Bulky electron-rich monodentate

FIGURE 5. Sterically bulky, electronically rich, monodentate alkylphosphines and a carbene for a general reaction of acyclic secondary amines with aryl halides... 

Reductive elimination can often be facilitated by the use of catalysts bearing bulky, monodentate phosphine hgands, and is beheved to be most rapid when the two coupling partners have opposite electronic properties (i.e. one electron-rich and one electron-poor) [14]. The rates of all three steps in the catalytic cycle are believed to be maximized by employing conditions that favor the formation of intermediates bearing a single phosphine hgand [15]. 

To this end, monodentate phosphine or bidentate PX (X=P, N, O) ligands have usually been employed as ancillary ligands for transition-metal-catalyzed reactions, with bulky tertiary alkyl phosphines proving particularly effective. Significant advances have been achieved in the use of less active aryl chlorides (bond strength C-Cl>C-Br>C-I) as chemical feedstock [5], with a number of processes mediated by palladium-bulky phosphine systems. This success is often explained by the effect of bulk and electron richness at the metal center along the catalytic cycle depicted in Fig. 1 [6]. 

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