The Sonogashira Catalysts

To enable cross-couphng of more demanding substrates, highly specialized catalysts with improved reactivity are often required. Thus, great attention has been focused on the development of novel Hgands comprising P, N, O, and S donor atoms typically to facilitate oxidative addition of the haloarene component to the Pd center. 

On the basis of the overwhelming success of homogeneous Pd catalysts on the laboratory scale, widespread research interest also covered industrial appHca-tions, and a number of chemical transformations have been implemented on the industrial scale [15]. In contrast to their laboratory counterparts, suitable industrial 

N-Heterocychc carbene (NHC) ligands have recently received considerable attention as phosphine mimics in the Sonogashira and numerous other metal-catalyzed 

Catalytic activity in the Sonogashira reaction has also been demonstrated for a variety of palladacycles (Table 9.4). They typically exhibit an interesting combination of characteristics including both high catalytic activity and stability and are usually encountered as precatalyst precursors of active Pd clusters in many C-C coupling reactions [80]. 

Herrmann et al. [87] reported using only 0.1 mol% of an sp -metallated phosphine palladacycle without a cocatalyst to cross-couple alkynes with bromoarenes at 90 °C. 

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The group of Pal [51] reported an interesting sequence to thiophene-annelated a-pyrones 29 and 30 in good yields based upon the Sonogashira catalyst system (Scheme 12.19). Although the mechanistic rationale for this pseudo-three-component synthesis remains obscure because an oxidation step has to be formally Involved, the cooperatlvlty of the palladium-copper system is remarkable. 

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