Carbometalation

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:

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+.

Scope

In one study methylphenylacetylene is reacted with phenylmagnesium bromide to a vinyl magnesium bromide which is quenched with water:[1][2]

Another demonstration of this reaction type is an alternative route to tamoxifen starting from diphenylacetylene and ethyllithium:[3]

The capturing electrophile here is triisopropyl borate forming the boronic acid R-B(OH)2. The second step completing tamoxifen is a Suzuki reaction.

Silylmetalation

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 study[4] a 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.

References

  1. ↑ 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
  2. ↑ 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
  3. ↑ 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
  4. ↑ 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