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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N<br />
Br<br />
SO 2CH 3<br />
+<br />
(HO) 2B CHO<br />
Pd catalyst<br />
DMF, H 2O<br />
KF or K 2CO 3<br />
N<br />
CHO<br />
SO 2CH 3<br />
Figure 3.9. The Suzuki coupl<strong>in</strong>g of an 8-bromo-6-(2-(methylsulfonyl)propan-2-<br />
yl)qu<strong>in</strong>ol<strong>in</strong>e and 3-formylphenylboronic acid<br />
(Source: Conlon et al. 2003)<br />
Leblond et al. also studied Pd/C <strong>catalysts</strong> for Suzuki coupl<strong>in</strong>gs, focus<strong>in</strong>g their<br />
studies on the conversions of aryl chlorides (Table 3.1, entry 5). They observed that<br />
addition of small amounts of triphenylphosph<strong>in</strong>e <strong>in</strong>hibited the <strong>reaction</strong> significantly,<br />
conclud<strong>in</strong>g that the heterogeneous metal particle surface was the true active catalyst,<br />
and that activation of metal halides was made possible by synergistic anchimeric and<br />
electronic effects occurr<strong>in</strong>g <strong>in</strong> the presence of adsorbed species on the catalyst surface<br />
(Leblond et al. 2001).<br />
The majority of the heterogeneous catalyst is related to silica materials s<strong>in</strong>ce<br />
they have excellent stability, high surface area, good accessibility, and organic groups<br />
can be anchored to the surface. However, it has limited stability <strong>in</strong> aqueous, especially<br />
basic conditions.<br />
Blanco et al. first prepared a hybrid organic–<strong>in</strong>organic material conta<strong>in</strong><strong>in</strong>g a<br />
macrocyclic triolef<strong>in</strong>ic <strong>palladium</strong>(0) complex covalently bonded to a silica matrix, 31<br />
(Figure 3.10), and tested catalytic activity <strong>in</strong> Suzuki <strong>cross</strong>-coupl<strong>in</strong>g <strong>reaction</strong>s with<br />
c<strong>in</strong>namyl bromide and activated aryl iodides, result<strong>in</strong>g high product formation (Table<br />
3.1, entry 6). Heterogeneous catalyst has been reused up to five cycles, although a<br />
progressive decrease <strong>in</strong> activity has been observed (Blanco et al. 2006).<br />
Baleizao et al. demonstrated that a silica or MCM-41 anchored oxime–<br />
carbapalladacycle complex, 32 (Figure 3.10), was highly active and reusable<br />
heterogeneous catalyst (Table 3.1, entry 7). They enabled to perform the Suzuki<br />
<strong>reaction</strong> of chloroarenes and phenylboronic acid <strong>in</strong> water and the catalyst could be<br />
reused eight times without loss of activity or leach<strong>in</strong>g of the Pd from the solid to the<br />
liquid phase (Baleizao et al. 2003, Baleizao et al. 2004).<br />
32