The Application Of Palladium-Catalysts In Hydrogenations And Aqueous Reactions

Development of highly efficient and selective reactions was an important part in modern organic chemistry. Catalytic reactions, especially the palladium catalyzed reactions, most attracted our attention. In this thesis, the application of palladium catalysts in organic synthesis were studies mainly focusing on two aspects. One is the chemical selectivity of the Pd/C catalyst in catalytic hydrogenations, the other is the application of palladium catalyst in aqueous reaction.As a traditional palladium catalyst, Pd/C was widely used in catalytic hydrogenation. In chapter 2, the catalytic hydrogenolysis of benzaldehydes to methylbenzenes was studied. A three stage novel'acetal pathway'was first revealed by a systematic study when lower alcohols were used as solvents. In the first stage, benzaldehyde carried out an acetalization catalyzed by aq. HCl and Pd/C together to yield dimethyl acetal. Then, dimethyl acetal carried out a Pd/C catalyzed"fast hydrogenolysis"to give the benzyl methyl ether. Finally, benzyl methyl ether underwent a Pd-C catalyzed"slow hydrogenolysis"to give methylbenzene. A solvent-controlled highly efficient procedure for hydrogenolysis of benzaldehydes to methylbenzenes was established.In chapter 3, by adding a few milliliters of CHCl3 in the conventional Pd/C catalytic hydrogenation condition, a self-adjusted system was established. Using this system, an unprecedented efficient and chemoselective DRA of benzaldehydes and primary amines was developed to directly yield N-substituted benzylamine hydrochlorides as single products in practically quantitative yields. A four-stage cyclic pathway was proposed.In chapter 4, a group of dioxime ligands were designed and synthesized. The ligands may form dinuclear complexes with palladium, by that a novel aqueous Suzuki coupling between halobenenzes and phenylboronic acids was catalyzed efficiently. The catalyst had a very high TON with very low loading (0.1 mol%) and can be recycled four times in suit without significant lose of the reactivity. In chapter 5, the aqueous homocoupling of bromoarenes was studied systematically by using different reducing agents. As a result, we found that glucose is the most efficient reducing agent and the phase transfer catalyst played a very important role in the coupling reaction. When phase transfer catalyst was used, the yield of the homocoupling was improved. The homocoupling reaction was carried out using palladium diacetate as catalyst without any ligand. Both bromopyridines and bromobenzenes can give good results.