Oxidative addition

Last updated

Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. [1] [2] [3] [4] 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. [5]

Contents

Role in transition metal chemistry

For transition metals, oxidative reaction results in the decrease in the dn to a configuration with fewer electrons, often 2e fewer. Oxidative addition is favored for metals that are (i) basic and/or (ii) easily oxidized. Metals with a relatively low oxidation state often satisfy one of these requirements, but even high oxidation state metals undergo oxidative addition, as illustrated by the oxidation of Pt(II) with chlorine:

[PtCl4]2− + Cl2 → [PtCl6]2−

In classical organometallic chemistry, the formal oxidation state of the metal and the electron count of the complex both increase by two. [6] One-electron changes are also possible and in fact some oxidative addition reactions proceed via series of 1e changes. Although oxidative additions can occur with the insertion of a metal into many different substrates, oxidative additions are most commonly seen with H–H, H–X, and C–X bonds because these substrates are most relevant to commercial applications.

Oxidative addition requires that the metal complex have a vacant coordination site. For this reason, oxidative additions are common for four- and five-coordinate complexes.

Reductive elimination is the reverse of oxidative addition. [7] Reductive elimination is favored when the newly formed X–Y bond is strong. For reductive elimination to occur the two groups (X and Y) should be mutually adjacent on the metal's coordination sphere. Reductive elimination is the key product-releasing step of several reactions that form C–H and C–C bonds. [5]

Mechanisms

Oxidative additions proceed by diverse pathways that depend on the metal center and the substrates.

Concerted pathway

Oxidative additions of nonpolar substrates such as hydrogen and hydrocarbons appear to proceed via concerted pathways. Such substrates lack π-bonds, consequently a three-centered σ complex is invoked, followed by intramolecular ligand bond cleavage of the ligand (probably by donation of electron pair into the sigma* orbital of the inter ligand bond) to form the oxidized complex. The resulting ligands will be mutually cis, [2] although subsequent isomerization may occur.

Concerted OA.png

This mechanism applies to the addition of homonuclear diatomic molecules such as H2. Many C–H activation reactions also follow a concerted mechanism through the formation of an M–(C–H) agostic complex. [2]

A representative example is the reaction of hydrogen with Vaska's complex, trans-IrCl(CO)[P(C6H5)3]2. In this transformation, iridium changes its formal oxidation state from +1 to +3. The product is formally bound to three anions: one chloride and two hydride ligands. As shown below, the initial metal complex has 16 valence electrons and a coordination number of four whereas the product is a six-coordinate 18 electron complex.

Oxidation of Vaska's complex with dihydrogen.png

Formation of a trigonal bipyramidal dihydrogen intermediate is followed by cleavage of the H–H bond, due to electron back donation into the H–H σ*-orbital, i.e. a sigma complex. [8] This system is also in chemical equilibrium, with the reverse reaction proceeding by the elimination of hydrogen gas with simultaneous reduction of the metal center. [9]

The electron back donation into the H–H σ*-orbital to cleave the H–H bond causes electron-rich metals to favor this reaction. [9] The concerted mechanism produces a cis dihydride, while the stereochemistry of the other oxidative addition pathways do not usually produce cis adducts.

SN1-type

Some oxidative additions proceed analogously to the well known bimolecular nucleophilic substitution reactions in organic chemistry. Nucleophilic attack by the metal center at the less electronegative atom in the substrate leads to cleavage of the R–X bond, to form an [M–R]+ species. This step is followed by rapid coordination of the anion to the cationic metal center. For example, reaction of a square planar complex with methyl iodide:

General SN2-type oxidative addition reaction.png

This mechanism is often assumed in the addition of polar and electrophilic substrates, such as alkyl halides and halogens. [2]

Ionic

The ionic mechanism of oxidative addition is similar to the SN2 type in that it involves the stepwise addition of two distinct ligand fragments. The key difference being that ionic mechanisms involve substrates which are dissociated in solution prior to any interactions with the metal center. An example of ionic oxidative addition is the addition of hydrogen chloride. [2]

Radical

In addition to undergoing SN2-type reactions, alkyl halides and similar substrates can add to a metal center via a radical mechanism, although some details remain controversial. [2] Reactions which are generally accepted to proceed by a radical mechanism are known however. One example was proposed by Lednor and co-workers. [10]

Initiation
[(CH3)2C(CN)N]2 → 2 (CH3)2(CN)C• + N2
(CH3)2(CN)C• + PhBr → (CH3)2(CN)CBr + Ph•
Propagation
Ph• + [Pt(PPh3)2] → [Pt(PPh3)2Ph]•
[Pt(PPh3)2Ph]• + PhBr → [Pt(PPh3)2PhBr] + Ph•

Applications

Oxidative addition and reductive elimination are invoked in many catalytic processes in homogeneous catalysis, e.g., hydrogenations, hydroformylations, hydrosilylations, etc. [5] Cross-coupling reactions like the Suzuki coupling, Negishi coupling, and the Sonogashira coupling also proceed by oxidative addition. [11] [12]

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.

A substitution reaction is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions are of prime importance in organic chemistry. Substitution reactions in organic chemistry are classified either as electrophilic or nucleophilic depending upon the reagent involved, whether a reactive intermediate involved in the reaction is a carbocation, a carbanion or a free radical, and whether the substrate is aliphatic or aromatic. Detailed understanding of a reaction type helps to predict the product outcome in a reaction. It also is helpful for optimizing a reaction with regard to variables such as temperature and choice of solvent.

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 (also known as organostannanes). A variety of organic electrophiles provide the other coupling partner. The Stille reaction is one of many palladium-catalyzed coupling reactions.

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(PPh3)], 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.

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">Carbon–hydrogen bond activation</span> Organic reactions in which the H in a C–H bond is substituted

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 organic chemistry, the Buchwald–Hartwig amination is a chemical reaction for the synthesis of carbon–nitrogen bonds via the palladium-catalyzed coupling reactions of amines with aryl halides. Although Pd-catalyzed C–N couplings were reported as early as 1983, Stephen L. Buchwald and John F. Hartwig have been credited, whose publications between 1994 and the late 2000s established the scope of the transformation. The reaction's synthetic utility stems primarily from the shortcomings of typical methods for the synthesis of aromatic C−N bonds, with most methods suffering from limited substrate scope and functional group tolerance. The development of the Buchwald–Hartwig reaction allowed for the facile synthesis of aryl amines, replacing to an extent harsher methods while significantly expanding the repertoire of possible C−N bond formations.

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.:

Transition metal hydrides are chemical compounds containing a transition metal bonded to hydrogen. Most transition metals form hydride complexes and some are significant in various catalytic and synthetic reactions. The term "hydride" is used loosely: some of them are acidic (e.g., H2Fe(CO)4), whereas some others are hydridic, having H−-like character (e.g., ZnH2).

Dioxygen complexes are coordination compounds that contain O2 as a ligand. The study of these compounds is inspired by oxygen-carrying proteins such as myoglobin, hemoglobin, hemerythrin, and hemocyanin. Several transition metals form complexes with O2, and many of these complexes form reversibly. The binding of O2 is the first step in many important phenomena, such as cellular respiration, corrosion, and industrial chemistry. The first synthetic oxygen complex was demonstrated in 1938 with cobalt(II) complex reversibly bound O2.

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.

<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.

In organometallic chemistry, metal sulfur dioxide complexes are complexes that contain sulfur dioxide, SO2, bonded to a transition metal. Such compounds are common but are mainly of theoretical interest. Historically, the study of these compounds has provided insights into the mechanisms of migratory insertion reactions.

<span class="mw-page-title-main">Metal-phosphine complex</span>

A metal-phosphine complex is a 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).

Hydrophosphination is the insertion of a carbon-carbon multiple bond into a phosphorus-hydrogen bond forming a new phosphorus-carbon bond. Like other hydrofunctionalizations, the rate and regiochemistry of the insertion reaction is influenced by the catalyst. Catalysts take many forms, but most prevalent are bases and free-radical initiators. Most hydrophosphinations involve reactions of phosphine (PH3).

In organometallic chemistry, a transition metal alkyne complex is a coordination compound containing one or more alkyne ligands. Such compounds are intermediates in many catalytic reactions that convert alkynes to other organic products, e.g. hydrogenation and trimerization.

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):

In organic chemistry, the Murai reaction is an organic reaction that uses C-H activation to create a new C-C bond between a terminal or strained internal alkene and an aromatic compound using a ruthenium catalyst. The reaction, named after Shinji Murai, was first reported in 1993. While not the first example of C-H activation, the Murai reaction is notable for its high efficiency and scope. Previous examples of such hydroarylations required more forcing conditions and narrow scope.

Metal-ligand cooperativity (MLC) is a mode of reactivity in which a metal and ligand of a complex are both involved in the bond breaking or bond formation of a substrate during the course of a reaction. This ligand is an actor ligand rather than a spectator, and the reaction is generally only deemed to contain MLC if the actor ligand is doing more than leaving to provide an open coordination site. MLC is also referred to as "metal-ligand bifunctional catalysis." Note that MLC is not to be confused with cooperative binding.

References

  1. ↑ Jay A. Labinger "Tutorial on Oxidative Addition" Organometallics, 2015, volume 34, pp 4784–4795. doi : 10.1021/acs.organomet.5b00565
  2. 1 2 3 4 5 6 Crabtree, Robert (2005). The Organometallic Chemistry of the Transition Metals. Wiley-Interscience. pp. 159–180. ISBN   0-471-66256-9.
  3. ↑ Miessler, Gary L.; Tarr, Donald A. Inorganic Chemistry (3rd ed.).[ ISBN missing ]
  4. ↑ Shriver, D. F.; Atkins, P. W. Inorganic Chemistry .[ ISBN missing ]
  5. 1 2 3 Hartwig, J. F. (2010). Organotransition Metal Chemistry, from Bonding to Catalysis. New York: University Science Books. ISBN   978-1-891389-53-5.
  6. ↑ IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) " oxidative addition ". doi : 10.1351/goldbook.O04367
  7. ↑ IUPAC , Compendium of Chemical Terminology , 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) " reductive elimination ". doi : 10.1351/goldbook.R05223
  8. ↑ Kubas, Gregory J. (2001-08-31). Metal Dihydrogen and σ-Bond Complexes: Structure, Theory, and Reactivity. Kluwer. ISBN   0-306-46465-9.
  9. 1 2 Johnson, Curtis; Eisenberg, Richard (1985). "Stereoselective Oxidative Addition of Hydrogen to Iridium(I) Complexes. Kinetic Control Based on Ligand Electronic Effects". Journal of the American Chemical Society. 107 (11): 3148–3160. doi:10.1021/ja00297a021.
  10. ↑ Hall, Thomas L.; Lappert, Michael F.; Lednor, Peter W. (1980). "Mechanistic studies of some oxidative-addition reactions: free-radical pathways in the Pt0-RX, Pt0-PhBr, and PtII-R′SO2X Reactions (R = alkyl, R′ = aryl, X = halide) and in the related rhodium(I) or iridium(I) Systems". J. Chem. Soc., Dalton Trans. (8): 1448–1456. doi:10.1039/DT9800001448.
  11. ↑ Korch, Katerina M.; Watson, Donald A. (2019). "Cross-Coupling of Heteroatomic Electrophiles". Chemical Reviews. 119 (13): 8192–8228. doi:10.1021/acs.chemrev.8b00628. PMC   6620169 . PMID   31184483.
  12. ↑ Corbet, Jean-Pierre; Mignani, Gérard (2006). "Selected Patented Cross-Coupling Reaction Technologies". Chemical Reviews. 106 (7): 2651–2710. doi:10.1021/cr0505268. PMID   16836296.

Further reading

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.