Studies On The Palladium-Catalyzed Cross-Coupling Reactions And Their Mechanisms

In this dissertation, the palladium-catalyzed cross-coupling reactions were studied, especially their mechanisms. We have developed a new catalytic system, proposed the mechanism of an unexpected reaction, and improved a synthetic method by omitting precious metal catalyst. The dissertation includes:1. The activation of sp₃ C-X bond. Oxidative addition of transition metal with a sp3 C-X bond is more difficult than with a sp₂ C-X bond. To solve the problem, a series of aminophosphine ligands were synthesized and applied successfully to the palladium-catalyzed cross-coupling reactions between boronic acids and ethyl bromoacetate. We optimized the reaction conditions and found that addition of water as a co-solvent was crucial for the cross-coupling reaction, and this can be explained by the equilibrium between C-Pd and O-Pd species. The addition of water partially inhibits the formation of biaryl products and thus favors the formation of cross-coupling intermediate, leading to the cross-coupling products. The reactions of a number of aryl boronic and alkenyl boronic acids were conducted, good to excellent yields were obtained.2. The mechanisms of base- and oxidant-free Heck-type reactions of aryl boronic acids with both electron-deficient and electron-rich alkenes were studied. In the reactions, one equivalent of Heck product was derived from one equivalent of boronic acid with the consumption of two equivalent of alkene. The alkene served both as reactant and hydrogen acceptor or oxidant.For the reactions of electron-deficient alkenes in the presence of trifluoroacetic acid, instead of the normal Heck reactions expected, they underwent direct protonlysis via an insertion of their C=C bond into Pd-H bond, forming a palladium enolate intermediate, or direct insertion of acetone to Pd-H bond generating the catalytic species. A catalytic amount of trifluoroacetic acid is crucial to the reaction.For the reactions of electron-rich alkenes, the insertion of their C=C bond into Pd-H species was followed by the generation of an active palladium species via theβ-OR elimination, proved by the evolution of ethene gas.3. Lewis acids-promoted olefination of aldehydes with organozinc reagents was developed. The olefination of benzaldehyde with organozinc reagents in the presence of transition metal catalyst was reported in 2002, but we proved the transition metal compounds did not affect the reactions: olefination was promoted by Lewis acids, cheap and readily available AlCl₃ was the best of chioce, (E)-alkenes were obtained in high yields and good stereoselectivity.