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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L<br />
R Pd X<br />
R Pd<br />
L<br />
OR'' X<br />
L<br />
L<br />
+<br />
OR'<br />
R'B(OH) 2(OR'')<br />
R'B(OH) 2 + OR''<br />
Transmetallation<br />
Figure 2.11. The role of base <strong>in</strong> transmetallation<br />
(Source: Genet and Saignac 1999)<br />
R<br />
L<br />
B(OH) 2(OR'') 2<br />
The most commonly used base <strong>in</strong> the <strong>cross</strong>-coupl<strong>in</strong>g <strong>reaction</strong> is Na2CO3 but this<br />
is often <strong>in</strong>effective with sterically demand<strong>in</strong>g substrates. In such <strong>in</strong>stances, Ba(OH)2 or<br />
K3PO4 has been used to generate good yields of the <strong>cross</strong>-coupl<strong>in</strong>g products. Other<br />
bases utilized <strong>in</strong> the Suzuki-Miyaura coupl<strong>in</strong>g <strong>reaction</strong> <strong>in</strong>clude Cs2CO3, K2CO3, KF and<br />
NaOH. However, 1-2 molar equivalents of bases are often required. The best choice of<br />
both solvent and amount of base chosen must be determ<strong>in</strong>ed on an <strong>in</strong>dividual basis.<br />
2.2.2.5. Additive Effect<br />
Additives have a critical role <strong>in</strong> Suzuki <strong>reaction</strong>s. Phase transfer catalysis<br />
enables <strong>reaction</strong>s between anions or molecules soluble <strong>in</strong> one phase (usually aqueous)<br />
and organic substrates soluble <strong>in</strong> organic phase. The phase-transfer <strong>catalysts</strong> such as<br />
crown esters, cryptates, ammonium salts such as tetrabutylammonium bromide (TBAB)<br />
or phosphonium salts could significantly promote the <strong>cross</strong>-coupl<strong>in</strong>g <strong>reaction</strong>.<br />
The role of the ammonium salt is thought to be twofold although not clear yet.<br />
Firstly, it facilitates solvation of the organic substrates <strong>in</strong> the solvent medium. Secondly,<br />
it enhances the rate of the coupl<strong>in</strong>g <strong>reaction</strong> by activat<strong>in</strong>g the boronic acid towards<br />
<strong>reaction</strong> by the formation of a boronate complex [ArB(OH)3] - [R4N] + (Leadbeater 2005,<br />
Leadbeater and Marco 2003, Zim et al. 2000). In addition, it is also proposed that the<br />
Pd<br />
L<br />
R'<br />
19