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7.1. C-H Arylation with Prefunctionalized Arylating Reagents A first key development in 2005 was the demonstration that Pd(OAc)2 catalyzes the ortho-phenylation of diverse arylpyridines, quinolines, pyrrolidinones, oxazolidinones, and benzodiazepines with diphenyliodonium salts (Scheme 25).71-73 The unsymmetrical mesityl/aryl-substituted iodonium reagents [MessI-Ar]BF4 could be used to install different aryl groups (eq 29). The large mesityl substituents served as ¡°dummy ligands¡± on the iodine(III), and the smaller aryl group was transferred with high selectivity (>99:1 in most cases). Generally, faster reaction rates and higher yields were observed with electron-deficient aryl groups. Several key experiments provided support for the mechanism in Scheme 26.17 First, the catalytic reaction showed a firstorder dependence on [IIII] (26), a second-order dependence on [Pd], and an inverse third-order dependence on [arylpyridine] (all consistent with the rate law derived for the sequence in Scheme 26). Second, Hammett studies with para-substituted IIII reagents showed a F value of +1.7, consistent with turnover-limiting oxidative addition. Finally, substitution of the ortho-C-H bond with a CsD bond resulted in an intra- but not an intermolecular kinetic isotope effect, indicating that C-H activation occurs after the turnover-limiting step. |
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