Alpha elimination

In chemistry, alpha elimination refers to particular types of elimination reactions. The definition of alpha elimination differs for organometallic and organic chemistry.

Organic chemistry

In organic chemistry, alpha-elimination refers to reactions of this type:^[1]

R₂CHX → R₂C: + HX

The reaction is employed to generate carbenes and nitrenes. The formation of dichlorocarbene from chloroform is an example.

Alpha eliminations contrasts with beta eliminations, which are commonly used to generate alkenes:

R₂CHCXR'₂ → R₂C=CR'₂ + HX

Both alpha- and beta-eliminations typically require strong base.

Organometallic chemistry

In organometallic chemistry, alpha elimination refers to reactions of this type (other spectator ligands omitted):^[2]

X-M-CH₂R → M=CHR + HX

Well studied case are found in organotantalum chemistry leading to an alkylidene derivatives. Specifically, tetraalkyl-monochloro-tantalum complex undergoes α-hydrogen elimination, followed by alkylation of the remaining chloride to give a derivative with a Ta=C bond.^[3]

Alpha elimination contrasts with β-hydride elimination, whereby an alkyl group bonded to a metal centre is converted into the corresponding metal-bonded hydride and an alkene.

Both α- and β-eliminations proceed via agostic intermediates.^[4]

References

^ Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, p. 1477, ISBN 978-0-471-72091-1
^ Elschenbroich, C. (2006). Organometallics. Weinheim: Wiley-VCH. ISBN 978-3-527-29390-2.
^ Schrock, Richard R. (1979-03-01). "Alkylidene complexes of niobium and tantalum". Accounts of Chemical Research. 12 (3): 98–104. doi:10.1021/ar50135a004. ISSN 0001-4842.
^ Brookhart, M.; Green, M. L. H.; Parkin, G. (2007). "Agostic interactions in transition metal compounds". Proceedings of the National Academy of Sciences. 104 (17): 6908–6914. doi:10.1073/pnas.0610747104. PMC 1855361. PMID 17442749.

[1] Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, p. 1477, ISBN 978-0-471-72091-1

[Elschenbroich-2] Elschenbroich, C. (2006). Organometallics. Weinheim: Wiley-VCH. ISBN 978-3-527-29390-2.

[Schrock-3] Schrock, Richard R. (1979-03-01). "Alkylidene complexes of niobium and tantalum". Accounts of Chemical Research. 12 (3): 98–104. doi:10.1021/ar50135a004. ISSN 0001-4842.

[4] Brookhart, M.; Green, M. L. H.; Parkin, G. (2007). "Agostic interactions in transition metal compounds". Proceedings of the National Academy of Sciences. 104 (17): 6908–6914. doi:10.1073/pnas.0610747104. PMC 1855361. PMID 17442749.

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v t e Organometallic chemistry
Principles	Crystal field theory Ligand field theory 18-electron rule d electron count Polyhedral skeletal electron pair theory Isolobal principle π backbonding Dewar–Chatt–Duncanson model Hapticity spin states Agostic interaction Metal–ligand multiple bond
Reactions	Oxidative addition / reductive elimination Migratory insertion β-hydride elimination Transmetalation Carbometalation
Types of compounds	Gilman reagents Grignard reagents Cyclopentadienyl complexes Transition metal indenyl complexes Transition metal fullerene complexes Metallocenes Metal tetranorbornyls Sandwich compounds Half sandwich compounds Transition metal acyl complexes Transition metal carbene complexes Transition metal carbyne complexes Transition metal alkene complexes Transition metal alkyne complexes Transition-metal allyl complexes Transition metal carbides Arene complexes of univalent gallium, indium, and thallium
Applications	Carbonylation Cativa process Grignard reaction Monsanto process Olefin metathesis Shell higher olefin process Ziegler–Natta process
Related branches of chemistry	Organic chemistry Inorganic chemistry Bioinorganic chemistry
Category Commons

Organic chemistry

Organometallic chemistry

See also

References