Migratory insertion

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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.: [1]

Contents

Overview

In the migratory insertion, a ligand that is viewed as an anion (X) ligand in and a ligand that is viewed as neutral couple, generating a new anionic ligand. The anion and neutral ligands that react are adjacent. If the precursor complex is coordinatively saturated, migratory insertion often result in a coordinatively unsaturated product. A new (neutral) ligand can then react with the metal leading to a further insertion. The process can occur many times on a single metal, as in olefin polymerization.

The anionic ligand can be: H− (hydride), R− (alkyl), acyl, Ar− (aryl), or OR− (alkoxide). The ability of these groups to migrate is called their migratory aptitude. The neutral ligand can be CO, alkene, alkyne, or in some cases, even carbene.

Diverse reactions apply to the migratory insertion. One mechanism involves the attack of the anionic ligand on the electrophilic part of the neutral ligand (the anionic ligand migrates to the neutral ligand). The other mechanism involves the neutral ligand inserts itself between the metal and the anionic ligand.

CO insertion

The insertion of carbon monoxide into a metal-carbon bond to form an acyl group is the basis of carbonylation reactions, which provides many commercially useful products. Mechanistic studies reveal that the alkyl group migrates intramolecularly to an adjacent CO ligand. [2] [3]

CO Insertion reaction pathway for an octahedral complex Insertion reaction for an octahedral complex.png
CO Insertion reaction pathway for an octahedral complex

Early studies were conducted on the conversion of CH3Mn(CO)5 to give the acetyl derivative. [4] Using 13CO, the products is cis [Mn(COCH3)(13CO)(CO)4] (scheme 1).

Scheme 1. Note the solvent changing in the backward reaction: heptane is a high-boiling solvent, suitable for the temperature indicated. CO insertion.png
Scheme 1. Note the solvent changing in the backward reaction: heptane is a high-boiling solvent, suitable for the temperature indicated.

CO insertion does not always involve migration. Treatment of CpFe(L)(CO)CH3 with 13CO yields a mix of both alkyl migration products and products formed by true insertion of bound carbonyls into the methyl group. Product distribution is influenced by the choice of solvent. [5]

Alkyl derivatives of square planar complexes undergo CO insertions particularly readily. Insertion reactions on square planar complexes are of particular interest because of their industrial applications. Since square planar complexes are often coordinatively unsaturated, they are susceptible to formation of 5-coordinate adducts, which undergo migratory insertion readily. [5] In most cases the in-plane migration pathway is preferred, but, unlike the nucleophilic pathway, it is inhibited by an excess of CO. [6]

Nucleophilic insertion and rearrangement of a square planar complex Square Planar.png
Nucleophilic insertion and rearrangement of a square planar complex

Effects on reaction rates

Reverse reaction

Decarbonylation of aldehydes, the reverse of CO insertion, is a well-recognized reaction:

RCHO → RH + CO

The reaction is not widely practiced in part because the alkanes are less useful materials than are the aldehyde precursors. Furthermore, the reaction is not often conducted catalytically because the extruded CO can be slow to dissociate. [9] Extrusion of CO from an organic aldehyde is most famously demonstrated using Wilkinson's catalyst: [10]

RhCl(PPh3)3 + RCHO → RhCl(CO)(PPh3)2 + RH + PPh3

Please see Tsuji-Wilkinson Decarbonylation Reaction for an example of this elementary organometallic step in synthesis

Insertion of other oxides

Many electrophilic oxides insert into metal carbon bonds; these include sulfur dioxide, carbon dioxide, and nitric oxide. These reactions have limited or no practical significance, but are of historic interest. With transition metal alkyls, these oxides behave as electrophiles and insert into the bond between metals and their relatively nucleophilic alkyl ligands. As discussed in the article on Metal sulfur dioxide complexes, the insertion of SO2 has been examined in particular detail. SO2 inserts to give both O-sulphinates and S-sulphinates, depending on the metal centre. [11] With square planar alkyl complexes, a pre-equilibrium is assumed involving formation of an adduct. [12]

Insertion of alkenes into metal-carbon bonds

The insertion of alkenes into both metal-carbon is important. The insertion of ethylene and propylene into titanium alkyls is the cornerstone of Ziegler–Natta catalysis, the main source of polyethylene and polypropylene. The majority of this technology involves heterogeneous catalysts, but it is widely assumed that the principles and observations on homogeneous systems are applicable to the solid-state versions. Related technologies include the Shell Higher Olefin Process which produces detergent precursors.

Steps in alkene polymerization. Step i involves binding of the monomer to the metal and step ii involves the migratory insertion step. These steps, which alternate from one side of the metal center to the other, are repeated many times for each polymer chain. The box represents a vacant (or extremely labile) coordination site. AlkeneInsMech'.png
Steps in alkene polymerization. Step i involves binding of the monomer to the metal and step ii involves the migratory insertion step. These steps, which alternate from one side of the metal center to the other, are repeated many times for each polymer chain. The box represents a vacant (or extremely labile) coordination site.

Mechanism

Factors affecting the rate of olefin insertions include the formation of the cyclic, planar, four-center transition state involving incipient formation of a bond between the metal and an olefin carbon. From this transition state, it can be seen that a partial positive charge forms on the β-carbon with a partial negative charge formed on the carbon initially bonded to the metal. This polarization explains the subsequently observed formation of the bond between the negatively charged carbon/hydrogen and the positively charged β-carbon as well as the simultaneous formation of the metal-α-carbon bond. This transition state also highlights the two factors that most strongly contribute to the rate of olefin insertion reactions: (i) orbital overlap of the alkyl group initially attached to the metal and (ii) the strength of the metal-alkyl bond. With greater orbital overlap between the partially positive β-carbon and the partially negative hydrogen/alkyl group carbon, the formation of the new C-C bond is facilitated. With increasing strength of the metal-alkyl bond, the breaking of the bond between the metal and the hydrogen/alkyl carbon bond to form the two new bonds with the α-carbon and β-carbon (respectively) is slower, thus decreasing the rate of the insertion reaction. [13]

Insertion of alkenes into M–H bonds

The insertion of alkenes into metal-hydrogen bonds is a key step in hydrogenation and hydroformylation reactions. The reaction involves the alkene and the hydride ligands combining within the coordination sphere of a catalyst. In hydrogenation, the resulting alkyl ligand combines with a second hydride to give the alkane. Analogous reactions apply to the hydrogenation of alkynes: an alkenyl ligand combines with a hydride to eliminate an alkene.

Migratory insertion of an alkene into a metal hydride. The middle image depicts the transition state. Notice that the less substituted carbon becomes bonded to the metal, at least usually. The reverse reaction is called beta-hydride elimination. Beta-hydrideElim&TS.png
Migratory insertion of an alkene into a metal hydride. The middle image depicts the transition state. Notice that the less substituted carbon becomes bonded to the metal, at least usually. The reverse reaction is called beta-hydride elimination.

Mechanism

In terms of mechanism, the insertion of alkenes into M–H bond and into M–C bonds are described similarly. Both involve four-membered transition states that place the less substituted carbon on the metal.

The reverse of olefin insertion into a metal-hydrogen bond is β-hydride elimination. The Principle of Microscopic Reversibility requires that the mechanism of β-hydride elimination follow the same pathway as the insertion of alkenes into metal hydride bonds. The first requirement for β-hydride elimination is the presence of a hydrogen at a position that is β with respect to the metal. β-elimination requires a vacant coordination position on the metal that will accommodate the hydrogen that is abstracted. [14]

Industrial applications

Carbonylation

Two widely employed applications of migratory insertion of carbonyl groups are hydroformylation and the production of acetic acid by carbonylation of methanol. The former converts alkenes, hydrogen, and carbon monoxide into aldehydes. The production of acetic acid by carbonylation proceeds via two similar industrial processes. More traditional is the Monsanto acetic acid process, which relies on a rhodium-iodine catalyst to transform methanol into acetic acid. This process has been superseded by the Cativa process which uses a related iridium catalyst, [Ir(CO)2I2]− (1). [15] [16] By 2002, worldwide annual production of acetic acid stood at 6 million tons, of which approximately 60% is produced by the Cativa process. [15]

The catalytic cycle of the Cativa process Cativa-process-catalytic-cycle.png
The catalytic cycle of the Cativa process

The Cativa process catalytic cycle, shown above, includes both insertion and de-insertion steps. The oxidative addition reaction of methyl iodide with (1) involves the formal insertion of the iridium(I) centre into the carbon-iodine bond, whilst step (3) to (4) is an example of migratory insertion of carbon monoxide into the iridium-carbon bond. The active catalyst species is regenerated by the reductive elimination of acetyl iodide from (4), a de-insertion reaction. [15]

Alkene polymerization

Industrial applications of alkene insertions include metal-catalyzed routes to polyethylene and polypropylene. Typically these conversions are catalyzed heterogeneously by titanium trichloride which are activated by aluminium alkyls. This technology is known as Ziegler–Natta catalysts. [17] In these reactions, ethylene coordinates to titanium metal followed by its insertion. These steps can be repeated multiple times, potentially leading to high molecular weight polymers.

Related Research Articles

<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.

In organic chemistry, hydroformylation, also known as oxo synthesis or oxo process, is an industrial process for the production of aldehydes from alkenes. This chemical reaction entails the net addition of a formyl group and a hydrogen atom to a carbon-carbon double bond. This process has undergone continuous growth since its invention: production capacity reached 6.6×106 tons in 1995. It is important because aldehydes are easily converted into many secondary products. For example, the resultant aldehydes are hydrogenated to alcohols that are converted to detergents. Hydroformylation is also used in speciality chemicals, relevant to the organic synthesis of fragrances and pharmaceuticals. The development of hydroformylation is one of the premier achievements of 20th-century industrial chemistry.

Reductive elimination is an elementary step in organometallic chemistry in which the oxidation state of the metal center decreases while forming a new covalent bond between two ligands. It is the microscopic reverse of oxidative addition, and is often the product-forming step in many catalytic processes. Since oxidative addition and reductive elimination are reverse reactions, the same mechanisms apply for both processes, and the product equilibrium depends on the thermodynamics of both directions.

<span class="mw-page-title-main">Wilkinson's catalyst</span> Chemical compound

Wilkinson's catalyst (chlorido­tris(triphenylphosphine)­rhodium(I)) is a coordination complex of rhodium with the formula [RhCl(P(C6H5)3], where 'Ph' denotes a phenyl group. It is a red-brown colored solid that is soluble in hydrocarbon solvents such as benzene, and more so in tetrahydrofuran or chlorinated solvents such as dichloromethane. The compound is widely used as a catalyst for hydrogenation of alkenes. It is named after chemist and Nobel laureate Sir Geoffrey Wilkinson, who first popularized its use.

The Monsanto process is an industrial method for the manufacture of acetic acid by catalytic carbonylation of methanol. The Monsanto process has largely been supplanted by the Cativa process, a similar iridium-based process developed by BP Chemicals Ltd, which is more economical and environmentally friendly.

<span class="mw-page-title-main">Cativa process</span> Method for the manufacture of acetic acid

The Cativa process is a method for the production of acetic acid by the carbonylation of methanol. The technology, which is similar to the Monsanto process, was developed by BP Chemicals and is under license by BP Plc. The process is based on an iridium-containing catalyst, such as the complex [Ir(CO)2I2]− (1).

beta-Hydride elimination

β-Hydride elimination is a reaction in which an alkyl group bonded to a metal centre is converted into the corresponding metal-bonded hydride and an alkene. The alkyl must have hydrogens on the β-carbon. For instance butyl groups can undergo this reaction but methyl groups cannot. The metal complex must have an empty site cis to the alkyl group for this reaction to occur. Moreover, for facile cleavage of the C–H bond, a d electron pair is needed for donation into the σ* orbital of the C–H bond. Thus, d0 metals alkyls are generally more stable to β-hydride elimination than d2 and higher metal alkyls and may form isolable agostic complexes, even if an empty coordination site is available.

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.

<span class="mw-page-title-main">Organoiridium chemistry</span> Chemistry of organometallic compounds containing an iridium-carbon bond

Organoiridium chemistry is the chemistry of organometallic compounds containing an iridium-carbon chemical bond. Organoiridium compounds are relevant to many important processes including olefin hydrogenation and the industrial synthesis of acetic acid. They are also of great academic interest because of the diversity of the reactions and their relevance to the synthesis of fine chemicals.

<span class="mw-page-title-main">Organocobalt chemistry</span> Chemistry of compounds with a carbon to cobalt bond

Organocobalt chemistry is the chemistry of organometallic compounds containing a carbon to cobalt chemical bond. Organocobalt compounds are involved in several organic reactions and the important biomolecule vitamin B12 has a cobalt-carbon bond. Many organocobalt compounds exhibit useful catalytic properties, the preeminent example being dicobalt octacarbonyl.

Hydroacylation is a type of organic reaction in which an electron-rich unsaturated hydrocarbon inserts into a formyl C-H bond. With alkenes, the product is a ketone:

<span class="mw-page-title-main">Organorhodium chemistry</span> Field of study

Organorhodium chemistry is the chemistry of organometallic compounds containing a rhodium-carbon chemical bond, and the study of rhodium and rhodium compounds as catalysts in organic reactions.

An insertion reaction is a chemical reaction where one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:

<span class="mw-page-title-main">Brookhart's acid</span> Chemical compound

Brookhart's acid is the salt of the diethyl ether oxonium ion and tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BAr′4). It is a colorless solid, used as a strong acid. The compound was first reported by Volpe, Grant, and Brookhart in 1992.

In organometallic chemistry, a transition metal alkene complex is a coordination compound containing one or more alkene ligands. The inventory is large. Such compounds are intermediates in many catalytic reactions that convert alkenes to other organic products.

<span class="mw-page-title-main">Transition metal alkyl complexes</span> Coordination complex

Transition metal alkyl complexes are coordination complexes that contain a bond between a transition metal and an alkyl ligand. Such complexes are not only pervasive but are of practical and theoretical interest.

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

β-Carbon elimination is a type of reaction in organometallic chemistry wherein an allyl ligand bonded to a metal center is broken into the corresponding metal-bonded alkyl (aryl) ligand and an alkene. It is a subgroup of elimination reactions. Though less common and less understood than β-hydride elimination, it is an important step involved in some olefin polymerization processes and transition-metal-catalyzed organic reactions.

<span class="mw-page-title-main">Transition metal acyl complexes</span>

Transition metal acyl complexes describes organometallic complexes containing one or more acyl (RCO) ligands. Such compounds occur as transient intermediates in many industrially useful reactions, especially carbonylations.

A Fischer carbene is a type of transition metal carbene complex, which is an organometallic compound containing a divalent organic ligand. In a Fischer carbene, the carbene ligand is a σ-donor π-acceptor ligand. Because π-backdonation from the metal centre is generally weak, the carbene carbon is electrophilic.

References

  1. ↑ Elschenbroich, C. (2006). Organometallics. Weinheim: Wiley-VCH. ISBN   978-3-527-29390-2.
  2. ↑ Hartwig, J. F. (2010). Organotransition Metal Chemistry, from Bonding to Catalysis. New York, NY: University Science Books. ISBN   978-1-891389-53-5.
  3. ↑ Yadav, M. S. (2005). Quick Review in Inorganic Chemistry. Anmol Publications. p. 244. ISBN   978-81-261-1898-4.
  4. ↑ F. Calderazzo, F. A. Cotton. “The Carbonylation of Methyl Manganese Pentacarbonyl and Decarbonylation of Acetyl Manganese Pentacarbonyl”. Inorg. Chem., 1962, 1, 30–36. https://doi.org/10.1021/ic50001a008.
  5. 1 2 3 Anderson, G. K.; Cross, R. J. (1984). "Carbonyl-Insertion Reactions of Square Planar Complexes". Acc. Chem. Res. 17 (2): 67–74. doi:10.1021/ar00098a005.
  6. 1 2 3 Cavell, K. J. (1996). "Recent Fundamental Studies on Migratory Insertion into Metal-Carbon Bonds". Coord. Chem. Rev. 155 (11): 209–243. doi:10.1016/S0010-8545(96)90182-4.
  7. 1 2 Alexander, J.J. (1985). "Insertions into transition metal-carbon bonds". In Hartley; Patai (eds.). The chemistry of the metal–carbon bond. Vol. 2. John Wiley & Sons. doi:10.1002/9780470771747.ch5.
  8. ↑ Shusterman, A. J.; Tamir, I.; Pross, A. (1988). "The Mechanism of Organometallic Migration Reactions. A Configuration Mixing (CM)Approach". J. Organomet. Chem. 340 (2): 203–222. doi:10.1016/0022-328X(88)80076-7.
  9. ↑ Fristrup, Peter; Kreis, Michael; Palmelund, Anders; Norrby, Per-Ola; Madsen, Robert (2008). "The Mechanism for the Rhodium-Catalyzed Decarbonylation of Aldehydes: A Combined Experimental and Theoretical Study". J. Am. Chem. Soc. 130 (15): 5206–5215. doi:10.1021/ja710270j. PMID   18303836. S2CID   207119793.
  10. ↑ Ohno, K.; Tsuji, J. (1968). "Organic Synthesis by means of Noble Metal Compounds. XXXV. Novel Decarbonylation Reactions of Aldehydes and Acyl Halides using Rhodium Complexes". J. Am. Chem. Soc. 90 (1): 99–107. doi:10.1021/ja01003a018.
  11. ↑ Douglas; McDaniel; Alexander (1994). Concepts and Models of Inorganic Chemistry (3rd ed.). John Wiley & Sons, Inc. ISBN   978-0-471-62978-8.
  12. ↑ Puddephatt, R.A.; Stalteri, M.A. (1980). "Competition between Insertion of Sulfur Dioxide into the Methyl- or Phenyl- Transition Metal Bond". Journal of Organometallic Chemistry. 193: C27–C29. doi:10.1016/S0022-328X(00)86091-X.
  13. ↑ Burger, B. J.; Thompson, M. E.; Cotter, W. D.; Bercaw, J. E. (1990). "Ethylene Insertion and β-hydrogen Elimination for Permethylscandocene Alkyl Complexes. A Study of the Chain Propagation and Termination Steps in Ziegler-Natta Polymerization of Ethylene". J. Am. Chem. Soc. 112 (4): 1566–1577. doi:10.1021/ja00160a041.
  14. ↑ Crabtree, R. H. (2009). The Organometallic Chemistry of the Transition Metals. John Wiley and Sons. p. 192. ISBN   978-0-470-25762-3.
  15. 1 2 3 Jones, Jane H. (2000). "The Cativa Process for the Manufacture of Acetic Acid". Platinum Metals Review . 44 (3): 94–105.
  16. ↑ Sunley, G. J.; Watson, D. J. (2000). "High Productivity Methanol Carbonylation Catalysis using Iridium - The Cativa Process for the Manufacture of Acetic Acid". Catalysis Today. 58 (4): 293–307. doi:10.1016/S0920-5861(00)00263-7.
  17. ↑ Kissin, Y. V. (2008). "Synthesis, Chemical Composition, and Structure of Transition Metal Components and Cocatalysts in Catalyst Systems for Alkene Polymerization". Alkene Polymerization Reactions with Transition Metal Catalysts. Amsterdam: Elsevier. pp. 207–290. ISBN   978-0-444-53215-2.
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