Studies On The Heck And Suzuki Coupling Reactions Of Aryl Halides

The carbon-carbon bond forming reactions are always the key steps in many syntheses of organic chemicals. The coupling reaction of the alkyl halides is one of the most fundamental methods. Since its invention, numerous efforts have been devoted to further development of the reactions because of its usefulness and especially the wide applicability to substrates. It has become an excellent tool for the modern organic synthesis. In the past years, the coupling reaction has been extensively utilized in the preparations of medicines, pesticides, and many fine chemicals.In the dissertation, the Heck reaction of aryl halides in the homogeneous and heterogeneous catalyst systems has been investigated. The Suzuki reaction which catalyzed by some supported palladium catalysts and other supported catalysts with non-palladium has been studied. In addition, the carbonylation reaction of alkyl halides catalyzed by cobalt complex has been explored.In the study of the Heck reaction in the homogeneous system, a phosphonium salt, PPh₄Br, was used as co-catalyst in the pailadacycle catalyst system for the Heck reaction of the aryl chlorides. The influence of different catalyst and co-catalyst on reaction performance was also discussed. The reaction conditions and the ratio of PPh₄Br/Pd were optimized. In the case of Palladacycle-PPh4Br catalyst system, the amount of 0.3 mol % palladium can afford high yield (88%) and conversion (90%) for the Heck reaction of chlorobenzene with styrene by using Na₂CO₃ as the base. The presence of PPh₄Br was able to decrease the required amount of palladium catalyst and stabilize the palladium complex during the reaction. Palladacycle-PPh₄Br is an efficient catalyst system for the most aryl halides, even some aryl chlorides with electron-donating groups.In the study of the Heck reaction in the heterogeneous system, LDH-F hydrotalcite was achieved, for the first time, as an efficient and selective basic support in palladium catalyst for the Heck reaction. Small amounts of Pd/LDH-F (0.1 mol % of Pd) are required to perform the Heck reaction of bromobenzne and styrene withhigh yield (86%) in the absence of any ligands. The ligand-free heterogeneous catalyst Pd/LDH-F exhibits higher activity and selectivity compared with the other supported palladium catalysts (including some supports modified by fluorine) in the Heck reaction of aryl bromides with styrene. The structure of catalysts and the leaching of Pd in the solution were characterized by XRD and ICP. The results revealed that the catalyst could easily be separated from the reaction mixture and reused after washing without loss in catalytic activity in four runs.The supported palladium catalysts e.g. Pd/LDH —F, which performed high activity in the Heck reaction also were applied in the Suzuki reaction of aryl halides. The effects of different catalysts and reaction conditions were discussed. The catalyst Pd/LDH-F also performed high activity in the Suzuki reaction. The almost equivalent production could be obtained in the Suzuki reaction of bromobenzene and phenylboronic acid catalyzed by Pd/LDH-F (0.1 mol % of Pd) under the optimal reaction conditions: a mixture solution of DMF/H2O (1/2, v/v), K2CO3 as base,, in air and at room temperature for 2 h. Similarly, the catalyst Pd/LDH-F could also be recovered and reused after washing without loss in activity in the Suzuki reactions.Because of the expensive price of palladium, an attempt was made to apply the other metals in the coupling reaction of aryl halides. Several transition metals were supported on the supports with hydrotalcite-like structure such as HAP, FAP and LDH et al. The relevant catalysts were utilized in the Suzuki reaction. A 70% yield was achieved in the Suzuki reaction of bromobenzene and phenylboronic acid under refluxing 6 h catalyzed by MnFAP in the mixing solvent of DMF/H2O (1/3, v/v). At the same reaction conditions, the catalyst CoFAP also afforded 40% yield. These results have not been reported in literature so far.In addition, a new approach for the synthesis of methyl acetoacetate by selective carbonylation of chloroacetone catalyzed by Na[Co(CO)4] was developed. The cobalt complex, Na[Co(CO)4], was prepared in situ and was used to instead of palladium catalyst. The reaction was carried out under mild conditions with good yield (65%) and the chloroacetone was almost completely converted. The influences of reaction temperature, pressure and base on reaction performance were discussed. Meanwhile,the possible reaction mechanism catalysed by cobalt complex was also proposed.