Transmetalation

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Transmetalation (alt. spelling: transmetallation) is a type of organometallic reaction that involves the transfer of ligands from one metal to another. It has the general form:

Contents

M1–R + M2–R′ → M1–R′ + M2–R

where R and R′ can be, but are not limited to, an alkyl, aryl, alkynyl, allyl, halogen, or pseudohalogen group. The reaction is usually an irreversible process due to thermodynamic and kinetic reasons. Thermodynamics will favor the reaction based on the electronegativities of the metals and kinetics will favor the reaction if there are empty orbitals on both metals. [1] There are different types of transmetalation including redox-transmetalation and redox-transmetalation/ligand exchange. During transmetalation the metal-carbon bond is activated, leading to the formation of new metal-carbon bonds. [2] Transmetalation is commonly used in catalysis, synthesis of main group complexes, and synthesis of transition metal complexes.

Types of transmetalation

There are two main types of transmetalation, redox-transmetalation (RT) and redox-transmetalation/ligand-exchange (RTLE). Below, M1 is usually a 4d or 5d transition metal and M2 is usually a main group or 3d transition metal. By looking at the electronegativities of the metals and ligands, one can predict whether the RT or RTLE reaction will proceed and what products the reaction will yield. For example, one can predict that the addition of 3 HgPh2 to 2 Al will yield 3 Hg and 2 AlPh3 because Hg is a more electronegative element than Al.

Redox-transmetalation

M1n+–R + M2 → M1 + M2n+–R.

In redox-transmetalation a ligand is transferred from one metal to the other through an intermolecular mechanism. During the reaction one of the metal centers is oxidized and the other is reduced. The electronegativities of the metals and ligands is what causes the reaction to go forward. If M1 is more electronegative than M2, it is thermodynamically favorable for the R group to coordinate to the less electronegative M2.

Redox-transmetalation/ligand-exchange

M1–R + M2–X → M1–X + M2–R.

In redox-transmetalation/ligand exchange the ligands of two metal complexes switch places with each other, bonding with the other metal center. The R ligand can be an alkyl, aryl, alkynyl, or allyl group and the X ligand can be a halogen, pseudo-halogen, alkyl, or aryl group. The reaction can proceed by two possible intermediate steps. The first is an associative intermediate, where the R and X ligands bridge the two metals, stabilizing the transition state. The second and less common intermediate is the formation of a cation where R is bridging the two metals and X is anionic. The RTLE reaction proceeds in a concerted manner. Like in RT reactions, the reaction is driven by electronegativity values. The X ligand is attracted to highly electropositive metals. If M1 is a more electropositive metal than M2, it is thermodynamically favorable for the exchange of the R and X ligands to occur.

Applications

Cross-coupling reactions

Transmetalation is often used as a step in the catalytic cycles of cross-coupling reactions. Some of the cross-coupling reactions that include a transmetalation step are Stille cross-coupling, Suzuki cross-coupling, Sonogashira cross-coupling, and Negishi cross-coupling. The most useful cross-coupling catalysts tend to be ones that contain palladium. Cross-coupling reactions have the general form of R′–X + M–R → R′–R + M–X and are used to form C–C bonds. R and R′ can be any carbon fragment. The identity of the metal, M, depends on which cross-coupling reaction is being used. Stille reactions use tin, Suzuki reactions use boron, Sonogashira reactions use copper, and Negishi reactions use zinc. The transmetalation step in palladium catalyzed reactions involve the addition of an R–M compound to produce an R′–Pd–R compound. Cross-coupling reactions have a wide range of applications in synthetic chemistry including the area of medicinal chemistry. The Stille reaction has been used to make an antitumor agent, (±)-epi-jatrophone; [3] the Suzuki reaction has been used to make an antitumor agent, oximidine II; [4] the Sonogashira reaction has been used to make an anticancer drug, eniluracil; [5] and the Negishi reaction has been used to make the carotenoid β-carotene via a transmetalation cascade. [6]

Figure 1. Synthesis of b-carotene by Negishi cross-coupling and transmetalation cascades. Transmetalation Cascade.png
Figure 1. Synthesis of β-carotene by Negishi cross-coupling and transmetalation cascades.


Lanthanides

Lanthanide organometallic complexes have been synthesized by RT and RTLE. Lanthanides are very electropositive elements.

Organomercurials, such as HgPh2, are common kinetically inert RT and RTLE reagents that allow functionalized derivatives to be synthesized, unlike organolithiums and Grignard reagents. [7] Diarylmercurials are often used to synthesize lanthanide organometallic complexes. Hg(C6F5)2 is a better RT reagent to use with lanthanides than HgPh2 because it does not require a step to activate the metal. [8] However, phenyl-substituted lanthanide complexes are more thermally stable than the pentafluorophenyl complexes. The use of HgPh2 led to the synthesis of a ytterbium complex with different oxidation states on the two Yb atoms: [9]

Yb(C10H8)(THF)2 + HgPh2 → YbIIYbIIIPh5(THF)4

In the Ln(C6F5)2 complexes, where Ln = Yb, Eu, or Sm, the Ln–C bonds are very reactive, making them useful in RTLE reactions. Protic substrates have been used as a reactant with the Ln(C6F5)2 complex as shown: Ln(C6F5)2 + 2LH → Ln(L)2 + 2C6F5H. It is possible to avoid the challenges of working with the unstable Ln(C6F5)2 complex by forming it in situ by the following reaction:

Ln + HgR2 + 2 LH → Ln(L)2 + Hg + 2 RH

Organotins are also kinetically inert RT and RTLE reagents that have been used in a variety of organometallic reactions. They have applications to the synthesis of lanthanide complexes, such as in the following reaction: [10]

Yb + Sn(N(SiMe3)2)2 → Yb(N(SiMe3)2)2 + Sn

Actinides

RT can be used to synthesize actinide complexes. RT has been used to synthesize uranium halides using uranium metal and mercury halides as shown:

U + HgX → UX + Hg      (X = Cl, Br, I) [11]

This actinide RT reaction can be done with multiple mercury compounds to coordinate ligands other than halogens to the metal:

2 U + 3 (C5H5)2Hg + HgCl2 → 2 (C5H5)3UCl + 4 Hg

Alkaline earth metals

Alkaline earth metal complexes have been synthesized by RTLE, employing the same methodology used in synthesizing lanthanide complexes. The use of diphenylmercury in alkaline-earth metal reactions leads to the production of elemental mercury. The handling and disposal of elemental mercury is challenging due to its toxicity to humans and the environment. This led to the desire for an alternative RTLE reagent that would be less toxic and still very effective. Triphenylbismuth, BiPh3, was discovered to be a suitable alternative. [12] Mercury and bismuth have similar electronegativity values and behave similarly in RTLE reactions. BiPh3 has been used to synthesize alkaline-earth metal amides and alkaline-earth metal cyclopentadienides. The difference between HgPh2 and BiPh3 in these syntheses was that the reaction time was longer when using BiPh3.

Related Research Articles

Inorganic chemistry deals with synthesis and behavior of inorganic and organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subjects of organic chemistry. The distinction between the two disciplines is far from absolute, as there is much overlap in the subdiscipline of organometallic chemistry. It has applications in every aspect of the chemical industry, including catalysis, materials science, pigments, surfactants, coatings, medications, fuels, and agriculture.

The lanthanide or lanthanoid series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and yttrium, are often collectively known as the rare-earth elements or rare-earth metals.

Organometallic chemistry Study of chemical compounds containing at least one bond between a carbon atom of an organic compound and a metal

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 alkaline, 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.

Organolithium reagent

Organolithium reagents are organometallic compounds that contain carbon – lithium bonds. These reagents are important in organic synthesis, and are frequently used to transfer the organic group or the lithium atom to the substrates in synthetic steps, through nucleophilic addition or simple deprotonation. Organolithium reagents are used in industry as an initiator for anionic polymerization, which leads to the production of various elastomers. They have also been applied in asymmetric synthesis in the pharmaceutical industry. Due to the large difference in electronegativity between the carbon atom and the lithium atom, the C-Li bond is highly ionic. Owing to the polar nature of the C-Li bond, organolithium reagents are good nucleophiles and strong bases. For laboratory organic synthesis, many organolithium reagents are commercially available in solution form. These reagents are highly reactive, and are sometimes pyrophoric.

The Stille reaction is a chemical reaction widely used in organic synthesis. The reaction involves the coupling of two organic groups, one of which is carried as an organotin compound. A variety of organic electrophiles provide the other coupling partner. The Stille reaction is one of many palladium-catalyzed coupling reactions.

The Suzuki reaction is an organic reaction, classified as a cross-coupling reaction, where the coupling partners are a boronic acid and an organohalide and the catalyst is a palladium(0) complex. It was first published in 1979 by Akira Suzuki, and he shared the 2010 Nobel Prize in Chemistry with Richard F. Heck and Ei-ichi Negishi for their contribution to the discovery and development of palladium-catalyzed cross-couplings in organic synthesis. This reaction is also known as the Suzuki–Miyaura reaction or simply as the Suzuki coupling. It is widely used to synthesize polyolefins, styrenes, and substituted biphenyls. Several reviews have been published describing advancements and the development of the Suzuki reaction. The general scheme for the Suzuki reaction is shown below, where a carbon-carbon single bond is formed by coupling an organoboron species (R1-BY2) with a halide (R2-X) using a palladium catalyst and a base.

The Sonogashira reaction is a cross-coupling reaction used in organic synthesis to form carbon–carbon bonds. It employs a palladium catalyst as well as copper co-catalyst to form a carbon–carbon bond between a terminal alkyne and an aryl or vinyl halide.

Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. Oxidative addition is often a step in catalytic cycles, in conjunction with its reverse reaction, reductive elimination.

Organopalladium chemistry is a branch of organometallic chemistry that deals with organic palladium compounds and their reactions. Palladium is often used as a catalyst in the reduction of alkenes and alkynes with hydrogen. This process involves the formation of a palladium-carbon covalent bond. Palladium is also prominent in carbon-carbon coupling reactions, as demonstrated in tandem reactions.

Metalation is a chemical reaction that formation of a bond to a metal. This reaction usually refers to the replacement of a halogen atom in an organic molecule with a metal atom, resulting in an organometallic compound. In the laboratory, metalation is commonly used to activate organic molecules during the formation of C—X bonds, which are necessary for the synthesis of many organic molecules.

A transition metal carbene complex is an organometallic compound featuring a divalent organic ligand. The divalent organic ligand coordinated to the metal center is called a carbene. Carbene complexes for almost all transition metals have been reported. Many methods for synthesizing them and reactions utilizing them have been reported. The term carbene ligand is a formalism since many are not derived from carbenes and almost none exhibit the reactivity characteristic of carbenes. Described often as M=CR2, they represent a class of organic ligands intermediate between alkyls (−CR3) and carbynes (≡CR). They feature in some catalytic reactions, especially alkene metathesis, and are of value in the preparation of some fine chemicals.

The Negishi coupling is a widely employed transition metal catalyzed cross-coupling reaction. The reaction couples organic halides or triflates with organozinc compounds, forming carbon-carbon bonds (c-c) in the process. A palladium (0) species is generally utilized as the metal catalyst, though nickel is sometimes used. A variety of nickel catalysts in either Ni0 or NiII oxidation state can be employed in Negishi cross couplings such as Ni(PPh3)4, Ni(acac)2, Ni(COD)2 etc.

Organozinc compound

Organozinc compounds in organic chemistry contain carbon to zinc chemical bonds. Organozinc chemistry is the science of organozinc compounds describing their physical properties, synthesis and reactions.

Organocopper compound Compound with carbon to copper bonds

Organocopper compounds in organometallic chemistry contain carbon to copper chemical bonds. Organocopper chemistry is the science of organocopper compounds describing their physical properties, synthesis and reactions. They are reagents in organic chemistry.

Organomanganese chemistry is the chemistry of organometallic compounds containing a carbon to manganese chemical bond. In a 2009 review, Cahiez et al. argued that as manganese is cheap and benign, organomanganese compounds have potential as chemical reagents, although currently they are not widely used as such despite extensive research. A key disadvantage of organomanganese compounds is that they can be obtained directly from the metal only with difficulty.

Metal-phosphine complex

A metal-phosphine complex is a In coordination complex containing one or more phosphine ligands. Almost always, the phosphine is an organophosphine of the type R3P (R = alkyl, aryl). Metal phosphine complexes are useful in homogeneous catalysis. Prominent examples of metal phosphine complexes include Wilkinson's catalyst (Rh(PPh3)3Cl), Grubbs' catalyst, and tetrakis(triphenylphosphine)palladium(0).

f-Block Metallocene refers to a class of organometallic sandwich compounds with f-Block metal and electron-rich ligands like cyclopentadienyl anion.

Palladium–NHC complex

In organometallic chemistry, palladium-NHC complexes are a family of organopalladium compounds in which palladium forms a coordination complex with N-Heterocyclic carbenes (NHCs). They have been investigated for applications in homogeneous catalysis, particularly cross-coupling reactions.

Nickel(II) precatalysts are a type of catalyst used in organic reactions. Many transformations are catalyzed by nickel in organometallic chemistry and in organic synthesis. Many of these transformations invoke a low valent (generally Ni(0)) species as the active catalyst. Unfortunately, unlike its counterpart, Pd(0), Ni(0) catalysts are predominantly confined to the glovebox due to their high instability to air and water, with the most common Ni(0) catalyst being Ni(cod)2. Additionally, Ni(cod)2 is more expensive than many Ni(II) salts and the quality varies significantly amongst suppliers. To make nickel catalysis more accessible and amenable to synthesis and industrial purposes, the use of air-stable Ni(II) precursors has emerged as an important development in this area of research. This page describes the more commonly employed nickel(II) precatalysts, their synthesis for those not commercially available, and the methods for their reduction to Ni(0) complexes.

Lanthanocene

A lanthanocene is a type of metallocene compound that contains an element from the lanthanide series. The most common lanthanocene complexes contain two cyclopentadienyl anions and an X type ligand, usually hydride or alkyl ligand.

References

  1. 1 2 Spessard, Gary O.; Miessler, Gary L. (2010). Organometallic Chemistry. New York, NY: Oxford University Press. ISBN   978-0195330991.
  2. ↑ Osakada, Kohtaro (2003). Fundamentals of Molecular Catalysis. Amsterdam: Elsevier. ISBN   0444509216.
  3. ↑ Gyorkos, Albert C.; Stille, John K.; Hegedus, Louis S. (1990). "The total synthesis of (±)-epi-jatrophone and (±)-jatrophone using palladium-catalyzed carbonylative coupling of vinyl triflates with vinyl stannanes as the macrocycle-forming step". J. Am. Chem. Soc. 112 (23): 8465–8472. doi:10.1021/ja00179a035.
  4. ↑ Molander, Gary A.; Dehmel, Florian (2004). "Formal Total Synthesis of Oximidine II via a Suzuki-Type Cross-Coupling Macrocyclization Employing Potassium Organotrifluoroborates". J. Am. Chem. Soc. 126 (33): 10313–10318. doi:10.1021/ja047190o. PMID   15315445.
  5. ↑ Cooke, Jason W. B.; Bright, Robert; Coleman, Mark J.; Jenkins, Kevin P. (2001). "Process Research and Development of a Dihydropyrimidine Dehydrogenase Inactivator: Large-Scale Preparation of Eniluracil Using a Sonogashira Coupling". Org. Process Res. Dev. 5 (4): 383–386. doi:10.1021/op0100100.
  6. ↑ Zeng, Fanxing; Negishi, Ei-Ichi (2001). "A Novel, Selective, and Efficient Route to Carotenoids and Related Natural Products via Zr-Catalyzed Carboalumination and Pd- and Zn-Catalyzed Cross Coupling". Org. Lett. 3 (5): 719–722. doi:10.1021/ol000384y. PMID   11259045.
  7. ↑ Vicente, José; Arcas, Aurelia; Gálvez López, María Dolores; Jones, Peter G. (2004). "Bis(2,6-dinitroaryl)platinum Complexes. 1. Synthesis through Transmetalation Reactions". Organometallics . 23 (14): 3521–3527. doi:10.1021/om049801r.
  8. ↑ Deacon, Glen B.; Forsyth, Craig M.; Nickel, Siegbert (2002). "Bis(pentafluorophenyl)mercury—a versatile synthon in organo-, organooxo-, and organoamido-lanthanoid chemistry". J. Organomet. Chem. 647 (1–2): 50–60. doi:10.1016/S0022-328X(01)01433-4.
  9. ↑ Bochkarev, Mikhail N.; Khramenkov, Vladimir V.; Radkov, Yury F.; Zakharov, Lev N.; Struchkov, Yury T. (1992). "Synthesis and characterization of pentaphenyldiytterbium Ph2Yb(THF)(μ-Ph)3Yb(THF)3". J. Organomet. Chem. 429: 27–39. doi:10.1016/0022-328X(92)83316-A.
  10. ↑ Cetinkaya, Bekir; Hitchcock, Peter B.; Lappert, Michael F.; Smith, Richard G. (1992). "The first neutral, mononuclear 4f metal thiolates and new methods for corresponding aryl oxides and bis(trimethylsilyl)amides". J. Chem. Soc., Chem. Commun. 13 (13): 932–934. doi:10.1039/C39920000932.
  11. ↑ Deacon, G.B.; Tuong, T.D. (1988). "A simple redox transmetallation synthesis of uranium tetrachloride and related preparations of uranium triiodide and chlorotris(cyclopentadienyl)uranium(IV)". Polyhedron . 7 (3): 249–250. doi:10.1016/S0277-5387(00)80561-6.
  12. ↑ Gillett-Kunnath, Miriam M.; MacLellan, Jonathan G.; Forsyth, Craig M.; Andrews, Philip C.; Deacon, Glen B.; Ruhlandt-Senge, Karin (2008). "BiPh3—A convenient synthon for heavy alkaline-earth metal amides". Chem. Commun. 37 (37): 4490–4492. doi:10.1039/b806948d. PMID   18802600.
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