chapter 2 palladium catalysts in suzuki cross- coupling reaction
chapter 2 palladium catalysts in suzuki cross- coupling reaction
chapter 2 palladium catalysts in suzuki cross- coupling reaction
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2.2.2.2. Halogen Effect<br />
Nearly all reports of <strong>palladium</strong>-catalyzed coupl<strong>in</strong>g <strong>reaction</strong>s utilized aryl<br />
bromides, iodides, and triflates (-OTf) succesfully as substrates. On the other hand,<br />
although chlorides are of lower cost and have wider diversity of available compounds,<br />
the use of aryl chlorides was not very common compared to other reactants because of<br />
their low reactivity toward Suzuki <strong>reaction</strong>s.<br />
The oxidative addition of <strong>palladium</strong> <strong>in</strong>to carbon-halogen bonds ma<strong>in</strong>ly depends<br />
on the strength of the bond between C-X. Bond energies for Ph-X; Cl: 96 kcalmol -1 ; Br:<br />
81 kcalmol -1 ; I: 65 kcalmol -1 (Grush<strong>in</strong> and Alper 1994). Differences <strong>in</strong> bond energies<br />
are due to size of halogens. The size of halogen atom <strong>in</strong>creases as we go down the<br />
periodic table. Fluor<strong>in</strong>e atom is the smallest and iod<strong>in</strong>e is the largest. The low reactivity<br />
of chlorides is related to the strength of the C-Cl bond. Fluor<strong>in</strong>e is unreactive with any<br />
of the known <strong>catalysts</strong>. Unlike chlor<strong>in</strong>e or fluor<strong>in</strong>e, aryl iodides need low activation<br />
energy and no ligands are required to undergo oxidative addition.<br />
As mentioned <strong>in</strong> section 2.2.1.1, the relative reactivity towards <strong>cross</strong>-coupl<strong>in</strong>g<br />
<strong>reaction</strong>s decreases <strong>in</strong> the order of (Littke and Fu 2002).<br />
2.2.2.3. Ligand Effect<br />
C-I > C-OTf > C-Br >> C-C1 >>> C-F<br />
In the case of aryl iodides and bromides conta<strong>in</strong><strong>in</strong>g electron-withdraw<strong>in</strong>g<br />
groups, even ligandless <strong>palladium</strong> catalyst precursors are sufficient to promote <strong>cross</strong>coupl<strong>in</strong>g<br />
<strong>in</strong> high turnovers. However, <strong>in</strong> the case of less reactive aryl chlorides and<br />
electron-rich aryl bromides, the presence of added ligands is necessary to effectively<br />
promote these <strong>cross</strong>-coupl<strong>in</strong>gs (Zim et al. 2000).<br />
S<strong>in</strong>ce the first report on coupl<strong>in</strong>g <strong>reaction</strong>s of aryl iodides or bromides with<br />
phenylboronic acid, Pd(PPh3)4 have become commonly used complex for Suzuki<br />
<strong>reaction</strong>s.<br />
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