Leading Questions

Palladium-catalyzed C-H arylation reaction is an arylation reaction catalyzed by palladium. Arylation reaction is the addition of aryl group to a substrate. The especial part of C-H arylation reaction is that C-H bond does not need to be converted to other functionalized group. The reaction is shown below: As the reaction scheme suggested, in order to add the aryl group to the benzene, the aryl group often has a halide group on it. The reaction is straightforward. the bromide on the aryl group reacts with the unactivated C-H group, forming a biaryl compound. This arylation reaction often takes place at sp2 C-H, sp2 C-I, and C-Br bonds. Thus aryl groups can also be added to alkenes. These two reactions are called Mizoroki-Heck reaction. In this reaction, the arylation reaction occurs at alkenyl hydrogens instead of aromatic hydrogens. From the above three reactions, it is not hard to find that the common palladium catalysts used in this kind of reaction is Pd(OAc)2 and Pd(Cl)2 under basic conditions such as potassium carbonate, CH3COOK, and n-butylamine. Thus, in our reaction, silver carbonate is used as the base. It is interesting that the addition of some catalytic acid such as pivalic acid can greatly improve the yield. Silver carbonate is used to create basic conditions for the reaction to proceed after reacting with the catalytic pivalic acid. In our reaction, 1 eq. of pivalic acid was used to maximize yield. The mechanism of the palladium-catalyzed C-H arylation reaction, however, is complicated. The first step involved the insertion of palladium in sp2 C-X (X=I, Br) bond. The mechanism of this step is not very clear but should happen in theory, similar to the transfer of a single electron as the the formation of Grignard reagents. We will use Heck reaction as an example of palladium-catalyzed C-H arylation to discuss the mechanism. After the palladium gets inserted in the C-X bond, it will react with the alkene. With the help of the palladium catalyst, the aryl group is attached to the C=C end where two hydrogens are bonded. Then palladium is attached to the other end of the C=C bond. The reason why the aryl group is attached to the end shown above is likely due to sterics. The other end is more sterically hindered, so the aryl group wants to be attached to the less sterically hindered end. The palladium then takes away one of the hydrogen bonded to the carbon where the aryl group is also bonded. After the hydrogen is taken off, the electrons get pushed back to the C-C bond, restoring the original C=C bond. Then palladium complex detaches from the newly formed arylated product. Palladium is still complexed with a hydrogen and bromine atom. The decomplexation of the two loses HBr and the palladium catalyst is regenerated. Works Cited 1. http://en.wikipedia.org/wiki/Heck_reaction (accessd April 18, 2014) 2. Lafrance, M.; Fagnou, K. J. Am. Chem. Soc. 2006, 128, 16496-16497.