Sara Eavani - Faculty of Chemistry, Razi University, Kermanshah, Iran
Ehsan Azadi - Faculty of Chemistry, Razi University, Kermanshah, Iran
Mohammad Joshaghani - Faculty of Chemistry, Razi University, Kermanshah, Iran

Catalytic activity of a Pd/biphenyl based- phosphine system in the Ullmann cross-coupling reaction of various para-substituted aryl bromides were investigated. The results showed that the bulky electron-rich phosphine efficiently modify the electronic and steric properties of palladium center and promote the cross-coupling reaction. Mechanistic study using the Hammett correlation revels that electron-withdrawing substituents decrease the electron density at the palladium center and decrease the rate of oxidative-addition step such that this step will become the rate determining step (rds). On the other hand, electron-donating groups increase the rate of oxidative-addition step such that the reductive-elimination step will become the rds.

Ullmann cross-coupling, Palladium catalyst, phosphine, Hammett correlation, Mechanistic study