Bo Qin and Uwe Schneider, University of Edinburgh, UK, have used elemental gallium as a catalyst in organic synthesis for the first time.
The researchers used elemental gallium together with a mildly oxidizing Ag(I) cocatalyst to catalyze C−C bond formations between allyl or allenyl boronic esters and acetals, ketals, or aminals. A transient Ga(I) catalytic species was formed through partial oxidation of Ga(0), which was confirmed by 71Ga NMR spectroscopy. The team used ultrasonic activation instead of conventional heating to improve the activity of the catalyst. Additionally, a crown ether was used to stabilize the Ga(I) intermediate.
The proposed reaction mechanism involves the formation of the catalytic Ga(I) species, which activates acetals or ketals by abstracting an alkoxide. The [GaI]−OR which is formed can, in turn, activate the boronic ester through C–B bond activation. The Ga(I) catalyst is regenerated when the two activated reactants undergo C–C bond formation. The reaction proceeds in high yields with almost complete regio- and chemoselectivity. According to the researchers, this scalable method is a rare example of ultrasonic activation in catalysis and may open up a new field in organic synthesis.
- Catalytic Use of Elemental Gallium for Carbon–Carbon Bond Formation,
Bo Qin, Uwe Schneider,
J. Am.Chem. Soc. 2016.