Investigation of aryldimethylsilanolates in palladium-catalyzed cross-coupling reactions and development of chiral bis-hydrazone ligands for asymmetric aryl-aryl couplings

Cross Coupling;Hiyama Coupling;Biaryl;Silanolate;Transmetalation;Hammett Study;Hydrazone Ligand;Asymmetric Aryl-Aryl Coupling

Chang, Wen-Tau

关键词: Cross Coupling;    Hiyama Coupling;    Biaryl;    Silanolate;    Transmetalation;    Hammett Study;    Hydrazone Ligand;    Asymmetric Aryl-Aryl Coupling;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/46779/Wen-Tau_Chang.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

The preparation of biaryls by palladium-catalyzed cross-coupling of aromatic bromides and chlorides with alkali-metal salts (potassium and sodium) of aryl- and heteroarylsilanols has been developed. The critical feature for the success of this method is the use of bis(tri-tert-butylphosphine)palladium catalyst. Under the optimized reaction conditions, a range of coupling partners bearing different functionalities have been examined and provide biaryl products in moderate to good yields. Refinement of the mechanism for this reaction is facilitated by the ability to prepare a variety of arylpalladium(II) silanolate intermediates from easily accessible aryldimethylsilanolates. These in situ generated complexes enable kinetic studies of the transmetalation step in the proposed catalytic cycle under stoichiometric conditions. Hammett analysis reveals that electron-donating groups on the silicon nucleophile and electron-withdrawing groups on the aryl halide electrophile accelerate this process. Further interpretation of the kinetic data leads to the proposal of an intramolecular, asynchronous, SEAr-type transmetalation pathway.The application of aryldimethylsilanolate to asymmetric biaryl cross-coupling reactions has been investigated. Pivotal to this study is the development of a novel synthetic route to access a number of chiral bis-hydrazone ligands featuring a 2,5-diarylpyrrolidine moiety. This endeavor enables systematic investigation of the ligand effect on the reactivity and enantioselectivity of the reaction. The origin of the chiral induction has been rationalized through computational modeling.Preliminary mechanistic studies indicate that reductive elimination is the stereodeteremining step. Efforts in the preparation of palladium complexes of an achiral bis-hydrazone ligand have laid the groundwork for future development.