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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3.2.2. Catalysts Anchored to Inorganic Solids<br />
It has been accepted that <strong>in</strong>organic solids, unlike polymers, prevent the<br />
<strong>in</strong>termolecular aggregations of the active surface species by means of their rigid<br />
structures. Moreover, <strong>in</strong>organic solids have the advantage that they can be prepared with<br />
larger surface areas, have periodic structured porosity, and can resist thermal<br />
regeneration (Baleizao et al. 2003, Baleizao et al. 2004).<br />
In literature recently, some <strong>cross</strong>-coupl<strong>in</strong>g processes have used <strong>palladium</strong><br />
supported on various types of supports such as sepiolites, silica, zeolites, zeolitic<br />
materials, layered double hydroxides and carbon.<br />
Palladium powder has also been used as a catalyst precursor for Suzuki<br />
<strong>reaction</strong>s. Kabalka et al. found that aryl iodides readily couple with arylboronic acids <strong>in</strong><br />
reflux<strong>in</strong>g methanol for 4 hour under air, <strong>in</strong> the presence of a catalytic amount of Pd<br />
powder (Kabalka et al. 2001).<br />
Kabalka and his research group developed a novel, solventless Suzuki coupl<strong>in</strong>g<br />
methodology <strong>in</strong>clud<strong>in</strong>g the use of a commercially available KF–Al2O3 mixture, doped<br />
with a ligandless Pd(0) catalyst for <strong>cross</strong>-coupl<strong>in</strong>g <strong>reaction</strong>s of aryl, allyl halides and<br />
aryl- or alkenyl boronic acids (Table 3.1, entry 4) (Kabalka et al. 1999). Moreover, with<br />
these <strong>reaction</strong> conditions optimized, it was observed that the use of microwave<br />
irradiation accelerated the <strong>reaction</strong>s by decreas<strong>in</strong>g the <strong>reaction</strong> times from hours to<br />
m<strong>in</strong>utes (Kabalka et al. 2000, Kabalka et al. 2003).<br />
With the advantage of its relatively <strong>in</strong>expensive, stable, easily removable and<br />
reusable characteristics, <strong>palladium</strong> on carbon has been found to be another effective<br />
precatalyst for Suzuki <strong>reaction</strong>s (Gruber et al. 2004, Dyer et al. 2001).<br />
The Pd/C catalyzed the Suzuki coupl<strong>in</strong>g of an 8-bromo-6-(2-<br />
(methylsulfonyl)propan-2-yl)qu<strong>in</strong>ol<strong>in</strong>e and 3-formylphenylboronic acid (Figure 3.9).<br />
The <strong>reaction</strong> was shown to give coupl<strong>in</strong>g product <strong>in</strong> excellent yield with low levels of<br />
residual <strong>palladium</strong> after a simple filtration and precipitation (Conlon et al. 2003).<br />
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