Carbon-Carbon Coupling Reactions Catalyzed by Heterogeneous Palladium Catalysts

摘要

Homogeneous palladium catalysis has gained enormous relevance in various coupling reactions such as Heck, Stille, Suzuki, Sonogashira, and Buchwald-Hartwig reactions. Many products could be synthesized by this methodology for the first time or in a much more efficient way than before. This type of catalysis provides high reaction rate and high turnover numbers (TON) and often affords high selectivities and yields. The properties of such Pd catalysts can be tuned by ligands, such as phosphines, amines, carbenes, dibenzylideneacetone (dba), etc. Proper ligand design has led to catalysts that tolerate weak leaving groups such as chloride, exhibit higher TON and reaction rates, have improved lifetimes, and are suitably stable to run the reactions without the exclusion of water or air and at lower temperatures. The structure of the catalytic species is often known, and structure-activity relations could be established. Recent developments of ligand-free Pd catalysts have provided interesting and practically important alternatives to ligand-assisted methodologies. On the other hand, homogeneous catalysis has a number of drawbacks, in particular, the lack of reuse of the catalyst or at least the problem of recycling of the catalyst.