Experimental Chemistry II
Experiment A4: Palladium Catalysis: The Suzuki Reaction
In this experiment, the Suzuki reaction was performed using phenylboronic acid, p-iodophenol and Pd/C catalysis in potassium carbonate aqueous media to synthesize biphenyl-4-ol. The yield of final product was only 11.94% and the measured melting point was 170-175℃. Since the appearance of the product was significantly different from expected light tan color and the measured melting point was higher than literature value, the experimental synthesized product may not be the desire final product, biphenyl-4-ol, or may contain large amount of impurities.
The Suzuki reaction was reported in 1979 by Akira Suzuki and N. Miyaura. It is commonly referred to as the palladium-catalyzed cross coupling of aryl halides with organoboron reagents leading to the formation of sp2-sp2 carbon-carbon bonds.1 It is a powerful cross coupling method that allows for the synthesis of conjugated olefins, styrenes, and biphenyls. The purpose of this experiment was to perform a palladium/charcoal catalyzed Suzuki reaction using phenylboronic acid and p-iodophenol in potassium carbonate aqueous media to synthesize a biphenyl product, biphenyl-4-ol. The overall reaction is shown as:
The mechanism of the Suzuki reaction is best viewed from a generated catalytic cycle, which is shown below:
Figure 1: A generalized palladium catalytic cycle for the Suzuki reactio.2 This cycle states the Suzuki reaction is generally achieved by three steps: oxidatice addition, transmetalation and reductive elimination.2 Oxidative addition is the rate determining step of the catalytic cycle. The palladium catalyst couples with the p-iodophenol which results in an organopalladium complex. This complex is initially in the cis conformation but isomerizes to the trans conformation. Stereochemistry with vinyl halides are maintained, while inversion of stereochemistry happens with allylic or benzylic halides.2 The role of base is to active phenylboronic acid and also to promote the formation of Ar-Pd-ArOH from ArOH-Pd-I. This step cannot occur in the absence of base and the exact mechanism have not been stated.2 Boron “ate” complexes ,via forming a quaternization of the boron with a negatively charged base, are mostly frequent seen explanation.3 An isomerization from trans complex to cis complex is required before the reductive elimination can undergo. The reductive elimination is the final step which gives the desired product and also reproduces the palladium catalyst. The regenerated palladium catalyst would participate in the reaction again to synthesize more products.2 In experiment, a ligandless palladium catalyst, Pd/C was used which is easier to handle and can be remove by simple filtration. The generalized catalytic cycle shown above is specific to palladium catalyst with ligand, such as triphenylphosphine. The procedure in this experiment could be applied to cross-coupling Suzuki reation of a variety of iodophenols and arylboronic acids, giving the corresponding hydroxybiphenyls. In many cases, the coupling products are obtained quantitatively.3
Modifications were applied in the experiments.
The filter paper was washed by 5mL of methanol and transferred the solution into 25mL Erlenmeyer flask since there was a little amount of crude product collected by suction filtration. 5mL of water was added to the solution instead of 10mL. There was no solid left in the flask before heating the solution on steam bath. The flask containing reaction solution was heated on the hot plate to evaporate the excess of methanol after it was cooling in the ice-bath. Due to limit of time, the flask was placed into the fume hood for one week after a few minutes of heating. The final product was obtained after one-week-long methanol evaporating process. Since the product was unable to measure the melting...