Carbometalation is an organometallic reaction involving the nucleophilic addition to alkenes and alkynes of a diverse range of organometallic reagents such as organolithium compounds, organocopper compounds and Grignard reagents according to the following general alkyne scheme:
Additional recommended knowledge
The addition can yield the cis or trans isomer and with unsymmetrical alkynes the organometallic compound can add in two different way thus control of regioselectivity is important.
In a follow-up step the sensitive metalalkenyl group is replaced by an electrophile E+.
In one study methylphenylacetylene is reacted with phenylmagnesium bromide to a vinyl magnesium bromide which is quenched with water:
Another demonstration of this reaction type is an alternative route to tamoxifen starting from diphenylacetylene and ethyllithium:
The capturing electrophile here is triisopropyl borate forming the boronic acid R-B(OH)2. The second step completing tamoxifen is a Suzuki reaction.
This reaction type can be extended to compounds of silicon bonded to a suitable metal in so-called silylmetalation as an extension to hydrosilylation. In one studya zinc ate complex is formed from dimethylphenylsilyllithium, 2,2'-biphenol, zinc chloride and the Grignard reagent of tert-butylchloride which is capable of silylmetalation to styrene with 100% regioselectivity:
The intermediate complex can be captured with many electrophiles such as propargyl bromide (pictured, forming the allene), benzoyl chloride and allyl bromide.
- ^ Arylmagnesiation of Alkynes Catalyzed Cooperatively by Iron and Copper Complexes Eiji Shirakawa, Takafumi Yamagami, Takahiro Kimura, Shigeru Yamaguchi, and Tamio Hayashi J. Am. Chem. Soc.; 2005; 127(49) pp 17164 - 17165; (Communication) doi:10.1021/ja0542136
- ^ In this reaction the Grignard reagent together with iron acetylacetonate and tributylphosphine first form an ill-defined aryliron intermediate and then by reaction with copper(I) chloride an intermediate cuprate
- ^ Carbolithiation of Diphenylacetylene as a Stereoselective Route to (Z)-Tamoxifen and Related Tetrasubstituted Olefins Neola F. McKinley and Donal F. O'Shea J. Org. Chem.; 2006; 71(25) pp 9552 - 9555; (Note) doi:10.1021/jo061949s
- ^ Regio- and Chemoselective Silylmetalation of Functionalized Terminal Alkenes Shinji Nakamura and Masanobu Uchiyama J. Am. Chem. Soc.; 2007; 129(1) pp 28 - 29; (Communication) doi:10.1021/ja066864n
|Concepts in organometallic chemistry|
|Principles||Electron counting, 18-Electron rule, Polyhedral skeletal electron pair theory, Isolobal principle, π backbonding, Hapticity|
|Reactions||Oxidative addition, Reductive elimination, Beta-hydride elimination, Transmetalation, Carbometalation|
|Types of compounds||Gilman reagent, Grignard reagents, Cyclopentadienyl complexes, Metallocenes, Sandwich compound, Transition metal carbene complexes|
|Applications||Monsanto process, Ziegler-Natta catalyst, Olefin metathesis|
|Related branches of chemistry||Organic chemistry - Inorganic chemistry - Bioinorganic chemistry|