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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CHAPTER 3<br />
THE HETEROGENEOUS SUZUKI CROSS-COUPLING<br />
REACTIONS<br />
Two types of <strong>catalysts</strong> are used <strong>in</strong> the Suzuki <strong>reaction</strong>s; heterogeneous <strong>catalysts</strong><br />
where metallic Pd is supported on a carrier and homogeneous <strong>catalysts</strong> consist<strong>in</strong>g of<br />
mononuclear Pd (ligand) complexes.<br />
Homogeneous <strong>palladium</strong> <strong>catalysts</strong> are widely used <strong>in</strong> many <strong>cross</strong>-coupl<strong>in</strong>g<br />
<strong>reaction</strong>s. These catalytic systems generally exhibit better activity and selectivity than<br />
heterogeneous systems. Although homogeneous techniques are certa<strong>in</strong>ly important <strong>in</strong><br />
terms of their substrate tolerance, catalyst load<strong>in</strong>g and reactivity, their complexity<br />
preclude the large-scale applications. Palladium ligands and precursors are also<br />
expensive, which severely restrict their <strong>in</strong>dustrial use and it is often difficult to remove<br />
these reagents at end of <strong>reaction</strong>s (Ruiz et al. 2006). Moreover, undesired byproducts<br />
and/or scrambled products may also form <strong>in</strong> the presence homogeneous <strong>catalysts</strong> (Kotha<br />
et al. 2002) (Figure 3.1.).<br />
R<br />
B(OH) 2<br />
Pd(PPh 3) 4<br />
R<br />
HO<br />
R<br />
R<br />
R<br />
self-coupl<strong>in</strong>g<br />
product<br />
scrambled<br />
product<br />
hydrolysis<br />
product<br />
Figure 3.1. Generally observed side-<strong>reaction</strong>s with Pd-catalyzed Suzuki-Miyaura <strong>cross</strong>coupl<strong>in</strong>g<br />
(Source: Kotha et al. 2002)<br />
In this aspect, heterogeneous <strong>catalysts</strong> have many advantages over homogeneous<br />
counterparts so that it can be easily removed from <strong>reaction</strong> mixtures and re-used several<br />
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