Metathesis Chemistry: Dehydrosilylation of Methane with a Scandium Catalyst". Angewandte Chemie International Edition. 42 (7): 803–805. doi:10.1002/anie.200390213. ISSN   1521-3773. PMID   12596205.
  10. ↑ Sadow, Aaron D.; Tilley, T. Don (2003-06-04). "Homogeneous Catalysis with Methane. A Strategy for the Hydromethylation of Olefins Based on the Nondegenerate Exchange of Alkyl Groups and σ-Bond Metathesis at Scandium". Journal of the American Chemical Society. 125 (26): 7971–7977. doi:10.1021/ja021341a. PMID   12823019.
  11. ↑ P. J. Pérez, Alkane C–H Activation by Single-Site Metal Catalysis, Springer, Dordrecht, 2012
  12. ↑ Caballero, Ana; Despagnet-Ayoub, Emmanuelle; Díaz-Requejo, M. Mar; Díaz-Rodríguez, Alba; González-Núñez, María Elena; Mello, Rossella; Muñoz, Bianca K.; Ojo, Wilfried-Solo; Asensio, Gregorio (2011-05-13). "Silver-Catalyzed C-C Bond Formation Between Methane and Ethyl Diazoacetate in Supercritical CO2". Science. 332 (6031): 835–838. Bibcode:2011Sci...332..835C. doi:10.1126/science.1204131. hdl: 10272/11557 . ISSN   0036-8075. PMID   21566191. S2CID   206533375.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.