A Guide to Sonogashira Cross-Coupling Reactions: The Influence of Substituents in Aryl Bromides, Acetylenes, and Phosphines
2012
Schilz, Marc | Plenio, Herbert
The conversion–time data for 168 different Pd/Cu-catalyzed Sonogashira cross-coupling reactions of five arylacetylenes (phenylacetylene; 1-ethynyl-2-ethylbenzene; 1-ethynyl-2,4,6-R₃-benzene (R = Me, Et, i-Pr)) and Me₃SiCCH with seven aryl bromides (three 2-R-bromobenzenes (R = Me, Et, i-Pr); 2,6-Me₂-bromobenzene and three 2,4,6-R₃-bromobenzenes (R = Me, Et, i-Pr)) with four different phosphines (P-t-Bu₃, t-Bu₂PCy, t-BuPCy₂, PCy₃) were determined using quantitative gas chromatography. The stereoelectronic properties of the substituents in the aryl bromides, acetylenes, and phosphines were correlated with the performance in Sonogashira reactions. It was found that the nature of the most active Pd/PR₃ complex for a Sonogashira transformation is primarily determined by the steric bulk of the acetylene; ideal catalysts are: Pd/P-t-Bu₃ or Pd/t-Bu₂PCy for sterically undemanding phenylacetylene, Pd/t-BuPCy₂ for 2- and 2,6-substituted arylacetylenes or Me₃SiCCH and Pd/PCy₃ for extremely bulky acetylenes and aryl bromides. Electron-rich and sterically demanding aryl bromides with substituents in the 2- or the 2,6-position require larger amounts of catalyst than 4-substituted aryl bromides. The synthesis of tolanes with bulky groups at one of the two aryl rings is best done by placing the steric bulk at the arylacetylene, which is also the best place for electron-withdrawing substituents.
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