C—H Activation Assisted by Carboxylate or Carbonate Bases

Recently, interesting new systems that include carboxylates or carbonate in their structure have been broadly studied. One of the most common complexes used is palladium acetate, [Pd(OAc)j]. With this catalyst precursor, the acetate is thought to play an active role in the C-4I activation step [7d]. Therefore, a number of computational studies oriented toward the understanding of the mechanism involved in the C—H activation step have appeared in recent years, and these will be the focus of the following section. 

Lledos [26] reported on a similar mechanism, also with [Pd(OAc) J, for the selective cyclopaUada-tion of iminophosphoranes RjP=NCH2/fr. The energy profile found for the exo cyclometalation (Fig. 25.16) exhibited an agostic intermediate (5) in analogy to the mechanism described by Macgregor and coworkers (see Fig. 25.11). 

Experimental studies by Davies and coworkers [27] showed that dmba-H reacts with [IrCljCp ] in presence of sodium acetate under mild conditions to form [Ir(q-Cp )(dmba)Cl] as the final cyclometal-lated product. To obsawe the final product, the use of acetate as a base was a must, while the use of other bases such as triethylamine remained unsuccessful. This methodology is also used for the cyclometalation of imines, and similar reactivity is seen with [RhCl Cp ] and [RuCl tp-cymene)] [28]. 

Based on these experimental results, Macgregor et al. [29] reported a computational study of this cyclometalation reaction focusing on the mechanism involved in the C—H activation step (Fig. 25.17). The model system considered was [lr(q -Cp)(K -OAc)(dmba-H)] (7). The lowest-energy pathway involves a 6-membered transition state, TS(7-9), with an activation barrier of 

FIGURE 25.15 C—H bond activation of benzene (right) and methane (left) by [Pd(K -02CH)2] [25]. 

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