Reductive elimination

Last updated

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. [1] [2]

Contents

General information

Reductive elimination is often seen in higher oxidation states, and can involve a two-electron change at a single metal center (mononuclear) or a one-electron change at each of two metal centers (binuclear, dinuclear, or bimetallic). [1] [2]

General Reductive Elimination.png

For mononuclear reductive elimination, the oxidation state of the metal decreases by two, while the d-electron count of the metal increases by two. This pathway is common for d8 metals Ni(II), Pd(II), and Au(III) and d6 metals Pt(IV), Pd(IV), Ir(III), and Rh(III). Additionally, mononuclear reductive elimination requires that the groups being eliminated must be cis to one another on the metal center. [3]

Cis vs. Trans.png

For binuclear reductive elimination, the oxidation state of each metal decreases by one, while the d-electron count of each metal increases by one. This type of reactivity is generally seen with first row metals, which prefer a one-unit change in oxidation state, but has been observed in both second and third row metals. [4]

Binuclear Reductive Elimination.png

Mechanisms

As with oxidative addition, several mechanisms are possible with reductive elimination. The prominent mechanism is a concerted pathway, meaning that it is a nonpolar, three-centered transition state with retention of stereochemistry. In addition, an SN2 mechanism, which proceeds with inversion of stereochemistry, or a radical mechanism, which proceeds with obliteration of stereochemistry, are other possible pathways for reductive elimination. [1]

Octahedral complexes

The rate of reductive elimination is greatly influenced by the geometry of the metal complex. In octahedral complexes, reductive elimination can be very slow from the coordinatively saturated center; and often, reductive elimination only proceeds via a dissociative mechanism, where a ligand must initially dissociate to make a five-coordinate complex. This complex adopts a Y-type distorted trigonal bipyramidal structure where a π-donor ligand is at the basal position and the two groups to be eliminated are brought very close together. After elimination, a T-shaped three-coordinate complex is formed, which will associate with a ligand to form the square planar four-coordinate complex. [5]

Octahedral Reductive Elimination.png

Square planar complexes

Reductive elimination of square planar complexes can progress through a variety of mechanisms: dissociative, nondissociative, and associative. Similar to octahedral complexes, a dissociative mechanism for square planar complexes initiates with loss of a ligand, generating a three-coordinate intermediate that undergoes reductive elimination to produce a one-coordinate metal complex. For a nondissociative pathway, reductive elimination occurs from the four-coordinate system to afford a two-coordinate complex. If the eliminating ligands are trans to each other, the complex must first undergo a trans to cis isomerization before eliminating. In an associative mechanism, a ligand must initially associate with the four-coordinate metal complex to generate a five-coordinate complex that undergoes reductive elimination synonymous to the dissociation mechanism for octahedral complexes. [6] [7]

Reductive Elimination from Square Planar Complexes.png

Factors that affect reductive elimination

Reductive elimination is sensitive to a variety of factors including: (1) metal identity and electron density, (2) sterics, (3) participating ligands, (4) coordination number, (5) geometry, and (6) photolysis/oxidation. Additionally, because reductive elimination and oxidative addition are reverse reactions, any sterics or electronics that enhance the rate of reductive elimination must thermodynamically hinder the rate of oxidative addition. [2]

Metal identity and electron density

First-row metal complexes tend to undergo reductive elimination faster than second-row metal complexes, which tend to be faster than third-row metal complexes. This is due to bond strength, with metal-ligand bonds in first-row complexes being weaker than metal-ligand bonds in third-row complexes. Additionally, electron-poor metal centers undergo reductive elimination faster than electron-rich metal centers since the resulting metal would gain electron density upon reductive elimination. [8]

Electron-Poor Metal Reductive Elimination.png

Sterics

Reductive elimination generally occurs more rapidly from a more sterically hindered metal center because the steric encumbrance is alleviated upon reductive elimination. Additionally, wide ligand bite angles generally accelerate reductive elimination because the sterics force the eliminating groups closer together, which allows for more orbital overlap. [9]

Reductive Elimination Bite Angles.png

Participating ligands

Kinetics for reductive elimination are hard to predict, but reactions that involve hydrides are particularly fast due to effects of orbital overlap in the transition state. [10]

Reductive Elimination Participating Ligands.png

Coordination number

Reductive elimination occurs more rapidly for complexes of three- or five-coordinate metal centers than for four- or six-coordinate metal centers. For even coordination number complexes, reductive elimination leads to an intermediate with a strongly metal-ligand antibonding orbital. When reductive elimination occurs from odd coordination number complexes, the resulting intermediate occupies a nonbonding molecular orbital. [11]

Reductive Elimination Coordination Number.png

Geometry

Reductive elimination generally occurs faster for complexes whose structures resemble the product. [2]

Photolysis/oxidation

Reductive elimination can be induced by oxidizing the metal center to a higher oxidation state via light or an oxidant. [12]

Oxidatively-Induced Reductive Elimination.png

Applications

Reductive elimination has found widespread application in academia and industry, most notable being hydrogenation, [13] the Monsanto acetic acid process, [14] hydroformylation, [15] and cross-coupling reactions. [16] In many of these catalytic cycles, reductive elimination is the product forming step and regenerates the catalyst; however, in the Heck reaction [17] and Wacker process, [18] reductive elimination is involved only in catalyst regeneration, as the products in these reactions are formed via β–hydride elimination.

Related Research Articles

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.

The Suzuki reaction or Suzuki coupling is an organic reaction that uses a palladium complex catalyst to cross-couple a boronic acid to an organohalide. 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 noble metal catalysis in organic synthesis. This reaction is sometimes telescoped with the related Miyaura borylation; the combination is the Suzuki–Miyaura reaction. It is widely used to synthesize polyolefins, styrenes, and substituted biphenyls.

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.

<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.

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.

<span class="mw-page-title-main">Wacker process</span> Chemical reaction

The Wacker process or the Hoechst-Wacker process refers to the oxidation of ethylene to acetaldehyde in the presence of palladium(II) chloride and copper(II) chloride as the catalyst. This chemical reaction was one of the first homogeneous catalysis with organopalladium chemistry applied on an industrial scale.

<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.

<span class="mw-page-title-main">Bite angle</span>

In coordination chemistry, the bite angle is the angle on a central atom between two bonds to a bidentate ligand. This ligand–metal–ligand geometric parameter is used to classify chelating ligands, including those in organometallic complexes. It is most often discussed in terms of catalysis, as changes in bite angle can affect not just the activity and selectivity of a catalytic reaction but even allow alternative reaction pathways to become accessible.

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

In organometallic chemistry, a metallacycle is a derivative of a carbocyclic compound wherein a metal has replaced at least one carbon center; this is to some extent similar to heterocycles. Metallacycles appear frequently as reactive intermediates in catalysis, e.g. olefin metathesis and alkyne trimerization. In organic synthesis, directed ortho metalation is widely used for the functionalization of arene rings via C-H activation. One main effect that metallic atom substitution on a cyclic carbon compound is distorting the geometry due to the large size of typical metals.

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">Organomolybdenum chemistry</span> Chemistry of compounds with Mo-C bonds

Organomolybdenum chemistry is the chemistry of chemical compounds with Mo-C bonds. The heavier group 6 elements molybdenum and tungsten form organometallic compounds similar to those in organochromium chemistry but higher oxidation states tend to be more common.

<span class="mw-page-title-main">Iron tetracarbonyl dihydride</span> Chemical compound

Iron tetracarbonyl dihydride is the organometallic compound with the formula H2Fe(CO)4. This compound was the first transition metal hydride discovered. The complex is stable at low temperatures but decomposes rapidly at temperatures above –20 °C.

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.:

Nucleophilic abstraction is a type of an organometallic reaction which can be defined as a nucleophilic attack on a ligand which causes part or all of the original ligand to be removed from the metal along with the nucleophile.

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.

In inorganic chemistry, the cis effect is defined as the labilization of CO ligands that are cis to other ligands. CO is a well-known strong pi-accepting ligand in organometallic chemistry that will labilize in the cis position when adjacent to ligands due to steric and electronic effects. The system most often studied for the cis effect is an octahedral complex M(CO)
5X where X is the ligand that will labilize a CO ligand cis to it. Unlike the trans effect, which is most often observed in 4-coordinate square planar complexes, the cis effect is observed in 6-coordinate octahedral transition metal complexes. It has been determined that ligands that are weak sigma donors and non-pi acceptors seem to have the strongest cis-labilizing effects. Therefore, the cis effect has the opposite trend of the trans-effect, which effectively labilizes ligands that are trans to strong pi accepting and sigma donating ligands.

In chemistry, compounds of palladium(III) feature the noble metal palladium in the unusual +3 oxidation state (in most of its compounds, palladium has the oxidation state II). Compounds of Pd(III) occur in mononuclear and dinuclear forms. Palladium(III) is most often invoked, not observed in mechanistic organometallic chemistry.

In homogeneous catalysis, C2-symmetric ligands refer to ligands that lack mirror symmetry but have C2 symmetry. Such ligands are usually bidentate and are valuable in catalysis. The C2 symmetry of ligands limits the number of possible reaction pathways and thereby increases enantioselectivity, relative to asymmetrical analogues. C2-symmetric ligands are a subset of chiral ligands. Chiral ligands, including C2-symmetric ligands, combine with metals or other groups to form chiral catalysts. These catalysts engage in enantioselective chemical synthesis, in which chirality in the catalyst yields chirality in the reaction product.

Heterobimetallic catalysis is an approach to catalysis that employs two different metals to promote a chemical reaction. Included in this definition are cases where: 1) each metal activates a different substrate, 2) both metals interact with the same substrate, and 3) only one metal directly interacts with the substrate(s), while the second metal interacts with the first.

References

  1. 1 2 3 Crabtree, Robert H. (2014). The Organometallic Chemistry of the Transition Metals (6 ed.). Wiley. p. 173. ISBN   978-1-118-13807-6.
  2. 1 2 3 4 Hartwig, John F. (2010). Organotransition Metal Chemistry, from Bonding to Catalysis. University Science Books. p. 321. ISBN   978-1-891389-53-5.
  3. ↑ Gillie, A.; Stille, J. K. (1980). "Mechanisms of 1,1-Reductive Elimination from Palladium". J. Am. Chem. Soc. 102 (15): 4933–4941. doi:10.1021/ja00535a018.
  4. ↑ Okrasinski, S. J.; Nortom, J. R. (1977). "Mechanism of Reductive Elimination. 2. Control of Dinuclear vs. Mononuclear Elimination of Methane from cis-Hydridomethyltetracarbonylosmium". J. Am. Chem. Soc. 99: 295–297. doi:10.1021/ja00443a076.
  5. ↑ Milstein, D. (1982). "The First Isolated, Stable cis-Hydridoalkylrhodium Complexes and Their reductive Elimination Reaction". J. Am. Chem. Soc. 104 (19): 5227–5228. doi:10.1021/ja00383a039.
  6. ↑ Driver, M. S.; Hartwig, J. F. (1997). "Carbon−Nitrogen-Bond-Forming Reductive Elimination of Arylamines from Palladium(II) Phosphine Complexes". J. Am. Chem. Soc. 119 (35): 8232–8245. doi:10.1021/ja971057x.
  7. ↑ Yamamoto, T.; Yamamoto, A.; Ikeda, S. (1971). "Study of Organo(dipyridyl)nickel Complexes. I. Stability and Activation of the Alkyl-Nickel Bonds of Dialkyl(dipyridyl)nickel by Coordination with Various Substituted Olefins". J. Am. Chem. Soc. 93: 3350. doi:10.1021/ja00743a009.
  8. ↑ Giovannini, R.; Stüdemann, T.; Dussin, G.; Knochel, P. (1998). "An Efficient Nickel-Catalyzed Cross-Coupling Between sp3 Carbon Centers". Angew. Chem. Int. Ed. 37 (17): 2387–2390. doi:10.1002/(SICI)1521-3773(19980918)37:17<2387::AID-ANIE2387>3.0.CO;2-M. PMID   29710957.
  9. ↑ Marcone, J. E.; Moloy, K. G. (1998). "Kinetic Study of Reductive Elimination from the Complexes (Diphosphine)Pd(R)(CN)". J. Am. Chem. Soc. 120 (33): 8527–8528. doi:10.1021/ja980762i.
  10. ↑ Low, J. J.; Goddard, III, W. A. (1984). "Reductive Coupling of Hydrogen-Hydrogen, Hydrogen-Carbon, and Carbon-Carbon Bonds from Palladium Complexes". J. Am. Chem. Soc. 106 (26): 8321–8322. doi:10.1021/ja00338a067.
  11. ↑ Crumpton-Bregel, D. M.; Goldberg, K. I. (2003). "Mechanisms of C-C and C-H Alkane Reductive Eliminations from Octahedral Pt(IV): Reaction via Five-Coordinate Intermediates or Direct Elimination?". J. Am. Chem. Soc. 125 (31): 9442–9456. doi:10.1021/ja029140u. PMID   12889975.
  12. ↑ Lau, W.; Huffman, J. C.; Kochi, J. K. (1982). "Electrochemical Oxidation-Reduction of Organometallic Complexes. Effect of the Oxidation State on the Pathways for Reductive Elimination of Dialkyliron Complexes". Organometallics. 1: 155–169. doi:10.1021/om00061a027.
  13. ↑ de Vries, J. G. (2007). The Handbook of Homogeneous Hydrogenation. Wiley. ISBN   978-3-527-31161-3.
  14. ↑ Paulik, F. E.; Roth, J. F. (1968). "Novel Catalysts for the Low-pressure Carbonylation of Methanol to Acetic Acid". Chem. Commun. (24): 1578. doi:10.1039/C1968001578A.
  15. ↑ Ojima, I.; Tsai, C.-H.; Tzamarioudaki, M.; Bonafoux, D. (2004). "The Hydroformylation Reaction". Organic Reactions. 56: 1–354. doi:10.1002/0471264180.or056.01. ISBN   0471264180.
  16. ↑ New Trends in Cross-Coupling: Theory and Applications Thomas Colacot (Editor) 2014 ISBN   978-1-84973-896-5
  17. ↑ de Vries, J. G. (2001). "The Heck reaction in the production of fine chemicals" (PDF). Can. J. Chem. 79 (5–6): 1086–1092. doi:10.1139/v01-033. hdl: 11370/31cf3b82-e13a-46e2-a3ba-facb1e2bffbf .
  18. ↑ Dong, J. J.; Browne, W. R.; Feringa, B. L. (2015). "Palladium-Catalyzed anti-Markovnikov Oxidation of Terminal Alkenes" (PDF). Angew. Chem. Int. Ed. 54 (3): 734–744. doi:10.1002/anie.201404856. PMID   25367376.
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.