Iodide ligands equivalents

More recently, Chiu and coworkers have developed a Ni complex containing a tetradentate pyridine/NHC ligand (complex 18, Eq. 22) which catalyzes the Suzuki reaction at catalyst loadings between 1 and 3 mol% [56]. Aryl iodides, bromides, and chlorides with both electron-rich and -poor aryl rings were compatible. However, electronically poor or electronically neutral aryl bromides performed much better than did electron-rich aryl bromides. It was also found that the use of 2 equivalents of PPh3 was crucial to achieving high yields with aryl chlorides. 

The scope of the Heck and related coupling reactions was substantially broadened by the development, in the last few years, of palladium/ligand combinations which are effective with the cheap and readily available but less reactive aryl chlorides [86, 87] rather than the corresponding bromides or iodides. The process still generates one equivalent of chloride, however. Of interest in this context, therefore, is the report of a halide-free Heck reaction which employs an aromatic carboxylic anhydride as the arylating agent and requires no base or phosphine ligands [89]. 

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