β-Hydride elimination
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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.[1] 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 (or vacant) 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.[2]
Role of β-Hydride Elimination in Reactions
[edit]The β-hydride elimination can either be a vital step in a reaction or an unproductive side reaction. The Shell higher olefin process relies on β-hydride elimination to produce α-olefins which are used to produce detergents. Illustrative of a sometimes undesirable β-hydride elimination, β-hydride elimination in Ziegler–Natta polymerization results in polymers of decreased molecular weight.[3] In the case of nickel- and palladium-catalyzed couplings of aryl halides with alkyl Grignard reagents, the β-hydride elimination can lower the yield. The production of branched polymers from ethylene relies on chain walking, a key step of which is β-hydride elimination. In Hydroformylation, β-hydride elimination can act as a side reaction that influences product regioselectivity.[4] For example, in the hydroformylation of open chain unsaturated ethers, it reverses the formation of branched metal-alkyl intermediates at high temperatures, leading to a greater yield of linear products.
In some cases, β-hydride elimination is the first in a series of steps. For instance in the synthesis of RuHCl(CO)(PPh3)3 from ruthenium trichloride, triphenylphosphine and 2-methoxyethanol, an intermediate alkoxide complex undergoes a β-hydride elimination to form the hydride ligand and the pi-bonded aldehyde which then is later converted into the carbonyl (carbon monoxide) ligand.
Mechanism for Metal-Alkyl Complexes
[edit]β-hydride elimination occurs commonly for metal-alkyl complexes. The process transforms a metal-bound alkyl group into an alkene with the release of a metal hydride.[5] Notably, β-hydride elimination can also occur for other ligands, such as alkoxides, via similar mechanisms.
The transformation can be divided into the following stages:
1) Alignment of the beta hydrogen
The initial step begins with the coordination of the substrate, typically an alkyl or aryl complex, to the metal center. This coordination aligns the reacting alkyl group near the hydride, enabling subsequent reactions.[6] The geometric arrangement of the alkyl relative to the metal, along with the presence of vacant sites on the metal, determines the feasibility of the elimination process.[7][8]
2) Hydride Transfer
After the coordination step, the elimination process proceeds with the transfer of a hydride (H-) from the metal to the β-carbon of the alkyl group. This step is often facilitated by the availability of d-electrons on the metal, which can overlap with the σ* orbital of the C–H bond on the β-carbon.[8] The hydride transfer initiates the breaking of the C–H bond while simultaneously forming a new C=C double bond, leading to the generation of an alkene. This creates a departing metal hydride species.
3) Formation of Alkene
As the hydride transfers, a π-bond is formed between the α and β carbons, resulting in the generation of an alkene. The stability and substituent effects at the β-position greatly influence the ease of this bond-migration process. The geometry of the alkaline product is significant as it dictates further reactions and the mechanism's overall outcome.
4) Regeneration of the Catalyst
After the formation of the alkene, the released metal hydride, can either re-enter the catalytic cycle to react with another substrate or be further modified through subsequent reactions. This regeneration allows the metal complex to continue facilitating additional reactions without being consumed.
Avoiding β-hydride elimination
[edit]Several strategies exist for avoiding β-hydride elimination. The most common strategy is to employ alkyl ligands that do not have any hydrogen atoms at the β position. Common substituents include methyl and neopentyl. β-Hydride elimination is also inhibited when the reaction would produce a strained alkene. This situation is illustrated by the stability of metal complexes containing norbornyl ligands, where the β-hydride elimination product would violate Bredt's rule.[9]
Bulky alkyl ligands, such as tert-butyl or trimethylsilyl, may prohibit the hydrogen atom from approaching a coplanar configuration with respect to the metal, and the α and β atoms. If the metal center does not have empty coordination sites, for example by the complex already having an 18-electron configuration, β-hydride elimination is not possible as well.
In some cases, the coligands can impose geometries that inhibit β-hydride elimination. For the above example, the unwanted β-hydride elimination is prevented by using a diphosphine where the two phosphorus atoms are fixed apart in space. One way of doing this is to use a trans spanning ligand such as Xantphos. As these metal complexes traditionally form square planar geometries, no vacant site cis to the alkyl group can be formed. Hence the β-hydride elimination is prevented. (See trans-spanning ligand.)
References
[edit]- ^ Elschenbroich, C. (2006). Organometallics. Weinheim: Wiley-VCH. ISBN 978-3-527-29390-2.
- ^ Crabtree, Robert H. (2005). The organometallic chemistry of the transition metals (4th ed.). Hoboken, N.J.: John Wiley. p. 58. ISBN 0-471-66256-9. OCLC 61520528.
- ^ Burger, Barbara J.; Thompson, Mark E.; Cotter, W. Donald; Bercaw, John E. (1990-02-01). "Ethylene insertion and .beta.-hydrogen elimination for permethylscandocene alkyl complexes. A study of the chain propagation and termination steps in Ziegler-Natta polymerization of ethylene". Journal of the American Chemical Society. 112 (4): 1566–1577. doi:10.1021/ja00160a041. ISSN 0002-7863.
- ^ Zhang, Baoxin; Peña Fuentes, Dilver; Börner, Armin (2022-12-02). "Hydroformylation". ChemTexts. 8 (1). doi:10.1007/s40828-021-00154-x. ISSN 2199-3793.
- ^ Hartwig, John F. (2010). Organotransition metal chemistry: from bonding to catalysis. Sausalito, Calif: University Science Books. ISBN 978-1-891389-53-5. OCLC 310401036.
- ^ Lu, Xiyan (2005-06-01). "Control of the β-Hydride Elimination Making Palladium-Catalyzed Coupling Reactions more Diversified". Topics in Catalysis. 35 (1): 73–86. doi:10.1007/s11244-005-3814-4. ISSN 1572-9028.
- ^ Spessard, Gary O.; Miessler, Gary L. (2016). Organometallic chemistry (3rd ed.). New York: Oxford University Press. ISBN 978-0-19-934267-9.
- ^ a b Theofanis, Patrick L.; Goddard, William A. III (2011-09-26). "Understanding β-Hydride Eliminations from Heteroatom Functional Groups". Organometallics. 30 (18): 4941–4948. doi:10.1021/om200542w. ISSN 0276-7333.
- ^ Bower, Barton K.; Tennent, Howard G. (1972). "Transition metal bicyclo[2.2.1]hept-1-yls". J. Am. Chem. Soc. 94: 2512–2514. doi:10.1021/ja00762a056.
