chapter 2 palladium catalysts in suzuki cross- coupling reaction

2.2.2.2. Halogen Effect
Nearly all reports of palladium-catalyzed coupling reactions utilized aryl
bromides, iodides, and triflates (-OTf) succesfully as substrates. On the other hand,
although chlorides are of lower cost and have wider diversity of available compounds,
the use of aryl chlorides was not very common compared to other reactants because of
their low reactivity toward Suzuki reactions.
The oxidative addition of palladium into carbon-halogen bonds mainly depends
on the strength of the bond between C-X. Bond energies for Ph-X; Cl: 96 kcalmol -1 ; Br:
81 kcalmol -1 ; I: 65 kcalmol -1 (Grushin and Alper 1994). Differences in bond energies
are due to size of halogens. The size of halogen atom increases as we go down the
periodic table. Fluorine atom is the smallest and iodine is the largest. The low reactivity
of chlorides is related to the strength of the C-Cl bond. Fluorine is unreactive with any
of the known catalysts. Unlike chlorine or fluorine, aryl iodides need low activation
energy and no ligands are required to undergo oxidative addition.
As mentioned in section 2.2.1.1, the relative reactivity towards cross-coupling
reactions decreases in the order of (Littke and Fu 2002).
2.2.2.3. Ligand Effect
C-I > C-OTf > C-Br >> C-C1 >>> C-F
In the case of aryl iodides and bromides containing electron-withdrawing
groups, even ligandless palladium catalyst precursors are sufficient to promote crosscoupling
in high turnovers. However, in the case of less reactive aryl chlorides and
electron-rich aryl bromides, the presence of added ligands is necessary to effectively
promote these cross-couplings (Zim et al. 2000).
Since the first report on coupling reactions of aryl iodides or bromides with
phenylboronic acid, Pd(PPh3)4 have become commonly used complex for Suzuki
reactions.
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