Palladacycles reagents

Controlling secondary reactions required using appropriate ratios among reagents. Special attention was devoted to norbomene although it acts as a catalyst its concentration must be kept sufficiently high to enable it to form the palladacycle by insertion, but not so high as to prevent its liberation at the end of the sequence. 

Later in 2010, Wu and co-workers developed a series of carbene adducts of cyclo-palladated ferrocenylimine (Figure 7) and evaluated their activity in the KTC reaction between aryl halides and aryl Grignard reagents. Palladacycle 68 was identified as the most active catalyst, generating a number of di- and tri-orf/io-substituted biaryls in good to excellent yield using 0.5 mol% catalyst loading and 2 equiv. of LiCl (Scheme 6). 

Dimetallation may be understood by participation of both palladacycles 167 and 168. In general, addition of dimetallic reagents occurs via the palladacycles 177 with cleavage of the proximal bond to provide 178. For example, borylsilylation of the MCP 179 with 180 gave 181 using a combination of Pd(OAc)2 with r-octyl isocyanide (XVII-6). On the other hand, addition via the distal bond cleavage was observed in the reaction of the same reagent 180 with the MCP 182, and the product 183 was obtained via 184 [57]. 

Halogen-bridged dimeric palladacycles can serve as precursors to monomeric derivatives that can readily be obtained by treating the dimers with various reagents, such as phosphines and organometals (Scheme 35). 

It emerges that the main steps of the former textbook mechanism proposed by Heck have been confirmed (Scheme 1.8). However, the catalytic cycle may involve intermediate palladium complexes whose structures differ from those originally proposed, depending on the experimental conditions. One must also take into account the fact that new reagents (aryl triflates), new ligands (bidentate ligands, carbenes, bulky phosphines, etc.) and new precursors (palladacycles) have been introduced a long time after Heck s proposal. 

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